A re-interpretation of hydration forces near charged surfaces

Abstract

The concept of hydration forces (bound water) has long been applied to colloid science and its biological applications^1–11. While these repulsive forces have been demonstrated experimentally in many cases^1,2,6–11 our understanding of them remains uncertain^3–5,12. Their best definition is perhaps as the discrepancy between experimental results and the currently accepted Gouy–Chapman–Stern/Derjaguin–Landau–Verwey–Overbeek (GCS/ DLVO) theories. We have found that a new repulsive force is predicted by application of the Debye–Hückel–Manning (DHM) approach to double-layer overlap^13–15 using the ion-binding/ion-bouncing model (IBBM)¹⁵. This new force arises when overlapping double layers approach each other in such a way that the co–ions are completely expelled and counter-ions are squeezed into a smaller volume. The magnitude of these repulsions is of the order of hundreds of atmospheres at close separations. We therefore question the current interpretations of these forces with the warning that the electrostatic model outlined below needs further elaboration for neutral and weakly charged surfaces^16,17.