Precipitation of calcium salts of surfactants

Abstract

The precipitation of calcium laurate from solution has been studied using a stopped-flow spectrophotometric technique. From measurements of turbidity at two different wavelengths, particle radii and number concentration were calculated using Rayleigh-Debye theory, the results being checked by electron microscopy and laser counting. The change from heterogeneous to homogeneous nucleation was observed on increasing the supersaturation above 5. Chronomal analysis indicated two possible rate determining steps in the growth of particles, and a diffusion coefficient (D= 6.4 × 10^–6 cm² s^–1) and an interfacial tension (σ= 34.4 mN m^–1) were obtained. Finally, the influence of micelles on the precipitation process was investigated using the calcium salt of sodium dodecyl sulphate. The results indicated that the micelles merely acted as reservoirs of Ca²⁺ and surfactant, the dissociation or breakdown of the micelle not being rate-determining.