Interfacial Dilational Properties of Tri-Substituted Alkyl Benzene Sulfonates

The dilational rheological properties of five types of surfactants: sodium 3,4-dihexylbenzene sulfonate (66), sodium 3,4-diheptylbenzene sulfonate (77), sodium 2-ethyl-4,5-dihexylbenzene sulfonate (266), sodium 2-propyl-4,5-dihexylbenzene sulfonate (366), and sodium 2-butyl-4,5-dihexylbenzene sulfonate (466) at the air-water and decane-water interfaces were investigated by a drop shape analysis method. The influence of hydrophobic chains located at different positions on the benzene ring on the interfacial molecular behavior was investigated. Experimental results showed that the hydrophobic chains that were located at different positions affected the dilational elasticity and dilational viscosity differently. The dilational elasticity appeared to increase with an increase in the length of the hydrophobic group and the dilational viscosity behaved differently. The short alkyl chain (2-4) ortho to the sulfonate group had little influence on the dilational viscosity, while an increase in the long meta alkyl group played an important role in the surface relaxation process, which resulted in higher surface dilational viscosity. The insertion of decane molecules from the oil phase weakened the strong interactions between the meta-alkyl chains and therefore the interfacial dilational parameters were obviously lower than those of the surface. We also found that the interfacial dilational viscosities of a pair of structural isomers, 77 and 266, were approximately equal.