Water Structure Change-Induced Expansion and Collapse of Zwitterionic Polymers Surface-Grafted onto Carbon Black

We demonstrate the expansion and collapse of surface-grafted zwitterionic polymer brushes in water caused by the addition of urea. We hypothesize that at low urea concentrations, this is an effect of an ion–dipole interaction between urea and the polymer, and at high urea concentrations, an effect of a change in water structure causing change in solvation of the brushes and hence a change in the dipole–dipole interaction, and that it is analogous to the effects of urea on protein stability.