The Schulman Method of Cosurfactant Titration of the Oil/Water Interface (Dilution Method): A Review on a Well-Known Powerful Technique in Interfacial Science for Characterization of Water-in-Oil Microemulsions

摘要

Microemulsions are thermodynamically stable, isotropic transparent mixtures of two immiscible liquids (polar and nonpolar) and amphiphile(s) (usually surfactant and/or cosurfactant). The cosurfactant plays an important role by blending with surfactants, and partitioned between the coexisting aqueous and oleic phases to control the bending elasticity of the interfacial layer to render stability to the dispersion. The microheterogeneity of such dispersions makes them useful in biological and technological applications. Various techniques viz., conductance, interfacial tension, SANS etc. were applied to estimate the distribution of cosurfactant between the interface and the bulk oil. However, a simple but ingenious method comprising repetitively oil dilution with cosurfactant titration till attainment of stable microemulsions was also used to measure the distribution of cosurfactant. This review summarizes formation and characterization of water-in-oil microemulsions stabilized by single or mixed surfactants of different charge types and polar head groups or mixed surface active ionic liquid and surfactant, and cosurfactants of different lipophilicities in both hydrocarbons and long chain alkyl ester oil, emphasizing interfacial composition, thermodynamics of formation and structural parameters by the dilution method coupled with transport properties, microstructures and states of water organization inside the pool etc. using instrumental techniques. Applicability of the data obtained from dilution method in different areas, viz. nanomaterial synthesis, enzyme activity, etc. have been reported. Indeed, this article presented a journey of development and significant contribution in utilizing this elegant and inexpensive method to accomplish the formation of microemulsion through consistent motivation by researchers of different countries.