Chemical Force Microscopy Study of Adhesion and Friction between Surfaces Functionalized with Self-Assembled Monolayers and Immersed in Solvents. 
Adhesive and frictional forces between surfaces modified with self-assembled monolayers (SAMs) and immersed in solvents were measured with chemical force microscopy as functions of surface functionality and solvent.  Si/SiO2 substrates were modified with SAMs of alkylsiloxanes (SiCl3(CH2)n-X), and gold-coated AFM tips were modified with SAMs of alkylthiolates (HS-(CH2)n-X).  SAMs of alkylsiloxanes terminated in a methyl or oxidized vinyl group; SAMs of alkanethiolates terminated in a methyl or carboxyl group.  Adhesive and frictional forces were measured in hexadecane, ethanol, 1,2-propanediol, 1,3-propanediol, and water.  The work of adhesion (W) was calculated with the Johnson-Kendall-Roberts theory of adhesive contact.  The JKR values agreed well with values derived from the Fowkes-van Oss-Chaudhury-Good surface tension model and from contact angle results.  Calculated values of W for all combinations of contacting surfaces and solvents spanned two orders of magnitude.  W correlated with the surface tension of the solvent for hydrophobic/hydrophobic interactions; hydrophilic/hydrophilic and hydrophobic/hydrophilic interactions were more complex.  Friction forces were fit to a modified form of Amonton's law.  For any solvent, friction coefficients were largest for the hydrophilic/hydrophilic contacting surfaces.  The friction coefficient for any contacting pair was largest in hexadecane.  In polar solvents, friction coefficients scaled with solvent polarity only for hydrophobic/hydrophobic contacting pairs.  Copyright 1999 Academic Press.