Dilational rheological properties of 2, 5-dialkyl benzene sulfonates at air-water and decane-water interfaces

The dilational properties of two anionic surfactants 5-6-4 and 5-6-t4 (2, 5-dialkyl benzene sulfonates) at the air-water and decane-water interfaces were investigated using the drop shape analysis method. The effects of aging time, oscillating frequency, bulk concentration, and surface/interfacial pressure on the dilational elasticity and dilational viscosity were explored. The results show that the slow process and diffusion exchange process control the nature of surface/interfacial films at low and high concentrations, respectively. At low concentrations, the strong interaction between the branched long alkyl chains dominates at the surface. At high concentrations, the surface elasticity of 5-6-4 is higher because of the slower diffusion-exchange process. The insertion of oil molecules disrupts the interactions between branched long alkyl chains and simultaneously enhances the diffusion-exchange process, resulting in lower maximum interfacial dilational elasticity and viscosity for both 5-6-4 and 5-6-t4 molecules. The rigid isobutyl group endows the 5-6-t4 molecule with higher dilational elasticity and viscosity at the interface.