YangeYang critical anomaly strength parameter from the direct two- phase isochoric heat capacity measurements near the critical point (original) (raw)
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Chemical Physics Letters, 2012
Using adiabatic scanning calorimetry, we have found the first experimental evidence of the Yang-Yang anomaly in liquid-liquid criticality from high-resolution two-phase isobaric heat capacity measurements for the binary mixture 3-pentanol + nitromethane. The results suggest a rather strong effect. The critical amplitude of the partial molar heat capacity is higher for the component with larger molecular volume, in accordance with the predictions of complete scaling as obtained from the customary observed asymmetric behavior of the coexistence-curve diameter. This consolidates complete scaling as the true formulation of fluid-fluid criticality. The quantitative analysis indicates that molecular size is not the only microscopic factor at play in asymmetric liquid-liquid criticality.
Yang – Yang Critical Anomaly y
2017
Isochoric heat capacity, cV, is a thermodynamic property of fluids and fluid mixtures with a variety of scientific applications. cV is distinctly different from the commonly measured isobaric heat capacity, cP, which is often used by engineers in process design calculations. On the one hand, cV and cP show similar behaviors as a function of temperature and pressure in the singlephase (liquid or vapor) region. On the other hand, only cV is measurable inside the vaporþ liquid coexistence region, while cP is not a measurable property. In terms of a process explanation, with a pure substance in a calorimeter when P is held constant in the vaporþ liquid coexistence region, then energy added by heating (DH) serves solely to evaporate liquid to the vapor phase without increasing the temperature of the sample (DT1⁄4 0); thus, the apparent heat capacityE(DH/DT)p would have an indeterminant value. In this study, we will take advantage of cV measurements that are made inside, outside and on th...
Thermodynamic consistency near the liquid-liquid critical point
The Journal of chemical …, 2009
The thermodynamic consistency of the isobaric heat capacity per unit volume at constant composition C p,x and the density near the liquid-liquid critical point is studied in detail. To this end, C p,x ͑T͒, ͑T͒, and the slope of the critical line ͑dT / dp͒ c for five binary mixtures composed by 1-nitropropane and an alkane were analyzed. Both C p,x ͑T͒ and ͑T͒ data were measured along various quasicritical isopleths with a view to evaluate the effect of the uncertainty in the critical composition value on the corresponding critical amplitudes. By adopting the traditionally employed strategies for data treatment, consistency within 0.01 K MPa −1 ͑or 8%͒ is attained, thereby largely improving the majority of previous results. From temperature range shrinking fits and fits in which higher-order terms in the theoretical expressions for C p,x ͑T͒ and ͑T͒ are included, we conclude that discrepancies come mainly from inherent difficulties in determining the critical anomaly of accurately: specifically, to get full consistency, higher-order terms in ͑T͒ are needed; however, the various contributions at play cannot be separated unambiguously. As a consequence, the use of C p,x ͑T͒ and ͑dT / dp͒ c for predicting the behavior of ͑T͒ at near criticality appears to be the best choice at the actual experimental resolution levels. Furthermore, the reasonably good thermodynamic consistency being encountered confirms that previous arguments appealing to the inadequacy of the theoretical expression relating C p,x and for describing data in the experimentally accessible region must be fairly rejected.
