Search

Ionic liquids have been suggested as new engineering fluids, namely in the area of heat transfer, as alternatives to current biphenyl and diphenyl oxide, alkylated aromatics and dimethyl polysiloxane oils, which degrade above 200 &deg, C and pose some environmental problems. Recently, we have proposed 1-ethyl-3-methylimidazolium methanesulfonate, [C2mim][CH3SO3], as a new heat transfer fluid, because of its thermophysical and toxicological properties. However, there are some interesting points raised in this work, namely the possibility of the existence of liquid metastability below the melting point (303 K) or second order-disorder transitions (&lambda, type) before reaching the calorimetric freezing point. This paper analyses in more detail this zone of the phase diagram of the pure fluid, by reporting accurate thermal-conductivity measurements between 278 and 355 K with an estimated uncertainty of 2% at a 95% confidence level. A new value of the melting temperature is also reported, Tmelt = 307.8 &plusmn, 1 K. Results obtained support liquid metastability behaviour in the solid-phase region and permit the use of this ionic liquid at a heat transfer fluid at temperatures below its melting point. Thermal conductivity models based on Bridgman theory and estimation formulas were also used in this work, failing to predict the experimental data within its uncertainty.

It has been established experimentally that the thermodynamic properties of fluids satisfy scaling laws with universal critical exponents asymptotically close to a single critical point of the vapor–liquid phase transition. In contrast to the conjecture of Woodcock, there is no reliable experimental evidence to doubt the existence of such a single critical point in the thermodynamic limit and of the validity of the scaling theory for critical thermodynamic behavior.

We use fluctuating hydrodynamics to derive expressions for the spatial and temporal spectra of concentration fluctuations in a ternary liquid mixture in equilibrium. Our results agree with those found by other investigators who have used a procedure in which arbitrary initial values are adopted. Our method of using fluctuating hydrodynamics has the advantage that it can also be extended to deal with fluctuations in systems out of equilibrium.

In this article we derive expressions for Casimir-like pressures induced by nonequilibrium concentration fluctuations in liquid mixtures. The results are then applied to liquid mixtures in which the concentration gradient results from a temperature gradient through the Soret effect. A comparison is made between the pressures induced by nonequilibrium concentration fluctuations in liquid mixtures and those induced by nonequilibrium temperature fluctuations in one-component fluids. Some suggestions for experimental verification procedures are also presented., Comment: 9 pages, 3 figures

There exists a formulation for the critical locus of aqueous solution of NaCl recommended by the International Association for the Properties of Water and Steam. In this article, we present slightly modified equations for the critical parameters of H2O + NaCl that incorporate a non-analytic concentration dependence predicted by theory.

In a recent publication, it has been shown how a non-analytic scaling theory of critical phenomena can describe the available experimental information for the thermodynamic properties of supercooled water, when it is assumed to exhibit a liquid–liquid critical point. In this article, we present a mean-field equation of state which also represents the experimental data for supercooled water. Compared to the scaling theory, the mean-field equation has the advantage of simplicity for practical calculations of the properties of supercooled water. The insensitivity to a particular form of a critical equation of state is due to a lack of experimental data asymptotically close to the liquid–liquid critical point. Hence, while the assumed existence of a liquid–liquid critical point in water can account for the observed thermodynamic behavior of supercooled water, the actual location of such a liquid–liquid critical point cannot be determined accurately from the available experimental thermodynamic property data.

Recently, we have started to develop mesoscopic nonequilibrium thermodynamics for a reaction far from equilibrium in the presence of a temperature gradient, so that the interplay between the chemical reaction, diffusion and thermal diffusion can be described within the same theoretical framework. In this article we show that the spatial symmetry properties of the deterministic solution, which were so characteristic for conditions close to equilibrium, are no longer valid far from equilibrium. This is expected to have some conceptual consequences for the spatial spectrum of the fluctuations of temperature and concentrations around their local equilibrium values.

The principle of isomorphism of critical phenomena asserts that the equation of state of fluid mixtures in the critical region has the same form as that of the pure components, provided that the mixture is not considered at a fixed composition but at a fixed value of a constant field variable related to the chemical potentials of the components of the mixtures. This principle has been successfully applied by various investigators to formulate nonclassical equations of state for fluid mixtures in the critical region. In this paper we show how the same principle can be applied to simple classical equations of state using the Peng–Robinson equation as a representative example. The isomorphic Peng–Robinson equation thus obtained is applied to represent phase equilibria properties of some binary mixtures of methane, butane and decane.

