I.R. studies of the librational band of aqueous electrolyte solutions

Abstract

In this research the l.R. Ubrational bands of water and aqueous solutions were investigated, quantitatively. Studies included the alkali metal and ammonium nitrate, perch 1 orate and halide, alkaline earth nitrate, transition metal nitrate and tetraalkylammonium nitrate solutions.A thin film cell ( ~5 µ) was used to obtain quantitative l.R. spectra of aqueous solutions. The effects of reflection loss, due to interference in the thin film, were investigated and corrections to the absorbance were made to account for this. This required the determination of refractive indices in the l.R. The set of librational bands for electrolyte solutions of increasing concentration possess an isosbestic point for almost every electrolyte studied. The alkali metal and ammonium nitrates, perchlorates, and halides, excepting NH4F, shifted the water librational band to lower frequency and increase the integrated Intensity. An approximate linear relationship is observed between the Ubrational frequency and the intensity for equimolar halide, nitrate and perchlorate solutions containing a common alkali metal cation. The frequency, half-width and Intensity of the observed Ubrational bands are found to be dependent on the nature of both the cation and the anion. Large weakly polarising cations and large weakly H —bonding anions lead to Ubrational bands of lowest frequency. The largest intensity Increases are caused by small strongly polarising cations and large anions.These observations are discussed in terms of direct water — Ion Interactions and not in terms of structure making or breaking effects of the ion. It Is proposed that an equilibrium exists between two spectroscoplcally distinct types of water; (a) water involved in the primary hydration of ion —pairs, termed 'A' water and(b) all remaining water, which has librational properties identical to pure water, termed 'B' water. The librational absorption of 'A' water increases and 'B' water decreases as the electrolyte concentration rises, until the former dominates the spectrum.The above proposal enabled 'A' water librational components to be separated from the total librational band of each electrolyte solution. The peak frequency, half-width and shape of these bands remains constant and the intensity increases linearly as the electrolyte concentration increases, suggesting that they arise from discrete stable absorbing species. Estimates of the number of water molecules involved in each hydrated Ion — pair are made and the values range from 4 to 6. The trends in the cation and an ion effects on the 'A' band frequencies, half-widths and intensities are the same as for the overall band. The librational frequency is thought to be related to the strength of ion—water interactions. The librational Intensity Is considered to be influenced both by the polarising effect of the neighbouring Ion on the water molecule and the amplitude of the water molecule's librational motion. The librational bands of weakly coordinating divalent and trivalent cations, Ca2+, Sr2+ and La3+ are little different from those of the monavalent Ions. In spite of the Increased charge on the Ion only the primary hydration appears 1ibratlonally distinct from the bulk water. The divalent transition metal nitrate solution llbrational bands are very similar in frequency and shape to the pure water librational band apart from a shoulder at 600 cm-1 which is assigned to the M - OH2 coordinate bond stretching vibration. The trivalent, aluminium and chromium nitrate solution llbrational bands are lower in intensity than the pure water bands. The weak peak at 600 cm-1 is assigned to the M - OH2 stretching vibration. The changes in the water llbrational band produced by these coordinating divalent and trivalent ions are discussed in terms of an M(H20)6 type complex ion of Th symmetry. The tetraalkylammonium nitrates produce a decrease in the frequency and an increase in the intensity of the librational bands considerably greater than equimolar solutions of the monatomic cations. This has been attributed to the greater extent of hydration possible about these large ions. A linear relationship is obtained between the intensity and ionic radius which supports this view.