INCREASING ION-PAIRING AND AGGREGATION IN SUPERCOOLED AND GLASSY DILUTE AQUEOUS-ELECTROLYTE SOLUTION AS SEEN BY FTIR SPECTROSCOPY OF ALKALI-METAL THIOCYANATES

FTIR spectra of aqueous solutions of lithium (1 M), sodium (0.04, 0.2, and 1 M), and potassium (1 M) thiocyanate in their glassy states at 83 K, and of 1 M sodium thiocyanate in supercooled solution from 298 to 230 K, are reported from 1950 to 2200 cm-1. In the CN stretching band region of the glassy solution derivative spectra, up to six discrete bands are observed. The band at almost-equal-to 2050 cm-1 is assigned to mainly "free" SCN-. Bands at 2071, 2068, and 2067 cm-1 are assigned to the N-bonded contact ion pairs Li+NCS-, Na+NCS-, and K+NCS-. For the 1 M glassy solutions, the relative intensity of the curve-fitted component bands at almost-equal-to 2070 cm-1 is K+ > Na+ > Li+, but for the band at almost-equal-to 2050 cm-1 it is the opposite. Additional bands at almost-equal-to 2115, almost-equal-to 2089, almost-equal-to 2030, and almost-equal-to 2015 cm-1 may originate from (neutral or charged) ion aggregates. Evidence for ion pairing or aggregation is seen already in supercooled 1 M NaSCN solution by development of a second peak at almost-equal-to 2084 cm-1 at high frequency of the band due to "free" SCN-. We surmise that water's density maximum and the anomalies of supercooled water and aqueous solutions are at the root of increasing ion pairing and/or aggregation in the supercooled and glassy states.