Surfactant Solubilizing Groups

Surface activity is not limited to aqueous systems, however. AH of the combiaations of aqueous and nonaqueous phases are known to occur, but because water is present as the solvent phase in the overwhelming proportion of commercially important surfactant systems, its presence is assumed in much of the common terminology of industry. Thus, the water-soluble amphipathic groups are often referred to as solubilizing groups. 

Carboxylate, sulfonate, sulfate, and phosphate ate the polar, solubilizing groups found in most anionic surfactants. In dilute solutions of soft water, these groups ate combined with a 12—15 carbon chain hydrophobe for best surfactant properties. In neutral or acidic media, or in the presence of heavy-metal salts, eg, Ca, the carboxylate group loses most of its solubilizing power. 

For many solubilized enzymes the greatest catalytic activity and/or changes in conformation are found at R < 12, namely, when the competition for the water in the system between surfactant head groups and biopolymers is strong. This emphasizes the importance of the hydration water surrounding the biopolymer on its reactivity and conformation [13], It has been reported that enzymes incorporated in the aqueous polar core of the reversed micelles are protected against denaturation and that the distribution of some proteins, such as chymotrypsine, ribonuclease, and cytochrome c, is well described by a Poisson distribution. The protein state and reactivity were found markedly different from those observed in bulk aqueous solution [178,179],... 

The aqueous phase pH determines the ionization state of the surface-charged groups on the protein molecule. Solubilization of the protein in RMs is found to be dominated by electrostatic interactions between the charged protein and the inner layer of the surfactant head groups [112]. Solubilization of protein is favored at pH values above the isoelectric point (pi) of the protein in the case of... 

Since the water pool serves as the reaction medium, the characteristics of the solubilized water present within the polar core would be expected to influence the nucleation process. The properties of the water pool of relevance here include the proportion of free versus bound water molecules and the polarity of the solubilized water. When a volume of aqueous solution containing a given reactant is introduced into an initially dry surfactant-oil solution, a portion of the added water molecules will be immobilized through hydration of the surfactant polar groups. With increasing immobilization of the water molecules, reactions that require ionic dissociation (e.g., those involving weak acids and bases) will become less favorable. Thus, under such circumstances, an increase in the water content should enhance nucleation (see Fig. 5e). In the case of reactions involving hydrolysis, water serves as a reactant, and therefore a decrease in the availability of free water will lead to lowered nucleation rates (Fig. 5e). 

As one can see, the number of modifications of the simple dodecane molecule that can lead to materials with good surfactant characteristics is limited primarily by the imagination and skill of the organic chemist—and by the time and money available for indulgence in creative molecular architecture. In each example discussed, a solubilizing group has been added to the basic... 

For nonionic surfactants with polyoxyethylene solubilizing groups, the HLB may be calculated from the formula... 

Other structural factors, such as the charge on the surfactant head group, can significantly affect the locus of solubilization. Materials containing aromatic rings, for example, may be solubilized in or near the core of anionic systems but in the palisades layer of cationic surfactants, due to electronic interactions between the ring and the cationic head group. 

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