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