Observation on the ion association equilibria in NaNO₃ droplets using micro-Raman spectroscopy

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 (ν₁-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 ν₁-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 < WSR < 200), there is the same amount of free hydrated ions transformed into SIPs as that of SIPs transformed into CIPs. As a result, the overall amount of SIPs remained unchanged over the concentration range. With a decrease in WSR from 45 to 0.8, the amounts of SIPs and free solvated NO₃^- ions kept decreasing, whereas the amount of CIPs rose to a maximum at WSR = 7 and then fell with a further decreasing WSR. Formation of CIAs started at WSR ∼ 45, and its amount continuously increased as the WSR is further reduced to 0.8. The effect of temperature on ion association structure in the NaNO₃ droplets was also studied in the present work. An increase in temperature promoted formation of both CIPs and CIAs, and the latter was more pronounced. At 80 °C, the most concentrated NaNO₃ droplets had a WSR approximately equal to 0.12 and were in amorphous state with cations and anions aggregated in a complicated manner, highly similar to ionic liquid.