title: AFM forces between mica and polystyrene surfaces in aqueous electrolyte solutions with and without gas bubbles.
abstract: Force curves between a flat mica substrate and a polystyrene microsphere were measured with an atomic force microscope (AFM) in carefully degassed water and aqueous NaCl, CaCl2, and AlCl3 solutions. The pH of the water used does not change significantly with degassing treatment, and its value remains close to 6. Electrolyte concentration ranges from 10-4 to 10-2M and pH from 4.7 to 5.1. We have found that the repulsive long-range electrostatic force between mica and polystyrene is attenuated by the presence of electrolytes and counterbalanced by a long-range attractive force, which we referred to as a hydrophobic force, which is longer-ranged than the ever present attractive van der Waals force. This force, which includes the adhesive bridging of residual air bubbles and newborn vapor cavities, and any other unknown forces, is reasonably well represented by a unique exponential law. Prefactor and decaying length are not very sensitive to electrolyte type, concentration, and pH, suggesting that any new force included in the law, in addition to adhesive bridges, should obey a non-classical electrostatic mechanism. However, we also know that liquid/solid contact angle and liquid/vapor surface tension increase with electrolyte concentration and valence increasing the stability of bubbles and cavities which in turn increase the bridging force. Clearly, these effects are hidden in the empirical force law.
authors.affiliation: Chemical Engineering Department and Surface Analysis Laboratory (ASIF), University of Concepción, PO Box 160-C, Correo 3, Concepción, Chile.
authors.forename: Jorge H
authors.initials: JH
authors.lastname: Saavedra
authors.forename: Sergio M
authors.initials: SM
authors.lastname: Acuña
authors.forename: Pedro G
authors.initials: PG
authors.lastname: Toledo
mesh_terms: D000077410:Aluminum Chloride; D017607:Aluminum Compounds / Q000737:chemistry*; D002122:Calcium Chloride / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D004573:Electrolytes / Q000737:chemistry; D005740:Gases / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D018625:Microscopy, Atomic Force; D011137:Polystyrenes / Q000737:chemistry*; D012965:Sodium Chloride / Q000737:chemistry*; D012996:Solutions; D013499:Surface Properties; D014867:Water / Q000737:chemistry
chemical_list: D017607:Aluminum Compounds; D002712:Chlorides; D004573:Electrolytes; D005740:Gases; D011137:Polystyrenes; D012996:Solutions; D014867:Water; D000077410:Aluminum Chloride; D012965:Sodium Chloride; D002122:Calcium Chloride
keywords: AFM; Extended DLVO; Hydrophilic–hydrophobic interaction; Hydrophobic force; Nanobubbles; Vapor cavities