After a review of the history and an assessment of the current status of the subject, we present light-scattering data to determine the critical correlation-function exponents for an aqueous electrolyte solution yielding the experimental values γ=1.238±0.012, ν=0.629±0.003, η=0.032±0.013. We conclude the paper with some comments concerning the temperature dependence of non-asymptotic effective critical-exponent values of fluids.

The asymptotic critical behavior of the thermal diffusivity of H2O, deduced from dynamic light-scattering measurements, is shown to be in good agreement with the mode-coupling theory for critical dynamics in fluids. Consistency of the thermal-diffusivity measurements with the critical enhancement of the viscosity of H2O is also demonstrated.

The International Association for the Properties of Water and Steam (IAPWS) encouraged an extensive research effort to update the IAPS Formulation 1985 for the Viscosity of Ordinary Water Substance, leading to the adoption of a Release on the IAPWS Formulation 2008 for the Viscosity of Ordinary Water Substance. This manuscript describes the development and evaluation of the 2008 formulation, which provides a correlating equation for the viscosity of water for fluid states up to 1173K and 1000MPa with uncertainties from less than 1% to 7% depending on the state point.

Experimental data are presented for the thermal conductivity of mixtures of carbon dioxide and ethane. The thermal conductivity has been measured with a parallel-plate instrument, as a function of temperature along isochores, so as to obtain detailed information on the behavior of the thermal conductivity of a mixture near the locus of vapor–liquid critical points. The experimental results are in agreement with theoretical predictions from the mode-coupling theory for critical dynamics in binary mixtures.

This note presents an update of an evaluation of the critical parameters of mixtures of carbon dioxide and ethane, previously reported by Abbaci et al. (Int. J. Thermophys. 13, 1043 (1993)).

In this Letter we consider a liquid mixture confined between two thermally conducting walls subjected to a stationary temperature gradient. While in a one-component liquid non-equilibrium fluctuation forces appear inside the liquid layer only, non-equilibrium fluctuations in a mixture induce a Casimir-like force on the walls. The physical reason is that the temperature gradient induces large concentration fluctuations through the Soret effect. Unlike temperature fluctuations, non-equilibrium concentration fluctuations are also present near a perfectly thermally conducting wall. The magnitude of the fluctuation-induced Casimir force is proportional to the square of the Soret coefficient and is related to the concentration dependence of the heat and volume of mixing., Comment: 5 pages, 1 figure

Because of the spatially long-ranged nature of spontaneous fluctuations in thermal non-equilibrium systems, they are affected by boundary conditions for the fluctuating hydrodynamic variables. In this paper we consider a liquid mixture between two rigid and impervious plates with a stationary concentration gradient resulting from a temperature gradient through the Soret effect. For liquid mixtures with a large Lewis number, we are able to obtain explicit analytical expressions for the intensity of the non-equilibrium concentration fluctuations as a function of the frequency $\omega$ and the wave number $q$ of the fluctuations. In addition we elucidate the spatial dependence of the intensity of the non-equilibrium fluctuations responsible for a non-equilibrium Casimir effect., Comment: 9 pages, 2 figures

We have examined experimental consequences of ‘complete scaling’, recently formulated by Fisher and co-workers [Phys. Rev. Lett. 85 (2000) 696; Phys. Rev. E 67 (2003) 061506], when extended to liquid–liquid coexistence in binary mixtures. A controversial issue of the proper definition of the order parameter in liquid mixtures is resolved. We show that a ‘singular’ diameter of liquid–liquid coexistence curves originates from two different sources: one is associated with a correlation between concentration and entropy, whereas another one is a correlation between concentration and molar density, generating a lower-order, and thus a more significant singular term.

Based on the principle of isomorphic critical behavior of fluid mixtures we derive a simple equation for closed solubility loops in a temperature–concentration diagram at constant pressure. The validity of the simple equation is tested by comparisons with experimental solubility data for liquid mixtures at atmospheric pressure and at elevated pressures. We also elucidate the nature of liquid–liquid phase separation in the vicinity of a double critical point. Specifically, we show how from an expansion around the critical double point one can develop a description of solubility loops as a function of pressure.

