Co-solvency

Rubino JT, Yalkowsky SH (1987) Co-solvency and co-solvent polarity. J Pharmacol Res 4 220-230... 

The parameter of interaction, %j23, between polymer and a two-component liquid can be used as a co-solvency criterion for linear polymers (or criterion of extreme swelling), more general, than the equality ( 2 = ) ... 

According to Schneider (see elsewhere in this volume) the physical origin of the holes has to be ascribed to the so-called co-solvency effect. In particular, when the two low-volatile components are chemically different, the critical pressures of the critical loci of the two binary border systems do not differ too much and when no specific interactions like, for instance, hydrogen bonding play a role, co-solvency is likely to have a strong influence on the multi-phase behavior in the ternary systems. For a detailed discussion on the relationschip between co-solvency and multiphase behavior, one is referred to else> bere in this volume and to [48] as well. 

Phase Equilibria. From recent research (Schneider and Peters) it became apparent that in the near-critical region of certain ternary carbon dioxide mixtures, due to co-solvency effects of the two solutes relative to each other, the fluid multiphase behavior can be quite complex. Phenomena like immiscibility windows and holes are not unusual, which have their consequences for separations in near-critical processing. Peters stressed that for many applications in supercritical technology carbon dioxide is not an appropriate choice since for many solutes it is a poor solvent that would require the use of a cosolvents. If safety and environmental constraints permit, it is certainly worthwhile to consider alternatives for carbon dioxide. Gulari, Schneider and Peters emphasized the importance of studying representative model systems in order to obtain insight into the systematic variations of the complex phase behavior that may occur in near-critical multicomponent mixtures. Debenedetti stressed the importance of focusing on complex fluids like emulsions. 

Solubility. Solubihty of a polymer will reflect the compatibility with the solvent. For a polymer to be dissolved there is a requirement for the forces between the solvent to match those between the elements of the polymer itself If the match is close, then it is possible to create a situation in which the dissolved polymer is in a collapsed state corresponding to the thermodynamic requirements of the theta condition. At this point the forces between the segments balance the forces between the segments and the solvent. If the strength of the interaction is increased, the quality of the solvent is improved and the effective size of the polymer increases it swells. Thermoplastic polymers can be completely dissolved in a good solvent, whereas thermoset materials are only able to swell to a size that reflects the cross-link density of the matrix. It is possible for two nonsolvents to combine to form a solvating mixture, and this phenomenon is termed co-solvency (see Miscibility). 

Fig. 9. Modeling by analogy to Fig. 8 but for the cases of co-solvency Oeft) and co-nonsolvency (right).

Supercritical fluid extraction using CO has long been an appealing technology to extract soybeans. At elevated temperatures and pressures (31°C and 73 bar, 88°F and 1,060 psi), a supercritical state is achieved where CO has the diffusivity of a gas and the density of a liquid and has significant oil solvency. A simplified flow diagram is... 

Figure 4.1 Relative solvency of 3M Co. s Novec hydrofluoroether cleaning solvents. Copyright 2000, 3M Co., reproduced with permission.)...

Williams, of Dow Coming Corporation, discusses the properties of linear volatile methyl siloxanes, a class of mild solvents having an unusual combination of environmentally benign qualities, that have the potential to replace less benign solvents in applications such as coating formulations or the removal of particulates, oils, fluxes, and aqueous contaminants. The chapter details data showing that their solvency can be tailored to specific applications by using azeotropes, co-solvents and surfactants. 

The high boiling point, increased solvency and low surface tension of 3M HFE-7100 make it suitable for use in vapor degreasing applications as a neat (pure), azeotropic or co-solvent parts cleaner. In addition, its chemical and thermal stability, non-flammability and low toxicity make it useful for other industrial applications such as specialty solvent and heat transfer applications. 

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