(622j) Using Solvent NMR Relaxation and Diffusion Measurements As Aids in Fingerprinting Microemulsions
In part owing to their ease of formation and stability, the effective use of microemulsions has increased dramatically as major industrial applications have expanded in a variety of fields. One important example is as drug carrier systems for oral, topical, and parenteral administration; microemulsions offer the advantage of ease of manufacturing and scale-up, and improved drug solubilization and bioavailability.
Thorough characterization of any colloidal system is essential but presents distinctive problems in the case of microemulsions because they are clear and, further, since microemulsion domains exist at fairly specific concentrations of the three basic components, dilution is almost never an option. Hence, investigation by conventional light scattering is difficult, if not impossible. X-ray and neutron scattering and dielectric measurements have been used to provide insight to the structure and dynamics of such systems but such measurements are not routine and, certainly, are not suited to normal laboratory analysis.
Solvent NMR relaxation and diffusion measurements are non-invasive and non-destructive. Not only (micro)emulsions but also swollen micelles can be measured without dilution, stored under thermally controlled conditions (e.g. ICH guidelines) and re-analyzed to understand thermodynamic stability. Emulsion and micellar microstructure can be investigated with exquisite clarity via NMR.
With the advent of small bench-top low field NMR devices, the technique has become very useful as a routine aid in formulation of suspensions and emulsions. Using a 13MHz device, we will present data obtained on a model microemulsion system to illustrate both the utility and simplicity of the technique in fingerprinting not only the composition of both the preparatory oil and water phases but also the microemulsion system so-formed from them.