LONG-RANGED ATTRACTIVE FORCES BETWEEN NEUTRAL SURFACES DUE TO AMPHIPHILE AGGREGATION

The interaction of two charged surfaces, neutralized by adsorption or deposition of charged amphiphilic molecules, has been investigated usi ng Metropolis Monte Carlo simulations. Each surface can be considered as a two-dimensional electrolyte (plasma), and it is well-known that t here is a weak attractive surface-surface interaction decaying as h(-3 ) (h being the separation) in the absence of any salt between the wall s. If a small fraction of the amphiphiles self-assemble and form highl y charged aggregates, the attractive interaction increases significant ly. The magnitude is much larger than the ordinary van der Waals attra ction and the force can be substantial out to several thousand angstro ms and is of course salt-dependent. That this can be a plausible mecha nism for the very long-ranged forces sometimes observed between mica s urfaces made hydrophobic by adsorption or deposition of charged amphip hiles.