The influence of nicotine form on its absorption

The influence of nicotine form on its absorption | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article The influence of nicotine form on its absorption Zhuo Wang, Huapeng Cui, Suxing Tuo, Wen Du, Zhiguo Wang This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7628503/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 06 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted 10 You are reading this latest preprint version Abstract Elucidating the pharmacokinetic profiles of different chemical forms of nicotine (free-base versus protonated)—particularly their differences in biological absorption—is critically important for balancing its potential clinical applications in neurodegenerative disease therapy with effective management of its inherent addiction risk. Current empirical observations regarding which form exhibits superior absorption remain significantly divergent. This study employs an aerosol delivery model to compare the permeability (simulated absorption) of the two nicotine forms in solution by quantifying the amount absorbed and the residual nicotine after aerosol penetration through a simulated solution barrier. The results demonstrate that free-base nicotine exhibits higher solution absorption than its protonated counterpart. The superior absorption of free-base nicotine is primarily attributed to its diffusion from the particulate phase to the gas phase within aerosols, facilitating efficient mass transfer across the gas–liquid interface. These findings provide an in vitro experimental basis for further evaluation of the potential differences in human absorption efficiency between nicotine forms, thereby supporting the development of nicotine-based pharmaceuticals. Physical sciences/Chemistry Biological sciences/Drug discovery Earth and environmental sciences/Environmental sciences Aerosol Nicotine forms Absorption pathways Solution permeation Figures Figure 1 Figure 2 Figure 3 Introduction Nicotine, as the primary psychoactive component in tobacco, has long attracted widespread attention due to its high addictive potential [ 1 ]. The adverse health effects of smoking are primarily attributed to harmful constituents in smoke, such as polycyclic aromatic hydrocarbons, nitrosamines, and tar, which can damage multiple organs including the lungs and heart, and significantly increase the risk of carcinogenesis [ 2 ]. In contrast, the core role of nicotine itself lies in its mechanism of addiction: as a potent agonist, nicotine rapidly crosses the blood-brain barrier and acts on nicotinic acetylcholine receptors (nAChRs) in the central nervous system. Due to its relatively slow metabolism, nicotine causes sustained activation of these receptors, leading to receptor desensitization and upregulation, and ultimately resulting in physiological dependence [ 3 ]. Beyond physiological dependence, nicotine addiction is also accompanied by prominent withdrawal symptoms, such as anxiety, irritability, poor concentration, and sleep disturbances, which severely disrupt daily life. Additionally, nicotine exerts a direct effect on the cardiovascular system, causing increased heart rate and blood pressure, and elevating the risk of cardiovascular diseases like myocardial ischemia and arrhythmia. Long-term addiction may further pose potential negative impacts on brain cognitive functions (e.g., memory and decision-making abilities), exacerbating health hazards [ 4 ]. However, recent studies have revealed that nicotine’s action on nAChRs may also possess therapeutic potential [ 5 – 7 ]. It can modulate ion channel signaling, promote neurotransmitter release, and activate the cholinergic anti-inflammatory pathway to regulate immune responses, thereby exhibiting neuroprotective properties. Epidemiological {8] and experimental evidence{9] suggests that nicotine may have certain efficacy in interventions for neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. It is noteworthy that the biological effects of nicotine exhibit dose dependency: moderate exposure may confer neuroprotection, whereas long-term high-dose exposure tends to induce dependence and neurological damage. Therefore, a thorough understanding of the absorption kinetics of nicotine in the human body is crucial for balancing its therapeutic potential against addiction risks. The chemical form (protonation state) of nicotine is an important factor influencing its absorption efficiency, yet existing research findings show significant discrepancies. Under physiological conditions, nicotine exists mainly in its unprotonated free base form (I) or mono-/di-protonated salt forms (II/III, i.e., nicotine salts). On one hand, Takano et al. [ 10 , 11 ] using a rat alveolar epithelial cell model found that increasing extracellular pH enhanced nicotine absorption; Burch et al. [ 12 ] also observed a positive correlation between higher smoke pH and increased nicotine absorption in smokers. These results suggest that the free base form of nicotine has higher bioavailability under alkaline conditions. On the other hand, patent data from Pax Labs [ 13 ] and clinical trials by O’Connell et al. [ 14 ] demonstrated that under the same administration conditions, nicotine benzoate or nicotine lactate salts yielded higher plasma concentrations and faster absorption rates compared to equivalent concentrations of free base nicotine. This implies that salt forms of nicotine may have superior absorption efficacy. The complexity of biological systems has thus far prevented a clear consensus regarding these conflicting results. To clarify the direct impact of nicotine form on its absorption capacity, this study employed an aerosol delivery model, generating aerosols of free base nicotine and protonated nicotine (nicotine salt), respectively. By measuring the amount of nicotine absorbed and residual after aerosol penetration through a simulated solution barrier, we compared the solution permeability (simulating absorption) of the two nicotine forms, thereby assessing their potential differences in human absorption efficiency. The findings of this study are expected to provide important in vitro experimental evidence for the application of nicotine in the pharmaceutical field, such as in the development of neuroprotective agents. Experimental Experimental Materials Propylene Glycol (99%, China National Pharmaceutical Group Corporation.), Glycerol (99%, China National Pharmaceutical Group Corporation.), Benzoic Acid (99%, China National Pharmaceutical Group Corporation.), Sodium Hydroxide (99%, China National Pharmaceutical Group Corporation.), Hydrochloric Acid (99%, China National Pharmaceutical Group Corporation.), Dichloromethane (99%, China National Pharmaceutical Group Corporation), Deionized Water. Experimental Methods Preparation of Test E-liquids: A base vehicle was prepared by mixing propylene glycol and glycerol in a 1:1 mass ratio, followed by the addition of deionized water to achieve a final water content of 5% (w/w). To this base vehicle, the following were separately added:① free-base nicotine, and② nicotine benzoate salt. Both test e-liquids were formulated to a final nicotine mass fraction of 5%. Preparation of Absorption Solutions: The following absorption solutions (20 mL each) were prepared and placed into individual absorption bottles:① Ethanol;② Ethanol with 1 mmol/L HCl;③ Ethanol with 1 mmol/L NaOH;④ Deionized water (H₂O);⑤ Aqueous solution with 1 mmol/L HCl;⑥ Aqueous solution with 1 mmol/L NaOH. Experimental Setup and Procedure: The aerosol delivery system was assembled as follows: test e-liquids containing different nicotine forms were loaded into an aerosol generator. The generator outlet was connected in series to an absorption bottle (maintained