Stability of Dilute Colloidal Silica Suspensions in the Vicinity of the Binodal Curve of the System 2-Butoxyethanol/Water

Abstract

In the temperature–composition diagram of the system 2-butoxyethanol (abbreviated C4E1)/water, SiO2[mass fraction of SiO2,y(SiO2) ≈ 0.01; volume fraction φ(SiO2) ≈ 10−3] there exists a flocculation temperature–composition curve running below the binodal curve forx > xc(x, mole fraction of C4E1;xc= 0.0598). The concentration of the colloidal particles is low enough to consider them an “impurity.” In the region of the phase diagram bound by the binodal and the flocculation curve the suspension of the colloidal SiO2particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase separation and the flocculation temperature increases with increasing values ofx − xcup to a maximum value ofx(xmax). Forx > xmaxthe suspensions are unstable at all temperatures studied. Forx < xcthe suspension of the SiO2particles is stable up to the temperature of phase separation of the C4E1/water mixture. The flocculation curve is assumed to reflect the influence of local concentration fluctuations with long range correlations on the stability of the SiO2suspensions. Forx > xcthese concentration fluctuations are water rich and interact with the hydrophilic surface of the colloidal SiO2particles by forming an adsorption layer. This layer is assumed to modify the interparticle potential energy–distance curve and to trigger flocculation. Forx < xcthe concentration fluctuations are C4E1rich and no adsorption layer is formed at the hydrophilic surface of the colloidal particles.