Novel results on the adsorption of ionic surfactants at the air/water interface — sodium- n-alkyl sulphates

Abstract

Equilibrium surface tension vs. concentration isotherms of aqueous, surface-chemically pure solutions of the homologous series of sodium n-alkyl sulphates (from C 7 to C 14) have been evaluated using a new surface equation of state approach. All adsorption parameters (standard free energy of adsorption, interaction parameter, saturation adsorption) and the CMC reveal a pronounced even/odd effect. Surprisingly, it was found that the cross-sectional areas of the short-chain homologues decrease with increasing chain length, reaching a minimum value for C 10 and C 11. However the cross-sectional areas of the longer-chain homologues (C n ≥ C 12) increase again drastically with increasing chain length. This novel effect is attributed to increasing electrostatic repulsion within the adsorption layer caused by increasing Debye lengths of the stronger surface-active homologues. The good agreement between the data for the cross-sectional area of sodium- n-dodecyl sulphate, as obtained from the new surface equation of state approach, from the Gibbs equation and from crystal structure analysis, make us suggest that it is unreasonable to apply an additional factor in the Gibbs equation for ionic amphiphiles as long as the counterion is very small in comparison with the oppositely charged amphiphilic molecule.