本研究藉由表面壓-面積及表面電位-面積等溫線、鬆弛曲線，以及布魯斯特角顯微鏡（Brewster angle microscope）影像，探討離子對雙親分子（ion pair amphiphile, IPA）hexadecyltrimethylammonium-dodecylsulfate（HTMA-DS）與不同添加物所形成之混合Langmuir單分子層的行為。目的是了解添加之雙鏈陽離子型界面活性劑（dialkyldimethylammonium bromide, DXDAB）的碳鏈長度及莫耳分率，對於HTMA-DS與HTMA-DS/膽固醇單分子層中分子間交互作用，以及混合單分子層之穩定性的影響。
HTMA-DS/DXDAB混合單分子層的等溫線顯示雙十六烷基二甲基溴化銨（dihexadecyldimethylammonium bromide, DHDAB）與雙十八烷基二甲基溴化銨（dioctadecyldimethylammonium bromide, DODAB）的添加，對於單分子層有明顯的凝縮效應。然而，XHTMA-DS = 0.5之混合單分子層等溫線出現不合理的分子極限佔據面積。這應是由於DXDA+ 取代了HTMA-DS中的HTMA+，並與DS－ 形成新的IPA DXDA-DS，而被取代的HTMA+ 脫附至水相中，造成界面上分子的損失。此外，由單分子層鬆弛曲線可知，DHDAB 和DODAB的添加有助於單分子層於緊密狀態下的穩定性。但添加之雙十四烷基二甲基溴化銨（ditetradecyldimethylammonium bromide, DTDAB）對單分子層的凝縮效應並不明顯，也無法促進單分子層的穩定性。
對於含43 mol% 膽固醇的混合單分子層，DXDAB的碳氫鏈長度對單分子層行為並沒有太大的影響，單分子層的等溫線行為及形態似乎由膽固醇所主導。此外，膽固醇的存在對於各單分子層均有提升分子間吸引力的效果，而可調節分子的排列行為。但當混合單分子層被壓縮到表面壓30 mN/m時，添加膽固醇並無法有效提升界面上混合單分子層的穩定性，這可能因為膽固醇的固醇環與HTMA-DS及DXDAB的疏水端之間產生立體障礙及不佳的配位性所造成的。

In this study, mixed Langmuir monolayer behavior of ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), with various additives was investigated by surface pressure-area and surface potential-area isotherms, relaxation curves, and Brewster angle microscopy (BAM) images. The purpose is to understand the effects of alkyl chain length and molar fraction of dialkyldimethylammonium bromide (DXDAB) on the molecular interaction in the HTMA-DS and HTMA-DS/cholesterol monolayers and on the corresponding mixed monolayer stability.
The isotherms of the mixed HTMA-DS/DXDAB monolayers indicated that the addition of dihexadecyldimethylammonium bromide (DHDAB) and dioctadecyldimethylammonium bromide (DODAB) led to an apparent condensing effect on the monolayers. However, an unreasonable value of limiting area per molecule was obtained from the isotherm of the mixed monolayer with XHTMA-DS = 0.5. This was because DXDA+ replaced the HTMA+ of HTMA-DS to form a new IPA, DXDA-DS, and the replaced HTMA+ desorbed into the aqueous subphase, resulting in the molecule loss at the interface. In addition, the monolayer relaxation curves suggested that the addition of DHDAB and DODAB could improve the stability of monolayers at condensed states. Nevertheless, with the addition of ditetradecyldimethylammonium bromide (DTDAB), no significant condensing effect was found for the monolayers and no enhancement in the monolayer stability was detected.
For the mixed monolayers containing 43 mol% cholesterol, no pronounced effect was found for the alkyl chain length of DXDAB on the monolayer behavior, and the isotherm behavior and morphology of the monolayers seemed dominated by cholesterol. Moreover, the presence of cholesterol in the monolayers could enhance the attractive interaction between the molecules and thus adjust the molecular packing behavior. However, when the monolayers were compressed to reach a surface pressure of 30 mN/m, the added cholesterol could not effectively improve the mixed monolayer stability at the interface, which was probably due to the steric barrier and mismatch occurring between the sterol ring of cholesterol and the hydrophobic parts of HTMA-DS and DXDAB.

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