Uses and applications of microemulsions
Since the discovery of microemulsions, they have attained increasing significance both in basic research and in industry. Due to their unique properties, namely, ultralow interfacial tension, large interfacial area, thermodynamic stability and the ability to solubilize otherwise immiscible liquids, uses and applications of microemulsions have been numerous. The objective of this review is to present briefly the possible applications of the novel compartmentallized systems of microemulsions. IT is well established that large amounts of two immiscible liquids (e.g. water and oil) can be brought into a single phase (macroscopically homogeneous but microscopically heterogeneous) by addition of an appropriate surfactant or a surfactant mixture. This unique class of optically clear, thermodynamically stable and usually low viscous solutions, called ‘microemulsions’1, have been the subject of extensive research over the last two decades primarily because of their scientific and technological importance. Microemulsions can have characteristic properties such as ultralow interfacial tension, large interfacial area and capacity to solubilize both aqueous and oil-soluble compounds. For detailed information one can consult several books and review articles2–5. The essential distinction between normal emulsion and microemulsion is their particle size and stability; the former is ‘kinetically stable’ whereas the latter is ‘thermodynamically stable’. The stability of the microemulsion can be influenced by addition of salt, other additives, temperature or pressure. Normal emulsions age by coalescence of droplets and Ostwald ripening (transfer of material from small droplets to larger ones), since these processes lead to a decrease in the free energy of dispersion (the system is inherently thermodynamically unstable). Thermodynamic stability of the microemulsions has been proposed by uckenstein and Chi6 who considered that the free energy of formation comprises interfacial free energy, interaction energy between droplets and entropy of dispersion. The interaction energy between droplets has been shown to be negligible and the free energy of formation can be zero or even negative if the interfacial tension is of the order of 10–2–10–3 mN/m. The concept of hydrophilic–lipophilic balance temperature (HLBT) or phase inversion temperature (PIT) at which maximum solubilization of oil in water and ultralow interfacial tensions are achieved has been also introduced6,7.