The present work is a continuation of our studies on predicting dilute transport properties of polyatomic gases consisting of sulfur hexafluoride (SF6), carbon tetrafluoride (CF4), and nitrous oxide (N2O). In this study, a good effective and isotropic pair interaction potential energy is obtained for each aforementioned system by the use of two-iteration inversion of the corresponding states of viscosity. A rather accurate correlation for the viscosity is reproduced from the inverted pair potential energy. The correlation for SF6 agrees with the literature data to within 1% in the temperature range 220 K

The calculations of phase equilibria in the CoCr system were performed using the CALPHAD method on the basis of a new two-sublattices model of sigma phase. This model enables us to utilise the results of ab initio calculations of total energy differences between the sigma phase structure and the Standard Element Reference (SER) structures of pure metal at the relaxed lattice parameters (Δ0Eiσ−SER). Total energies were calculated by Full-Potential Linear Augmented Plane Waves (FLAPW) method in the General Gradient Approximation (GGA). The entropy contribution to the Gibbs energy of the pure elements in the sigma phase structure, and the excess Gibbs energy of mixing of the sigma phase were adjusted to the experimental phase equilibrium data.

An accurate equation of state is presented for the thermodynamic properties of mixtures of H2O and D2O in the critical region. The equation incorporates a crossover from fluctuation-induced singular behavior near the locus of vapor–liquid critical points to regular behavior outside the critical region. It is a generalization of a crossover equation-of-state previously developed for pure H2O and pure D2O extended to the mixtures on the basis of the principle of isomorphism of critical behavior. The equation represents the thermodynamic properties of mixtures of H2O and D2O in the same range of temperatures and densities as for the pure components. A comparison is made with available experimental data for the pressure, for the isochoric heat capacity, for the enthalpy of mixing, and for the isotope fraction ratio of mixtures of H2O and D2O. An analysis is presented of the near-critical behavior of various thermodynamic properties, such as the isobaric heat capacity, the partial molar volumes, Henry’s consta...

It has been generally assumed that the asymptotic critical behavior of real fluids can be characterized in terms of the same physical variables as that of the lattice gas. This assumption implies that, below Tc, the second derivative of the pressure with respect to temperature should asymptotically diverge like the isochoric heat capacity CV, while the second derivative of the chemical potential with respect to the temperature should remain finite at the critical point. The validity of this assumption has recently been questioned on the basis of an analysis of experimental two-phase CV data in terms of the so-called Yang–Yang relation. In this paper we show how such an analysis may be affected by the presence of a small amount of impurity as well as by other nonasymptotic deviations from lattice-gas symmetry. When corrections for a small amount of impurity are applied and allowance is made for the leading asymmetric Wegner correction, the experimental CV data are not inconsistent with previous treatments ...

We report the results of a computer simulation study of the thermodynamic properties and the thermal conductivity of supercooled water as a function of pressure and temperature using the TIP4P-2005 water model. The thermodynamic properties can be represented by a two-structure equation of state consistent with the presence of a liquid-liquid critical point in the supercooled region. Our simulations confirm the presence of a minimum in the thermal conductivity, not only at atmospheric pressure, as previously found for the TIP5P water model, but also at elevated pressures. This anomalous behavior of the thermal conductivity of supercooled water appears to be related to the maximum of the isothermal compressibility or the minimum of the speed of sound. However, the magnitudes of the simulated thermal conductivities are sensitive to the water model adopted and appear to be significantly larger than the experimental thermal conductivities of real water at low temperatures.

