Characterization of nonionic surfactant micellar structures in organic solvents by small angle X-ray scattering (SAXS)

The micellar structure behavior of nonionic surfactants in nonpolar solvents has attracted great interest in the last years because of its application in the solubilization of several substances, material synthesis and stabilizers of insoluble reactive species in nonpolar solvents. In the present work we report a study on the determination of critical micellar concentration (CMC) and characterization of reverse micelles formed by a nonylphenolpolyethoxylated surfactant (Ultranex-50) in organic solvents (octane, decane and dodecane), employing ethylene glycol monobutyl ether as polar additive. In order to determine CMC and to characterize reverse micelle structural behavior under several factors (type of solvent, surfactant concentration, temperature and polar additive molar ratio), use was made of small angle X-ray scattering (SAXS), employing the generalized indirect Fourier transformation (GIFT). Aiming to determine CMC of systems composed of Ultranex-50, ethylene glycol monobutyl ether, and nonpolar solvents, we have acquired data from SAXS with the surfactant concentration varying from 0.005 to 0.2 mol/L, with an ethylene glycol monobutyl ether molar ratio of 2:1. First of all, we verified that the formation of aggregates in nonpolar solvents occurred at a surfactant concentration magnitude of 10−2 mol/L. From the collected data, we could visualize that for the studied systems the shape of the reverse micelle was predominantly globular. We have observed that micellar size decreases upon heating. Finally, when reducing ethylene glycol monobutyl ether molar ratio, we noticed that the micellar aggregates turned from almost spherical globular structures to less symmetrical ones.