Compression icing of room-temperature NaX solutions (X = F, Cl, Br, I)

Abstract

In situ Raman spectroscopy revealed that transiting H₂O/NaX (∼64) solutions into an ice VI phase and then into an ice VII phase at a temperature of 298 K requires excessive pressures with respect to pure water. The increase of the critical pressures varies with the solute type in the Hofmeister series order: X = I > Br > Cl > F ∼ 0. The results suggest that the solute hydration creates electric fields that lengthen and soften the O:H nonbond and meanwhile shorten and stiffen the H–O bond through O–O Coulomb repulsion. Compression, however, does the opposite to solute electrification upon the O:H–O bond relaxation. Therefore, compression of the aqueous solutions recovers the electrification-deformed O:H–O bond first and then proceeds to the phase transitions, which requires excessive energy for the same sequence of phase transitions. Ice exclusion of solute disperses the frequencies of characteristic phonons and the critical pressures with unlikely new bond formation.