at 37°C using a water bath) containing 20 mL of absorption solution, a trapping unit equipped with a Cambridge filter pad, and finally a puffing device (Fig. 1 ). The puffing device was programmed to operate under a square-wave puff profile with the following parameters: puff volume of 55 mL, puff duration of 3s, and puff interval of 30 s. Each experiment consisted of 20 consecutive puffs. All experiments were performed in duplicate. The procedure was carried out as follows: ① The initial mass of the test e-liquid was recorded: ② The system was connected, and the puffing protocol was initiated.;③ After completion, the system was disconnected, and the test e-liquid was reweighed. The difference in mass was recorded as e-liquid consumption༛④ The Cambridge filter pad was collected and extracted with 10 mL of dichloromethane. The extract was analyzed via gas chromatography–mass spectrometry (GC–MS) to quantify the amount of particulate-phase nicotine trapped༛⑤ For ethanol-based absorption solutions, the solution was directly sampled and analyzed by GC–MS to determine the amount of nicotine absorbed. Results and Discussion 2.1 Comparison of Absorption Capacity of Different Nicotine Forms in Ethanol-Based Absorption Solutions Using ethanol as the absorption solution, the captured amounts of different nicotine forms (free-base nicotine and nicotine benzoate) in both the absorption bottle and the Cambridge filter pad were determined. The absorption differences between the two nicotine forms were compared by calculating the ratio of the amount captured on the filter pad to that absorbed in the solution (pad-to-solution nicotine capture ratio). The results are presented in Table 1 and illustrated in Fig. 2 . The raw data and data processing procedure are available in the Supporting Information (SI) Table S1 . Table 1 Comparison of the absorption of different nicotine forms in ethanolic-based absorption solutions with varying pH values Absorption Solution Test E-liquid Nicotine Captured in Absorption Solution Nicotine Captured on Filter Pad Nicotine Capture Ratio (Filter Pad/Absorption Solution) Mean Value Nicotine Salt/Free-base Nicotine Ratio Ethanol Nicotine 1 1.35 1.71 1.26 1.23 1.59 Nicotine 2 1.52 1.82 1.20 Nicotine Salt 1 0.80 1.68 2.10 1.96 Nicotine Salt 2 0.83 1.50 1.81 1 mmol/L HCl in ethanol solution Nicotine 1 1.52 1.88 1.24 1.28 1.49 Nicotine 2 1.37 1.81 1.32 Nicotine Salt 1 0.72 1.52 2.12 1.91 Nicotine Salt 2 0.89 1.51 1.71 1 mmol/L NaOH in ethanol solution​ Nicotine 1 1.72 1.90 1.10 1.09 1.55 Nicotine 2 1.80 1.93 1.07 Nicotine Salt 1 0.85 1.56 1.85 1.69 Nicotine Salt 2 0.96 1.46 1.52 The experimental results indicate that under all absorption conditions (ethanol, ethanolic 1 mmol/L HCl solution, and ethanolic 1 mmol/L NaOH solution), the capture efficiency of nicotine salt on the Cambridge filter pad was significantly higher than that of free-base nicotine. This implies that the solution absorption efficiency of free-base nicotine was higher than that of nicotine salt. Specifically, for the ethanolic absorption solution, the ratio of the amount of free-base nicotine captured on the Cambridge filter pad to that absorbed in the solution was 1.23, whereas the ratio for nicotine salt was 1.96–1.59 times that of the free-base form. This trend was consistently observed in both acidic (ethanolic 1 mmol/L HCl solution) and alkaline (ethanolic 1 mmol/L NaOH solution) absorption media. The corresponding pad-to-solution capture ratios for free-base nicotine were 1.28 (acidic) and 1.09 (alkaline), while those for nicotine salt were 1.91 (acidic) and 1.69 (alkaline). Collectively, these results demonstrate that compared to free-base nicotine, a higher proportion of nicotine salt aerosol particles remained in the aerosol phase (i.e., were captured by the Cambridge filter pad) after passing through the absorption solution, corresponding to a lower absorption efficiency into the solution. This discrepancy originates from differences in the aerosol kinetic behavior and absorption pathways of the two nicotine forms [ 15 – 17 ]. Free-base nicotine, with lower polarity and higher volatility, readily volatilizes from aerosol particles into the gas phase, subsequently diffusing across the gas-liquid interface and dissolving into the absorption solution—hence its higher solution absorption efficiency. In contrast, nicotine salt, constrained by ionic bonding, exhibits minimal volatility. Its absorption primarily depends on the direct contact and dissolution of nicotine-containing aerosol particles with the absorption solution, resulting in relatively lower solution absorption efficiency. Furthermore, comparison across absorption solutions with different pH values revealed that the pad-to-solution capture ratios for both free-base and nicotine salt were similar in acidic and neutral solutions (free-base: 1.28 and 1.23; bound-form: 1.91 and 1.96, respectively). However, in the alkaline absorption solution (ethanolic 1 mmol/L NaOH), the ratios decreased for both forms (free-base: 1.09; bound-form: 1.69), indicating enhanced solution absorption of nicotine under alkaline conditions. This phenomenon is related to the polarity of the absorption solution and the distribution of nicotine species. As a weak base, nicotine exists predominantly in the protonated (bound) form under acidic and neutral conditions, whereas the deprotonated (free-base) form becomes predominant under alkaline conditions (1 mmol/L NaOH) [ 18 ]. Ethanol, being a weakly polar solvent, exhibits a greater dissolution capacity for molecular organic species (e.g., free-base nicotine) than for ionic compounds (e.g., nicotine salts). Therefore, the increased proportion of the free-base form under alkaline conditions improves the compatibility between nicotine and ethanol, thereby promoting solution absorption. Notably, however, the ratio of the pad-to-solution capture ratio of nicotine salt to that of free-base nicotine (bound value / free value) did not vary significantly across different pH conditions (acidic: 1.49; neutral: 1.59; alkaline: 1.55). This suggests that the facilitative effect of alkaline conditions on absorption was consistent for both nicotine forms. The bulk absorption solution likely possessed sufficient buffering capacity to maintain the local chemical environment, thereby limiting the influence of the initial chemical form of nicotine on its ultimate dissolution behavior in the solution. This further supports the conclusion that the differences in solution absorption between nicotine forms primarily stem from disparities in aerosol kinetic behavior and absorption pathways, with the intrinsic solubility playing a comparatively minor role. 2.2 Comparison of Absorption Capacity of Different Nicotine Forms in Aqueous-Based Absorption Solutions Given that human body fluids are essentially aqueous systems, the aforementioned experiments were repeated using aqueous-based absorption solutions to enhance physiological relevance. Since the measurement methodology employed in this study does not permit direct quantification of nicotine absorbed in aqueous solutions via gas chromatography–mass spectrometry (GC–MS), a normalization approach was adopted to evaluate absorption efficiency. The amount of nicotine captured by the Cambridge filter pad and the total consumption of nicotine solution in each experiment were determined. This allowed for the calculation of the normalized residual amount of nicotine—defined as the quantity of nicotine captured by the filter pad per unit of nicotine solution (or nicotine salt solution) consumed, which represents the fraction of aerosolized nicotine not absorbed by the solution. By systematically comparing this normalized residual amount between different initial forms (free-base