本研究利用反射吸收式紅外光譜技術（infrared reflection-absorption spectroscopy, IRRAS），探討不同碳鏈對稱性的離子對雙親分子（ion pair amphiphile, IPA）添加雙十八烷基二甲基溴化銨（dioctadecyldimethylammonium bromide, DODAB）與膽固醇之情況下，氣/液界面上混合單分子層之分子排列特性和穩定性。研究中使用的IPA是由十六烷基三甲基溴化銨（hexadecyltrimethylammonium bromide, HTMAB）和十六烷基硫酸鈉（sodium hexadecylsulfate, SHS）製備而成的HTMA-HS，並比較分別使用HTMA-HS與HTMA-DS為主材料之混合單分子層的分析結果。實驗結果顯示，由於HTMA-HS分子有兩條對稱的碳氫鏈，與HTMA-DS相較之下，分子間作用較強，且疏水性較高，因此HTMA-HS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。在IPA/DODAB/膽固醇的混合單分子層中，膽固醇會造成凝縮效應和不規則效應，兩者影響力之強弱與表面分子密度以及IPA的碳氫鏈對稱性有關，因此產生不同的主導效應。由於在兩種IPA的混合系統中，膽固醇的主導效應皆不隨著DODAB的添加量而改變，顯示IPA碳氫鏈對稱性除了影響分子間的交互作用，造成單分子層中分子排列的差異外，也間接影響添加劑的效應。由DODAB所產生的分子取代行為造成HTMA+在壓縮氣/液界面的過程中從界面上脫附，降低分子碳氫鏈的擺動自由度，也提高單分子層的疏水性。從實驗結果推測在含HTMA-HS之混合系統中，取代反應所造成分子損失的程度較高。而適量地添加膽固醇能增強單分子層中分子間的凡得瓦作用，抑制HTMA+從氣/液界面上脫附；但添加過多的膽固醇時，穩定單分子層結構的能力則會減弱。鬆弛實驗中，添加DODAB和膽固醇對於兩種IPA所形成之混合單分子層的穩定度皆有提升的作用。由於含HTMA-DS之混合單分子層的分子排列相對鬆散，添加劑對其穩定度之影響較顯著。比較液胞雙層膜和單分子層中分子碳氫鏈的排列，結果顯示雙層膜中的分子排列會較單分子層規則，膽固醇對分子碳氫鏈造成之不規則效應也較不顯著。推測是液胞雙層膜中的分子排列會受到雙層膜彼此之間作用，分子碳氫鏈的擺動自由度因而降低，且膽固醇在雙層膜和單分子層中插排的位置可能不同所致。

The molecular packing characteristics of IPA monolayers with addition of dioctadecyldimethylammonium bromide (DODAB) and cholesterol at the air/water interface were analyzed with the infrared reflection-absorption spectroscopy. In this study, two IPAs, hexadecyltrimethylammonium-hexadecylsulfate (HTMA-HS) and hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS) were adopted. The results showed that the hydrophobic tails of HTMA-DS would have a higher degree of freedom than that of HTMA-HS. The addition of DODAB would cause displacement behavior, resulting in a more ordered molecular packing and molecular loss. In IPA/DODAB/cholesterol monolayers, two significant cholesterol effects, condensing effect and disordering effect, were observed in this study. The dominating effect varied with the molecular packing status, and the symmetry of IPA alkyl chain structure played a key role in the cholesterol effect. Besides, the desorption of HTMA+ was decreased as cholesterol was added, probably because the hydrophobic interaction between the molecules was enhanced. The results have implications for controlling the surface character of catanionic vesicles composed of IPA, DODAB, and cholesterol.

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