Modulated Differential Scanning Calorimetry (MDSC) studies on low-temperature freezing of water adsorbed on clays, apparent specific heat of soil water and specific heat of dry soil

Low temperature exothermic peaks (LTEP) were previously observed in the DSC experiments on cooling of homoionic clays (Kozlowski, 2011). According to a straightforward hypothesis these thermal events might be attributed to the crystallization of the adsorbed “unfreezable” water. In the present paper, results of additional experiments by use of Modulated Differential Scanning Calorimetry (MDSC) are presented. Decomposition of the total heat capacity measured on cooling into the reversible and nonreversible components indicates that the LTEP are most likely a manifestation of freezing or a similar transition, yet quantitative determination of this freezable fraction would require realistic data on the specific heat of the adsorbed water as a function of its relative content and temperature. Detailed analysis of the apparent specific heat of soil water as a function of temperature shows that a model, in which the amount of water approximately equal to the hygroscopic water content w0.95 (equivalent to 2–3 pseudo-layers) remains unfrozen to − 70 °C, proves consistent with the obtained results and referenced data on the specific heat of the adsorbed water. The hypothesis that the LTEP are related to the freezing of the water hydrating exchangeable cations seems the most reasonable in view of the obtained results and some recent data from references.

► Modulated Differential Scanning Calorimetry was used to investigate clays to − 70 °C.
► Low temperature exothermic peaks (LTEP) do not involve any reversible component.
► The specific heats of dry soils were obtained between − 70 and + 30 °C.
► The apparent specific heat of exclusively water was analyzed to − 70 °C.
► Only a small part of the adsorbed water freezes at the LTEP.