nicotine and nicotine salt), the relative solution absorption efficiency in aqueous-based absorption solutions was indirectly assessed (a higher residual value indicates lower solution absorption efficiency). The relevant experimental results are summarized in Table 2 and illustrated in Fig. 3 . The raw data and data processing procedure are available in the Supporting Information Table S2. Table 2 Comparison of the absorption capacity of nicotine/nicotine salt in aqueous-based absorption solutions with different pH values Absorption Solution Test E-liquid E-liquid Consumption (g) Nicotine Capture on Filter Pad Nicotine Captured per 1 g E-liquid Consumed Mean Value Nicotine Salt/Free-base Nicotine Ratio H 2 O Nicotine 1 0.128 0.174 1.36 1.39 1.54 Nicotine 2 0.145 0.204 1.41 Nicotine Salt 1 0.092 0.197 2.15 2.14 Nicotine Salt 2 0.088 0.188 2.12 1 mmol/L HCl in H 2 O Nicotine 1 0.146 0.201 1.38 1.39 1.57 Nicotine 2 0.153 0.214 1.40 Nicotine Salt 1 0.090 0.199 2.20 2.18 Nicotine Salt 2 0.093 0.200 2.16 1 mmol/L NaOH in H 2 O Nicotine 1 0.146 0.200 1.37 1.40 1.55 Nicotine 2 0.149 0.215 1.44 Nicotine Salt 1 0.094 0.204 2.19 2.18 Nicotine Salt 2 0.092 0.201 2.18 As evidenced by the results presented above, under all aqueous absorption conditions (water, aqueous 1 mmol/L HCl solution, and aqueous 1 mmol/L NaOH solution), the Cambridge filter pad capture efficiency of nicotine salt was significantly higher than that of free-base nicotine. This indicates that the solution absorption efficiency of free-base nicotine is higher than that of nicotine salt. Specifically, when using water as the absorption solution, the ratio of nicotine salt to free-base nicotine captured per unit of nicotine-containing test solution consumed was 1.54, indicating that a greater amount of nicotine salt was retained on the filter pad and thus exhibited lower solution absorption. This trend was consistently observed in both acidic (aqueous 1 mmol/L HCl) and alkaline (aqueous 1 mmol/L NaOH) solutions, with corresponding ratios of 1.57 and 1.55, respectively. In contrast to the observations in ethanolic solutions, the pH of the aqueous absorption solution did not exert a noticeable influence on the relative absorption behavior of the two nicotine forms. Although the physicochemical properties of the absorption medium (i.e., solvent type and pH) modulated the absolute absorption quantity of nicotine, they did not alter the relative absorption hierarchy between the two chemical forms. This universal behavior stems from the fundamental difference in the mass transfer mechanisms of the two nicotine species. Free-base nicotine, characterized by lower molecular polarity and higher volatility, readily partitions from the aerosol particle phase into the gas phase, subsequently undergoing efficient dissolution into the absorption solution via gas-liquid interfacial diffusion-dominated transfer. Consequently, it demonstrates superior solution absorption performance. In contrast, the absorption of nicotine salt is severely constrained by strong ionic association, which significantly suppresses its volatility. Its release and dissolution are predominantly dependent on direct particle–solution contact and solid–liquid interfacial dissolution kinetics, resulting in systematically lower solution absorption efficiency. In summary, while environmental parameters such as solvent polarity and pH modulate the absolute extent of nicotine absorption, the chemical state of nicotine (free-base vs. bound-form) remains the central mechanistic determinant of relative absorption efficiency. This is achieved by governing the contribution of two competing mass transfer pathways: volatilization–diffusion versus particle dissolution. 3. Conclusion This study employed an aerosol delivery model to investigate the solution penetration (simulated absorption) capacity of free-base nicotine versus protonated nicotine (nicotine salt). It was demonstrated that free-base nicotine is more readily absorbed into solution compared to its bound-form counterpart. The type of solution (ethanol or water) and its pH value did not alter the relative absorption strength between the two nicotine forms. The observed difference in absorption efficiency is attributed to their distinct absorption pathways: free-base nicotine, due to its ability to transfer from the aerosol particle phase to the gas phase and subsequently diffuse into the solution, achieves higher absorption. Through the aerosol delivery model, this study confirms that the solution penetration efficiency of free-base nicotine is significantly higher than that of protonated nicotine. The relative absorption strength ratio remained constant across both ethanolic and aqueous systems over a wide pH range, indicating that the physicochemical properties of the absorption medium modulate only the absolute absorption quantity without altering the fundamental inter-form difference. The underlying mechanism originates from the divergence in mass transfer pathways: free-base nicotine, leveraging its lower polarity and higher volatility, undergoes efficient absorption via a multi-stage route of "aerosol particle phase to gas phase to gas-liquid diffusion". In contrast, protonated nicotine, constrained by ionic bonding, relies solely on a single-rate-limiting pathway of "direct particle-solution contact and dissolution". These findings reveal that the protonation state of nicotine governs its interphase transport kinetics by regulating the competition between two pathways—volatilization-diffusion and interfacial dissolution. This study provides important in vitro experimental evidence for evaluating the therapeutic potential and addiction risks of nicotine, thereby supporting its future applications in the pharmaceutical field. Declarations Supporting Information The raw data and the detailed process of data analysis are provided in Table S1-S2. AUTHOR INFORMATION Corresponding author Zhiguo Wang. E-mail: [email protected] Author Contributions Zhuo Wang：Conceptualization, Methodology, Curation, Formal Analysis, Validation, Writing - Original Draft. Huapeng Cui：Conceptualization, Methodology, Data Curation, Formal Analysis, Validation. Suxing Tuo：Conceptualization, Methodology, Investigation, Supervision, Writing-Review & Editing. Wen Du：Conceptualization, Methodology, Supervision, Project Administration, Writing-Review & Editing, Writing - Review & Editing. Zhiguo Wang*：Conceptualization, Methodology, Supervision, Project Administration, Writing-Review & Editing. + These authors contributed equally to this work Notes This study is a pure academic research. All experimental data and results are publicly available free of charge, and there are no conflicts of interest. FUNDING DECLARATION This work was supported by the Postdoctoral Foundation of China Tobacco Hunan Industrial Co., Ltd(KY2024JC0007), and completed with the technical support of the Zhengzhou Tobacco Research Institute of CNTC. The research results do not involve any commercial interests. References Le Foll, B. et al. Tobacco and nicotine use. Nat. Rev. Dis. Primers . 8 , 19 (2022). Li, Y. P. & Hecht, S. S. Carcinogenic components of tobacco and tobacco smoke: a 2022 update. Food Chem. Toxicol. 65 , 113179 (2022). Fisher, M. L., Pauly, J. R., Froeliger, B. & Turner, J. R. Translational research in nicotine addiction. CSH Perspect. Med. 11 (6), a039776 (2021). Betts, J. M. et al. Expanding the scope of the withdrawal syndrome: Anhedonia as a core nicotine withdrawal symptom. J. Psychopathol. Clin. Sci. 134 (5), 540–553 (2025). Soares, E. N. et al. Nicotinic acetylcholine receptors in glial cells as molecular target for parkinson’s disease. Cells 13 (6), 474 (2024). Peng, H. 