IDEAL BEHAVIOR OF SODIUM ALKYL SULFATES AT VARIOUS INTERFACES . THERMODYNAMICS OF ADSORPTION AT OIL-WATER INTERFACE

Adsorption of sodium decyl sulfate and sodium dodecyl sulfate has been studied at very high areas per molecule at the hydrocarbon-0.1 M NaCl interface for a homologous series of n-alkanes (C6-C16). Ideal gaseous behavior was observed in all systems studied. The chain length of the oil had no apparent effect on adsorption in the ideal region. The standard thermodynamic functions for adsorption at the dodecane-0.1 M NaCl interface were determined and evaluated. Based on comparisons of the data obtained for both air-water and hydrocarbon-water interfaces and on evaluation of the results in terms of the major forces involved in the adsorption process, it is suggested that sodium dodecyl sulfate dimers, which are present in the aqueous bulk, dissociate at the oil-water interface and that the monomer is the exclusive adsorbed species. The importance of hydrophobic bonding and entropy as a driving force to adsorption at the oil-water interface is discussed.