Pressure of fluids and solids composed of particles interacting with a short-range repulsive potential

Thermodynamic stability of soft-core Lennard-Jones fluids and their mixtures

Thermodynamic stability of model single component and binary mixture fluids is considered with the Fisher-Ruelle (FR) stability criteria, which apply in the thermodynamic limit, and molecular dynamics (MD) simulation for finite periodic systems. Two soft-core potential forms are considered, phi(6,1)(r)=4[1/(a+r(6))(2)-1/(a+r(6))] and phi(2,3)(r)=4[1/(a+r(2))(6)-1/(a+r(2))(3)], where r is the separation between the particle centers. According to FR these are unstable in the thermodynamic limit if a>a(c)=1/2 and a>a(c)=(7/32)(1/3), respectively. MD simulations with single-component particles show, however, that this transition on typical simulation times is more gradual for finite periodic systems with variation in a on either side of a(c). For a<a(c), asymmetric density fluctuations are stabilized by the periodic boundary conditions. Also for binary mixtures of (stable) Lennard-Jones and phi(2,3) particles, phase separation into regions richer in one component than the other was observed for a<a(c). Binary systems with interactions similar to a model polymer-colloid fluid in the "depletion" limit equilibrated particularly slowly for a>a(c), with the unstable component in the mixture breaking up into many long-lived microdroplets which conferred apparent equilibrium thermodynamic behavior (i.e., negligible N-dependence of the average potential energy per particle) in this period.

Friction of -bead macromolecules in solution: effects of the bead-solvent interaction

The role of the bead-solvent interaction has been studied for its influence on the dynamics of an N-bead macromolecule which is immersed into a solution. Using a Fokker-Planck equation for the phase-space distribution function of the macromolecule, we show that all the effects of the solution can be treated entirely in terms of the friction tensors which are assigned to each pair of interacting beads in the chain. For the high-density as well as for the critical solvent, the properties of these tensors are discussed in detail and are calculated by using several (realistic) choices of the bead-solvent potential. From the friction tensors, moreover, an expression for the center-of-mass friction coefficient of a (N-bead) chain macromolecule is derived. Numerical data for this coefficient for "truncated" Lennard-Jones bead-solvent potential are compared with results from molecular dynamic simulations and from the phenomenological theoretical data as found in the literature.

Phase behavior and structure of Janus fluids

The equilibrium phase behavior of Janus fluids is examined based on a model potential for the interaction between their constituents. Janus fluids consist of axisymmetric particles possessing two different "faces," e.g., one hydrophobic and one hydrophilic surface, and the interaction depends on the relative orientation. Starting from a short range, isotropic potential we make an ansatz for an anisotropic model interaction potential. Two types of symmetries of the particles are considered. One leads to a polar phase. The Helmholtz free energy and the pressure are calculated by the help of an augmented van der Waals approximation. A qualitative phase diagram is obtained. The appearance of a polar phase and the corresponding transition temperature are examined adapting a Landau-de Gennes expansion of the orientational part of the free energy. Monte Carlo simulations are performed and the results are compared with the ones obtained by the analytical description.

Thermophysical properties of gases, liquids, and solids composed of particles interacting with a short-range attractive potential

A short-range polynomial interaction potential is introduced which has both a repulsive core and an attractive part. It is cut off smoothly such that its first and second derivatives vanish at the cutoff distance. The potential therefore enables efficient simulation studies of a model material that exhibits similarities to a full (but computationally expensive) classical Lennard-Jones system. Thermophysical properties of the model are calculated by (nonequilibrium) molecular dynamics computer simulations and compared with analytical results. Among the quantities studied is the pressure as a function of the density for various temperatures. Equations of state for the fluid and the solid are tested. The coexistence of gaseous, (metastable) liquid, and fcc solid phases is found for a range of temperatures. Bulk and shear moduli are computed. The response of the system to a shear deformation with a constant shear rate is analyzed. The liquid shows viscoelastic behavior that can be described with a Maxwell model. The solid behaves as an elastic medium up to a finite deformation and then undergoes a transition to plastic flow, which is stick-slip-like at small shear rates and continuous at higher ones.