Divergent freezing behavior in nanoconfinement: organic vs. salt aqueous mixtures

Aqueous mixtures confined in mesoporous materials exhibit solid-liquid phase behavior that departs from predictions based on bulk mixture thermodynamics and confinement effects. We investigated confined organic and salt-based aqueous mixtures to identify the molecular origins of these deviations. Organics induce weaker melting-point and enthalpy depressions, whereas salt solutions show stronger effects than predicted. We propose that this divergence originates from solute adsorption or depletion within the non-freezable δ layer coating the pore walls-organic molecules adsorbing onto silica, while ions are excluded-thereby altering local composition and water activity in the pore center.

We examined binary aqueous mixtures of dimethyl sulfoxide, formamide, and sodium chloride confined in SBA-15 mesoporous silica (∼8.5 nm pores) using calorimetry, positron annihilation spectroscopy, and Raman spectroscopy to probe phase transitions and molecular organization. All-atom molecular dynamics simulations quantified solute-surface interactions.

The proposed hypothesis explains the divergent behavior of confined mixtures through solute-specific adsorption and exclusion at the silica interface. Simulations and Raman spectroscopy confirm that dimethyl sulfoxide and formamide adsorb onto silanol-covered pore walls, stabilized by hydrogen bonding, whereas NaCl ions are excluded. These contrasting interactions produce heterogeneous radial solute partitioning, altering local composition and phase behavior. While all mixtures exhibit eutectic crystallization in the bulk, only the confined salt solution retained a first-order eutectic transition, consistent with solute partitioning within the non-freezable δ layer. This mechanism provides a molecular basis for phase behavior in nanoconfined aqueous systems, with implications for cryobiology, prebiotic chemistry, and materials engineering.