Inverse micellar

THE W/O MICROEMULSIONS CONTAINING 502 HYDROCARBON ARE A DIRECT CONTINUATION OF THE INVERSE MICELLAR AREA AT 02 HYDROCARBON AND THE THREE STRUCTURE-FORMING ELEMENTS FOR THE AREA ARE SIMILAR... 

For some systems, at high concentration, inverse phases are observed. That is, one may generate an inverse hexagonal columnar phase (columns of water encapsulated by amphiphiles), or an inverse micellar phase (a bulk LC sample with spherical water cavities). 

Bicontinuous cubic phase Lamellar phase Bicontinuous cubic phase Reverse hexagonal columnar phase Inverse cubic phase (inverse micellar phase)... 

Figure 2.2 Representation of formation of (a) micellar and (b) inverse micellar vesicular assemblies from diblock and triblock copolymers, respectively.

Water-in-fluorocarbon emulsions, stabilised with fluorinated nonionic surfactants, were investigated by small angle neutron scattering (SANS) spectroscopy [8,99]. The results indicated that the continuous oil phase comprised an inverse micellar solution, or water-in-oil microemulsion, with a water content of 5 to 10%. However, there was no evidence of a liquid crystalline layer at the w/o interface. A subsequent study using small angle x-ray scattering (SAXS) spectroscopy gave similar results [100]. 

Several studies have reported on the enhanced bioavailability of cutaneous drugs using o/w and w/o MEs compared to conventional emulsions, gels or solutions, mesophases, micellar and inverse micellar systems, and vesicles [93], Moreover, a diverse range of drug molecules such as ketoprofen, apomorphine, estradiol, lido-caine [94-97], indomethacin and diclofenac [98], prostaglandin Ei [99], aceclofenac... 

Hence the different phases may be visualized as a series of association structures with increasing complexity from the monomeric to the liquid crystalline state. The transfer from the monomeric state to the inverse micellar structure is discussed for two special cases and it is shown that packing constraints may prevent the formation of inverse micelles. Instead a liquid crystalline phase may form. 

This article will, in addition to a short description of the essential features of surfactant systems in general, concentrate on the energy conditions in premicellar aggregates, the transition premicellar aggregates/inverse micellar structures and the direct transition premicellar aggregates/lyotropic liquid crystals. 

The inverse micellar solubility areas in systems of water, surfactants and a third amphiphilic substance frequently are of a shape according to Fig. l. iZ/ Such shapes are also found in W/0 microemulsions —when water solubility is plotted against cosurfactant/surfactant fraction. 

Figure I. The solubility area of water in a hydrocarbon (CeH,) perUa(ethylene gly-c< ) dodecyl ether (C,t[EO]s) solution at 30°C. Key IM, inverse micellar solution.

Figure 2. The phase diagram water/benzene/pentafethylene glycol) dodecyl ether at 30°C. Key IM, inverse micellar solution LLC, lamellar liquid crystal and unmarked, aqueous micellar solution.

The two inverse micellar solutions will first briefly be described followed by an evaluation of the available experimental and theoretical information. 

For soap/alcohol combinations — g will depend not only on the soap counter ion but also on the alcohol/soap ratio. Furthermore, when a certain alcohol/soap ratio is exceeded (=2 for the potassium oleate system) S becomes Independent of the water content of the lamellar phase. This condition applies for Inverse structures and the water/pentanol/potassium oleate inverse micellar system will be examined for the structure determining ratio in Table I. 

A pentanol/potasslum oleate ratio of 15 that Is typical of the inverse micellar solution gives the corresponding value 1.02. Formally the two values are straddling the value v/a l = 1 in the correct directions, but it is obvious that they are extremely similar and the application of the zeroth order approachto these systems must be viewed with caution. The pronounced influence of a partition of cosurfactants between the Interface and the organic bulk is evident. 

It has been shown that the presence of strong Intermolecular forces may have a drastic effect on the stability of premlcellar aggregates In Inverse micellar systems. 

Delacroix, H., Gulik-Krzywicki, T., and Seddon, J.M. (1996). Freeze fracture electronmicro-scopy of lyotropic lipid systems quantitative analysis of the inverse micellar cubic phase of space group Fd3m (Q227). J. Mol. Biol. 258, 88-103. 

Solubility of Sodium Chloride in an Inverse Micellar Solution of Pentanol... 

The solubilization of an aqueous sodium chloride solution by potassium oleate in the pentanol isotropic solution was determined. The presence of sodium chloride increased the minimum concentration for solubilization, reduced the maximum solubilization at high pentanohpotassium oleate ratios, and altered this ratio to lower values for maximal solubilization of the electrolyte solution. The increased minimum amount of electrolyte solution for solubilization arose from the fact that no micelles were present at the lowest fractions of water in the pentanol solution. The increased potassium oleate. pentanol ratio for maximal solubilization of the electrolyte was related to the destabilization of the lamellar liquid crystal with which the inverse micellar pentanol solution of high water content was in equilibrium. 

