Abstract
The gradual precipitation of surfactant calcium salts was observed from solutions of linear alkylbenzene sulfonate (LAS) and α-sulfohexadecanoic acid methyl ester (SFM) where either was the sole surfactant in the presence of calcium ions. When LAS and SFM were present in the same solution, their precipitation was retarded and detergency enhanced. In the present study, an investigation was made of interactions between LAS and SFM. Based on thermodynamic considerations, the influence of SFM on the alkyl distribution of LAS as monomer was predicted. It was considered that, should a mixed aggregate of LAS and SFM be formed in the presence of calcium ions, the surfactant with the lower alkyl chain of LAS would constitute the greater part of the monomer. The alkyl length distribution of monomeric LAS was determined from the initial permeation rate of each LAS constituent in solution in a semipermeable tube, this rate being assumed proportional to monomer concentration. LAS monomer was measured in various solutions with and without calcium ions or SFM. Experimental data confirmed the formation of mixed aggregates of LAS and SFM which we concluded to be responsible for preventing the precipitation of LAS and SFM in solution containing calcium ions.
