IR Spectroscopy of aqueous alkali halides. Factor analysis

The infrared spectra of ten salts in aqueous solutions were obtained in their solubility range using attenuated total reflection (ATR). The salts are: LiCl, NaCl, KCl, CsCl, MgCl2, NaBr, KBr, NaI, KI, and CsI. The salts, which are completely ionized in water, do not absorb in the IR. Only their interactions with water are observed and analyzed. Factor analysis (FA) applied to the spectra showed that two principal species are present in the solutions: pure water and salt-solvated water. The modifications of the water spectrum are dependent on the nature of the salts, their concentrations and are mainly of first-order. Second-order effects were observed but, being weak, were not investigated. The modifications of the IR-ATR spectrum from pure water to a salt solution are proportional to the modifications of the imaginary part of the refractive index spectrum (k(v)). This indicates that the anomalous dispersion effects do not interfere with the chemical analysis of the IR-ATR spectra. In every salt studied, the two main species present in the solutions remained stable throughout the salt solubility ranges. Comparison between the IR spectra of the different salts solutions indicates that both anions and cations are in strong interaction with water molecules and are solvated together in stable clusters.