链式与面式两种不对称疏水结构表面活性剂之间的相互作用热力学

本文通过等温滴定量热法(ITC)、电导法和浊度法研究了阴离子生物表面活性剂脱氧胆酸钠(NaDC)及其与相反电荷的十二烷基三甲基溴化铵(DTAB)在水溶液中的自组装热力学.ITC结果支持了NaDC在水溶液中先生成预胶束再形成稳定胶束的分步聚集模型,由此得到了NaDC的预胶束和胶束化过程的一系列热力学参数,并讨论了它们形成的热力学机理.进一步研究了具有头-尾链式和疏水-亲水刚性面式非对称结构的DTAB/NaDC混合体系的聚集热力学行为,得到了富NaDC临界混合胶束浓度(cmcmix)、富DTAB临界胶束浓度(CM)及对应过程的转变焓.结果表明,NaDC面式结构与DTAB链式结构的对称性差异以及相反电荷的相互作用,导致混合体系有别于单一表面活性剂或头-尾链式结构的混合体系的聚集行为.混合溶液的聚集行为受控于表面活性剂浓度和摩尔分数的变化.富NaDC胶束化过程为熵驱动,而富DTAB的两种胶束形态转变过程为熵焓共同驱动的热力学机理.这些结果对于从热力学角度认识胆汁酸盐的自组装机理以及与传统的头-尾链式结构的表面活性剂相互作用机理和相行为有重要的意义.

Thermodynamics of interaction between two kinds of surfactants with asymmetric hydrophobic moieties of alkyl tail and rigid face

In this work, the thermodynamics of molecular self-assembly for an anionic biosurfactant, sodium deoxycholate （NaDC） and their mixture with an oppositely charged surfactant, docecyltrimethylamonium bromide （DTAB） in aqueous solution was investigated by isothermal titration calorimeter （ITC）, companied with turbidity and conductivity measurements. From the perspective of interaction energy, the results for single surfactant NaDC obtained from ITC provided a direct evidence for the stepwise association model to form the premicelles in the first step and the stable micelles in the second step. We obtained a series of thermodynamic parameters of premicellar and micellar formation, and thus discussed the thermodynamic mechanisms of molecular aggregation in these two processes. Furthermore, we investigated the thermodynamics of intermolecular interaction for the mixed systems composed of oppositely charged surfactants （DTAB/NaDC）. The mixed critical micelle concentration （cmcmix） and the transition concentration between two micellar morphologies （CM） and their corresponding changes of enthalpy in the both processes were derived from ITC measurements in rich-NaDC and in rich-DTAB region, respectively. The results show that aggregation behaviors of DTAB/NaDC differ from single surfactant system or the normal mixed surfactants with charged ＂head-and-tail＂, due to the asymmetry in molecular structures caused by the rigid steroid skeleton of NaDC composed of a hydrophilic and a hydrophobic surfaces. As a result, the occurrence of various aggregation processes in mixed solution can be controlled by the total concentrations of two surfactants and their molar ratio. Conclusively the process of mixed micellization is driven by entropy, and the transition between two kinds of micellar morphologies is driven jointly by entropy and enthalpy. Therefore, from the perspective of interaction themodynamics, this work gave a deep insight in understanding the themodynamics of molecular self- assembly and their phase behavior for mixed systems with bile salts and the positively charged surfactant.