Behavior of Binary Liquid Solutions

Behavior of Binary Liquid Solutions Property changes of mixing and excess properties find greatest application in the description of liquid mixtures at low reduced temperatures, i.e., at temperatures well below the critical temperature of each constituent species. The properties of interest to the chemical engineer are V (= AV), (= AH), S, AS, G, and AC. The activity coefficient is also of special importance because of its application in phase equilibrium calculations. 

The volume change of mixing (V = AV), the heat of mixing (H = AH), and the excess Gibbs energy G are experimentally accessible, AV and AH by direct measurement and G (or In Ji) indirectly by reduction of vapor/liquid equilibrium data. Knowledge of H and 

The equations developed in preceding sections are for PVT systems in states of internal equilibrium. The criteria for internal thermal and mechanical equilibrium simply require uniformity of temperature and pressure throughout the system. The criteria for phase and chemical reaction equilibria are less obvious. 

If a closed PVT system of uniform T and P, either homogeneous or heterogeneous, is in thermal and mechanical equilibrium with its surroundings, but is not at internal equilibrium with respect to mass transfer or chemical reaction, then changes in the system are irreversible and necessarily bring the system closer to an equilibrium state. The first and second laws written for the entire system are 

The inequality applies to all incremental changes toward the equilibrium state, whereas the equality holds at the equilibrium state where change is reversible. 

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The behavior of binary liquid solutions is clearly displayed by plots of M , AM, and In y, vs. Xi at constant T and P. The volume change of mixing (or excess volume) is the most easily measured of these quantities and is normally small. However, as illustrated by Fig. 4-1, it is subject to individumistic behavior, being sensitive to the effects of molecular size and shape and to differences in the nature and magnitude of intermolecular forces. 

A brief discussion of sohd-liquid phase equihbrium is presented prior to discussing specific crystalhzation methods. Figures 20-1 and 20-2 illustrate the phase diagrams for binary sohd-solution and eutectic systems, respectively. In the case of binary solid-solution systems, illustrated in Fig. 20-1, the liquid and solid phases contain equilibrium quantities of both components in a manner similar to vapor-hquid phase behavior. This type of behavior causes separation difficulties since multiple stages are required. In principle, however, high purity... 

The lifetime (Ti) of a vibrational mode in a polyatomic molecule dissolved in a polyatomic solvent is, at least in part, determined by the interactions of the internal degrees of freedom of the solute with the solvent. Therefore, the physical state of the solvent plays a large role in the mechanism and rate of VER. Relaxation in the gas phase, which tends to be slow and dominated by isolated binary collisions, has been studied extensively (11). More recently, with the advent of ultrafast lasers, vibrational lifetimes have been measured for liquid systems (1,4). In liquids, a solute molecule is constantly interacting with a large number of solvent molecules. Nonetheless, some systems have been adequately described by isolated binary collision models (5,12,13), while others deviate strongly from this type of behavior (14-18). The temperature dependence of VER of polyatomic molecules in liquid solvents can show complex behavior (16-18). It has been pointed out that a change in temperature of a liquid solute-solvent system also results in a change in the solvent s density. Therefore, it is difficult to separate the influences of density and temperature from an observed temperature dependence. 

Since then. Dr. Woldfarth s main researeh has been related to polymer systems. Currently, his research topics are molecular thermodynamics, continuous thermodynamics, phase equilibria in polymer mixtures and solutions, polymers in supercritical fluids, PVT behavior and equations of state, and sorption properties of polymers, about which he has published approximately 100 original papers. He has written the following books Vapor-Liquid Equilibria of Binary Polymer Solutions, CRC Handbook of Thermodynamic Data of Copolymer Solutions, CRC Handbook of Thermodynamic Data of Aqueous Polymer Solutions, CRC Handbook of Thermodynamic Data of Polymer Solutions at Elevated Pressures, CRC Handbook of Enthalpy Data of Polymer-Solvent Systems, and CRC Handbook of Liquid-Liquid Equilibrium Data of Polymer Solutions. 

Adidharma and Radosz provides an engineering form for such a copolymer SAFT approach. SAFT has successfully applied to correlate thermodynamic properties and phase behavior of pure liquid polymers and polymer solutions, including gas solubility and supercritical solutions by Radosz and coworkers Sadowski et al. applied SAFT to calculate solvent activities of polycarbonate solutions in various solvents and found that it may be necessary to refit the pure-component characteristic data of the polymer to some VLE-data of one binary polymer solution to calculate correct solvent activities, because otherwise demixing was calculated. GroB and Sadowski developed a Perturbed-Chain SAFT equation of state to improve for the chain behavior within the reference term to get better calculation results for the PVT - and VLE-behavior of polymer systems. McHugh and coworkers applied SAFT extensively to calculate the phase behavior of polymers in supercritical fluids, a comprehensive summary is given in the review by Kirby and McHugh. They also state that characteristic SAFT parameters for polymers from PVT-data lead to... 

Usually, but not always, yT is the extreme (maximum or minimum) value assumed by 7/ for a component in a binary mixture. Hence, the 7,° are often used as measures of the magnitudes of nonidealities of binary liquid mixtures. Another measure is provided by gF or g /RT for the equimolar mixture for many binary solutions, this is near to the maximum (or minimum) value. For liquid solutions exhibiting positive deviations from ideal-solution behavior (activity coefficients greater than unity), a yl" of about 5 or an equimolar tFfRTof about 0.5 is considered large. ... 

Polymer solutions show deviations from the ideal behavior described in the previous section. In addition to intermolecular polymer-solvent interactions that make AH 0, chain connectivity causes ASm to deviate from the ideal case (eq. 3). When dealing with polymer solutions where one of the components is much larger than the other, significant changes need to be introduced in the treatment of binary liquid-liquid mixtures (2). 

For gas-liquid solutions which are only moderately dilute, the equation of Krichevsky and Ilinskaya provides a significant improvement over the equation of Krichevsky and Kasarnovsky. It has been used for the reduction of high-pressure equilibrium data by various investigators, notably by Orentlicher (03), and in slightly modified form by Conolly (C6). For any binary system, its three parameters depend only on temperature. The parameter H (Henry s constant) is by far the most important, and in data reduction, care must be taken to obtain H as accurately as possible, even at the expense of lower accuracy for the remaining parameters. While H must be positive, A and vf may be positive or negative A is called the self-interaction parameter because it takes into account the deviations from infinite-dilution behavior that are caused by the interaction between solute molecules in the solvent matrix. 

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