PHASE-BEHAVIOR OF 2-COMPONENT SELF-ASSEMBLED MONOLAYERS OF ALKANETHIOLATES ON GOLD

This paper examines the relationship between the composition of two-co mponent self-assembled monolayers (SAMs) of alkanethiolates on gold an d the composition of the solutions from which they were formed. The SA Ms were prepared by competitive adsorption of a long-chain alkanethiol (HS(CH2))(21)CH3) and a short-chain alkanethiol (HS(CH2)(11)OH) from solutions in ethanol. Under conditions in which the alkanethiolates in a SAM and the alkanethiols in solution are close to equilibrium, the relationship between the composition of the solution and the compositi on of the SAM suggests that the monolayer tends, thermodynamically, to exist as a single phase predominantly composed of either long-chain o r short-chain thiolates. A derivation of the thermodynamic relationshi p between the compositions of the SAM and solution is described that i ncludes intermolecular interactions between components in the SAM; the ory and experiment agree qualitatively. This analysis concludes that, for a two-component system of alkanethiolates on gold well-equilibrate d with alkanethiols in solution, a single phase is preferred at equili brium; phase-separated, two-component monolayers of the sort extensive ly studied in Langmuir systems are not observed.