The specific heat of dry homoionic forms of bentonites SWy-2 and Stx-1b: An effect of the fraction of particles less than 2 μm

摘要

The specific heat of Ca~(2+)-, Na~+- and K~+-forms of SWy-2 and Stx-1b has been determined in the range between -150 ℃ and 100 ℃ by use of Quasi Isothermal Modulated Differential Scanning Calorimetry. According to expectations, a strong temperature dependency is observed, but the results of ANOVA also show high statistical significance of the kind of initial clay and the kind of the exchangeable cation. The effect of the latter proved to be indirect, as the contribution from the heat capacity of the exchangeable cations appeared less significant, although its detailed analysis meets difficulties associated with a lack of data on the specific heat of the cations and its relation to the specific heat of the corresponding metals in bulk. A relatively high and statistically significant correlation of the specific heat at a given temperature and the fraction of particles less than 2 μm may indicate a contribution of surface atoms to the specific heat capacity, analogical to that observed and theoretically substantiated in relation to nanopartides of metals and simple organic compounds. This thesis is supported by the kind of temperature-size dependence, resembling that reported for nanoparticles of cuprum oxide by Wang et al. (2006). However, a proposed simple power function model not taking into account the size effect can predict specific heat of montmorillonites with the standard error of the estimate SEE equal to 0.038 J g~(-1)K~(-1).