AN EXPLANATION FOR THE UNEXPECTED INTERACTIONS OF SILICA NANOPARTICLES USING THE SOFT PARTICLE MODEL

Abstract

Under geothermal conditions, silica scaling is essentially the deposition of colloidal silica: when cooled geothermal brine is reinjected, monomeric (dissolved) silica will polymerise and form silica nanoparticles. These will transport and deposit on the mineral surfaces, and may eventually block the fluid pathways in the aquifer.
Silica deposition in geothermal reinjection may be treated as a two-step process: the first step is the initial attachment between silica particles and “clean” mineral surfaces on which no (monomeric or polymerised) silica has already deposited, and the other is the later attachment between suspended and previously deposited particles. If it is assumed that the initial attachment is instantaneous, and the deposition rate is controlled by the later attachment, which is essentially an aggregation problem.
It has been shown that DLVO theory can predict the stability of colloids for certain chemical compositions with reasonable accuracy. However, amorphous silica particle is an exception. To understand this, a hypothesis has been proposed (De Gennes 1987, Healy 1994), assuming that the unique behaviours are induced by the hairy layer (also called the gel layer) made of polymer chains of silicone anchored on the surface of the colloidal silica. Ohshima (2015) proposed a general model to quantify the electrostatic interactions caused by ion-penetrable surface layers of polyelectrolytes. The present work applies Ohshima’s model to explain the experimental observations of Škvarla (2013). The resulting model provides an explanation of the non-DLVO behaviour of silica particles.