The cross-sectional headgroup area of nonionic surfactants; the influence of polydispersity

In this work, we have measured surface tension as a function of concentration of pure and technical nonionic ethoxylated surfactants. Critical micelle concentration(CMC) values, surface tension at the CMC and the cross-sectional headgroup areas are compared. It is shown that the CMC does not depend significantly on the polydispersity of the polyoxyethylene chain. However, the surfactant headgroup area, as determined from the surface tension plots, is much smaller for polydisperse surfactants than for homologue pure surfactants. This is proposed to be due to selective adsorption of the shorter polyoxyethylene species at the surface. For the same reason, the surface tension at the CMC is lower for technical surfactants than for homologue pure species.