Professor Georges A. Guiochon's 75th birthday.
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Chromatography↗
Biomedical subjects
Mark R Schure
Publications and source records attributed to Mark R Schure.
Chain conformation and solvent partitioning in reversed-phase liquid chromatography: Monte Carlo simulations for various water/methanol concentrations.
Many structural models for the stationary phase in reversed-phase liquid chromatography (RPLC) systems have been suggested from thermodynamic and spectroscopic measurements and theoretical considerations. To provide a molecular picture of chain conformation and solvent partitioning in a typical RPLC system, a particle-based Monte Carlo simulation study is undertaken for a dimethyl octadecyl (C(18)) bonded stationary phase on a model siliceous substrate in contact with mobile phases having different methanol/water concentrations. Following upon previous simulations for gas-liquid chromatography and liquid-liquid phase equilibria, the simulations are conducted using the configurational-bias Monte Carlo method in the Gibbs ensemble and the transferable potentials for phase equilibria force field. The simulations are performed for a chain surface density of 2.9 micromol/m(2), which is a typical bonded-phase coverage for mono-functional alkyl silanes. The solvent concentrations used here are pure water, approximately 33 and 67% mole fraction of methanol and pure methanol. The simulations show that the chain conformation depends only weakly on the solvent composition. Most chains are conformationally disordered and tilt away from the substrate normal. The interfacial width increases with increasing methanol content and, for mixtures, the solvent shows an enhancement of the methanol concentration in a 10 Angstrom region outside the Gibbs dividing surface. Residual surface silanol groups are found to provide hydrogen bonding sites that lead to the formation of substrate bound water and methanol clusters, including bridging clusters that penetrate from the solvent/chain interfacial region all the way to the silica surface.
Chromatography, Liquid↗
Retention in gas-liquid chromatography with a polyethylene oxide stationary phase: molecular simulation and experiment.
Configurational-bias Monte Carlo simulations in the isobaric-isothermal Gibbs ensemble were carried out to investigate the partitioning of normal alkanes, primary and secondary alcohols, symmetric alkyl ethers and arenes between a helium vapor phase and a polyethylene oxide stationary phase (M(W)=382 g mol(-1)). The united-atom version of the transferable potentials for phase equilibria force field was used to model all solutes, polyethylene oxide and helium. The Gibbs free energies of transfer and Kovats retention indices of the solutes were calculated directly from the partition constants at two different temperatures, 353 and 393 K. Chromatographic experiments on a Carbowax 20M retentive phase were performed for the same set of solutes and temperatures ranging from 333 to 413 K. The predicted retention indices for alcohols, ethers and arenes are overestimated by about 120, 70 and 20 retention index units, respectively, pointing to an overestimation of the first-order electrostatic interactions in the model system. Molecular-level analysis shows that hydrogen-bonding and dipole-dipole interactions lead to orientational ordering for the alcohol and ether analytes, whereas the weaker dipole-quadrupole interactions for the arene solutes are not sufficient to induce orientational ordering. The retention indices of alcohols and ethers decrease with increasing temperature because of the large entropic cost of hydrogen-bonding and orientational ordering. In contrast, the retention indices for arenes increase with increasing temperature because the entropic cost of cavity formation is smaller for arenes than for comparable alkanes.
Alcohols↗
How does column packing microstructure affect column efficiency in liquid chromatography?
Full three-dimensional computer simulations of the fluid flow and dispersion characteristics of model nonporous chromatographic packings are reported. Interstitial porosity and packing defects are varied in an attempt to understand the chromatographic consequences of the packing microstructure. The tracer zone dispersion is calculated in the form of plate height as a function of fluid velocity for seven model particle packs where particles are selectively removed from the packs in clusters of varying size and topology. In an attempt to examine the consequences of loose but random packs, the velocities and zone dispersion of seven defect-free packs are simulated over the range 0.36< or =epsilon< or =0.50, where epsilon is the interstitial porosity. The results indicate that defect-free loose packings can give good chromatographic efficiency but the efficiency can vary depending on subtle details of the pack. When the defect population increases, the zone dispersion increases accordingly. For a particle pack where 6% of the particles are removed from an epsilon=0.36 pack, approximately 33% of the column efficiency is lost. These results show that it is far more important in column packing to prevent defect sites leading to inhomogeneous packing rather than obtaining the highest density pack with the smallest interstitial void volume.
Chromatography, High Pressure Liquid↗
Conformation and solvation structure for an isolated n-octadecane chain in water, methanol, and their mixtures.
