NMR Self-Diffusion Studies and Stability Studies of Emulsions
Abstract: The aim of this study is to characterize emulsion systems by self-diffusion PGSE NMR methods. The specifics of the emulsions studied are all taken from the literature and they are of the W/O or O/W type stabilized with different emulsifiers. We have investigated what characteristics of emulsions that the NMR self-diffusion technique can shed light on. The diffusion behavior of the liquid inside the droplet (restricted diffusion) and of the liquid outside the droplet (normal diffusion with obstruction effects) are so different that the W/O or O/W type can be established. We present experimental data for the restricted diffusion inside the droplets and the results of fits of Murday and Cotts1 equation for restricted diffusion to the data. This equation extended with a log-normal size distribution function gives the emulsion droplet size distribution from the restricted diffusion data by numerical methods. For emulsions with large inner-surfaces the transverse relaxation time for the liquid in the continuous medium is affected by local field gradients around the droplets and the stimulated spin echo technique is used to evaluate the diffusion coefficients. The microemulsion structure in the continuous phase was studied in an emulsion system for the transitions Winsor I - Winsor II - Winsor III. The structure changed from normal micelles in the Winsor I region to bicontinuous in the vicinity of and in the Winsor III area to reverse micelles in the Winsor II region. We have also studied a W/O/W multiple emulsion with the NMR technique and the droplet size distribution for the water in the starting W/O emulsion and the water in the multiple emulsion could be evaluated and they were practically the same. Information about the state of the oil in both emulsion may also be obtained. In addition, the NMR technique may be used to obtain information regarding the water exchange permeability across the oil film in the multiple emulsion. Finally, we have studied emulsion stability in an emulsion system where the Winsor transitions were obtained by changing the weight ratio nonionic cosurfactant/SDS. In this system emulsion samples scattered in the phase diagram and along the three-phase region were examined with regard to creaming and coalescence. In the vicinity of the three-phase area the emulsions were unstable in agreement with the theory of Kabalnov and Wennerström. It is the low spontaneous curvature of the surfactant film in the bicontinuous microemulsion that drives the equilibrium phase changes. From samples scattered in the phase diagram it is evident that the W/O emulsions coalesce fast, on a timescale of days, thus the cosurfactant appears to be inefficient in producing a steric stabilization. The O/W emulsions can take years to coalesce as a result of their charged surfaces. Conductivity studies in nonhomogenized and homogenized O/W emulsion samples show that the conductivity is lower in some homogenized samples, which effect is attributed to the higher counterion binding to the emulsion droplet surface.
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