### The effect of salt and pH on the protonation state of fatty aniline

- 주제어 (키워드) Langmuir monolayers , Fatty aniline , Sum-frequency vibrational spectroscopy , SFG , Mean-field theory , Chemical equilibrium , Poisson-Boltzmann equation
- 발행기관 서강대학교 일반대학원
- 지도교수 김도석
- 발행년도 2024
- 학위수여년월 2024. 2
- 학위명 석사
- 학과 및 전공 일반대학원 물리학과
- 실제URI http://www.dcollection.net/handler/sogang/000000077096
- UCI I804:11029-000000077096
- 본문언어 영어
- 저작권 서강대학교 논문은 저작권 보호를 받습니다.

#### 초록

Langmuir monolayers of fatty aniline (4-Hexadecyl aniline, C16-aniline), at different pHs and concentrations of added NaCl were investigated by sum-frequency vibrational spectroscopy. The SFG spectra in the OH regions were complicated due to the present of NH modes in this region. The SFG spectra of C16-aniline at pH 2 to 6 and constant ionic strength of 10mM (where the phase of dc-induced signal was suppressed to a few degrees) were fitted with multi-peak Lorentzian model following theory of SFG for charged interface. By combining the mean-field theory incorporating the chemical equilibrium of the aniline headgroup, and the result of fitting, the pKa of aniline was determined to be 4.9 ± 0.4. The protonation fraction of aniline headgroup was estimated to be about less than 5% at pH6 and added NaCl concentration up to 1M. The generalized Poisson Boltzmann equation could be used to quantitatively explain observed SFG spectra in OH region for fatty aniline as low as pH of 2 without any salt addition.

more#### 목차

Chapter 1. Introduction 1

Chapter 2. Theory 3

2.1. Pressure-area (π-A) Isotherm 3

2.2 Nonlinear optical phenomena 5

2.3 Sum-frequency generation 5

2.4 Sum frequency generation from a charged interface 11

2.5 Poisson- Boltzmann Equation 15

2.6 Ionization fraction for GPB 23

Chapter 3. Experimental setup and sample preparation 26

3.1. Broadband SFG setup 26

3.2 Material and sample preparation 31

Chapter 4. Experimental Result 33

4.1 π A isotherms of 4-hexadeylaniline (C22 H39 N) on pure water 33

4.2 π A isotherms of 4-hexadeylaniline (C22 H39 N) at low pH 34

4.3 π A isotherms of 4-hexadeylaniline (C22 H39 N) on salt solution 35

4.4 Peak assignment of Aniline 35

4.5 pKa of aniline at the interface 38

4.6 Sum-frequency spectra of 4-hexadeylaniline (C22 H39 N) at different pHs 42

4.7 Sum-frequency Spectra of 4-hexadeylaniline (C22 H39 N) at neutral pH with different salt concentrations 46

4.8 Extracting χ(2)BIL of water 49

Chapter 5. Summary and Conclusion 52

6. Reference 53