Publications by authors named "Leo Lue"

28 Publications

  • Page 1 of 1

Exploring the Role of Anti-solvent Effects during Washing on Active Pharmaceutical Ingredient Purity.

Org Process Res Dev 2021 Apr 12;25(4):969-981. Epub 2021 Mar 12.

EPSRC Continuous Manufacturing & Advanced Crystallisation (CMAC) Future Manufacturing Research Hub, University of Strathclyde, Glasgow G1 1RD, U.K.

Washing is a key step in pharmaceutical isolation to remove the unwanted crystallization solvent (mother liquor) from the active pharmaceutical ingredient (API) filter cake. This study looks at strategies for optimal wash solvent selection, which minimizes the dissolution of API product crystals while preventing the precipitation of product or impurities. Selection of wash solvents to avoid both these phenomena can be challenging but is essential to maintain the yield, purity, and particle characteristics throughout the isolation process. An anti-solvent screening methodology has been developed to quantitatively evaluate the propensity for precipitation of APIs and their impurities of synthesis during washing. This is illustrated using paracetamol (PCM) and two typical impurities of synthesis during the washing process. The solubility of PCM in different binary wash solutions was measured to provide a basis for wash solvent selection. A map of wash solution composition boundaries for precipitation for the systems investigated was developed to depict where anti-solvent phenomena will take place. For some crystallization and wash solvent combinations investigated, as much as 90% of the dissolved PCM and over 10% of impurities present in the PCM saturated mother liquor were found to precipitate out. Such levels of uncontrolled crystallization during washing in a pharmaceutical isolation process can have a drastic effect on the final product purity. Precipitation of both the product and impurities from the mother liquor can be avoided by using a solvent in which the API has a solubility similar to that in the mother liquor; for example, the use of acetonitrile as a wash solvent does not result in precipitation of either the PCM API or its impurities. However, the high solubility of PCM in acetonitrile would result in noticeable dissolution of API during washing and would lead to agglomeration during the subsequent drying step. Contrarily, the use of -heptane as a wash solvent for a PCM crystal slurry resulted in the highest amount of precipitation among the solvent pairs evaluated. This can be mitigated by designing a multi-stage washing strategy where wash solutions of differing wash solvent concentrations are used to minimize step changes in solubility when the mother liquor and the wash solvent come into contact.
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http://dx.doi.org/10.1021/acs.oprd.1c00005DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8057229PMC
April 2021

Simple corrections for the static dielectric constant of liquid mixtures from model force fields.

Phys Chem Chem Phys 2020 Oct;22(38):21741-21749

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, UK.

Pair-wise additive force fields provide fairly accurate predictions, through classical molecular simulations, for a wide range of structural, thermodynamic, and dynamical properties of many materials. However, one key property that has not been well captured is the static dielectric constant, which characterizes the response of a system to an applied electric field and is important in determining the screening of electrostatic interactions through a system. A simple correction has been found to provide a relatively robust method to improve the estimate of the static dielectric constant from molecular simulations for a broad range of compounds. This approach accounts for the electronic contribution to molecular polarizability and assumes that the charges that couple a molecule to an applied electric field are proportional to the effective force field charges. In this work, we examine how this correction performs for systems at different temperatures and for binary mixtures. Using a value for the electronic polarizability, based on the experimental index of refraction, and a charge scaling factor, determined at a single temperature, we find that the static dielectric constant can be predicted remarkably well, in comparison to the experimentally measured values. This provides good evidence that the effective charges that appear in pair-wise additive force fields developed to reproduce the potential energy surface of a system are not the same as those that determine the static dielectric constant; however, they can be captured in a relatively simple manner, which is dependent on the particular force field.
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http://dx.doi.org/10.1039/d0cp04034gDOI Listing
October 2020

Interaction between charged lipid vesicles and point- or rod-like trivalent ions.

Colloids Surf B Biointerfaces 2019 Jun 20;178:525-529. Epub 2019 Feb 20.

