Surface dilational rheology of betaine surfactants: Effect of molecular structures

Abstract

Measurements of the surface dilational rheological properties have been carried out for aqueous solutions of four zwitterionic surfactants, hexadecanol glycidyl ether glycine betaine (C16PB), guerbet alcohol hexadecyl glycidyl ether glycine betaine (C16GPB), hexadecanol polyoxyethylene(3) glycidyl ether glycine betaine (C16(EO)3PB) and guerbet alcohol hexadecyl polyoxyethylene(3) glycidyl ether glycine betaine (C16G(EO)3PB), using the oscillating bubble method. The influences of surface ageing time, surface pressure, dilational frequency and bulk concentration on surface rheological properties have been expounded. It is found that the whole hydrophilic part of betaine molecule inclines to flat on the surface when betaine adsorbs, which will lead to higher dilational modulus. The nature of adsorption film is determined by in-surface processes, such as the variation of orientation of hydrophilic group. The branched hydrophobic alkyl chains of betaine molecules improve the intermolecular interactions, not only results in higher moduli, but causes a maximum in dynamic elastic modulus at high concentration. This indicates that the strength of surface film is strongly related to molecular arrangement, not simply determined by the surface molecule concentration. When ethylene oxide groups are incorporated in the hydrophobic group, the enhancement of steric hindrance will decrease both elastic modulus and viscous modulus.