LONG-RANGE ELECTROSTATIC ATTRACTION BETWEEN SIMILAR, CHARGE-NEUTRAL WALLS

Long-range attractive forces between electrically neutral, glass surfa ces, a sphere and a plane are measured directly with an atomic force m icroscope. Charge neutrality is achieved by immersion in cationic surf actant solutions. The forces are much larger in magnitude than dispers ion forces. Over a wide range of ionic strength (Debye length kappa(-1 )), achieved by addition of salt [rho(total) = (10(-5) - 10(-2))M], th e energy of interaction decays asymptotically as rho(total) exp(-2 kap pa D)/kappa D. Both the pseudoexponential behavior and the dependence of the prefactor upon the ionic strength suggest an electrostatic mech anism.