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, rp=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/rp dependence where rp is the pore radius for rp≳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 rp. 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 rp decreases and both bands disappear at rp=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 rp 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, rp≤2.5 nm, we suggest that NaNO3 exists as disordered aggregates.

Calorimetric investigations of NaNO3 and NaNO2 embedded into porous glasses

“…Results presented above remain in a good agreement with those obtained for glasses with different pore values (2.5-10 nm), presented in work [8].…”

“…A decreasing of the melting temperature can be accomplished by reducing of the pore sizes. Similar phenomena observed in NaNO 3 crystals were explained [8] as a new phase formation with physical properties different from those observed for the bulk crystals which was a result of an interaction between the material embedded into pores and the pores surface themselves. Two anomalies of the specific heat clearly seen for PG160 porous glasses show a broadening of the stability modulated phase temperature range.…”

“…4. The specific heat anomaly connected with the structural phase transition for NaNO 3 was observed at about 548 K. It is worth noticing that at work [8] no anomaly of the specific heat connected with these phase transitions was observed due to the calorimeter low sensitivity and resolution. A decreasing of the pore dimensions caused a diffusing of the specific heat anomaly and a very small shift of C p (T) maximum to lower temperatures.…”

“…[8]. A disappearance of the structural phase transition and a drop of the melting temperature with the decreasing pore sizes were observed in the calorimetric investigations.…”

Molecular Dynamics of Tert-butyl Chloride Confined to CPG (7.4, 15.6 nm)

“…A crystal growing inside a pore, because of relative strong reduced size, experiences larger compressive stress, so the structure of the solid TBC is in the form of a disordered aggregate rather than in its conventional solid structure, and solid-solid transitions do not occur at all [23,28,29]. Probably the critical size of the crystal is required.…”