Forces between Hydrophobic Solids in Concentrated Aqueous Salt Solution

Abstract

Much research has focused on the discovery and description of long-ranged forces between hydrophobic solids immersed in water. Here we show that the force between high contact-angle solids in concentrated salt solution (1M KCl) agrees very well with vanderWaals forces calculated from Lifshitz theory for separations greater than 5nm. The hydrophobic solids are octadecyltrichlorosilane-coated glass, with an advancing contact angle of 108°. Thus, in 1M salt solution, it is unnecessary to invoke the presence of a hydrophobic force at separations greater than 5nm. Through measurement in salt solution, we avoid the necessity of accounting for large electrostatic forces that frequently occur in pure water and may obscure resolution of other forces.