The Effect of Grafting Density on Binding Isotherms of Cationic Surfactants to a Polyacrylic Acid Brush

Abstract

Adsorption (binding) isotherms of three cationic surfactants (hexadecyl trimethylammonium bromide, dodecyl trimethylammonium bromide, and decyl trimethylammonium bromide) bound to poly(acrylic acid) brushes were determined by means of optical reflectometry. The adsorption occurs reversibly. The shape of the isotherms and the maximum degree of binding depends strongly on the grafting density of the brush. Dense brushes exchanged at most about 40% of their counterions for surfactant ions and had isotherms that could be fitted to the Langmuir equation. For more dilute brushes the isotherms had a quite different shape: the quantity of surfactant could increase up to complete exchange in a second step occurring at higher concentrations. It is shown that these differences are due to the bulkiness of the surfactant ions; the volume required for full loading is so large that it forces the polymer chains to stretch too much. A lamellar structure of the complex occurring in the fully loaded brush is proposed.