Effect of novel alkanolamides on the phase behavior and surface properties of aqueous surfactant solutions

The surface tension properties and phase behavior of a new series of alkanolamides, alkanoyl N-methyl ethanolamides (NMEAs) and their mixtures with sodium dodecyl sulfate (SDS) were investigated. NMEAs alone do not form micelles in aqueous solutions but reduce considerably the surface tension until macroscopic phase separation occurs. According to Gibbs isotherms, the surface layer is less compact for the NMEA with the shortest alkanoyl chain. The critical micelle concentration (CMC) of SDS solutions is greatly reduced upon addition of a small amount of NMEA and the magnitude of this effect increases with the length of the alkanoyl group. The results indicate the presence of attractive interactions between SDS and NMEA molecules inside micelles. The mixing of SDS with NMEA-16 causes a reduction in the melting temperature of the solid similar to freezing-point depression in a binary system. On the other hand, the eutectic temperature is higher in SDS-conventional dodecanoyl mono ethanolamide (DMA) systems in which the mixture is in a solid state at room temperature over a wide range of mixing fractions. Among NMEAs, surface tension decay is faster as the alkanoyl chain length decreases. Only for the dodecanoyl chain could a diffusion-controlled adsorption be identified at low concentrations. When small amounts of NMEA are added to SDS aqueous solutions, the surface tension decay is retarded; however, and at long times a lower surface tension is reached. For NMEA/ SDS and DMA/SDS systems, an adsorption barrier is likely present. The magnitude of this barrier seems to depend on the SDS/alkanolamide ratio.