Probing molecular motion and microstructure into emulsion gels by PFG NMR and advanced microscopy for microstructural observations

Emulsion gels are soft-solid materials with a composite structure consisting of an oil phase within a gel matrix. In recent years, the interest in the application of gel-like systems as functional colloids has attracted great attention especially in the food and pharmaceutical industries due to their tunable morphology and microstructure, excellent stability, and promising functional properties, such as controlled release of encapsulated bioactive compounds. The presence of the oil phase in emulsion gels can strongly affect water diffusion in a complex way that is far from being fully elucidated. Here, an experimental investigation based on the use of pulsed-field gradient (PFG) NMR with a low-field benchtop NMR instrument, in combination with advanced microscopy, has been carried out to investigate the dynamic behavior of water molecules in agarose gel-based systems. Results on agarose gels (that is, without the oil/surfactant components) at two different concentrations (1% and 2% wt) showed a single water diffusion environment, which could be ascribed to free water diffusing within the gel pores. Conversely, the PFG NMR signal attenuation data for water in agarose emulsion gels showed a non-linear behavior, which can be described very well by a model that assumes species exchanging between two compartments with different diffusivities, a fast diffusion attributed to water within the gel pores (unbound water), and a much slower diffusion attributed to a complex interconnected microstructure formed through a synergic connection between the hydrophilic and hydrophobic surfactants, the latter being also alike to the oil.