pH-Dependent Interfacial Tension and Dilatational Modulus Synergism of Oil-Soluble Fatty Acid and Water-Soluble Cationic Surfactants at the Oil/Water Interface
pH-Dependent Interfacial Tension and Dilatational Modulus Synergism of Oil-Soluble Fatty Acid and Water-Soluble Cationic Surfactants at the Oil/Water Interface
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DOI：
10.1021/acs.langmuir.1c01889
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发表时间：
2021-09-23
期刊：
影响因子：
3.9
通讯作者：
Tilton, Robert D.
中科院分区：
文献类型：
--
作者：
Hsieh, Tsung-Lin;Law, Stephen;Tilton, Robert D.
虽然表面活性剂混合物中界面张力协同作用的概念已得到充分确立，但很少有人关注混合表面活性剂体系的界面流变学协同效应的可能性。此外，界面张力协同作用最常用于单相表面活性剂混合物的研究。在这里，我们定义了膨胀模量协同作用，并报告了二元表面活性剂体系的界面张力等温线和复杂膨胀模量的研究，其中两种表面活性剂从相对侧进入油/水界面。使用可在油/水界面电离的油溶性脂肪酸表面活性剂（棕榈酸，PA）和季铵水溶性阳离子表面活性剂（十四烷基三甲基溴化铵，TTAB），通过水相pH值调节二元界面相互作用。通过悬滴法测量二元表面活性剂体系以及相应的单一表面活性剂体系的界面张力和膨胀模量，以确定协同效应。从两个角度探讨了膨胀模量协同作用可能发生的可能性：一个是固定的总表面活性剂浓度，另一个是固定的界面张力。发现水的pH值对界面张力协同作用和膨胀模量协同作用均具有控制作用。界面张力协同作用的条件与储能模量协同作用的条件一致：在 pH 7 下测试的所有条件下都观察到了张力和储能模量协同作用，其中 PA 大部分是去质子化的，对于所检查的两个角度而言，但在 pH 3 下测试的任何条件下都观察到，其中 PA 大部分是质子化的。损耗模量协同表现出更复杂的行为，例如频率和界面张力依赖性，但同样仅在 pH 7 时观察到。pH 7 下的张力和模量协同归因于电离 PA 和阳离子 TTAB 之间吸引力的增加以及油/水界面处阴阳离子络合物的形成。
While the concept of interfacial tension synergism in surfactant mixtures is well established, little attention has been paid to the possibility of synergistic effects on the interfacial rheology of mixed surfactant systems. Furthermore, interfacial tension synergism is most often investigated for mixtures of surfactants residing in a single phase. Here, we define dilatational modulus synergism and report a study of interfacial tension isotherms and complex dilatational moduli for a binary surfactant system with the two surfactants accessing the oil/water interface from opposite sides. Using an oil-soluble fatty acid surfactant (palmitic acid, PA) that may be ionized at the oil/water interface and a quaternary ammonium water-soluble cationic surfactant (tetradecyltrimethylammonium bromide, TTAB), the binary interfacial interaction was tuned by the aqueous phase pH. Interfacial tensions and dilatational moduli were measured by the pendant drop method for the binary surfactant system as well as the corresponding single-surfactant systems to identify synergistic effects. The possible occurrence of dilatational modulus synergism was probed from two perspectives: one for a fixed total surfactant concentration and the other for a fixed interfacial tension. The aqueous pH was found to have a controlling effect on both interfacial tension synergism and the dilatational modulus synergism. The conditions for interfacial tension synergism coincided with those for the storage modulus synergism: both tension and storage modulus synergisms were observed under all conditions tested at pH 7 where PA was mostly deprotonated, for both perspectives examined, but not for any conditions tested at pH 3 where PA is mostly protonated. The loss modulus synergism exhibited more complex behaviors, such as frequency and interfacial tension dependences, but again was only observed at pH 7. The tension and modulus synergism at pH 7 were attributed to the increased attraction between ionized PA and cationic TTAB and the formation of catanionic complexes at the oil/water interface.