STABILITY OF MONODISPERSE ZINC-SULFIDE COLLOIDAL DISPERSIONS

The stability of monodisperse, spherical colloidal particles of zinc s ulfide, 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-bas e (AB) interactions have to be considered for better explaining the st ability of ZnS colloidal dispersions. Theoretical interaction energy-d istance curves are computed and compared to experimental determination s 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 componen ts 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 potentia l of the particles is typical of indifferent electrolytes, whereas Ca2 + cations appear to specifically interact with the ZnS surface. The st ability of the suspensions is lowest for concentrations around 10(-2) M, whereas higher concentrations seem to stabilize the suspensions. Af ter calculation of the surface free-energy components of the particles , potential energy of interaction curves are computed for different in terparticle distances. A comparison is carried out between the predict ions of both classical and extended DLVO models and experimental stabi lity data. A good qualitative agreement between theoretical and experi mental results is found when the latter model is used. The inclusion o f(Lewis) acid-base interactions between the particles is thus a useful tool to adequately describe the stability of ZnS suspensions. The res ults support the previous findings (van Oss, C. J.; et al. Clays Clay Min. 1990, 38, 151) on the suitability of adding acid-base (Lewis) for ces to electrostatic and Lifshitz-van der Waals forces to have a power ful theory capable of predicting many aspects of the behavior of collo idal suspensions.