Phase Equilibrium in Aqueous Systems

TAD Tada, E. dos S., Loh, W., and Pessoa-Filho, P. de A., Phase equilibrium in aqueous two-phase systems containing ethylene oxide-propylene oxide block copolymers and dextran, Fluid Phase Equil, 218, 221, 2004. 

SA3 Sadeghi, R., Vapor-liqrrid equilibrium in aqueous systems contaiiring poly(vinyl pyrrohdone) and sodirtm citrate at different temperatures. Experimeirt and modeling. Fluid Phase Equil, 249, 33, 2006. 

SA2 Sadeghi, R., Rafiei, H R., and Motamedi, M., Phase equilibrium in aqueous two-phase systems containing poly(vinylpyrrolidone) and sodium citrate at different temperatures. Experimental and modeling, Thermochim. Acta, 451, 163, 2006. 

Sadeghi R (2006) Vapor-liquid equilibrium in aqueous systems containing poly(vinylpyrrolidine) and sodium citrate at different temperatures - experimental and modeling. Fluid Phase Equilib 249 33-41... 

Virtuoso LS, Velio KASF, de Oliveira AA, Junqueira CM, Mesquita AF, Lemes NHT, de Carvalho RMM, da Silva MCH, da Silva LHM (2012) Measurement and modeling of phase equilibrium in aqueous two-phase systems L35 + sodium citrate + water, L35 + sodium tartrate + water, and L35 + sodium hydrogen sulfite + water at different temperatures. J Chem Eng Data 57 462-468... 

Three Phase Equilibrium in the System H2S—Propane—3M MDEA (Aqueous)... 

Franck, E. U. "Equilibrium in Aqueous Electrolyte Systems at High Temperatures and Pressures" in "Phase Equilibria and Fluid Properties in the Chemical Industry" Storvick, F. S. Sandler, S. I., Eds., ACS Symposium Series 60, American Chemical Society,... 

Phenomena at Liquid Interfaces. The area of contact between two phases is called the interface three phases can have only a line of contact, and only a point of mutual contact is possible between four or more phases. Combinations of phases encountered in surfactant systems are L—G, L—L—G, L—S—G, L—S—S—G, L—L, L—L—L, L—S—S, L—L—S—S—G, L—S, L—L—S, and L—L—S—G, where G = gas, L = liquid, and S = solid. An example of an L—L—S—G system is an aqueous surfactant solution containing an emulsified oil, suspended solid, and entrained air (see Emulsions Foams). This embodies several conditions common to practical surfactant systems. First, because the surface area of a phase increases as particle size decreases, the emulsion, suspension, and entrained gas each have large areas of contact with the surfactant solution. Next, because interfaces can only exist between two phases, analysis of phenomena in the L—L—S—G system breaks down into a series of analyses, ie, surfactant solution to the emulsion, solid, and gas. It is also apparent that the surfactant must be stabilizing the system by preventing contact between the emulsified oil and dispersed solid. Finally, the dispersed phases are in equilibrium with each other through their common equilibrium with the surfactant solution. 

In summary, whether a reaction equilibrium or a phase equilibrium approach is adopted depends on the size of the micelles formed. In aqueous systems the phase equilibrium model is generally used. In Section 8.5 we see that thermodynamic analyses based on either model merge as n increases. Since a degree of approximation is introduced by using the phase equilibrium model to describe micellization, micelles are sometimes called pseudophases. 

Here we summarize the results of a systematic theoretical examination of amino and imino tautomers in the systems schematically shown in Figure 3-17 based on DFT model chemistry in the gas phase and in water. In particular the equilibrium constant Ki = [amino tautomer]/[imino tautomer] for the process of Figure 3-17 was studied. The imino tautomers exhibit in general larger dipole moments and, hence are expected to have greater affinity to water and therefore a smaller Kj. In Table 3-14 the Kj values in the gas phase and in aqueous solution, obtained applying the MD module of DRF90 [87] for the solutes in 100 water molecules, are listed. 

Tautomeric equilibrium in aqueous cw-malonaldehyde, see reaction 1 in Figure 8-4, is a prototypical reaction extensively studied in the gas phase but still relatively unknown in solution. In fact, despite the large number of NMR experiments [52,53,54] and quantum chemical calculations [55] with the polarized continuum model (PCM), [1] the actual stability of czT-malonaldehyde is not well clarified, although the trans isomer should be the predominant form in water. Secondly, the involvement of the light proton in the reaction may in principle provide relevant quantum effects even in condensed phase. All these complications did not prevent this reaction to be used as a prototypical system for theoretical studies of intramolecular proton transfer in condensed phase by several investigators [56,57,58,59,60] including ourselves. 

