The adsorption and configuration of octyl hydroxamic acid on pyrochlore and calcite

Abstract

XPS, adsorption density and ToF-SIMS measurements were performed on pyrochlore and calcite treated with octyl hydroxamic acid (OHA). XPS results indicated a slight shift of the binding energies of the Ca 2p electrons (0.19eV) on calcite after OHA treatment but the N1s binding energy spectrum of OHA did not change, showing a physical adsorption mechanism. On OHA-treated pyrochlore, the binding energies of both Nb 3d and Ca 2p electrons shifted (0.24 and 0.35eV, respectively), and the N1s electron binding energy peak of OHA split from 400.3eV to 400.7 and 398.5eV, representing the neutral RNHOH and the ionized RNHO− groups. The ratio of the integrated areas of the 400.7eV peak to the 398.5eV peak was 3.8 to 1. Therefore OHA possibly adsorbed on pyrochlore by both physisorption and chemisorption, with the number of OHA molecules involved in the adsorption at 3.8 to 1. The adsorption isotherm of OHA on pyrochlore showed two plateaus, with the first plateau corresponding to a vertically oriented monolayer of OHA. The second plateau was 4 times higher than the first plateau. Therefore, the adsorption isotherm was consistent with XPS results, in that the vertically oriented monolayer (first plateau) formed the chemisorbed layer, and the second plateau contained mostly physisorbed OHA. The adsorption isotherm of OHA on calcite showed a lower plateau which corresponded to horizontally oriented OHA. ToF-SIMS positive ion images on OHA treated mixtures of pyrochlore and calcite confirmed that OHA preferentially adsorbed on pyrochlore.