Inverse micelle stability

The volume of water which can be taken up and stabilized In these swollen Inverse micelles Is limited, usually only a small fraction of a mole percent of the total liquid present In the system. 

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). 

The stability of inverse micelles has been treated by Eicke (8,9) and by Muller (10) for nonaqueous systems, while Adamson (1) and later Levine (11) calculated the electric field gradient in an inverse micelle for a solution in equilibrium with an aqueous solution. Ruckenstein (5) later gave a more complete treatment of the stability of such systems taking both enthalpic (Van der Waals (VdW) interparticle potential, the first component of the interfacial free energy and the interparticle contribution of the repulsion energy from the compression of the diffuse part of the electric double layer) and entropic contributions into consideration. His calculations also were performed for the equilibrium between two liquid solutions—one aqueous, the other hydrocarbon. 

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. 

Equation 4 predicts a square dependence of the energy on the electric potential of the interface of the inverse micelle. Addition of electrolyte will not change the surface potential much a slightly reduced stability may be expected. The higher surfactant alcohol ratio that was observed (Figure 2) will increase the surface potential. 

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. 

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. 

Figure 3 Different types of steric stabilization of metal nanoparticles. (a) polymer molecules on the particles surface (b) micelle (c) inverse micelle (d) ligand stabilized particle...

Several transition metals such as V, Nb, Ta, and Pd can form stable bulk hydrides, so-called interstitial hydrides the bonding in the hydride phase is not ionic but mostly metallic in character, and the hydrogen to metal ratio is not necessarily stoichiometric. Especially, nanoparticles of noble metals such as Pd are relatively easy to prepare by various methods, such as vapor phase deposition on substrates, reductions of salts in solution (electrochemically or electroless), and the inverse micelle templated growth. They are not easily oxidized, and, in recent years, several methods have been developed to precisely control the size of the particles or clusters. Furthermore, growth in solution in the presence of surfactants and stabilizers allows control over the shape of the final particles [35, 36, 42]. 

The Implication of this change Is straightforward. Polymerization In the low water content means no conflicting space demands between the polymer and the Inverse micelles because the latter do not exist. Since polystyrene Is completely soluble In styrene It would be reasonable to expect polymerization to be possible In this region with no stability problems. 

Our interest in this phenomenon is mainly the role of the water molecules for the stability of such aggregates an interesting problem against the suggestion by Eicke (7) that small amounts of water are essential for the stability of inverse micelles of aerosol OT. 

Light scattering measurements and theoretical treatment strongly support the idea that attractive interactions between inverse micelles play an important role in the stability of oil rich microemulsions. In the system containing pentanol, attractions between (i)/o micelles can be sufficient to give rise to a phase separation between two microemulsion phases. 

The ultrasonification process is connected with the rapidly increased oil-water interfacial area as well as the significant re-organization of the droplet clusters or droplet surface layer. This may lead to the formation of additional water-oil interface (inverse micelles) and, thereby, decrease the amount of free emulsifier in the reaction medium. This is supposed to be more pronounced in the systems with non-ionic emulsifier. Furthermore, the high-oil solubility of non-ionic emulsifier and the continuous release of non-micellar emulsifier during polymerization influence the particle nucleation and polymerization kinetics by a complex way. For example, the hairy particles stabilized by non-ionic emulsifier (electrosteric or steric stabilization) enhance the barrier for entering radicals and differ from the polymer particles stabilized by ionic emulsifier. The hydro-phobic non-ionic emulsifier (at high temperature) can act as hydrophobe. 

Nomenclature Thermo- dynamic stability Surfactant concentration relative to erne Existence of micelles/ inverse-micelles Primary Locus of Nucleation ... 

Isopar-M SMO AIBN 1 1 6 30-50 47 Baade and Reichert [49] Inverse micelles not detected. Polymerization in monomer droplets. Kinetic latex stability. Solution-like kinetics, with interfacial reactions Inverse- Suspension... 

The transition to inverse micelles is important in microemulsions stabilized by a carboxylate, a soap. For these systems (Friberg, 1978) the presence of electrolyte in the water leads to a change of the solubility region the minimiora water content (CD, Fig. 3) will be enhanced and the maximiora water solubilization (B, Fig. 3) will occur at higher surfac-tant/cosurfactant ratio. 

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