Faculty of Health Sciences, University of Ljubljana, Zdravstvena 5, SI-1000 Ljubljana, Slovenia. Electronic address:

This work examines the influence of the charge distribution of trivalent cations on their interaction with soft anionic particles, using a combination of experimental measurements and theoretical modelling. In particular, we perform electrophoresis measurements to determine the zeta-potential of anionic liposomes in the presence of spermidine and lanthanum cations. We work in a range of electrolyte concentration where a reversal in the electrophoretic mobility of the liposomes is expected; however, unlike the case of lanthanum cations, spermidine does not induce mobility reversal of liposomes. As a result, the charge distribution within the counterion appears to be a key factor. This conclusion is supported by a theory that accounts for intra-ionic correlations, which has previously been successfully used to describe the colloidal electric double layer. It allows us to model spermidine as rod-like ions and lanthanum cations as point-like ions in order to test the importance of the ionic geometry in the interactions with soft particles such as lipid vesicles.
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http://dx.doi.org/10.1016/j.colsurfb.2019.02.010DOI Listing
June 2019

Anomalous heat transport in binary hard-sphere gases.

Phys Rev E 2019 Mar;99(3-1):030102

School of Engineering, University of Aberdeen, Aberdeen AB24 3UE, United Kingdom.

Equilibrium and nonequilibrium molecular dynamics (MD) are used to investigate the thermal conductivity of binary hard-sphere fluids. It is found that the thermal conductivity of a mixture can not only lie outside the series and parallel bounds set by their pure component values, but can lie beyond even the pure component fluid values. The MD simulations verify that revised Enskog theory can accurately predict nonequilibrium thermal conductivities at low densities and this theory is applied to explore the model parameter space. Only certain mass and size ratios are found to exhibit conductivity enhancements above the parallel bounds and dehancement below the series bounds. The anomalous dehancement is experimentally accessible in helium-hydrogen gas mixtures and a review of the literature confirms the existence of mixture thermal conductivity below the series bound and even below the pure fluid values, in accordance with the predictions of revised Enskog theory. The results reported here may reignite the debate in the nanofluid literature on the possible existence of anomalous thermal conductivities outside the series and parallel bounds as this Rapid Communication demonstrates they are a fundamental feature of even simple fluids.
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http://dx.doi.org/10.1103/PhysRevE.99.030102DOI Listing
March 2019

The dielectric constant: Reconciling simulation and experiment.

J Chem Phys 2019 Feb;150(8):084108

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.

In this paper, we present a simple correction scheme to improve predictions of dielectric constants by classical non-polarisable models. This scheme takes into account electronic polarisation effects, through the experimental refractive index of the liquid, and a possible mismatch between the potential energy surface and the dipole moment surface. We have described the latter effect by an empirical scaling factor on the point charges, the value of which was determined by fitting the dielectric constant of methanol. Application of the same scaling factor to existing benchmark datasets, comprising four different models and a wide range of compounds, led to remarkable improvements in the quality of the predictions. In particular, the observed systematic underestimation of the dielectric constant was eliminated by accounting for the two missing terms in standard models. We propose that this correction term be included in future development and validation efforts of classical non-polarisable models.
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http://dx.doi.org/10.1063/1.5080927DOI Listing
February 2019

Why different water models predict different structures under 2D confinement.

J Comput Chem 2018 Sep 18;39(25):2051-2059. Epub 2018 Sep 18.

School of Chemical Engineering and Analytical Sciences, University of Manchester, Manchester M13 9PL, United Kingdom.

Experiments of nanoconfined water between graphene sheets at high pressure suggest that it forms a square ice structure (Algara-Siller et al., Nature, 2015, 519, 443). Molecular dynamics (MD) simulations have been used to attempt to recreate this structure, but there have been discrepancies in the structure formed by the confined water depending on the simulation set-up that was employed and particularly on the choice of water model. Here, using classical molecular dynamics simulations, we have systematically investigated the effect that three different water models (SPC/E, TIP4P/2005 and TIP5P) have on the structure of water confined between two rigid graphene sheets with a 0.9 nm separation. We show that the TIP4P/2005 and the TIP5P water models form a hexagonal AA-stacked structure, whereas the SPC/E model forms a rhombic AB-stacked structure. Our work demonstrates that the formation of these structures is driven by differences in the strength of hydrogen bonds predicted by the three water models, and that the nature of the graphene/water interaction only mildly affects the phase diagram. Considering the available experimental data and first-principle simulations we conclude that, among the models tested, the TIP4P/2005 and TIP5P force fields are for now the most reliable when simulating water under confinement. © 2018 Wiley Periodicals, Inc.
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http://dx.doi.org/10.1002/jcc.25369DOI Listing
September 2018

A diagrammatic analysis of the variational perturbation method for classical fluids.

