Surface chemical properties in aqueous solutions of sodium alkylbenzene sulfonates

Abstract

Surface tension measurements have been made on aqueous solutions of sodium alkylbenzene sulfonates in the presence of sodium chloride. The total free energy change in adsorption, ΔG total, obtained from the above data by assuming two different types of adsorption models, has been discussed in the light of the treatment recently developed for adsorbed ionic monolayers. The adsorbed molecules at an air/water interface would seem to be immobile. With an immobile model, the free energy change in adsorption, ΔG 0, exclusive of electrical energy and cohesional energy of the monolayer, involved in transferring a molecule from the bulk to the surface has been estimated in the presence of sodium chloride of a constant concentration. The value of ΔG 0 does not depend on the fractional coverage, but increases approximately 1.3 kcal/mole in going from decyl- to dodecylbenzene sulfonate. This shows that Traube's rule holds good even for surfactants having a hydrocarbon chain to which a benzene ring is attached. The effect of the concentration of the electrolyte on the free energy of adsorption was obtained when the concentration of sodium chloride was changed up to 0.10 M. Addition of the excess electrolyte raises ΔG total whereas it lowers ΔG 0.