STABILITY OF DILUTE COLLOIDAL SILICA SUSPENSIONS IN THE VICINITY OF THE BINODAL CURVE OF THE SYSTEM 2-BUTOXYETHANOL WATER/

In the temperature-composition diagram of the system 2-butoxyethanol ( abbreviated C4E1)/water, SiO2 [mass fraction of SiO2, gamma(SiO2) appr oximate to 0.01; volume fraction phi(SiO2) approximate to 10(-3)] ther e exists a flocculation temperature-composition curve running below th e binodal curve for x > x(c) (x, mole fraction of C4E1;x(c) = 0.0598). The concentration of the colloidal particles is low enough to conside r them an ''impurity.'' In the region of the phase diagram bound by th e binodal and the flocculation curve the suspension of the colloidal S iO2 particles is unstable and the particles flocculate reversibly. The difference between the temperature of phase separation and the floccu lation temperature increases with increasing values of x - x(c) up to a maximum value of x (x(max)). For x > x(max) the suspensions are unst able at all temperatures studied. For x < x(c) the suspension of the S iO2 particles is stable up to the temperature of phase separation of t he C4E1/water mixture. The flocculation curve is assumed to reflect th e influence of local concentration fluctuations with long range correl ations on the stability of the SiO2 suspensions. For x > x(c) these co ncentration fluctuations are water rich and interact with the hydrophi lic surface of the colloidal SiO2 particles by forming an adsorption l ayer. This layer is assumed to modify the interparticle potential ener gy-distance curve and to trigger flocculation. For x < x(c) the concen tration fluctuations are C4E1 rich and no adsorption layer is formed a t the hydrophilic surface of the colloidal particles. (C) 1997 Academi c Press.