Authors:
Leo Lue

Soft Matter 2018 Jun;14(23):4721-4734

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow, UK.

The statistical mechanics of classical fluids can be approached from the particle perspective, where the focus is on the various positions and interactions of the particles, and from the field perspective, where the focus is on the form of the interaction fields generated by the particles. In this work, we combine these two perspectives by examining the variational perturbation method for classical fluids, which has been widely used to describe nonuniform electrolyte systems. Most of this work has been for low orders of the approximation, it has been limited to cases where the electrostatic interactions are weak. We present an exact diagrammatic representation of the method, which greatly facilitates the enumeration and evaluation of higher order corrections to the free energy functional. This framework is able to encapsulate several different approximate theories. Performing a cumulant expansion, leads to the Debye-Hückel and higher order corrections. Including the contribution of chain diagrams leads to a theory closely related to the splitting theory [Hatlo and Lue, Europhys. Lett., 2010, 89, 25002], which has been shown to be accurate from the weak through to the strong coupling limits. Including all chain and ring diagrams leads to the hypernetted chain approximation; this is a more direct route to the conventional derivation, which also requires a renormalization of the Mayer f-bonds to the total correlation functions. These approximations to the variational perturbation method are applied to the classical one-component plasma in order to assess their relative accuracy and understand their relationship to each other. Strategies for developing improved approximations are discussed.
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http://dx.doi.org/10.1039/c8sm00676hDOI Listing
June 2018

Molecular Dynamics Investigation of the Influence of the Hydrogen Bond Networks in Ethanol/Water Mixtures on Dielectric Spectra.

J Phys Chem B 2018 02 22;122(4):1505-1515. Epub 2018 Jan 22.

Department of Chemical and Process Engineering, University of Strathclyde , James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.

The dielectric response of fluids to electromagnetic radiation in the microwave region originates from processes occurring at the molecular level. Understanding these processes in more detail is relevant to many fields, such as microwave heating, fluid mixing, and separation technologies. In this work, we use molecular dynamics (MD) simulations to study the dielectric spectra of ethanol/water mixtures. We compare our predictions with experimental results at different compositions. We show how the dielectric response can be estimated to a high level of accuracy using three dielectric relaxations: a dominant and slower process at microwave frequencies and two faster processes. A deeper study of the dynamics of the hydrogen bond network formed in these systems reveals how collective processes between the individual species are the origin of the final dielectric response. Our results agree with the "wait-and-switch" mechanism, which describes the dynamics of the hydrogen bond network as the combination of two processes: the fast breakage and formation of individual hydrogen bonds and the subsequent reorganization of the entire network once this process becomes energetically favorable. Since the dielectric response is related to dipole reorientations in the system, it is directly linked to these mechanisms.
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http://dx.doi.org/10.1021/acs.jpcb.7b12220DOI Listing
February 2018

Communication: The cluster vapor to cluster solid transition.

J Chem Phys 2016 May;144(17):171102

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.

Until now, depletion induced transitions have been the hallmark of multicomponent systems only. Monte Carlo simulations reveal a depletion-induced phase transition from cluster vapor to cluster solid in a one-component fluid with competing short range and long range interactions. This confirms a prediction made by earlier theoretical work. Analysis of renormalized cluster-cluster and cluster-vapor interactions suggests that a cluster liquid is also expected within a very narrow range of model parameters. These insights could help identify the mechanisms of clustering in experiments and assist the design of colloidal structures through engineered self-assembly.
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http://dx.doi.org/10.1063/1.4948784DOI Listing
May 2016

Interactions between charged surfaces mediated by stiff, multivalent zwitterionic polymers.

Soft Matter 2016 05;12(19):4397-405

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, UK.

