Clifford E. Woodward

A density functional theory for polymers: Application to hard chain–hard sphere mixtures in slitlike pores

A density functional theory for polymer fluids is derived. The form of the theory is identical to a continuum self‐consistent‐field theory but we show that with the correct ‘‘mean field,’’ the theory is exact. We apply the theory to a polymer fluid made up of tangential hard spheres and borrow from a successful density functional theory for simple hard spheres to obtain a nonlocal functional expression for this mean field. The latter depends only upon the monomer density. This approximation is used to treat the problem of a mixture of such polymers and simple hard spheres, contained in slitlike pores. Comparison with simulations show that the theory, though simple in structure, and containing a single adjustable parameter, is surprisingly accurate.

Monte Carlo density functional theory of nonuniform polymer melts

A theory for nonuniform polymer melts is presented, which combines density functional theory with Monte Carlo methods. The theory treats the ideal gas functional exactly via a single chain simulation and uses the weighted density approximation for the excess free energy functional. The bulk fluid properties required in the theory are obtained from a generalized Flory equation of state. The predictions of the theory are compared to Monte Carlo simulations for the density profiles of semiflexible polymer melts confined between flat plates. Good agreement between theory and simulation is found for 3mers and 20mers and for several densities and molecular stiffnesses.

Electric double layer forces in the presence of polyelectrolytes

An electric double layer is studied by means of Monte Carlo simulations and mean‐field theory. The counterions of the uniformly charged surfaces are modeled as flexible polyelectrolytes. For this particular model system it turns out that the traditional double layer repulsion becomes attractive for a wide range of systems. The main reason for this attraction is an entropically driven bridging mechanism, and its magnitude is significant compared to ordinary double layer or van der Waals forces. The polyelectrolyte Poisson–Boltzmann theory developed here behaves in a qualitatively correct manner, also predicting an attractive interaction extending over several nanometers. These results may have some relevance to technical and biological systems, where sometimes puzzling force behavior is seen in the presence of polyelectrolytes.

Widom's method for uniform and non-uniform electrolyte solutions

We develop and apply test particle methods to calculate chemical potentials in uniform and non-uniform electrolytes using computer simulations. Our techniques are based on the well-known Widom method, but account for non-electroneutral particle fluctuations, implicitly suppressed in a usual Canonical Ensemble Monte Carlo simulation. These fluctuations are shown to contribute greatly to the Widom average for a single ionic species. We compare our results with similar Grand Canonical Monte Carlo simulations for 1:1, 2:1 and 2:2 uniform electrolytes, and 1:1 electrolytes at a charged planar interface. In general, we find good agreement between the methods, all within the statistical fluctuations. The advantages of using the test particle approaches are discussed.

DENSITY FUNCTIONAL THEORY FOR INHOMOGENEOUS POLYMER SOLUTIONS

The structure of polymer solutions confined between hard walls is investigated via density functional theory. A generalized Flory theory for polymer mixtures incorporating an equation of state for dimers is implemented in the theory without introducing any adjustable parameters. The structure predicted by the theory is in good agreement with computer simulations for confined melts, with polymer lengths ranging from 3 mers to 20 mers and for several volume fractions. Similarly good agreement is found for 8‐mer/monomer mixtures.

A simple analysis of ion-ion correlation in polyelectrolyte solutions

In the Poisson–Boltzmann theory of macroion screening in electrolyte solutions the chief simplifying assumption is the neglect of correlations within the ionic atmosphere. Using a generalized van der Waals density functional analysis incorporating ion–ion correlation in a local approximation we obtain a simple correlation corrected Poisson–Boltzmann theory. For point charges in the salt free case, this local correlation approximation leads to asymptotic instability (a ‘‘structuring catastrophe’’) though the correction is well behaved to low orders in perturbation theory. Results in zeroth order and in a zeroth order field approximation are compared with an extended series of Monte Carlo simulations within a cell model (and a planar electrode model). An excellent agreement is found over a wide range of coupling strengths.

A Classical Density Functional Theory of Ionic Liquids

We present a simple, classical density functional approach to the study of simple models of room temperature ionic liquids. Dispersion attractions as well as ion correlation effects and excluded volume packing are taken into account. The oligomeric structure, common to many ionic liquid molecules, is handled by a polymer density functional treatment. The theory is evaluated by comparisons with simulations, with an emphasis on the differential capacitance, an experimentally measurable quantity of significant practical interest.

A self‐consistent‐field integral equation theory for nonuniform polymer fluids

An integral equation theory is developed for nonuniform polymer fluids. It is based on an exact formulation of the density functional theory for polymers, which is essentially identical in form to the popular self‐consistent‐field approximations. A nonuniform site–site Ornstein–Zernike equation is obtained. As well, the Triezenberg–Zwanzig–Wertheim–Lovett– Mou–Buff equations are generalized to polymer fluids. The Percus–Yevick and mean spherical approximations are suggested as plausible closures to these equations.

Dinuclear ruthenium(II) antimicrobial agents that selectively target polysomes in vivo

Wide-field fluorescence microscopy at high magnification was used to study the intracellular binding site of Rubb16 in Escherichia coli. Upon incubation of E. coli cells at the minimum inhibitory concentration, Rubb16 localised at ribosomes with no significant DNA binding observed. Furthermore, Rubb16 condensed the ribosomes when they existed as polysomes. It is postulated that the condensation of polysomes would halt protein production, and thereby inhibit bacterial growth. The results of this study indicate that the family of inert dinuclear ruthenium complexes Rubbn selectively target RNA over DNA in vivo. Selective RNA targeting could be advantageous for the development of therapeutic agents, and because of differences in ribosome structure between bacteria and eukaryotic cells, the Rubbn complexes could be selectively toxic to bacteria. In support of this hypothesis, the toxicity of Rubb16 was found to be significantly less to liver and kidney cell lines than against a range of bacteria.

Generalized van der Waals theory of hard sphere oscillatory structure

The recently developed generalized van der Waals (GvdW) theory, a free energy density functional theory based on cell theory and van der Waals approximations, is here applied to the prediction of hard sphere oscillatory structures at a hard wall, between two hard walls, and around a hard sphere. Three different functional forms of the crucial free volume factor are compared. The results confirm that the fine‐grained GvdW theory containing a nonlocal entropy functional yields structures reproducing packing oscillations not only qualitatively but to quantitative accuracy. The error depends on the choice of free volume factor and can be made small except at high density where the range and magnitude of oscillations are overestimated. Evidence of early onset of a hard sphere freezing transition is seen.