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Supplementary Files SITheinfluenceofnicotineformonitsabsorption.docx Cite Share Download PDF Status: Published Journal Publication published 06 Mar, 2026 Read the published version in Scientific Reports → Version 1 posted Editorial decision: Revision requested 06 Jan, 2026 Reviews received at journal 29 Dec, 2025 Reviews received at journal 25 Dec, 2025 Reviewers agreed at journal 25 Dec, 2025 Reviewers agreed at journal 23 Dec, 2025 Reviewers invited by journal 23 Dec, 2025 Editor assigned by journal 14 Nov, 2025 Editor invited by journal 31 Oct, 2025 Submission checks completed at journal 14 Oct, 2025 First submitted to journal 14 Oct, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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13:54:12","extension":"xml","order_by":10,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":65886,"visible":true,"origin":"","legend":"","description":"","filename":"417e68917368423f92f4a235d3b916231structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/0882621d9fcc44031d146145.xml"},{"id":98999602,"identity":"0624dfbc-8804-4dc5-840b-d84f057de354","added_by":"auto","created_at":"2025-12-25 13:54:13","extension":"html","order_by":11,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":75404,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/89ef6aacd3891e4bb5544e69.html"},{"id":98999603,"identity":"2954b4e4-bac1-435e-bf9c-7cfc7132c7ad","added_by":"auto","created_at":"2025-12-25 13:54:13","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":26181,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of the measurement apparatus. 1: Aerosol generator. 2: Absorption bottle. 3: Cambridge filter pad.4: Puffing device\u003c/p\u003e","description":"","filename":"floatimage141.png","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/ab0a0ffb9e4ab11b53d78d8c.png"},{"id":98999607,"identity":"45308713-4592-4e14-b227-4e399d3bc46e","added_by":"auto","created_at":"2025-12-25 13:54:14","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":21184,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of absorption of different nicotine forms in ethanol-based absorption solutions across varying pH levels. Blue bars represent the results for free-base nicotine, yellow bars indicate the results for nicotine salt, and red data points illustrate the comparative ratio between the two forms.\u003c/p\u003e","description":"","filename":"Onlinefloatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/55c6a8b22415a5857f5189d6.png"},{"id":98999591,"identity":"bcde3001-9c5a-4912-82b3-ea1c430ad6a4","added_by":"auto","created_at":"2025-12-25 13:54:10","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":19446,"visible":true,"origin":"","legend":"\u003cp\u003eComparison of absorption of different nicotine forms in H\u003csub\u003e2\u003c/sub\u003eO-based absorption solutions across varying pH levels. Blue bars represent the results for free-base nicotine, yellow bars indicate the results for nicotine salt, and red data points illustrate the comparative ratio between the two forms.\u003c/p\u003e","description":"","filename":"Onlinefloatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/9b1af7e6f0681ebe8edfaafd.png"},{"id":104250980,"identity":"f331d6ea-53a4-4250-ad35-7fb2d03c7e5c","added_by":"auto","created_at":"2026-03-09 16:11:42","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":888070,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/68639384-57e3-48ef-ab08-e1fd5db21c3c.pdf"},{"id":99312548,"identity":"8ba02a09-14de-4801-9c55-13e2fb6c0325","added_by":"auto","created_at":"2025-12-31 16:19:04","extension":"docx","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":26576,"visible":true,"origin":"","legend":"","description":"","filename":"SITheinfluenceofnicotineformonitsabsorption.docx","url":"https://assets-eu.researchsquare.com/files/rs-7628503/v1/d9756b77a2454d3abc02469b.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"The influence of nicotine form on its absorption","fulltext":[{"header":"Introduction","content":"\u003cp\u003eNicotine, as the primary psychoactive component in tobacco, has long attracted widespread attention due to its high addictive potential [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The adverse health effects of smoking are primarily attributed to harmful constituents in smoke, such as polycyclic aromatic hydrocarbons, nitrosamines, and tar, which can damage multiple organs including the lungs and heart, and significantly increase the risk of carcinogenesis [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. In contrast, the core role of nicotine itself lies in its mechanism of addiction: as a potent agonist, nicotine rapidly crosses the blood-brain barrier and acts on nicotinic acetylcholine receptors (nAChRs) in the central nervous system. Due to its relatively slow metabolism, nicotine causes sustained activation of these receptors, leading to receptor desensitization and upregulation, and ultimately resulting in physiological dependence [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Beyond physiological dependence, nicotine addiction is also accompanied by prominent withdrawal symptoms, such as anxiety, irritability, poor concentration, and sleep disturbances, which severely disrupt daily life. Additionally, nicotine exerts a direct effect on the cardiovascular system, causing increased heart rate and blood pressure, and elevating the risk of cardiovascular diseases like myocardial ischemia and arrhythmia. Long-term addiction may further pose potential negative impacts on brain cognitive functions (e.g., memory and decision-making abilities), exacerbating health hazards [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eHowever, recent studies have revealed that nicotine\u0026rsquo;s action on nAChRs may also possess therapeutic potential [\u003cspan additionalcitationids=\"CR6\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. It can modulate ion channel signaling, promote neurotransmitter release, and activate the cholinergic anti-inflammatory pathway to regulate immune responses, thereby exhibiting neuroprotective properties. Epidemiological {8] and experimental evidence{9] suggests that nicotine may have certain efficacy in interventions for neurodegenerative diseases such as Alzheimer's disease and Parkinson's disease. It is noteworthy that the biological effects of nicotine exhibit dose dependency: moderate exposure may confer neuroprotection, whereas long-term high-dose exposure tends to induce dependence and neurological damage. Therefore, a thorough understanding of the absorption kinetics of nicotine in the human body is crucial for balancing its therapeutic potential against addiction risks.\u003c/p\u003e \u003cp\u003eThe chemical form (protonation state) of nicotine is an important factor influencing its absorption efficiency, yet existing research findings show significant discrepancies. Under physiological conditions, nicotine exists mainly in its unprotonated free base form (I) or mono-/di-protonated salt forms (II/III, i.e., nicotine salts). On one hand, Takano et al. [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] using a rat alveolar epithelial cell model found that increasing extracellular pH enhanced nicotine absorption; Burch et al. [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] also observed a positive correlation between higher smoke pH and increased nicotine absorption in smokers. These results suggest that the free base form of nicotine has higher bioavailability under alkaline conditions. On the other hand, patent data from Pax Labs [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e] and clinical trials by O\u0026rsquo;Connell et al. [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e] demonstrated that under the same administration conditions, nicotine benzoate or nicotine lactate salts yielded higher plasma concentrations and faster absorption rates compared to equivalent concentrations of free base nicotine. This implies that salt forms of nicotine may have superior absorption efficacy. The complexity of biological systems has thus far prevented a clear consensus regarding these conflicting results.