Configurational-bias Monte Carlo simulations in the isobaric-isothermal ensemble (T = 323 K and p = 10 atm) were carried out to probe structural properties of an isolated n-octadecane chain solvated in water, methanol, water-rich, or methanol-rich mixtures and, for comparison, of an isolated chain in the gas phase and for neat liquid n-octadecane. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to represent n-octadecane and methanol and the TIP-4P model was used for water. In all six environments, broad conformational distributions are observed and the n-octadecane chains are found to predominantly adopt extended, but not all-trans conformations. In addition, a small fraction of more collapsed conformations in which the chain ends approach each other is observed for aqueous hydration, the water-rich solvent mixture and the gas phase, but the simulation data do not support a simple two-state picture with folded and unfolded basins of attraction. For chains in these three "poor" solvent environments, the dihedral angles near the center of the chain show an enhancement of the gauche population. The ensemble of water-solvated chains with end-to-end contacts is preferentially found in a U-shaped conformation rather than a more globular state. An analysis of the local solvation structures in the water-methanol mixtures shows, as expected, an enrichment of the methyl group of methanol near the methylene and methyl segments of the n-octadecane chain. Interestingly, these local bead fractions are enhanced by factors of 2.5 and 1.5 for methyl and methylene segments reflecting the more hydrophobic nature of the former segments.
Alkanes↗
Temperature dependence of hydrogen bonding: an investigation of the retention of primary and secondary alcohols in gas-liquid chromatography.
Configurational-bias Monte Carlo simulations in the Gibbs Ensemble were carried out to investigate the analyte partitioning of n-pentane, n-hexane, n-heptane, 1-propanol, and 2-propanol into a dioctyl ether retentive (stationary) phase used in gas-liquid chromatography. The united-atom version of the TraPPE (transferable potentials for phase equilibria) force field was used to model all analytes and the solvent. The analyte partition coefficients, Gibbs free energies of transfer, and Kovats retention indexes were calculated at four different temperatures ranging from 303.15 to 348.15 K. Although hydrogen bonding is a major contributor to the retention of the alcohol analytes over the entire temperature range, its importance for the separation factor between the primary and secondary alcohol decreases substantially with increasing temperature. The enthalpies and entropies for hydrogen bond formation were also estimated from the temperature dependence of the corresponding equilibrium constants. In agreement with experimental measurements, it is observed that the hydrogen bond involving 1-propanol is enthalpically favored, but entropically disfavored compared to 2-propanol.
Journal Article↗
Molecular simulation study of the bonded-phase structure in reversed-phase liquid chromatography with neat aqueous solvent.
The dramatic loss of retention in reversed-phase liquid chromatography when switching to 100% aqueous solvent and stopping flow (depressurizing) has long intrigued separation scientists. Recent experimental evidence suggests that the observed loss of retention is due to the loss of pore wetting with subsequent loss of solvent penetration in the porous matrix. One of the prevalent explanations of this phenomenon has been that the bonded phase chains, typically octadecyl silane bound to porous silica, would undergo significant conformational changes, viz. collapse, under pure aqueous conditions. As a definitive means toward elucidating the conformation of bonded-phase chains under pure aqueous conditions, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for a system of dimethyl octadecyl silane of intermediate coverage bound to the (111) face of beta-cristobalite and immersed in pure water. The results of two sets of simulations, which were started from two entirely different starting configurations as a validity check toward reaching the same equilibrium distribution of states, show that chains are neither clustering together nor laying on the surface but rather have a broad distribution of orientations and of conformational states. The interface between the bonded and solvent phases is rough on a molecular level, and clusters of water molecules are sometimes found to adsorb at the silica surface. This computational study lends further evidence that the driving force for the loss of retention when switching to pure aqueous conditions and depressurizing is not the collapse of bonded-phase chains.
Alkanes↗
Simulation studies on the effects of mobile-phase modification on partitioning in liquid chromatography.
Various driving forces have been suggested to explain retention and selectivity in reversed-phase liquid chromatography (RPLC). To provide molecular-level information on the retention mechanism in RPLC, configurational-bias Monte Carlo simulations in the Gibbs ensemble were carried out for model systems consisting of three phases: an n-hexadecane retentive phase, a mobile phase with varying water-methanol composition, and a helium vapor phase as reference state. Liquid n-hexadecane functions as a model of a hydrophobic stationary phase, and a wealth of experimental data exists for this system. Gibbs free energies for solute transfers from gas to retentive phase, from gas to mobile phase, and from mobile to retentive phase were determined for a series of short linear alkanes and primary alcohols. Although the magnitude of the incremental Gibbs free energy of transfer for a methylene segment is always larger for the gas- to retentive-phase transfer than the gas- to mobile-phase transfer, it is found that the partitioning of alkanes and alkyl tail groups is mostly affected by the changes in the aqueous mobile phase that occur when methanol modifiers are added. In contrast, the partitioning of the alcohol headgroup is sensitive to changes in both the n-hexadecane and the mobile phases. In particular, it is found that hydrogen-bonded aggregates of methanol are present in the n-hexadecane phase for higher methanol concentrations in the mobile phase. These aggregates strongly increase alcohol partitioning into the retentive phase. The simulation data clearly demonstrate that due to modification of the retentive-phase hydrocarbons by solvent components, neither the solvophobic theory of RPLC, advocated by Horvath and co-workers, nor the lipophilic theory of RPLC, advocated by Carr and co-workers, can adequately describe the separation mechanism of the hexadecane model system of a retentive phase studied here nor the more complex situation present in actual RPLC systems.
