Excess adsorption on hydrophobic and hydrophilic solid-liquid interfaces. Positive excess adsorption of non-ionic surfactant. Part 1

Abstract

The extent of adsorption (Gamma(1)(2)) of non-ionic surfactant Tween-20 onto hydrophilic alumina and silica and hydrophobic charcoal surfaces has been measured as a function of surfactant concentration (X-2) in mole fraction unit at different temperatures. For each isotherm, Gamma(1)(2) at low range of X-2 increases with increase in surfactant concentration. But, it reaches maximum (Gamma(m)(2)) when X-2 reaches a critical value X-2(m). With increase in temperature, Gamma(m)(2) increases for charcoal and silica surfaces while it decreases for alumina surface. At 40 degrees C, Gamma(1)(2) in charcoal Surface be ins to decrease when X-2 exceeds X-2(m). At 30 degrees C for charcoal surface, Gamma(1)(2) increases sharply with increase in X-2 without reaching any limiting value due to the multilayer adsorption of Tween-20. Similar formation of multilayers is also observed at alumina-water interface when the Surface coverage is significantly low. Formation of multilayers of Tween-20 molecules on the surface of silica is not observed even when X-2 is relatively high. Using the integrated form of the Gibbs adsorption equation, the standard free energy change Delta G(0) for the adsorption of non-ionic surfactant per square meter of powdered surface due to the change of the bulk activity of Tween-20 from zero to unity has been evaluated and critically examined.