Theoretical modeling of cationic surfactants aggregation at the silica/aqueous solution interface: Effects of pH and ionic strength

Abstract

A theory of ionic surfactant aggregation on oppositely charged surfaces is presented. In the proposed model the adsorbed phase is considered as a mixture of singly dispersed surfactant molecules, monolayered and bilayered aggregates of various sizes and the ions of simple electrolyte added to the aqueous solution. The electrostatic interactions between the adsorbed ions (surfactant ions and simple ions) and the charged surface are described by using the triple layer model (TLM) formalism. The calculated surfactant adsorption isotherms, surface charge isotherms, and electrophoretic mobilities of silica particles are compared with the experimental results obtained for two cationic surfactants (dodecylpyridinium chloride and cetylpyridinium chloride) adsorbed on Aerosil OX50 at two different values of pH. On the basis of the theoretical results the evolution of the adsorbed phase structure and the charge of silica particles with an increasing surface coverage is discussed.