Attractive potential microemulsions

At larger concentration, the system cannot be described as a droplets dispersion a more complicated structure, possibly bi-con-tinuous, appears and deviation from dilution line is observed. (The same occurs for g < -20 even at low tf> values (13)). The measured values of g and R are reported for different microemulsions in Table I. The g values smaller than 8 (hard-sphere-like systems) can be accounted for by introducing a supplementary attractive potential. 

The contact point = c is a critical consolute point. The calculated critical values of the virial coefficient and of the droplet volume fraction (B =-21 and <(ic JO. 13) for a hard-sphere model with an attractive potential are in qualitative agreement with the experimental observations (Figure 2). Around those critical values, a very large turbidity is observed. If the temperature is varied, the microemulsion separates into two turbid microemulsions. Angular variations of the scattered intensity and of the diffusion coefficient are observed (16) but the correlation function remains exponential. All these features are characteristic of the vicinity of a critical consolute point. The data can be fitted with theoretical predictions (17) ... 

The first attempt to compile all the factors contributing to the stability of a microemulsion is due to Ruckenstein and Chi [15], who summarized calculations of enthalpic components (van der Waals attractive potential, electrical double layer repulsive potential, and the interfacial stretching and bending free energy) and entropic contributions from the location of droplets. These calculations as well as those following [16] were useful because they revealed the importance of extremely low interfacial tension. 

The variation of ( )/I versus (p, normalized to 1 for ( ) = 0, is represented in Figure 6 for several microemulsion series. The slopes of the straight lines are the virial coefficients 2B [from eq. (7)]. All the results are reported in Table 3. If the particles interact as hard spheres, 2B is equal to 8 (5). One can see in Table 3 that this is not the case for microemulsions. Either 2B < 8 indicating the existence of a supplementary attractive potential, or 2B > 8, a case of supplementary repulsive potential. 

Electrochemical redox studies of electroactive species solubilized in the water core of reverse microemulsions of water, toluene, cosurfactant, and AOT [28,29] have illustrated a percolation phenomenon in faradaic electron transfer. This phenomenon was observed when the cosurfactant used was acrylamide or other primary amide [28,30]. The oxidation or reduction chemistry appeared to switch on when cosurfactant chemical potential was raised above a certain threshold value. This switching phenomenon was later confirmed to coincide with percolation in electrical conductivity [31], as suggested by earlier work from the group of Francoise Candau [32]. The explanations for this amide-cosurfactant-induced percolation center around increases in interfacial flexibility [32] and increased disorder in surfactant chain packing [33]. These increases in flexibility and disorder appear to lead to increased interdroplet attraction, coalescence, and cluster formation. 

Microemulsions are potentially exploitable in any situation where the mixing of oil and water is desired. The possibility of using them to enhance tertiary oil recovery has recently attracted a great deal of attention. 

In previous papers, the experimental values of B for several (jj/o microemulsions have been measured. As already pointed out, these values are function of both the radius of the micelles and the alcohol chain length (9-10). The attractive interactions between micelles increase as the micellar radius increases and as the alcohol chain length is shorter. We have proposed an interaction potential between w/o micelles which allows to account for the scattering results (10). This potential V(r) results from the possibility of penetration of the micelles. V(r) is proportional to the volume of interpenetration of micelles. The penetration is limited by the molecules of alcohol located inside the interfacial film, r is the distance between two micelles. 

The production of microemulsions is comparatively simple and cost-effective, and thus, they have attracted a great interest as drug-delivery vehicles. Microemulsions have the capability of transporting lipophilic substances through an aqueous medium, and can also carry hydrophilic substances across lipoidal medium. Based on this attribute, potential of microemulsions has been explored for oral, transdermal, parenteral, topical, and pulmonary administration of lipophilic and hydrophilic drugs. In the last decade, microemulsions have also been explored for their potential as vehicles for topical ocular drug delivery."- ... 

As the potential between inverse droplets becomes strongly attractive, a critical behavior is observed. The curves of reduced compressibility versus the micellar volume fraction for three microemulsions formed with water, dodecane, and SDS (sodium dodecylsulfate) and heptanol or hexanol or pentanol, illustrate this effect [22,23]. The heptanol microemulsion is close to a hard-sphere system. In the case of pentanol, both dnld(f) and nfdcfr are close to zero, which is the signature of a critical point. Thus for this system it appears that attractions between droplets are strong enough to induce a phase separation into a micelle-rich phase and a micelle-poor phase. 

