Thermal hysteresis induced by ammonium polyacrylate as antifreeze polymer

Growth and melting rates of a single crystal of ice in ammonium polyacrylate (NH4PA) aqueous solutions were measured at different solution temperatures, and the morphology of the ice crystals was observed. Thermal hysteresis, defined as the difference between the melting temperature and the non-equilibrium freezing temperature of ice, was confirmed in NH4PA solutions at concentrations below 25.0 mM. The higher the NH4PA concentration, the larger the thermal hysteresis, although the thermal hysteresis induced by NH4PA was much smaller than that induced by antifreeze proteins, antifreeze glycoproteins, or poly(vinyl alcohol). A single crystal of ice grown in the NH4PA solutions at temperatures within the thermal hysteresis region exhibited the basal faces. When the solution temperature was below the non-equilibrium freezing temperature, the ice crystal grew faster in the a-axis direction than in the c-axis direction, while retaining the basal faces.