On the phase equilibrium Stockmayer fluids

The fluid phase equilibrium of the Stockmayer fluid is investigated using a thermodynamic perturbation theory approach. The reference and the perturbation potential are the Lennard–Jones potential and the dipolar–dipolar interactions, respectively. They are assumed to be represented by the modified Benedict–Webb–Rubin equation of state [J.K. Johnson, J.A. Zollweg, K.E. Gubbins, Mol. Phys. 78 (1993) 591–618] and the Padé approximant [G. Stell, J.C. Rasaiah, H. Narang, Mol. Phys. 27 (1974) 1393–1414], respectively. The asymmetry found in an analogous study [M.E. van Leeuwen, B. Smit, E.M. Hendriks, Mol. Phys. 78 (1993) 271–283] based on the BWR equation of state [J.J. Nicolas, K.E. Gubbins, W.B. Streett, D.J. Tildesley, Mol. Phys. 37 (1979) 1429–1454] is now not observed on the vapour–liquid equilibrium coexistence curves of Stockmayer fluids with dipolar strength of μ^*2 = 1, 2, 3, and 4. Results agree with computer simulations for dipolar strength of μ^*2 = 1; however as strength dipole increases, liquid densities are over-estimated.     

The structure of hyperspherical fluids in various dimensions

The structure of hard, hyperspherical fluids in dimension one, two, three, four, and five has been examined by calculating the pair correlation function using a Monte Carlo simulation. The pair correlation functions match known results in one, two, and three dimensions. The contact value of the pair correlation functions in all the different dimensions agrees well with the theory of Song, Mason, and Stratt [J. Phys. Chem. 93, 6916 (1989)]. The decrease in ordering as the dimension is increased is readily apparent in the structure of the pair correlation function.     

Equation of state and liquid-vapor equilibrium of polarizable Stockmayer fluids

In this work we develop the concept of an effective potential to obtain the equation of state of polarizable Stockmayer (PSM) fluids. This potential consists of a Lennard-Jones function with appropriate energy and distance parameters that depend on the reduced dipolar moment μ(?) and polarizability α(?). The approach deals accurately with polarizable SM fluids with μ(?)≤2.0 and α(?)≤0.1. However, prediction of second virial coefficients is reliable up to μ(?)≤4.0. When the low-density sphericalized potential is used at moderate and large densities, the effect of the dipole-dipole attraction is overestimated in agreement with an effect previously found in the literature. This effect can be traced back to a frustration mechanism due to the interaction between three and more dipoles. We propose a model to account for this frustration effect and are able to reproduce the vapor-liquid equilibrium of polarizable SM fluids in agreement with simulated results from the literature. Molecular dynamics simulations were carried out to show that the effective SM fluid has a radial distribution function very close to that of the true SM system.     

Gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids: A molecular dynamics study

The Maxwell construction together with molecular dynamics simulation is used to study the gas-liquid phase coexistence of quasi-two-dimensional Stockmayer fluids. The phase coexistence curves and corresponding critical points under different dipole strength are obtained, and the critical properties are calculated. We investigate the dependence of the critical point and critical properties on the dipole strength. When the dipole strength is increased, the abrupt disappearance of the gas-liquid phase coexistence in quasi-two-dimensional Stockmayer fluids is not found. However, if the dipole strength is large enough, it does lead to the formation of very long reversible chains which makes the relaxation of the system very slow and the observation of phase coexistence rather difficult or even impossible.     

Heat capacities of Stockmayer fluids from Monte Carlo simulations and perturbation theory

The heat capacities of dipolar fluids are investigated using a thermodynamic perturbation theory approach and the NVT and NpT Monte Carlo simulation methods. The theoretical results are compared to corresponding simulation data. The comparison shows that the applied perturbation theory is appropriate for the heat capacity calculations. As an application, the isobaric heat capacity of ammonia is also studied by the Stockmayer fluid model.     

Simulations of polyelectrolyte dynamics in an externally applied electric field in confined geometry

We consider the dynamics of charged polymers in free solution in a slit geometry under the influence of an electrical field, applied at an angle to the plane parallel walls of the confinement. The simulations are carried out using the Brownian dynamics method with explicit counterions and implicit hydrodynamics. The hydrodynamic interactions between all the particles and the plane parallel walls are taken into account using a diffusion matrix which depends on slit geometry and the actual polyelectrolyte-solute conformations. We observe a selective transport of the charged polymers, as a function of the degree of polymerization and slit height.     

