Adsorption of sodium alkyl sulfate homologues at the air/solution interface

Experimental results are presented on the adsorption of sodium alkyl sulfate homologues (nC = 8-14) at the air/solution interface. The adsorption isotherms calculated from equilibrium surface-tension vs concentration data and the critical micelle concentration change regularly with the length of the alkyl chain; the odd/even effect was not observed. The isotherms were analyzed using a model-independent approach. The analysis indicates that the total driving force of adsorption reaches a plateau value and becomes constant in the function of the adsorbed amount in the case of each homologue. With the use of different electrostatic models, it was demonstrated that this behavior is consistent with a saturation-type hydrophobic driving-force contribution, which can be interpreted by the development of a liquidlike alkane environment in the adsorbed layer above a "critical" adsorbed amount.