Solution critical behavior

NEW SOURCE OF CORRECTIONS TO SCALING FOR MICELLAR SOLUTION CRITICAL BEHAVIOR G. Martinez-Mekler, G. F. AI-Noaimi and A. Robledo ... 

New source of corrections to scaling for micellar solution critical behavior... 

Jacob J, Kumar A, Anisimov M A, Povodyrev A A. and Sengers J V 1998 Crossover from Ising to mean-field critical behavior in an aqueous electrolyte solution Phys. Rev. E 58 2188... 

M. L. Japas, J. M. H. Levelt Sengers. Critical behavior of a conducting ionic solution near its consolute point. J Chem Phys 94 5361-5368, 1994. 

The introduced THEOS did not bring about precipitation in protein solutions. This behavior differs from that observed with common silica precursors. For example, TEOS added in such small amounts caused precipitation. By using THEOS, we could prepare homogeneous mixtures. When its amount introduced into the albumin solution was less than 5 wt.%, there was no transition to a gel state (Table 3.1). A gradual increase in THEOS concentration resulted in a rise in the solution viscosity. The transition to a gel state took place as soon as a critical concentration was reached. Its value, as demonstrated in Ref. 

Further heating led to liquid-liquid phase separation observed as a cloud point. This is an example of LCST (lower critical solution temperature) behavior. 

This chapter deals with critical phenomena in simple ionic fluids. Prototypical ionic fluids, in the sense considered here, are molten salts and electrolyte solutions. Ionic states occur, however, in many other systems as well we quote, for example, metallic fluids or solutions of complex particles such as charged macromolecules, colloids, or micelles. Although for simple atomic and molecular fluids thermodynamic anomalies near critical points have been extensively studied for a century now [1], for a long time the work on ionic fluids remained scarce [2, 3]. Reviewing the rudimentary information available in 1990, Pitzer [4] noted fundamental differences in critical behavior between ionic and nonionic fluids. 

While the early work on molten NH4CI gave only some qualitative hints that the effective critical behavior of ionic fluids may be different from that of nonionic fluids, the possibility of apparent mean-field behavior has been substantiated in precise studies of two- and multicomponent ionic fluids. Crossover to mean-field criticality far away from Tc seems now well-established for several systems. Examples are liquid-liquid demixings in binary systems such as Bu4NPic + alcohols and Na + NH3, liquid-liquid demixings in ternary systems of the type salt + water + organic solvent, and liquid-vapor transitions in aqueous solutions of NaCl. On the other hand, Pitzer s conjecture that the asymptotic behavior itself might be mean-field-like has not been confirmed. 

As binary PPE/SAN blends form the reference systems and the starting point for the foaming analysis, their miscibility will be considered first. As demonstrated in the literature [41, 42], both miscibility and phase adhesion of PPE/SAN blends are critically dependent on the composition of SAN, more precisely on the ratio between styrene and acrylonitrile (AN). Miscibility at all temperatures occurs up to 9.8 wt% of AN in SAN, whereas higher contents above 12.4 wt% lead to phase separation, independent of the temperature. Intermediate compositions exhibit a lower critical solution temperature behavior (LCST). Taking into account the technically relevant AN content SAN copolymers between 19 and 35 wt%, blends of SAN and PPE are not miscible. As the AN content of the SAN copolymer, selected in this work, is 19 wt%, the observed PPE/SAN blends show a distinct two-phase structure and an interfacial width of only 5 nm [42],... 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

In recent years, studies of the phase behavior of salt-water systems have primarily been carried out by Russian investigators (headed by Prof. Vladimir Valyashko) at the Kurnakov Institute in Moscow, particularly for fundamental understanding of the phase behavior of such systems. Valyashko [37,39,42,43], Ravich [38], Urosova and Valyashko [40], and Ravich et al. [41] have given a classification of the existence of two types of salts, depending on whether the critical behavior is observed in saturated solutions. Type 1 does not exhibit critical behavior in saturated solutions. The classic example of Type 1 is the NaCl-water system and has been studied by many authors [36,37,44-47]. The Type 2 systems exhibit critical behaviors in saturated solutions, and therefore have discontinuous solid-liquid-vapor equilibria. Table 1 shows the classification of binary mixtures of salt-water systems. 

