Adhesion of colloidal ZnS on silicon. Effects of ionic strength and radio frequency electric field

Abstract

The adhesion of zinc sulfide particles to silicon plates in the presence of aqueous solutions of several electrolytes, NaCl, MgCl₂, and LaCl₃, has been studied. The results are discussed in the light of the extended DLVO approach, in terms of energy-balance plots versus the distance between the interacting surfaces. Moreover, we were also interested in finding out whether high-frequency (43 MHz) electric field pretreatment of the surface influenced the adhesion process. The adhesion of ZnS was quite dependent on the NaCl concentration, being higher as the concentration of NaCl was increased. The effect of applying a radio frequency (RF) electric field during the adhesion process was only noticeable at the lowest NaCl concentration studied (0.1 mM). RF pretreatment 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, and LaCl₃ aqueous solutions showed that the maximum adhesion was obtained in the presence of Mg²⁺ ions, while the minimum adhesion was obtained in the presence of La³⁺. When these two electrolytes were used, application of an RF electric field inhibited the adhesion. However, no change was observed when the electric field was applied in the presence of univalent electrolyte NaCl at a concentration of 1 mM. The total free energy of interaction between the colloidal particles and the substrate, as well as its components, electrodynamic Lifshitz-van der Waals, electrostatic, and acid-base (Lewis), gives, at least for moderate electrolyte concentrations, a qualitative explanation for the adhesion of ZnS to Si plates.

Keywords:

• Adhesion of ZnS on silicon
• extended DLVO theory
• zeta potential
• surface free energy