Effect of variations in the fatty acid chain on functional properties of oligofructose fatty acid esters

Oligofructose fatty acid esters are surfactants that considerably lower the surface tension of an air/water interface, provide the interface with a high dilatational modulus and lead to a high foam stability. In this study, we investigate the effect of the molecular structure of oligofructose fatty acid esters on their functional properties. We varied the length and degree of saturation of the fatty acid chain, and the number of fatty acids esterified to the oligofructose part. A tensiometer was used to establish CAC curves, light scattering to determine the size of micellar aggregates and an ellipsometer to determine the amount of material that adsorbed at the interface. Esters with a more hydrophobic character had a lower CAC and had a higher efficiency. Oligofructose mono- and di-esters with fatty acid chain lengths between C8 and C16 formed spherical micelles, while esters with a fatty acid chain length of C18 formed larger aggregates. The effectiveness was similar for all esters. Using the Gibbs adsorption model, we did not find major differences in the area per molecule for the different esters. Ellipsometry experiments also did not indicate major differences in the area per molecule for the oligofructose mono-esters. The area per molecule of oligofructose esters was larger than that of sucrose esters and independent of the degree of saturation of the fatty acid chain. We conclude that the amount of interfacial area occupied by one molecule is determined by the oligofructose part.