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28 Mar 2013

Volume 138, Issue 12, Articles (12xxxx)

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J. Chem. Phys. 138, 124701 (2013); http://dx.doi.org/10.1063/1.4794685 (9 pages)

Luying Wang, Randall S. Dumont, and James M. Dickson
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back to top Polymers and Soft Matter

Perturbation theory for non-spherical fluids based on discretization of the interactions

Francisco Gámez and Ana Laura Benavides

J. Chem. Phys. 138, 124901 (2013); http://dx.doi.org/10.1063/1.4794783 (7 pages)

Online Publication Date: 22 March 2013

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An extension of the discrete perturbation theory [A. L. Benavides and A. Gil-Villegas, Mol. Phys. 97(12), 1225 (1999)10.1080/00268979909482924] accounting for non-spherical interactions is presented. An analytical expression for the Helmholtz free energy for an equivalent discrete potential is given as a function of density, temperature, and intermolecular parameters with implicit shape dependence. The presented procedure is suitable for the description of the thermodynamics of general intermolecular potential models of arbitrary shape. The overlap and dispersion forces are represented by a discrete potential formed by a sequence of square-well and square-shoulders potentials of shape-dependent widths. By varying the intermolecular parameters through their geometrical dependence, some illustrative cases of square-well spherocylinders and Kihara fluids are considered, and their vapor-liquid phase diagrams are tested against available simulation data. It is found that this theoretical approach is able to reproduce qualitatively and quantitatively well the Monte Carlo data for the selected potentials, except near the critical region.
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65.20.De General theory of thermodynamic properties of liquids, including computer simulation
61.20.Ja Computer simulation of liquid structure

Comparing different coarse-grained potentials for star polymers

Roberto Menichetti and Andrea Pelissetto

J. Chem. Phys. 138, 124902 (2013); http://dx.doi.org/10.1063/1.4796143 (9 pages)

Online Publication Date: 25 March 2013

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We compare different coarse-grained single-blob models for star polymers. We find that phenomenological models inspired by the Daoud-Cotton theory reproduce quite poorly the thermodynamics of these systems, even if the potential is assumed to be density dependent, as done in the analysis of experimental results. Using the numerically determined coarse-grained potential, we also determine the minimum value fc of the functionality of the star polymer for which a fluid-solid transition occurs. By applying the Hansen-Verlet criterion we find 35 < fc ≲ 40. This result is confirmed by an analysis that uses the modified (reference) hypernetted chain method and is qualitatively consistent with previous work.
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61.25.he Polymer solutions
64.70.D- Solid-liquid transitions
65.20.De General theory of thermodynamic properties of liquids, including computer simulation

Method of evaluating curvature-dependent elastic parameters for small unilamellar vesicles using molecular dynamics trajectory

Takenobu Nakamura and Wataru Shinoda

J. Chem. Phys. 138, 124903 (2013); http://dx.doi.org/10.1063/1.4795579 (7 pages)

Online Publication Date: 27 March 2013

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A numerical method is proposed for evaluating the curvature dependency of elastic parameters of a spherical vesicle based on a calculation of the pressure profile across the membrane. The proposed method is particularly useful for small unilamellar vesicles (SUVs), in which the internal structure of the membrane is asymmetric owing to the high curvature. In this case, the elastic energy is insufficiently described as a perturbation from a planar membrane. The calculated saddle-splay curvature modulus of SUVs, which is about 16 nm in diameter, is found to be much higher than that of a planar membrane. A comparison of the free energy change in the initial stage of vesicle-to-bicelle transformation with the Fromherz theory demonstrates that the elastic parameters estimated for SUVs provide better estimation of the free energy than those estimated for a planar membrane.
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87.16.dm Mechanical properties and rheology
87.16.-b Subcellular structure and processes
87.10.Tf Molecular dynamics simulation
87.16.D- Membranes, bilayers, and vesicles

Capacitance and optical studies of elastic and dielectric properties in an organosiloxane tetrapode exhibiting a NB phase

S. Polineni, J. L. Figueirinhas, C. Cruz, D. A. Wilson, and G. H. Mehl

J. Chem. Phys. 138, 124904 (2013); http://dx.doi.org/10.1063/1.4795582 (11 pages)

Online Publication Date: 28 March 2013

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Biaxial (NB) and uniaxial nematic (NU) phase behavior was detected and confirmed for an organosiloxane tetrapode material using capacitance and birefringence measurements. Elastic constants, permittivities at two distinct low frequencies, and birefringencies were determined as a function of temperature over both the NU and the NB phase ranges. The NU-NB transition is clearly observed in the birefringencies and conoscopy data. A temperature dependent cross-over frequency is also detected in this material for the permittivities, allowing the electrical switching of both planar and homeotropic aligned samples.
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77.22.Ch Permittivity (dielectric function)
77.84.Nh Liquids, emulsions, and suspensions; liquid crystals
78.20.Fm Birefringence
64.70.mj Experimental studies of liquid crystal transitions
61.30.Eb Experimental determinations of smectic, nematic, cholesteric, and other structures
62.10.+s Mechanical properties of liquids
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