Surfactant solutions aggregates

Lindman, B., Olsson, U., and Soderman, O. (1995) Surfactant solutions aggregation phenomenon and microheterogeneity, in Dynamics of Solutions and Fluid Mixes by NMR (ed. ).-). Delpuech), John Wiley Sons,... 

Nilsson, P. G. Lindman, B. Nuclear magnetic resonance self-diffusion and proton relaxation studies of nonionic surfactant solutions. Aggregate shape in isotropic solutions above the clouding temperature. J. Phys. Chem. 88,1984,4764-4769. 

The basic mechanism for surfactants to enhance solubility and dissolution is the ability of surface-active molecules to aggregate and form micelles [35], While the mathematical models used to describe surfactant-enhanced dissolution may differ, they all incorporate micellar transport. The basic assumption underlying micelle-facilitated transport is that no enhanced dissolution takes place below the critical micelle concentration of the surfactant solution. This assumption is debatable, since surfactant molecules below the critical micelle concentration may improve the wetting of solids by reducing the surface energy. 

An aqueous dispersion of a disperse dye contains an equilibrium distribution of solid dye particles of various sizes. Dyeing takes place from a saturated solution, which is maintained in this state by the presence of undissolved particles of dye. As dyeing proceeds, the smallest insoluble particles dissolve at a rate appropriate to maintain this saturated solution. Only the smallest moieties present, single molecules and dimers, are capable of becoming absorbed by cellulose acetate or polyester fibres. A recent study of three representative Cl Disperse dyes, namely the nitrodiphenylamine Yellow 42 (3.49), the monoazo Red 118 (3.50) and the anthraquinone Violet 26 (3.51), demonstrated that aggregation of dye molecules dissolved in aqueous surfactant solutions does not proceed beyond dimerisation. The proportion present as dimers reached a maximum at a surfactant dye molar ratio of 2 5 for all three dyes, implying the formation of mixed dye-surfactant micelles [52]. 

Surfactant solutions critical micelle concentration distribution of reactants among particles surfactant aggregation numbers interface properties and polarity dynamics of surfactant solutions partition coefficients phase transitions influence of additives... 

A Proposed Theory. In earlier publications (1-3), a theory was proposed to correlate solubilization rate, interfacial tension and size of the surfactant aggregate (1) the interfacial tension lowering between the oil-surfactant solution interface is a function of the rate of solubilization of oil, and (2) the rate of solubilization (AS/At) is a function of the effective volume for solubilization ... 

In general we found that the oil solubilization rate is a function of the surfactant aggregate size. The maximum eff for a series of surfactant solutions seems to occur at the condition that the surfactant associates to the maximum aggregate volume without increasing the density of the aggregate. The eff value seems to parallel the final solubilization value of the surfactant. 

It is well known that SDS and many commercial surfactants can not be used to recover oil. In our opinion these surfactants can not associate to form large enough aggregates is one important reason. In most cases, when electrolyte is added to the surfactant solution to increase the size of the aggregate, surfactant separation occurs before large enough aggregates can be built up. 

Surfactants can aggregate in nonpolar solvents in the presence of small amounts of water with the tails oriented towards the bulk nonpolar solution and head groups interacting with water in the center (Fig. 2). The water pool formed in reverse micelles has been used as a medium to study chemical and biological reactions [22]. 

Consider the formation of a mixed micelle in aqueous solution from a binary surfactant solution consisting of a nonionic and an anionic surfactant. The process is depicted as the aggregation of ng molecules of nonionic surfactant B, of n molecules of anionic surfactant A", and in addition there will be counterions, C" ", of the anionic surfactant in the amount of an where a is the fraction of the counterions associated or bound with the surfactant anions in the micelle. The process as depicted is... 

There s another example of water-in-oil compartmentation, which can circumvent this problem water-in-oil emulsions. These can be prepared by adding to the oil a small amount of aqueous surfactant solution, with the formation of more or less spherical aggregates (water bubbles) having dimensions in the range of 20-100 p,m in diameter. These systems are generally not thermodynamically stable, and tend to de-nfix with time. However, they can be long-lived enough to permit the observation of chemical reactions and a kinetic study. 

Exploiting the properties of aqueous surfactant solutions in which the surfactants aggregate to form micelles consisting of apolar cores comprised of the hydrophobic tail groups stabilized by coronae formed by the hydrophilic surfactant heads (Fendler and Fendler, 1975 Bunton, 1991). The apolar core plays the role of the organic solvent, whereas the palisade layer can provide a medium of intermediate polarity. 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

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