Effect of Temperature and Added Salts on the Adsorption of Selected Polyelectrolyte at the Surface of Water

Abstract

Abstract

Surface tensions of aqueous solutions of oppositely charged sodium poly(styrene sulfonate)/benzyldimethylhexadecyl ammonium chloride and sodium poly(styrene sulfonate)/polyallylamine hydrochloride have been measured by the Wilhelmy plate method as a function of temperature and addition of NaCl and CaCl2 salts. The surface activity of the pure polyelectrolytes is very low. Upon mixing the polyelectrolyte with the oppositely charged surfactant, benzyldimethylhexadecyl ammonium chloride, complexation takes place which showed significant surface activity. The same trend was found when the polyelectrolyte was mixed with the oppositely charged polyelectrolyte polyallylamine hydrochloride. The surface tension of the polyelectrolyte/surfactant mixture is lower than that of the pure surfactant but only at low concentrations of the polyelectrolyte ( 0.1 g /100 ml). At relatively higher concentrations it is higher due to diminishing of the complexation process and thus the surface activity. It is found that the complexation process between either the polyelectrolyte/surfactant or the polyanion/polycation is enhanced by increasing temperature or addition of salts. The surface excess entropy was calculated from the temperature dependence of the surface tension to get more insight into the ordering at the surface. A downward peak was observed in the behavior of the surface tension as a function of the poly(styrene sulfonate) for the case of the polyanion/polycation mixture at equal amounts (0.1 g/100ml) of the two polyelectrolytes. This peak may indicate that the complexation process is at its maximum and therefore the surface activity is at its maximum value. Finally, it has to be mentioned that studies on the surface activity of the complex formed from the polyanion/polycation mixture are lacking and thus a major part of this study is absolutely needed.