Adsorption of pi molecules and ions onto surfactant monolayers probed by sum- frequency vibrational spectroscopy
- 주제어 (키워드) Cation-pi interaction , charge inversion , co-ion effect , sum-frequency vibrational spectroscopy
- 발행기관 서강대학교 일반대학원
- 지도교수 김도석
- 발행년도 2025
- 학위수여년월 2025. 8
- 학위명 박사
- 학과 및 전공 일반대학원 물리학과
- 실제 URI http://www.dcollection.net/handler/sogang/000000081728
- UCI I804:11029-000000081728
- 본문언어 영어
- 저작권 서강대학교 논문은 저작권 보호를 받습니다.
초록 (요약문)
The adsorption of π molecules and ions to model membrane lipid surfactants plays a crucial role in various biological processes. Among these interactions, cation-π interactions—strong non-covalent interactions between π-electron-rich molecules and cationic species—are essential for signal transduction, protein stabilization, enzymatic catalysis, wet adhesion, and biomolecular condensation. In this study, sum-frequency vibrational spectroscopy (SFVS) is used to investigate the interactions of cationic surfactants, including octadecylguanidine hydrochloride (ODG), octadecylamine (ODA), and 1,2-dipalmitoyl-3-trimethylammonium-propane (DPTAP), with π molecules such as phenol, indole, and benzylamine. The results reveal that the presence of these π molecules in the aqueous subphase stabilizes the surfactant monolayer, and due to their high membrane permeability, they penetrate into the monolayer of the cationic surfactants. Beyond cation-π interactions, electrostatic interactions between charged surfactant monolayers and ions are explored. Strong adsorption of counterions to the lipid headgroups not only screens the surface charge but also can lead to charge inversion, where the adsorbed counterions overcompensate the initial surface charge, resulting in a net charge reversal. While charge inversion is typically associated with multivalent ions, this study demonstrates that even monovalent hydroxide ions (OH⁻) can induce charge inversion at DPTAP monolayers. It was also studied that fluoride anions, despite their weak screening ability, enhance the OH signal of the interfacial water in the SFG spectra. This enhancement varies with alkali cations: both NaF and LiF show a maximum OH signal at 0.1 mM, while KF exhibits no increase, highlighting a specific co-ion effect. Together, these findings provide new insights into how molecular interactions at charged interfaces—through cation–π interactions, charge inversion, and co-ion effects—govern the physicochemical behavior of membrane surfaces, with implications for biological and biomimetic systems. Keyword: Cation-π interaction, charge inversion, co-ion effect, sum-frequency generation spectroscopy, π molecules, laser heating.
more목차
1. Introduction 1
2. Theory 8
2.1 Langmuir monolayer of surfactant molecules 8
2.2 Pressure-area (𝝅 −A) isotherm 9
2.3 Sum-frequency generation spectroscopy 11
3. Experimental setup and sample preparation 18
3.1. Broadband SFVS setup (Homodyne SFG) 18
3.2 Phase-sensitive SFVS set up (Heterodyne SFG) 20
3.3 Materials and sample preparation 26
4. Experimental Result 28
4.1 Adsorption of π molecules to the cationic surfactant molecules 28
4.1.1 π molecules at the water surface (phenol, indole and benzylamine) 28
4.1.2 Adsorption of π molecules onto ODG monolayer 31
4.1.3 Adsorption of π molecules onto ODA monolayer 36
4.1.4 Adsorption of π molecules to the DPTAP monolayer 40
4.1.5 Adsorption of π molecules to the DPPC monolayers 50
4.2 Adsorption of ions to the cationic surfactant molecules 54
4.2.1 Counterion adsorption (charge inversion study) 54
4.2.2 Co-ion effect in the electric double layer 63
5. Summary and Conclusion 71
References 74