An equation of state is presented for the thermodynamic properties of cold and supercooled water. It is valid for temperatures from the homogeneous ice nucleation temperature up to 300 K and for pressures up to 400 MPa, and can be extrapolated up to 1000 MPa. The equation of state is compared with experimental data for the density, expansion coefficient, isothermal compressibility, speed of sound, and heat capacity. Estimates for the accuracy of the equation are given. The melting curve of ice I is calculated from the phase-equilibrium condition between the proposed equation and an existing equation of state for ice I., 23 pages, 30 figures. Updated after peer review; now includes a section on the vapor pressure of supercooled water. Includes ancillary material (C++ and MATLAB code, tables with property data)

Correlations in fluids in nonequilibrium steady states are long ranged. Hence, finite-size effects have important consequences in the nonequilibrium thermodynamics of fluids. One consequence is that nonequilibrium temperature fluctuations induce nonequilibrium Casimir-like pressures proportional to the square of the temperature gradient. Hence, fluctuations cause a breakdown of the concept of local thermal equilibrium. Furthermore, transport coefficients become dependent on boundary conditions and on gravity. Thus nonequilibrium fluctuations affect some traditional concepts in nonequilibrium thermodynamics., Comment: 15 pages, 3 figures, 1 table. Physical Review E, accepted

To examine the nature of criticality in ionic fluids we have analyzed experimental liquid–liquid coexistence and susceptibility data for various ionic solutions. We show that ionic fluids generally exhibit crossover or, at least, a tendency to crossover from Ising behavior asymptotically close to the critical point to mean-field behavior upon increasing distance from the critical point. This crossover is governed by two physical parameters: a rescaled coupling constant which reflects the strength and range of intermolecular interactions and a “cutoff” length. We conclude that the crossover critical behavior in ionic fluids is primarily governed by the cutoff length, which emerges as a new length scale that cannot be identified with the effective molecular-interaction range. An analogy between crossover critical phenomena in ionic fluids and in polymer solutions is discussed.

We have discovered a mean-field multicritical point on the critical locus in an aqueous solution of 3-methylpyridine and sodium bromide. Light-scattering measurements indicate Ising-like asymptotic critical behavior at the lower salt concentrations. However, the temperature range of Ising critical behavior shrinks with increasing salt concentration and the critical behavior becomes mean-field-like at a concentration of about 17% mass fraction of NaBr. Emergence of a new characteristic length scale diverging at this point and a simultaneous pronounced increase in the background scattering suggests mean-field tricritical behavior associated with the formation of a microheterogeneous phase due to clustering of ions and molecules.

The Soret coefficient ST and collective (mass) diffusion coefficient Dc of polystyrene dissolved in the good-solvent toluene has been measured over a range of concentrations and molecular masses with an optical beam-deflection method. Our measurements indicate that ST scales inversely with the polymer translational diffusion coefficient in dilute solutions, exhibits a power-law scaling with polymer concentration, and an independence of polymer molecular mass in semidilute solutions. These findings are consistent with the known scaling of 1/Dc in dilute and semidilute polymer solutions, the relative insensitivity of the thermal-diffusion coefficient Dth of polystyrene in toluene to polymer concentration, and the relation ST=Dth/Dc from irreversible thermodynamics. We are able to represent our ST and Dc data by theoretically motivated reduced-concentration master curves, but the concentration-molecular mass scaling variables are found to be different for each transport property, a result contrary to theoret...

A theoretically based crossover model, which incorporates a crossover from singular thermodynamic behavior at the critical point to regular thermodynamic behavior far away from the critical point, is presented for the thermodynamic properties of ammonia. The equation is capable of representing the thermodynamic properties of ammonia between 398 and 500 K in an appreciable range of densities around the critical density. The crossover equation of state was constructed so that it can be used in conjunction with the classical equation of state developed by Tillner-Roth et al. (TR EOS), which works well outside the critical region. We have also compared our crossover equation of state with the widely used equation of state for ammonia developed earlier by Haar and Gallagher (HG EOS).

An equation, that incorporates a crossover from singular thermodynamic behavior asymptotically close to the critical point to regular thermodynamic behavior far away from the critical point, is presented for calculating the thermodynamic properties of sulfurhexafluoride in the critical region. The equation is constructed by applying a crossover transformation to a truncated classical Landau expansion. The equation is capable of representing the thermodynamic properties of sulfurhexafluoride at temperatures from 310 to 390 K in an appreciable range of densities around the critical density.

