Micellar properties, effect

The effect of added inorganic salts on the micellar properties of the nonionic and cationic forms of dimethyl dodecyl amine oxide has been deterrnined (2). 

Micellar properties are affected by changes in the environment, eg, temperature, solvents, electrolytes, and solubilized components. These changes include compHcated phase changes, viscosity effects, gel formation, and Hquefication of Hquid crystals. Of the simpler changes, high concentrations of water-soluble alcohols in aqueous solution often dissolve micelles and in nonaqueous solvents addition of water frequendy causes a sharp increase in micellar size. 

Although the proposed theory has been used effectively in several practical applications, no experimental proof has been given that the oil solubilization rate is a function of surfactant aggregate size. In view of the importance of solubilization and the existence of practical methods of measuring and controlling surfactant aggregate size, we decided to correlate the solubilization rate with micellar properties for some anionic and nonionic surfactants. 

The purpose of this article is to review studies carried out on hemes incorporated inside the micellar cavity, and examine the effect of micellar interaction on the electronic and structural properties of the heme. A comparison of these results with those on the metalloproteins is clearly in order to assess their suitability as models. The article begins with a general introduction to micellar properties, the incorporation of hemes in the micellar cavity, and then discusses results on hemes inside the micelles with different oxidation and spin states, and stereochemistry. The experimental techniques used in the studies on these aqueous detergent micelles are mostly NMR and optical spectroscopy. The present article has therefore a strong emphasis on NMR spectroscopy, since this technique has been used very extensively and purposefully for studies on hemes inside micellar cavities. 

In investigating temperature effects on drug solubilization in micellarsystems, changes in the micellar properties as well as those in the aqueous solubility of the solute signiLcantly affect the solubilization... 

Briganti, G., S. Puvvada, and D. Blankschtein. 1991. Effect of urea on micellar properties of aqueous solutions of nonionic surfactants . Phys. Cherr65 8989-8995. 

Lavasanifar A., J. Samuel, and G. S. Kwon. 2001. The effect of alkyl core structure on micellar properties of polyethylene oxide)-block-poly(aspartamide) derivative .oll. Surf. B Bioint.22 115-126. 

McDonald, C. andC. K. V fong. 1974. The effect oftemperature on the micellar properties of a polyoxypropylene-polyoxyethylene polymer in wataf. Pharm. Pharmalol. 26 556-557. 

In this present study we report the effect of graded doses of y-irradiation on the properties of the micelles of Pluronic FI27 and on the temperature induced micellar changes which lead to the eventual gelation of these solutions. In view of the batch variability of the micellar properties of poloxamers (3) comparisons have been made with solutions of the same batch which have not been subjected to irradiation. 

Effect of Radiation Dose on Micellar Properties. Figure 1 shows the concentration dependence of the micellar diffusion coefficient at 40° as determined by quasi-elastic light scattering (QELS) for solutions subjected to radiation doses of up to 4.56 Mrad. Limiting diffusion coefficients, D0>were obtained by extrapolation of data for dilute solutions (<0.05%) to zero concentration, the critical micelle concentration (CMC) being negligibly low for this poloxamer ( 1 ). 

Table 1. Effect of Radiation Dose on the Micellar Properties of...

Effect of Temperature on Micellar Properties. Figure 5 compares the influence of temperature on the diffusion properties of the micelles in solutions previously irradiated with a dose of 4.56 Mrad with those not subjected to radiation treatment. Hydrated radii calculated from the limiting diffusion coefficients for micelles not treated with radiation remain independent of temperature over the range 25° to 40° (Table III). 

Table III. Effect of Temperature on the Micellar Properties of Irradiated and Non-irradiated solutions...

Since the x-value in the relationship of Rp oc Cg depends on the size of monomer-swollen micelles and the latter is, in turn, related to the solubilizing power of the monomer-free micelles and the hydrophobic properties of the monomers, the "micellar size effect" should predict the following ... 

There have been only a few neutron scattering studies that have examined the effects of additives on micellar properties but these do include some studies concerned with pharmaceutically relevant additives such as sugars or drugs. Despite limited work in this area, it should be noted that the technique of SANS is ideally suited to such studies, particularly when the surfactant and/or the additive are available in both hydrogenous and deuterated forms. 

Table 6.3 Effect of substituents on the micellar properties of some diphenylmethane drugs° R2...

The counterion associated with the charged group of ionic surfactants has a significant effect on the micellar properties. There is an increase in micellar size for a particular cationic surfactant as the counterion is changed according to the series Ch < Br < T, and for a particular anionic surfactant according to Na < < Cs. ... 

Table 6.5 Effect of electrolyte on the micellar properties of dodecyltrimethylammonium bromide CH3(CH2),...

Temperature has a comparatively small effect on the micellar properties of ionic surfactants. The temperature dependence of the cmc of sodium lauryl (dodecyl) sulfate shown in Fig. 6.29 is typical of the effect observed. 

T. Amarson and P. H. Elworthy. Effects of stme-tural variations of nonionic surfactants on micellar properties and solubilization surfactants based on emcyl and behenyl (C22) alcohols. /. Pharm. Pharmacol, 32, 381-5 (1980)... 

Potentiometric H2O Attwood D and Natarajan, R., Effect of pH on the micellar properties... 

E. Effects of Electrostatic Charges of Surfactants and Cosurfactants on Reversed Micellar Properties... 

These results show that simplified molecular dynamics simulations can qualitatively account for micellization quite well. However, the computation time necessary for even such simple models is too great to allow the model to be useful for the calculation of other micellar properties or phase behavior or for an in-depth study of solubilization. Stochastic dynamics simulations, in which the solvent effects are accounted for through a mean-field stochastic term in the equations of motion, can also be used to study surfactant self-assembly [22], but the most efficient approach to date is still the one based on lattice Monte Carlo simulations, which are discussed next. 

Tiible 2 Effect of Various System Parameters on Micellar Properties... 

A comparison of all the results reported here reveals that all the models reviewed predict qualitatively similar micellar behavior. Although the magnitudes of quantities such as the cmc or average aggregation number may be quite diflFerent for the different models (e.g., h t-i completely phase separates in Larson s model but forms well-behaved micelles in the simulations of Desplat and Care [31], because of the head-solvent attraction they used), each model predicts similar trends in these properties. This confirms the assumption that the solvophobic effect (i.e., the dislike of the solvophobic tail beads for the solvent) is the major driving force for micellization but also indicates that other forces are present that control specific micellar properties. 

The effects of solubilized additives on the micellar properties of nonaqueous surfactant systems vary according to the structures of Ae components. Such changes, however, are often greater than those found in aqueous solutions, so that due care must be exercised in evaluating the effects of even small additions on the aggregation characteristics of surfactants in nonaqueous solvents. 

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