Irani, C. A. Cozewith, "Lower Critical Solution Temperature Behavior of Ethylene Propylene Copolymers in Multicomponent Solvents," J. Appl. Polym. Sci., 31, 1879 (1986). 

Binary-Liquid Option. As an alternative to this study of critical behavior in a pure fluid, one can use quite a similar technique to investigate the coexistence curve and critical point in a binary-liquid mixture. Many mixtures of organic liquids (call them A and B) exhibit an upper critical point, which is also called a consolute point. In this case, the system exists as a homogeneous one-phase solution for all compositions if Tis greater than... 

Liquid-Liquid Phase Separations and Critical Behavior of Electrolyte Solutions Driven by Long-Range and Short-Range Interactions... 

Electrolyte solutions are of long-standing interest, and in many respects our understanding of their thermodynamics is in a mature state. The discoveiy of liquid-liquid phase equilibria in such systems has, however, introduced new features. " Although already reported in 1903," and studied in more detail in 1963, such phenomena have remained almost unnoticed. New impetus in the this field has now come from interest in the critical properties of ionic fluids. Experiments at high temperatures have indicated that, at least on a first study, ionic fluids appear to exhibit classical critical behavior, as opposed to the /smg-like criticality of uncharged fluids. Recent experiments using liquid-liquid immiscibilities with critical points... 

Table 1 Liquid-liquid imtruniscibilities and critical behavior of electrolyte solutions...

Wang, Z.-Y., Zhang, Q.-Z., Konno, M., and Saito, S. 1991. Sol-gel transition of alginate solution by the additions of various divalent cations critical behavior of relative viscosity. Chem. Phys. Lett. 186(4,5) 463-466. 

We propose the study of Lennard-Jones (LJ) mixtures that simulate the carbon dioxide-naphthalene system. The LJ fluid is used only as a model, as real CO2 and CioHg are far from LJ particles. The rationale is that supercritical solubility enhancement is common to all fluids exhibiting critical behavior, irrespective of their specific intermolecular forces. Study of simpler models will bring out the salient features without the complications of details. The accurate HMSA integral equation (Ifl) is employed to calculate the pair correlation functions at various conditions characteristic of supercritical solutions. In closely related work reported elsewhere (Pfund, D. M. Lee, L. L. Cochran, H. D. Int. J. Thermophvs. in press and Fluid Phase Equilib. in preparation) we have explored methods of determining chemical potentials in solutions from molecular distribution functions. 

In the present article, we review recent progress in this subject area. In Sec. 2, we give a short overview on the chemical composition of the low melting salts and ILs. In Sec. 3 we address the problem of the electrolyte solution structure at conditions of low reduced temperature, where phase separations are known to occur. In Sec. 4, we consider experimental and theoretical results concerning the location of the two-phase regime in solutions of ionic fluids. In Sec. 5 we finally review theoretical and experimental results on near-critical behavior of ionic fluids. 

Japas, M.L., and Levelt Sengers, J.M.H. Critical Behavior of a Conducting Ionic Solution near its Critical Point. J. Phys. Chem., 1990, 94, p. 5361-68. 

Schroer, W., Kleemeier, M., Plikat, M., Weiss, V., and Wiegand, S. Critical behavior of ionic solutions in non-polar solvents with a liquid liquid phase transition. J. Phys. Condensed Matter, 1996, 8, p. 9321-7. 

Although by starting with Eq. 11.2-2 one can proceed directly to the calculation of the liquid-liquid phase equilibrium state, this equation does not provide in.sight into the reason that phase separation and critical solution temperature behavior occur. To obtain this insight it is necessary to study the Gibbs energy versus composition diagram for various mixtures. For an ideal binary mixture, we have (Table 9.3-1)... 

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