THE ROLE OF SURFACE CHARGE AND EXCHANGE CATION SPECIATION ON THE STRUCTURE OF INTERFACIAL WATER IN NONTRONITE SUSPENSIONS

Attenuated total reflection infrared spectroscopy (ATR-FTIR) was used to investigate the structure of water at the surface of suspensions of the nontronites, NAu-1 and NAu-2. Raw ATR spectra were converted to absorption index (k) spectra via the Kramers-Kronig transform to allow direct comparison of samples with different indices of refraction. Difference spectra produced from these k spectra allowed subtle shifts in the O-H stretching region to be discerned, thereby providing information about differences in the degree of hydrogen bonding. Suspensions of both NAu-1 and NAu-2 exchanged with either Na⁺ or K⁺ exhibit increased hydrogen-bonding at the mineral/water interface as compared to bulk water. NAu-1, which has greater total and tetrahedral charge than NAu-2, shows no change in water structure upon reduction of structural Fe or the addition of a small excess of electrolyte. These observations suggest that the ordering of interfacial water in NAu-1 suspensions is dominated by the highly charged mineral surface. Reduction of structural Fe in NAu-2 results in changes to the interfacial water structure that are dependent on the exchange cation species. In this case, reduction produces a significant increase in tetrahedral charge, which alters the interactions of the exchange cations with the surface.