Negative adsorption of inorganic salts, sucrose and urea at solid-liquid interfaces

Abstract Adsorption of water vapor on the surfaces of alumina and silica powders was studied by isopiestic vapor pressure technique. The curves obtained from the plot of moles ( n 1 ) of water adsorbed per gram of oxide powder against water activity ( p/p 0 ) resemble the type II BET isotherm. Adsorption of water vapor on the oxide powders exhibits hysteresis at lower ranges of p/p 0 . The specific surface area ( A w ) calculated from the BET plot is ten time larger than the surface area ( A f ) per gram calculated by the palmitic acid adsorption method. Hydration of silica and alumina was also studied in the presence of electrolytes, sucrose and urea. Water remains bound as positive excess for relatively high concentrations of a solute in solution. In the lower region of concentration, the solute is bound as positive excess. The azeotropic point where the surface excess of water becomes zero for the different solutes studied occurs when the mole fraction of the solvent remains in the range 0.95 to unity. The absolute binding of water and solute, Δ n 1 and Δ n 2 , respectively, per gram of the powder were evaluated and the dependence of these parameters on the nature of the electrolytes, urea and sucrose was critically analysed. Using the values of Δ n 1 , and Δ n 2 , the theoretical surface area ( A m ) of the powdered particles was calculated assuming the existence of a monolayer. A m / A w thus represents approximately the number of layers of bound components at the solid-liquid interfaces. This ratio for alumina remains between 1 to 3 and for silica it is 1 to 5 depending on the nature of the solute present. The number of bound water layers calculated from the ratio A m / A f are on the other hand large in magnitude.