Emulsion Droplet Sizing Using Nmr: Application to Water-in-crude Oil Emulsions

Emulsion droplet sizing is an essential measurement requirement of a very broad range of industries including food, pharmaceuticals, personal care products, agrochemicals and crude oil dispersions. Nuclear magnetic resonance (NMR), courtesy of its ability to measure restricted self-diffusion via application of pulsed field magnetic field gradients (PFG), is able to readily measure emulsion droplet size distributions for concentrated, opaque, contaminated and often complex structured emulsion systems. Over the past decade the MR Engineering (MRE) research group of Prof Mike Johns at the University of Cambridge, UK, and more recently at the University of Western Australia, have adapted this method for on-line measurement purposes; enabling measurement on flowing streams, reducing total acquisition times from 10s of minutes to under 5s, eliminating the assumption of a specified droplet size distribution shape and adapting the technique for various bench-top NMR apparatus. These developments are summarised in a recent review article (Johns, 2009) and will be briefly elucidated in the presentation along with some exemplar applications. Current research is focussed on sizing water-in-crude oil emulsions (an increasing oil production problem) on a range of bench-top NMR apparatus. At the extreme this features use of the Earth’s magnetic field, opening up the possibility of assembling an NMR emulsion droplet measurement device for under $200. Developments in this respect will be outlined and improvements in the accuracy of this emulsion sizing application detailed for a variety of Australian crude oil samples. INTRODUCTION Emulsion droplet sizing is an essential measurement requirement of a very broad range of industries including food, pharmaceuticals, personal care products, agrochemicals and crude oil dispersions. Emulsions are colloidal systems which consist of discrete dispersions of droplets of one liquid phase within another immiscible liquid phase. Emulsions are usually characterised as either macroemulsions or microemulsions, referring to a thermodynamically unstable system of droplet diameters of 1-100μm and a thermodynamically stable system of droplet diameters of 0.001-0.01μm respectfully. Generally macroemulsions are referred to as emulsions while microemulsions retain their name, we will follow this convention in this paper. Emulsions are often formed in the presence of surfactants, which act to stabilize the system by providing an electrostatic and steric barrier. When fine solids act as the surfactants the emulsion is referred to as a Pickering emulsion. Since emulsions are thermodynamically unstable the phases will eventually separate via a combination of mechanisms e.g. coalescence, flocculation, Ostwald ripening and density difference. Emulsions are generally categorized as oil-inwater (O/W) or water-in-oil (W/O) emulsions, however a wide range of alternate forms E.O. Fridjonsson, M.L. Johns 2 of emulsions exist e.g. oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W). These latter forms of emulsions are a growing area of emulsion research. The droplet size distribution (DSD) of an emulsion is a fundamental physical characteristic determining stability, rheology, texture and other functionalities and as such it is an essential measurement requirement of a very broad range of industries. The principal method of determining DSD using nuclear magnetic resonance (NMR) is by measurement of the restricted diffusion of the discrete phase molecules by the application of pulsed magnetic field gradients (PFG). PFG-NMR techniques can readily measure emulsion droplet size distributions for concentrated, opaque, contaminated and complex structured emulsion systems. NMR BACKGROUND The basic PFG-NMR pulse sequence (Fig. 1a) consists of two equal magnetic field gradients separated by an observation time (∆), placed on either side of the 180 rf pulse of a spin echo (SE) pulse sequence. The first pulsed magnetic field gradient dephases the spin precession frequency of the nuclei (typically H), while the second pulsed magnetic field gradient re-phases the spin precession frequency. Any molecular displacement during ∆ in the applied gradient direction will cause a net phase difference proportional to the molecular motion, which causes signal attenuation of the measured spin echo. Due to T2 relaxation, it is common practice to replace the spin echo (SE) pulse sequence with a stimulated echo (STE) pulse sequence (Fig 1b), because the latter is affected by T1 relaxation which is slower than T2 relaxation resulting in an improved signal-to-noise (S-to-N) ratio. Fig. 1: Figure shows the basic NMR timing diagrams of (a) PFG-SE pulse sequence and (b) PFG-STE pulse sequence. When the molecules experience free motion during the experimental time (∆), the signal attenuation is dependent on the molecular diffusion coefficient according to the Stejskal-Tanner equation (Tanner & Stejskal, 1968),