The interaction between like-charged objects in electrolyte solutions can be heavily altered by the presence of multivalent ions which possess a spatially distributed charge. In this work, we examine the influence of stiff, multivalent zwitterionic polymers on the interaction between charged surfaces using a splitting field theory previously shown to be accurate for the weak to the intermediate to the strong electrostatic coupling regimes. The theory is compared to Monte Carlo simulations and good agreement is found between both approaches. For surface separations shorter than the polymer length, the polymers are mainly oriented parallel to the surfaces, and the surface-surface interaction is repulsive. When the surface separation is comparable to the length of polymers, the polymers have two main orientations. The first corresponds to the polymers adsorbed onto the surface with their centers located near to or in contact with the surface; the second corresponds to polymers which are perpendicular to the charged surfaces, bridging both surfaces and leading to an attractive force between them. Increasing the surface charge density leads to more pronounced attraction via bridging. At surface separations greater than the polymer length, the polymers in the center of the system are still mainly perpendicular to the surfaces, due to "chaining" between zwitterions that enable them to bridge the surfaces at larger separations. This leads to an attractive interaction between the surfaces with a range significantly longer than the length of the polymers.
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http://dx.doi.org/10.1039/c6sm00236fDOI Listing
May 2016

Ions confined in spherical dielectric cavities modeled by a splitting field-theory.

Authors:
Leo Lue Per Linse

J Chem Phys 2015 Apr;142(14):144902

Physical Chemistry, Department of Chemistry, Lund University, P.O. Box 124, S-221 00 Lund, Sweden.

The properties of ions confined within spherical dielectric cavities are examined by a splitting field-theory and Monte Carlo simulations. Three types of cavities are considered: one possessing a uniform surface charge density, one with a uniform volume charge density, and one containing mobile ions. In all cases, mobile counterions are present within the dielectric sphere. The splitting theory is based on dividing the electrostatic interaction into long- and short-wavelength contributions and applying different approximations on the two contributions. The splitting theory works well for the case where the dielectric constant of the confining sphere is equal to or less than that of the medium external to the sphere. Nevertheless, by extending the theory with a virial expansion, the predictions are improved. However, when the dielectric constant of the confining sphere is greater than that of the medium outside the sphere, the splitting theory performs poorly, only qualitatively agreeing with the simulation data. In this case, the strong-coupling expansion does not seem to work well, and a modified mean-field theory where the counterions interact directly with only their own image charge gives improved predictions. The splitting theory works best for the system with a uniform surface charge density and worst for the system with a uniform volume charge density. Increasing the number of ions within the sphere, at a fixed radius, tends to increase the ion density near the surface of the sphere and leads to a depletion region in the sphere interior; however, varying the ion number does not lead to any qualitative changes in the performance of the splitting theory.
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http://dx.doi.org/10.1063/1.4917256DOI Listing
April 2015

Green's function for a spherical dielectric discontinuity and its application to simulation.

Authors:
Per Linse Leo Lue

J Chem Phys 2014 Jan;140(4):044903

Department of Chemical and Process Engineering, University of Strathclyde, James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.

We present rapidly convergent expressions for the Green's function of the Poisson equation for spherically symmetric systems where the dielectric constant varies discontinuously in the radial direction. These expressions are used in Monte Carlo simulations of various electrolyte systems, and their efficiency is assessed. With only the leading term of the expansion included, a precision of the polarization energy of 0.01 kJ mol(-1) or better was achieved, which is smaller than the statistical uncertainty of a typical simulation. The inclusion of the dielectric inhomogeneity leads to a 2.5-fold increase of the computational effort, which is modest for this type of model. The simulations are performed on six types of systems having either (i) a uniform surface charge distribution, (ii) a uniform volume charge distribution, or (iii) mobile ions, which were neutralized by mobile counterions. The ion density distributions are investigated for different dielectric conditions. These spatial distributions are discussed in terms of the importance of (i) the direct mean-field Coulomb interaction, (ii) the surface charge polarization at the dielectric discontinuity, and/or (iii) the change in the attractive Coulomb correlations.
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http://dx.doi.org/10.1063/1.4862148DOI Listing
January 2014

Mapping continuous potentials to discrete forms.

J Chem Phys 2014 Jan;140(3):034105

School of Engineering, University of Aberdeen, Fraser Noble Building, Kings College, Aberdeen AB24 3UE, United Kingdom.

