本研究利用反射吸收式紅外光譜技術（infrared reflection-absorption spectroscopy, IRRAS），探討氣／液界面上的離子對雙親分子（ion pair amphiphile, IPA）單分子層在添加雙十八碳鏈陽離子型界面活性劑與膽固醇之情況下的分子排列特性。IPA是由十六烷基三甲基溴化銨（hexadecyltrimethylammonium bromide, HTMAB）和十二烷基硫酸鈉（sodium dodecylsulfate, SDS）製備而成的hexadecyltrimethylammonium-dodecylsulfate（HTMA-DS），在氣／液界面上的HTMA-DS單分子層經壓縮後可形成緊密的膜，但是單分子層鬆弛行為的分析顯示有自由分子損失的情形。隨著IPA／雙十八碳鏈陽離子型界面活性劑混合單分子層中界面活性劑的比率增加，在壓縮過程中分子碳氫鏈的排列逐漸形成更緊密有秩序的狀態，在混合單分子層的鬆弛過程中，自由分子的損失則變得較不明顯，可能的原因是添加雙十八碳鏈陽離子型界面活性劑可以有效提升分子碳氫鏈間的疏水作用。當提高添加在IPA或IPA／雙十八碳鏈陽離子型界面活性劑混合單分子層中的膽固醇莫耳分率時，雖然混合單分子層無法被壓縮至緊密規則的分子排列狀態，但在混合單分子層的鬆弛過程中，能減少自由分子的損失。推測是膽固醇提升了分子間的疏水作用而減少自由分子的損失，但膽固醇的環狀結構與單分子層中分子碳氫鏈間不佳的配位性，似乎阻礙了分子碳氫鏈緊密有序的排列。

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.

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