Explanation for the Increased Induction Times in Binary Mixed Anionic Surfactant Mixtures

Abstract

The induction time for precipitation of pure sodium dodecylsulfate (NaDS) solutions by CaCl2 both below and above the cmc at a specific temperature depends only on the supersaturation ratio of the precipitating species in the bulk solution, rather than the means of achieving the supersaturation (changes in calcium ion concentration or in surfactant concentration). For mixed NaDS/sodium decylsulfate (NaDeS) systems precipitated by CaCl2, the induction time is only a function of the supersaturation ratio calculated based on Ca(DS)2, which is formed from these solutions, since its solubility product is orders of magnitude less than that of Ca(DeS)2. The increase in induction time in mixtures of these surfactants, compared to pure systems of the same total surfactant content, is only due to the change in the concentration of the precipitating species (DS) because of changes in the molar ratio of the surfactants present and thermodynamic changes due to mixed micellization. For mixed NaDS/sodium octylbenzenesulfonate (NaOBS) systems precipitated by CaCl2, the precipitation of calcium surfactant salts can result in either Ca(DS)2 or Ca(OBS)2, depending on the conditions, due to the similar KSP values of these two salts. Precipitation of Ca(OBS)2 occurs in systems of approximately 0−50 mol % NaDS, and inhibition of precipitation is not found in mixed NaDS/NaOBS systems for this range of surfactant compositions. For precipitation of Ca(DS)2 in systems between 50 and 100 mol % NaDS, the value of the induction time as a function of supersaturation ratio calculated based on Ca(DS)2 is slightly larger than that found in the pure NaDS systems. Inhibition in these mixtures is mainly due to the change in concentration of the precipitating species (monomer DS) because of changes in surfactant composition in the solution phase and changes in the precipitation phase boundary because of the thermodynamic changes resulting from mixed micellization; however, there is also a small amount of kinetic inhibition.