CHAIN-LENGTH AND SOLVENT EFFECTS ON COMPETITIVE SELF-ASSEMBLY OF FERROCENYLHEXANETHIOL AND 1-ALKANETHIOLS ONTO GOLD

Competitive self-assembly of ferrocenylhexanethiol (Fc-C6-SH) and 1-al kanethiols onto gold from ethanol and 1-hexanol solution is interprete d using a model based on coupled Frumkin isotherms. Alkanethiols with long alkyl chains are always preferentially adsorbed relative to Fc-C6 -SH. In the limit of low ferrocene coverage, that preference is descri bed quantitatively in terms of a parameter DELTA(DELTAG2 - DELTAG1)' t hat describes the incremental change per methylene unit in the charact eristic free energy difference controlling competitive adsorption. Tha t incremental change is found to be -1.9 kJ mol-1 per methylene unit i ncrease in alkanethiol chain length for adsorption from ethanol and -0 .8 kJ mol-1 per methylene unit increase for adsorption from 1-hexanol. These values are smaller than would be anticipated from independent e stimates of the incremental change in cohesive energy of n-alkanes per methylene unit derived from vapor pressure, fusion enthalpy, vaporiza tion enthalpy, and sublimation enthalpy data. Finally, data on ferroce ne coverage as a function of exposure time to a coating solution indic ate that monolayer composition can change over time during self-assemb ly, reaching a limiting composition only after exposure times of betwe en several hours and several days. It is postulated that the preferenc e for adsorption of long-chain alkanethiols is manifest primarily in t he slow relaxation of an initially-formed disordered monolayer to a mo re ordered structure.