For W/O microemulsions, deviations from hard-sphere behavior are sometimes observed. The reason for this is that here there is a stronger tendency for attractive interactions and the formation of anisometric shapes. Here viscosity data can be used to determine the shape of the corresponding aggregates or to extract information regarding the interaction potential that exists between the droplets. From temperature-dependent experiments the binding enthalpy of the droplets can be determined. 

It can be seen from Table 6 that anionic, cationic, and nonionic surfactants have all been exploited in formulating microemulsions for materials synthesis. Anionic and nonionic surfactants appear to be the most popular types of surfactants, with Aerosol OT (AOT) and the polyoxyethylated alkylphenyl ether surfactants (e.g., NP-5) leading. Part of the attraction of AOT and the NP surfactants is related to the fact that they permit microemulsion formulation without the need for cosurfactants. Also, a large body of information is already available on the phase behavior and structure of AOT microemulsions [121], and this makes it convenient to work with this anionic surfactant. A unique advantage of the nonionic surfactants is the fact that their use does not involve the introduction of (potentially undesirable) counterions. The ability to alter the size of the hydrophilic (oxyethylene) groups and/or the hydrophobic (alkyl) groups provides additional flexibility in surfactant selection. 

Upon further addition of acrylamide, the interaction potential becomes so attractive that transient clusters form. Above a threshold volume fraction, a large increase in the electrical conductivity is observed, which is an indication of a percolation phenomenon [25] (Fig. 2). The percolation threshold decreases with increasing AM/H2O ratio, i.e., with increasing attractive interactions, in good agreement with theoretical analyses [26] and data obtained for other microemulsions containing alcohols as cosurfactants [27-29]. As shown in Sec. III.C, this percolating structure has an effect on the formation of polymer latex particles and the polymerization mechanism. 

The properties of microemulsions make them attractive for cosmetic formulations from several points of view. First, the transparent appearance gives a perception of a clean system for a large majority of potential customers this is an essential property. The small droplet size and the transparency make them look like solutions, but the fact that they contain colloidal size droplets of oil in water, or vice versa, significantly enhances their use in cosmetics. 

The potential of microemulsions as a means of preparing liposomal solutions has not been realized in spite of the fact that the method offers significant advantages over existing methods. Instead, the publications so far have been concerned with simple surfactant solutions, with the process as follows. A micelle-forming surfactant (Sm) and a liposome-forming one (Sl) are combined at a concentration Sm > cmcsM (where cmc = critical micelle concentration) [43-45]. The liposome/micelle fraction Sl/(Sl + Sm) has attracted some attention [46]. 

Mieroemulsion research has been stimulated by the great potential of microemulsions for practical applications primarily because their use for enhanced oil recovery seemed an attractive alternative due to the work of Shah and colleagues (1971), and over the years a wide spectrum of other applieations—ranging from mimdane to sophisticated—have emerged and many more are under aetive development. Moreover, as we learn further about the interesting characteristics of microemulsions, new vistas will emerge. 

Like many other topics covered here, microemulsions hold a great deal of promise in many practical applications. To date, most research in the area has been closely associated with the formation and destruction of microemulsions in the context of secondary and tertiary petroleum recovery, although more interest is being shown in cosmetic applications. While the concept of microemulsions is very attractive for potential use in many other areas, especially pharmaceuticals, their sensitivity to composition makes their application much more problematical. Since in many cases (i.e., drug delivery) one or more component (which may be somewhat surface-active) may be determined by the intended function of the system, the options for the formulator may be drastically reduced or at least greatly complicated. 

A. This value of the radius is physically impossible due to the fact that the length of one SDS molecule is about 22 A, therefore the hydrodynamic radius of the microemulsion droplets must be larger than 22 A. Thus, the simple hard core potential assumption is not valid for the water in oil microemulsions. It is more lilely that the real potential contains both attractive and repulsive parts. Overall, must increase with the volume fraction in... 

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