@article {2620, title = {Structural and dynamical properties of ionic liquids: The influence of charge location}, journal = {Journal of Chemical Physics}, volume = {130}, number = {10}, year = {2009}, note = {ISI Document Delivery No.: 420IFTimes Cited: 8Cited Reference Count: 31Spohr, H. V. Patey, G. N.}, month = {Mar}, pages = {11}, type = {Article}, abstract = {The properties of ionic liquids depend on the chemical structure of the constituent ions. An important difference between molten inorganic salts and room temperature ionic liquids (RTILs) is that in RTILs the charge is frequently not located at the center of mass. This paper describes a molecular dynamics investigation of the influence of charge location on the structure and transport properties of ionic liquids. The model considered consists of univalent spherical ions with the cation charge moved away from its center of mass. It is shown that the charge location has an important influence on the liquid properties. As the charge is moved off center, the electrical conductivity initially increases, and the shear viscosity decreases. However, when the charge exceeds a certain displacement, this behavior is reversed. With further charge displacement, the conductivity decreases sharply and the viscosity increases rapidly. This behavior reversal can be traced to the formation of directional ion pairs that are present in sufficient numbers, and have lifetimes sufficiently long to strongly influence the liquid properties. We suggest that the influence of directional ion pairing can explain what appear to be anomalously low conductivities and high viscosities observed for some RTILs. The rotational and reorientational motions of the cations are examined, and shown to be strongly influenced by ion-pair formation when the charge is far off center. The temperature dependence of the transport properties is considered for selected systems, and deviations from Arrhenius behavior are found to be most important for the conductivity. Based on our results, this possibly indicates that directional ion pairs create an additional "barrier" to charge transport in some ionic liquids.}, keywords = {ANION, DIFFUSION, electrical conductivity, HYDROGEN-BONDS, liquid metals, liquid structure, liquid theory, molecular dynamics method, MOLECULAR-DYNAMICS, PHYSICOCHEMICAL PROPERTIES, pressure, TEMPERATURE, TRANSPORT-PROPERTIES, VISCOSITIES, VISCOSITY}, isbn = {0021-9606}, url = {://000264281800024}, author = {Spohr, H. V. and Patey, G. N.} } @article {445, title = {Molecular dynamics simulations of a ferroelectric nematic liquid under shear flow}, journal = {Journal of Chemical Physics}, volume = {117}, number = {18}, year = {2002}, note = {ISI Document Delivery No.: 610YYTimes Cited: 5Cited Reference Count: 47}, month = {Nov}, pages = {8551-8564}, type = {Article}, abstract = {We perform nonequilibrium molecular dynamics simulations to examine the change in orientational order of a ferroelectric nematic liquid crystal under shear flow, specifically planar Couette flow. The orientational order is found to decrease with increasing shear rate. This drop in orientational order is not due to structural changes but is rather a result of the director dynamics being flow unstable. Employing both nonequilibrium and equilibrium molecular dynamics with the director orientation fixed, we determine the shear and twist viscosities which relate the pressure tensor to the shear or strain rate tensor in a nematic liquid crystal under flow. The Miesowicz viscosities are then obtained from linear combinations of the shear and twist viscosities. The short-range spatial structure of the ferroelectric nematic liquid crystal is similar to that of a ferroelectric tetragonal I lattice. The relative magnitudes of the Miesowicz viscosities can be understood by examining the shear stress response of this lattice to a shear deformation. (C) 2002 American Institute of Physics.}, keywords = {BOUNDARY-CONDITIONS, COMPUTER-SIMULATION, CRYSTALS, FLUIDS, MONTE-CARLO, PHASE-TRANSITIONS, rheology, STATE, VISCOSITY}, isbn = {0021-9606}, url = {://000178990700045}, author = {McWhirter, J. L. and Patey, G. N.} } @article {447, title = {Nonequilibrium molecular dynamics simulations of a simple dipolar fluid under shear flow}, journal = {Journal of Chemical Physics}, volume = {117}, number = {6}, year = {2002}, note = {ISI Document Delivery No.: 577BXTimes Cited: 12Cited Reference Count: 71}, month = {Aug}, pages = {2747-2761}, type = {Article}, abstract = {Nonequilibrium molecular dynamics (NEMD) simulations are used to investigate the behavior of a fluid of dipolar Lennard-Jones (LJ) particles under planar Couette flow. Such systems can be viewed as simple models for magnetic ferrofluids. Various issues that arise in such simulations are discussed. These include the importance of the long-range dipolar forces, the influence of different thermostats, and the induction of orientational order. The shear viscosity is