A global crossover equation of state that incorporates the classical van der Waals behavior (including the ideal-gas limit and the high-density hard-sphere limit) far away from the critical point and the singular behavior near the critical point has been developed. It is based on a transformation derived from the renormalization-group theory of critical fluctuations to the classical van der Waals equation of state. The crossover van der Waals equation contains only one non-universal system-dependent parameter associated with the range of intermolecular forces and it shows how the density fluctuations affect all thermodynamic properties more and more significantly when the fluid approaches the critical point. In particular, the critical compressibility factor which for the classical van der Waals equation is Z c =0.375, becomes suppressed to a more realistic value of 0.26–0.29 as is typical for simple fluids.

The Flory theory for phase separation in polymer solutions has been modified to incorporate critical fluctuations. The fluctuations change the shape of the coexistence curve near the critical demixing point and shift the critical temperature. Asymptotically close to the critical point Ising-like critical behavior is revealed, whereas classical (mean-field) Flory behavior is recovered far away from the critical region. The effects of fluctuations strongly depend on the degree of polymerization and disappear at the theta point in the infinite chain-length limit. The dependence of the effective critical exponents and critical amplitudes on the degree of polymerization in the crossover Flory model has been analyzed.

Experimental data reported in the literature indicate a strong dependence of the critical temperature of aqueous solutions of sodium chloride on salt concentration, especially in the limit of pure water. Critical pressures and densities also show strong dependences on salt concentration. We propose a new set of equations for the critical locus and compare the results with evaluations reported previously by a number of investigators. The new equations yield a better description of the experimental data from the limit of pure water to the highest salt concentration for which data are available (0.3 mass fraction of sodium chloride).

A description is presented of the construction and operating procedure of an improved guarded parallel-plate instrument for measuring the thermal conductivity of fluids in the critical region. With the improved arrangement, it has been possible to reduce the temperature differences across the fluid layer between the two horizontal plates to values as low as 0.3 mK, thus allowing the thermal conductivity of fluids to be measured at temperatures as close as 20 mK to the critical temperature. The instrument has been used to measure the thermal conductivity of methane and of a mixture of methane and ethane in the close vicinity of the critical point.

Excess Gibbs energy models which are widely used in modelling thermodynamic properties of multicomponent liquid phases are based on the assumption that long-range density or concentration fluctuations can be neglected. This assumption is no longer valid near a system's critical point, where large density or concentration fluctuations effectively mask the identity of the system and produce universal phenomena which have been well studied in simple liquid–vapor and liquid–liquid systems. Based on the Landau–Ginzburg–Wilson theory of fluctuations, a crossover procedure has been developed to incorporate the effects of critical fluctuations into a classical excess Gibbs energy model. As an example, we have applied our crossover procedure to the Non-Random Two-Liquid (NRTL) excess Gibbs energy model. This crossover procedure involves the use of transformed variables for temperature and concentration and the addition of a fluctuation term to the classical excess Gibbs energy. The resulting transformed Gibbs energy has the universal scaling behavior near the consolute critical point and has a smooth crossover to classical behavior far away from the consolute critical point.

We have investigated the kinetics of asphaltene aggregation in toluene+n-heptane mixtures with a dynamic light-scattering method adapted to opaque fluids. Two types of aggregation kinetics have been observed: diffusion-limited aggregation and reaction-limited aggregation. At asphaltene concentrations in toluene below the critical micelle concentration of about 3 g l −1 the aggregation kinetics appears to be solely limited by diffusion. Far above the critical micelle concentration reaction-limited aggregation takes place at least in the initial stage of particle growth. At a concentration of asphaltenes in toluene slightly above the critical micelle concentration, a crossover between these two types of aggregation kinetics has been observed. We propose a simple theoretical expression for the crossover kinetics and also a universal scaled description of the observed phenomena.

We have observed a sharp and nonmonotonic crossover of the susceptibility (osmotic compressibility) from mean-field to Ising critical behavior in semidilute solutions of polystyrene in deuterocyclohexane as the temperature decreases from the {Theta} region down to the critical temperature of the phase separation. We describe this crossover in terms of a competition between the long-range but finite intramolecular correlations of monomers in the polymer chain and the diverging correlation length of concentration fluctuations. {copyright} {ital 1997} {ital The American Physical Society}

Experimental thermal-conductivity measurements, obtained for an equimolar mixture of methane and ethane very close to the critical point, confirm that the thermal conductivity of a mixture does not diverge at the vapor-liquid critical point but crosses over to a finite limiting behavior at the critical point. A recently developed extension of the mode-coupling theory for dynamic critical phenomena in mixtures gives a good quantitative description of the experimental thermal-conductivity data.

