Computer modelling of Newton Black Films: The physical origin of the Hydration force

Water is the solvent in which most of physical, chemical and biological processes occur. An understanding of the nature of the interactions of charged species in solution is essential to explain biomolecule interactions, protein folding or the stability of foams, emulsions and colloidal suspensions. We have reported new results that clarify the origin of one of the most elusive forces in soft matter physics, the "hydration force". Since the early 70's experiments on bilayers, membranes and biomolecules, have shown the existence of a strong repulsive force -the "hydration force"-. The quest for the physical mechanism responsible for the hydration force has been the focus of intense work, debate and controversy in recent years. In our work, we clarify the physical origin of the "hydration force" in ionic Newton Black Films and the precise role played by water.

We have performed large scale computer simulations of realistic models of thin water films coated by ionic surfactants (Newton Black Films). These simulations were performed in the HPCx supercomputer. Our investigations show that water exhibits a strong anomalous dielectric response. Near the surfactants water acquires a very high polarisation that is not consistent with the local relation between water polarisation and the electrostatic field. This effect is responsible for the failure of the Poisson-Boltzmann theory to describe the electrostatics of Newton Black Films. The anomalous dielectric response of water results in a strong electrostatic repulsion between charged species (figure 1), which is the origin of the hydration force in ionic bilayers.

References:
· F. Bresme and J. Faraudo, Computer simulations of Newton Black Films, Langmuir, 20, 5127 (2004).
· J. Faraudo and F. Bresme, Anomalous dielectric behaviour of water in ionic Newton Black Films, Phys Rev Lett, 92, 236102 (2004).
· J. Faraudo and F. Bresme, Origin of the short-range strong repulsive force between ionic surfactant layers, Phys. Rev. Lett., 94,077802 (2005).


J Faraudo and F Bresme (Imperial College)