Heat Capacity and Volume of Colloidal Silica in the Presence of Electrolytes and Cationic Surfactants

An exploratory study was made of the densities and heat capacities of colloidal dispersions and suspensions of fine silica particles in water (Ludox HS40) using flow instruments. The flocculation induced by inorganic electrolytes (NaCl and CaCl 2) is accompanied by an increase in the apparent specific volume of silica, and this change in volume varies with the flocculation time. The effect on the apparent specific heat capacity is less clear. Specific adsorption of a small quantity of cationic surfactants, octylammonium bromide (OABr) and dodecyltrimethylammonium bromide (DTAB), also causes flocculation and is accompanied by an increase in volume and a decrease in heat capacity. A larger adsorption of surfactant on the silica restabilizes the dispersion; the apparent specific quantities of the restabilized silica are then of the same magnitude as those of pure silica in water. The adsorption isotherms of the surfactants were determined by ultrafiltration. The apparent molar volumes and heat capacities of adsorbed surfactants, V a s and C a p,s, can be evaluated only in the restabilized zone. The sign and magnitude of these values are consistent with the formation of hemimicelles or admicelles on the surface of SiO 2. The advantages and limitations of these techniques are discussed.