Micelles of ionic surfactants

The importance of the material exchange process can hardly be overemphasized since it is the mechanism whereby the equUibrium miceUar size and polydispersity are reached and maintained, the reversed micelles of ionic surfactants become charged, polar and amphiphilic solubilizates are transported, and hydrophilic reactants can come in... 

It was recently ascertained that the behavior of the adsorbed film of two surfactants in equilibrium with their micelle can be explained by assuming both the surface region and the micelle particle to be mixtures of the surfactants (1 - ) - Further, the application of the regular solution theory to the mixtures was shown to be useful to describe the nonideal behavior of ionic surfactants ( - ) However, the above treatments are incomplete from the thermodynamic viewpoint, because they do not consider the dissociation of surfactants and ignore the presence of solvent (T). In addition, it is impossible to suppose that the regular solution theory is applicable to both the adsorbed film and the micelle of ionic surfactants accompanied by the electrical double layer ( ). 

This change Is caused by the interaction between the aromatic hydrocarbon and the polar group of the surfactant. This phenomenon has been clarified for normal micelles of ionic surfactants (24) for Inverse micelles material Is available only for nonlonlc surfactants. Christenson and collaborators (25-27) made an extensive study using NMR and calorimetry. The results of both studies agree showing a partition coefficient for benzene between the surfactant polar group and Its hydrocarbon chain of approximately 3. 

Polymers with bound charges, micelles of ionic surfactants, globular proteins, and similar materials are referred to as polyelectrolytes, and have the properties that separate them from normal low-molecular weight electrolytes... 

Synthetic polymers are widely applied to modify the surface properties of materials, and their adsorption mechanism is very different from small ions or molecules discussed in previous sections. Moreover, special methods are applied to study polymer adsorption, thus, polymer adsorption became a separate branch of colloid chemistry. Polymers that carry ionizable groups are referred to as polyelectrolytes. Their adsorption behavior is more sensitive to surface charging than adsorption of neutral polymers. Polyelectrolytes are strong or weak electrolytes, and the dissociation degree of weak polyelectrolytes is a function of the pH. The small counterions form a diffuse layer similar to that formed around a micelle of ionic surfactant. 

By experimentally measuring the turbidity as a function of concentration, one can obtain the molecular weight of solute and the second virial coefficient, B2, which characterizes interactions between solute molecules (or particles). In systems containing charged particles, such as e.g. micelles of ionic surfactants, the second virial coefficient describes the effective charge of particles. The molecular weight and the second virial coefficient can be determined by plotting the quantity Hc/x as a function of concentration, c. 

Fig. 1. Schematic presentation of a micelle of ionic surfactant in solution the fill symbols designate the polar heads of surfactant open symbols represent the counterions.

Harkins conclusions relate to the polymerization of hydrocarbon monomers which are miscible with their polymers and which have very low s(4ubilities in water, but which can be solubilized in larger quantities in the interior of the micelles of ionic surfactants. Most of the wartime work relates to the etmilsion copolymerization of butadiene and styrene, monomers which fulfil these critma. Model experiments concentrate on styrene homopolymerization because it can be handled more conveniently and is less toxic than some other common monomers. However, conclusions based on experiments with styrene may need modification before they can be extended to more polar monomers (e.g. methyl methacrylate, vinyl acetate) which have significantly higher solubilities in water or which have only limited miscibility with their polymer (e.g. acrylonitrile, vinyl chloride) or which produce polymras with a significant degree of crystallinity (e.g. vinylidene chloride, tetrafluoroethylene). 

Moreira L, Firoozaqbadi A (2010) Molecular thermodynamics modelling of specific ion effects on micellization of ionic surfactants. Langmuir 26 15177-15191 Nandi PK, Robinson DR (1972a) The effects of salts on the free energy of the peptide group. J Am Chem Soc 94 1299-1308... 

Expectedly, the outlook is quite different in case of non-ionic surfactants, like those belonging to poly(oxyethylene)alkyl or alkylphenyl ethers [61]. The hydrophilic part of these molecules can be in the form of chains longer than the corresponding hydrophobic part. An example is Triton X-100. As a result of the above, the structure of the polar interior of a reverse micelle of such amphiphiles does not resemble that of a reverse micelle of an ionic surfactant. The resemblance, indeed, is more with the interior of a normal micelle (of ionic surfactants). In reverse micelles of surfactants like Triton X-100, the polar interior can be invaded to an extent by a non-polar solvent like cyclohexane. 

Salt effects on the micellization of ionic surfactants are well known they can be substantial. It would therefore be no surprise to find related effects in the interaction of ionic... 

Paruchuri, V. K., Nguyen, A.V., and Miller, J. D. 2004. Zeta potentials of self-assembled surface micelles of ionic surfactants adsorbed at hydrophobic graphite surfaces. Colloids Surf. A 250 519. 

In Fig. 31, we present SLS data for micelles of ionic surfactant (sodium dodecyl dioxyethylene sulfate SDP-2S) in the presence of added NaQ (the upper line) and a mixture of NaCl and AIQ3 (the lower line) in both cases, the ionic strength of the solutions is the same, / = 24 mM. The surfactant concentration is small enough to have only small spherical micelles in the solution. The ratio... 

Micelles of ionic surfactants are aggregates composed of a compressive core surrounded by a less compressive surface structure/ and with a rather fluid environment (of viscosity 8-17 cP for solubilized nitrobenzene in SDS and cetyltrimethylammonium bromide micelles). Copper ions attached to micelles have essentially the same hydration shell near the micellar surface as in the bulk phase, and do not penetrate into the nonpolar part of the micelle. In addition, it is known that the volume change caused by binding of divalent metal ions to micelles is very small. The rate of rotation of the hydrated Na ion at the micellar surface is unlikely to change by more than 35% upon adsorption from the bulk to the Stem layer of SDS micelles. ... 

According to the mass action model of micellization of ionic surfactants [24], the equilibrium between surfactant monomers and micelles is given by [25)... 

Misra, P.K., Misra, B.K., Behera, G.B. Micellization of ionic surfactants in tet-rahydrofuran-water and acetonitrile-water mixed solvent systems. Colloids Surf. 1991, 57(1-2), 1-10. 

The effect of temperature changes on the micellization of ionic surfactants is not as simple a relationship as that found for most nonionic materials, and it is to be expected that the effects on solubilization will be correspondingly more complex. It has been reported that micellar solutions of dodecylamine hydrochloride saturated with xylene passed from a clear, isotropic solution to a turbid dispersion as the temperature was increased. It was noted in Chapter 4 that many ionic surfactants pass through a minimum in cmc near room temperature it would be interesting to know whether a maximum in solubilizing power is attained in the same temperature region as the minimum in cmc. 

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