Department of Chemistry, University of Houston, Houston, Texas 77204-5003, USA.
QUILL Research Centre, The Queen's University of Belfast, Belfast BT9 5AG, Northern Ireland, United Kingdom.
J Chem Phys. 2018 May 21;148(19):193841. doi: 10.1063/1.5009674.
Sum frequency generation (SFG) spectroscopy is a nonlinear vibrational spectroscopic technique used in the study of interfaces, due to its unique ability to distinguish surface molecules that have preferential ordering compared to the isotropic bulk. Here, a series of alkyltrioctylphosphonium chloride ionic liquids, systematically varied by cation structure, were characterized at the air-liquid interface by SFG. The effect on surface structure resulting from molecular variation (i.e., addition of cyano- and methoxy-functional groups) of the cation alkyl chain was investigated. SFG spectra in the C-H stretching region (2750-3100 cm) for [P ][Cl], where = 4, 5, 8, 10, 12, or 14, showed characteristic changes as the alkyl chain length was increased. Spectral profiles for = 4, 5, 8, or 10 appeared similar; however, when the fourth alkyl chain was sufficiently long (as in the case of = 12 or = 14), abrupt changes occurred in the spectra. Molecular dynamics (MD) simulation of a slab of each ionic liquid (with = 8, 10, or 12) confirmed defects, with enhancement for the long alkyl chain and an abrupt increase of occurrence from = 8 to = 10. A comparison of the tilt angle distribution from the simulation and the SFG analysis show a broad distribution of angles. Using experimental SFG spectra in conjunction with MD simulations, a comprehensive molecular picture at the surface of this unique class of liquids is presented.
和频发生（SFG）光谱学是一种非线性振动光谱技术，用于研究界面，由于其独特的能力，可以区分具有优先有序的表面分子，而不是各向同性的体相。在这里，一系列烷基三辛基氯化磷离子液体通过阳离子结构系统地变化，通过 SFG 在气液界面进行了表征。通过研究阳离子烷基链分子变化（即添加氰基和甲氧基官能团）对表面结构的影响。在 C-H 伸缩区域（2750-3100 cm）的 SFG 光谱中，[P ][Cl]，其中 = 4、5、8、10、12 或 14，随着烷基链长度的增加显示出特征变化。 = 4、5、8 或 10 的光谱轮廓相似；然而，当第四个烷基链足够长时（如 = 12 或 = 14），光谱发生了突然变化。每个离子液体（ = 8、10 或 12）的薄片的分子动力学（MD）模拟证实了缺陷的存在，长烷基链增强， = 8 到 = 10 的出现急剧增加。模拟和 SFG 分析的倾斜角分布比较表明存在广泛的角度分布。使用实验 SFG 光谱结合 MD 模拟，呈现了这种独特类液体表面的综合分子图像。
