Mechanisms of Physical Stabilization of Concentrated Water-In-Oil Emulsions Probed by Pulse Field Gradient Nuclear Magnetic Resonance and Rheology through a Multiscale Approach

Abstract

The long-term physical stability of surfactant-stabilized (Span 80 and Tween 20) concentrated water-in-mineral oil (W/O) emulsions in the presence of an electrolyte (NaCl) was studied. Pulse field gradient NMR and rheology (bulk and interfacial) were used to probe the response at the macroscopic, microscopic, and molecular levels, rendering a multiscale approach. The results show that: (1) Emulsions prepared with NaCl exhibit higher values of the elastic shear modulus ( G_{with NaCl}^' > G_{without NaCl}^') even after ∼20 days. (2) The stabilization effect of salt against the coarsening of droplets is not due to the differences in droplet size (and thus G') or the energy incorporated through emulsification. (3) NaCl relaxes the liquid-liquid interface via a salting-in effect, which results in a lower interfacial shear elasticity ( G_{with NaCl}^s' < G_{without NaCl}^s') and a higher resistance to coarsening events because of the changes in the adsorption density of the layer.