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A combination of the aggregation-volume-bias and configurational-bias Monte Carlo algorithms and the umbrella sampling technique was applied to investigate two different binary vapor-liquid nucleation systems: water/ethanol and water/ n-nonane. Structural analysis reveals that the formation of an ethanol-enriched surface region is responsible for the observed mutual enhancement and also the shortcomings of the CNT predictions for the water/ethanol systems.

Adsorption and ordering at the vapor-liquid interfaces of mutually saturated water/1- butanol solutions at a temperature of 298.15K are investigated using configurational-bias Monte Carlo simulations. The calculated surface tension of the water phase decreases by about 35% upon saturation with 1-butanol solutions.

The importance of adsorption at the gas-liquid interface on retention in gas-liquid chromatography has been controversial since the pioneering work of Martin in the 1960s. In particular, experimental studies using chromatographic and static techniques to quantify partitioning and adsorption of polar analytes on nonpolar liquid phases yielded conflicting results. In this work, Monte Carlo simulations were carried out for a free-standing liquid slab of squalane surrounded by a helium vapor to investigate interfacial adsorption effects for n-pentane, n-hexane, n-heptane, 1-butanol, and benzene solutes at infinite dilution. The simulations indicate preferential adsorption for the flexible alkane and alcohol solutes in a narrow region just inside the Gibbs dividing surface, but no such effect was observed for the rigid benzene solute. Nevertheless, the extent of the interfacial enrichment is small, as measured by the partition coefficient between the bulk liquid and the interfacial region ([K.sub.bulk-interface] [approximately equal to] 1.5). In addition, a region that is slightly depleted for all solute molecules is found to separate the interfacial and bulk regions of the squalane slab. Thus, adsorption at the gas-liquid interface should not contribute significantly to the retentive behavior observed in gas-liquid chromatography on nonpolar capillary columns but might play a role in packed-bed columns with low bonded-phase loadings. The origin for the small enrichments and more favorable free energies for solutes at the interface is that the enthalpies of solvation decrease to a smaller relative extent than the entropies of solvation compared to the bulk liquid.

In an attempt to elucidate the molecular basis for concentration (isotherm) effects on retention in gas-liquid chromatography, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out to investigate changes in analyte partitioning caused by overloading a model chromatographic system with either an alkane or an alcohol. Squalane was used as the stationary-phase material, and the analytes included n-pentane, n-hexane, n-heptane, 1-butanol, and 1-pentanol. Three systems were studied that differed in the mobile-phase composition: (i) a helium vapor, (ii) a n-hexane vapor, and (iii) a 1-pentanol-saturated helium vapor. While the amount of helium that partitions into the stationary phase is very small, both n-hexane and 1-pentanol partition strongly into and thereby swell the stationary phase. Although the swelling of the stationary phase leads to a reduction in the partition coefficients for the alkane solutes for both the n-hexane-and 1-pentanol-swollen stationary phases, the effects on the alcohol solutes differ markedly. Whereas saturation by n-hexane causes a decrease of the alcohol partition contants (to an extent similar to that for the alkane solutes), the saturation by 1-pentanol causes a dramatic increase of the alcohol partition coefficients; e.g., the Kovats index of 1-butanol increases by more than 150 Kovats units. The formation of hydrogen-bonded alcohol aggregates in the liquid phase is the microscopic origin for the dramatic effect of 1-pentanol saturation on the retention of alcohols.

A novel Monte Carlo method, called the aggregation-volume-bias Monte Carlo algorithm (AVBMC), is presented, which greatly enhances the efficiency of sampling the phase space of fluid systems consisting of strongly associating molecules. The efficiency of AVBMC algorithm is demonstrated for a large variety of processes and systems, including the vaporization of a liquid methane droplet or a water cluster, an investigation of the temperature-and pressure-dependent properties of super heated hydrogen fluoride vapor, and the vapor-liquid coexistence curve of acetic acid.

Research is presented concerning the computation of temperature-composition and pressure-composition phase diagrams for four molecular models of a binary mixture of supercritical ethane and n-heptane. Comparison with a binary mixture of helium and n-heptane is made.

Simulation experiments demonstrate predicting the relative retention times of 10 alkane isomers based on their topology in a helium-squalane gas-liquid chromatographic system.

The solubilities of small solutes in normal alkanes depend on the solvent chain length. Gibbs-ensemble Monte Carlo simulations were conducted to investigate the vapor-liquid partitioning of methane in normal alkanes and related model solvents. The simulations revealed that the increase in solvent density with increasing solvent chain length is the main cause of the solvent chain length dependence for normal alkanes.