The monomer analog of a common water-soluble charged macromolecule shows surface tilt and cooperative adsorption that would be expected for larger surfactant molecules—the small organic ion behaves as an embryonic surfactant. Yet the organic ion competes with inorganic ions ( ${\mathrm{Na}}^{+}$, ${\mathrm{Mg}}^{++}$, etc.) for access to the surface, and therefore its tilt increases with the size of the competing co-ion. Similar ordering of charged units along a polyelectrolyte chain would be sterically frustrated. This suggests a new contribution to surface energetics when a charged macromolecule adsorbs.

• Received 8 September 1997

DOI:https://doi.org/10.1103/PhysRevLett.80.3646