Study of dynamic interfacial tension of alkyl sulphates with different alkyl chain lengths at the water/hexane interface

The adsorption behaviour of alkyl sulphates at the water/oil interface is rather different from that frequently studied at the water/air interface. First of all typical impurities in form of homologous alcohols, immanent for this type of surfactants at the surface of aqueous solutions, are not essential at the water/oil interface due to their high solubility in the oil phase. In the interfacial layers the oil molecules, here hexane, are incorporated into the structure, and have essential impact on the equation of state at low interfacial coverage and in particular at the shorter alkyl chain surfactants. For the longer chain alkyl sulphates (NaC14SO4 and NaC16SO4) and at high interfacial coverage the hexane molecules are squeezed out of the adsorption layer due to the stronger hydrophobic interaction between the surfactants’ alkyl chains. The adsorption characteristics are discussed in the framework of the two well-known thermodynamics Frumkin and reorientation models.