以紅外光譜法探討離子對雙親分子／雙十八碳鏈陽離子型界面活性劑／膽固醇之混合Langmuir單分子層的分子排列特性

Abstract

This study investigated the molecular packing characteristic of ion pair amphiphile (IPA) monolayers with the addition of dioctadecyl chain cationic surfactant and cholesterol at the air/water interface by the infrared reflection-absorption spectroscopy (IRRAS). The IPA, hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), was prepared from hexadecyltrimethylammonium bromide (HTMAB) and sodium dodecylsulfate (SDS). The IPA monolayer at the air/water interface could form a condensed film after the compression stage. However, the analysis of monolayer relaxation behavior indicated the loss of free molecules. With the increase of the surfactant molar fraction in the mixed monolayers of IPA/dioctadecyl chain cationic surfactant, the molecular alkyl chains became much packed during the compression stage, and the loss of free molecules was less significant in the relaxation process. This is probably because the presence of dioctadecyl chain cationic surfactant could effectively enhance the hydrophobic interaction between the molecular alkyl chains. When the molar fraction of cholesterol added into the IPA or mixed IPA/dioctadecyl chain cationic surfactant monolayers was increased, the mixed monolayers could not be compressed to a state with well-packed molecules, but the loss of free molecules in the monolayer relaxation process was decreased. Cholesterol might enhance the hydrophobic interaction between molecules and thus decrease the loss of free molecules. However, the mismatch of the rigid cyclic structure of cholesterol and the molecular alkyl chains in the monolayers seemed to restrict the ordered arrangement of the alkyl chains.