\u003c/p\u003e \u003cp\u003eTo clarify the direct impact of nicotine form on its absorption capacity, this study employed an aerosol delivery model, generating aerosols of free base nicotine and protonated nicotine (nicotine salt), respectively. By measuring the amount of nicotine absorbed and residual after aerosol penetration through a simulated solution barrier, we compared the solution permeability (simulating absorption) of the two nicotine forms, thereby assessing their potential differences in human absorption efficiency. The findings of this study are expected to provide important in vitro experimental evidence for the application of nicotine in the pharmaceutical field, such as in the development of neuroprotective agents.\u003c/p\u003e"},{"header":"Experimental","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eExperimental Materials\u003c/h2\u003e \u003cp\u003ePropylene Glycol (99%, China National Pharmaceutical Group Corporation.), Glycerol (99%, China National Pharmaceutical Group Corporation.), Benzoic Acid (99%, China National Pharmaceutical Group Corporation.), Sodium Hydroxide (99%, China National Pharmaceutical Group Corporation.), Hydrochloric Acid (99%, China National Pharmaceutical Group Corporation.), Dichloromethane (99%, China National Pharmaceutical Group Corporation), Deionized Water.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eExperimental Methods\u003c/h3\u003e\n\u003cp\u003ePreparation of Test E-liquids: A base vehicle was prepared by mixing propylene glycol and glycerol in a 1:1 mass ratio, followed by the addition of deionized water to achieve a final water content of 5% (w/w). To this base vehicle, the following were separately added:① free-base nicotine, and② nicotine benzoate salt. Both test e-liquids were formulated to a final nicotine mass fraction of 5%.\u003c/p\u003e \u003cp\u003ePreparation of Absorption Solutions: The following absorption solutions (20 mL each) were prepared and placed into individual absorption bottles:① Ethanol;② Ethanol with 1 mmol/L HCl;③ Ethanol with 1 mmol/L NaOH;④ Deionized water (H₂O);⑤ Aqueous solution with 1 mmol/L HCl;⑥ Aqueous solution with 1 mmol/L NaOH.\u003c/p\u003e \u003cp\u003eExperimental Setup and Procedure: The aerosol delivery system was assembled as follows: test e-liquids containing different nicotine forms were loaded into an aerosol generator. The generator outlet was connected in series to an absorption bottle (maintained at 37\u0026deg;C using a water bath) containing 20 mL of absorption solution, a trapping unit equipped with a Cambridge filter pad, and finally a puffing device (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe puffing device was programmed to operate under a square-wave puff profile with the following parameters: puff volume of 55 mL, puff duration of 3s, and puff interval of 30 s. Each experiment consisted of 20 consecutive puffs. All experiments were performed in duplicate.\u003c/p\u003e \u003cp\u003eThe procedure was carried out as follows: ① The initial mass of the test e-liquid was recorded: ② The system was connected, and the puffing protocol was initiated.;③ After completion, the system was disconnected, and the test e-liquid was reweighed. The difference in mass was recorded as e-liquid consumption༛④ The Cambridge filter pad was collected and extracted with 10 mL of dichloromethane. The extract was analyzed via gas chromatography\u0026ndash;mass spectrometry (GC\u0026ndash;MS) to quantify the amount of particulate-phase nicotine trapped༛⑤ For ethanol-based absorption solutions, the solution was directly sampled and analyzed by GC\u0026ndash;MS to determine the amount of nicotine absorbed.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cp\u003e \u003cb\u003e2.1 Comparison of Absorption Capacity of Different Nicotine Forms in Ethanol-Based Absorption Solutions\u003c/b\u003e \u003c/p\u003e \u003cp\u003eUsing ethanol as the absorption solution, the captured amounts of different nicotine forms (free-base nicotine and nicotine benzoate) in both the absorption bottle and the Cambridge filter pad were determined. The absorption differences between the two nicotine forms were compared by calculating the ratio of the amount captured on the filter pad to that absorbed in the solution (pad-to-solution nicotine capture ratio). The results are presented in Table\u0026nbsp;\u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The raw data and data processing procedure are available in the Supporting Information (SI) Table \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab1\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of the absorption of different nicotine forms in ethanolic-based absorption solutions with varying pH values\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"7\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAbsorption Solution\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTest E-liquid\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eNicotine Captured in Absorption Solution\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNicotine Captured on Filter Pad\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNicotine Capture Ratio (Filter Pad/Absorption Solution)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMean Value\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c7\"\u003e \u003cp\u003eNicotine Salt/Free-base Nicotine Ratio\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eEthanol\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.35\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.71\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.26\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.23\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.59\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.82\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.20\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.68\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.96\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.81\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003e1 mmol/L HCl in ethanol solution\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.88\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.28\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.49\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.81\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e2.12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.91\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.89\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.51\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.71\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003e1 mmol/L NaOH in ethanol solution​\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.72\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.90\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.10\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c7\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e1.80\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.07\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.56\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.85\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\" morerows=\"1\" rowspan=\"2\"\u003e \u003cp\u003e1.69\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e0.96\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1.46\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e1.52\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe