Use of ATR-FTIR with D/H substitution for assessing water uptake in alkali perchlorate matrices

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 the LiClO₄ matrix, evidenced and validated by the isotopically substituted IR spectral features, even though the presence of the other salts (i.e. NaClO₄ in the former mixture and KClO₄ in the latter one) is verified by the 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.