Lee Yuh-Lang, Yang Yaw-Chia, Shen Yu-Jen
Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan, ROC.
J Phys Chem B. 2005 Mar 17;109(10):4662-7. doi: 10.1021/jp045251z.
Mixed monolayers of stearic acid (SA) and octadecylamine (ODA) at the air/water interface were investigated in this article. The miscibility of the two compounds was evaluated by the measurement of surface pressure-area per molecule (pi-A) isothems and the direct observation of Brewster angle microscopy (BAM) on the water surface. The two compounds were spread individually on the subphase (method 1) or premixed first in the spreading solvent and then cospread (method 2). The effect of spreading method on the miscibility of the two compounds was also studied. The results show that the mixed monolayers prepared by method 1 cannot get a well-mixed state. The isotherms of mixed monolayers preserve both characteristics of SA and ODA and exhibit two collapse points. The calculated excess surface area is very small. Besides, distinguished domains corresponding to those of pure SA and ODA can be inspected from the BAM images. Such results indicate that SA and ODA cannot get a well-mixed phase via 2-dimensional mixing. On the contrary, in the mixed monolayer prepared by cospreading, the two compounds exhibit high miscibility. In the pi-A isotherms, the individual characteristics of SA and ODA disappear. The calculated excess area exhibits a highly positive deviation which indicates the existence of special interaction between the two compounds. The low compressibility of isotherm implies the highly rigid characteristic of the mixed monolayer. which was also sustained by the striplike collapse morphology observed from the BAM. The rigid characteristic of SA/ODA mixed monolayer was attributed to the formation of "catanionic surfactant" by electrostatic adsorption of headgroups of SA and ODA or to the formation of salt by acid-base reaction.
本文研究了硬脂酸（SA）和十八烷基胺（ODA）在空气/水界面的混合单分子层。通过测量表面压力-每分子面积（π-A）等温线以及对水面进行布鲁斯特角显微镜（BAM）直接观察，评估了这两种化合物的混溶性。这两种化合物分别铺展在亚相上（方法1），或者先在铺展溶剂中预混合然后共铺展（方法2）。还研究了铺展方法对这两种化合物混溶性的影响。结果表明，通过方法1制备的混合单分子层无法达到良好的混合状态。混合单分子层的等温线保留了SA和ODA的特征，并呈现出两个崩塌点。计算得到的过剩表面积非常小。此外，从BAM图像中可以观察到与纯SA和ODA相对应的明显区域。这些结果表明，SA和ODA无法通过二维混合形成良好的混合相。相反，在共铺展制备的混合单分子层中，这两种化合物表现出高度的混溶性。在π-A等温线中，SA和ODA的各自特征消失。计算得到的过剩面积呈现出高度正偏差，这表明这两种化合物之间存在特殊相互作用。等温线的低压缩性意味着混合单分子层具有高度刚性的特征，这也通过从BAM观察到的条纹状崩塌形态得到证实。SA/ODA混合单分子层的刚性特征归因于SA和ODA头基的静电吸附形成“阴阳离子表面活性剂”，或者归因于酸碱反应形成盐。
