Stability of microemulsions

By small-angle neutron scattering experiments on water/AOT/hydrocarbon microemulsions containing various additives, the change of the radius of the miceUar core with the addition of small quantities of additives has been investigated. The results are consistent with a model in which amphiphilic molecules such as benzyl alcohol and octanol are preferentially adsorbed into the water/surfactant interfacial region, decreasing the micellar radius, whereas toluene remains predominantly in the bulk hydrocarbon phase. The effect of n-alcohols on the stability of microemulsions has also been reported [119],... 

The above concept of duplex film can be used to explain both the stability of microemulsions and the bending of the interface. Considering that initially the flat duplex film has different tensions (i.e., different values) on either side of it, then the deriving force for film curvature is the stress of the tension gradient which tends to make the pressure or tension in both sides of the curved film the same. This is schematically shown in Figure 1. For example if ir > ir on the flat... 

The effect of adding a surfactant, (NaDDS), was also investigated. One such case only is shown in Fig. 6 where BE is replaced by a 5 1 mixture of BE-NaDDS. The main effect of NaDDS is to increase the miscibility range of the oil in water. Various ratios of BE-NaDDS were used and, as a first approximation, the change in the phase diagram is directly proportional to the concentration of NaDDS. The addition of a surfactant probably stabilizes the microstructures which were already present in the ternary system BE-DEC-H O and decreases the quantity of BE needed to solubilize DEC. Therefore the presence of a surfactant is useful but not essential to the stability of microemulsions. 

Any surfactant adsorption will lower the oil-water interfacial tension, but these calculations show that effective oil recovery depends on virtually eliminating y. That microemulsion formulations are pertinent to this may be seen by reexamining Figure 8.11. Whether we look at microemulsions from the emulsion or the micellar perspective, we conclude that the oil-water interfacial free energy must be very low in these systems. From the emulsion perspective, we are led to this conclusion from the spontaneous formation and stability of microemulsions. From a micellar point of view, a pseudophase is close to an embryo phase and, as such, has no meaningful y value. 

Several theories have been proposed to account for the thermodynamic stability of microemulsions. The most recent theories showed that the driving force for microemulsion formation is the ultralow interfacial tension (in the region of 10 4-10 2 mN m 1). This means that the energy required for formation of the interface (the large number of small droplets) A Ay is compensated by the entropy of dispersion —TAS, which means that the free energy of formation of microemulsions AG is zero or negative. 

Micellar aggregates are considered in chapter 3 and a critical concentration is defined on the basis of a change in the shape of the size distribution of aggregates. This is followed by the examination, via a second order perturbation theory, of the phase behavior of a sterically stabilized non-aqueous colloidal dispersion containing free polymer molecules. This chapter is also concerned with the thermodynamic stability of microemulsions, which is treated via a new thermodynamic formalism. In addition, a molecular thermodynamics approach is suggested, which can predict the structural and compositional characteristics of microemulsions. Thermodynamic approaches similar to that used for microemulsions are applied to the phase transition in monolayers of insoluble surfactants and to lamellar liquid crystals. 

Role of Cosurfactants/Cosolvents in Formation and Stabilization of Microemulsion Systems... 

ROLE OF COSURFACTANTS/COSOLVENTS IN FORMATION AND STABILIZATION OF MICROEMULSIONS... 

The use of butyl cellosolve as cosurfactant instead of 2-pentanol increases the stability of microemulsions during polymerization. This is partly due to the fact that the solubility of butyl cellosolve in lystyrene is hiAer than the solubility of 2-pentanol in polystyrene as experimentally verified by Gan and Chew (17). 

It should be noted that high concentrations of ionic species can alter the phase stability of microemulsions based upon ionic surfactant systems. Nonionic surfactant systems are much less susceptible to this effect. The curvature of the interfacial film of the microemulsion droplet is determined by a balance between the electrostatic interactions of the head groups and repulsive interactions of the surfactant tail group. Addition of ionic solutes can upset this delicate balance and induce phase separation. By changing the structure of the surfactant or through the addition of cosurfactants one can restore this balance and thus allow the dissolution of high concentrations of ionic species. 

Polyoxyethylene alkyl ethers are nonionic surfactants widely used in topical pharmaceutical formulations and cosmetics, primarily as emulsifying agents for water-in-oil and oil-in-water emulsions and the stabilization of microemulsions and multiple emulsions. 

