Aqueous solution of polysaccharides forms hydrogel in a characteristic concentration and temperature range. Polysaccharides, such as agarose, κ-carrageenan, gellan gum, form thermoreversible gel by cooling and gel-sol and sol-gel transition can be detected by thermal measurements. In contrast, polysaccharides, such as methylcellulose and curdlan form hydrogel by heating. The former is thermo-reversible and the latter is thermo-irreversible. Besides the above polysaccharides, it is found that “non-gelling” polysaccharides, such as xanthan gum and hyalulonan, form hydrogel, if aqueous solution is annealed at a temperature higher than gel-sol transition and subsequently cooled. In the present paper, recent studies on thermal properties of polysaccharide physical hydrogels are reviewed. Attention is especially paid for change of gelling mechanism affected by annealing in the sol state.

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Molten globule (MG) state, which is structurally distinct from both the native (N) and denatured (D) states, was originally proposed as an equilibrium intermediate state of denaturation of some proteins with a compact conformation, a considerable native-like secondary structure, and a largely fluctuating tertiary structure. Recently, our group has developed isothermal acid-titration calorimetry (IATC), a calorimetric method for evaluating the enthalpy change accompanying the pH-induced transition of protein using isothermal titration calorimeter. By this method, the pH-induced transition from N to MG state of cytochrome c was directly observed by calorimetry and was confirmed to be a two-state transition with small enthalpy change. Also, the MG state was detected in the thermal transition from N to D state by highly precise differential scanning calorimetry.

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We have reviewed our recent heat capacity and neutron scattering works on various ionic liquids with 1-butyl-3-methylimidazolium (bmim) and its related cations. The heat capacity data revealed that above ionic liquids exhibit glass transitions with large C_p jumps below room temperatures. The temperature dependence of the mean square displacements of the ionic liquids, which were determined from the neutron scattering data, indicated that fast (THz) relaxations take place above their glass transition temperatures. The relation between the glass transition and fusion temperatures of the ionic liquids is consistent with the empirical law (T_g/T_fus=2/3) for molecular liquids. The relation between T_g and anion size suggested that the configurational motions of cations, surrounded by a relatively-rigid anion framework, is dominant for the glass transitions of ionic liquids. The temperature dependence of the configurational entropy and the size of cooperatively rearranging region (CRR) of the ionic liquids are similar to those of molecular liquids. Both heat capacity and inelastic neutron scattering data showed that the ionic glasses exhibit so-called boson peaks around 2meV. By summarizing the above results, it is concluded that the ionic liquids with bmim and its related cations are similar to molecular liquids.

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Large amount of toluene and carbon tetrachloride can be absorbed into 1-dimensional tunnels in copper(II) trans-1, 4-cyclohexanedicarboxylate Cu₂(OOC-C₆H₁₀-COO) under the saturated vapor pressure at room temperature. The absorbed toluene and carbon tetrachloride can be desorbed easily by evacuation above room temperature, and the absorption/desorption is completely reversible. The absorption/desorption phenomena were studied for the empty, partly and fully toluene and carbon tetrachloride absorbed samples by adiabatic calorimetry between 13K and 300K and by powder X-ray diffractiometry with high-energy synchrotron radiation at SPring-8 of JASRI. The first-order phase transition was observed in the empty sample. The partly toluene-absorbed samples showed smaller heat capacity anomaly at higher temperatures than the empty sample. On the other hand, the partly carbon tetrachloride-absorbed samples showed smaller heat capacity anomaly at lower temperatures than the empty sample. The difference in the two cases was discussed on the basis of structural and thermal data.

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Thermal conductivity via electron and spin degrees of freedom has been a valuable tool to study superconducting properties of high-T_c cuprates and quantum fluctuation of one-dimensional spin liquids. It turned out that presence of metallic quasiparticles is necessary for the superconducting phase transition for the high-T_c cuprates. It is also demonstrated that spin excitations carry a large amount of heat in the one-dimensional quantum magnet, indicating their excellent spatial coherence like phonons and electrons in metals. The thermal conductivity measurement will be useful more generally in studying exotic properties of strongly-correlated electron systems.

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