Dilational Properties of N-Acyltaurates with Aromatic Side Chains at Water–Air and Water–Decane Interfaces

Abstract

A series of novel surfactants, sodium N-2-(alkylphenoxy)-tetradecanolytaurinates (12 + nB-T, where 12 means the carbon number of alkyl chain in the main chain and n is the carbon number of alkyl chains in the side chain attached to the phenoxyl group, n = 0, 2, 6) bearing identical polar groups, were designed and synthesized. The dilational properties of the three surfactants at the water–air and water–decane interfaces were investigated by drop shape analysis. The influences of oscillating frequency and bulk concentration on dilational properties were explored, and the effect of alkyl chain length in the side chain on interfacial behavior has been expounded. The longer alkyl chains in the side chains lead to a higher surface dilational elasticity at low bulk concentrations, owing to strong interaction among the longer hydrophobic alkyl chains. However, at high bulk concentrations, the longer the hydrophobic alkyl chain is, the easier the diffusion-exchange between the bulk and the surface becomes. As a result, the surface dilational elasticity of surfactant with the shortest chain is the highest at a corresponding concentration range. The insertion of oil molecules will reduce interfacial molecular interactions and ease down the diffusion exchange process. Consequently, dilational elasticity decreases at low concentrations and increases at high bulk concentrations, respectively. Moreover, the maximum of dilational viscous components at two types of interfaces is approximately equal for all three surfactants, which may be attributed to the offset between the variation of interfacial molecular interaction and diffusion exchange process.