本研究利用反射吸收式紅外光譜分析技術（infrared reflection-absorption spectroscopy，IRRAS），分析液相的pH值對氣液界面上十八碳胺（octadecylamine，ODA）單分子層行為的影響。藉由IRRAS分析所得之ODA分子的νasCH2吸收峰強度、吸收頻率及半高寬，可評估ODA單分子層的相態行為。此外，也配合理論模式分析界面上ODA分子的位向。結果顯示當液相pH = 11時，ODA分子可穩定存在於氣液界面上，且在壓縮界面上ODA單分子層的過程中，可以偵測到單分子層的相態變化。由分子位向的理論分析，發現當ODA每分子佔據面積因界面的壓縮而逐漸變小時，ODA分子的平均位向漸趨於垂直於氣液界面。當液相pH值為5.6或3時，ODA分子則受到質子化的影響而離開氣液界面。由於pH = 3時的質子化效應比pH = 5.6時顯著，使得液相pH = 3時界面上的ODA分子過少，在壓縮界面的過程中並不會造成相態的轉變。此外，藉由理論分析可以得知界面上ODA單分子層的界面密度，在液相pH = 3時比pH = 5.6或11時小，證實在液相pH = 3時，界面上ODA分子會因為質子化而溶入水相中。

　　This study investigated the influence of pH on the octadecylamine (ODA) monolayer behavior at the air/liquid interface by the infrared reflection-absorption spectroscopy (IRRAS). With the reflectance-absorbance (RA) intensity, wavenumber, and full width at half maximum of the ODA νasCH2 band obtained from the IRRAS analysis, the phase behavior of an ODA monolayer was evaluated. Moreover, the orientation of ODA molecules at the interface was analyzed with a theoretical model. The results indicated that for a subphase with pH = 11, the ODA molecules could stably stay at the air/liquid interface. During the compression stage of the ODA monolayer at the interface, the monolayer phase changes could be detected. It was found from the theoretical analysis of molecular orientation that when the area per ODA molecule became smaller because of the interface compression, the average ODA molecular orientation became vertical to the air/liquid interface. When the subphase pH was 5.6 or 3, then the ODA molecules would leave the air/liquid interface due to the protonation effect. The protonation effect was more significant at pH 3 than at pH 5.6. As a result, for a subphase with pH = 3, there were few ODA molecules at the interface and no phase change was observed during the interface compression stage. In addition, based on the theoretical analysis, one could conclude that the surface density of an ODA monolayer at the interface with a subphase pH = 3 was lower than that with a subphase pH = 5.6 or 11. This result proved that the ODA molecules at the interface with a subphase pH = 3 would dissolve into the subphase due to the protonation effect.

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