Department of Chemical and Environmental Engineering, Mason Laboratory, Yale University, New Haven, Connecticut 06520-8286, USA.
Langmuir. 2010 Oct 5;26(19):15177-91. doi: 10.1021/la102536y.
Specific ion effects are ubiquitous in biological and colloidal systems. The addition of electrolytes to ionic surfactant solutions has pronounced effects on micellar properties, such as critical micelle concentration (cmc), micellar size, and shape. Ions play an important role in colloid stability and aggregation behavior of ionic surfactant solutions. Despite extensive experimental data, there is no well established molecular theory on specific ion effects. Published molecular thermodynamic theories for ionic surfactants do not properly account for ion-specific effects such as the inversion of the lyotropic series for the cmc of alkyl sulfates and carboxylates. In this work, we present a molecular thermodynamic theory for ionic surfactant solutions to take into account the headgroup-counterion specificity and address ion-specific effects on the cmc and aggregation number. We assume that the charged headgroup and the counterion at the Stern layer form solvent-shared ion pair with different degrees of cosphere overlap. The thickness of the Stern layer is estimated from molecular structures of hydrated surfactant heads and hydrated counterions, and from the knowledge of the qualitative strength of headgroup-counterion interaction in line with Collins' concept of matching water affinities. Our proposed thermodynamic model properly predicts the cmc of both anionic and cationic surfactants of various counterions, and the effect of different inorganic salts on micellization of ionic surfactants.
特定离子效应在生物和胶体系统中普遍存在。电解质的加入对离子表面活性剂溶液的胶束性质有显著影响，如临界胶束浓度（cmc）、胶束大小和形状。离子在胶体稳定性和离子表面活性剂溶液的聚集行为中起着重要作用。尽管有广泛的实验数据，但对于特定离子效应还没有建立完善的分子理论。已发表的离子表面活性剂的分子热力学理论不能正确考虑离子特异性效应，例如烷基硫酸盐和羧酸盐的 cmc 的溶致系列倒置。在这项工作中，我们提出了一种离子表面活性剂溶液的分子热力学理论，以考虑头基-抗衡离子的特异性，并解决 cmc 和聚集数上的离子特异性效应。我们假设带电荷的头基和 Stern 层中的抗衡离子以不同程度的共球重叠形成溶剂共享离子对。Stern 层的厚度是根据水合表面活性剂头和水合抗衡离子的分子结构以及与 Collins 的匹配水亲和力概念一致的头基-抗衡离子相互作用的定性强度知识来估计的。我们提出的热力学模型正确预测了各种抗衡离子的阴离子和阳离子表面活性剂的 cmc，以及不同无机盐对离子表面活性剂胶束化的影响。
