本研究利用表面壓－每分子佔據面積等溫線法、熱力學分析、鬆弛實驗和反射吸收式紅外光譜技術（infrared reflection-absorption spectroscopy, IRRAS），探討膽固醇的添加對三碳鏈dihexadecyldimethylammonium-hexadecylsulfate（DHDA-DS）及雙碳鏈hexadecyltrimethylammonium-dodecylsulfate（HTMA-DS）之離子對雙親分子（ion pair amphiphile, IPA）Langmuir單分子層行為的影響。實驗結果顯示，DHDA-DS單分子層的分子排列較HTMA-DS單分子層規則，在氣/液界面上的穩定性也較佳。推測是因為DHDA-DS分子有三條碳氫鏈，與HTMA-DS相較分子間作用較強，且疏水性較高。在IPA/膽固醇混合單分子層中，膽固醇會造成凝縮效應和不規則效應，兩者影響之強弱與表面分子密度及分子間的作用有關，因此對於不同IPA的主導效應不同。在DHDA-DS混合單分子層中，膽固醇的影響以不規則效應為主；在HTMA-DS混合單分子層中，壓縮初期以凝縮效應為主，但隨著單分子層壓縮，逐漸轉為由不規則效應主導。在鬆弛實驗中，添加膽固醇有助於提升HTMA-DS混合單分子層的穩定性，但對於DHDA-DS混合單分子層反而會降低其穩定性。由於含HTMA-DS混合單分子層的分子排列相對鬆散，因此膽固醇對單分子層穩定性的影響十分顯著。而DHDA-DS具三碳鏈的結構，分子間的作用較強，分子排列較緊密，添加膽固醇後，其固醇環造成的立體障礙反而會讓單分子層中分子的排列變得鬆散，使得單分子層的穩定性下降。由DHDA-DS混合單分子層的鬆弛行為與相同材料所製備出之陰陽離子液胞（catanionic vesicle）的物理穩定性可知，當膽固醇的含量增加時，並無助於提升液胞的物理穩定性。添加膽固醇後的液胞雙層膜及混合單分子層中分子碳氫鏈的排列皆變得較不規則，應是因為膽固醇的固醇環會產生立體障礙，讓排列變得鬆散，使得液胞及單分子層的穩定性下降。

This study investigated the effect of added cholesterol on the Langmuir monolayer behavior of two ion pair amphiphiles (IPAs), double-chained hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) and triple-chained dihexadecyldimethylammonium-dodecylsulfate (DHDA-DS), at the air/water interface with surface pressure-area isotherms and infrared spectra. The results showed that stronger hydrophobic interaction between the molecules was found in the DHDA-DS monolayer than in the HTMA-DS monolayer, because DHDA-DS was composed of triple alkyl chains. In the mixed IPA/cholesterol Langmuir monolayers, there were two significant cholesterol effects observed, condensing effect and disordering effect. Condensing effect made the molecular alkyl chains around the steroid ring become ordered by enhancing the van der Waals interaction. However, the steroid ring produced a steric barrier to inhibit the molecular alkyl chains approaching each other and enhanced the degree of freedom of the alkly chains, inducing the disordering effect. The dominating effect associated with cholesterol addition in the mixed IPA/cholesterol Langmuir monolayers varied with the molecular surface density and the interaction between the molecules.

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