College of Chemistry and Chemical Engineering, Hunan Normal University , Changsha 410081, China.
J Phys Chem B. 2012 Aug 30;116(34):10330-41. doi: 10.1021/jp303682d. Epub 2012 Aug 22.
Electrostatic interactions play an important role in setting the aqueous two-phase separation behaviors of mixtures of oppositely charged surfactants. The aqueous mixture of cetyltrimethylammonium bromide (CTAB) and sodium dodecylsulfonate (AS) is actually a five-component system, comprised of CTAB, AS, complex salt (cetyltrimethylammonium dodecylsulfonate, abbreviated as CTA(+)AS(-)), NaBr, and water. In the three-dimensional pyramid phase diagram, the aqueous two-phase region with excess AS or with excess CTAB extends successively from the region very near to the NaBr-H2O line through the CTAB-AS-H2O conventional mixing plane to the CTA(+)AS(-)-AS-H2O side plane or to the CTA(+)AS(-)-CTAB-H2O side plane, respectively. Large or small molar ratios between the counterions and their corresponding surfactant ions for oppositely charged surfactants located in the NaBr side or the CTA(+)AS(-) side of the pyramid imply strong or weak electrostatic screening. Electrostatic screening of counterions alters the electrostatic attractions between the oppositely charged head groups or the electrostatic repulsions between the like-charged head groups in excess, and the electrostatic free energy of aggregation thus affects the aqueous two-phase separation modes. Composition analysis, rheological property investigation, and TEM images suggest that there are two kinds of aqueous two-phase systems (ATPSs). On the basis of these experimental results and Kaler's cell model, two kinds of phase separation modes were proposed. Experimental results also indicate that all of the top phases are surfactant-rich, and all of the bottom phases are surfactant-poor; the density difference between the top phase and the bottom phase in one ATPS is very small; the interfacial tension (σ) of the ATPS is ultralow.
静电相互作用在确定相反电荷表面活性剂混合物的水相两相间分离行为方面起着重要作用。十六烷基三甲基溴化铵（CTAB）和十二烷基磺酸钠（AS）的水混合物实际上是一个五组分体系，由 CTAB、AS、复合盐（十六烷基三甲基十二烷基磺酸钠，简称 CTA(+)AS(-)）、NaBr 和水组成。在三维角形相图中，具有过量 AS 或过量 CTAB 的水相两相间区域分别从非常靠近 NaBr-H2O 线的区域通过 CTAB-AS-H2O 常规混合平面延伸到 CTA(+)AS(-)-AS-H2O 侧平面或 CTA(+)AS(-)-CTAB-H2O 侧平面。位于角形图的 NaBr 侧或 CTA(+)AS(-)侧的相反电荷表面活性剂的抗衡离子与其相应的表面活性剂离子的大或小摩尔比表示强或弱静电屏蔽。抗衡离子的静电屏蔽改变了相反电荷头基之间的静电吸引力或过量带相同电荷的头基之间的静电排斥，因此静电聚集自由能会影响水相两相间的分离模式。组成分析、流变性质研究和 TEM 图像表明存在两种水相两相间系统（ATPS）。基于这些实验结果和 Kaler 的细胞模型，提出了两种相分离模式。实验结果还表明，所有的顶相都是表面活性剂富相，所有的底相都是表面活性剂贫相；一个 ATPS 中的顶相和底相之间的密度差非常小；ATPS 的界面张力（σ）超低。
