Solubilizates, hydrophobic

Hydrophobic solubilizates such as styrene (S) decrease the saponification rate of the EUP. Accordingly, the EUP-molecules in micelles containing S are more resistant against hydrolytic degradation than molecularly dissolved EUP-mole-cules. Obviously, the access of the base to the hydrophobic interior of these micelles and microemulsion droplets is more difficult. 

Solubilization is the formation of a thermodynamically stable, isotropic solution of a substance (the solubilizate), normally insoluble or slightly soluble in water, by the addition of a surfactant (the solublizer). The micelles of the surfactant cause solubilization of the substrate, producing an isotropic solution of the chemical. The solubilizate can be incorporated in the surfactant micelle in different ways, depending on the nature of the substrate and the surfactant micelles. For hydrophobic substrates, the molecules become incorporated in the hydrocarbon core of the micelle. With more polar substrates, the molecules may become incorporated in the hydrophilic PEO chains of the micelle or they may be simply adsorbed at the micelle surface. 

Lawrence (1937) first noted that the site of incorporation of solubilized molecules depended on their relative hydrophobic and hydrophilic tendencies. The solubilizate may be entrapped in the hydrocarbon core of the micelle, be oriented radially in the micelle with its polar group buried (deep penetration) or near the surface (short penetration), or be adsorbed on the surface of the micelle. Additionally, for non-ionic surfactants, incorporation of the solubilizate can occur in the polyoxyethylene shell of the surfactant. Fig. 3 illustrates several modes of solubilizate incorporation. It is important to realize, however, that solubilization, like micelle formation, is not a static but a dynamic equilibrium process. 

A neutral molecule solubilized in the micelle can be located in several positions or microenvironments. As early as the 1930s it was suggested by Lawrence that the site of a solubilized molecule would be dependent on the hydrophobic/hydrophilic composition of the solubilizate. Two extremes are easily identified the core of the micelle providing a hydrocarbon-like microenvironment, and the palisade layer providing an aqueous or water-rich interfacial environment. It seems logical to assume, then, that nonpolar solutes like alkanes would prefer the micellar core and that polar molecules would be anchored at the surface. However, this is an oversimplification available data tend to contradict it. First, the solubility of alkanes in micelles is significantly lower than expected if compared to solubility in hydrocarbon solvents. Second, the size of a micelle is normally such that part of the solute would be close to the surface at any time. Sepulveda et al. state that for SDS micelles at least half of the solute will be within 4 to 5 A of the surface. We should also consider the timescale of the experiments, as the timescale for intramicellar migration is short. The rate constants of entry and exit of molecules to and from micelles is of the order 1(F and... 

Lamellar liquid crystalline phases should be formed at the surface in order to attain good lubrication of a surfactant system. Surfactant molecular structure is one of the critical variables affecting the shape of the surfactant aggregates and, hence, the lubrication properties of surfactant systems. The aggregate shape is also determined by other factors, such as hydrophilic/hydrophobic additives, salt, solubilizates, and temperature. In addition, the aggregates present at the surface are also sensitive to surface properties as well as to co-ions. 

NMR methods. NMR can be used to obtain information on solubilization by measuring the shift in the peak positions on addition of the solubilizate. For example, by measuring IH NMR shift for a compound with an aromatic ring versus the concentration of a surfactant that contains no aromatic ring, e.g. SDS, one can determine the location of the solubilizate. This leads to an upheld shift of the IH peak, indicating a more hydrophobic environment. 

Phenolic solutes have been reported to decrease the cloud point of cetomacrogol [187]. A systematic study of the effect of phenols, cresols and xylenols on the cloud point of cetomacrogol has been carried out by Donbrow and Azaz [188] (see Chapter 6). All of these solubilizates caused a pronounced depression of the cloud point to room temperature, the effect of a phenol being inversely related to its hydrophobicity. 

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