The characterisation of microemulsions using NMR measurements of diffusion

Law, Susan (2015). The characterisation of microemulsions using NMR measurements of diffusion. University of Birmingham. Ph.D.

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Abstract

This thesis investigates how nuclear magnetic resonance (NMR) measurements of diffusion are used to characterise reverse micelles (RMs) in water-in-oil (w/o) microemulsions. The average droplet sizes were determined at varying water to surfactant ratios (ω) and droplet volume fractions (ød), by converting the diffusion coefficients from the surfactant molecules into hydrodynamic radii using the Stokes-Einstein equation. The size distributions of AOT/n-octane/water RMs were obtained by the application of a constrained form of the inverse Laplace transform. The AOT/iso-octane/water/pentanol and CTAB/hexanol/water systems were also studied, where the alcohols act as co-surfactants. Molecular exchange processes were observed between the RMs and the continuous phase, at varying NMR experimental parameters at different ω and ød. There was a decrease in droplet sizes with the addition of pentanol to the AOT/iso-octane/water system, which was observed with consideration to the changes in the viscosity of the continuous phase due to the partitioning and exchange of co-surfactant molecules.

Molecular simulations of solvated RMs were set up at the same values of ω as the NMR experiments. The simulations, which were constructed with all-atom forcefields, displayed the interactions between the molecules in the continuous and dispersed phases and showed the RM shape fluctuations and development. The AOT/iso-octane/water/pentanol droplets formed split RMs and were smaller than RMs in the AOT/iso-octane/water system at values of ω < 20. These observations were reflected in the experimental results. The CTAB/hexanol/water droplets formed oblate RMs initially, which continued to fragment into smaller droplets, due to hexanol molecules penetrating the micelle interface.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):