ADHESION OF COLLOIDAL ZNS ON SILICON - EFFECTS OF IONIC-STRENGTH AND RADIO-FREQUENCY ELECTRIC-FIELD

The adhesion of zinc sulfide particles to silicon plates in the presen ce of aqueous solutions of several electrolytes, NaCl, MgCl2, and LaCl 3, has been studied. The results are discussed in the light of the ext ended DLVO approach, in terms of energy-balance plots versus the dista nce between the interacting surfaces. Moreover, we were also intereste d in finding out whether high-frequency (43 MHz) electric field pretre atment of the surface influenced the adhesion process. The adhesion of ZnS was quite dependent on the NaCl concentration, being higher as th e concentration of NaCl was increased. The effect of applying a radio frequency (RF) electric field during the adhesion process was only not iceable at the lowest NaCl concentration studied (0.1 mM). RF pretreat ment in such conditions reduced the adhesion of ZnS to Si. The type of counterion in the solution determined the surface properties of both the adherent and the substrate. The results using 1 mM NaCl, MgCl2, an d LaCl3 aqueous solutions showed that the maximum adhesion was obtaine d in the presence of Mg2+ ions, while the minimum adhesion was obtaine d in the presence of La3+. When these two electrolytes were used, appl ication of an RF electric field inhibited the adhesion. However, no ch ange was observed when the electric field was applied in the presence of univalent electrolyte NaCl at a concentration of 1 mM. The total fr ee energy of interaction between the colloidal particles and the subst rate, as well as its components, electrodynamic Lifshitz-van der Waals , electrostatic, and acid-base (Lewis), gives, at least for moderate e lectrolyte concentrations, a qualitative explanation for the adhesion of ZnS to Si plates.