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Philip J Camp

Publications and source records attributed to Philip J Camp.

17 recordsLinked to original sources
PubMed2006

Aggregation kinetics and the nature of phase separation in two-dimensional dipolar fluids.

The kinetics of aggregation in a monolayer of dipolar particles are studied using stochastic dynamics computer simulations. Transient concentrations of end defects (at low density) and Y-shaped defects (at high density) clearly exceed those at equilibrium. Although very large dipole moments are expected to disfavor such defects at equilibrium, it is found that the transient defect concentrations increase with increasing dipole moment. The results suggest that the conditions for defect-driven condensation--as proposed by Tlusty and Safran [T. Tlusty and S. A. Safran, Science 290, 1328 (2000)]--could be met by kinetic trapping, giving rise to a metastable phase transition between isotropic fluid phases.

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PubMed2006

Self-assembly of peptide scaffolds in biosilica formation: computer simulations of a coarse-grained model.

The self-assembly of model peptides is studied using Brownian dynamics computer simulations. A coarse-grained, bead-spring model is designed to mimic silaffins, small peptides implicated in the biomineralization of certain silica diatom skeletons and observed to promote the formation of amorphous silica nanospheres in vitro. The primary characteristics of the silaffin are a 15 amino acid hydrophilic backbone and two modified lysine residues near the ends of the backbone carrying long polyamine chains. In the simulations, the model peptides self-assemble to form spherical clusters, networks of strands, or bicontinuous structures, depending on the peptide concentration and effective temperature. The results indicate that over a broad range of volume fractions (0.05-25%) the characteristic structural lengthscales fall in the range 12-45 nm. On this basis, we suggest that self-assembled structures act as either nucleation points or scaffolds for the deposition of 10-100 nm silica-peptide building blocks from which diatom skeletons and synthetic nanospheres are constructed.

Computer Simulation↗
PubMed2006

Deposition of magnetic colloidal particles on graphite and mica surfaces driven by solvent evaporation.

The deposition of colloidal magnetite particles onto graphite and mica surfaces induced by solvent evaporation is studied using atomic force microscopy. After evaporation under ambient conditions we observe polydisperse beadlike aggregates; the mean aggregate diameter is larger on graphite than on mica. After evaporation at elevated temperatures we observe a variety of effects, including enhanced particle aggregation and spinodal-like deposition patterns. To explain these trends, we propose mechanisms involving the wetting properties of the solvent. We have also made a brief study of the effects of applied magnetic fields on the formation of aggregates. A field applied parallel to the surface enhances aggregation and favors deposition patterns characteristic of hole-nucleation processes. A perpendicular field leads to a reduction in aggregate size and favors a homogeneous distribution of particles on the surface. These effects are explained in terms of the likely orientation of the dipolar particles on the surface.

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PubMed2006

The assembly of rotaxane-like dye/cyclodextrin/surface complexes on aluminium trihydroxide or goethite.

Simple azo-dyes carrying phosphonic acid and arsonic acid substituents such as 4-(4-hydroxyphenyl azo)phenylphosphonic acid (5) and 4-(4-hydroxyphenylazo)phenylarsonic acid (6) bind more strongly to high surface area oxides such as aluminium trihydroxide and goethite than their carboxylic and sulfonic acid analogues and the phosphonate-functionalized dyes have been shown to have greater humidity fastness when printed onto commercial alumina-coated papers. Adsorption isotherm measurements provide evidence for the formation of ternary dye/cyclodextrin/surface complexes. Dyes which form such ternary complexes show higher light fastness when printed onto alumina coated papers in an ink formulation containing alpha-cyclodextrin.

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PubMed2006

Two-dimensional structure in a generic model of triangular proteins and protein trimers.

