Interfacial curvature microemulsions

R. Strey Microemulsion, Microstructure and Interfacial Curvature. Colloid Polym. Sci. 272, 1005 (1994). 

Strey, R. (1994) Microemulsion microstructure and interfacial curvature. Colloid Polymer Sci., 272(8), 1005-1019. 

A theoretical basis for different shapes of microemulsions (even for small W/O or O/W volume fractions) has been established on the basis of the relationship between shape and interfacial curvature [350,351]. It is reasonable to expect that the relevant properties of the surfactant film are represented by a bending elasticity with a spontaneous curvature, Co (as was demonstrated for binary systems). If the elastic modulii k, ksT, the fluctuations in curvature of the film are very small, and the entropy associated with them can be neglected. The actual morphology is the result of the competition between the tendency to minimize the bending free energy (which prefers spheres of optimal radius of curvature, = l/c ) and the necessity to use up all of the water, oil, and surfactant... 

A microemulsion is thus anomalous, because it is relatively fluid and much less viscous than expected. This has been linked with the flexibility of the structure and its transient behavior, as early mentioned byWinsor and Shinoda for the surfactant phase, as they called it (they did not use the word microemulsion). This is particularly true in the case in which extremely low interfacial tension allows easy deformation and low interfacial curvature, a feature that can be attained by adjusting the physicochemical formulation. 

A schematic diagram presenting the comparative effects of spontaneous curvature and elasticity of the interfacial film in W/O microemulsions is shown in Fig. 3.8. In a W/O microemulsion, therefore, a maximum water solubilization can be achieved when the interfacial bending stress of rigid interfaces, as also the attractive interdroplet interaction of fluid interfaces is minimized, keeping the interfacial curvature and elasticity at an optimum level. 

Choi, S.M., Chen, S.H., Sottmann, T., and Strey, R. 1997 Measurement of interfacial curvatures in microemulsions using small-angle neutron scattering, Physica B Condens. Mat. 241—243 976—978. 

FIGURE 15.2 Evolution of interfacial curvature of microemulsion systems according to parameter R. 

The relationship between the interfacial curvatures and phase behavior in bicontinuous microemulsions - a SANS study... 

In some systems, inversion from water-in-oil to oil-in-water microemulsions is prevented by geometrical restrictions and the nature of the interfacial curvature. Thus, no core of free water is formed and when the surfactant becomes fully saturated, the addition of more water leads to phase separation. 

Many reports are available where the cationic surfactant CTAB has been used to prepare gold nanoparticles [127-129]. Giustini et al. [130] have characterized the quaternary w/o micro emulsion of CTAB/n-pentanol/ n-hexane/water. Some salient features of CTAB/co-surfactant/alkane/water system are (1) formation of nearly spherical droplets in the L2 region (a liquid isotropic phase formed by disconnected aqueous domains dispersed in a continuous organic bulk) stabilized by a surfactant/co-surfactant interfacial film. (2) With an increase in water content, L2 is followed up to the water solubilization failure, without any transition to bicontinuous structure, and (3) at low Wo, the droplet radius is smaller than R° (spontaneous radius of curvature of the interfacial film) but when the droplet radius tends to become larger than R° (i.e., increasing Wo), the microemulsion phase separates into a Winsor II system. 

Surfactants form semiflexible elastic films at interfaces. In general, the Gibbs free energy of a surfactant film depends on its curvature. Here we are not talking about the indirect effect of the Laplace pressure but a real mechanical effect. In fact, the interfacial tension of most microemulsions is very small so that the Laplace pressure is low. Since the curvature plays such an important role, it is useful to introduce two parameters, the principal curvatures... 

The interfacial tension is a key property for describing the formation of emulsions and microemulsions (Aveyard et al., 1990), including those in supercritical fluids (da Rocha et al., 1999), as shown in Figure 8.3, where the v-axis represents a variety of formulation variables. A minimum in y is observed at the phase inversion point where the system is balanced with respect to the partitioning of the surfactant between the phases. Here, a middle-phase emulsion is present in equilibrium with excess C02-rich (top) and aqueous-rich (bottom) phases. Upon changing any of the formulation variables away from this point—for example, the hydrophilie/C02-philic balance (HCB) in the surfactant structure—the surfactant will migrate toward one of the phases. This phase usually becomes the external phase, according to the Bancroft rule. For example, a surfactant with a low HCB, such as PFPE COO NH4+ (2500 g/mol), favors the upper C02 phase and forms w/c microemulsions with an excess water phase. Likewise, a shift in formulation variable to the left would drive the surfactant toward water to form a c/w emulsion. Studies of y versus HCB for block copolymers of propylene oxide, and ethylene oxide, and polydimethylsiloxane (PDMS) and ethylene oxide, have been used to understand microemulsion and emulsion formation, curvature, and stability (da Rocha et al., 1999). 

At low temperatures an O/W microemulsion (0/Wm) is formed which is in equilibrium with an excess oil phase. This condition is termed a Winsor I system. At high temperatures the headgroup requires less space on the interface and, thus, a negative curvature can result. A phase inversion o ccurs and a W/O microemulsion (W/Om) is formed which is in equilibrium with an excess water phase. This situation is termed a Winsor II system. At intermediate temperatures three phases - a water phase, a microemulsion D and an oil phase - are in equilibrium. This is called a Winsor III system. Here the curvature of the interfaces is more or less zero. Hence, the interfacial tension is minimum as depicted in Figure 3.24 (right) for the system C12E5, tetradecane and water. 

Here y is a generalized interfacial tension, Ci and C2 are bending stresses associated with the curvatures ci and eg, respectively A is the internal interfacial area per unit volume of microemulsion ui and ni are the chemical potentials per molecule and the number of molecules of species i, respectively 0 is the volume fraction of the dispersed phase and P2 and pi are the pressures inside the globules and in the continuous phase in the space between the globules. Here the actual physical surface of the globule (to the extent to which it can be defined) of radius r is selected as the Gibbs dividing surface. 

Microemulsions consist of oil, water and an oil-water interfacial Him. DLS and SLS have been used to determine the translational diffusion coefficient and the interaction potential of microemulsions [45—47). The thickness of the inter-facial film and its curvature were measured by the contrast variation method in neutron scattering [48,491. This method is based on changing the scattering strength by changing the relative amount of light and heavy water in the microemulsion. 

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. 

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