The optimal conversion of a continuous inter-particle potential to a discrete equivalent is considered here. Existing and novel algorithms are evaluated to determine the best technique for creating accurate discrete forms using the minimum number of discontinuities. This allows the event-driven molecular dynamics technique to be efficiently applied to the wide range of continuous force models available in the literature, and facilitates a direct comparison of event-driven and time-driven molecular dynamics. The performance of the proposed conversion techniques are evaluated through application to the Lennard-Jones model. A surprising linear dependence of the computational cost on the number of discontinuities is found, allowing accuracy to be traded for speed in a controlled manner. Excellent agreement is found for static and dynamic properties using a relatively low number of discontinuities. For the Lennard-Jones potential, the optimized discrete form outperforms the original continuous form at gas densities but is significantly slower at higher densities.
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http://dx.doi.org/10.1063/1.4861669DOI Listing
January 2014

Crystal templating through liquid-liquid phase separation.

Chem Commun (Camb) 2015 Jan;51(6):1139-42

School of Chemistry, WestCHEM, University of Glasgow, Glasgow G12 8QQ, UK.

Controlled induction of crystal nucleation is a highly desirable but elusive goal. Attempts to speed up crystallization, such as high super saturation or working near a liquid-liquid critical point, always led to irregular and uncontrollable crystal growth. Here, we show that under highly nonequilibrium conditions of spinodal decomposition, water crystals grow as thin wires in a template-less formation of "Haareis". This suggests that such nonequilibrium conditions may be employed more widely as mechanisms for crystal growth control.
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http://dx.doi.org/10.1039/c4cc07880bDOI Listing
January 2015

Cluster formation in fluids with competing short-range and long-range interactions.

J Chem Phys 2014 Mar;140(12):124508

Department of Chemical and Process Engineering, University of Strathclyde, Glasgow G1 1XJ, United Kingdom.

We investigate the low density behaviour of fluids that interact through a short-ranged attraction together with a long-ranged repulsion (SALR potential) by developing a molecular thermodynamic model. The SALR potential is a model of effective solute interactions where the solvent degrees of freedom are integrated-out. For this system, we find that clusters form for a range of interaction parameters where attractive and repulsive interactions nearly balance, similar to micelle formation in aqueous surfactant solutions. We focus on systems for which equilibrium behaviour and liquid-like clusters (i.e., droplets) are expected, and find in addition a novel coexistence between a low density cluster phase and a high density cluster phase within a very narrow range of parameters. Moreover, a simple formula for the average cluster size is developed. Based on this formula, we propose a non-classical crystal nucleation pathway whereby macroscopic crystals are formed via crystal nucleation within microscopic precursor droplets. We also perform large-scale Monte Carlo simulations, which demonstrate that the cluster fluid phase is thermodynamically stable for this system.
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http://dx.doi.org/10.1063/1.4869109DOI Listing
March 2014

Recycling of uranyl from contaminated water.

J Phys Chem B 2013 Sep 5;117(37):10846-51. Epub 2013 Sep 5.

Faculty of Health Sciences, University of Ljubljana , Zdravstvena 5, SI-1000 Ljubljana, Slovenia.

Many separation processes are related to the behavior of ions close to charged surfaces. In this work, we examine uranyl ions, which can be considered as rod-like molecular ions with a spatially distributed charge, embedded in a system of like charged surfaces. The analysis of the system is based on an approximate field theory which is accurate from the weak to the strong electrostatic coupling regimes. The numerical results show that close to the charged surface the ions are oriented parallel to the surface, whereas at distances greater than half of the ion length, they are randomly oriented. Due to the restriction of the orientational degrees of freedom, the density of ions at the charged surface decreases to zero. For large surface charge densities, the force between like charged surfaces becomes attractive, as a result of charge correlations. The theoretical results are in good agreement with Monte Carlo simulation results.
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http://dx.doi.org/10.1021/jp404822fDOI Listing
September 2013

Density functional theory for Yukawa fluids.

J Chem Phys 2012 Aug;137(6):064115

Institute for Theoretical Physics, Utrecht University, Leuvenlaan 4, 3584 CE Utrecht, The Netherlands.

We develop an approximate field theory for particles interacting with a generalized Yukawa potential. This theory improves and extends a previous splitting field theory, originally developed for counterions around a fixed charge distribution. The resulting theory bridges between the second virial approximation, which is accurate at low particle densities, and the mean-field approximation, accurate at high densities. We apply this theory to charged, screened ions in bulk solution, modeled to interact with a Yukawa potential; the theory is able to accurately reproduce the thermodynamic properties of the system over a broad range of conditions. The theory is also applied to "dressed counterions," interacting with a screened electrostatic potential, contained between charged plates. It is found to work well from the weak coupling to the strong coupling limits. The theory is able to reproduce the counterion profiles and force curves for closed and open systems obtained from Monte Carlo simulations.
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http://dx.doi.org/10.1063/1.4742154DOI Listing
August 2012

Macroion solutions in the cell model studied by field theory and Monte Carlo simulations.

