Cationic surfactants, reverse aggregate

Keywords Cationic surfactants, reverse aggregate of Interfacial water, structure of Reverse micelles Water-in-oil microemulsions... 

Abstract The enzymatic polymerization of ADP was carried out in sodium bis(2-ethylhexyl)sulfosuccinate (AOT)-reversed micellar solutions. The poly (adenylic acid), poly(A), being formed in the water pools precipitated out of the AOT solution together with the enzyme, whose activity was maintained for a long time. The process of the precipitation was studied in comparison with the polymerization in cationic surfactant reversed micelles and the precipitate aggregates were observed by atomic force microscopy (AFM). 

In the following years some more studies appeared in the literature concerning trypsin activity in reverse micelles in relation to various characteristics of the reaction medium. Fadnavis et al. studied the pH dependence of hydrolytic activity of trypsin in CTAB reverse micelles toward a positively charged model ester substrate [74]. It was found that enzyme activity variations as a function of w are pH dependent. In 2005, Dasgupta and coworkers related the catalytic activity of trypsin in reverse micelles formulated with cationic surfactants with the concentration of the water-pool components and the aggregate size to delineate the independent role of both parameters [75]. Finally, in 2006, the influence of ethylene glycol on the thermostability of trypsin in AOT reverse micelles was examined and was found to exhibit a positive effect [76]. 

The collapse of the gels is reversible as can be inferred from Figure 10.15, which presents the volume change on addition of an anionic surfactant to a DNA gel, which was collapsed by a cationic surfactant the anionic surfactant interacts strongly with the cationic surfactant, forming different mixed aggregates, and effectively extracts the cationic surfactant from the gel. 

Highly monodisperse reversed micelles are formed by sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) dissolved in hydrocarbons that are in equilibrium with monomers whose concentration (cmc) is 4 X 10 M, have a mean aggregation number of about 23, a radius of 15 A, exchange monomers with the bulk in a time scale of 10 s, and dissolve completely in a time scale of 10 s [1,2,4,14], Other very interesting surfactants able to form reversed micelles in a variety of apolar solvents have been derived from this salt by simple replacing the sodium counterion with many other cations [15,16],... 

Surfactants are classified on the basis of the charge carried by the polar headgroup as anionic, cationic, nonionic, and amphoteric. Surfactant headgroups are dipoles, especially ionic ones that exist as ion pairs in hydrocarbon solvents. Electrostatic dipole-dipole attraction between headgroups in hydrocarbon solvents is the driving force for the formation of reverse micelles, or micellar aggregates, see Fig. 3.1 and Fig. 3.2. 

Surfactants are organic molecules that possess a nonpolar hydrocarbon tail and a polar head. The polar head can be anionic, cationic, or nonionic. Because of the existence of the two moieties in one molecule, surfactants have limited solubility in polar and nonpolar solvents. Their solubility is dependent on the hydrophile-lipophile balance of their molecular structure. At a critical concentration, they form aggregates in either type of solvent. This colloidal aggregation is referred to as micellization, and the concentration at which it occurs is known as the critical micelle concentration. The term micelle was coined by McBain (7) to designate the aggregated solute. In water or other polar solvents, the micellar structure is such that the hydrophobic tails of the surfactant molecules are clustered together and form the interior of a sphere. The surface of the sphere consists of the hydrophilic heads. In nonpolar solvents, the orientation of the molecules is reversed. 

The self-assembling process involves noncovalent or weak interactions (van der Waals, electrostatic, and hydrophobic interactions, hydrogen and coordination bonds, and r-7T stacking). This process corresponds to a variation of reversed pore structures. Most ordered mesoporous materials are derived from the thermodynamically stable and ordered a regates spontaneously driven by the noncovalent interactions between molecules. These aggregates come from the cationic, anionic and nonionic surfactants, neutral amines, block copolymers, or their mixtures (Figure 13.1). They are disordered on the atomic or... 

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