Some Structural Aspects of Microemulsions and Co-Solubilized Systems
Authors: Shah D.O. , Walker R.D. , Hsieh W.C. , Shah N.J. , Dwivedi S. , Nelander J. , Pepinsky R. , Deamer D.W. 
Published: December 2007

American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc.
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Abstract
Microemulsions are optically clear, transparent, and stable dispersions of oil, water, surfactant and cosurfactant mixed in specific proportions. Stability and transparency have been used as sufficient criteria for defining microemulsions by several investigators. Using a combination of physical techniques such as electrical resistance, high resolution NMR (220 Mc), spin-lattice relaxation time, and viscosity measurements we have shown that two isotropic clear systems with identical compositions except one containing n-pentanol and the other n-hexanol, ate structurally quite dissimilar systems. The isotropic clear system containing pentanol was proposed to be a cosolubilized system (i.e. similar to molecular solution) whereas the hexanol containing system to be a microemulsion. The results suggested that a difference of one carbon atom in the cosurfactant molecule can significantly influence the microstructure as well as the macroscopic properties (e.g. viscosity) of such systems. properties (e.g. viscosity) of such systems. Upon increasing the water to oil ratio, these systems undergo a phase transition, namely from an isotropic to a birefringent state. The birefringent liquid crystalline region exhibited unusual rheological properties. Using X-ray scattering and properties. Using X-ray scattering and freeze-etching electron microscopy, it was shown that these systems consisted of the lamellar structure and that upon shearing the parallel orientation of the lamellae was transformed into a disordered state. This disordering of the lamellae presumably increases the viscosity of the system.

Introduction
Microemulsions are optically isotropic, clear, and stable dispersions of oil, water, surfactant and cosurfactant. Such oil-in-water or water-in-oil microemulsions have been examined by low-angle X-ray measurements, light-scattering techniques, ultracentrifugation, electron microscopy, high resolution NMR spectroscopy and viscosity and have been shown to consist of droplets 100 to 1000 R in diameter. We have shown previously that upon increasing the water to oil ratio, one can convert a water-in-oil microemulsion to an oil-in-water type.