Microstructure evolution during nano-emulsification by NMR and microscopy

Hypothesis: Microstructure evolution in emulsions as a function of composition is of great interest but fundamentals have not yet been fully elucidated. Here, pulsed-field gradient (PFG)NMR diffusion measurements have been combined with confocal laser scanning microscopy (CLSM)to assess evolution of dynamics and microstructure during nano-emulsification. Experiments: Diffusion coefficients of emulsions made of water, mineral oil and surfactants (Span 20 and Tween 80)were measured as a function of water composition and compared with the morphological features of the emulsions obtained by CLSM. Findings: In the absence of water, two phases are visible from CLSM, and two diffusion components are observed with PFG NMR, a major fast component attributed to a continuous oil phase containing the more hydrophobic surfactant Span 20 with traces of Tween 80, and a minor slow component attributed to a dispersed phase of the more hydrophilic surfactant Tween 80 with traces of mineral oil and Span 20. At the inversion point (25 wt% water)the two-component diffusion behavior of the oil-rich phase is drastically reversed in terms of populations, with the slow diffusion process becoming dominant. This suggests a significant structuring of the oil-rich phase in the presence of surfactants enhanced by water, which can be explained by the formation of aggregates in the oil phase as reverse micelles or of a lamellar structure, and ties in well with the rheological measurements.