Systems with Lower Critical Temperatures

It is well-known that ethoxylated and propoxylated non-ionic surfactants and polymers exhibit high solubility in water at low temperatures and partial miscibility at elevated temperatures. This behavior derives from 

FIGURE 4. 36 Foamability as function of temperature of an aqueous solution of octyl phenyl ethoxylate (OP.EO97) at weight fraction of 0.01 surfactant (by Ross-Miles technique). (Reprinted with permission from Fineman, M. et al. J. Phys. Chem., 56, 963. Copyright 1952 American Chemical Society.) 

More than two decades later, Kruglyakov [41] and others [122, 123] argned that the phenomenon of rednction in foamability of ethoxylated nonionic snrfactants at temperatnres above the cloud point may concern the formation of oil lenses in foam films by non-ionic-rich cloud phase drops. Clear evidence that snch drops contribnte to an antifoam effect is presented by Koretskaya [123], who showed that removal of the cloud phase drops by hltration restores the foamability despite the reduction in overall surfactant concentration cansed by snch a procednre. 

Chaisalee et al. [125] again ascribed these effects to bridging of foam films by drops of the dispersed concentrated conjugate phase. However, measurements of the relevant bridging coefficients, although positive, are in violation of Equation 4.31. This presumably reflects experimental error. 

Lenses of the surfactant-rich conjugate phase at the air-water surface 

---

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. 

This effect shows the existence of two systems with different critical temperatures. In the imdeidoped and optimally doped phases only one transition temperature (Ti) is found. As fig. 85 indicates, the values fit without discontinuity to the optimally doped Tc values. The lower Td values in the overdoped range decrease with x so that, e.g., ATc Ri3.5K at x=6.976. More details about the splitting can be seen in fig. 86, showing the susceptibility curve of a x = 6.990 sample synthesized with 0. Also drawn is the susceptibility curve of the same sample after a site-selective isotope exchange with 0... 

Chand, A. McQuillan, A. R. Fenby, D. V. Thermodynamic study of systems with lower critical solution temperatures H20 + (C2H5)3N, D20 + (C2H5)3N Fluid Phase Equilib. 1979, 2, 263-274... 

Figure 2.7 The temperature-composition diagram for a system with a lower critical temperature, such as water and ethylamine...

Since there had not been any measurements of thermal diffusion and Soret coefficients in polymer blends, the first task was the investigation of the Soret effect in the model polymer blend poly(dimethyl siloxane) (PDMS) and poly(ethyl-methyl siloxane) (PEMS). This polymer system has been chosen because of its conveniently located lower miscibility gap with a critical temperature that can easily be adjusted within the experimentally interesting range between room temperature and 100 °C by a suitable choice of the molar masses [81, 82], Furthermore, extensive characterization work has already been done for PDMS/PEMS blends, including the determination of activation energies and Flory-Huggins interaction parameters [7, 8, 83, 84],... 

Fig. Ill-1. The interfacial tension, cr, as a function of the composition, x, of phases in contact and temperature, T, in a binary two-phase system with the upper critical temperature (a), lower critical temperature (b), critical temperature of mixing and a closed region where the separation into two phases occurs (c)...

In systems with the upper critical temperature, rcu, the surface tension decreases with increasing temperature, and consequently the excess of entropy within the surface layer is positive (Fig. III-l, a). In systems with the lower critical temperature, TCL, (Fig. III-l, b) the increase in interfacial tension is observed above the point at which system separates into two phases the value of tj is hence negative. The latter may serve as evidence for the existence of strong coorientation between molecules within the interfacial layer, which is due to the presence of directed chemical bonding, such as hydrogen bonding. 

Liihmann and Finkelmann also made the first published report of a nematic phase in a binary nonionic/water system (148). This was formed by discshaped micelles of the surfactant H2C=CH-CH2-O-0-0-O-CH2-COO(CH2CH2O)7CH3, being observed between the Li and phases in a narrow band of 34-38% surfactant between 7.5 and 23.4°C (Figure 21.24). The phase exists up to 72% surfactant (and to 80% in a bi-phasic region with water). Clouding is seen over a wide concentration range (up to > 90% surfactant) with a lower critical temperature of 33.2°C. 

We add here that there are not only polymer solutions with UCST behavior but also with lower critical solution temperature (LCST). They behave in opposite way with respect to temperature as systems with UCST that is they are homogeneous at low temperatures and decay upon heating into a polymer-poor and a polymer-rich phase. Phase separation proceeds above critical temperature. Aqueous solution of poly(Wisopropylacrylamide) (PNIPAM), Figure 4, is an example for a system exhibiting LCST behavior. The phase diagram of an aqueous solution is depicted in Figure 5. 

Systems with No Critical Solution Temperature. A large number of liquid pairs form systems without upper or lower critical points. In these cases, a solid phase forms before the appearance of a lower C.S.T. on cooling, and on heating, a vapor-liquid critical condition (vapor phase of the same composition and density as one of the liquid phases) occurs. Ether... 

Examples of transition curves (ac susceptibility) which undergo a return to the normal state at a lower critical temperature and the corresponding re-entrant reduced transition temperature vs Ce concentration curve for the (LaCelAU system are shown in figs. 11.9(a) and (b) (Maple et al., 1972). The data are also compared with the predictions of the Abrikosov-Gor kov (AG) theory (Abrikosov and Gor kov, 1961) (dashed line) which hold for superconducting matrix-impurity systems for which there is no Kondo effect > 0) and for which the degeneracy of the Hund s rule impurity ion multiplet is not lifted by a crystalline electric field. 

This entry is organized as follows In section Historical Aspects of Reliability, Durability and Cost Issues, historical aspects are first described to provide essential points of SOFC stack/system development. The technological features of the first-generation cells, namely, sealless tubular cells, will be described in comparison with the second- and the third-generation cells in critical technological issues these are materials selection of interconnect (oxide or metal), sealing scheme, redox issues of nickel cermets, metal support cells, trade-off relation between reliability and performance, and materials chemistry associated with lowering operation temperature. 

TA4 Tager, A.A., Lirova, B.I., Smolyanskii, A.L., and Plomadil, L.A., Phase equilibriums in systems with a lower critical temperature of mixing studied by an IR spectroscopic method, Vysokomol. Soedin., Ser. B, 17, 61, 1975. 

---