Enthalpy Fluctuations Research Articles

Time-domain and frequency-domain differential calorimetry

The heat capacityC p of a sample can be considered as a frequency dependent quantity; its behaviour can reflect the dynamics of enthalpy fluctuations. In order to take into account the dynamic nature of the measured quantity, calorimetry can mimic experimental methods as those of dielectrometry, performing experiments in time domain or in frequency domain. In this paper, an instrument is presented which is based on a calorimetric method meeting these requirements, and thus allowing to study sample dynamics of very viscous systems as glasses and some supercooled liquids. Moreover, experimental procedures permitting investigation of samples undergoing chemical and/or physical transformations by simultaneous measurements of enthalpy variation, heat capacity and, under certain conditions, thermal conductivity, are discussed.

Ionic Dynamics of Alkali Chloride Systems in the Supercooled and Glassy States: Analyses of Inherent Structures

Abstract Molecular dynamics simulations have been performed for two alkali chlorides, LiCl and LiCl-KCl, to investigate the relation between the finite-temperature real properties and the corresponding configurations of potential minima (Stillinger's inherent structures). The thermodynamic properties and the single-particle dynamical properties relevant to not vibrational but diffusive motions have been analyzed on the basis of quenched inherent structures. Each system include so large enthalpy gap and so large enthalpy fluctuation in their quenched configurations at around the glass transition temperature that both systems are found to be fragile liquids in Angell's scheme. It is found that the mean square displacements in the successive inherent structures exhibit Brownian behavior or linear time-dependence from very early time t = 0.01 ps, so that interbasin transitions represent the ionic diffusive behavior. The interbasin transitions can successfully describe Chemla's effect and an inverse phenomenon observed in real MD trajectory of the low-temperature liquids and glasses. Furthermore, we have revealed that there are three types of short-time intermittent interbasin transitions, namely, the jump-type transitions, non-jump-type residual motions, and residence in basins, all of which are hidden under vibrational motions at finite temperatures. As for the chemical difference, there are more prominent residual motions which are distinguished from the jump-type transitions in the LiCl-KCl glass than in the LiCl glass.

Vortical mode instability of shear layers with temperature and density gradients

The linear inviscid instability of a low Mach number shear layer with imposed temperature and density gradients is analyzed in terms of its vortical, acoustic, and entropy modes. The conditions under which the acoustic mode is decoupled from both the vortical and entropy modes in the linear instability region of the shear layer are first discussed. It is then shown that for a two-dimensiona l parallel shear layer, satisfying these conditions, the vortical mode also decouples from the entropy mode. The instability of such a shear layer is then identified with the instability of the vortical mode. Two examples that illustrate the strong influence of the steady-state temperature profile on the growth rates of the unstable vortical mode are presented and discussed. Finally, it is demonstrated that the initial value of the unstable vortical mode is determined by the acoustic mode. Nomenclature A = eigenfunction in Eq. (12) a — local speed of sound a, = reference speed of sound B ' = Bernoulli enthalpy fluctuation B = Fourier component of B ' C — constant in the solution of the unstable vortical wave Cf, = specific heat at constant pressure cpl} = phase velocity of vortical wave F —Fourier

Frequency spectrum of enthalpy fluctuations associated with macromolecular transitions.

A multifrequency calorimeter has been designed to measure the amplitude and time regime of the enthalpic fluctuations associated with structural or conformational transitions in biological macromolecular systems. The heat capacity function at constant pressure is directly proportional to the magnitude of the enthalpic fluctuations in a system. Biological macromolecules undergo thermally induced transitions of different kinds. Within the transition region, these systems exhibit relatively large enthalpy fluctuations that give rise to the characteristic peaks observed by conventional differential scanning calorimetry. The multifrequency calorimeter developed in this laboratory has been designed to measure the frequency spectrum of the enthalpy fluctuations, thus allowing us to estimate thermodynamic parameters as well as relaxation times. This information is obtained from the attenuation in the amplitude or phase-angle shift of the response of the system to a periodic temperature oscillation. This instrument has been used to study the gel-liquid crystalline transition of phosphatidylcholine bilayers. The frequency-temperature response surface for large dimyristoyl phosphatidylcholine vesicles has been measured in the frequency range 0.04-1 Hz. The data are consistent with two enthalpic relaxation processes with time constants on the order of 3.8 s and 80 ms at the midpoint of the main gel-liquid crystalline transition.

Correlation Coefficients of the Fluctuations in Enthalpy or Intermolecular Potential Energy and Volume of Organic Liquids and Water

Combining a theory presented here with available data on thermodynamic properties of liquids, correlation coefficients of the fluctuations in enthalpy and volume, as well as those in intermolecular potential energy and volume are evaluated for 16 organic liquids and water. Fluctuations in intermolecular potential energy and volume of nonpolar liquids have a large correlation coefficient of about 0.8 or more at 25°C. Any attractive polar interaction between constituent molecules has the effect of reducing the correlation coefficients. Correlation coefficients for water are exceptionally small near room temperature, increase monotonously with temperature, and reach the room temperature value for polar organic liquids only at about 150°C.

Mixing enthalpy and composition fluctuations in ternary III–V semiconductor alloys

The mixing enthalpy of ternary tetrahedral semiconductor alloys is fairly well described by regular solution theory, with a thermodynamic interaction parameter that is sensitive to the lattice spacing of the binary constituents. We derive an estimate of the interaction parameter from a model which ascribes the mixing enthalpy to bond distortions associated with the alloy formation, and relates these to the macroscopic elastic properties of the crystal. Numerical estimates are given for the 18 alloys with cations Al, Ga, In and anions P, As, Sb and are compared with experimental values and alternative models. To within a single adjustable parameter, the predictions agree with experiment and are consistent with those of the delta lattice parameter (DLP) model. A further calculation of the elastic energy associated with composition fluctuations (clustering) in these alloys indicates that this energy is sufficient to suppress clustering above the critical mixing temperature.

