Phase behavior cosolvent effect

Cosolvent effects on SCF solution behavior allow the tailoring of solvents for extractions and separations. The strong interactions in these systems currently defy prediction by popular computational methods. Only by understanding these interactions at a molecular level will we be able to guide the development of phase equilibria models successfully. One way of exploring the molecular level interactions is with spectroscopy of various kinds and we have demonstrated here an attempt to look at the cosolvent/solute interaction. 

A limited number of studies have considered the use of surfactant and cosolvent mixtures to enhance the recovery of NAPLs (Martel et al., 1993 Martel and Gelinas, 1996). Martel et al. (1993) and Martel and Gelinas (1996) employed ternary phase diagrams to select surfactant+cosolvent formulatons for treatment of NAPL-contaminated aquifers. The surfactant+cosolvent formulations used in their work, which included lauryl alcohol ethersulfate/n-amyl alcohol, secondary alkane sulfonate/n-butanol, and alkyl benzene sulfonate/n-butanol, were shown to be effective solubilizers of residual trichloroethene (TCE) and PCE in soil columns (Martel et al., 1993). However, very little information is available regarding the effect of cosolvents on the solubilization capacity and phase behavior of ethoxylated nonionic surfactants. 

Phase behavior studies with poly(ethylene-co-methyl acrylate), poly (ethylene-co-butyl acrylate), poly(ethylene-co-acrylic add), and poly(ethylene-co-methacrylic acid) were performed in the normal alkanes, their olefinic analogs, dimethyl ether, chlorodifluoromethane, and carbon dioxide up to 250 °C and 2,700 bar. The backbone architecture of the copolymers as well as the solvent quality greatly influences the solution behavior in supercritical fluids. The effect of cosolvent was also studied using dimethyl ether and ethanol as cosolvent in butane at varying concentrations of cosolvent, exhibiting that the cosolvent effect diminishes with increasing cosolvent concentrations. 

It is well known that the phase behavior of solutes in SCCO2 can be modified by the addition of a small amount of cosolvent, such as ethanol. The main effect of a cosolvent is the solubility enhancement that results from an increase in the density of SCCO2 + cosolvent mixture or intermolecular interactions between the cosolvent and particular solutes. Selectivity of a separation can be improved by cosolvent addition only if there are specific intermolecular interactions between the cosolvent and one or more of the mixmre components, as solubility of aU mixture components is enhanced due to the density effect. 

Equations for the KBIs in ternary mixtures are available in matrix form [2]. Explicit equations are obtained here which will allow us to analyze interesting features of ternary mixtures, such as the effect of a third component on the phase behavior of a binary mixture and the effect of a cosolvent (entrainer) on supercritical binary mixtures. Only the former problem is examined in the present paper. The calculations will he carried out for an interesting ternary mixture, namely AA -dimethylformamide-methanol-water, in order to extract information about the intermolecular interactions. In the next section explicit equations for the KB integrals will he derived and applied to the above ternary mixture. Finally, the results obtained will be used to shed some light on the local structure and the intermolecular interactions in the above mixture. 

Cosolvent and co-surfactant effects on phase behavior and polarity... 

As we can see from the preceding discussion, including the cosolvent (alcohol) effect requires not only more experimental work, but also simulation work to find the parameters to describe the effect. In most practical cases, we just add the minimum amount of cosolvent, or alcohol (generally less than the amount of the surfactant used), when we select chemicals for a project. Alcohol adsorption is thought to be less than surfactant (CamiUeri, 1983). Trushenski et al. (1974) found that the adsorption loss of the isopropyl alcohol cosurfactant is negligible. Alcohol can work as a tracer. Thus, the chromatographic separation between surfactant and alcohol makes it more complex to include the alcohol effect in phase behavior calculation. 

Variables identified as important in the achievement of the low IFT in a W/O/S/electrolyte system are the surfactant average MW and MW distribution, surfactant molecular structure, surfactant concentration, electrolyte concentration and type, oil phase average MW and structure, temperature, and the age of the system. Salager et al. (1979b) classified the variables that affect surfactant phase behavior in three groups (1) formulation variables those factors related to the components of the system-surfactant structure, oil carbon number, salinity, and alcohol type and concentration (2) external variables temperature and pressure (3) two-position variables surfactant concentration and water/oil ratio. Some of the factors affecting IFT-related parameters are briefly discussed in this section. Some other factors, such as cosolvent, salinity, and divalent, are discussed in Section 7.4 on phase behavior. Healy et al. (1976) presented experimental results on the effects of a number of parameters. 

McHugh M A, Rindfleisch F, Kuntz PT, Schmaltz C, Buback M. Cosolvent effect of alkyl acrylates on the phase behavior of poly(alkyl acrylates)-supercritical CO2 mixtures. Pol5mier 1998 39 6049-6052. 

McHugh and coworkers have presented an extensive investigation of the effect of polar and hydrogen bonding cosolvents on the phase behavior of... 

