Adsorption of sodium dodecyl sulfate and cosurfactant at the planar cyclohexane-brine interface. Validity of the saturation adsorption approximation and effects of the cosurfactant chain length

Abstract

The interfacial excess concentrations of sodium dodecyl sulfate (SDS) and two different cosurfactanta (pentanol and hexanol) in mixed monolayers at the interface between water (with 0.2 M NaCl) and an oil phase consisting of cyclohexane and cosurfactant are determined from surface tension measurements using the Gibba adsorption equation. The data are analyzed in two ways: assuming saturation adsorption and fitting the data to the Szyszkowski equation. This analysis reveals that a constant interfacial concentration in the vicinity of the critical micelle concentration is a good approximation even though the cosurfactant interfacial concentration does not reach its saturation value in the experimental cosurfactant concentration range. For both pentanol and hexanol, at low concentrations a strong competition between cosurfactant and SDS is observed, whereas at higher concentrations the composition of the interface remains practically constant. Under these conditions a high packing density in the mixed monolayer is observed.