Vapor/liquid equilibrium solute/solvent systems

Vapor-liquid equilibrium experiments were performed with an improved Othmer recirculation still as modified by Johnson and Furter (2). Temperatures were measured with Fisher thermometers calibrated against boiling points of known solutions. Equilibrium compositions were determined with a vapor fractometer using a type W column and a thermal conductivity detector. The liquid samples were distilled to remove the salt before analysis with the gas chromatograph the amount of salt present was calculated from the molality and the amount of solvent 2 present. Temperature measurements were accurate to 0.2°C while compositions were found to be accurate to 1% over most of the composition range. The system pressure was maintained at 1 atm. 1 mm... 

The salt effects of potassium bromide and a series office symmetrical tetraalkylammonium bromides on vapor-liquid equilibrium at constant pressure in various ethanol-water mixtures were determined. For these systems, the composition of the binary solvent was held constant while the dependence of the equilibrium vapor composition on salt concentration was investigated these studies were done at various fixed compositions of the mixed solvent. Good agreement with the equation of Furter and Johnson was observed for the salts exhibiting either mainly electrostrictive or mainly hydrophobic behavior however, the correlation was unsatisfactory in the case of the one salt (tetraethylammonium bromide) where these two types of solute-solvent interactions were in close competition. The transition from salting out of the ethanol to salting in, observed as the tetraalkylammonium salt series is ascended, was interpreted in terms of the solute-solvent interactions as related to physical properties of the system components, particularly solubilities and surface tensions. 

Isobaric vapor-liquid equilibrium data at atmospheric pressure are reported for the four systems of the present investigation in Tables I-VI. Salt concentrations are reported as mole fraction salt in the solution, while mixed-solvent compositions are given on a salt-free basis. A single fixed-liquid composition was used for potassium iodide and sodium acetate potassium acetate used three—all chosen from the region of ethanol-water composition where relative volatility is highest. In the... 

Our work gives insight into the many problems that would be met in trying to account for the influence of the concentration of water and acid both on reactions and physicochemical processes that take place in a solvent such as sulfolane. Our results also indicate some possible methods for solving such problems. For example, our present vapor-liquid equilibrium data on solutions of water and acid in sulfolane were correlated with solution composition along lines previously used for the system NH3-Cu(II) salts in aqueous solution (35) in this latter system... 

Equation-of-state approaches are preferred concepts for a quantitative representation of polymer solution properties. They are able to correlate experimental VLE data over wide ranges of pressure and temperature and allow for physically meaningful extrapolation of experimental data into unmeasured regions of interest for application. Based on the experience of the author about the application of the COR equation-of-state model to many polymer-solvent systems, it is possible, for example, to measure some vapor pressures at temperatures between 50 and 100 C and concentrations between 50 and 80 wt% polymer by isopiestic sorption together with some infinite dilution data (limiting activity coefficients, Henry s constants) at temperatures between 100 and 200 C by IGC and then to calculate the complete vapor-liquid equilibrium region between room temperature and about 350 C, pressures between 0.1 mbar and 10 bar, and solvent concentration between the common polymer solution of about 75-95 wt% solvent and the ppm-region where the final solvent and/or monomer devolatilization process takes place. Equivalent results can be obtained with any other comparable equation of state model like PHC, SAFT, PHSC, etc. 

Such a process depends upon the difference in departure from ideally between the solvent and the components of the binary mixture to be separated. In the example given, both toluene and isooctane separately form nonideal liquid solutions with phenol, but the extent of the nonideality with isooctane is greater than that with toluene. When all three substances are present, therefore, the toluene and isooctane themselves behave as a nonideal mixture and then-relative volatility becomes high. Considerations of this sort form the basis for the choice of an extractive-distillation solvent. If, for example, a mixture of acetone (bp = 56.4 C) and methanol (bp = 64.7°Q, which form a binary azeotrope, were to be separated by extractive distillation, a suitable solvent could probably be chosen from the group of aliphatic alcohols. Butanol (bp = 117.8 Q, since it is a member of the same homologous series but not far removed, forms substantially ideal solutions with methanol, which are themselves readily separated. It will form solutions of positive deviation from ideality with acetone, however, and the acetone-methanol vapor-liquid equilibria will therefore be substantially altered in ternary mixtures. If butanol forms no azeotrope with acetone, and if it alters the vapor-liquid equilibrium of acetone-methanol sufficiently to destroy the azeotrope in this system, it will serve as an extractive-distillation solvent. When both substances of the binary mixture to be separated are themselves chemically very similar, a solvent of an entirely different chemical nature will be necessary. Acetone and furfural, for example, are useful as extractive-distillation solvents for separating the hydrocarbons butene-2 and a-butane. 

The major requirements for a successful ebulliometiy experiment are thermal stability, equilibration of both concentration and temperature, temperature measurement and control and pressure measurement and control. It is an advantage of ebulliometiy to know very exactly the constant pressure applied since pressure constancy is a prerequisite of any successful experiment. Commercially sold ebulliometers have seldom been used for polymer solutions. For application to polymer solutions, the operating systems have been individually constmcted. The above-mentioned reviews explain some of these in detail which will not be repeated here as ebulliometiy is not really a practiced method to obtain solvent activities and thermodynamic data in polymer solutions. However, ebulliometiy is a basic method for the investigation of vapor-liquid equilibrium data of common binaiy liquid mixtures, and we again point to the review by Williamson,where an additional number of equilibrium stills is shown. 

