Self-organizing structures of phosphatidylcholine in nonaqueous solvents: tailoring gel-like systems

Abstract

In this paper, we investigated the role played by solvent type and additives (water [W] and citric acid [CA]) on the self-assembly of phosphatidylcholine (PtdCho) into gels. Soybean lecithin (L) served as the PtdCho source used in this study. Lecithin was combined with different oils: hexadecane (HEX), sunflower oil, and medium-chain triacylglycerol to explore its ability to form organogels. Among the solvents, only HEX was able to form a translucent self-sustainable gel with lecithin. It was found that the lower water solubility and viscosity of HEX favored gel formation. Small-angle X-ray scattering revealed different structures arisen from lecithin organization depending on the organic medium type. In addition, the gel properties of the L-HEX binary system were also tailored through inclusion of the primers W or CA. Oscillatory rheological behavior of organogels was effectively described by a single relaxation-time Maxwell model, with good fitting at low and intermediate frequencies and with deviations at higher frequencies, indicating the presence of reverse wormlike micelles. Organogels were thermoreversible and, upon CA addition, a substantial increase in zero-shear viscosity was observed. Incorporation of W led to a similar microstructure to that obtained only with L, whereas CA promoted a different ordering of PtdCho assemblies. Formation and properties of PtdCho-based organogels were dictated by using different oil types or by changing the polarity of the medium with the incorporation of primers. A better understanding of phospholipid-based nonlamellar mesophases in organic solvent offers the prospect of adapting the gel properties to the desired application and functionality.