Kinetics of water freezing from inorganic salt solution confined in mesopores

Highlights: • Activation energy for transition during cooling can be of negative value. • Water freezing changes from homogeneous to heterogeneous nucleation. • Presence of salt results in greater demand for energy to create a new phase. • The smaller pore diameter, the greater absolute value of activation energy. • NaCl aqueous solution surrounding silica does not exhibit the eutectic transition. -- Abstract: In the presented study the kinetics of pore-confined water freezing is analysed using differential scanning calorimetry. The process is investigated in mesopores of two silica gels with pore diameter equal to 6.5 nm and 11.3 nm. Based on isoconversional study of ice crystallization caused by cooling of pure water and 1%, 3% NaCl aqueous solutions, the negative value of activation energy is obtained. The observed variation of effective activation energy might be explained by the nucleation theory, namely, the dominant mechanism changes from homogeneous through heterogeneous nucleation to diffusion in the final stage of phase transition. At the low concentrations the bulk solution surrounding the silica gel particles does not exhibit the eutectic solidification.