Cationic surfactants adsorption on sand as a function of the surface charge of the substrate and hydrocarbon tail length of the surfactant

The adsorption of a surfactant molecule in the solid - liquid interface can be represented by a model that includes three fundamental zones, the electrostatic concentration zone, the electrostatic - hydrophobic concentration zone and the hydrophobic concentration zone. The first zone is ruled by the surfactant-substratum interaction (adsorption process begins, with a monolayer morphology), the third zone is influenced by the surfactant- surfactant interaction (superficial morphology is basically a totally formed bilayer or a tridimensional aggregate also known as hemimicele) and the second zone represents a transition between both zones (monolayer stops forming and bilayer or the corresponding aggregate begins to form). This research studies the effects of the hydrocarbon tail length of the surfactant and the pre-treatment of the adsorbent (in order to modify its surface charge) in the adsorption of cationic surfactants on sand. The phenomenon is studied creating adsorption isotherms using the depletion method, and the residual concentration of surfactant in the solution was determined by optical methods. Results show the influence of treatment of the substratum in the adsorption process and alsothat the relation between the area per molecule of surfactant in the gas - liquid interface and the area per molecule of surfactant in the solid - liquid interface not only decreases as the hydrocarbon tail length of the surfactant increases but it is a quantitative measure of the type of aggregate that is formed in the adsorbent surface.