OMEGA-TERMINATED ALKANETHIOLATE MONOLAYERS ON SURFACES OF COPPER, SILVER, AND GOLD HAVE SIMILAR WETTABILITIES

Long-chain alkanethiols (HS(CH2)nX) adsorb from solution onto the surfaces of freshly evaporated copper, silver, and gold films and form oriented monolayers. Both polar and nonpolar tail groups (X) can be accommodated in these adsorptions. Adsorption on all three metals generates self-assembled monolayers (SAMs) exhibiting similar wetting properties. X-ray photoelectron spectroscopy (XPS) data suggest that the omega-terminated n-alkanethiolate monolayers, like those derived from simple alkanethiols, are composed of trans-extended chains having orientations on copper and silver that are closer to the perpendicular to the surface than are those on gold. These observations suggest that variations in the structure of the underlying polymethylene region of these SAMs have little effect on the interfacial free energy of the SAM as manifested by wetting. We have also characterized monolayers ("mixed monolayers") prepared by exposure of all three metals to mixtures of HS(CH2)11OH and HS(CH2)11CH3. On all three, the wettability of the interfaces covers the range between the extremes: theta(a)H2O = approximately 10-degrees and approximately 115-degrees. Values of the advancing contact angle of water can be related to their composition by Cassie's expression. The similarity in wettabilities of these surfaces and the fact that wettability is related to surface composition by a simple linear relationship both argue that CH2OH and CH2CH3 functional groups behave approximately independently at the monolayer-air (water) interface.