Phase Behavior of Ionic Microemulsions

Abstract

Non-polar oils and water can form thermodynamically stable quasi-homogeneous (colloidal) mixtures (called microemulsions) in the presence of relatively large amounts (several %) of ionic surfactants. If the surfactant contains a single hydrocarbon chain (e.g. Sodium Dodecyl Sulphate) the presence of a non-ionic cosurfactant (e.g. hexanol) and electrolyte (concentration of the order 0.1 M) is essential. With a double chain surfactant (e.g. Aerosol OT) the cosurfactant can be missed. At increasing concentrations of electrolyte and/or cosurfactant the nature of the microemulsion changes from droplets of oil in water via a presumably bicontinuous pattern to droplets of water in oil. It should be obvious that thermodynamic stability requires the interfacial tension between water and oil to be low (order of 0.01 0.1 mN m -1) so that the dispersion entropy can offset the interfacial free energy. At these low interfacial tensions the influence of curvature on the interfacial tensions becomes important. It turns out that a given amount of surfactant (and co-surfactant) can only disperse a limited amount of oil in water or of water in oil or of water and oil into one another and therefore a microemulsion may be in equilibrium with non colloidal oil and/or water phases. In the bicontinuous microemulsion oil and water may have a geometrically irregular interface or they may form lamellae of more or less constant thickness or other structures, such as a "molten cubic phase". These equilibria lead to very interesting, but rather complicated phase diagrams. It will be discussed by what mechanisms the various components of the mixture influence the interfacial tension and promote the stability of the microemulsion and how this depends on the chemical nature of the components.

Formation Kinetics of Oil-Rich, Nonionic Microemulsions

“…Although these emulsions might be kinetically stable for weeks or even months, they are inherently unstable from a thermodynamic perspective. Employing a larger amount of surfactantand thereby covering the complete interface between polar and nonpolar substancethermodynamically stable, nanostructured microemulsions may form. − Although their equilibrium properties like phase behavior, − low oil/water interfacial tension, , and multifarious nanostructure − have been studied in great detail for the past quarter of a century, far less is known about their formation kinetics and the formation of the internal interface in particular. In older related studies the pressure- and temperature-induced micelle formation was investigated in (pseudo)­binary water/surfactant mixtures − as well as the transformation of spherical micelles to elongated ones. , For these transitions similar studies have also been conducted using block copolymers. − Beyond that, most of the other related experiments deal with the kinetics of structural transformations, e.g., the well-studied micelle-to-vesicle transition ,− induced mainly by changes in the composition (using the stopped-flow technique) − or the lamellar-to-sponge (L 3 ) transition due to changes in temperature or pressure .…”

The Formation of Middle‐Phase Microemulsions of Polar Oils

“…[2] 1 The middle-phase microemulsion formed with a balanced HLB is unique. It has a bicontinuous water-oil structure, and an interfacial tension between the oil and water interfaces that is almost zero ('ultra-low' interfacial tension).…”

Influence of NaCl on the Structure and Dynamics of Phospholipid Layers

“…This is in line with the very short ranged dipolar field of the zwitterionic head group that is barely shielded by NaCl ions on long ranges but has short range compensations [37] compared to isolated ions in the Debye-Hückel [38] theory (κ −1 2.2 Å). So the only effect of the NaCl ions is on the viscosity of the water and only weakly changes the dipole field effect on the bending rigidity in contrast to simply charged head groups [39,40]. This can be understood as the smallscale analogue of rough surfaces sliding against each other.…”