Long-Range Electrostatic Attraction between Similar, Charge-Neutral Walls

Long-range attractive forces between electrically neutral, glass surfaces, a sphere and a plane are measured directly with an atomic force microscope. Charge neutrality is achieved by immersion in cationic surfactant solutions. The forces are much larger in magnitude than dispersion forces. Over a wide range of ionic strength (Debye length ${\kappa }^{-1\mathrm{}}$), achieved by addition of salt [ ${\rho }_{\mathrm{total}}=({10}^{-5\mathrm{}}-{10}^{-2\mathrm{}})M$], the energy of interaction decays asymptotically as ${\rho }_{\mathrm{total}}\exp (-2\mathrm{}\kappa D)/\kappa D$. Both the pseudoexponential behavior and the dependence of the prefactor upon the ionic strength suggest an electrostatic mechanism.