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

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 (0.19 eV) on calcite after OHA treatment but the N 1s 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 0.35 eV, respectively), and the N 1s electron binding energy peak of OHA split from 400.3 eV to 400.7 and 398.5 eV, representing the neutral RNHOH and the ionized RNHO− groups. The ratio of the integrated areas of the 400.7 eV peak to the 398.5 eV 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.

At low concentrations, octyl hydroxamic acid formed a vertically oriented monolayer on pyrochlore surface, but a horizontally oriented monolayer on calcite surface.Figure optionsDownload as PowerPoint slideHighlights
► Hydroxamic acid chemisorbed on pyrochlore, but physically adsorbed on calcite.
► At low concentration, hydroxamic acid formed a vertically oriented monolayer on pyrochlore.
► At low concentration, hydroxamic acid formed a horizontally oriented monolayer on calcite.