Effect of urea and alkylureas on micelle formation by a nonionic surfactant with short hydrophobic tail at 25 degrees C

The effect of urea, N-monomethylurea, and N,N'-dimethylurea on the micelliz ation of the nonionic ethoxylated tenside pentaethyleneglycolmonohexyl ethe r (C6E5) was investigated through intradiffusion coefficient measurements ( Pulsed-gradient spin-echo-NMR method) at 25 degrees C. The presence of the additive causes, in all cases, an almost linear increase in the critical mi cellar composition (cmc), while a decrease in the aggregation number occurs . Furthermore, both the monomer-monomer and micelle-micelle interactions in crease. The analysis of the intradiffusion coefficients shows the presence of additive molecules in the hydration shell of the tenside, in both its mo nomeric and micellar forms. A comparison with the results obtained for the water-pentaethyleneglycol (PEG5)-urea or alkylurea systems suggests that th e additive molecules are mainly positioned near the tenside ethoxylic chain . The number of alkylurea molecules in the solvation shell of each C6E5 and PEGS molecule was computed and commented. The experimental cmc values in w atcr-C6E5-urea mixtures are in quite good agreement with those predicted th rough a molecular thermodynamic model.