Swollen micellar models for solubilization

A critical appraisal of microemulsions for drug delivery: part I

Microemulsions (MEs) are thermodynamically stable, optically transparent isotropic solutions of oil and water successfully formulated by using a combination of suitable surfactant and cosurfactant. While the selection of oil is based primarily on the solubility of drug in it, surfactant is generally selected on the basis of its hydrophilic–lipophilic balance value. MEs are characterized by ultra-low interfacial tension between the immiscible phases and offer the advantage of spontaneous formation, thermodynamic stability and ease of manufacture. The solubilization power of MEs for lipophilic, hydrophilic and amphiphilic solutes form a viable approach for enhancing bioavailability of hydrophobic drugs and percutaneous permeation of poorly permeable drugs, mainly due to the large area to volume ratio available for mass transfer.

Thermodynamic Approaches to Microemulsions

The thermodynamics of microemulsions is treated by decomposing the Helmholtz free energy into a sum of a free energy F0 of a dispersion in a continuous medium containing fixed, noninteracting globules and a free energy DeltaF due to the entropy of dispersion of globules in the continuous medium and to the interactions among them. The pressure p1 in the continuous medium of the system involving fixed, noninteracting globules is determined in two different ways. In one of them, it is calculated by minimizing the total free energy with respect to the volume fraction phi of the dispersed medium, while in the other it is considered equal to the external pressure. The equivalence between the conventional thermodynamics of a multicomponent mixture and a thermodynamics based on modeling a microemulsion as a dispersion is used to derive the basic equations. Equations are obtained for single microemulsions as well as microemulsions coexisting with an excess dispersed phase and with both excess phases. One demonstrates that the conventional Laplace equation is not valid for a microemulsion, and new equations are derived. One concludes that the approach involving the determination of p1 via the optimization of the total free energy is the proper one. Considering that DeltaF is dominated by the entropy of dispersion of globules in the continuous medium, equations are established relating the interfacial free energy at the surface of the globules to the radius of the globules and phi, for the case in which a microemulsion coexists with an excess dispersed phase. These equations reveal that for phi approximately 0.5 the above state cannot be stable and that a transition to another state involving a microemulsion coexisting with both excess phases is likely to occur. Copyright 1998 Academic Press.

Equilibrium of a Microemulsion That Coexists With Oil or Brine

Huh, Chun; SPE; Exxon Production Research Co.

Abstract

When salinity, or an equivalent variable, is increased, microemulsions generally undergo orderly transitions from a lower-to middle- to upper-phase. Even though the significance of such multiphase behavior has been well recognized in the design of surfactant flood processes, their quantitative nature in terms of the molecular structures of the surfactant lipophile, hydrophile, and the oil and brine salinity has not been fully understood. A theory of lower- and upper-phase microemulsions that gives reasonable predictions of their interfacial tensions (IFT's) and phase behavior is presented. In the theory, the surfactant monomers adsorbed at oil/brine interface cause the interface to bend as a result of an imbalance between the hydrophile/brine interaction on the one hand and lipophile/oil interaction on the other. With sufficient imbalance, high local curvature causes small drops of one phase to disperse into the other. In addition, interactions between these drops are taken into account for the microemulsion equilibrium. The theory also offers a possibility of being able to describe the hydrophile/lipophile-balanced state (optimal salinity state of Healy and Reed) in terms of the tendency of surfactant layer at the oil/brine interface to bend.

Introduction

Understanding the phase behavior of microemulsions is an important step in designing surfactant flooding processes and interpreting the results when they are applied to recover tertiary oil. It is well established that the phase behavior of many microemulsion systems, even those containing a large phase behavior of many microemulsion systems, even those containing a large number of different components can be represented qualitatively using pseudoternary diagrams similar to those in Fig. 1. Fig. 1a shows the pseudoternary diagrams similar to those in Fig. 1. Fig. 1a shows the lower-phase microemulsion in equilibrium with excess oil, Figs. 1b and 1c the middle-phase microemulsion in equilibrium with both oil and brine, and Fig. 1d the upper-phase microemulsion coexistent with excess water. Even though not all microemulsions conform to this simple picture, it serves as a good approximation frequently enough to use it as a basis for discussing microemulsion phase behavior. Transitions such as those shown by Fig. 1 can be produced by changing any of a large number of variables in a systematic manner. The phase shifts from "a" to "d" generally occur with increases in the salinity of the brine, the alkyl chain length of the surfactant, the aromaticity of the oil, the addition of a highly oil- soluble alcohol and a temperature increase (for non-ionic surfactants). The shifts also occur with decreases in the chain length of oil, the number of hydrophilic groups (e.g., ethylene oxide) of the surfactant, the addition of a highly water-soluble alcohol, and a temperature decrease (for most ionic surfactants).

Since microemulsion phase transitions will be determined by the manner in which microemulsion structure depends on changes in the variables described above, many experimental studies have been made to determine microemulsion structure. Ultracentrifuge and light-scattering measurements show that the lower-phase microemulsion consists of spherical oil drops with radius of about 50 to 1,000 k in water. As it moves toward the middle-phase state (see Figs. 1a and 1b), the drop radius grows. On the other hand, the upper-phase microemulsion consists of small water drops in oil, and as it moves toward the middle-phase state (Figs. 1c and 1d), the drop radius again grows. Very little is known about the structure of middle-phase microemulsions.

SPEJ

p. 829