The application of UNIFAC to the solid-liquid equilibrium of sohds, such as naphthalene and anthracene, in nonaqueous mixed solvents provided quite accurate results [11]. Unfortunately, the accuracy of UNIFAC regarding the solubility of solids in aqueous solutions is low [7-9]. Large deviations from the experimental activity coefficients at infinite dilution and the experimental octanol/water partition coefficients have been reported [8,9] when the classical old version of UNIFAC interaction parameters [4] was used. To improve the prediction of the activity coefficients at infinite dilution and of the octanol/water partition coefficients of environmentally significant substances, special ad hoc sets of parameters were introduced [7-9]. The reason is that the UNIFAC parameters were determined mostly using the equihbrium properties of mixtures composed of low molecular weight molecules. Also, the UNIFAC method cannot be applied to the phase equilibrium in systems containing... 

Helgeson, H.C., Brown, T.H. and Leeper, R.H., 1969a. Handbook of Theoretical Activity Diagrams Depicting Chemical Equilibrium in Geologic Systems Involving an Aqueous Phase at One Atmosphere and 0—300 C. Freeman and Cooper, San Francisco, Calif., 253 pp. 

The thermodynamic functions that describe this equilibrium include the equilibrium constant, the enthalpy, the free energy, and the heat capacity. These are all predictable, and can be derived by a variety of routes, each route yielding the same values for the functions. The equation describing the reaction is sufficient to allow for the initiation of all appropriate calculations. In contrast, the rate of the reaction, and the temperature dependence of the rate of the reaction are inherently unpredictable, and require empirical measurement. In particular, the equation describing the reaction stoichiometry cannot, a priori, enable the kinetic equations to be predicted. Detailed knowledge of the reaction mechanism would be required. This distinction between the inherent predictability of equilibrium conditions, and the empirical nature of kinetic conditions, must be borne in mind when considering the phase behavior of aqueous systems. 

U.v.-Visible Absorption Spectroscopy.—A review of currently available u.v.-visible spectrophotometers and accessories has been compiled by Tayler, Several sample cells have been reported allowing absorption spectra to be recorded under non-ambient sample conditions. A high-temperature cell, designed for a Cary model 15 spectrophotometer, has been employed in an investigation of the octahedral-tetrahedral equilibrium in aqueous solutions of cobalt(ii) compounds. A cell for a double-beam instrument (Beckman Acta M-VII) enabled studies of aqueous systems with temperatures up to 325 °C at maximum pressures of 12 MPa.Sample pressure and temperature variation was also possible in a study of volatile uranyl complexes in the gas phase using a home-built spectrophotometer. ... 

Although any of the designs mentioned above will provide the location of phase boundaries (versus temperature and pressure), it is also important to know the compositions of the two phases in equilibrium. Note that while tie lines (lines connecting phases in equilibrium on T-x or p-x diagrams) are horizontal for simple binary mixtures, this is not true for phase separation in multicomponent systems (most notably polymer-fluid systems where the polymer sample contains chains of various lengths). Consequently, ports which allow withdrawal of samples following phase separation and equilibration are an important feature of view cells. Such ports also allow for the measurement of partition coefficients of solutes between, for example, aqueous and CO2 phases. 

A solution with constant Th(N03)4 concentration of 0.0251 M was mixed with different concentrations of NaF at (25 + 0.1)°C to obtain molar ratios of F to Th of 1 to 10. The solubility and solid phase characterisations of the resulting precipitates were investigated at pH values ranging from 2.52 to 6.30. The solubility results are presented in a tabular form. The authors did not detect any soluble Th at F Th ratios of > 4.0 and the aqueous Th concentrations at F Th ratios of < 1.5 are veiy similar to the Th added initially. The authors report the presence of colloids at F Th ratios of < 3.8. The authors do not report any thermodynamic data, nor can any thermodynamic data be calculated because of the paucity of data points (only 4 points), lack of evidence for the presence of solid phase, and of equilibrium in this system. However, the authors provide convincing evidence, based on chemical, thermogravimetric, and X-ray diffraction analyses, that the compound that precipitates at higher NaF concentrations is NaThFs-HzO. 

Tsimpanogiarmis et al., [11] reported a comprehensive comparison of all the experimental data for the solubility of methane in the aqueous phase under hydrate equilibrium conditions against a nttmber of models that were based on the van der Waals-Platteeuw theory. The component-specific EoS for methane reported by Sun and Duan [13] has been considered. In addition, a nttmber of models that describe the gas solubihty in aqueous systems have been considered [14-17]. 

After equilibrium in the system has been attained, the aqueous feed phase is separated from the receiver phase emulsion globules, which are further demulsi-fied (e.g., by adding n-butanol) to form an organic liquid membrane layer and an aqueous receiver layer. After the separation of these two liquid layers the aqueous receiver solution is analyzed. 

The chemical potential of the solid crystal salt B is in phase equilibrium with the dissolved salt B in the liquid or aqueous phase. In aqueous systems we are primarily dealing with salts of strong electrolytes, which in water dissociate completely to the constituent cations and anions of the salt. The chemical potential of the dissolved salt is then given by... 

---