Effect of ureas and alkylureas on the sphere-to-rod transition in aqueous ionic micellar solutions

The effects of urea, thiourea, mono-, di-, and tetramethylurea (alkylureas) on the sphere-to-rod transition (s → r) in a typical system of 3.5% (w/v) sodium dodecylsulfate (SDS) + 0.28 M NaCl + x M 1-pentanol in aqueous solution have been investigated using viscosity and dynamic light scattering measurements. In this system, 1-pentanol at a concentration of 0.04 M, is known to promote s → r transition, whereas the presence of urea or its derivatives are found to promote/retard the transition. This effect in the structural transition is due to the increased dielectric constant of the medium and adsorption of ureas on dodecylsulfate anion (DS−). The former effect ameliorates hydrophobic interactions, as the electrostatic repulsion would be decreased. The adsorption effect continuously increases with increase in the number of methyl group in the urea molecule, which in turn, results in increased hydration of the micelles.