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Attention is drawn to the consequences of the finding that adsorbates are in a liquidlike state at temperatures below their bulk freezing point. If the relative pressure is based on the saturation vapor pressure of the undercooled liquid, differences between the N[2] and Kr surface areas and those between the BET and Dubinin-Raduskevich areas become minimal. On application of inappropriate relative pressure values in constructing isotherms, apparent supersaturation may be indicated.

Adsorption of water has been studied extensively above 0 degrees C, but the problem of the phase transition of adsorbates in porous materials is presented at temperatures below 0 degrees C. Experiments that show how phase transitions modify length change-adsorption relationships are described. Adsorption-length change measurements from - 40 to -0.5 and isotherms at -20 and -0.5 degrees C were obtained. In another group of experiments different forms of isosteres were also obtained; the principle that water should be allowed to enter the sample at the low temperature was usually followed. Isotherms and length change- adsorption characteristics below 0 degrees C are qualitatively similar to those above 0 degrees C, but a decrease in adsorptive capacity and length change was observed. These results and those obtained from the isosteres lead to the conclusion that capillary condensation and menisci theories are operative and that a solid meniscus exists. It follows therefore that the saturated vapor pressure of frozen adsorbate is greater than that of bulk ice and that its volumetric expansion on freezing is similar to the bulk phase transition. It is shown how these phenomena can predict migration of water and generation of pressure within the porous body when it is subjected to cooling and warming cycles.