Water transport mechanisms in wheat gluten based (nano)composite materials

An in depth investigation of water transport mechanisms has been undertaken on extruded wheat gluten (WG)/clay materials, which have been shown to display either a nanocomposite or a microcomposite structure depending on the nature of the nanoclay used (i.e. unmodified sodium montmorillonite (named HPS) or organically modified montmorillonite (Cloisite®30B), respectively). The interplay of two concomitant phenomena has been evidenced: first a plasticization of protein chains by water that favors water diffusivity and, second, a clusterization of water as revealed by the Zimm and Lundberg and Guggenheim–Anderson–de Boer theories leading to a slowdown and finally a decrease in water mobility within the polymer. Comparison between liquid and vapor water diffusivity showed a strong impact of the state of the water phase in contact, the water liquid diffusivity being three fold higher than water vapor diffusivity. This Schroeder’s paradox could be related to the microporous structure of the extruded wheat gluten materials in which liquid water moves due to an additional capillary phenomenon resulting in a higher apparent diffusivity. As well predicted by the Bharadwaj’s tortuosity-based mathematical model, the achievement of a well-exfoliated structure (as observed in the case of the WG-HPS system) has no effect on water diffusivity, whether the phase in contact is liquid or vapor. On the contrary, such a structure led to a significant reduction of the liquid water uptake that might be ascribed to water hydrogen bondings established between the hydrophilic sites of wheat gluten and the unmodified montmorillonite, in turn reducing their availability for water.