Oil structuring: polymer bridging mechanism for structuring soft materials using natural emulsions as templates

Abstract

Using a bridging flocculation mechanism in the design of oleogels materials constitutes an alternative framework to achieve desired rheological properties of oil-in-water emulsions. Aggregation by polymer bridging generates a droplet network linked together by firmly bound polymer bridges. In this dissertation, we used negatively charged polysaccharides, sodium alginate, xanthan gum, and ι-carrageenan as the structuring agents and soybean oleosomes as the template. Bridging flocculation between polysaccharides and oleosomes was induced by mixing and adjusting the pH to values where both are oppositely charged, leading to electrostatic-driven interactions. Our results indicate that polysaccharides with flexible polymer chains, such as sodium alginate and ι-carrageenan, are effective bridging flocculants. In contrast, polysaccharides with a more rigid backbone, such as xanthan gum, resulted in depletion flocculation characterized by phase separation between oleosome droplets and xanthan molecules. Bridging flocculation is more effective at an optimum dosage between polysaccharides and oleosomes, expressed as a mass ratio (g polysaccharide/g oleosome) or as an equivalent per droplet surface area (mg/m2). Sodium alginate presented a higher bridging ability than ι-carrageenan, with its optimum bridging ratio at 0.005 g/g and ι-carrageenan at 0.01 g/g. This was confirmed by quantitative analysis of oleosome content upon centrifugation recovery, where sodium alginate yielded more compact and concentrated gels than ι-carrageenan. Differences in structural conformations between sodium alginate and ι-carrageenan account for the difference in bridging ability. Sodium alginate presents a co-block arrangement of alternating charged and uncharged parts. The negatively charged blocks adsorb strongly onto the oleosome interface at several charged units. At the same time, the uncharged parts impart a high degree of flexibility, allowing the polymer chains to bridge several droplets together. On the other hand, ι-carrageenan is less flexible than alginate, making the individual carrageenan chains more effective for oleosome surface coating but less effective for bridging neighboring droplets. This difference in bridging ability between sodium alginate and ι-carrageenan will influence the structure of the aggregated network and, as a result, will be responsible for the mechanical behavior in rheological measurements. Sodium alginate produced more heterogeneous and interconnected structures, while ι-carrageenan produced smaller and less interconnected clusters. This difference in microstructure and the effect of the structural conformations in the polysaccharide chains becomes relevant at medium and large deformations in amplitude sweeps oscillatory rheology. At deformations between 3- 100%, sodium alginate presented steeper slopes in the moduli G’, indicating sudden microstructure fracture. In contrast, ι-carrageenan presented less steep slopes indicating yielding rather than fracture behavior in the decrease of the moduli G’. At deformations between 200- 300%, the moduli presented an overshoot indicating a “cage effect” where individual droplets are immobilized due to crowding by surrounding droplets. This effect was more clearly prominent in conditions leading to the densest structures, such as in the compacted gels upon centrifugation performed at the optimum bridging ratios for sodium alginate (0.005 g/g) and ι-carrageenan (0.01 g/g). This study offers many perspectives on how to construct the macroscopic functional properties of oleogels in accordance with their application using the molecular architecture of polysaccharides.