Effect of the Surfactant C12mimBr on the Aggregation Behavior of Gemini12-2-12 at an Air/Water Interface, Investigated Using an Interfacial Dilational Rheology Method

The dynamic interfacial tension, dilational rheological properties, and interfacial relaxation processes of quaternary ammonium Gemini surfactant C-12-(CH2)(2)-C-12 center dot 2Br (Gemini12-2-12) solutions and Gemini12-2-12/ionic liquid surfactant C(12)mimBr mixed systems at an air/water interface were investigated using an interfacial dilational rheology method at low frequencies (0.02-0.50 Hz). The effect of the C(12)mimBr on the interfacial properties of the Gemini12-2-12/C(12)mimBr mixed systems, and the mechanism responsible for the influence of C(12)mimBr on the aggregation behavior of the Gemini12-2-12 at the air/water interface, are discussed here. The experimental results showed that with increasing the amount of C(12)mimBr, the time required to achieve interfacial adsorption equilibrium for the mixed systems was reduced, the dilational moduli and phase angle in the mixed systems decreased, and the interfacial adsorption films were inclined to become elastic. Simultaneously, the relaxation processes at the interface or near the interface changed significantly, the slow relaxation process disappeared, and a fast relaxation process dominated the properties of the interfacial films. Moreover, the contribution of the fast relaxation process increased with increasing the concentration of C(12)mimBr. The abovementioned changes in the interfacial properties were mainly attributed to the participation of the C(12)mimBr in the formation of the interface, and the competitive adsorption of the two surfactants at the air/water interface. At lower concentrations of C(12)mimBr, the C(12)mimBr molecules filled the vacancies between the Gemini12-2-12 molecules when the Gemini12-2-12 molecules were loosely arranged at the interface, and mixed-adsorption films formed from C(12)mimBr and Gemini12-2-12 spread on the air/water interface. With increasing the concentration of C(12)mimBr, the alkyl chains of the Gemini12-2-12 molecules that were wrapped around each other at the air/water interface untangled, and the Gemini12-2-12 molecules underwent desorption from the interface. At the same time, C(12)mimBr molecules replaced Gemini12-2-12 molecules, because of their low steric hindrance and strong hydrophobic effects; ultimately, C(12)mimBr molecules almost entirely occupied the air/water interface.