Mechanism of adsorption of long-chain alkylamines on silicates. A spectroscopic study. 1. Quartz

The mechanism of adsorption of long-chain alkylamines at pH 6-7 onto quartz was studied using FTIR and XPS spectroscopy. The spectroscopic data were c orrelated with zeta potential and Hallimond flotation results. For the firs t time it was shown that (1) amine cation in the first monolayer is H-bonde d with surface silanol group and this H-bond becomes stronger after the bre ak in the adsorption characteristics (isotherm, zeta potential, floatabilit y); (2) at the break the origin of the adsorbed amine species changes quali tatively, and along with alkylammonium ion attached to deprotonated silanol group, molecular amine appears at the surface and, as a result, monolayer thick patches of well-oriented and densely packed adsorbed amine species fo rm rendering the surface highly hydrophobic; and (3) at higher amine concen tration, bulk precipitation of molecular amine takes place. The counterion was found to influence both these steps. A model of successive two-dimensio nal and three-dimensional precipitation was suggested to explain amine adso rption on a silicate surface.