Journal Article↗
Simulation of ordered packed beds in chromatography.
A computer simulation of chromatographic dispersion in an ordered packed bed of spheres is conducted utilizing a detailed fluid flow profile provided by the Lattice Boltzmann technique. The ordered configurations of simple cubic, body-centered cubic, and face-centered cubic are employed in these simulations. It is found that zone broadening is less for the fcc structure than the sc and bcc structures and less than a random packed bed analyzed in a previous study in the low flow velocity region used for experimental chromatography. The factors which contribute to the performance of the ordered pack beds are analyzed in detail and found to be dependent both on the nearest surface to surface distance and on the distribution of velocities found in the various packing geometries. The pressure drops of the four configurations are compared and contrasted with the pressure drop from monolithic columns.
Algorithms↗
Simulation of packed-bed chromatography utilizing high-resolution flow fields: comparison with models.
A computer simulation of a section of the interior region of a liquid chromatographic column is performed. The detailed fluid flow profile is provided from a microscopic calculation of low Reynolds number flow through a random packed bed of nonporous spherical particles. The fluid mechanical calculations are performed on a parallel processor computer utilizing the lattice Boltzmann technique. Convection, diffusion, and retention in this flow field are calculated using a stochastic-based algorithm. This computational scheme provides for the ability to reproduce the essential dynamics of the chromatographic process from the fundamental considerations of particle geometry, particle size, flow velocity, solute diffusion coefficient, and solute retention parameters when retention is utilized. The simulation data are fit to semiempirical models. The best agreement is found for the "coupling" model of Giddings and the four-parameter Knox model. These models are verified over a wide range of particle sizes and flow velocities at both low and high velocity. The simulations appear to capture the essential dynamics of the chromatographic flow process for non-dimensional flow velocities (Péclet number) less than 500. Since the same packing geometry is utilized for different particle size studies, the interpretation of the parameter estimates from these models can be extended to the physical column model. The simulations reported here agree very well with a number of experiments reported previously.
Journal Article↗
Molecular simulation of concurrent gas-liquid interfacial adsorption and partitioning in gas-liquid chromatography.
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(bulk-interface) approximately 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.
Journal Article↗
Temperature effects on the retention of n-alkanes and arenes in helium-squalane gas-liquid chromatography. Experiment and molecular simulation.
Experiments and molecular simulations were carried out to study temperature effects (in the range of 323 to 383 K) on the absolute and relative retention of n-hexane, n-heptane, n-octane, benzene, toluene and the three xylene isomers in gas-liquid chromatography. Helium and squalane were used as the carrier gas and retentive phase, respectively. Both the experiments and the simulations show a markedly different temperature dependence of the retention for the n-alkanes compared to the arenes. For example, over the 60 K temperature range studied, the Kovats retention index of benzene is found to increase by about 16 or 18+/-10 retention index units determined from the experiments or simulations, respectively. For toluene and the xylenes, the experimentally measured increases are similar in magnitude and range from 14 to 17 retention index units for m-xylene to o-xylene. The molecular simulation data provide an independent method of obtaining the transfer enthalpies and entropies. The change in retention indices is shown to be the result of the larger entropic penalty and the larger heat capacity for the transfer of the alkane molecules.
Alkanes↗
Accuracy estimation of multiangle light scattering detectors utilized for polydisperse particle characterization with field-flow fractionation techniques: a simulation study.
The coupling of field-flow fractionation (FFF) and multiangle light scattering (MAIS) detectors is complementary in that the MALS system allows particle characterization when a narrow dispersity particle population is present in the detector. The fractionation process provides this narrow dispersity. Utilizing discrete particle simulations of FFF and optical calculations based on both the Mie theory of particle scattering and Rayleigh-Gans-Debye (RGD) scattering theory, the extent of polydispersity that can be tolerated for accurate particle quantitation is explored. It is found that flow, electrical, and sedimentation FFF provide adequate separation for accurate particle quantitation by MALS. The Mie theory is more accurate than the RGD theory, which is known to deviate at higher particle size. Low error in the measurement of mean diameters is found when only the particle diameter is of interest. It is shown that the reconstruction of the particle size distribution from time slice data is distorted due to errors in concentration, which result from finite polydispersity and other effects. A number of procedures are evaluated in restoring the size distribution to higher accuracy. None of these procedures is deemed of general purpose and none of these is reliable. The best results are obtained when fractionation is conducted under the minimal possible outlet polydispersity and when steric effects are minimized. In addition, best results are had for inherently narrow dispersity colloidal materials.
Journal Article↗