Abstract
Colloidal assemblies of phospholipids in oil are known to be highly sensitive to changes in system composition and temperature. Despite the fundamental biological and high industrial relevance of these aggregates, the mechanisms behind the structural changes, especially in real oils, are not well understood. In this work, small-angle X-ray scattering (SAXS) was combined with molecular dynamics simulations to characterize the effects of oleic acid, water, and temperature on self-assembled structures formed by lecithin in rapeseed oil. SAXS showed that adding water to the mixtures caused the precipitation of liquid-crystalline phases with lamellar or hexagonal geometry. The combination of SAXS and molecular dynamics simulations revealed that stable spherical reverse micelles in oil had a core radius of about 2 nm and consisted of approximately 60 phospholipids centered around a core containing water and sugars. The presence of oleic acid improved the stability of reverse micelles against precipitation due to the increase in the water concentration in oil by allowing the reverse micelle cores to expand and accommodate more water. The shape and size of the reverse micelles changed at high temperatures, and irreversible elongation was observed, especially in the presence of oleic acid. The findings show the interdependency of the structure of the reverse micellar aggregates on system composition, in particular, oleic acid and water, as well as temperature. The revealed characteristics of the self-assembled structures have significance in understanding and tuning the properties of vegetable oil-based emulsions, food products, oil purification, and drug delivery systems.
Highlights
The self-assembly of amphiphilic surfactants in solution is an interesting phenomenon that has wide significance in fields ranging from fundamental biology to chemical engineering and biotechnology.[1]
To simplify the experimental composition, vegetable oil was modeled as triolein, lecithin was modeled as a pure 1,2-dioleylphosphatidylcholine (DOPC), and sugar was modeled as sucrose
A similar increase at low q was observed in the small-angle X-ray scattering (SAXS) intensity of bulk water, which on the other hand was almost constant above q = 0.5 nm−1
Summary
The self-assembly of amphiphilic surfactants in solution is an interesting phenomenon that has wide significance in fields ranging from fundamental biology to chemical engineering and biotechnology.[1]. Most widely studied and applied self-assembling lecithin systems include ternary mixtures of lecithin, water, and an organic solvent of low polarity (oil), which typically take the form of an emulsion with the hydrophobic tail of the lecithin molecule residing in the nonpolar oil phase and the hydrophilic headgroup being in contact with the water.[3] Especially in water-in-oil systems at very low water concentrations, lecithin molecules assemble into spherical or cylindrical reverse micelles. Lecithin/water/oil systems have a wide range of applications as food stabilizers,[4] drug carriers,[5] and nanoreactors,[6] among other things
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