experimental results indicate that under all absorption conditions (ethanol, ethanolic 1 mmol/L HCl solution, and ethanolic 1 mmol/L NaOH solution), the capture efficiency of nicotine salt on the Cambridge filter pad was significantly higher than that of free-base nicotine. This implies that the solution absorption efficiency of free-base nicotine was higher than that of nicotine salt. Specifically, for the ethanolic absorption solution, the ratio of the amount of free-base nicotine captured on the Cambridge filter pad to that absorbed in the solution was 1.23, whereas the ratio for nicotine salt was 1.96\u0026ndash;1.59 times that of the free-base form. This trend was consistently observed in both acidic (ethanolic 1 mmol/L HCl solution) and alkaline (ethanolic 1 mmol/L NaOH solution) absorption media. The corresponding pad-to-solution capture ratios for free-base nicotine were 1.28 (acidic) and 1.09 (alkaline), while those for nicotine salt were 1.91 (acidic) and 1.69 (alkaline). Collectively, these results demonstrate that compared to free-base nicotine, a higher proportion of nicotine salt aerosol particles remained in the aerosol phase (i.e., were captured by the Cambridge filter pad) after passing through the absorption solution, corresponding to a lower absorption efficiency into the solution.\u003c/p\u003e \u003cp\u003eThis discrepancy originates from differences in the aerosol kinetic behavior and absorption pathways of the two nicotine forms [\u003cspan additionalcitationids=\"CR16\" citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. Free-base nicotine, with lower polarity and higher volatility, readily volatilizes from aerosol particles into the gas phase, subsequently diffusing across the gas-liquid interface and dissolving into the absorption solution\u0026mdash;hence its higher solution absorption efficiency. In contrast, nicotine salt, constrained by ionic bonding, exhibits minimal volatility. Its absorption primarily depends on the direct contact and dissolution of nicotine-containing aerosol particles with the absorption solution, resulting in relatively lower solution absorption efficiency.\u003c/p\u003e \u003cp\u003eFurthermore, comparison across absorption solutions with different pH values revealed that the pad-to-solution capture ratios for both free-base and nicotine salt were similar in acidic and neutral solutions (free-base: 1.28 and 1.23; bound-form: 1.91 and 1.96, respectively). However, in the alkaline absorption solution (ethanolic 1 mmol/L NaOH), the ratios decreased for both forms (free-base: 1.09; bound-form: 1.69), indicating enhanced solution absorption of nicotine under alkaline conditions. This phenomenon is related to the polarity of the absorption solution and the distribution of nicotine species. As a weak base, nicotine exists predominantly in the protonated (bound) form under acidic and neutral conditions, whereas the deprotonated (free-base) form becomes predominant under alkaline conditions (1 mmol/L NaOH) [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Ethanol, being a weakly polar solvent, exhibits a greater dissolution capacity for molecular organic species (e.g., free-base nicotine) than for ionic compounds (e.g., nicotine salts). Therefore, the increased proportion of the free-base form under alkaline conditions improves the compatibility between nicotine and ethanol, thereby promoting solution absorption.\u003c/p\u003e \u003cp\u003eNotably, however, the ratio of the pad-to-solution capture ratio of nicotine salt to that of free-base nicotine (bound value / free value) did not vary significantly across different pH conditions (acidic: 1.49; neutral: 1.59; alkaline: 1.55). This suggests that the facilitative effect of alkaline conditions on absorption was consistent for both nicotine forms. The bulk absorption solution likely possessed sufficient buffering capacity to maintain the local chemical environment, thereby limiting the influence of the initial chemical form of nicotine on its ultimate dissolution behavior in the solution. This further supports the conclusion that the differences in solution absorption between nicotine forms primarily stem from disparities in aerosol kinetic behavior and absorption pathways, with the intrinsic solubility playing a comparatively minor role.\u003c/p\u003e \u003cp\u003e \u003cb\u003e2.2 Comparison of Absorption Capacity of Different Nicotine Forms in Aqueous-Based Absorption Solutions\u003c/b\u003e \u003c/p\u003e \u003cp\u003eGiven that human body fluids are essentially aqueous systems, the aforementioned experiments were repeated using aqueous-based absorption solutions to enhance physiological relevance. Since the measurement methodology employed in this study does not permit direct quantification of nicotine absorbed in aqueous solutions via gas chromatography\u0026ndash;mass spectrometry (GC\u0026ndash;MS), a normalization approach was adopted to evaluate absorption efficiency. The amount of nicotine captured by the Cambridge filter pad and the total consumption of nicotine solution in each experiment were determined. This allowed for the calculation of the normalized residual amount of nicotine\u0026mdash;defined as the quantity of nicotine captured by the filter pad per unit of nicotine solution (or nicotine salt solution) consumed, which represents the fraction of aerosolized nicotine not absorbed by the solution. By systematically comparing this normalized residual amount between different initial forms (free-base nicotine and nicotine salt), the relative solution absorption efficiency in aqueous-based absorption solutions was indirectly assessed (a higher residual value indicates lower solution absorption efficiency). The relevant experimental results are summarized in Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and illustrated in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The raw data and data processing procedure are available in the Supporting Information Table S2.\u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eComparison of the absorption capacity of nicotine/nicotine salt in aqueous-based absorption solutions with different pH values\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"8\"\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c6\" colnum=\"6\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c7\" colnum=\"7\"\u003e\u003c/div\u003e \u003cdiv align=\"left\" class=\"colspec\" colname=\"c8\" colnum=\"8\"\u003e\u003c/div\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eAbsorption Solution\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTest E-liquid\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eE-liquid Consumption (g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eNicotine Capture on Filter Pad\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eNicotine Captured per 1 g E-liquid Consumed\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eMean Value\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colspan=\"2\" nameend=\"c8\" namest=\"c7\"\u003e \u003cp\u003eNicotine Salt/Free-base Nicotine Ratio\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003eH\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.128\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.174\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.36\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e1.