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 NaNO(3) droplets (5-60 μm) on a polytetrafluoroethylene (PTFE) substrate with molar water-to-solute ratios (WSRs) of 0.8-28 and bulk NaNO(3) solutions with WSRs of 35-200 by combining micro-Raman spectroscopy and component band analysis. Concentrations of the NaNO(3) 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 (ν(1)-NO(3)(-)) 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 ν(1)-NO(3)(-) 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 NaNO(3) 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 NO(3)(-) 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 NaNO(3) 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 NaNO(3) 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.