Organogel formation via supramolecular assembly of oleic acid and sodium oleate

Abstract

To create materials with novel functionalities, the formation of gels within hydrophobic media has become popular. This is often accomplished through the assembly of low molecular weight organogelators into a variety of complex phases through intermolecular interactions. In the case of edible materials, the assembly of saturated fatty acids to form fat crystal networks is often used for structuring. Here, the first example of structuring with unsaturated fatty acids is reported, namely mixtures of oleic acid and sodium oleate, to structure edible lipid phases. Small-angle scattering demonstrates that the resultant structures, which vary with oleic acid and sodium oleate molar ratio, comprise either inverse micellar or lamellar phases, combined with the formation of crystalline space-filling networks. Network formation was found for filler concentrations above 10 wt%. Rheological measurements show that gel strength depends on the ratio of oleic acid to sodium oleate, and is greater when only oleic acid is used. The addition of up to 1.5 wt% of water enhanced the strength of the organogels, probably through supplementary hydrogen bonding but, for concentrations greater than 2.0 wt%, the assembly was inhibited leading to collapse of the gel.

Conventional and Unconventional Crystallization Mechanisms

“…Organogels are a class of materials that can hold large volumes of organic liquids in self-assembled networks and have many practical applications in the pharmaceutical, chemical, and food industries regarding lipid technology [32]. Organogels are formed by structurants of low molecular weight and some organic solvents that are supported, through a three-dimensional network, forming the gel.…”

“…The formation of these structures through supramolecular assembly is of great interest, since they can form semisolid phases that are produced at low cost [32]. Several researchers have focused their studies on the properties of structurants to form organogels [22,[36][37][38].…”

“…The network is stabilized by weak interactions between the chains, such as hydrogen bonds, van der Waals forces, and π staking [40][41][42][43]. Although it is known that organogels are formed through weak intermolecular interactions between the structurant molecules, which generate three-dimensional networks [44], there is still a lack of fundamental understanding of the type of interactions that are required [32].…”

“…Organogel networks may range from the assembly of surfactants into solution by physical interactions (e.g., micellization, lyotropism, and crystallization) [48], also known as molecular organogels, to the formation of flexible polymer networks (e.g., swelling), known as polymer organogels [49]. The molecular organogels known today are at the interface between complex fluids [50] and solids and, regardless of the nature of the structure, are composed of thermoreversible semisolid materials [32].…”

“…The assembly is theoretically driven largely by hydrophilic interactions between the hydrophilic head groups, as the strength of the interactions increased by the addition of small amounts of water. For water concentrations of more than 2%, the assembly was inhibited [32].…”