Adsorption of an ethoxylated amine surfactant on mica and its effect on the surface forces

• The adsorption of tetraoxyethylene dodecylamine (C12NHE4) on mica and its effect on the interparticle forces between negatively charged muscovite mica surfaces has been studied employing the surface force technique. The pH of the solution has a decisive influence on the amount adsorbed, the structure of the adsorbed layer, and the interparticle forces. The main parameters which determine the structure of the adsorbed C12NHE4 -- layer are the area per lattice charge compared to the size of the molecule, the affinity between the polar group and the surface, and the affinity between the polar group and water. Below a pH of about 8 the positively charged ammonium group binds electrostatically and a surfactant monolayer is formed on each surface. At a surfactant concentration of 10-4 M, (a factor of 4 below the cmc) the surfaces are nearly uncharged. The attractive force between the monolayer coated surfaces is of a similar magnitude as the expected van der Waals force. At a pH value of 9-10 the majority of the surfactants are uncharged. Consequently, the head-group repulsion decreases, and, at a surfactant concentration of 10-4 M, an uncharged bilayer builds up on each surface. At short separations a steric force/hydration force arises from compression and dehydration of the polar groups that are directed towards the solution. At even higher pH values the affinity between the polar group and the surface decreases resulting in a disordered adsorbed layer. A repulsive electrostatic double-layer force dominates at large distances, and a steric force/hydration force is present at short surface separations. As the pH is lowered again the outer layer desorbs, which results in a reappearance of a strong attractive force at short separations.

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