Forces between polystyrene surfaces in water-electrolyte solutions: Long-range attraction of two types?

A great deal of effort has recently been focused on the experimental studies of the phenomenon of long-ranged attraction between identically charged colloidal (polystyrene) particles immersed in an electrolyte. The theoretical validation suggested the need for revision of the existing and established colloidal theories assuming the observed attraction is of electrostatic origin. We, however, demonstrate that for a number of reasons (first of all hydrophobicity and roughness of particles) the Derjaguin-Landau-Verwey-Overbeek (DLVO) behavior should not be expected for polystyrene surfaces. Indeed, the force measurements with an atomic force microscope-related set-up suggest that even within one pair of the interacting surfaces, attractive interaction of various types can be observed. There is usually a difference between the first approach and the later ones. The first approach is characterized by a short-range jump into a contact. Depending on conditions (electrolyte concentration, previous contacts of surfaces, etc.) there exists a late attraction of two types between polystyrene surfaces. The force of the first type is characterized by an abrupt jump from the maximum of a repulsive force, which is typically of longer-range than on the first approach. This is most likely due to submicroscopic bubbles trapped and/or formed due to previous contacts (and separation) of the surfaces. The attraction of the second type is weak and exponentially decaying. (C) 2001 American Institute of Physics.