Reversed micelles critical micelle concentration

A spiropyran compound bearing a pyridinium group and a long alkyl chain behaves as a surfactant. The components shown in Scheme 1 exhibit reverse photochromism in polar solvents. The colored merocyanine form is more stable than the spiropyran form in the dark. Upon photoirradiation at A>510 nm, the polar merocyanine form is converted to the hydrophobic spiropyran form so that the CMC (critical micelle concentration) of the surfactant decreases. Consequently, when the initial concentration is set between the CMC of the two forms, photoirradiation induces a sudden formation of micelles at a certain conversion to the spiropyran form corresponding to the CMC of the mixed micelle of the two forms. 

Critical Micelle Concentration (cmc) is the surfactant concentration below which the formation of reverse micelles does not occur, while the number of surfactant molecules per micelle is referred to as the aggregation number, n. The cmc is obtained through physical measurements, and varies from 0.1-1.0 mmol dm in water or the nonpolar solvents. 

The ideal permeation enhancer is safe, effective, pharmacologically inactive, chemically inert, and has a reversible effect (does not compromise the mucosal barrier function for extended periods) [10]. It is noticeable that the majority of the most widely investigated permeation enhancers have surfactant-like properties, and those that are water soluble seem to be most active at concentrations above the critical micelle concentration. The following have been investigated as a means of enhancing buccal permeability. 

Common surfactants that have been used in MEKC, are listed in Table 3.1 with the respective critical micelle concentrations the most popular are SDS, bile salts, and hydrophobic chain quaternary ammonium salts. Selectivity can also be modulated by the addition to the aqueous buffer of organic solvents (methanol, isopropanol, acetonitrile, tetrahydrofuran, up to a concentration of 50%). These agents will reduce the hydrophobic interactions between analytes and micelles in a way similar to reversed-phase chromatography. Organic modifiers also reduce the cohesion of the hydrophobic core of the micelles, increasing the mass transfer kinetics and, consequently, efficiency. Nonionic... 

However, increasing surfactant concentration has the drawback of reducing globule drop size and increasing the interfacial area available for mass transfer of both solute and water. This effect is enhanced in the case where the surfactant molecules themselves have an affinity for water [95,96]. If the surfactant concentration exceeds the critical micelle concentration (cmc), water transport in W/O/W systems by reversed micelles can occur [89,97]. An increase in concentration of some surfactants such as SPAN 80 also leads to an increase in the entrainment of the external phase during permeation promoted by an excess of surfactant molecules [71,98]. Miesiac et al. [99] found that in the case of penicillin G separation, the choice of surfactant could control not only the extraction rate, but also the back transfer rates of the hydrolysis products. 

Solubilization can be defined as the preparation of a thermodynamically stable isotropic solution of a substance normally insoluble or very slightly soluble in a given solvent by the introduction of an additional amphiphilic component or components. The amphiphilic components (surfactants) must be introduced at a concentration at or above their critical micelle concentrations. Simple micellar systems (and reverse micellar) as well as liquid crystalline phases and vesicles referred to above are all capable of solubilization. In liquid crystalline phases and vesicles, a ternary system is formed on incorporation of the solubilizate and thus these anisotropic systems are not strictly in accordance with the definition given above. 

We have examined the stmcture of both ionic and nonionic micelles and some of the factors that affect their size and critical micelle concentration. An increase in hydrophobic chain length causes a decrease in the cmc and increase of size of ionic and nonionic micelles an increase of polyoxyethylene chain length has the opposite effect on these properties in nonionic micelles. About 70-80% of the counterions of an ionic surfactant are bound to the micelle and the nature of the counterion can influence the properties of these micelles. Electrolyte addition to micellar solutions of ionic surfactants reduces the cmc and increases the micellar size, sometimes causing a change of shape from spherical to ellipsoidal. Solutions of some nonionic surfactants become cloudy on heating and separate reversibly into two phases at the cloud point. 

The fluorescent probe 4-aminophthalimide (63) was employed by several authors to study micelles. Samanta and coworkers187,188 used it to study micellization of common cationic (SDS), anionic (cetyltrimethylammonium bromide) and neutral (Triton-X 100) aqueous surfactants, and the same systems were also studied by Datta, Mandal and coworkers189 19°. The critical micelle concentration could be determined and it was found that the probe binds to the micelle water interface or to the cyclodextrin cavities that have also been studied187,188. Water-in-oil microemulsions of Triton-X 100 in a mixture of benzene and hexane showed190 the probe to reside in the water core of the reversed micelles, the polarity of which differs much from that of bulk water. 

Micelle formation has been studied for sodium salts of fatty acids containing terminal double bonds using electrical conductivity . Here two critical micelle concentration points were observed of which the first point at 0.044 moles litres" was critical in terms of the number average degree of polymerisation of the polymers produced. At concentrations up to the second point the molecular weight change was significantly smaller. The photopolymerisation of acrylamide in reverse micelles was found to be first order with respect to monomer concentration whilst the order was found to depend upon the oil concentration in the... 

The initial work at Bartlesville has concentrated on measurements of enthalpy changes from dilution and adsorption for surfactant systems. From the observed dilution enthalpy changes, critical micelle concentrations have been determined, and standard state enthalpies of micel lization have been calculated. In the studies on adsorption, several properties are of interest the enthalpy of adsorption, the amount of surfactant adsorbed, the surface area of the solid and determining whether the adsorption is reversible. The kinetics of adsorption and desorption are also of interest. 

Salting Out (Surfactant) In salting out, the addition of electrolyte to a solution of nonionic surfactant causes the critical micelle concentration to decrease. Salting in refers to the reverse. 

Formation of microemulsion was carried out using a small quantity of ionic surfactant, CTAB (cetyltrimethylammonium bromide), and de-ionized water in excess dried toluene. Above critical micelle concentration (cmc) these three components form reversed micelles. As the size of such micelle system is related to the ratio of water/surfactant (W) added, tailored amount of water to surfactant ratios in order to control the sizes of the nano-composites were investigated. 

To overcome this problem, our laboratories have initiated a program of study in the area of micellar liquid chromatography (MLC). The mobile phase in a MLC experiment consists of a surfactant that is at a concentration above the critical micellization concentration (cmc). We have learned that the addition of a co-surfactant to a micellar mobile phase will result in the formation of lamellar liquid crystals at the surface of the reversed phase (i.e., Cjg) material... 

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