Reactive scattering in hyperspherical coordinates

Preliminary results are presented for transition probabilities in the H + H₂ system derived from an adiabatic representation in terms of surface functions on hyperspheres. Special attention is given to the resonance structure for transition probabilities in the first vibrational level.     

Phosphatidylcholine-based nonaqueous photorheological fluids: effect of geometry and solvent

We report a detail study on photoinduced rheological changes in nonaqueous photorheological (PR) fluids obtained with 1,2-diacyl-sn-glycero-3-phosphocholine-based reverse wormlike micelles, 1-palmitoyl-2-oleophosphatidylcholine (POPC)/cyclohexane/H₂O, POPC/isooctane/H₂O, and L-α-dioleophosphatidylcholine (DOPC)/isooctane/H₂O systems. Initially, the mixtures form highly viscoelastic fluids of long, reverse wormlike micelles as confirmed by the rheological measurements. When photosensitive molecule, trans-CA, is added to these mixtures, they exhibit photosensitivity, and the viscoelasticity increases that decreases on UV irradiation. The nature of the substituent on the benzene ring of trans-CA influences the rheology. When hydroxycinnamic acid (HCA) (hydrophilic, ?OH group attached to the benzene ring of CA) is added to DOPC/isooctane/H₂O phase, separation occurs; while with the methoxycinnamic acid (MOCA) and methylcinnamic acid (MCA) (hydrophobic groups, ?OCH₃, and –CH₃ attached to the benzene ring of CA respectively) led to higher viscoelasticity. The study on the effects of position of the substitution on CA reveals the viscosity enhancement is in the order of p-?>?m-?>?o-isomers. The different geometries obtained because of substitution and photoinduced trans-cis isomerisation is responsible for the different rheology as confirmed by the dynamic rheology, the UV absorption, and the ¹H NMR spectrums. The ¹H NMR spectra revealed a change in the solubilization site of CA with irradiation. cis-CA (high solubility in water) is solubilized in the vicinity of water (at the hydrophilic end of DOPC) as compared with trans-CA. This disrupts the 3D networks of reverse wormlike micelles, decreasing the η ₀, plateau modulus, G _0, and relaxation time, τ _R of solution.     

Binding energy and geometry of e+ A (A=Li,Na) by the hyperspherical approach

We calculate the binding energy and geometry of the weakly bound e(+)Li and e(+)Na systems within the framework of hyperspherical coordinates. The Schrodinger equation in hyperangular coordinates is solved at a series of fixed hyper-radii using B-splines and the resulting coupled hyper-radial equation is solved using the slow variable discretization method developed by Tolstikhin et al. [J. Phys. B 29, L389 (1996)]. Great efforts are made in optimizing the distribution of B-splines to overcome the slow convergence of the binding energy and geometrical quantities. This approach allows us to obtain the results with improved convergence that are in good agreement with the best values reported to date. In addition, an analysis of the structure of the two systems is also made and the e(+)Na system is seen to exhibit quantum halo features.     

Kinetic energy functional in hyperspherical coordinates

Hyperspherical coordinates and a generator coordinate representation are employed to find a simple form of the kinetic energy for a general three-particle problem. An expression is developed for the determination of adiabatic hyperangular states in a local potential using the finite element method.     

Selective IR-multiphoton excitation of hyperspherical modes

Selective IR-multiphoton excitation of the hyperspherical modes of an ABA-type model molecule is simulated by the Floquet and by the quasi-resonant-approximation techniques. The results are analogous to selective IR-multiphoton excitation of processor modes in the Hénon- Heiles model system.     

Dynamics of four particles in hyperspherical coordinates

A possible extension to four particles of the hyperspherical coordinates initially introduced by Smith and Whitten for three particles is presented. Corresponding exact classical and quantum-mechanical Hamiltonians are then derived.     

Adiabatic hyperspherical study of three-particle systems

In an effort to unravel the dependence of the dynamics of three-particle systems on the relative mass of the particles, an adiabatic treatment in hyperspherical coordinates is used to study the ground state and some doubly excited states of several three-particle systems. Surprisingly, the binding energy of the ground state and of several doubly excited states exhibits a nearly linear dependence on the atomic reduced mass.     