These inverse micelles will solubilize electrolytes in their aqueous core but the presence of the electrolytes also will influence the stability of the inverse micelle. A change in the stability of the inverse micelle will be reflected in modifications of the solubility region of the inverse micellar solution. This chapter will relate the changes in solubility areas from addition of electrolytes to the water to the structure of inverse micelles and other association complexes in the pentanol solution. 

The results showing augmentation of the surfactant alcohol ratio for maximum aqueous solubility with added electrolytes are not amenable to a similarly simple explanation, and the influence of the presence of electrolytes must be discussed against the relative stability of the inverse micelles and of the lyotropic liquid crystalline phase with which the inverse micellar solution is in equilibrium (7). 

However, experimental evidence has shown (7) that inverse micellar systems are rarely in equilibrium with aqueous micellar solutions but rather with a lamellar liquid crystalline phase. The presence of an electrolyte will influence the stability of both the inverse micelles and the lamellar liquid crystalline phase. This influence will be estimated now. 

The discussion of the relative stability of solutions with inverse micelles and of liquid crystals containing electrolytes may be limited to the enthalpic contributions to the total free energy. The experimentally determined entropy differences between an inverse micellar phase and a lamellar liquid crystalline phase are small (12). The interparticle interaction from the Van der Waals forces is small (5) it is obvious that changes in them owing to added electrolyte may be neglected. The contribution from the compression of the diffuse electric double layer is also small in a nonaqueous medium (II) and their modification owing to added electrolyte may be considered less important. It appears justified to limit the discussion to modifications of the intramicellar forces. 

The energy of the electric double layer is directly dependent on the square of the surface potential (Equation 4) and the observed increase of the potassium oleate alcohol ratio should enhance the stability of the inverse micelle. The stability of the inverse micelle is not the only determining factor. Its solution with a maximal amount of water is in equilibrium with a lamellar liquid crystalline phase (7) and the extent of the solubility region of the inverse micellar structure depends on the stability of the liquid crystalline phase. 

A reduction of the stability of the liquid crystalline phase means a reduced region where it is stable and a corresponding increase of the region for the inverse micellar solution. The present results agree with these predictions, and it is justifiable to relate the changes in stability areas mainly to modifications of the potential distribution within the electric double layers. 

The changes in stability regions for inverse micellar solutions where added electrolytes appear were given a rational explanation using the associated structures determined in the inverse micelle solution with no electrolyte. 

A hydrophobically modified polybetaine proved to be an efficient pour point depressant (PPD), to inhibit the deposition of wax, and to improve the viscosity of waxy crude oil from the Kumkol-Akshabulak oil field (western Kazakhstan) [282]. The inhibition of wax deposits in the presence of the hydrophobic polybetaine was interpreted in terms of its interference with the wax crystalhzation process, due to the formation of inverse micellar structures. While the zwitterionic parts on the polymer backbone stabilize the... 

Figure 2. Starting with the conditions of the three basic components—water, surfactant, and co-surfactant—the microemulsions are easily shown to be a part of the inverse micellar area...

Osmotic flux of water to and from the internal droplets, possibly associated with inverse micellar species in the oil phase... 

Pileni has also pioneered the use of the surfactant-as-reactant approach in the preparation of nanoparticles. For example, in the preparation of CoFc204 nanoparticles with sizes between 2 and 5 nm, instead of preparing inverse-micellar dispersions of the Co and Fe salts, Moumen and Pileni [48] prepared the dodecylsul-fonate (DS) analogs Fe(DS)2 and Co(DS)2. These were made to form micellar solutions, to raise the pH, aqueous methylamine solution was added. Stirring for 2 h resulted in a magnetic precipitate. Due to the low yield of Fe(II) to Fe(III) oxidation under these conditions, an excess of Fe(DS)2 is required. 

The phase behavior observed in the quaternary systems A and B is also evidenced in ternary systems. Figure 4 shows the phase diagrams for systems made of AOT-water and two different oils. The phase diagram with decane was established by Assih (14) and that with isooctane has been established in our laboratory. At 25°C the isooctane system does not present a critical point and the inverse micellar phase is bounded by a two-phase domain where the inverse micellar phase is in equilibrium with a liquid crystalline phase, as for system B or system A when the W/S ratio is below 1.1. In the case of decane, a critical point has been evidenced by light scattering (15). Assih and al. have observed around the critical point a two-phase region where two microemulsions are in equilibrium. A three-phase equilibrium connects the liquid crystalline phase and this last region. 

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