Sorption Microcalorimetry: Method Development and Applications

Abstract: A double twin isothermal microcalorimeter has been designed and its performance has been tested experimentally. The calorimeter makes it possible to continuously and isothermally scan a solid sample in a broad range of relative vapor pressures at near-equilibrium conditions. The method is suited for measurements with water vapor as well as organic vapors and has been tested at temperatures between 25 and 40ºC. With the technique it is possible to simultaneously and independently in one experiment obtain the sorption isotherm of a sample along with the corresponding differential enthalpies of sorption. The method provides a rather unique combination of information on both partial molar enthalpy and chemical potential change and thus also the partial molar entropy change for the vapor in the sorption process. This is sufficient to characterize the sorption process thermodynamically. It is demonstrated that the method can be applied to the study of a wide range of physicochemical phenomena associated with the uptake of vapor by a substance/material including capillary condensation, hydrate and solvate formation, crystallization and lyotropic phase transitions. The calorimetric results agree well with data from other established techniques such as desiccator storage, sorption microbalance, osmotic stress, solution calorimetry and DSC. The advantage of the present technique is that the sorption data are continuous and cover the whole range of water activities; the isotherms are supported by differential enthalpies of sorption obtained simultaneously for the same sample. It is shown that with the present instrument the critical vapor pressures of hydrate formation and phase boundaries of the lyotropic phase transitions can be determined along with corresponding differential enthalpies of sorption. It is demonstrated that the results of the sorption calorimetric measurements, supported by the data obtained with more specific techniques, can help in understanding of the phase behavior of substances that can self–associate upon hydration.