STABILITY OF MONODISPERSE ZINC-SULFIDE COLLOIDAL DISPERSIONS

The stability of monodisperse, spherical colloidal particles of zinc sulfide, in the presence of NaCl and CaCl2 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 zeta potential of the particles and their surface free-energy components were determined as a function of electrolyte concentration, using, respectively, electrophoretic mobility measurements and the thin-layer wicking method. The effect of NaCl concentration on the zeta potential of the particles is typical of indifferent electrolytes, whereas Ca2+ 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.