Electric and adsorption properties of pharmaceutical polymers. Part I: Electrokinetics of Aquacoat

The characterization of the electrical surface properties of Aquacoat, a polymer latex of great interest in pharmaceutical sciences, is described. The technique used is electrophoresis. Analysis was carried out of the effect of pH, electrolyte and surfactant concentration on the electrophoretic mobility of the latex particles. Increasing the pH of the dispersion medium provokes a monotonous increase in the value of the negative mobility. The electrolytes LiCl, KCl and NaCl give rise to larger mobilities when their concentration in solution is increased up to ca. 10^–3 M, and a similar behavior is found in the presence of Na₂SO₄. The effect of raising the concentration of CaCl₂ is to decrease the absolute value of the mobility as a consequence of double layer compression. Sodium dodecyl sulphate seems to adsorb on the particle surface increasing its negative charge, but when its concentration is close to 10^–3 M saturation of the surface appears to take place, and an approximately constant mobility is suggested by data, whatever the pH of the medium. Finally, the mobility variations with LaCl₃ concentration indicate adsorption of the La³⁺ cation when it is hydrolyzed (pH5), whereas non-hydrolyzed lanthanum has little effect on the particle charge.