Micellar phase, inverse

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)... 

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 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. 

In conclusion, the same phase behavior is evidenced when we change the alcohol or the W/S ratio in a quaternary system, and the oil in a ternary system. This behavior can be characterized by two types of phase diagrams. In the first type no critical point occurs. In this case, the inverse micellar phase is bounded by a two-phase region where it is in equilibrium with a liquid crystalline phase. 

The second type is characterized by the occurence of a critical point. In this case, the inverse micellar phase is bounded by a region where two micellar phases are in equilibrium. 

J.M. Seddon, J. Robins, T. Gulik-Krzywicki, H. Delacroix, Inverse micellar phases of phospholipids and glycolipids. Phys. Chem. Chem. Phys. 2, 4485-4493 (2000)... 

Related to these structures and also of relevance for the preparation of nanoparticles are some amphiphile-based nanostructured phases which do not possess any long range order. For example, another type of bicontinuous phase with relation to the inverse bicontinous cubic phases is the so-called sponge phase (L3). Its curved bilayer structure is disordered so that the water channels adopt a sponge-like structure. Sometimes this phase is referred to as a "melted cubic (v2) phase". Moreover, also dispersions of inverse micellar phases (L2) have been described which may be regarded as "melted I2 phase". Although such disordered phases do not represent a liquid crystalline phase in a strict sense they are included here since they are of relevance for nanoparticulate drug delivery purposes. 

The results obtained by DLS on samples composed of DU TC and PT TC at 50 50 ratio (La inverse micellar phase) stabilized by TM concentrations ranging from 0.5 wt% to 5 wt% are summarized in Table 1. A first important remark is on the visual aspect of the emulsions, which are white emulsions with no coalescence or phase separation within weeks [7],[9] hence, a good stabilization has been obtained. Although, the widths of the distributions, W, reflect that the quality of the stabUizatimi is better in the case of DU than for PT, similarities on the behavior between both Upids can be distinguished. At a given TM concentration, the hydrodynamic radius, Rj], has been found to increase with the added salt cOTicentration. This shows that the La internally self-assembled... 

Figure 15.3 shows a typical ternary phase diagram of soybean oil (triglyceride), sunflower oil monoglyceride and water at 25 °C [8]. It clearly shows the LC phase and the inverse micellar (L2) phase. This inverse micellar phase is relevant to the formation of water-in-oil emulsions. The interfacial tension between the micellar L2 phase and water is about 1-2 mN m and that between the L2 and oils even lower. The L2 phase is proposed to form an interfacial film during emulsification, and the droplet size distribution should then be expected to be related mainly to the interfacial and rheological properties of the L2 phase. 

Pilman E, Larsson K, Torenberg E. 1980. Inverse micellar phases in ternary systems of polar Upids/fats/water and protein emulsification of such phases to W/OAV— microemulsion—emulsions. / Dispersion Sci Technol 1 267-281. 

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]. 

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. 

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.

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. 

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. 

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 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. 

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

The solubilisation of oil or water in a micellar solution of non-ionic surfactant, a) two-phase diagram (O - oil, W - water, 0, - oil in micellar solution, - water in inverse micellar solution, D - phase separation temperature region), b) interfacial tension as a function of T, according to Shinoda Friberg 1975... 

Quaternary phase diagram, L, - micellar solution, Lj - inverse micellar solution, M -micFoemulsion, after Friberg (1983)... 

C.J. Hawker and J.M.J. Fechet, Preparation of polymers with controlled molecular architecture. A new covergent approach to dendritic macromolecules, J. Am. Chem. Soc., 1990, 112, 7638 F. Zeng and S.C. Zimmerman, Dendrimers in supramolecular chemistry from molecular recognition to self-assembly, Chem. Rev., 1997, 97, 1681 D.J.P. Yeardley, G. Ungar, V. Percec, M.N. Holerca and G. Johnsson, Spherical supramolecular minidendrimers self-organized in an inverse micellar -like thermotropic body-centered cubic liquid crystalline phase, J. Am. Chem. Soc., 2000, 122, 1684. 

Figure 2 Phase diagram of a binary amphiphile-water mixture obtained from a Ginzburg- Landau model with a vector order parameter for the amphiphile orientation (50,51]. The phases L and L2 are micellar liquids, is a lamellar phase. H and H denote hexagonal and inverse hexagonal phases, respectively, I is an fee crystal of spherical micelles, and V is a simple cubic bicontinuous phase. (From Ref. 51.)...

Very early, the Swedish school attempted to determine the extent and shape of the region of existence of microemulsions in quaternary systems [76-78]. By examination of sections of the phase diagram at several levels of oil, Friberg and coworkers established a direct connection between the microemulsion areas and the inverse micellar solutions described by Ekwall [1]. Thus, prior to describing the phase diagrams of the quaternary systems, those of ternary systems made of water, sodium dodecylsulfate (SDS), and an alcohol are first presented here. 

Conversely, a Winsor II diagram and phase behavior (also noted 2) corresponds to the opposite situation, in which the polyphasic equilibrium consists of an inverse micellar organic solution S2 (that eventually solubilizes enough water to become a microemulsion) in equilibrium with an essentially pure aqueous phase. 

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