Formation and determination of the amount of ice formed in water dispersed in various materials

Abstract For some materials containing dispersed water such as emulsions, clouds, vegetables, biological tissues and food, the conditions of formation of ice and the amount of ice formed when they are submitted to temperature variations between ambient temperature and sub ambient ones are predicted. In these materials, water is not pure and very often electrolytes are dissolved. Experiments on water in oil emulsions performed by calorimetry (DSC) are described thoroughly as far they are good candidates for predicting what is going on for the other materials cited. The formation of ice and its amount appear to be temperature and composition dependent. Due to nucleation phenomena, delays in the freezing are observed and furthermore water available in the dispersed aqueous solutions could freeze in two steps, one involving the freezing of a partial amount of water and the other one involving the crystallization of the dissolved salt that induces the freezing of the remaining water. As far supercoolings are involved, freezing can occur at a temperature less than the eutectic one. At any temperature T the total amount of ice expected to form in a solution of composition x , is given by the ratio of the segments LM and LS, the points L and S being respectively situated on either the equilibrium ice/solution curve or its extension and on the line x  = 0 (pure ice). Point M ( x , T ) represents the solution understudy. The calculation needs the knowledge of the extension of the equilibrium curve. This extension has been obtained from thermodynamic data on the activity of pure ice versus temperature and the humidity of the solutions versus composition. An example of such a determination is given for NaCl + water solutions for which experimental determination of the amount of ice formed at T  = −33 °C below the eutectic point have been determined by studying the amount of ice formed in emulsions of these dispersed solutions maintained at the fixed temperature T . Experimental results obtained by DSC compared to the calculations made from the knowledge of the extension of the equilibrium curve, show a good agreement.