Molecular picture of hydroxide anion hydration in aqueous solutions studied by FT-IR ATR spectroscopy

Hydration of hydroxide anion has been studied in aqueous solutions of alkali metal hydroxides (LiOH, NaOH, KOH) by means of attenuated total reflectance (ATR) FT-IR spectroscopy. H2O vibrational spectra provide valuable information about the structural state of water molecules in hydration shells, resulting from oscillator couplings in the system. Spectral data have been analysed in a way that led to removal of the contribution of bulk water, in order to separate the spectra of solute-affected water only. The analysis has been focused on the infinite dilution limit behaviour of the absorption spectrum. Changes induced in the affected spectra by temperature have been studied for NaOH solutions at 15–35 °C. The stretching vibration fundamental has been found to be primarily affected by counter-cation; the only indication of OH− hydration in this part of the spectrum has been a red-shift of all four components of this band. However, affected H2O spectra in alkali solutions show the presence of “absorption continua”, or very wide absorption bands in the ranges, in which bulk water shows negligible own absorption. They could be adequately resolved into analytical components. “Continuum” type bands have been correlated with stretching vibration component bands influenced by the cation, as indicated by 2D IR correlation spectra. Their intensity has increased significantly with temperature.