Use of ATR-FTIR with D/H Substitution for Assessing Water Uptake in Alkali Perchlorate Matrices

Abstract

Use of FTIR spectroscopy together with isotopic exchange offers a powerful approach towards understanding cation dependence of water uptake in alkali perchlorate salts at room temperature. ATR-FTIR data reveal distinct vibrational features of H2O, HDO, and D2O isotopologues entrapped in LiClO4 and NaClO4 matrices, as opposed to the case of KClO4, where no discernible IR-spectral evidence is provided for the presence of water. Moreover, precipitates of the co-dissolved aqueous mixtures of LiClO4 + NaClO4 and LiClO4 + KClO4 always end up having water trapped within LiClO4 matrix, evidenced and validated by the isotopically substituted IR spectral features, even though presence of the other salts (i.e. NaClO4, in the former mixture and KClO4 in the latter one) are verified by use of both X-ray Diffraction and X-ray Photoelectron Spectroscopic data. A ternary mixture has also been investigated which supports the surprisingly strong cation dependence of the water uptake of these alkali perchlorate salts. Our results indicate that the order of water uptake is LiClO4 > NaClO4 >> KClO4 crystals, which can qualitatively be correlated with charge density of their corresponding cations.