Synergistic effects in mixtures of two identically charged ionic surfactants with different critical micelle concentrations
Synergistic effects in mixtures of two identically charged ionic surfactants with different critical micelle concentrations
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DOI：
10.1039/c1sm06064c
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发表时间：
2011-01-01
期刊：
影响因子：
3.4
通讯作者：
Aratono, Makoto
中科院分区：
文献类型：
--
作者：
Bergstrom, L. Magnus;Aratono, Makoto
两种带相同电荷的单价离子表面活性剂混合物中的临界胶束浓度 (cmc) 和活度系数作为表面活性剂组成的函数的表达式源自非线性泊松-玻尔兹曼 (PB) 理论。对于不添加盐的特殊情况，推导出简单表达式 cmc(alpha) = xcmc(1)(alpha) + (1 - x)cmc(2)(alpha)，其中指数参数 alpha > 1 取决于表面活性剂分子中离子种类的数量以及自组装界面的曲率。将理论预测值与通过一些不同实验技术获得的两种阳离子表面活性剂双十二烷基二甲基溴化铵(DDAB)和十二烷基三甲基溴化铵(DTAB)在水中且不添加盐的混合物的cmc值进行比较。结果表明，PB 理论与实验数据的一致性明显好于理想行为或常规混合理论的预测。我们发现，当溶液中 DDAB 摩尔分数 y = 0.005 时，协同效应最大。根据PB理论，这个非常低的y值对应于自组装界面聚集体中DDAB的摩尔分数等于x = 0.995。此外，我们对 cmc 以上自组装界面聚集体中表面活性剂组成的计算表明，发现从小胶束到大双层聚集体的转变始终发生在 DDAB 摩尔分数等于约 x = 0.41-0.42 时，与溶液中表面活性剂摩尔比无关。实验观察有力地支持了这样一个事实：游离表面活性剂的浓度以及自组装界面聚集体中的表面活性剂组成可以通过非线性泊松-玻尔兹曼理论精确计算。另一方面，自组装界面聚集体中表面活性剂摩尔分数的变化引起的胶束到双层的转变既不符合理想的表面活性剂行为，也不符合常规混合物理论的协同行为。
Expressions for the critical micelle concentration (cmc) and activity coefficients as functions of surfactant composition in mixtures of two identically charged monovalent ionic surfactants are derived from the nonlinear Poisson-Boltzmann (PB) theory. For the special case of no added salt, the simple expression cmc(alpha) = xcmc(1)(alpha) + (1 - x)cmc(2)(alpha) is deduced, where the exponential parameter alpha > 1 depends on the number of ionic species in a surfactant molecule as well as the curvature of the self-assembled interface. Theoretical predictions are compared with cmc values obtained with some different experimental techniques for mixtures of the two cationic surfactants didodecyldimethylammonium bromide (DDAB) and dodecyltrimethylammonium bromide (DTAB) in water and in the absence of added salt. It is demonstrated that the PB theory generates significantly better agreement with experimental data than predicted by ideal behaviour or the regular mixture theory. We find that maximum synergistic effects occur at a DDAB mole fraction in solution y = 0.005. According to the PB theory, this very low value of y corresponds to a mole fraction of DDAB in the self-assembled interfacial aggregates equal to x = 0.995. Moreover, our calculations of the surfactant composition in the self-assembled interfacial aggregates above cmc demonstrate that the transition from small micelles to large bilayer aggregates is found to consistently occur at a mole fraction of DDAB equal to about x = 0.41-0.42, irrespective of the surfactant molar ratio in solution. Experimental observations strongly support the fact that concentrations of free surfactant, as well as the surfactant composition in the self-assembled interfacial aggregates, may be accurately calculated from the non-linear Poison-Boltzmann theory. On the other hand, a micelle-to-bilayer transition induced by changes in surfactant mole fraction in the self-assembled interfacial aggregates is consistent with neither ideal surfactant behaviour nor synergistic behaviour according to the regular mixture theory.