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 SAMs were prepared by competitive adsorption of a long-chain alkanethiol (HS(CH2)21CHs) and a short-chain alkanethiol (HS(CH2)110H) 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 between the compositions of the SAM and solution is described that includes intermolecular interactions between components in the SAM; theory and experiment agree qualitatively. This analysis concludes that, for a two-component 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. Introduction'Mixed SAMs" on gold-self-assembled monolayers (SAMs) comprising two alkanethiolates prepared by m-chemisorption from a solution containing two alkanethiols-are useful in studying phenomena involving organic surface^.^-'^ A number of techniques can establish the average composition of a mixed SAMa2-11 X-ray photoelectron spectroscopy, XPS, can ascertain the compositionover an area thesizeofthe X-ray spot (approximately 1 mmz), and scanning electron microscopy can sometimes detect heterogeneities in composition of a SAM on the dimension of the order of 1 pm;14 phase separation has not been observed on these scales. In short, neither has the extent to which the components of a mixed SAM segregate into separate phases within a SAM been established nor has the question of the relationship between the composition of the solution used to form the SAM and its heterogeneity been addressed experimentally or theoretically. The problem is a difficult one experimentally, because there are few techniques well adapted for direct visualization of phase-separated regions with small sizes in an organic monolayer, especially on optically opaque substrates. Scanning probe microscopies (especially atomic and lateral force microscopies)17 are techniques that are applicable but only to certain types of SAMs.17-19In this paper, we investigate the phase behavior of twocomponent SAMs from both theoretical and experimental perspectives. The approach we have taken is thermodynamicto search for characteristic signatures of phase separation in the dependence of average composition on temperature, concentration, and time-rather than spectroscopic and based on imaging. We focused on the question of whether two-component SAMs form phase-separated domains. In the theoretical section, we derive a thermodynamic relationship b...

Voltammetry of immobilized cytochrome c on novel binary self-assembled monolayers of thioctic acid and thioctic amide modified gold electrodes

“…It is considered that the surface composition of SAMs is usually controlled by the ratio of two thiols in an adsorption solution [41][42][43][44][45][46]. The effect of the mixing ratio in the absorption solution on the amount and electron transfer rate constant of adsorbed cyt c attributed to the change of the surface structure of SAMs.…”

“…In fact, binary SAMs exhibit either homogeneous mixing or phase separation depending on the relative interactions between adsorbed molecules, which can be classified to three cases. Firstly, when the interaction between A-A or B-B is stronger than that of A-B, a monolayer is separated into two types of domains, as mentioned above [41][42][43][44][45][46].…”

Self-Assembled Monolayers of Symmetrical and Mixed Alkyl Fluoroalkyl Disulfides on Gold. 1. Synthesis of Disulfides and Investigation of Monolayer Properties

“…Given that the mobility of the thiolates is high enough at temperatures below which desorption becomes competitive, phase separation into monodomains should be feasible. A simulation 24 as well as a thermodynamical treatment of a possible phase separation in monolayers 25 suggest the formation of monodomains in the case of mobile molecules. Since phase separation would require independent behavior of the two parts of the molecule, this would unequivocally prove the cleavage of the sulfursulfur bond of the disulfide in the reaction with the gold and the existence of thiolates on the gold surface.…”

Binary functionalization of H:Si(111) surfaces by alkyl monolayers with different linker atoms enhances monolayer stability and packing

“…In ref 4 , it was shown that, for example, the wetting characteristics of gold surfaces can be tuned by mixed monolayers which contain both hydrophobic and hydrophilic components. In another work, the use of different ratios of two types of SAMs, namely n-alkanephosphonic and fluorinated phosphonic acid, which decrease and increase the WF, respectively, can tune the WF of indium-tin-oxide (ITO) over in the range of 5 -5.75 eV (the WF of bare ITO is ~ 5.65 eV).…”