Authors:
Leo Lue Per Linse

J Chem Phys 2011 Dec;135(22):224508

Department of Chemical and Process Engineering, University of Strathclyde James Weir Building, 75 Montrose Street, Glasgow G1 1XJ, United Kingdom.

Aqueous solutions of charged spherical macroions with variable dielectric permittivity and their associated counterions are examined within the cell model using a field theory and Monte Carlo simulations. The field theory is based on separation of fields into short- and long-wavelength terms, which are subjected to different statistical-mechanical treatments. The simulations were performed by using a new, accurate, and fast algorithm for numerical evaluation of the electrostatic polarization interaction. The field theory provides counterion distributions outside a macroion in good agreement with the simulation results over the full range from weak to strong electrostatic coupling. A low-dielectric macroion leads to a displacement of the counterions away from the macroion.
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http://dx.doi.org/10.1063/1.3665450DOI Listing
December 2011

Exact on-event expressions for discrete potential systems.

J Chem Phys 2010 Sep;133(12):124506

School of Chemical Engineering and Analytical Science, The University of Manchester, Oxford Road, Manchester M13 9PL, United Kingdom.

The properties of systems composed of atoms interacting though discrete potentials are dictated by a series of events which occur between pairs of atoms. There are only four basic event types for pairwise discrete potentials and the square-well/shoulder systems studied here exhibit them all. Closed analytical expressions are derived for the on-event kinetic energy distribution functions for an atom, which are distinct from the Maxwell-Boltzmann distribution function. Exact expressions are derived that directly relate the pressure and temperature of equilibrium discrete potential systems to the rates of each type of event. The pressure can be determined from knowledge of only the rate of core and bounce events. The temperature is given by the ratio of the number of bounce events to the number of disassociation/association events. All these expressions are validated with event-driven molecular dynamics simulations and agree with the data within the statistical precision of the simulations.
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http://dx.doi.org/10.1063/1.3486567DOI Listing
September 2010

One-component plasma of point charges and of charged rods.

Phys Rev E Stat Nonlin Soft Matter Phys 2009 Dec 8;80(6 Pt 1):061107. Epub 2009 Dec 8.

School of Chemical Engineering and Analytical Science, The University of Manchester, PO Box 88, Sackville Street, Manchester M60 1QD, United Kingdom.

An approximate theory is developed to describe the properties of mobile particles with extended charge distributions in the presence of a neutralizing fixed background charge. Long-wavelength fluctuations of the electric potential are handled within a variational perturbation approximation, and the short-wavelength fluctuations are handled within a cumulant (fugacity) expansion. The distinct treatment of these two contributions to the free energy enables the theory to provide quantitative predictions for the properties of these systems from the weak- to the strong-coupling regimes. With this theory, we study three different variations in the classical one-component plasma model: a plasma of point charges, a plasma of particles consisting of 8 linearly bonded point charges (8-mer), and a plasma of line charges. The theory was found to agree well with the available computer simulation data for the electrostatic interaction energy of these systems for all values of the plasma coupling parameter examined ( Gamma=0 to 400). In addition, we find that both the 8-mer rod and the line charge systems form a strongly ordered nematic phase, which is entirely driven by electrostatic interactions. The nematic phase only exists within a finite range of lengths of the charged particles. If the particles are too short or too long, the nematic phase does not appear. Finally, we find that the nematic phase is stable over a broader range of conditions for the line charge system than for the 8-mer rod system; consequently, the phase behavior of the one-component plasma is sensitive to the manner in which the charge is distributed on the particles.
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http://dx.doi.org/10.1103/PhysRevE.80.061107DOI Listing
December 2009

The properties of dimers confined between two charged plates.

J Chem Phys 2010 Mar;132(11):114102

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, M60 1QD Manchester, United Kingdom.