Motivated by the diversity and complexity of two-dimensional (2D) crystals formed by triangular proteins and protein trimers, we have investigated the structures and phase behavior of hard-disk trimers. In order to mimic specific binding interactions, each trimer possesses an "attractive" disk which can interact with similar disks on other trimers via an attractive square-well potential. At low density and low temperature, the fluid phase mainly consists of tetramers, pentamers, or hexamers. Hexamers provide the structural motif for a high-density, low-temperature periodic solid phase, but we also identify a metastable periodic structure based on a tetramer motif. At high density there is a transition between orientationally ordered and disordered solid phases. The connections between simulated structures and those of 2D protein crystals--as seen in electron microscopy--are briefly discussed.

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PubMed2005

Dipolar interactions, molecular flexibility, and flexoelectricity in bent-core liquid crystals.

The effects of dipolar interactions and molecular flexibility on the structure and phase behavior of bent-core molecular fluids are studied using Monte Carlo computer simulations. Some calculations of flexoelectric coefficients are also reported. The rigid cores of the model molecules consist of either five or seven soft spheres arranged in a "V" shape with external bend angle gamma. With purely repulsive sphere-sphere interactions and gamma = 0 degrees (linear molecules) the seven-sphere model exhibits isotropic, uniaxial nematic, and untilted and tilted smectic phases. With gamma > or = 20 degrees the untilted smectic phases disappear, while the system with gamma > or = 40 degrees shows a direct tilted smectic-isotropic fluid transition. The addition of electrostatic interactions between transverse dipole moments on the apical spheres is generally seen to reduce the degree of molecular inclination in tilted phases, and destabilizes the nematic and untilted smectic phases of linear molecules. The effects of adding three-segment flexible tails to the ends of five-sphere bent-core molecules are examined using configurational-bias Monte Carlo simulations. Only isotropic and smectic phases are observed. On the one hand, molecular flexibility gives rise to pronounced fluctuations in the smectic-layer structure, bringing the simulated system in better correspondence with real materials; on the other hand, the smectic phase shows almost no tilt. Lastly, the flexoelectric coefficients of various nematic phases--with and without attractive sphere-sphere interactions--are presented. The results are encouraging, but a large computational effort is required to evaluate the appropriate fluctuation relations reliably.

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PubMed2005

Transient cavities and the excess chemical potentials of hard-spheroid solutes in dipolar hard-sphere solvents.

Monte Carlo computer simulations are used to study transient cavities and the solvation of hard-spheroid solutes in dipolar hard-sphere solvents. The probability distribution of spheroidal cavities in the solvent is shown to be well described by a Gaussian function, and the variations of fit parameters with cavity elongation and solvent properties are analyzed. The excess chemical potentials of hard-spheroid solutes with aspect ratios x in the range of 15< or =x< or =5, and with volumes between 1 and 20 times that of a solvent molecule, are presented. It is shown that for a given molecular volume and solvent dipole moment (or temperature) a spherical solute has the lowest excess chemical potential and hence the highest solubility, while a prolate solute with aspect ratio x should be more soluble than an oblate solute with aspect ratio 1x. For a given solute molecule, the excess chemical potential increases with increasing temperature; this same trend can be observed in hydrophobic solvation. A scaled-particle theory based on the solvent equation of state and a fitted solute-solvent interfacial tension shows excellent agreement with the simulation results over the whole range of solute elongations and volumes considered. An information-theoretic model based on the solvent density and radial distribution function is less successful, being accurate only for small solute volumes and low solvent densities.

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PubMed2005

Dynamics in a two-dimensional core-softened fluid.

The dynamical properties of a model one-component core-softened fluid with purely repulsive interactions are found to be very complex. At low temperature the fluid structure exhibits cluster motifs including dimers, stripes, and polygons, depending on density. Single-particle diffusion and the velocity, shear-stress, and wave-vector-dependent current correlation functions have all been calculated using molecular dynamics simulations. The results highlight the presence of well-resolved single-particle and collective motions, which is remarkable for what is essentially a "simple" one-component fluid.

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PubMed2005

Toward understanding mobile proton behavior from first principles calculation: the short hydrogen bond in crystalline urea-phosphoric acid.

