THE POSSIBLE CROSSOVER EFFECTS OF NANO3 CONFINED IN POROUS-MEDIA - FROM BULK TO CLUSTERS

Differential scanning calorimetry (DSC) and Raman spectra are reported for NaNO3 bulk and for NaNO3 confined in porous silica with pore radi i, r(p)=2.5, 5, 10, 20 nm. Raman spectra are also;given for a 6 M solu tion 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 me lting transition is observed for NaNO3 confined in 2.5 nm pores. Above this pore size, their appears to be a deviation in the melting, trans ition dependence on r(p). The internal modes observed in the Raman spe ctra 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 upsilon(1) band. The external TO mode observed at 100 cm(-1) and the l ibrational 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 NaN O3 spectra appears in the spectra of confined NaNO3 and increases in i ntensity as r(p) decreases. We assign this band to a new phase of NaNO 3 which is stabilized by the surface hydroxyl groups of the porous sil ica. For NaNO3 confined in pores, r(p) less than or equal to 2.5 nm, w e suggest that NaNO3 exists as disordered aggregates.