We consider two like-charged planar surfaces immersed in solution of oppositely charged dimer counterions with a bond length l. To analyze this system, we extend and employ a self-consistent field theory that has been shown to be accurate from the weak to the intermediate through to the strong coupling regimes. In the limit of very short dimers, the results converge to the results for pointlike divalent ions. Near the surfaces, the dimers lie parallel to the charged plates. In the intermediate coupling regime, the dimers are aligned perpendicularly to the surface when they are a distance l from a surface. In the weak coupling regime, the interactions are only repulsive. At slightly higher couplings, there is a minimum in the variation of the free energy with distance at approximately the bond length of the dimers, which arises from bridging conformations of the dimers. In the intermediate coupling regime, an additional minimum in the free energy is observed at much smaller distances, which is due to the correlations between the dimers. For large dimer bond lengths, this minimum is metastable with respect to the previous minimum. However, as the bond length decreases, this minimum becomes the stable, while the minimum associated with the dimer bond length becomes metastable and eventually disappears. For shorter dimer bond length the attractive interaction is the result of correlations between counterions and charges on the surfaces. We find that dimers can mediate attractive interaction between like-charged surfaces in the intermediate coupling regime. The analysis of orientations confirms the bridging mechanism for sufficiently long dimers, whereas at high electrostatic couplings charge correlations contribute to the attraction.
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http://dx.doi.org/10.1063/1.3354121DOI Listing
March 2010

Thermodynamic pressures for hard spheres and closed-virial equation-of-state.

J Chem Phys 2010 Feb;132(8):084507

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom.

Hard-sphere molecular dynamics (MD) simulation results, with six-figure accuracy in the thermodynamic equilibrium pressure, are reported and used to test a closed-virial equation-of-state. This latest equation, with no adjustable parameters except known virial coefficients, is comparable in accuracy both to Padé approximants, and to numerical parameterizations of MD data. There is no evidence of nonconvergence at stable fluid densities. The virial pressure begins to deviate significantly from the thermodynamic fluid pressure at or near the freezing density, suggesting that the passage from stable fluid to metastable fluid is associated with a higher-order phase transition; an observation consistent with some previous experimental results. Revised parameters for the crystal equation-of-state [R. J. Speedy, J. Phys.: Condens. Matter 10, 4387 (1998)] are also reported.
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http://dx.doi.org/10.1063/1.3328823DOI Listing
February 2010

Collision statistics in sheared inelastic hard spheres.

Phys Rev E Stat Nonlin Soft Matter Phys 2009 Apr 22;79(4 Pt 1):041308. Epub 2009 Apr 22.

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom.

The dynamics of sheared inelastic-hard-sphere systems is studied using nonequilibrium molecular-dynamics simulations and direct simulation Monte Carlo. In the molecular-dynamics simulations Lees-Edwards boundary conditions are used to impose the shear. The dimensions of the simulation box are chosen to ensure that the systems are homogeneous and that the shear is applied uniformly. Various system properties are monitored, including the one-particle velocity distribution, granular temperature, stress tensor, collision rates, and time between collisions. The one-particle velocity distribution is found to agree reasonably well with an anisotropic Gaussian distribution, with only a slight overpopulation of the high-velocity tails. The velocity distribution is strongly anisotropic, especially at lower densities and lower values of the coefficient of restitution, with the largest variance in the direction of shear. The density dependence of the compressibility factor of the sheared inelastic-hard-sphere system is quite similar to that of elastic-hard-sphere fluids. As the systems become more inelastic, the glancing collisions begin to dominate over more direct, head-on collisions. Examination of the distribution of the times between collisions indicates that the collisions experienced by the particles are strongly correlated in the highly inelastic systems. A comparison of the simulation data is made with direct Monte Carlo simulation of the Enskog equation. Results of the kinetic model of Montanero [J. Fluid Mech. 389, 391 (1999)] based on the Enskog equation are also included. In general, good agreement is found for high-density, weakly inelastic systems.
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http://dx.doi.org/10.1103/PhysRevE.79.041308DOI Listing
April 2009

Transport properties of highly asymmetric hard-sphere mixtures.

J Chem Phys 2009 Apr;130(16):164507

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88 Sackville Street, Manchester M60 1QD, United Kingdom.

