Stability of Monodisperse Zinc Sulfide Colloidal Dispersions

Abstract

The stability of monodisperse, spherical colloidal particles of zinc sulfide, in the presence of NaCl and CaCl 2 solutions, has been studied in this work. The so-called extended DLVO theory of stability is used to explain the data. In this model, it is proposed that Lewis acid-base (AB) interactions have to be considered for better explaining the stability of ZnS colloidal dispersions. Theoretical interaction energy-distance curves are computed and compared to experimental determinations of the stability of the suspensions, obtained from time evolution of both their optical absorbance and particle diameter. Previously, the ζ potential of the particles and their surface free-energy components were determined as a function ofelectrolyte concentration, using, respectively, electrophoretic mobility measurements and the thin-layer wicking method. The effect of NaCl concentration on the ζ potential of the particles is typical of indifferent electrolytes, whereas Ca 2+ cations appear to specifically interact with the ZnS surface. The stability of the suspensions is lowest for concentrations around 10 -2 M, whereas higher concentrations seem to stabilize the suspensions. After calculation of the surface free-energy components of the particles, potential energy of interaction curves are computed for different interparticle distances. A comparison is carried out between the predictions of both classical and extended DLVO models and experimental stability data. A good qualitative agreement between theoretical and experimental results is found when the latter model is used. The inclusion of (Lewis) acid-base interactions between the particles is thus a useful tool to adequately describe the stability of ZnS suspensions. The results support the previous findings (van Oss, C. J. ; et al. Clays Clay Min. 1990, 38, 151) on the suitability of adding acid-base (Lewis) forces to electrostatic and Lifshitz-van der Waals forces to have a powerful theory capable of predicting many aspects of the behavior of colloidal suspensions.