The complex dielectric constant of liquid water was measured at 9.61 GHz down to −18 °C by means of a two resonant cavities apparatus. The static dielectric constant of bulk samples was also measured at 27.5 MHz down to −16.5 °C using a resonant circuit technique. From the analysis of the experimental results it follows that water in the metastable region has practically a single dielectric relaxation time τ. An analysis of the dynamic properties of water using our results and available data in literature, is presented. The main result is that self-diffusion DS, shear viscosity η, and τ below 0 °C are related to the same mechanism. For T≳0°C another mechanism affecting η rises.
To gain insight into the thermodynamics of protein denaturation, the complex heat capacity, C p * () C p ′ -iC p ′′) of lysozyme-water system has been measured at pH 2.5 in the 293-368 K range by using temperaturemodulated scanning calorimetry (TMSC), a technique in which the thermally reversible enthalpy changes are measured separately and simultaneously with the thermally irreversible enthalpy changes. The plot of C p ′ against the temperature T shows a broad peak, which is similar to that observed in C p,DSC , measured here and elsewhere by differential scanning calorimetry (DSC), a technique which gives the sum of both the reversible and irreversible contributions in the apparent heat capacity value. This peak in C p,DSC has been generally attributed to endothermic heat absorption on reversible and irreversible unfolding processes and irreversible thermal denaturation. It is shown that the observed C p ′ peak results from heat absorption when the equilibrium constant between the native lysozyme state and a conformationally different intermediate state increases with T. The plot of C p ′ versus T is subdivided into four regions, corresponding to the dominance of a certain process. Thermal denaturation of lysozyme was found to occur according to a scheme, native state T unfolded (intermediate) state f denatured state. This conclusion is consistent with the general view that the first step of denaturation of small one-domain globular protein like lysozyme is a reversible conformational (unfolding) transition, and the second step is irreversible denaturation. It is shown that when kept isothermally at T > 341 K, i.e., within the transition temperature range, C p ′ of lysozyme decreases. This decrease is exponential in time and corresponds to a rate constant, which varies according to the Arrhenius-type equation, with a preexponential factor of 5 × 10 20 s -1 and energy of 167 kJ/mol. The overall kinetics of the denaturation reaction is of the first order.
The heat capacity C(p) of the liquid state of water confined to 2 nm radius pores in Vycor glass was measured by temperature modulation calorimetry in the temperature range of 253-360 K, with an accuracy of 0.5%. On nanoconfinement, C(p) of water increases, and the broad minimum in the C(p) against T plot shifts to higher temperature. The increase in the C(p) of water is attributed to an increase in the phonon and configurational contributions. The apparent heat capacity of the liquid and partially frozen state of confined water was measured by temperature scanning calorimetry in the range of 240-280 K with an accuracy of 2%, both on cooling or heating at 6 K h(-1) rate. The enthalpy, entropy, and free energy of nanoconfined liquid water have been determined. The apparent heat capacity remains higher than that of bulk ice at 240 K and it is concluded that freezing is incomplete at 240 K. This is attributed to the intergranular-water-ice equilibrium in the pores. The nanoconfined sample melts over a 240-268 K range. For 9.6 wt % nanoconfined water concentration ( approximately 50% of the maximum filling) at 280 K, the enthalpy of water is 81.6% of the bulk water value and the entropy is 88.5%. For 21.1 wt % (100% filling) the corresponding values are 90.7% and 95.0%. The enthalpy decrease on nanoconfinement is a reflection of the change in the H-bonded structure of water. The use of the Gibbs-Thomson equation for analyzing the data has been discussed and it is found that a distribution of pore size does not entirely explain our results.
To obtain physical insight into the slowing of the molecular dynamics when a macromolecule grows as a result of polymerization, (i) the dielectric relaxation spectra, and (ii) the heat evolved during the growth of a linear chain structure by means of the reaction of diepoxide with a monoamine have been measured simultaneously and continuously over time, at a fixed temperature of 314.2 K, during fixed-rate heating to 341.5 K and thereafter cooling from this temperature. An instrument was designed for the purpose. The studies yield information, almost simultaneously, on the changes in the dc conductivity, on the static and dynamic behaviours of the dipolar relaxation, and on the number of covalent bonds, n, in the states of the structures formed by irreversible polymerization. The dc conductivity decreases with increase in n, but this decrease cannot be attributed entirely to the increase in viscosity. The decrease in the ion population has a significant effect on the change in dc conductivity. Both the equilibrium permittivity and the limiting high-frequency permittivity decrease on polymerization. This decrease is attributed mainly to a decrease in the dipolar orientational correlation factor in the former case, and to a predominant increase in the phonon frequencies in the latter case. It is found that the slowing of the molecular dynamics that occurs here on increase in n is more than compensated by the acceleration of the dynamics on increase in the thermal energy. This effect is interpreted in terms of the changes in the configurational entropy, , which leads to relations expressing the dependence of on n, as well as on the temperature. A faster molecular dynamics of the Johari - Goldstein relaxation evolves as n increases. This dynamics does not depend on the decrease in with decrease in n, but depends only on the change in with temperature. The dielectric behaviour of the completely polymerized state obtained after repeated thermal cycling of the initially molecular liquid has been studied, and the results are related to the molecular dynamics observed during the growth of the macromolecules.
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