Interactions between mica surfaces in dodecyl- and octylammonium chloride solutions

Abstract

The interactions between mica surfaces in octyl- and dodecylammonium chloride solutions were studied by using a surface force apparatus. As expected, the difference in chain length between dodecyl- and octylammonium ions strongly influences the adsorption and hence the interaction between mica surfaces immersed in such solutions. On the basis of the measured surface- and adhesion forces, a good picture of the relatively complex adsorption mechanism is obtained. In dodecylammonium chloride solution an electrically neutral surface is formed at about 10 −5 mole/liter, caused by the adsorption of DAH + and partly also H + cations. The density of the DAH + monolayer gradually increases at higher concentration. Simultaneously a charged layer of DAH + ions, oriented with the polar group toward the water phase, develops on top of the monolayer. This layer of charged surfactant molecules becomes pushed out from the surface at short separations (<10 nm) under the influence of an attractive hydrophobic force acting between the hydrophobic monolayers. A condensed bilayer-structured adsorbate is formed on top of the hydrophobic monolayer at higher concentrations. In contrast to this, athe octylammonium chloride case gives much weaker double-layer forces and the surface charge increases only slightly at higher concentrations. As in the DAH + case, the density of the monolayer increases at higher concentrations; however, the adhesion is lower. The hydrophobic force acting between mica surfaces neutralized by thin (0.5 nm) monolayers of dodecylammonium ions was much weaker than the corresponding force between mica surfaces coated with a monolayer of a double-chained (dimethyldioctadecylammonium) surfactant. This shows that the density of hydrocarbons on the surface is important for the strength of the hydrophobic force.