Equilibrium solubilization

Figure 4 Equilibrium solubilization of a water-insoluble drug by a CD. Abbreviations CD, cyclodextrin aq, aqueous.

Equilibrium Solubilization of Benzene in Micellar Systems and Micellar-Enhanced Ultrafiltration of Aqueous Solutions of Benzene... 

Ongoing research is investigating mechanisms of nonionic surfactant sorption onto soil, nonionic surfactant solubilization of hydrophobic organic compounds (HOCs) from soil, and microbial degradation of HOCs in soil-aqueous systems with nonionic surfactants. The equilibrium solubilization of HOC from soil can be described by a physicochemical model with parameters obtained from independent experiments. The microbial degradation of phenanthrene in soil-aqueous systems is inhibited by addition of alkyl ethoxylate, alkylphenyl ethoxylate, and Tween-type surfactants at doses that result in micellization and solubilization of phenanthrene from soil. 

In the next section a simple model is described that provides some understanding of the essentials of solubilization. Then some measurement techniques, experimental results, and theories of equilibrium solubilization are presented for micelles and microemulsions. Finally, dynamic phenomena such as solubilization rates and solubilization by intermediate phases formed after the solute contacts a surfactant solution are discussed. 

Here a is the drop radius, k a specific solubilization rate determined experimentally, c, the concentration of surfactant in micelles, and 0g and 9 the ratios of concentrations of the soluble species in the bulk and at the interface to the equilibrium solubilization capacity at c,. This equation for interfacially controlled transport is the counterpart to the well-known von Smoluchowski equation for diffusion-controlled transport ... 

Attempts are being made to determine non-aqueous phase liquid (NAPL) mole fraction and micelle-aqueous partition coefficients with rhamnolipids to understand equilibrium solubilization behaviour in surfactant-enhanced soil remediation situations [51]. A modification of Raoult s law was used for surfactant-enhanced solubilization but deviation from this ideal behaviour depended on the hydrophobicity of the compounds and the NAPL-phase mole fraction. Micelle-water partition coefficients were non-Unear in relation to the NAPL-phase mole fraction. Also enhancements by the surfactant were... 

The solubilization of diverse solutes in micelles is most often examined in tenns of partitioning equilibria, where an equilibrium constant K defines the ratio of the mole fraction of solute in the micelle (X and the mole fraction of solute in the aqueous pseudophase. This ratio serves to define the free energy of solubilization -RT In K). 

The product is equal to the equilibrium constant X for the reaction shown in equation 30. It is generally considered that a salt is soluble if > 1. Thus sequestration or solubilization of moderate amounts of metal ion usually becomes practical as X. approaches or exceeds one. For smaller values of X the cost of the requited amount of chelating agent may be prohibitive. However, the dilution effect may allow economical sequestration, or solubilization of small amounts of deposits, at X values considerably less than one. In practical appHcations, calculations based on concentration equihbrium constants can be used as a guide for experimental studies that are usually necessary to determine the actual behavior of particular systems. 

The two basic requirements for efficient bromine storage in zinc-bromine batteries, which need to be met in order to ensure low self-discharge and more over a substantial reduction of equilibrium vapor pressure of Br2 of the polybromide phase in association with low solubillity of active bromine in the aqueous phase. As mentioned by Schnittke [4] the use of aromatic /V-substitucnts for battery applications is highly problematic due to their tendency to undergo bromination. Based on Bajpai s... 

An example for a partially known ternary phase diagram is the sodium octane 1 -sulfonate/ 1-decanol/water system [61]. Figure 34 shows the isotropic areas L, and L2 for the water-rich surfactant phase with solubilized alcohol and for the solvent-rich surfactant phase with solubilized water, respectively. Furthermore, the lamellar neat phase D and the anisotropic hexagonal middle phase E are indicated (for systematics, cf. Ref. 62). For the quaternary sodium octane 1-sulfonate (A)/l-butanol (B)/n-tetradecane (0)/water (W) system, the tricritical point which characterizes the transition of three coexisting phases into one liquid phase is at 40.1°C A, 0.042 (mass parts) B, 0.958 (A + B = 56 wt %) O, 0.54 W, 0.46 [63]. For both the binary phase equilibrium dodecane... 