The relation between the changes of thermodynamic quantities at the glass transition and the long-wavelength fluctuations of glass structure

The thermodynamic relation between the discontinuity of the longitudinal elastic compliance at the glass transition and the long-wavelength density fluctuations in the glass is rederived using the formal framework of linear response theory. Analogous new relations are derived for the discontinuities of the specific heat and the thermal expansion coefficient. From these relations a new interpretation of the Prigogine–Defay ratio in terms of the correlation between enthalpy and volume fluctuations is obtained.

Relaxation in the glassy state: volume, enthalpy, and thermal density fluctuations

Relaxation in the glassy state: volume, enthalpy, and thermal density fluctuations

Noise from equilibrium enthalpy fluctuations in resistors

The theory of equilibrium enthalpy fluctuations in resistors is presented. Measurements of excess noise in an electrolyte resistor are compared with the Green’s function for change in resistance caused by changes in power dissipation. The two measurements are found to obey a relationship predicted by thermodynamics.

Some ways to use thermodynamic information to characterize linkage systems

In systems of coupled reactions, processes linked to the measured process whether they be free-swinging equilibria such as ionization of protein groups, or vehicles for site to site linkage such as conformation changes, are detectable and made available for quantitative study through compensation relationships between enthalpy and entropy often surprisingly linear. In protein systems thus far studied the most common effector of such behavior is a change of pH. This perturbs a measured protein process with only small net change in chemical potentials of the ionizing groups and the proton buffer species involved since cratic and unitary parts nearly balance. However, enthalpy and entropy changes can be large and will tend toward a linear relationship on pH variation with a proportionality constant near the mean experimental temperature. The ionized and un-ionized states of each acidic or basic group form a two-state system linked to the measured process. Other free-swinging processes such as the change in state populations of water produce similar results but the water process is complicated by the fact that the standard chemical potentials (unitary parts) of the macro states of bulk water are equal near 25/sup 0/C, the usual mean experimental temperature. Coupled multistate systems such asmore » these are a common source of enthalpy-entropy compensation behavior, but a more fundamental souce lies in the second-law requirement that those parts of ..delta..H and ..delta..S reserved to accommodate enthalpy and entropy fluctuations between system and surroundings at constant T and thus not able to contribute to ..delta..G in isothermal, isobaric processes, must cancel each other through the relationship ..delta..H/sub compensation/ = T/sub mean/ ..delta..S/sub compensation/.« less

Reconciliation between Thermodynamics and Noise Measurements on Metal Film Resistors

The deviation of a resistor's $I\ensuremath{-}V$ curve from linearity as a result of Joule heating and its temperature coefficient of resistivity are shown to be sufficient to predict the low-frequency limiting spectral density of resistance fluctuations caused by spontaneous enthalpy fluctuations. The prediction agrees with previous experiments. Previous predictions that substantially exceeded measured noise in metal film resistors are shown to rely on incorrectly normalized current-jump data.

A general relation between resistance fluctuations from enthalpy fluctuations and resistance response to current changes

The spectral shape and magnitude of spontaneous resistance fluctuations due to enthalpy fluctuations are shown to be determined by the resistance response to current impulses for a broad class of resistors, including any resistor containing only one type of conducting material, regardless of the resistor geometry. Implications for the design and interpretation of experiments are discussed.

Molecular conformation in molten and glassy polymers

Abstract As a consequence of enthalpy fluctuations in a small thermodynamic system, the conformation in molten and glassy polymers is predicted to be more nearly an irregularly folded molecule with 5 to 10 nearest neighbors than a random coil with 50 to 100 nearest neighbors.

Calculating the plane wake behind a body

The empirical theory of turbulence serves as a basis in this study of velocity, enthalpy, turbulence energy, and mean-squared enthalpy (density) fluctuation profiles along the wake axis. The problem is reduced to a system of ordinary differential equations. The asymptotic behavior of the solution to this system is analyzed. The results of calculations are compared with known test data.

Enthalpy and density fluctuations in high-speed wakes.

Thus, it appears that for the present accelerator the decelerating eddy currents are small in comparison to the total transverse accelerating current of nearly 500 amp, and no large velocity decreases in the end regions would be expected. Because eddy currents also may decelerate the flow by heating it, several attempts were made to determine their effect on the plasma velocity at the accelerator entrance and exit. In each instance there was no indication that the velocity was changed significantly. Unfortunately, it was not possible to measure the impact pressure at the exit of the acceleration section. However, a careful accelerator performance analysis by Wilson (of the type described in Ref. 7) indicated that the impact pressure measured just aft of the channel exit (3 cm aft of electrode pair 117) was reasonably well predicted by a modified one-dimensional MHD channelflow theory, which neglects the possible existence of eddy currents. This, together with the previously noted constancy of impact pressures during the open circuit tests, suggests that eddy currents did not cause a significant velocity decrease in the downstream unpowered section. In addition, wall static pressures, which were measured throughout the length of the channel, gave no evidence of velocity changes in either of the unpowered sections. Thus, eddy currents, which served to decelerate the flow, were present, but to a much lesser extent than was reported by Leonard and Fay. More generally, it may be concluded from the present tests that accelerator exit-velocity losses may be minimized by keeping the exit magnetic field gradients small as has been pointed out by Leonard and Fay. Further, one could attempt to quench the plasma before allowing it to pass through a strong magnetic field gradient if such gradients are unavoidable.