Surface free energy components of monodisperse zinc sulfide

Abstract

Apolar, Lifshitz-van der Waals ( γ LW ) and polar, acid-base (Lewis) ( γ AB ) surface free energy components, of monosized, spherical ZnS colloidal particles were experimentally determined using the thin-layer wicking technique. The results were compared with those obtained on a pure, commercial sample of zinc sulfide. The fact that the synthetized sample showed a very high electron-donor ( γ s −) component suggested that some species, related in principle to surface oxidation of ZnS during the synthesis reaction, could be formed on the surface of the particles. To check for this possibility, the surface free energy components of pure and oxidized ZnS, and of zinc oxide and sulfate, were determined by both the contact angle and thin-layer wicking methods. The similarity between γ s components obtained with both methods for pure ZnS lead to the conclusion that both techniques give reliable results. The high γ s − value determined for synthetic ZnS particles cannot be explained simply by surface oxidation of the colloidal particles, since partially oxidized ZnS, or even zinc oxide or sulfate, gave very similar γ s −, always lower than that found for synthetic spherical ZnS. The fact that the Lifshitz-van der Waals ( γ s LW ) component of the surface free energy is somewhat higher for the latter material is believed to be related to elemental sulfur present on the surface of the particles. The general compatibility of the results obtained by both methods suggests that the thin-layer wicking technique is useful to further characterize these or other systems, particularly if the examined material is sufficiently monodisperse.