International Journal of Thermophysics, 2020
PVT properties and the thermal-pressure coefficient, V = (P∕ T) V , of 2-propanol have been simultaneously measured in the near-and supercritical regions using hightemperature and high-pressure nearly constant-volume piezo-calorimeter. Measurements were made along 9 liquid and 3 vapor isochores between (234.3 and 697.69) kg•m −3 and at temperatures from (317.17 to 522.36) K at pressures up to 6.05 MPa. For each measured isochore (), the values of phase transition temperature (T S) and pressure (P S) at the liquid-gas phase equilibrium curve have been determined using the isochoric P − T break point and thermal-pressure coefficient abruptness techniques. The measured saturated liquid (′ S) and vapor (′′ S) densities near the critical point and thermal-pressure coefficient abruptness disappearance were used to estimate the values of the critical parameters (T C = 508.72 K, P C = 4.840 MPa, and C = 268.82 kg•m −3) of 2-propanol. The expanded uncertainty of the pressure (P), density (), and thermal-pressure coefficient (V) measurements at the 95 % confidence level with a coverage factor of k = 2 are estimated to be 0.05 %, 0.16 %, and 1.5 %, respectively. Scaling-type critical anomaly of the asymptotic behavior of thermal-pressure coefficient in the immediate vicinity of the critical point was experimentally observed. The measured pressures (PVT) and thermal-pressure coefficients (V VT) have been used to calculate of the other key thermodynamic properties such as internal pressure (P int), enthalpy (ΔH vap) and entropy (ΔS vap) of vaporization, saturation (C sat), isobaric (C P), and isochoric (C V) heat capacities.
International Journal of Thermophysics, 2007
Isochoric heat-capacity measurements for pure methanol are presented as a function of temperature at fixed densities between 136 and 750 kg·m −3 . The measurements cover a range of temperatures from 300 to 556 K. The coverage includes the one-and two-phase regions, the coexistence curve, the near-critical, and the supercritical regions. A high-temperature, high-pressure, adiabatic, and nearly constant-volume calorimeter was used for the measurements. Uncertainties of the heat-capacity measurements are estimated to be 2-3% depending on the experimental density and temperature. Temperatures at saturation, T S (ρ), for each measured density (isochore) were measured using a quasi-static thermogram technique. The uncertainty of the phase-transition temperature measurements is 0.02 K. The critical temperature and the critical density for pure methanol were extracted from the saturated data (T S , ρ S ) near the critical point. For one near-critical isochore (398.92 kg·m −3 ), the measurements were performed in both cooling and heating regimes to estimate the effect of thermal decomposition (chemical reaction) on the heat capacity and phase-transition properties of methanol. The measured values of C V and saturated densities (T S , ρ S ) for methanol were compared with values calculated from various multiparametric equations of state (EOS) (IUPAC, Bender-type, polynomial-type, and nonanalytical-type), scaling-type (crossover) EOS, and various correlations. The measured C V data have been analyzed and interpreted in terms of extended scaling equations for 163 0195-928X/07/0200-0163/0 © 2007 Springer Science+Business Media, LLC 164 Polikhronidi et al.
A new relation between the internal pressure and isochoric heat capacity jump of liquids along the coexistence curve near the critical point was found. Our previously reported one-and two-phase isochoric heat capacities and specific volumes at saturation were used to calculate internal pressure of molecular liquids (water, carbon dioxide, alcohols, n-alkanes, DEE, etc.).The internal pressure derived from the calorimetric measurements was compared with the values calculated from the reference (NIST, REFPROP) and crossover equations of state. Locus of the isothermal and isochoric internal pressure maxima and minima was studied using calorimetric data and the reference and crossover equations of state near the critical point. The maximum of the internal pressure of light and heavy water around the temperature of 460 K along the liquid saturation curve was found. We also found very simple relation between the internal pressure, ΔP int sat , and isochoric heat capacity, ΔC V , jumps near the critical point.
Molecular thermodynamics of solutions in the normal and critical regions — I. Theory
Chemical Engineering Science, 1973
The purpose of this work is to obtain parameters suitable for the correlation of activity coefficients which-unlike for example the van Laar constants-are independent of the pressure and the temperature of the system. Thus if the pressure-temperature-composition behaviour of a liq*lid mixture is known at one temperature, parameters correlated on the basis of these data may be used in the prediction of the pressure-temperature-composition behaviour at all other temperatures and pressures. First a working equation for the calculation of activity coefficients is derived using the methods of molecular thermodynamics. The method is based on the van der Waal's two-fluid model and a statistical mechanical formulation of the principle of corresponding states in conjunction with a simple two parameter equation of state. The two parameters in the equation of state are left as parameters to be determined in the derived working equation. The parameters are determined by fitting the working equation to pressure-temperature-composition data at one isotherm. The parameters obtained in this manner are then used in the derived working equation to predict activity coefficients at all other temperatures and pressures of the system.