The dynamics of the intermolecular short hydrogen bond in the molecular complex of urea and phosphoric acid are investigated using plane-wave density functional theory. Results indicate migration of the proton toward the center of the hydrogen bond as temperature is increased, in line with recent experimental measurements. Computed vibrational frequencies show favorable agreement with experimental measurement. An analysis of existing neutron diffraction data leads us to conclude that the effective potential well experienced by the proton is temperature-dependent. Inspired by our computations and theoretical analysis, we offer a possible explanation for the proton migration phenomenon.

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PubMed2005

[BDTA]2[Cu(mnt)2]: an almost perfect one-dimensional magnetic material.

[BDTA]2[Cu(mnt)2] (BDTA = benzo-1,3,2-dithiazolyl, mnt = maleonitriledithiolate) was crystallized in the space group P with an inversion center on Cu giving a stacked structure with each metal complex anion sandwiched by two cations. Short intermolecular S...S contacts give rise to a one-dimensional chain lateral to the stacking axis. Variable-temperature magnetic susceptibility and EPR measurements indicate that the salt behaves as an ideal one-dimensional Heisenberg antiferromagnetic material from 2 K < or = T < or = 300 K, with a coupling constant of J/k(B) = 16-17 K; the very low temperature magnetic properties are in quantitative agreement with the predictions of quantum field theory. DFT calculations are consistent with the formation of a one-dimensional magnetic chain with interstack interactions mediated by the BDTA counterions.

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PubMed2004

Structure and dynamics in a monolayer of dipolar spheres.

The structure and dynamics in a monolayer of dipolar soft spheres have been investigated using molecular dynamics simulations. This is a basic model of colloidal ferrofluid monolayers, and other magnetic liquids in planar geometries, which can exhibit self-assembled chainlike aggregates due to strong dipole-dipole interactions. The effects of such chaining on the structure, single-particle translational and rotational motions, and the collective rotational motions are examined. The signatures of aggregation in the various structural and dynamical functions considered in this study could prove useful in experimental investigations of strongly dipolar materials.

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PubMed2004

Constant-volume heat capacity in a near-critical fluid from Monte Carlo simulations.

We consider a near-critical fluid of hard spheres with short-range interactions (approximately r(-6)) and obtain its constant-volume heat capacity C(V) by means of Monte Carlo calculations in the canonical ensemble. The question addressed is whether or not the heat capacities of the finite-size systems studied in simulations can provide a reliable indication of nonclassical criticality. For the model fluid considered here this is found to be the case. The heat capacity along the critical isochore shows a peak near the critical temperature, with a system size dependence that is consistent with the known Ising universality class of the model. The relevance of our results to recent attempts to determine the universality class of ionic fluids through calculations of C(V) is briefly discussed.

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PubMed2004

A theoretical study of the staggered and eclipsed forms of the dinuclear complex Mn Re(CO)10.

Two possible conformers of the dinuclear complex Mn Re(CO)10, each of C(4v) symmetry, with eclipsed and staggered conformations, have been analyzed theoretically. Using both the B3LYP and BP86 density functionals we find that the staggered form is lower in energy. A determination of the B3LYP potential energy surface as a function of the Mn-Re distance is presented for both conformers. The computed bond lengths, bond angles, and rotational constant for the staggered conformation compare favorably with the results from microwave experiments. The harmonic frequencies for the staggered structure have been determined using several basis sets, with both analytical and finite difference methods. These unscaled vibrational frequencies, together with their intensities for both infrared and Raman activity, are used to assign the three most intense experimental IR and Raman bands, and in particular, the nu(CO) region. The lowest A(2) vibration was calculated to occur at 41 cm(-1) in the staggered conformer; this frequency becomes imaginary in the (saddle point) eclipsed form. Several fundamentals remain to be observed experimentally.

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PubMed2004

Computer simulations of bent-core liquid crystals.

