本研究利用反射吸收式紅外光譜技術（infrared reflection-absorption spectroscopy, IRRAS），探討在添加雙十六碳鏈陰離子型界面活性劑（dihexadecyl phosphate, DHDP）與膽固醇之情況下，氣／液界面上的離子對雙親分子（ion pair amphiphile, IPA）單分子層的穩定性及分子排列特性。本研究所使用的IPA是hexadecyltrimethylammonium-dodecylsulfate（HTMA-DS），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。
　　在HTMA-DS／DHDP雙成分混合系統中，增加DHDP含量可提升混合單分子層中分子排列的規則性與單分子層的穩定性。這可能是由於DHDP分子的碳氫鏈較長，使得單分子層中分子間的作用較強。此外，DHDP分子的頭基截面積小，可避免形成立體障礙，且弱解離的磷酸根頭基可避免因頭基間相斥而無法緊密排列。
　　添加膽固醇於HTMA-DS／DHDP混合單分子層中，會造成單分子層中分子的碳氫鏈排列處於較鬆散的狀態。除了固醇環形成的立體障礙，以及膽固醇分子影響分子碳氫鏈之trans和gauche結構的分布之外，膽固醇與分子之間的配位性不佳也會造成分子碳氫鏈的自由度上升，這現象在DHDP含量較高之單分子層中較顯著，因此添加膽固醇對於等莫耳混合之HTMA-DS／DHDP單分子層中的分子排列影響較明顯。在固定單分子層佔據面積的鬆弛實驗中，添加膽固醇明顯提升了HTMA-DS含量較高之混合單分子層的穩定性，但對於DHDP含量較高之混合單分子層的穩定性並無影響，可能是因為膽固醇對HTMA-DS分子間的交互作用有較明顯的增強效果。

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.

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