Dilational rheological properties of novel zwitterionic surfactants containing benzene ring and polyoxyethylene group at water–decane interface

The interfacial dilational rheological behaviors of four zwitterionic surfactants with benzene ring and polyoxyethylene group, p-(n-lauryl)-benzyl polyoxyethylene ether carboxybetaine C12BExCB (x = 0, 1, 2, 3), were investigated via the drop shape analysis method. The influences of time, oscillating frequency and bulk concentration on dilational properties were explored. The experimental results show that the number of ethylene oxide groups is one of the principal factors to control the interfacial film. The dilational properties of C12BExCB are quite different from those of common surfactants: the phase angle decreases with aging time, the slope of the logε − logω curve and phase angle of C12BExCB decrease in a wide concentration range, and the dilational modulus of C12BExCB passes through a plateau value with the increasing concentration for surfactant with more ethylene oxide groups. These phenomena become more and more apparent with increasing ethylene oxide groups and it cannot be attributed to the diffusion-exchange process between the bulk and the interface. It is reasonable to consider that ethylene oxide groups are flexible and can be compressed and expanded, just like a spring. The compression and expansion of the ethylene oxide groups in the interfacial layer and the exchange between interface and sub-layer play a crucial role for C12BExCB. Possible schematic diagrams of adsorbed molecules with time and concentration at the water–decane interface are proposed. The results of static modulus measurements and dynamic interfacial viscoelasticity for CyBE2CB (y = 8, 10, 12) with different length of hydrophobic chain can support our provided mechanism strongly.