The phase behavior of model linear and bent-core molecules has been studied using isothermal-isobaric Monte Carlo computer simulations. The molecular model consists of seven Lennard-Jones spheres rigidly arranged in a "V" shape, with external bond angle, gamma. With gamma=0 degrees (linear molecules), we find isotropic, nematic, untilted smectic A, and two layered phases in which the molecules are tilted with respect to the layer normal. The latter two phases correspond to distinct branches in the equation of state, and possess different types of ordering within and between the layers; these phases are tentatively assigned as being smectic B and crystal. Apart from the possible existence of a tilted smectic B, the phase behavior of this system is broadly in line with earlier simulation studies on related linear molecular models. In the gamma=20 degrees system, isotropic, nematic, and tilted smectic- B phases are observed. Interestingly, the range of stability of the nematic phase is enhanced compared to the gamma=0 degrees system. In simulations of the gamma=40 degrees system, nematic phases are absent, and only isotropic and tilted phases are in evidence. The in-layer structure of the tilted phases shows a very clear change from smectic- B to smectic- A ordering upon increasing the temperature. In all instances of a tilted phase, the degree of molecular tilt is in the region of 30+/-5 degrees, with respect to the smectic layer normal, which corresponds closely to typical experimental observations in real bent-core liquid crystals. In our model, the tilt provides efficient packing of the spheres and favorable attractive interactions between molecules. The relevance of the present simulation model to real bent-core liquid crystals is discussed critically.

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PubMed2003

Structure and phase behavior of a two-dimensional system with core-softened and long-range repulsive interactions.

The structure and phase behavior of a two-dimensional system with purely repulsive core-softened and long-range interactions are studied using Monte Carlo computer simulations. The pair interactions are of the form, u(r)=4epsilon[(sigma/r)(12)-(sigma/r)(6)]+epsilon(')(sigma/r)(3), with the energy parameter, epsilon(')=(8sqrt[6]/9)epsilon chosen to give a stationary point of inflection in the pair potential at r=6(1/6)sigma. This potential approximates the effective interparticle interactions for a two-dimensional dipolar system in a strong field aligned perpendicular to the plane. The low-temperature portion of the phase diagram is sketched out, and the static properties of the various phases are analyzed in some detail. At low temperatures a variety of interesting states are in evidence, including: fluids with chainlike, striped, and 6-10 sided polygon structural motifs; low-density and high-density triangular crystalline phases; and defective Kagomé lattices. It is shown that clustering is driven by the presence of the repulsive shoulder in the pair potential. Other features, such as the presence of a disordered phase with a network structure, are due to the long-range (1/r(3)) repulsive tail in the potential. The relevance of the simulation results to experimental work, including materials synthesis, is briefly discussed.

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PubMed2003

Phase diagrams of hard spheres with algebraic attractive interactions.

The phase diagrams of systems made up of hard spheres interacting with attractive potentials of the form -1/r(3+sigma) are calculated using Monte Carlo simulations, second-order thermodynamic perturbation theory, and an augmented van der Waals theory. In simulations of the systems with sigma=0.1, 1, and 3, fluid-solid coexistence results are obtained using the Gibbs-Duhem integration technique; simulation data for the vapor-liquid coexistence envelopes and critical points are taken from previously published work [P. J. Camp and G. N. Patey, J. Chem. Phys. 114, 399 (2001)]. It is shown that the agreement between the theoretical and simulated phase diagrams improves as the range of the potential is increased, reflecting the decreasing role of short-range correlations in determining the bulk thermodynamics. In the extreme case of sigma=0.1 both theories are in excellent agreement with simulations. Phase diagrams for systems with sigma=4, 5, and 6 are computed using second-order thermodynamic perturbation theory. The results indicate that the vapor-liquid transition becomes metastable with respect to freezing when sigma > or approximately equal to 5, in broad agreement with results for the hard-sphere attractive Yukawa system which is commonly used to model colloidal particles, globular proteins, and nanoparticles.

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