The static and dynamic properties of binary mixtures of hard spheres with a diameter ratio of sigma(B)/sigma(A)=0.1 and a mass ratio of m(B)/m(A)=0.001 are investigated using event driven molecular dynamics. The contact values of the pair correlation functions are found to compare favorably with recently proposed theoretical expressions. The transport coefficients of the mixture, determined from simulation, are compared to the predictions of the revised Enskog theory using both a third-order Sonine expansion and direct simulation Monte Carlo. Overall, the Enskog theory provides a fairly good description of the simulation data, with the exception of systems at the smallest mole fraction of larger spheres (x(A)=0.01) examined. A "fines effect" was observed at higher packing fractions, where adding smaller spheres to a system of large spheres decreases the viscosity of the mixture; this effect is not captured by the Enskog theory.
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http://dx.doi.org/10.1063/1.3120488DOI Listing
April 2009

Density of states for a short overlapping-bead polymer: clues to a mechanism for helix formation?

Phys Rev E Stat Nonlin Soft Matter Phys 2008 Sep 19;78(3 Pt 1):031803. Epub 2008 Sep 19.

School of Chemical Engineering and Analytical Science, The University of Manchester, P. O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom.

The densities of states are evaluated for very short chain molecules made up of overlapping monomers, using a model which has previously been shown to produce helical structure. The results of numerical calculations are presented for tetramers and pentamers. We show that these models demonstrate behaviors relevant to the behaviors seen in longer, helix-forming chains, particularly "magic numbers" of the overlap parameter, where the derivatives of the densities of states change discontinuously, and a region of bimodal energy probability distributions, reminiscent of a first-order phase transition in a bulk system.
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http://dx.doi.org/10.1103/PhysRevE.78.031803DOI Listing
September 2008

The role of image charges in the interactions between colloidal particles.

Soft Matter 2008 Jul;4(8):1582-1596

School of Chemical Engineering and Analytical Science, University of Manchester, PO Box 88, Sackville Street, Manchester, United KingdomM60 1QD.

The dielectric interiors of colloidal particles are responsible for dispersion (van der Waals) interactions. However, these dielectric regions also alter the manner in which charges, such as on ions or other colloidal particles, interact with each other, due to the induction of charges at the dielectric interfaces. The impact of these induced charges can be represented in terms of "image charges". These image charges result in an ion depletion layer in the vicinity of low dielectric bodies. This depletion layer is responsible for the increase in the surface tension of water upon the addition of electrolytes. In the case of colloidal particles, this depletion layer also leads to an "electrostatic depletion force" with a range of the order of a Bjerrum length. The relevance of this force to the salting out of proteins is discussed. This electrostatic depletion force is directly analogous to the entropically driven depletion force (due to excluded volume). Although image charge effects have been known, their influence on the behavior of colloidal systems, especially in the presence of mobile ions, has generally not been accounted for (e.g., DLVO theory). We review the previous theoretical and simulation studies of how image charges influence the properties of electrolyte and colloidal systems and discuss the relevance of these effects on experimental systems.
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http://dx.doi.org/10.1039/b803783cDOI Listing
July 2008

Structure and aggregation of a helix-forming polymer.

J Chem Phys 2007 Apr;126(14):144911

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, Manchester, M60 1QD, United Kingdom.

We have studied the competition between helix formation and aggregation for a simple polymer model. We present simulation results for a system of two such polymers, examining the potential of mean force, the balance between intermolecular and intramolecular interactions, and the promotion or disruption of secondary structure brought on by the proximity of the two molecules. In particular, we demonstrate that proximity between two such molecules can stabilize secondary structure. However, for this model, observed secondary structure is not stable enough to prevent collapse of the system into an unstructured globule.
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http://dx.doi.org/10.1063/1.2717924DOI Listing
April 2007

Helical structures from an isotropic homopolymer model.

Phys Rev Lett 2006 May 24;96(20):207802. Epub 2006 May 24.

School of Chemical Engineering and Analytical Science, The University of Manchester, P.O. Box 88, Sackville Street, Manchester M60 1QD, United Kingdom.

We present Monte Carlo simulation results for square-well homopolymers at a series of bond lengths. Although the model contains only isotropic pairwise interactions, under appropriate conditions this system shows spontaneous chiral symmetry breaking, where the chain exists in either a left- or a right-handed helical structure. We investigate how this behavior depends upon the ratio between bond length and monomer radius.
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http://dx.doi.org/10.1103/PhysRevLett.96.207802DOI Listing
May 2006