Surface dilational properties of sodium 4-(1-methyl)-alkyl benzene sulfonates: Effect of alkyl chain length

The dilational properties of absorbed film of three sodium 4-(1-methyl)-alkyl benzene sulfonates at the air-water interface have been investigated by drop shape analysis method. The influences of time, dilational frequency and surfactant bulk concentration on surface dilational modulus and phase angle have been expounded. The nature of interfacial film depends on the molecular structure of surfactants. For 2 Phi-14 and 2 Phi-16, the diffusion-exchange process controls the dilational properties and the film shows viscoelastic character. As an increase of the hydrophobic chain, the dilational modulus of 2 Phi-18 is independent on applied frequency in the studied bulk concentration except for 1 x 10(-5) mol/L, which indicates the adsorbed layer behaves nearly purely elastic. Furthermore, the phase angle of 2 Phi-14 and 2 Phi-16 are remarkably higher than those of 2 Phi-18, confirming that the presence of strong interactions between hydrophobic chains of 2 Phi-18. Best agreements with the experimental data have been obtained from the curves of dilational modulus vs. surface pressure.