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.145\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.204\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.41\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.092\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.197\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.15\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e2.14\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.088\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.188\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.12\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003e1 mmol/L HCl in H\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.146\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.201\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.38\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e1.39\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.57\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.153\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.214\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.090\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.199\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.20\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e2.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.093\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e\u003cb\u003e1 mmol/L NaOH in H\u003c/b\u003e\u003csub\u003e\u003cb\u003e2\u003c/b\u003e\u003c/sub\u003e\u003cb\u003eO\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.146\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.200\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.37\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e1.40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c8\" morerows=\"3\" rowspan=\"4\"\u003e \u003cp\u003e1.55\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.149\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.215\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e1.44\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 1\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.094\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.204\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.19\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colspan=\"2\" morerows=\"1\" nameend=\"c7\" namest=\"c6\" rowspan=\"2\"\u003e \u003cp\u003e2.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003e\u003cb\u003eNicotine Salt 2\u003c/b\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003e0.092\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c4\"\u003e \u003cp\u003e0.201\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c5\"\u003e \u003cp\u003e2.18\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAs evidenced by the results presented above, under all aqueous absorption conditions (water, aqueous 1 mmol/L HCl solution, and aqueous 1 mmol/L NaOH solution), the Cambridge filter pad capture efficiency of nicotine salt was significantly higher than that of free-base nicotine. This indicates that the solution absorption efficiency of free-base nicotine is higher than that of nicotine salt. Specifically, when using water as the absorption solution, the ratio of nicotine salt to free-base nicotine captured per unit of nicotine-containing test solution consumed was 1.54, indicating that a greater amount of nicotine salt was retained on the filter pad and thus exhibited lower solution absorption. This trend was consistently observed in both acidic (aqueous 1 mmol/L HCl) and alkaline (aqueous 1 mmol/L NaOH) solutions, with corresponding ratios of 1.57 and 1.55, respectively. In contrast to the observations in ethanolic solutions, the pH of the aqueous absorption solution did not exert a noticeable influence on the relative absorption behavior of the two nicotine forms.\u003c/p\u003e \u003cp\u003eAlthough the physicochemical properties of the absorption medium (i.e., solvent type and pH) modulated the absolute absorption quantity of nicotine, they did not alter the relative absorption hierarchy between the two chemical forms. This universal behavior stems from the fundamental difference in the mass transfer mechanisms of the two nicotine species. Free-base nicotine, characterized by lower molecular polarity and higher volatility, readily partitions from the aerosol particle phase into the gas phase, subsequently undergoing efficient dissolution into the absorption solution via gas-liquid interfacial diffusion-dominated transfer. Consequently, it demonstrates superior solution absorption performance. In contrast, the absorption of nicotine salt is severely constrained by strong ionic association, which significantly suppresses its volatility. Its release and dissolution are predominantly dependent on direct particle\u0026ndash;solution contact and solid\u0026ndash;liquid interfacial dissolution kinetics, resulting in systematically lower solution absorption efficiency.\u003c/p\u003e \u003cp\u003eIn summary, while environmental parameters such as solvent polarity and pH modulate the absolute extent of nicotine absorption, the chemical state of nicotine (free-base vs. bound-form) remains the central mechanistic determinant of relative absorption efficiency. This is achieved by governing the contribution of two competing mass transfer pathways: volatilization\u0026ndash;diffusion versus particle dissolution.\u003c/p\u003e \u003cp\u003e \u003cb\u003e3. Conclusion\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThis study employed an aerosol delivery model to investigate the solution penetration (simulated absorption) capacity of free-base nicotine versus protonated nicotine (nicotine salt). It was demonstrated that free-base nicotine is more readily absorbed into solution compared to its bound-form counterpart. The type of solution (ethanol or water) and its pH value did not alter the relative absorption strength between the two nicotine forms. The observed difference in absorption efficiency is attributed to their distinct absorption pathways: free-base nicotine, due to its ability to transfer from the aerosol particle phase to the gas phase and subsequently diffuse into the solution, achieves higher absorption.\u003c/p\u003e \u003cp\u003eThrough the aerosol delivery model, this study confirms that the solution penetration efficiency of free-base nicotine is significantly higher than that of protonated nicotine. The relative absorption strength ratio remained constant across both ethanolic and aqueous systems over a wide pH range, indicating that the physicochemical properties of the absorption medium modulate only the absolute absorption quantity without altering the fundamental inter-form difference. The underlying mechanism originates from the divergence in mass transfer pathways: free-base nicotine, leveraging its lower polarity and higher volatility, undergoes efficient absorption via a multi-stage route of \"aerosol particle phase to gas phase to gas-liquid diffusion\". In contrast, protonated nicotine, constrained by ionic bonding, relies solely on a single-rate-limiting pathway of \"direct particle-solution contact and dissolution\".\u003c/p\u003e \u003cp\u003eThese findings reveal that the protonation state of nicotine governs its interphase transport kinetics by regulating the competition between two pathways\u0026mdash;volatilization-diffusion and interfacial dissolution. This study provides important in vitro experimental evidence for evaluating the therapeutic potential and addiction risks of nicotine, thereby supporting its future applications in the pharmaceutical field.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eSupporting Information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe raw data and the detailed process of data analysis are provided in Table S1-S2.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAUTHOR INFORMATION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCorresponding author\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eZhiguo Wang. E-mail: [email protected]\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eZhuo Wang：Conceptualization, Methodology, Curation, Formal Analysis, Validation, Writing - Original Draft.