We report a configurational-bias Monte Carlo simulation study to investigate the effect of chain length, system size, and temperature on the preferential adsorption and phase behavior of mixed self-assembled monolayers (SAMs) of alkanethiols on Au(111). Simulations were performed in both semigrand canonical and canonical ensembles. The semigrand canonical simulations were carried out at conditions corresponding to low concentration of HS(CH2)8CH3−HS(CH2)9CH3 and HS(CH2)8CH3−HS(CH2)10CH3 alkanethiol mixtures in solutions at 300 and 373 K. The canonical ensemble simulations were performed at 300 K for a fixed composition of HS(CH2)9CH3−HS(CH2)9+ n CH3 mixed SAMs, where n = 1, 2, ..., 10. Continuous and discrete models were used in these simulations for describing the alkanethiol chains, and an analytical potential was used to model their interactions with the gold surface. Results show the preferential adsorption of long chains on the surface over in the solution and the occurrence of phase segregation when the two components in the mixed SAMs differ by more than three carbon atoms at a fixed surface composition (1:1). Calculated translational, orientational, and conformational orders of the mixed SAMs reveal a more pseudocrystalline structure consisting of closely packed and inclined chains at 300 K than at 373 K. The effect of system size on various properties was also studied.

A miniature electronic nose (ENose) has been designed and built at the Jet Propulsion Laboratory (JPL), Pasadena, CA, and was designed to detect, identify, and quantify ten common contaminants and relative humidity changes. The sensing array includes 32 sensing films made from polymer carbon-black composites. Event identification and quantification were done using the Levenberg-Marquart nonlinear least squares method. After successful ground training, this ENose was used in a demonstration experiment aboard STS-95 (October-November, 1998), in which the ENose was operated continuously for six days and recorded the sensors' response to the air in the mid-deck. Air samples were collected daily and analyzed independently after the flight. Changes in shuttle-cabin humidity were detected and quantified by the JPL ENose; neither the ENose nor the air samples detected any of the contaminants on the target list. The device is microgravity insensitive.

We report a theoretical study of the adsorption behavior of water–methanol mixtures in slit activated carbon micropores. The adsorption isotherms are obtained for a pore of width 2 nm at a temperature of 298 K from grand canonical ensemble Monte Carlo simulations. The water molecules are modeled using the four point transferable intermolecular potential functions (TIP4P) and methanol by the optimized potentials for liquid simulations (OPLS). Carboxyl (COOH) groups are used as active sites on a structured carbon surface. The effect of the relative contributions from dispersion and hydrogen bonding interactions of adsorbates, and of the chemical activation of adsorbents on adsorption behavior is investigated. The adsorption of the mixture components in activated carbon pores occurs by continuous filling, without the sharp capillary condensation observed in graphite pores. Water is preferentially adsorbed over methanol in activated carbon pores for a wide range of pressures, except at lower pressures. The hydrophilic nature of activated carbon pores results in the complexation of both water and methanol molecules with the active sites on the surfaces, leading to bulklike water behavior over the entire pore width. Solvation forces are also calculated as a function of pore size. The negative values found for the solvation force for all pore sizes reflect the hydrophilic interactions of the mixtures with the activated carbon surfaces.

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An electronic nose that uses an array of 32 polymer-carbon black composite sensors has been developed, trained, and tested. By selecting a variety of chemical functionalities in the polymers used to make sensors, it is possible to construct an array capable of identifying and quantifying a broad range of target compounds, such as alcohols and aromatics, and distinguishing isomers and enantiomers (mirror-image isomers). A model of the interaction between target molecules and the polymer-carbon black composite sensors is under development to aid in selecting the array members and to enable identification of compounds with responses not stored in the analysis library.

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In this work, we carried out dual-control-volume grand canonical molecular dynamics simulations of the transport diffusion of liquid water and methanol to vacuum under a fixed chemical potential gradient through a slit pore consisting of Au(111) surfaces covered by −CH3 or −OH terminated self-assembled monolayers (SAMs). Methanol and water are selected as model fluid molecules because water represents a strongly polar molecule while methanol is intermediate between nonpolar and strongly polar molecules. Surface hydrophobicity is adjusted by varying the terminal group of −CH3 (hydrophobic) or −OH (hydrophilic) of SAMs. We observed for the first time from simulations the convex and concave interfaces of fluids transporting across the slit pores. Results show that the characteristics of the interfaces are determined by the interactions between fluid molecules and surfaces. The objective of this work is to provide a fundamental understanding of how these interactions affect transport diffusion.