The energetics and kinetics of the adsorption of a variety of sulfur-containing hydrocarbons onto Au(111) have been explored using helium beam reflectivity and temperature-programmed desorption (TPD) techniques. Simple alkanethiols as well as dialkyl sulfides, dialkyl disulfides, and other sulfur-containing organics were found to adsorb with a low coverage physisorption enthalpy about 20% greater than the heat of vaporization in the bulk. In contrast to the dialkyl sulfides that only physisorb, alkanethiols and dialkyl disulfides also interact chemically with the gold with a chemisorption enthalpy of 126 kJ/mol that is independent of alkyl chain length. The presence of sterically hindering substituent groups on the carbon atom adjacent to the sulfur atom produces, however, a reduction in the chemisorption enthalpy of up to 15%. Temperature-programmed desorption of nonequilibrated, high exposure layers of alkanethiols with eight carbon atoms or longer displayed a second, higher energy, chemisorption peak at 148 kJ/mol which could be eliminated by annealing, leaving only the lower energy chemisorption peak. The chemisorption rate of molecules in the physisorbed precursor state has also been measured. Arrhenius plots for the chemisorption of a series of alkanethiols from their physisorbed precursors show that the chemisorption activation energy is 28.8 kJ/mol and is independent of chain length.

Helium atom reflectivity has been used to study the adsorption of a series of n-alkanes, 1-alkenes, and cyclic hydrocarbons on a Au(111) surface. Using this technique, both adsorption and desorption could be observed with high sensitivity under UHV conditions to determine adsorption energies and initial sticking coefficients. For the long-chain n-alkanes studied (C6H14−C12H26), the physisorption energy increases linearly with the chain length by 6.2 ± 0.2 kJ/mol per additional methylene unit. The physisorption energies of the 1-alkenes (C6H12−C11H22) show a similar linear dependence on chain length but are slightly higher than those of the corresponding alkanes. A bond-additive model is presented which is capable of predicting the adsorption energy of 25 saturated and unsaturated hydrocarbons on the basis of four fitted parameters with an average error of 1.9%. Of the molecules considered, 84% of the calculated adsorption energies differ from the experimental value by less than twice the average error. When 10 sulfur-containing compounds and two fitting parameters are added, the average error grows to 2.6%. For all linear hydrocarbons studied, the physisorption sticking coefficient is a function of the reduced surface temperature T*, which is defined as the temperature measured in units of the peak desorption temperature as observed by temperature programmed desorption. The sticking coefficient of each species is close to unity at low temperatures, starts to decrease at T* = 0.8, and reaches zero as the crystal temperature approaches the peak desorption temperature.

We present a study using several techniques of the growth of decanethiol monolayers deposited on singlecrystal gold surfaces. Through independent measurements of coverage, energetics, and structure as a function of the growth rate and temperature, we provide a quantitative, in-depth description of the molecular processes by which these aliphatic molecules ''self-assemble'' into highly ordered structures in the absence of a solvent. We find that the multiple-energy scales present in these systems produce distinct adsorption mechanisms, structures, and growth regimes, indicating a complexity that is likely to be a general characteristic of this broad class of self-assembling systems. ͓S0163-1829͑98͒03019-7͔

A versatile method for measuring the partition coefficients of volatile analytes with an aqueous pseudophase using headspace gas chromatography is reported. A "three-phase" model accounts for all equilibria present in the system, including the partitioning of the analyte in the gas and aqueous phases to the pseudophase. This method is applicable to a wide variety of volatile analytes and aqueous pseudophases, providing that sufficient pseudophase may be used to reduce the analyte partial pressure. Generally, the method offers good reproducibility and high sensitivity. The associations of five volatile analytes (hydrogen sulfide, methanethiol, dimethyl sulfide, dichloromethane, and ethyl ether) with various cyclodextrins were examined. All analytes were found to partition preferentially to the cyclodextrin pseudophase compared to the aqueous phase. In addition, several analyte-cyclodextrin combinations formed insoluble complexes in solution that enhanced the extraction of the analyte from the gas and aqueous phases. Derivatization of the cyclodextrins generally decreased the extent of analyte-cyclodextrin interaction.

No abstract