Scriven (78) proposed the role of the electrical double layer and molecular interactions in the formation and stability of microemulsions. According to them, the total interfacial tension (y ) can be expressed in the form... 

Stability of Microemulsions. The first attempt to describe the microemulsion stability in terms of different free energy components was made by Ruckenstein and Chi (55) who evaluated the enthalpic (Van der Waals potential, interfacial free energy and the potential due to the compression of the diffuse double layer) and entropic... 

A similar interpretation of phase diagrams has been recently proposed by Safran and Turkevich (18). These authors have considered the effects of interaction and curvature on the stability of microemulsions. They suggest that unstabilities of spherical microemulsion droplets lead the system to separate with water in order to prevent micellar growth above a limit radius R. Taking into account interactions with a phenomenologic treatment, they show that phase separation due to interaction is also possible and they found a critical radius Rc. If Rc is greater than R a water phase is formed and if Rc is lower than R phase separation gives rise to two micellar phases with a critical point. This theoretical treatment reflects very well the behavior we observe but it is not in full accordance with our experimental results. The main difference is that when interactions are not preponderant the phase separation does not occur with a water phase but with a lamellar phase. 

Thermodynamic stability The thermodynamic stability of microemulsions helps in improving the shelf-life of the product making them carriers of choice. 

Another important consideration is the physical stability of microemulsions in the presence of tonicity adjusting agents, preservatives and electrolytes. It is also necessary to... 

Olefins were hydroformylated in water-in-carbon dioxide microemulsions in the presence of organometallic catalysts formed in situ from Rh(CO)2acac and 3,3 ,3 -Phosphinidynetris(benzenesulfonic acid), trisodium salt (TPPTS) in the presence of synthesis gas. The microemulsions were supported by sodium salt of bis(2,2,3,3,4,4,5,5-octafluoro-1 -pentyl)-2-Sulfosuccinate (H(CF2)4CH200CCH2 CH(S03Na)COOCH2(CF2)4H (di-HCF4). The effects of the presence of salts and acid on the stability of microemulsions and activity were also investigated. 

In order to emphasize the role of the inter facial films and to highlight the most recent viewpoints on the stability of microemulsions, sponge phases, and dilute lamellar phases, some of the experimental facts about phase behavior of microemulsion systems containing alcohol are reviewed in this chapter. The systems investigated consist of water, oil, alcohol, and sodium dodecylsulfate (SDS). In the next section, the theoretical aspects of the stability of surfactant phases are briefly discussed. Then in Secs. Ill and IV the effects of varying alcohol and oil chain lengths and the addition of a water-soluble polymer are examined. The examination of multiphase regions provides the location of lines of critical points or critical endpoints. This chapter also deals with the study of several physical properties in the vicinity of critical points. 

Once more considering the thermodynamic stability of microemulsions, as well as the fact that the droplet size in microemulsions is frequently much smaller than that of the corresponding emulsion, microemulsions have also been applied as drug vehicles for cyclosporine oral administration (Sandimmune Neoral). In a number of investigations, it was found that the bioavailability of cyclosporine may be further improved with this formulation at the same time as the pharmacokinetic variability is reduced [43-45,47-49]. An example of this type of results is given in Fig. 6. In line with the findings of Tarr and Yalkowsky [46] on the effect of droplet size on the absorption from emulsions, these results seem to indicate that one contribution to the enhanced absorption from the microemulsion formulation is the small droplet size in this system. However, as shown... 

The thermodynamic stability of microemulsions and their ability to solubilize both polar and nonpolar active materials make these systems an attrachve alternative for drug delivery vehicles. Applications of microemulsions in pharmaceuticals are discussed in... 

Ruckenstein, E., The origin of thermodynamic stability of microemulsions, Chem. Phys. Lett., 57, 517, 1978. 

The arguments put forward to explain thermodynamic stability of microemulsions can also explain the stability of these microlatexes [6.22]. Concerning the former, it has been shown that the dispersion entropy of the droplets becomes larger than the extremely unfavourable contribution from interfacial energy, when the droplet radius goes below a critical radius. Moreover, the formation of microemulsions requires very low interfacial tensions. 

AN EXPERIMENTAL INVESTIGATION OF STABILITY OF MICROEMULSIONS AND POLYDISPERSITY EFFECTS USING LIGHT-SCATTERING SPECTROSCOPY AND SMALL ANGLE X-RAY SCATTERING... 

We shall use this new histogram approach which automatically takes into account the effects of polydispersity to study the formation and stability of microemulsions. 

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