In a multiphase formulation, such as an oil-in-water emulsion, preservative molecules will distribute themselves in an unstable equilibrium between the bulk aqueous phase and (i) the oil phase by partition, (ii) the surfactant micelles by solubilization, (iii) polymeric suspending agents and other solutes by competitive displacement of water of solvation, (iv) particulate and container surfaces by adsorption and, (v) any microorganisms present. Generally, the overall preservative efficiency can be related to the small proportion of preservative molecules remaining unbound in the bulk aqueous phase, although as this becomes depleted some slow re-equilibration between the components can be anticipated. The loss of neutral molecules into oil and micellar phases may be favoured over ionized species, although considerable variation in distribution is found between different systems. 

Note that in equilibria (2) the subscripts per and cyt are omitted where substrate S is concerned. This is obvious when the binding is measured to a solubilized transport protein, but also in the case where the enzyme is embedded in the membrane of closed vesicular structures, internal and external substrate will have equal concentrations at equilibrium (see Eig. 5). Consequently, the binding is independent of the orientation of the enzyme in the membrane. 

A detailed descriphon of octanol-water distribuhon coefficient measurements by shake-flask can be found in publications by Dearden [2] and Hansch [24], The method usually involves the following solubilization of the compound in a mixture of mutually presaturated buffered water and octanol, agitation unhl equilibrium has been reached, careful separation of octanol and aqueous phases, and direct measurement of the solute concentration in both phases. Although seemingly simple, the method has a number of caveats making it inappropriate for some compounds. 

Depicted in Fig. 2, microemulsion-based liquid liquid extraction (LLE) of biomolecules consists of the contacting of a biomolecule-containing aqueous solution with a surfactant-containing lipophilic phase. Upon contact, some of the water and biomolecules will transfer to the organic phase, depending on the phase equilibrium position, resulting in a biphasic Winsor II system (w/o-ME phase in equilibrium with an excess aqueous phase). Besides serving as a means to solubilize biomolecules in w/o-MEs, LLE has been frequently used to isolate and separate amino acids, peptides and proteins [4, and references therein]. In addition, LLE has recently been employed to isolate vitamins, antibiotics, and nucleotides [6,19,40,77-79]. Industrially relevant applications of LLE are listed in Table 2 [14,15,20,80-90]. 

Two situations are found in leaching. In the first, the solvent available is more than sufficient to solubilize all the solute, and, at equilibrium, all the solute is in solution. There are, then, two phases, the solid and the solution. The number of components is 3, and F = 3. The variables are temperature, pressure, and concentration of the solution. All are independently variable. In the second case, the solvent available is insufficient to solubilize all the solute, and the excess solute remains as a solid phase at equilibrium. Then the number of phases is 3, and F = 2. The variables are pressure, temperature and concentration of the saturated solution. If the pressure is fixed, the concentration depends on the temperature. This relationship is the ordinary solubility curve. 

Conversely, the racemic film system appears to be solubilized by the achiral fatty acid component. At compositions of 10-33% palmitic acid, the ESP of the racemic system varies linearly with film composition, indicating that the monolayer in equilibrium with the racemic crystal is a homogeneous mixture of racemic SSME and palmitic acid. At compositions of less than 33% palmitic acid, the ESP is constant, indicating that three phases consisting of palmitic acid monolayer domains, racemic SSME monolayer domains, and racemic SSME crystals exist in equilibrium at the surface. 

The cyclodextrins are stable bodies in aqueous solution, unlike the micelles, which are transitory and are in a state of dynamic equilibrium with the monomer surfactants. However, in many aspects the inclusion of analytes in the cyclodextrin cavity is reminiscent of the solubilization of hydrophobic molecules in micelles in aqueous solution. 

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