Some factors affecting supercooling and the equilibrium freezing point in soil-water systems

摘要

For six monomineral, homoionic clayey soils, the temperature of spontaneous nucleation T_(sn) and the equilibrium freezing point T_f were determined by use of the Differential Scanning Calorimetry (DSC) technique. The temperature of spontaneous nucleation T_(sn) was determined on the cooling run, as the initial temperature of the observed exothermic peak. The temperature of equilibrium freezing (or melting) T_f was interpreted as the initial temperature of the last non-zero thermal impulse in the diagram of real thermal impulses distribution q(T) obtained on warming. The supercooling ψ was calculated as the difference between T_f and T_(sn). The obtained results testify the strong dependency of the equilibrium freezing point T_f on the water content w. It has been proved that T_f can be expressed as a power function of the water content w and the plasticity limit W_p, with an asymptote at w equal to the unfreezable water content w_(nf). In contrary, a scatter of results was observed for T_(sn) and ψ. which could be related to the effect of factors other than the water content. The best fitted model expresses the temperature of non-equilibrium freezing T_(sn) as a function of the water content w, the plastic limit w_P and an extensive parameter of the sample, i.e. its mass m, the effect of which proved fully statistically significant. The results give evidence of the strong effect of both soil plasticity and the sample mass on the temperature of spontaneous nucleation and the supercooling. By use of auxiliary empirical function, relating the unfreezable water content w_(nf) to the plastic limit W_p, it was possible to calculate such a mass m_(ψ = 0) of a soil sample, for which the supercooling equals zero. At high water contents the predicted supercooling tends to zero for very large sample masses, from about 10~5 kg in a practically uncohesive soil (w_P= 1%) to 10~8 kg in an extremely cohesive soil (w_P= 100%).