气液界面阳离子型Gemini表面活性剂-硬脂酸混合单分子膜性质

Properties of Mixed Monolayer of Cationic Gemini Surfactant and Stearic Acid at the Air/Water Interface

研究了阳离子型Gemini表面活性剂([C18H37(CH3)2N+—(CH2)3—N+(CH3)2C18H37],2Br-,简写为18-3-18,2Br-1)和硬脂酸(SA)混合单分子膜在气液界面上的性质;测定了这一混合体系的表面压-平均分子面积(-πA)等温线;计算了混合单分子膜的压缩比(κ)以及过剩面积(Aex)与混合单分子膜的组成(硬脂酸的摩尔分数,xSA)的关系;给出了过剩面积和理想混合面积之比与混合单分子膜组成的关系;并用原子力显微镜(AFM)观察了单分子膜在不同组分下的形貌。结果显示:表面压起始增加值所对应的平均分子面积随xSA增加而降低,并且混合单分子膜由液态扩张膜向凝固态膜转变,混合单分子膜出现严重的负偏离现象。由混合单分子膜的可混合性和非理想性可看出,18-3-18,2Br-1与SA在气液界面上存在较强的相互作用,它们之间容易混合。AFM图像还说明18-3-18,2Br-1分子难以成膜,SA分子易于成膜,在18-3-18,2Br-1中加入SA可以增加18-3-18,2Br-1成膜的可能性。

The mixed monolayer behavior of cationic Gemini surfactant （[C18H37（CH3）2N^-（CH2）3-N^＋（CH3）2C18H37],2Br^-1, abbreviated as 18-3-18,2Br^-1） and stearic acid （SA） at the air/water interface was investigated in this paper. The surface pressure （π）-area per molecule （A） isotherms of the mixed system were measured, and the compressibility was calculated. The miscibility and nonideality of the mixed monolayers were examined by calculating the excess area as a function of composition of stearic acid. The atomic force microscopy （AFM） was also used for observing the morphology of the LB films. The result suggests that lift-off values of mean molecular area decrease with the increasing SA ratio in mixed monolayers, and the mixed monolayer is transited from liquid-expanded monolayers to solid state monolayers, and the negative deviations from ideality are observed, which suggests that the existence of attractive interactions between 18-3-18, 2Br^-1 and SA molecules in the mixed monolayers at the interface. The AFM images also indicate that 18-3-18, 2Br^-1 and SA are miscible at the air/water interface. In addition, film formarion by 18-3-18, 2Br^-1 molecules is difficult, while it is easy by SA molecules. The addition of SA into 18-3-18, 2Br^-1 increases the probability of the films formation by 18-3-18, 2Br^-1.