Room-temperature NaI/HO compression icing: solute-solute interactions.

In situ Raman spectroscopy revealed that transiting the concentrated NaI/HO solutions to an ice VI phase and then into an ice VII phase at 298 K proceeds in a way different from that activated by the solute type. Unlike the solute type that raises both the critical pressures P and P, for the liquid-VI, the VI-VII transition simultaneously occurs in the Hofmeister series order: I > Br > Cl > F ∼ 0; concentration increase raises the P faster than the P that remains almost constant at higher NaI/HO molecular number ratios. Concentration increase moves the P along the liquid-VI phase boundary and it finally merges with P at the triple-phase junction featured at 350 K and 3.05 GPa. The highly-deformed H-O bond is less sensitive to the concentration because of the involvement of anion-anion repulsion that weakens the electric field in the hydration shells. Observations confirm that the salt solvation lengthens the O:H nonbond and softens its phonon but relaxes the H-O bond contrastingly. Compression, however, has the opposite effect from that of salt solvation. Therefore, compression recovers the polarization-deformed O:H-O bond first and then proceeds to the phase transitions. The anion-anion interaction discriminates the effect of NaI/HO concentration from that of the solute type at an identical concentration on the phase transitions.