\u003c/p\u003e\n\u003cp\u003eHuapeng Cui：Conceptualization, Methodology, Data Curation, Formal Analysis, Validation.\u003c/p\u003e\n\u003cp\u003eSuxing Tuo：Conceptualization, Methodology, Investigation, Supervision, Writing-Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003eWen Du：Conceptualization, Methodology, Supervision, Project Administration, Writing-Review \u0026amp; Editing, Writing - Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003eZhiguo Wang*：Conceptualization, Methodology, Supervision, Project Administration, Writing-Review \u0026amp; Editing.\u003c/p\u003e\n\u003cp\u003e\u003csup\u003e\u0026nbsp;\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003eThese authors contributed equally to this work\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNotes\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis study is a pure academic research. All experimental data and results are publicly available free of charge, and there are no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFUNDING DECLARATION\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by the Postdoctoral Foundation of China Tobacco Hunan Industrial Co., Ltd(KY2024JC0007), and completed with the technical support of the Zhengzhou Tobacco Research Institute of CNTC. The research results do not involve any commercial interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eLe Foll, B. et al. Tobacco and nicotine use. \u003cem\u003eNat. Rev. Dis. Primers\u003c/em\u003e. \u003cb\u003e8\u003c/b\u003e, 19 (2022).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eLi, Y. P. \u0026amp; Hecht, S. S. Carcinogenic components of tobacco and tobacco smoke: a 2022 update. \u003cem\u003eFood Chem. 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K. \u0026amp; Zhou, J. W. The structure and function of glial networks: beyond the neuronal connections. \u003cem\u003eNeurosci. Bull.\u003c/em\u003e \u003cb\u003e39\u003c/b\u003e (3), 531\u0026ndash;540 (2023).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWills, L. \u0026amp; Kenny, P. J. Addiction-related neuroadaptations following chronic nicotine exposure. \u003cem\u003eJ. Neurochem\u003c/em\u003e. \u003cb\u003e157\u003c/b\u003e (5), 1652\u0026ndash;1673 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eAlhowail, A. Molecular insights into the benefits of nicotine on memory and cognition. \u003cem\u003eMol. Med. Rep.\u003c/em\u003e \u003cb\u003e23\u003c/b\u003e (6), 398 (2021).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eWang, Q. et al. 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Res.\u003c/em\u003e \u003cb\u003e17\u003c/b\u003e (10), 1270\u0026ndash;1278 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eGoniewicz, M. L., Lingas, E. O. \u0026amp; Hajek, P. Patterns of electronic cigarette use and user beliefs about their safety and benefits: an internet survey. \u003cem\u003eDrug Alcohol Rev.\u003c/em\u003e \u003cb\u003e32\u003c/b\u003e (2), 133\u0026ndash;140 (2013).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eTalih, S. et al. Effects of user puff topography, device voltage, and liquid nicotine concentration on electronic cigarette nicotine yield: measurements and model predictions. \u003cem\u003eNicotine Tob. Res.\u003c/em\u003e \u003cb\u003e17\u003c/b\u003e (2), 150\u0026ndash;157 (2015).\u003c/span\u003e\u003c/li\u003e \u003cli\u003e\u003cspan\u003eBrunnemann, K. D. \u0026amp; Hoffmann, D. Chemical studies on tobacco smoke. XXV. The pH of tobacco smoke. \u003cem\u003eFood Cosmet. Toxicol.\u003c/em\u003e \u003cb\u003e12\u003c/b\u003e, 115\u0026ndash;124 (1974).\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"Aerosol, Nicotine forms, Absorption pathways, Solution permeation","lastPublishedDoi":"10.21203/rs.3.rs-7628503/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7628503/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eElucidating the pharmacokinetic profiles of different chemical forms of nicotine (free-base versus protonated)\u0026mdash;particularly their differences in biological absorption\u0026mdash;is critically important for balancing its potential clinical applications in neurodegenerative disease therapy with effective management of its inherent addiction risk. Current empirical observations regarding which form exhibits superior absorption remain significantly divergent. This study employs an aerosol delivery model to compare the permeability (simulated absorption) of the two nicotine forms in solution by quantifying the amount absorbed and the residual nicotine after aerosol penetration through a simulated solution barrier. The results demonstrate that free-base nicotine exhibits higher solution absorption than its protonated counterpart. The superior absorption of free-base nicotine is primarily attributed to its diffusion from the particulate phase to the gas phase within aerosols, facilitating efficient mass transfer across the gas\u0026ndash;liquid interface. These findings provide an in vitro experimental basis for further evaluation of the potential differences in human absorption efficiency between nicotine forms, thereby supporting the development of nicotine-based pharmaceuticals.\u003c/p\u003e","manuscriptTitle":"The influence of nicotine form on its absorption","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-12-25 13:53:51","doi":"10.21203/rs.3.rs-7628503/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-01-06T07:26:02+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-29T12:23:05+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-12-25T16:36:48+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"275437885236504505914723269403484946607","date":"2025-12-25T14:32:41+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"233247779125394827115927441890017099869","date":"2025-12-23T15:56:45+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-12-23T11:55:24+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2025-11-14T10:37:27+00:00","index":"","fulltext":""},{"type":"editorInvited","content":"","date":"2025-10-31T05:09:10+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-10-14T08:58:53+00:00","index":"","fulltext":""},{"type":"submitted","content":"Scientific Reports","date":"2025-10-14T08:55:00+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"scientific-reports","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scirep","sideBox":"Learn more about [Scientific Reports](http://www.nature.com/srep/)","snPcode":"","submissionUrl":"","title":"Scientific Reports","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Scientific Reports","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"2b45ba32-4070-47c4-b87b-a33734095314","owner":[],"postedDate":"December 25th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":60190716,"name":"Physical sciences/Chemistry"},{"id":60190717,"name":"Biological sciences/Drug discovery"},{"id":60190718,"name":"Earth and environmental sciences/Environmental sciences"}],"tags":[],"updatedAt":"2026-03-09T16:07:17+00:00","versionOfRecord":{"articleIdentity":"rs-7628503","link":"https://doi.org/10.1038/s41598-026-42860-x","journal":{"identity":"scientific-reports","isVorOnly":false,"title":"Scientific Reports"},"publishedOn":"2026-03-06 15:59:21","publishedOnDateReadable":"March 6th, 2026"},"versionCreatedAt":"2025-12-25 13:53:51","video":"","vorDoi":"10.1038/s41598-026-42860-x","vorDoiUrl":"https://doi.org/10.1038/s41598-026-42860-x","workflowStages":[]},"version":"v1","identity":"rs-7628503","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7628503","identity":"rs-7628503","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}