Liquid Density and Critical Properties of Hydrocarbons Estimated from Molecular Structure
Journal of Chemical & Engineering Data, 2002
Correlations for estimation of thermophysical properties are needed for the design of processes and Ž . equipment related to phase equilibria. The normal boiling point NBP is a fundamental characteristic of chemical compounds, involved in many correlations used to estimate important properties. Modern simulation packages usually require the NBP and a standard liquid density from which they can estimate all other necessary properties and begin the design of particular processes, installations and flowsheets. The present work contributes a correlation between the molecular structure and the normal boiling point of hydrocarbons. Its main features are the relative simplicity, sound predictions, and applicability to diversified industrially important structures, whose boiling points and numbers of carbon atoms span a wide range. An achievement of particular Ž interest is the opportunity revealed, for reducing the number of the compounds required for the derivation the . learning set , through multivariate analysis and molecular design. The high accuracy achieved by the correlation opens up a possibility for systematic studies of chemical engineering applications in which the effects of small changes are important. This also defines a path towards the more general problem of the influence of uncertainties in calculated thermophysical parameters on the final outcome of computer aided simulation and design. q 0378-3812r99r$ -see front matter q 1999 Elsevier Science B.V. All rights reserved.
Physical Chemistry Chemical Physics, 2011
We validate here the Two-Phase Thermodynamics (2PT) method for calculating the standard molar entropies and heat capacities of common liquids. In 2PT, the thermodynamics of the system is related to the total density of states (DoS), obtained from the Fourier Transform of the velocity autocorrelation function. For liquids this DoS is partitioned into a diffusional component modeled as diffusion of a hard sphere gas plus a solid component for which the DoS(u) -0 as u -0 as for a Debye solid. Thermodynamic observables are obtained by integrating the DoS with the appropriate weighting functions. In the 2PT method, two parameters are extracted from the DoS self-consistently to describe diffusional contributions: the fraction of diffusional modes, f, and DoS . This allows 2PT to be applied consistently and without re-parameterization to simulations of arbitrary liquids. We find that the absolute entropy of the liquid can be determined accurately from a single short MD trajectory (20 ps) after the system is equilibrated, making it orders of magnitude more efficient than commonly used perturbation and umbrella sampling methods. Here, we present the predicted standard molar entropies for fifteen common solvents evaluated from molecular dynamics simulations using the AMBER, GAFF, OPLS AA/L and Dreiding II forcefields. Overall, we find that all forcefields lead to good agreement with experimental and previous theoretical values for the entropy and very good agreement in the heat capacities. These results validate 2PT as a robust and efficient method for evaluating the thermodynamics of liquid phase systems. Indeed 2PT might provide a practical scheme to improve the intermolecular terms in forcefields by comparing directly to thermodynamic properties. a Calculated from mean squared deviation-equation A.II.2a, ESIw. b Calculated from Green-Kubo formalism-equation A.II.2b, ESIw. c Ref. 92-94. d Values for F3C, SPC/E and TIP4P-Ew below taken from ref. 39.
Molecules
A universally applicable method for the prediction of the isobaric heat capacities of the liquid and solid phase of molecules at 298.15 K is presented, derived from their “true” volume. The molecules’ “true” volume in A3 is calculated on the basis of their geometry-optimized structure and the Van-der-Waals radii of their constituting atoms by means of a fast numerical algorithm. Good linear correlations of the “true” volume of a large number of compounds encompassing all classes and sizes with their experimental liquid and solid heat capacities over a large range have been found, although noticeably distorted by intermolecular hydrogen-bond effects. To account for these effects, the total amount of 1303 compounds with known experimental liquid heat capacities has been subdivided into three subsets consisting of 1102 hydroxy-group-free compounds, 164 monoalcohols/monoacids, and 36 polyalcohols/polyacids. The standard deviations for Cp(liq,298) were 20.7 J/mol/K for the OH-free compun...