LEE Lee, S.-H. and McHugh, M.A., The effect of hydrogen bonding on the phase behavior of poly(ethylene-co-acrylic acid)-ethylene-cosolvent mixtures at high pressures, Korean 7. Chem. Eng., 19, 114, 2002. 

Table I summarizes the qualitative changes in the phase behavior of microemulsions containing ionic surfactants. Some details of the effects of different variables are available in Ref. 13 and various chapters in this book. The phase transitions are generally understood in terms of relative strengths of hydrophilic and hydrophobic properties of the surfactant film in the microemulsion. The phase behavior depends strongly on the type and structure of the surfactant. For example, microemulsions containing nonionic surfactants are less sensitive to salinity but are more sensitive to temperature than those with ionic surfactants. The partitioning of cosolvents such as alcohols between the surfactant film, the organic phase, and the aqueous phase also affects the phase behavior. Microemulsions can be tailored for specific applications by adjusting an appropriate variable. For example, as indicated in Table 1, the effect of salinity on the phase behavior can be counterbalanced by an increase in the pH of an appropriate microemulsion [18,19].

KIM Kim, K. and Byun, H.-S., Cosolvent effect on phase behavior of poly(butyl methacrylate)-C02-butyl methacrylate system at high piessme, Hwahak Konghak, 38, 479, 2000. 

BYU Byun, H.-S., Kim, K., Kim, N.H., and Kwak, C., Cosolvent effect and phase behavior of poly(octadecyl methacrylate)-C02 mixtures at high pressiue, J. Korean Ind. Eng. Chem., 12, 212, 2001. 

Another important requirement for the development of new polymer processes based on SCCO2 is knowledge about the phase behavior of the mixture involved, which enables the process variables to be tuned properly to achieve maximum process efficiency. Determining parameters in the phase behavior of a system are the solvent quality, the molecular weight, chain branching, and chemical architecture of the polymer, as well as the effect of endgroups and the addition of a cosolvent or an antisolvent An overview of the available literature on the phase behavior of polymers in supercritical fluids has been published by Kirby and McHugh [50]. 

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. 

This study supports the assumption that the alcohol influences the phase behavior of oil-brine-surfactant system and its effect is related to the saturation concentration in the water-phase. On the other hand, the ternary phase diagram with a constant amount of the cosolvent seems to be more valuable for selecting microemulsion formulations than the pseudo-ternary representation in which both alcohol and surfactant are varied simultaneously. 

The cosolvent effect on the dimensions and shape of the particles is due to the formation of a layer around the monomer droplets and this layer controls the transfer of the monomer molecules from inside the droplets to the surrounding aqueous phase and can absorb the monomer molecules that are dispersed in the solvent e.g. in the presence of toluene, the size of the drop limits the dimension of the particle and also reduces the probability of collision, responsible of the coalescence. Moreover, the cosolvent acts as a hydrophobic layer and reduces the potential energy of the interface, leading to an overall stabilization of the emulsion and hence to regularly shaped particles a high polydispersity and irregular particle shapes were found for those samples prepared in the absence of toluene, while very low polydispersity and a regular round shape were found for the others this behavior is consistent with the study of Tanrisever et al. [284] on the polymerization kinetics of PMMA. 

Frequently, phase separation effected upon incorporation of additives or cosolvents is a major concern when developing novel fuel formulations. Microemulsion-based mixtures can overcome this problem, and has been the focus of more recent works. In that respect, Friberg and Force, in 1976, patented a diesel-based microemulsion formulation that could be used as fuel, with reduced NO emissious when compared to pure diesel [52]. Subsequent works have focused on phase behavior, stability, and performance of different mixtures, most of which involving surfactant-based mixtures [26,39], but it is worth considering the recent advances in the use of microemulsified systems incorporating other fluids like vegetable oils and alcohols. 

BY2 Byttn, H.-S., Bang, C.-H., and Lim, J.-S., Effect of cosolvent concentration on phase behavior for the poly(isodecyl acrylate) in supercritical carbon dioxide, propane, propylene, butane, 1-butene and dimethyl ether, J. Macromol. Sci., Part B Phys., 47, 150, 2008. 

Schmitt (1984) verified the entrainer behavior reported by Kurnik and Reid. Schmitt and Reid (1984) show that very small amounts of an entrainer in the SCF-rich phase have very little effect on the solubility of a second component in that phase. This observation is consistent with the work of Kohn and Luks for ternary mixtures at cryogenic temperatures. The data of Kurnik and Reid have been corroborated for the naphthalene-phenanthrene-carbon dioxide system (Gopal et al., 1983). Lemert and Johnston (1989, 1990) also studied the solubility behavior of solids in pure and mixed solvents at conditions close to the upper critical end points. Johnston finds that adding a cosolvent can reduce the temperature and pressure of the UCEP while simultaneously increasing the selectivity of the solid in the SCF-rich phase. In these studies Johnston found the largest effects with a cosolvent capable of hydrogen bonding to the solute. 

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