found to be somewhat sensitive to the treatment of the dipolar interactions with spherical truncation giving lower values than Ewald sums. The system considered is characterized by a moderate dipole moment, and strong orientational order does not develop at low shear rates. However, the direction of the weak polarization correlates with the shear-induced distortion of the fluid structure. At very high shear rates the fluid behavior depends on the thermostat applied. While the results are not strongly sensitive to details of the rotational thermostat, this is not the case for translational motion. A translationally biased thermostat leads to a string phase, although string formation requires significantly higher shear rates for the dipolar fluid than for the corresponding LJ system. For the dipolar fluid, the string phase is accompanied by strong orientational ordering perpendicular to the flow for Ewald sums and with the flow for the spherically truncated case. In the Ewald systems the order was ferroelectric for conducting boundary conditions, and an antiferroelectric domain structure formed in the vacuum case. For the truncated potential individually polarized chains are randomly oriented with or against the flow. The string phase and the orientational ordering disappears when the translationally unbiased thermostat of Evans and co-workers is employed. Some NEMD simulations with oscillating shear were also performed. These yielded layer structures and again strong orientational order in the dipolar case. (C) 2002 American Institute of Physics.}, keywords = {BOUNDARY-CONDITIONS, COMPUTER, HARD-SPHERE FLUID, INDUCED ORDER, LIQUID-CRYSTAL, MONTE-CARLO, NEMATIC PHASES, PAIR-CORRELATION-FUNCTION, STRUCTURAL-CHANGES, VISCOSITY}, isbn = {0021-9606}, url = {://000177042800034}, author = {McWhirter, J. L. and Patey, G. N.} } @article {4364, title = {Improved initial density dependence of the viscosity and a corresponding states function for high pressures}, journal = {Physica a-Statistical Mechanics and Its Applications}, volume = {260}, number = {1-2}, year = {1998}, note = {ISI Document Delivery No.: 135WLTimes Cited: 24Cited Reference Count: 50}, month = {Nov}, pages = {31-48}, type = {Article}, abstract = {The initial density correction to gaseous viscosity using accurate realistic potentials of the noble gases is evaluated using the Rainwater-Friend theory. It is shown that this theory works satisfactorily for densities up to about 2 mol dm(-3). Due to the superimposability of the noble gas potential functions, a universal function of the reduced second viscosity virial coefficient is obtained over the entire reduced temperature range. At densities beyond the range of the theory, a variant of the excess viscosity is developed, by which the viscosity of the different gases can be easily calculated above the critical temperature for pressures up to 900 MPa. The accuracy of this method is within the experimental uncertainties. (C) 1998 Elsevier Science B.V. All rights reserved.}, keywords = {11 POLYATOMIC GASES, ARGON, COEFFICIENT, COEFFICIENTS, EQUILIBRIUM, high-pressure excess viscosity, NOBLE-GASES, ORGANIC VAPORS, PHYSICAL-PROPERTIES, Rainwater-Friend theory, second viscosity virial, TEMPERATURE, THERMAL-CONDUCTIVITY, TRANSPORT-PROPERTIES, VISCOSITY}, isbn = {0378-4371}, url = {://000076825600004}, author = {Najafi, B. and Ghayeb, Y. and Rainwater, J. C. and Alavi, S. and Snider, R. F.} } @article {4099, title = {Quantitative description of analyte migration behavior based on the dynamic complexation model in capillary electrophoresis}, journal = {Canadian Journal of Chemistry-Revue Canadienne De Chimie}, volume = {75}, number = {5}, year = {1997}, note = {ISI Document Delivery No.: XF492Times Cited: 20Cited Reference Count: 27}, month = {May}, pages = {507-517}, type = {Article}, abstract = {A theory based on dynamic complexation is used to describe analyte migration behavior in capillary electrophoresis (CE). This theory is based on a one-phase system, instead of the commonly accepted two-phase system. The migration behavior of an analyte is described by three parameters (the electrophoretic mobility of the free analyte, the electrophoretic mobility of the analyte-additive complex, and the equilibrium constant (formation constant) that determines the fractions of the free analyte and the complex at a certain additive concentration). Varying the additive concentration shifts the equilibrium and changes the viscosity of the background electrolyte. Viscosity correction is crucial in interpreting the observed migration behavior of analytes. While electroosmotic flow in a capillary often varies from one capillary to another, the viscosity of a buffer is characteristic of the buffer composition and is constant for each buffer. The electrophoretic mobility of a certain species and the equilibrium constant are intrinsic properties and are less sensitive to changes in the environment. Understanding these relationships is indispensable in CE method development and method validation. A universal resolution equation is proposed, with a separation factor that has taken both the electrophoretic mobilities and equilibria into consideration. This resolution equation gives clear guidance for the optimization of CE separations. A group of nucleosides and their phosphates are used as analytes, and P-cyclodextrin is used as the additive in the model system studied in this paper. Both the observed analyte migration behavior and the resolution of analytes agree well with this theory.