Micelles physicochemical properties

Because of their preferential use as detergents, the main interest in the physicochemical properties of the salts of a-sulfo fatty acid esters is related to their behavior in aqueous solution and at interfaces. In principle these are surface-active properties of general interest like micelle formation, solubility, and adsorption, and those of interest for special applications like detergency, foaming, and stability in hard water. 

The structure and dynamics of the reversed micelle hosting the solubilizate, as well as the physicochemical properties (structure, dynamics, and reactivity) of the solubilizate, are modified. 

Dynamic light-scattering experiments or the analysis of some physicochemical properties have shown that finite amounts of formamide, A-methylformamide, AA-dimethyl-formamide, ethylene glycol, glycerol, acetonitrile, methanol, and 1,2 propanediol can be entrapped within the micellar core of AOT-reversed micelles [33-36], The encapsulation of formamide and A-methylformamide nanoclusters in AOT-reversed micelles involves a significant breakage of the H-bond network characterizing their structure in the pure state. Moreover, from solvation dynamics measurements it was deduced that the intramicellar formamide is nearly completely immobilized [34,35],... 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

Incidentally, it is of interest to note that solutions of water-containing reversed micelles could be employed to study the physicochemical properties of nanosize solid water. 

Since very often the physicochemical properties of solubilizates are modified when they are entrapped in reversed micelles, almost all the experimental techniques can been used, and have been used, to study solubilization [28,31,118-122]. 

Sometimes, the physicochemical properties of ionic species solubilized in the aqueous core of reversed micelles are different from those in bulk water. Changes in the electronic absorption spectra of ionic species (1 , Co ", Cu " ) entrapped in AOT-reversed micelles have been observed, attributed to changes in the amount of water available for solvation [2,92,134], In particular, it has been observed that at low water concentrations cobalt ions are solubihzed in the micellar core as a tetrahedral complex, whereas with increasing water concentration there is a gradual conversion to an octahedral complex [135],... 

Interesting phenomena are observed by increasing the concentration of reversed micelles, changing the temperature or pressure, applying high electric fields, or adding suitable solutes, In some conditions, in fact, a dramatic increase in some physicochemical properties has been observed, such as viscosity, conductance, static permittivity, and sound absorption [65,80,173,233,243,249,255,264-269],... 

It was mentioned previously that the narrow range of concentrations in which sudden changes are produced in the physicochemical properties in solutions of surfactants is known as critical micelle concentration. To determine the value of this parameter the change in one of these properties can be used so normally electrical conductivity, surface tension, or refraction index can be measured. Numerous cmc values have been published, most of them for surfactants that contain hydrocarbon chains of between 10 and 16 carbon atoms [1, 3, 7], The value of the cmc depends on several factors such as the length of the surfactant chain, the presence of electrolytes, temperature, and pressure [7, 14], Some of these values of cmc are shown in Table 2. 

Estimation is easier and less time-consuming because use is made of empirical relationships between the BCF and physicochemical properties of the compound, such as water solubility (S) [42-48], Km, (solid organic carbon/water partition coefficient) [48], Kmw (membrane water partition coefficient), iipw (liposome water partition coefficient) [49], critical micelle concentration (CMC) [45], steric factors, molecular weight [47,48], and others. The most common regression method is the estimation of BCF from the octanol-water partition coefficient (Kovl) [18,42,44-48,50,51],... 

Part II starts with the possibilities of ACE for characterizing the relevant physicochemical properties of drugs such as lipophilicity/hydrophilicity as well as thermodynamic parameters such as enthalpy of solubilization. This part also characterizes interactions between pharmaceutical excipients such as amphiphilic substances (below CMC) and cyclodextrins, which are of interest for influencing the bioavailability of drugs from pharmaceutical formulations. The same holds for interactions of drugs with pharmaceutical vehicle systems such as micelles, microemulsions, and liposomes. 

Electrokinetic chromatography (EKC) using microemulsion is one of the most powerful tools for the rapid measurement of log P w with high reproducibility. Because it is relatively easy to manipulate the pseudostationary phases of EKC, a lot of phases have been reported for the measurement not only of physicochemical properties but also of the separation selectivity, such as polymer micelles (64) and double-chain surfactant vesicles (56-58,60,61). These phases are also interesting in terms of the correlation to bioactivity. 

N Chen, Y Zhang, S Terabe, T Nakagawa. Effect of physicochemical properties and molecular structure on the micelle—water partition coefficient in micellar electrokinetic chromatography. J. Chromatogr. A 678 327-332 (1994). 

Solubilization oftropicamide, a poorly water-soluble mydriatic/cycloplegicdrug, by poloxamers or Pluronics was studied (Saettone et al., 1988). The polymers evaluated as solubilizers for the drug included L-64, P-65, F-68, P-75, F-77, P-84, P-85, F-87, F-88, and F-127. The authors measured a range of physicochemical properties, such as solubility oftropicamide in polymer solutions, partition coefLcient of the drug between isopropyl myristate and copolymer solutions, critical micelle concentration of the copolymers, and viscosity of the copolymeric solutions containing tropicamide. 

Harada, A., Togawa, H. and Kataoka, K. (2001) Physicochemical properties and nuclease resistance of antisense-oligodeoxynucleotides entrapped in the core of polyion complex micelles composed of poly(ethylene glycol)-poly (L-Lysine) block copolymers. Eur. J. Pharm. Sci., 13, 35—42. 

Penetration enhancers have different mechanisms of action depending on their physicochemical properties. Some examples of penetration enhancers and their mechanisms are bile salts (micellization and solubilization of epithelial lipids), fatty acids such as oleic acid (perturbation of intracellular lipids) [25,26], azone (l-dodecylazacycloheptan-2-one) (increasing fluidity of intercellular lipids), and surfactants such as sodium lauryl sulfate (expansion of intracellular spaces). The complete list of enhancers and their mechanism of actions are discussed in detail in Chapter 10. 

The ability of micelles or related aggregates to alter reaction rates and selectivity has been an area of active research for the past several decades. Reactants are partitioned into the aggregates by coulombic and hydrophobic interactions the observed rate accelerations are largely a result of the increased localization of the reactants and also of the typical physicochemical properties of the micellar environment, which are significantly different from those of the bulk solvents. This unique ability of the aggregate systems has therefore prompted several scientists to employ micellar media for catalytically carrying out specific reactions. 

Bile salts are amphiphilic molecules that are surface active and self-associate to form micelles in aqueous solution. They increase corneal permeability by changing the rheological properties of the bilayer [231], A number of bile salts such as deoxy-cholate, taurodeoxycholate, and glycocholate have been tested so far, and it was suggested, that a difference in their physicochemical properties (solubilizing activity, lipophilicity, Ca2+ sequestration capacity) is probably related to their performance as permeability-enhancing agents [36]. 

As already mentioned, to rationalize the shape and size of the aggregate is difficult and also the number of studies on the relationship between structure and the physicochemical properties of these luminescent metalloaggregates remain limited so far (50,104,114,115), and mostly dealing with spherical micelles, even though few vesicular systems have been also reported (116). In addition, these aggregates have been studied mainly in aqueous solutions, while there are only few known metallosurfactants which aggregate in organic solvents (116,117). 

The solubilizing capacity for a given surfactant system is a complex function of the physicochemical properties of the two components which, in turn, influence the location or sites where the drug is bound to the micelle. The molar volume of the solubilizate together with its lipophilicity are important factors, the former reducing and the latter increasing solubilization. ... 

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