PNNL

Abstract

Ion association ratios as a function of concentration were estimated in single NaNO3 droplets (5-60 µm) on a polytetrafluoroethylene (PTFE) substrate with molar water-to-solute ratios (WSRs) of 0.8-28 and bulk NaNO3 solutions with WSRs of 35-200 by combining micro-Raman spectroscopy and component band analysis. Concentrations of the NaNO3 droplets were accurately controlled by adjusting relative humidity (RH) in a sample chamber. As the WSRs decreased from 200 to 0.8, symmetric stretching band (?1-NO3 -) was observed to shift from 1047 to 1058 cm-1 along with a change in full width at half-maximum (fwhm) from ~10 to ~16 cm-1, indicative of formation of ion pairs with different structures. Through the component band analysis of the ?1-NO3 - band, five bands centered at 1040.0, 1042.9, 1048.5, 1053.5, and 1057.0 cm-1 were identified and assigned to coupled wagging modes of water molecules hydrated with nitrate ions, free hydrated nitrate anions, solvent-shared ion pairs (SIPs), contact ion pairs (CIPs), and the complex ion aggregates (CIAs), respectively. There were large amounts of SIPs and CIPs in dilute NaNO3 solution even at an extremely low concentration (WSR ~ 200), and each accounted for 50% and 20% of total nitrate species, respectively. This finding is in good agreement with earlier reported observations. In the dilute solutions (45