XPS study of the hematite-aqueous solution interface

The interaction between the surface of hematite colloidal platelets and Na(+) and Cl(-) ions was investigated by XPS using the cryogenic technique. Pastes in ionic strengths of 0 and 10 mm NaCl contained about 10 at. % water and water/NaCl atomic ratios of 3 to 6. These results fall within the range of values obtained for a variety of minerals studied with this technique. Pastes in 100 mm NaCl background electrolyte, however, contained an unusually larger water content of 25 at. %, yielding a Na(+): Cl(-): H(2)O ratio of 1 :1 :2. This result is in strong contrast with other minerals at the same ionic strength, which typically reveal about 10 at. % water. Substituting Na(+) for Cs(+) in the hematite paste with 100 mm CsCl yielded, on the other hand, the same amount of water as in the pastes with 0 and 10 mm NaCl, and underpinned the role of Na+ ions in the large water content of the hematite paste. As surface concentrations of Na(+) and Cl(-) exceeded those of hematite surface structural hydroxyl groups and Na 1s and Cl 2p spectra exhibited energy loss features, the electrolyte ions are proposed to be distributed in a three-dimensional array in the fast-frozen paste, possibly in a hydrohalite-like (NaCl center dot 2H(2)O) phase. In addition, because the fast-frozen solution of 100 mm NaCl yields a water/NaCl ratio of about 70, hematite is proposed to play an important role in the stabilization of this three-dimensional distribution of Na(+) and Cl(-) ions. The role of the neutrally charged {1001} plane, a predominant feature in the hematite particles of this study, is notably discussed in the light of recent molecular models showing that this plane can stabilize several layers of hydrated ions up to 15 angstrom from the surface. Copyright (C) 2008 John Wiley & Sons, Ltd.