Phase behavior of two-component self-assembled monolayers of alkanethiolates on gold

Abstract

This paper examines the relationship between the composition of two-component self-assembled monolayers (SAMs) of alkanethiolates on gold and the composition of the solutions from which they were formed. The were prepared by competitive adsorption of a long-chain alkanethiol (HS(CHz)zrCHl) and a short-chain alkanethiol (HS(CHz)rrOH) 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 composition of the SAM suggests that the monolayer tends, thermodynamically, to exist as a single phase predominantly composed of either long-chain or short-chain thiolates. A derivation of the thermodynamic relationship betwecn the compositions of the SAM and solution is described that includes intermolecular interactions between components in the SAM; theory and experiment agre,equalitatively. This analysis concludes that, for a two-cornponent system of alkanethiolates on gold well-equilibrated with alkanethiols in solution, a single phase is preferred at equilibrium; phase-separated, two-component monolayers of the sort extensively studied in Langmuir systems are not observed. Introduc6on Mired SAMs on gold--elf-assembled monolayers (SAMs) compnsing two alkanethiolates prepared by cochemisorption from a solution containing two alkanethiols-are useful in studying phenomena involving organic surfaces.2-I3 A number of techniques can establish the auerage composition of a mixed SAM.2-I t X-ray photoelectron spectroscopy, XPS, can ascertain the composition over an area thesize of the X-rayspot (approximately I mm2), and scanning electron microscopy can sometimes detect heterogeneities in composition of a SAM on thedimension of the order of I prn;tl phase separation has not been observed on thesc scales. In short, neither has thc extcnt to which the components of a mixed SAM segregate into separate phases within a SAM been established nor has the question of the relationship betwecn the composition of the solution used to form the SAM and its heterogeneity bcen addressed experimentally or theoretically. Thc problem is a difficult one experimentally, because there are few techniques well adapted for dircctvisualization of phase-separated regions with small sizes in an organic monolayer, especially on optically opaque sub,strates. Scanning probe microscopies (especially atomic and lateral force microscopies)17 are techniques that are applicable but only to certain types of SAMs.rT-re In this psper, we investigate the phase behavior of twocomponent from both theoretical and experimental perspectives. The approach we have taken is thermodynamic-to scarch for characteristic signatures of phase separation in the dependencc of average composition on temperature, con@ntration, and tim-rather than spcctroscopic and based on irraging. We focused on the question of whether two-component form phasc-separated domains. In the theoretical section, we derive a thermodynamic relationship between the composition of the solution and the composition of the sAM in terms of interactions bctween nearest neighbors within the SAM. This relationsbip prcdicts that phase separation in a SAM by a mechanism involving theequilibrium of species in thc sAM with those in solution will be detectable through a sharp transition between the properties characteristic of one monolayer to those characteristic of thc second.