離子對雙親分子/雙十六碳鏈陰離子型界面活性劑/膽固醇之混合Langmuir單分子層行為的反射吸收式紅外光譜分析

Abstract

This study investigated the stability and the molecular packing characteristic of ion pair amphiphile (IPA) Langmuir monolayers with the addition of dihexadecyl phosphate (DHDP) and cholesterol by the infrared reflection-absorption spectroscopy (IRRAS). The IPA used in this study was hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), which was prepared from hexadecyltrimethylammonium bromide (HTMAB) and sodium dodecylsulfate (SDS). In mixed HTMA-DS/DHDP monolayers, the molecular packing order and the stability of the monolayers can be enhanced by increasing the DHDP content. This is probably because of the stronger interaction between molecules in the monolayers was due to the longer hydrocarbon chains of DHDP. In addition, the small cross-section area of DHDP headgroup could avoid the steric barrier formation, the weak dissociation phosphate headgroup of DHDP can prevent the loose packing due to the repulsive interaction. When cholesterol was added into mixed HTMA-DS/DHDP monolayers, the packing of molecular hydrocarbon chains in the monolayers became loose. Besides the steric barrier induced by sterol ring and the distribution of trans/gauche structure of the molecular hydrocarbon chains affected by cholesterol, the mismatch between cholesterol and the molecules also result in the increased freedom degree of the molecular hydrocarbon chains. This phenomenon became pronounced in the DHDP-rich monolayers. As a result, the cholesterol effect on the molecular packing was more significant in the mixed HTMA-DS/DHDP monolayer with a 1:1 mixing ratio. In the relaxation experiments at a constant monolayer occupied area, the stability of HTMA-DS-rich monolayers could be significantly improved by adding cholesterol. However, the stability of DHDP-rich monolayers seems unaffected by cholesterol, probably because cholesterol had better effect on the interaction between HTMA-DS molecules.