We have developed a crossover scaling model for describing the thermodynamic properties of near-critical binary fluids. We have validated this model for the vapor-liquid equilibrium properties of mixtures of methane and ethane. We then apply the model to a most intriguing binary fluid, namely aqueous solutions of sodium chloride, so as to describe densities and concentrations at vapor-liquid equilibrium and isochoric specific heat capacities in the one-phase region. The description shows crossover from asymptotic Ising-like critical behavior to classical (mean-field) behavior. As the salt concentration increases, the region of Ising behavior decreases, although pure classical behavior is never attained. We suggest that processes of association/dissociation may be responsible for the failure of the model to describe compositions of the vapor phase of the solutions at very low concentrations.

This paper is concerned with theological properties of a phase-separating mixture in the presence of macroscopic shear flow. Specifically, we have investigated the shear viscosity of a nonionic micellar solution in shear-induced quasi-stationary states, where a dynamical balance between the growing concentration domains and the suppression by shear is established. During the phase separation, when concentration domains of the two phases with viscosity difference are formed, the measured viscosity of all mixtures as a function of temperature is found to satisfy to a good approximation a novel theological equation proposed by Onuki. In addition, we have determined that the observed characteristic temperature related to the crossover from the shear-affected regime to the shear-unaffected regime depends on the shear rate asS p , withp=1:vz≃0.51, wherev andz are universal critical exponents associated with the range and decay rate of the critical fluctuations. This power law depdendence onS. expected for a mixture at the critical composition, appears to be valid also for all off-critical mixtures.

A recently developed generalized isomorphism approach enables us to account for the critical behavior of thermodynamic and transport properties of fluid mixtures that exhibit continuous crossover from vapor-liquid critical phenomena to liquid-liquid critical phenomena. As an application we show here how the method can be used to predict thermodynamic and transport properties of methane + n-hexane mixtures in the one-phase region near their critical end points based on available liquid-liquid-vapor equilibria data for the mixtures and on the properties of the pure components.

The heat capacity of supercooled water, measured down to -37 {\deg}C, shows an anomalous increase as temperature decreases. The thermal diffusivity, i. e., the ratio of the thermal conductivity and the heat capacity per unit volume, shows a decrease. These anomalies may be associated with a hypothetical liquid-liquid critical point in supercooled water below the line of homogeneous nucleation. However, while the thermal conductivity is known to diverge at the vapor-liquid critical point due to critical density fluctuations, the thermal conductivity of supercooled water, calculated as the product of thermal diffusivity and heat capacity, does not show any sign of such an anomaly. We have used mode-coupling theory to investigate the possible effect of critical fluctuations on the thermal conductivity of supercooled water, and found that indeed any critical thermal-conductivity enhancement would be too small to be measurable at experimentally accessible temperatures. Moreover, the behavior of thermal conductivity can be explained by the observed anomalies of the thermodynamic properties. In particular, we show that thermal conductivity should go through a minimum as temperature is decreased, as Kumar and Stanley observed in the TIP5P model of water. We discuss physical reasons for the striking difference between the behavior of thermal conductivity in water near the vapor-liquid and liquid-liquid critical points., Comment: References added, typos corrected. Extrapolation for viscosity improved; results essentially unchanged

We present an equation of state for the thermodynamic properties of mixtures of methane and ethane in the critical region that incorporates the crossover from singular thermodynamic behavior near the locus of vapor-liquid critical points to regular thermodynamic behavior outside the critical region. The equation of state yields a satisfactory representation of the thermodynamic-property data for the mixtures in a large range of temperatures and densities.

Based on reliable. carefully selected data sets. equations for the thermal conductivity and the viscosity of the refrigerant R 112a are presented. They are valid at temperatures from 240 to 440 K, pressures up to 20 MPa. and densities up to 1050 kg · m−3. including the critical region.

