Interfacial dilational rheology of extended surfactants with short hydrophobic groups: role of ionic heads and hydrophobic tails

To clarify the influence of molecular structure on interfacial behaviors of extended surfactants, interfacial rheological properties of three extended surfactants with different ionic heads (C 8 P 10 E 5 C and C 8 P 10 E 5 S) and different hydrophobic tails (C 8 P 10 E 5 C and phP 10 E 5 C) were investigated by the droplet analysis method. The effects of adsorption time, interfacial pressure, oscillating frequency, and concentration on the dilational modulus and phase angle of C 8 P 10 E 5 C, C 8 P 10 E 5 S, and phP 10 E 5 C were determined. Results indicate that interfacial films of these three extended surfactants exhibit predominantly elastic behavior. The modulus and phase angle of C 8 P 10 E 5 C and C 8 P 10 E 5 S with different ionic heads exhibit little variation at variable experimental conditions, indicating that different ionic heads have a negligible effect on interfacial film properties. At low interfacial pressures, the adsorption layer is in a local thermodynamic quasi-equilibrium state with negligible intrinsic relaxation processes. At high interfacial pressures, the diffusion exchange process may play a more important role in the interfacial film of C 8 P 10 E 5 C. Compared to C 8 P 10 E 5 C, phP 10 E 5 C with a different hydrophobic group maintains a higher dilational modulus over a wider concentration range due to its rigid phenyl groups. Compared to ionic heads, hydrophobic tails exert a more pronounced influence on the interfacial rheological behavior of extended surfactants. These findings are expected to contribute to the rational design of extended surfactants with a short hydrophobic group.