Fatty amide ethoxylates:: Synthesis and self-assembly

A series of fatty acid monoethanolamide ethoxylates was synthesized. Tensiometry and fluorescence were used to study the solution behavior of this surfactant series. The effect of the amide bond, the effect of unsaturations in the hydrocarbon chains, and the effects of the oxyethylene chain length were investigated. The presence of the amide group was found to lead to a decrease in the critical micelle concentration (CMC), an effect that is probably due to the formation of hydrogen bonds between neighboring molecules. The surface tension at the CMC was higher when an amide bond was present in the surfactant. This probably reflects the increase in size of the polar group (decrease in the critical packing parameter value) which is unfavorable for close alignment of surfactant molecules at the air-water interface. The presence of unsaturations in the hydrophobic tail gave an increase in the CMC. This may partly be due to the surfactants becoming more hydrophilic and partly to the increased bulkiness of the chains, rendering packing into closed aggregates and formation of hydrogen bonds between amide groups more difficult. The cross-sectional headgroup areas of the surfactants increased with the degree of unsaturation as a consequence of more unfavorable packing at the surface.