A temperature gradient in a liquid mixture causes a concentration gradient through the Soret effect. We have developed an instrument to measure the Soret effect by observing the bending of a laser beam propagating horizontally through the liquid mixture subjected to a temperature gradient in the vertical direction. Our design of the liquid cell, with a long path length and controlled temperature uniformity, enables us to measure Soret coefficients with an accuracy of 1–3 %, higher than that obtained by previous investigators. In addition, by measuring the dynamic response of the beam deflection after imposition of the temperature gradient, we can also determine the mutual diffusion coefficient. We have applied the technique to mixtures of toluene and n‐hexane over the temperature range 5–45 °C and to mixtures of ethanol and water at 25 °C. We have verified that the measured value of the Soret coefficient is independent of the magnitude of the temperature gradient imposed up to 14 K/cm. The Soret coefficients obtained for mixtures of toluene and n‐hexane differ from the values obtained by previous investigators with a thermogravitational column method, but they are in good agreement with the results recently obtained by Kohler and Muller with a forced Rayleigh scattering method. For mixtures of ethanol and water, our Soret coefficients agree with the results obtained earlier by Kolodner et al., also with an optical beam‐bending technique.

We have measured the thermal conductivity of methane at temperatures from 308 K down to 190.585 K, which is just 21 mK above the critical temperature, and at densities up to 14 mol L−1. The data were obtained with an improved guarded parallel‐plate cell with a new cryostat that was built especially for measurements in the critical region of methane. The new experimental data have a higher accuracy than those reported previously in the literature and enable us to examine the validity of the currently available theoretical description of the asymptotic and nonasymptotic behavior of the thermal conductivity of fluids in the critical region. Equations for the thermal conductivity of methane in a wide range of temperatures and densities are also presented.

We extend the application of the mode‐coupling theory for the dynamics of critical fluctuations in fluids into the non‐asymptotic critical region. An approximate solution of the mode‐coupling equations yields a set of representative equations for the thermal conductivity and the viscosity of one‐component fluids which incorporates the crossover from asymptotic singular behavior near the critical point to the regular behavior of these transport properties far away from the critical point. The equations for the critical enhancements of the thermal conductivity and the viscosity depend on the background transport properties of the fluid, the equilibrium thermodynamic properties of the fluid, and one fluid‐dependent wave‐number cutoff qD, which is indicative of the crossover length scale. We compare our crossover model with experimental data for the thermal diffusivity, the thermal conductivity, and the viscosity of carbon dioxide and ethane.

A general isomorphism approach to critical phenomena in binary fluid mixtures that may exhibit complex critical-line behavior is developed by relating the two relevant scaling fields to linear combinations of three physical field variables. These physical field variables are related to the temperature and chemical potentials of the two components. The proposed approach includes crossover from vapor-liquid critical behavior to liquid-liquid critical behavior and incorporates also the critical behavior near other special points on critical loci. It is shown that the key factor which determines the apparent behavior of the thermodynamic and transport properties of near-critical mixtures is the shape of the critical locus. The number of system-dependent coefficients that determine the asymptotic critical behavior is elucidated. The choice of zero-points of entropy and energy in binary mixtures is also discussed. The approach provides a powerful tool for predicting thermodynamic and transport properties of fluid mixtures in the critical region.

An analysis of experimental data for the susceptibility of fluids and of liquid mixtures in the critical region has been performed to elucidate the character of the nonasymptotic critical behavior. While fluids and fluid mixtures exhibit an ultimate crossover to Ising-like asymptotic behavior, the effective susceptibility exponent {gamma}{sub eff} approaches the universal value {gamma}{congruent}1.24 either from {ital above} ({ital A}) or from {ital below} ({ital B}). We conclude that simple fluids belong to type {ital B}, whereas more complex systems may belong to type {ital A} and show a sharper nonmonotonic crossover from mean-field to Ising-like behavior. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.

We have investigated the dynamic behavior of a nonionic micellar solution of tetra-ethylene glycoln-decylether (C10 E4) in water near its critical point in the presence of shear. The non-Newtonian behavior of the viscosityν can be represented byν ν* = [ 1 +a(Sτ4)=]ω2, whereν* is the viscosity in the absence of shear,S is the shear rate. τ4 is the lifetime of the critical Iluctuations,a is a system-dependent constant, and ω = 0.02 In addition, we have found that, before attaining a steady state, the sheared mixture undergoing phase separation shows significant shear-dependent rheological effects due to the presence of concentration domains.