Gemini阳离子表面活性剂在水溶液中胶团化行为的温度效应与焓／熵补偿

Temperature Dependence and Enthalpy-Entropy Compensation of Micellization of Gemini Surfactants in Aqueous Solutions

测定了Gemini阳离子表面活性剂Cm-s-Cm@2Br(m=8,10,12;s=2,6及m=12;s=3,4)水溶液的电导,从电导(к)～表面活性剂浓度(c)曲线的转折点可求得临界胶团浓度cmc.实验发现,Gemini阳离子表面活性剂的胶团化倾向明显强于其'单体分子”(即单离子头基单烷烃链表面活性剂).根据质量作用模型计算了胶团化过程的吉布氏能、焓和熵的改变,结果表明Gemini表面活性剂聚集机理和其对应的'单体分子”类似,主要来自熵驱动.所有的焓/熵补偿图均呈现良好的线性关系,补偿直线在y轴的截距随s减小而变小,这意味着具有较小s的Gemini表面活性剂倾向于生成稳定的胶团.

Experiments on electrical conductivity in aqueous solutions of cationic gemini surfactants as a function of concentration and temperature were carried out, where two species of alkyldimethylammonium bromide [CmH2m+1N(CH3)(2) . Br, m = 8, 10, 12] were linked at the polar head groups via a hydrocarbon spacer [(CH2)(s), s =2,3,4,6]. Critical micelle concentration (cmc) was obtained from the break point in the curve of conductivity kappa vs concentration c. The results indicate that the trend of micelle formation of the gemini surfactants is significantly stronger than the corresponding 'monomer', i.e. alkyltrimethylammonium bromide. The changes of Gibbs free energy, enthalpy and entropy in the process of micellization were evaluated based on a mass action model. The results show that micellization of the gemini surfactants is an entropy - driven process, which is similar to conventional surfactants. All plots of enthalpy - entropy compensation exhibit excellent linearity. The intercept of compensation of the gemini surfactants decreases with the decrease of s from 6 to 2. This implies that the gemini surfactants with short spacer chain tend to form stable micelles.