COMPARISONS OF SELF-ASSEMBLED MONOLAYERS ON SILVER AND GOLD - MIXED MONOLAYERS DERIVED FROM HS(CH2)21X AND HS(CH2)10Y (X, Y = CH3, CH2OH) HAVE SIMILAR PROPERTIES

This paper describes the preparation of self-assembled monolayers (SAMs) by adsorption of alkanethiols on evaporated silver, and compares these SAMs with analogous SAMs prepared on gold. The work concentrated on SAMs derived from mixtures of alkanethiols having long and short chains and terminating in hydrophobic and hydrophilic tails: HS(CH2)10CH3,HS(CH2)21CH3, HS(CH2)10CH2OH, and HS(CH2)21CH2OH. The thickness and the compositions of these SAMs were established by X-ray photoelectron spectroscopy (XPS). As is observed with self-assembled alkanethiolate monolayers on gold, the composition of the monolayers parallels but does not equal the solution composition. Less pronounced differences are observed between compositions in solution and on the surface on silver than have previously been reported on gold. Rates of exchange of surface thiolates with thiols in solution are similar on silver and gold. This similarity suggests that the differences between the compositions of the mixed SAMs formed from solutions having the same composition by adsorption on silver and gold are due to factors important during the initial formation of these monolayers, rather than during their subsequent equilibration. Although mixtures of thiols are probably not adsorbed in a completely random fashion on silver, no evidence could be found for the formation of discrete, separate phases on the surface (i.e., islands). Measurements of surface wettability (via contact angle measurement with water and hexadecane) exhibit departures from Cassie's expression, indicating appreciable disorder in the hydrocarbon chains at the monolayer/liquid interface. The wettability of SAMs on silver by water and hexadecane were similar to those formed on gold having the same composition.