Formation of monolayers by the coadsorption of thiols on gold: variation in the length of the alkyl chain

Abstract

when X = Y = oH' the pure monolayers a. *etid by water, but the mixed monolayers are less hydrophilic because nonpolar polymethylenecha insareexposedatt hesurface. whinX =cHr, Y = oH (n=2L,m= ll),averysharptr ansitionoccurs from a monolayer composed largely of the longer, methyl-terminated component to the shorter, hydroxyl-terminated component as the mole fraction of HS(CHz) r roH in-the adsorption solution is increased. From solutions .ontuining two thiols, adsorption of the thiol with the longer chain is preferred. Tiris preference is greater when the monolayers are adsorbed from ethanol than from isooctane' The mixed monolayers do not uit , ideal two-dimensional solutions. T-he adsorption isotherms suggest a positive excess free energy of mixing of the two components in the monolayer. The compositions of the monolayers appear to be determined largely by thermodynamics, although in some cases there is also a kinetic contribution. The two components in the mixed monolayers do not phase-segregate into macroscopic islands (greater than a few tens of angstroms across) but are probably not randomly dispersed within the monolayer, ThL wettability of mixed, methyl-terminatea can be partially rationalized by the geometric m€an approximition, but a full description probably requires inclusion of the entropy of mixing at the monolayer-liquid interface. The hysteresis in the contact angle on these monolayers cannot be explained by theories of wetting based on macroscopic heterogeneity. Contact angles ur? -o.. sensitive than optical ellipsometry or X-ray photoelectron spectroscopy to certain types of changes in the coriposition and structure of these monolayers.