Representation of cusps in a hyperspherical basis set

The wave functions of Coulomb systems have cusps at points corresponding to two particle coelescences. In this paper, we derive series representing the cusps in terms of hyperspherical harmonics multiplied by functions of the hyperradius. When the hyperspherical method is applied to Coulomb systems, the harmonics which appear in these series should be included in the hyperangular basis set.     

Integral equation study of a Stockmayer fluid adsorbed in polar disordered matrices

Based on replica integral equations in the (reference) hypernetted chain approximation we investigate the structural features and phase properties of a dipolar Stockmayer fluid confined to a disordered dipolar matrix. The integral equations are applied to the homogeneous high-temperature phase where the system is globally isotropic. At low densities we find the influence of dipolar interactions between fluid (f) and matrix (m) particles to be surprisingly similar to the previously investigated effect of attractive isotropic (fm) interactions: the critical temperature of the vapor-liquid transition decreases with increasing (fm) coupling, while the critical density increases. The anisotropic nature of the dipolar (fm) interactions turns out to play a more dominant role at high fluid densities where we observe a pronounced sensitivity in the dielectric constant and a strong degree of local orientational ordering of the fluid particles along the local fields generated by the matrix. Moreover, an instability of the dielectric constant, which is a precursor of ferroelectric ordering occurring both in bulk Stockmayer fluids and in fluids in nonpolar matrices, is observed only for very small dipolar (fm) couplings.     

Influence of confinement on the electrostatic interaction between charged colloids: a (N,V,T) Monte Carlo study within hyperspherical geometry

Monte Carlo simulations within closed hyperspherical geometry are used to analyze the ionic distribution around two confined charged colloids to determine the origin of the net attraction recently reported in the literature. A scaling procedure is used to compare our numerical results obtained with small ideal colloids with the conclusion of the measurements performed with large silica colloids. Although no electrostatic attraction is detected under confinement, our simulations exhibit a significant reduction of the electrostatic repulsion between charged colloids confined between two weakly charged walls. After rescaling to reproduce the electrostatic repulsion between large confined colloids, our numerical results are qualitatively consistent with the reported attraction because we reasonably expect a reduction of the electrostatic force between such confined colloids below the order of magnitude of their van der Waals attraction.     

Structure, dynamics, and the free energy of solute adsorption at liquid-vapor interfaces of simple dipolar systems: molecular dynamics results for pure and mixed Stockmayer fluids

We have performed molecular dynamics simulation studies of the structural, thermodynamic, and dynamical properties of liquid-vapor interfaces of pure and binary Stockmayer fluids of different polarity. The density profiles, the width of the liquid-vapor interface, and the orientational structure of the interfaces are calculated to characterize the structural aspects of the interfaces. Among the thermodynamic properties, we have computed the surface tension and also the free energy of transfer of a charged solute across the liquid-vapor interface for both pure and mixed fluids. Among the dynamical properties of the interfaces, we have calculated the time dependence of the velocity and angular velocity autocorrelation functions, continuous and intermittent survival probabilities, mean square displacements, diffusion coefficients, and also the dipole correlation functions and orientational relaxation times of interfacial solvent molecules. It is found that the width of the interfaces decreases with increase of concentration of the more polar component. The dipole vectors of the interfacial molecules tend to align parallel to the surfaces and this alignment is enhanced with increasing dipole moment of the fluid molecules. Also, the surface tension shows an increasing trend with increase of dipole moment of the molecules. The dynamical properties of the interfaces are found to be different from those of the corresponding bulk liquid phases. In general, the molecules at the interfaces are found to rotate and translate in the parallel direction at a somewhat faster rate than the bulk molecules. Also, on increase of concentration of the more polar component, the diffusion and orientational relaxation of interfacial molecules are found to show a weaker slowing down than those of the bulk molecules, which can be attributed to the preferential presence of the more polar component in the bulk liquid regions. The temporal behavior of the interfacial survival probabilities reveals a decrease of the survival times with increasing polarity, which can be attributed to a corresponding decrease in the interfacial thickness. Results are presented for both continuous and intermittent survival times and the origins of their differences are discussed. The free energy calculations reveal no minimum at the interfaces for adsorption of a charged solute, which shows that the ions would prefer to stay in the interior of the liquid phases, rather than at interfaces, for these model dipolar systems.