Force Measurements between Semifluorinated Thiolate Self-Assembled Monolayers: Long-Range Hydrophobic Interactions and Surface Charge

摘要

Long-range interactions between self-assembled monolayers (SAMs) of semifluorinated alkanethiols have been studied by direct froce measurements in water and aqueous NaCl solutions. SAMs prepared from three different thils, with identical fluorinated head groups but varying hydrocarbon spacer lengths, were investigated: CF_3(CF_2)_9(CH_2)_xSH, where x = 2, 11, or 17. Force measurements show that the interactions in water and electrolyte solutions are composed of both double-layer interactions emerging from what appears to be charges adsorbed onto the surfaces and long-range "hydrophobic" attractions, in excess of the expected van der Waals forces. The three investigated thiols produce similar results in force measurements, though the contact angles with water the slightly different. The "hydrophobic" attraction has the form of step-like attractive discontinuities in the force profile at separations ranging from 20 to 40 nm, caused by bridging of microscopic bubbles residing at the surfaces. The shape or rgane of these discontinuities are not significantly affected by replacement of the water with either 1 mM or 1M NaCl solutions. The origin of the charges causing the electrostatic double-layer interaction is unclear, but some possible causes are discussed.