The easiest of the colligative properties to visualize is the effect of solute molecules on the vapor pressure exerted by a liquid. In a closed system, the solvent and its vapor reach dynamic equilibrium at a partial pressure of solvent equal to the vapor pressure. At this pressure, the rate of condensation of solvent vapor equals the rate of evaporation from the liquid. 

Included among the salts chosen for study were those that cause salting-out (NaBr, NaF, KCl, Li Cl) and salting-in (HgC ) of methanol in aqueous solutions. To test the technique described above, the vapor-liquid equilibria of systems of constant ratios of salt to solvent 2 were measured. For example, in cases where methanol is salted out, the experiments were done at constant salt-to-water ratios, and when methanol is salted in (salting-out of water), constant salt-to-methanol ratios were used. This was done by preparing a solution of a fixed salt molality and using it as component 2 in the equilibrium still. Thus, references to molality refer to the ratio moles of salt to 1000 g of solvent 2. 

When a nonvolatile solute is added to a solvent, the vapor pressure of the resulting solution will be lower than the vapor pressure of the pure solvent. Why does this occur To answer this, you have to remember back to our discussion of the equilibrium between vapor and liquid in a closed system. If you recall, equilibrium is reached when the rate that particles are escaping the liquid is equal to the rate that the particles are returning to the liquid. Also recall that these transitions take place on the surface of the liquid. Now, suppose you add a nonvolatile solute to the solvent. (The AP curriculum specifies that only the effects of nonvolatile solutes are required.) Because the solute is nonvolatile, it will not enter the gaseous phase above the liquid. What this means is that at the surface of the liquid, where particles of the solvent can escape, there are now some particles of the solute mixed in with the solvent. As a result, the number of solvent molecules that are able to escape will be less than the number that could have escaped in the pure solvent (they are blocked by the solute particles). The addition of a nonvolatile solute decreases the ability of solvent molecules to form vapors. This means that not as much vapor can form, which also means that there will be fewer gaseous molecules returning to the liquid state. Thus, the vapor pressure is reduced. 

When a nonvolatile solute is added to a solvent, the vapor pressure of the resulting solution will be lower than the vapor pressure of the pure solvent. Why does this occur To answer this, you have to remember back to our discussion of the equilibrium between vapor and liquid in a closed system. If you recall, equilibrium is reached when the rate that particles are escaping... 

We arrive at a similar limiting law from observing the behavior of solutions. For simplicity, we consider a solution composed of a volatile solvent and one or more involatile solutes, and examine the equilibrium between the solution and the vapor. If a pure liquid is placed in a container that is initially evacuated, the liquid evaporates until the space above the liquid is filled with vapor. The temperature of the system is kept constant. At equilibrium, the pressure established in the vapor is the vapor pressure of the pure liquid (Fig. 13.1a). If an involatile material is dissolved in the liquid, the equilibrium vapor pressure p over the solution is observed to be less than over the pure liquid (Fig. 13.1b). 

Osmosis is a colligative property and its theoretical treatment is similar to that for the lowering of vapor pressure. The membrane can be regarded as equivalent to the liquid-vapour interface, i.e. one that permits free movement of solvent molecules but restricts the movement of solute molecules. The solute molecules occupy a certain area at the interface and therefore inhibit solvent egress from the solution. Just as the development of a vapor pressure in a closed system is necessary for liquid-vapour equilibrium, the development of an OSMOTIC pressure on the solution side is necessary for equilibrium at the membrane. 

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. 

Many similarities exist between a saturated solution and a pure liquid and its vapor in a closed flask (see Section 12.2). For the liquid-vapor system, rates of vaporizing and condensing are equal for the solution, rates of dissolving and recrystallizing are equal. In the liquid-vapor system, particles leave the liquid to enter the space above it, and their concentration (pressure) increases until, at equilibrium, the space is saturated with vapor at a given temperature. In the solution, particles leave the solute to enter the solvent, and their concentration increases until, at equilibrium, the solvent is saturated with solute at a given temperature. 

For systems of the present type it is possible to obtain equilibrium information from two sources in the usual manner via the vapor pressmes of the solvent above the solutions within range HI (chemical potential of the solvent) and additionally from the saturatitMi composition w of the polymer (chemical potential of the polymer). The thermodynamic craisistency of these data was documented [52] by predicting w (liquid/soUd equilibrium) from the information of liquid/gas equilibria. This match of thermodynamic information from different sources is a further argument for the suitability to the present approach for the modeling of polymer-containing mixtures. 

We take up the topic of multicomponent equilibria by drawing a distinction between systems in which several or all components are present in the two equilibrated phases, and those in which only one component plays a key role by distributing itself in significant amounts between the phases in question. Vapor-liquid equilibria of mixtures and other similar multicomponent systems involving the appearance of several solutes in each phase are the prime example of the former, while distributions of a single component occur in a number of different contexts, which we take up in turn below. They include the equilibrium of a single gas with a liquid solvent, or a solid (gas... 

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