A &wry is developed for the innuence of pore form on the equilibrium freezing and melting temperatures of water in porous materials. This is supported by calorimetry and uses the hysteresis between freezing and melting temperatures for two clays and undricd synthetic organic textile fibres in which the pore geometry is that of hterWCting cylidders.For most water-saturatcd clay systems, analysis of the low temperature endotherms gives a better appreciation of the pore size distribution than analysis of the exothermic data, unless the width of rectangular pores is known. Freezing and melting temperature hysteresisPore size determines the equilibrium conditions for the solid, liquid and gas phases in porous materials (Defay et d., 1966). There will be hysteresis between freezing and melting temperatures if interfacial curvatures during freezing and fusion are different (Everett and Haynes, 1965).

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Suitable container design permits very high temperature and differential temperature resolution in DSC even when relatively large (_~ 0.14 cm 3) samples are used; and thus energy signals associated with phase change occurring over large temperature intervals may be analysed in differential elements.An original and powerful high resolution low temperature DSC technique (the authors plot") for studying and comparing pore size distributions (PSD) in water saturated samples (for pores having Kelvin radii between 1,2 and about 500 nm) is given, together with an application which shows that the PSD in a water saturated organic ion exchanger could be obtained from analysis of three Gaussian distributions which precisely generate a curve which is an excellent empirical fit to the recorded low temperature exotherm obtained by freezing pure water within the sample.The temperature at which liquids and solids change phase in porous materials depends upon pore size [1]. For water the lowering of freezing or melting temperatures is inversely proportional to the pore Kelvin radius [2]. For cylindrical pores the Kelvin radius is equivalent to the radius of the cylinder, whereas for parallel-sided fissures it corresponds to the wall to wall distance [3].Low temperature high resolution DSC is a powerful tool for investigating water and solution behaviour in porous materials [2] and knowledge of the relationship between phase transition temperature and pore size allows the distribution of pore sizes in wet porous materials to be determined. The shape of a pore determines the extent to which hysteresis occurs between the equilibrium freezing and melting temperatures of water (or salt eutectic) in the pore space [2]. For materials which swell in contact with water or for materials which contain "ink-bottle" shaped pores, monitoring of the freezing and melting behaviour of pure water in the substance gives quick and valuable information about the geometry of the pore spaces-these applications being suitable for studying pore sizes up to about 500 nm Kelvin radius, which size corresponds, for ice, to a lowering melting temperature of about 0.1 ~ [2]. In this report a description is given of how DSC investigations may be optimized for this type of study by appropriate choice of experimental conditions, some modifications to standard equipment and suitable sample container-design.Exotherms or endotherms corresponding to water changing phase (under equilibrium conditions) within porous materials, are curves which occur over a temper-