The possible crossover effects of NaNO3 confined in porous media: From bulk to clusters

Abstract

Differential scanning calorimetry (DSC) and Raman spectra are reported for NaNO₃ bulk and for NaNO₃ confined in porous silica with pore radii, r_p=2.5, 5, 10, 20 nm. Raman spectra are also given for a 6 M solution of NaNO₃. The melting transition for the confined NaNO₃ exhibits a 1/r_p dependence where r_p is the pore radius for r_p≳5 nm. No melting transition is observed for NaNO₃ confined in 2.5 nm pores. Above this pore size, their appears to be a deviation in the melting transition dependence on r_p. The internal modes observed in the Raman spectra for the confined material are in agreement with those of the bulk solid except for a feature observed on the low frequency side of the ν₁ band. The external TO mode observed at 100 cm⁻¹ and the librational mode at 175 cm⁻¹ for NaNO₃ both decrease in intensity and broaden as r_p decreases and both bands disappear at r_p=2.5 nm. An additional peak at 70 cm⁻¹ not observed in the solution or bulk NaNO₃ spectra appears in the spectra of confined NaNO₃ and increases in intensity as r_p decreases. We assign this band to a new phase of NaNO₃ which is stabilized by the surface hydroxyl groups of the porous silica. For NaNO₃ confined in pores, r_p≤2.5 nm, we suggest that NaNO₃ exists as disordered aggregates.