The Presence of a Second Liquid Phase

The CMC of the surfactant in the aqueous phase is changed very little by the presence of a second liquid phase in which the surfactant does not dissolve appreciably and which, in turn, either does not dissolve appreciably in the aqueous phase or is solubilized only in the inner core of the micelles (e.g., saturated aliphatic hydrocarbons). When the hydrocarbon is a short-chain unsaturated, or aromatic hydrocarbon, however, the value of the CMC is significantly less than that in air, with the more polar hydrocarbon causing a larger decrease (Rehfeld, 1967 Vijayendran, 1979 Murphy, 1988). This is presumably because some of this second liquid phase adsorbs in the outer portion of the surfactant micelle and acts as a class I material (Section C). On the other hand, the more polar ethyl acetate increases the CMC of sodium dodecyl sulfate slightly, presumably either because it has appreciable solubility in water and thus increases its solubility parameter, with consequent increase in the CMC of the surfactant, or because the surfactant has appreciable solubility in the ethyl acetate phase, thus decreasing its concentration in the aqueous phase with consequent increase in the CMC. 

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This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Among the factors known to affect the CMC in aqueous solution are (1) the structure of the surfactant, (2) the presence of added electrolyte in the solution, (3) the presence in the solution of various organic compounds, (4) the presence of a second liquid phase, and (5) temperature of the solution. Some examples of the effects of these factors are apparent from the data in Table 3-2. 

Analogous expressions have been derived (Rosen, 1986) for the existence of synergism in mixed micelle formation in the presence of a second liquid phase and for the conditions at the point of maximum synergism. 

Experimentally the high-temperature and low-temperature LEV lines for a polymer-solvent mixture superpose onto the vapor pressure curve of the pure solvent. The EEV line at high temperatures is bounded at one end by a liquid -I- vapor fluid transition in the presence of a second liquid phase and, at the other end, by the intersection of the higher-temperature branch of the critical mixture curve where a liquid -I- liquid fluid transition occurs in the presence of a vapor phase. 

Van Elk et al. [27] used a similar mathematical model, based on the penetration model for three reactants in an ideally stirred reactor, to study the dynamic behavior of the gas-liquid homogeneous hydroformylation process. The influence of mass and heat transfer on the reactor stability in the Idnetically controlled regime was analyzed and it brought to mind the existence of a dynamically unstable (limit circle) state under certain operating conditions. This model needs to be extended to account for the presence of a second liquid phase in biphasic hydroformylation. 

In all the above mentioned cases conversion can only take place when the components are transferred to the catalytic phase or at least to the interface in which the reaction proceeds. Transport from one phase to the other(s) requires a driving force, i.e., the existence of concentration gradients. Figure 2 shows schematically the principal steps of a homogeneously catalyzed gas-liquid-liquid reaction (eq.(7)), where the reaction product P, is formed by the reaction between a gaseous reactant Ai and reactant A2 in the liquid phase 1 in presence of a second liquid phase which contains the catalyst. Both liquid phases are immiscible and Ai is only soluble in liquid phase 1. 

The presence of a second liquid (hydrocarbon) phase increases the — AG°d value by a few kJ/mol, with the increase being largest for cyclohexane (of the hydrocarbons investigated) and becoming smaller with increase in the chain length of the hydrocarbon. 

The interaction parameters in the presence of a second liquid (hydrocarbon) phase, (Sjj and 3, for mixed monolayer formation at the aqueous solution-hydrocarbon interface and for mixed micelle formation in the aqueous phase, respectively, can be evaluated (Rosen, 1986) by equations analogous to 11.1, 11.2 and 11.3, and 11.4, respectively. The necessary data are obtained from interfacial tension-concentration curves. 

FIGURE 6.6. The wetting of a solid by a liquid in the presence of a second fluid phase will be controlled by the relative interfacial interactions among the three phases. 

Liquid crystals, as the name implies, are condensed phases in which molecules are neither isotropically oriented with respect to one another nor packed with as high a degree of order as crystals they can be made to flow like liquids but retain some of the intermolecular and intramolecular order of crystals (i.e., they are mesomorphic). Two basic types of liquid crystals are known lyotropic, which are usually formed by surfactants in the presence of a second component, frequently water, and thermotropic, which are formed by organic molecules. The thermotropic liquid-crystalline phases are emphasized here they exist within well-defined ranges of temperature, pressure, and composition. Outside these bounds, the phase may be isotropic (at higher temperatures), crystalline (at lower temperatures), or another type of liquid crystal. Liquid-crystalline phases may be thermodynamically stable (enantiotropic) or unstable (monotropic). Because of their thermodynamic instability, the period during which monotropic phases retain their mesomorphic properties cannot be predicted accurately. For this reason it is advantageous to perform photochemical reactions in enantiotropic liquid crystals. 

The previous sections describe how mixing is accomplished in a liquid phase. However, many industrial processes carried out in stirred tank reactors involve mixing of solids, gases and other liquids in a continuous liquid phase. The presence of a second phase will affect both the power consumption and the flow pattern in the tank. In the sequel, the mixing phenomena caused by the presence of gas bubbles, liquid droplets and solid particles are discussed. 

Polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water. It has been proposed that the increase of compressibility with the decrease of temperature is related to a second critical point at the end of a coexistence line between two liquid phases, a low-density liquid (LDL) and a high-density liquid (HDL) [6]. This critical point would be located in the supercooled experimentally inaccessible region [7-9], In contrast, it is possible to explain the existing anomalies without invoking the presence of a critical point [9-11] and support the presence of a second critical without the need of polyamorphism [12],... 

Polymer blends usually consist of immiscible components and if miscible in the melt the polymers crystallize in mutually separate crystalline phases. The presence of a second phase can provide nudeation sites regardless of whether it is liquid or solid. Heterogeneous nuclei may... 

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