}, keywords = {ACIDS, BETA-CYCLODEXTRIN, BINDING, capacity factor, CHIRAL SEPARATION, CORRECTION, dynamic complexation capillary electrophoresis, ELECTROKINETIC CHROMATOGRAPHY, mobility, nucleoside and, nucleotide separation, RESOLUTION, resolution equation, VISCOSITY}, isbn = {0008-4042}, url = {://A1997XF49200005}, author = {Peng, X. J. and Bebault, G. M. and Sacks, S. L. and Chen, D. D. Y.} } @article {2929, title = {DYNAMIC PROPERTIES OF A FERROELECTRIC NEMATIC LIQUID-CRYSTAL}, journal = {Physical Review E}, volume = {47}, number = {4}, year = {1993}, note = {ISI Document Delivery No.: KZ507Times Cited: 10Cited Reference Count: 18}, month = {Apr}, pages = {2954-2957}, type = {Note}, abstract = {Molecular-dynamics simulations are used to investigate the dynamical properties of a ferroelectric nematic liquid crystal. The results are also relevant to electrorheological fluids and to ferrofluids. Translational and rotational diffusion constants parallel and perpendicular to the director are reported and discussed. Dipole-dipole reorientational correlation functions are calculated and compared with theoretical models.}, keywords = {ANISOTROPY, ELECTROSTATIC SYSTEMS, FLUIDS, MOLECULAR-DYNAMICS, PERIODIC BOUNDARY-CONDITIONS, SIMULATION, VISCOSITY}, isbn = {1063-651X}, url = {://A1993KZ50700094}, author = {Wei, D. Q. and Patey, G. N.} } @article {2908, title = {THE EQUILIBRIUM PAIR DISTRIBUTION FUNCTION OF A GAS - ASPECTS ASSOCIATED WITH THE PRESENCE OF BOUND-STATES}, journal = {Journal of Chemical Physics}, volume = {99}, number = {11}, year = {1993}, note = {ISI Document Delivery No.: MJ902Times Cited: 1Cited Reference Count: 36}, month = {Dec}, pages = {9111-9121}, type = {Article}, abstract = {At thermal equilibrium the momentum distribution of atoms in a gas is usually assumed to be Maxwellian, whether classically or quantally. However, if an atom is bound in a diatomic molecule, the atom{\textquoteright}s momentum distribution is non-Maxwellian. This paper explores the consequent singlet and pair particle distribution functions in a gas having both unbound atoms and bound pairs of atoms. Comment is made on the range of behavior associated with whether the chemical equilibrium constant for diatom formation is small or large. Calculations of distribution functions and their moments for atoms which are members of dimers are presented for some specific model potentials.}, keywords = {2ND, DIMER, formation, INITIAL DENSITY DEPENDENCE, KINETIC-THEORY, PHASE-SPACE SUBDIVISION, POTENTIALS, TEMPERATURE, THERMAL-CONDUCTIVITY, TRANSPORT-PROPERTIES, VIRIAL-COEFFICIENT, VISCOSITY}, isbn = {0021-9606}, url = {://A1993MJ90200074}, author = {Rainwater, J. C. and Snider, R. F.} } @article {7112, title = {EXCESS PARTIAL MOLAR ENTHALPIES IN THE WATER-RICH REGION OF THE ISOBUTYRIC ACID - WATER-SYSTEM}, journal = {Canadian Journal of Chemistry-Revue Canadienne De Chimie}, volume = {69}, number = {7}, year = {1991}, note = {ISI Document Delivery No.: FY816Times Cited: 8Cited Reference Count: 24}, month = {Jul}, pages = {1065-1069}, type = {Article}, abstract = {The excess partial molar enthalpies of isobutyric acid (IBA), H(m)E(IBA), and those of H2O, H(m)E(H2O), were measured in aqueous solutions of IBA. The temperature dependence of H(m)E(IBA) at the infinite dilution suggested that the structure enhancement of the solvent H2O by IBA is weaker than those by tert-butanol (TBA) or 2-butoxyethanol (BE). The concentration dependence of H(m)E(IBA), and that of the enthalpic IBA-IBA interaction, N{partial H(m)E(IBA)/partial n(B)}, shows that there are two distinct mixing schemes bounded at about x(B) = 0.03, before reaching the two phase separation. Namely, the IBA-IBA interaction is repulsive below this boundary, while above this boundary it becomes attractive leading eventually to phase separation at a higher concentration. The transition between the two schemes is associated with a peak(negative) anomally in the fourth derivative of the free energy, N2 {partial H-2(m)E(IBA)/partial n(B)2}.}, keywords = {2-BUTOXYETHANOL, CRITICAL SOLUTION TEMPERATURE, excess partial molar enthalpies, EXPONENT, INTERFACIAL-TENSION, ISOBUTYRIC ACID WATER, LIQUID WATER, MIXING SCHEME, TERT-BUTANOL MIXTURES, TRANSITION, TRANSITION IN, ULTRALOW, VICINITY, VISCOSITY}, isbn = {0008-4042}, url = {://A1991FY81600005}, author = {Siu, W. W. Y. and Wong, T. Y. H. and Chao, L. C. F. and Koga,Yoshikata} }