Size of micelles

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

The diffusion coefficients obtained with another fhiorophore (NBD derivative) were slightly different. The values of the aggregations numbers were found to be often overestimated because incorporation of the fluorescent probe may require extra surfactant molecules. However, the relative size differences between the micelles are in good agreement with the values reported in the literature. In addition to the size of micelles, FCS can give information on the size distribution. 

The term mixed micelle refers to those micelles composed of two or more surface active agents. The sizes of micelles in a solution obey a distribution function that is characteristic of their chemical composition and the ionic nature of the solution in which they reside. 

Pyrene carboxaldehyde and a series of pyrene carboxylic acids were found useful as fluorescence probes in describing the constitution of inverted micelles of certain calcium alkarylsulfonates in hydrocarbon media. 1-Pyrene carboxaldehyde is a convenient probe for studying the particle sizes of micelles in the region of lOOA. A series of graded probes, pyrene carboxylic acids with varying alkyl chain length, have been used to determine internal fluidity and micro-polarity as a function of distance from the polar core of these Inverted micelles. Pyrene exclmer to monomer fluorescence intensity ratio and fluorescene lifetime provided the means of measurement of internal fluidity and micropolarity, respectively. 

Reversed micelles can be used to concentrate water-soluble materials in the water pool of SCCO2. The extraction of water-soluble vitamins into reversed micelles has been examined. The efficiency of extraction was strongly affected by the extraction temperature and the concentration of reversed micelles, and the selectivity depended on the size of micelles. Water-soluble vitamins could be efficiently and rapidly extracted. The selective extraction of a model mixture of vitamins from pharmaceutical preparations was also demonstrated. Moreover, the usefulness of the proposed method for the determination of vitamins in various commercial tablets was also demonstrated. Using this method, the surfactant remains mixed with the extracted com-... 

An interesting example of extension of the size of micelles is provided by the system composed of hexadecyltrimethylammonium bromide acting as a surfactant, to which n-alkane acting as solubilizing agent was added [29]. 

Important parameters that control the size of micelles are the degree of polymerization of the polymer blocks, NA and NB, and the Flory-Huggins interaction parameter %. The micellar structure is characterized by the core radius Rc, the overall radius Rm, and the distance b between adjacent blocks at the core/shell-interface as shown in Fig. 1. b is often called grafting distance for comparisons to polymer brush models, b2 is the area per chain which compares to the area per head group in case of surfactant micelles. In the case of spherical micelles, the core radius Rc and the area per chain b2 are directly related to the number of polymers per micelles, i.e., the aggregation number Z=4nR2clb2. 

If the solute induces the formation of small aggregates surrounding the solute, this would produce two sizes of micelles. The surface area would be reduced as surfactant is used in the aggregates about the solute therefore, the size of the water would be decreased. 

Figure 4. Effect of surfactant concentration with varying surfactant ratio on size of micelles and on polymerization rate...

I. Rates of Polymerization with Recipes of Identical Size of Micelles. ... 

High p(n) associates with a suitable size of micelle. 

Dendrimers are highly branched polymer constmcts formed from a central core which defines their initial geometry. Their branchlike stmcture (Fig. 8.4) leads to spheres which in higher generations appear to be the size of micelles and ultimately nanospheres of small dimensions. They can be functionalised and in this way layered systems can be formed by using different monomers for succeeding... 

The size of micelles. Ionic micelles suitable to solubilize natural hydrocarbons would have median diameters of about 60 A. 

In all the measurements carried out in this study, for different ratios of NaDDS NaDOC, the dilution curves (Figure 1) of surfactant solution exhibited a clear break that corresponded with the c.m.c. as determined by other methods. This observation agrees with literature reports (1-11,18). The present data, however, show for the first time that mixed micelle systems also behave the same way as pure micellar systems, as measured by calorimetry. Further, because the aggregation number, N, of NaDDS Is much larger than that of NaDOC (16), Table I, the variation of enthalpy around the c.m.c. region is not related to the size of micelles. 

The CMC s of all micelle-forming surfactants are usually low (about 10 5 to 10 2 kmol m 3), i.e. low concentrations of molecular solutions correspond to the micelles 3 solution equilibrium. This means that the existence of particles with sizes d, different from the size of micelles, dm, is thermodynamically unfavorable (see Chapter VI, 1). The transition from particles of size dm to those with smaller sizes, hence, results in the increase in free energy of the system, and the A5r(log d) curve contains a minimum in the colloidal range at d=dm (see Fig. VI-1). 

For many nonionic surfactants, that are liquid, the Krafft point does not exist, and one uses the cloud point as a thermal characteristic. An increased cloudiness above the cloud point is related to the increased size of micelles and the separation of system into two phases due to the dehydration of polar groups at elevated temperatures. 

Figure 18 shows a plot of the effective volume fraction of the micelles as a function of the stepwise thickness transitions for anionic micellar solutions of sodium dodecyl sulfate (with a mean micellar diameter of 4.8 nm) and nonionic micellar solutions of ethoxylated alcohol with 30 ethoxy groups and 12—15 carbon atoms (with a mean micellar diameter of 10 nm) (61). These curves show the effect of both the effective concentration and the size of micelles on film thickness transitions. (The curves of the different systems can be compared because the mechanism of film mi-... 

At the c.m.c. one size of micelle predominates, for which i is 50—100, while xt becomes of the same order as x. The first term on the right-hand side of equation (11.4) can therefore be neglected, and an approximate expression for the standard frcc-cncrgy change of miccllisation per mole of surfactant is... 

Influences of the size of micelles cannot be expected in these reactions, since benzyl radicals are relatively long-lived and not reactive with respect to solvent molecules. However, such effects are observed when phenyl radicals are formed within micelles. Phenyl radicals in hydrocarbon solvents always give benzene as a product formed by abstraction of hydrogen from the solvent. This is also the case in large micelles of polyoxyethylene ( = 23) dodecylether (Brij-35) after intermediate generation of phenyl radicals by irradiation of diphenylmercury (125) (see Table 5) ... 

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