Systems with Higher Critical Temperatures

Mass transport in distillation and fractionation towers can sometimes be adversely affected by the generation of unwelcome, but transient, foam, which is a product of the intrinsic properties of the relevant liquids rather than any inadvertent contaminant. Ross and coworkers have drawn attention to the role played by partial miscibility of those liquids in determining that foam behavior (see, e.g., references [134-137]). Their studies concerned both binary and ternary mixtures of low molecular weight molecules, most of which were non-aqueous. Unlike the aqueous eth-oxylated and propoxylated non-ionic surfactant and polymer systems considered in Section 4.6.3.2, these binary systems often exhibit higher critical temperatures so that miscibility occurs with increasing temperature. 

Interfadal Tensions, Entry, Spreading, and Bridging Coefficients for the Partially Miscible System Methyl Acetate-Ethylene Glycol at 20 C 

Source Ross, S., Nishioka, G. Foaming behaviour of partially miscible liquids as related to their phase diagrams, in Foams Proceedings of a Symposium Organized by the Soc. Chem. Ind., Colloid and Surface Chem. Group, Brunei Univ., 1975 (Akers, R.J., ed.). Academic Press, London, p 17,1976. 

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To be able also to measure off-critical mixtures down to the binodal within a convenient temperature range, a mixture with a higher critical temperature than for the previous investigation of the critical behavior has been chosen. The system chosen was PDMS (Mw = 16.4 kg mol Afw/Mn = 1.10) and PEMS (Afw =... 

Most highly polar and ionic species are not amenable to processing with desirable solvents such as carbon dioxide or any other solvent such as water that has a higher critical temperature well above the decomposition temperature of many solutes. In such instances, the combination of the unique properties of supercritical fluids with those of micro-emulsions can be used to increase the range of applications of supercritical fluids. The resulting thermodynamically stable systems generally contain water, a surfactant and a supercritical fluid (as opposed to a non-polar liquid in liquid micro-emulsions). The possible supercritical fluids that could be used in these systems include carbon dioxide, ethylene, ethane, propane, propylene, n-butane, and n-pentane while many ionic and non-ionic surfactants can be used. The major difference between the liquid based emulsions and the supercritical ones is the effect of pressure. The pressure affects the miscibility gaps as well as the microstracture of the micro-emulsion phase. 

Ordinarily, solutions which exhibit positive deviations from Raoult s law are formed from their constituents with an absorption of heat. AHs is positive, therefore, and yA will be smaller at higher temperatures. For mixtures with negative deviations, the AHs is ordinarily negative. In both cases, therefore, the solutions ordinarily more nearly approach Raoult s law as the temperature is increased. Obvious exceptions to this rule are systems with lower critical solution temperatures, where, at least in the neighborhood of the lower C.S.T., the Raoult s law deviations become greater with increasing temperature. 

The cloud point curves of the epoxy monomer/PEI blend and BPACY monomer/PEI blend exhibited an upper critical solution temperature (UCST) behavior, whereas partially cured epoxy/PEI blend and BPACY/PEI blend showed bimodal UCST curves with two critical compositions, ft is attributed to the fact that, at lower conversion, thermoset resin has a bimodal distribution of molecular weight in which unreacted thermoset monomer and partially reacted thermoset dimer or trimer exist simultaneously. The rubber/epoxy systems that shows bimodal UCST behavior have been reported in previous papers [40,46]. Figure 3.7 shows the cloud point curve of epoxy/PEI system. With the increase in conversion (molecular weight) of epoxy resin, the bimodal UCST curve shifts to higher temperature region. 

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