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 using Metropolis Monte Carlo simulations. Each surface can be considered as a two-dimensional electrolyte (plasma), and it is well-known that there is a weak attractive surface-surface interaction decaying as h(-3) (h being the separation) in the absence of any salt between the walls. If a small fraction of the amphiphiles self-assemble and form highly charged aggregates, the attractive interaction increases significantly. The magnitude is much larger than the ordinary van der Waals attraction and the force can be substantial out to several thousand angstroms and is of course salt-dependent. That this can be a plausible mechanism for the very long-ranged forces sometimes observed between mica surfaces made hydrophobic by adsorption or deposition of charged amphiphiles.