C. Caccamo

Ionic radii and diffraction patterns of molten alkali halides

The mean spherical approximation for a fluid of charged hard spheres of different radii is applied to the evaluation of diffraction patterns for the whole family of molten alkali halides. The ionic radii entering the model are determined from the liquid‐state compressibility. The liquid‐state radii are shown to be characteristic temperature‐dependent lengths for each ion through the family of salts, and a good one‐parameter fit of the whole set of compressibilities at each temperature is obtained through the assumption that the radius ratios are those for basic radii yielded by the Born–Mayer theory of alkali halide crystals. The analysis also provides new evidence against the older sets of crystal ionic radii. Calculations of partial structure factors and of x‐ray diffraction patterns are then reported for all the salts and compared with the results of x‐ray and neutron diffraction experiments wherever possible. The model emerges as having semiquantitative value in the prediction of x‐ray diffraction fro...

Theory and simulation of short-range models of globular protein solutions

We report theoretical and simulation studies of phase coexistence in model globular protein solutions, based on short-range, central, pair potential representations of the interaction among macro-particles. After reviewing our previous investigations of hard-core Yukawa and generalized Lennard-Jones potentials, we report more recent results obtained within a DLVO-like description of lysozyme solutions in water and added salt. We show that a one-parameter fit of this model, based on static light scattering and self-interaction chromatography data in the dilute protein regime, yields demixing and crystallization curves in good agreement with experimental protein-rich?protein-poor and solubility envelopes. The dependence of cloud and solubility point temperatures of the model on the ionic strength is also investigated. Our findings highlight the minimal assumptions on the properties of the microscopic interaction sufficient for a satisfactory reproduction of the phase diagram topology of globular protein solutions.

Phase coexistence in a DLVO model of globular protein solutions

Globular protein solutions of lysozyme in water and added salt are modelled according to the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory, in order to determine their fluid–fluid and fluid–solid coexistence lines. Calculations are based on both computer simulations and theoretical approaches. Our results indicate that, when the potential parameters are obtained by fitting physical properties directly deducible from either static or dynamic light scattering data, the fluid–fluid phase coexistence predictions agree quite well with the experiments. Our description of the solid phase allows only a qualitative reproduction of the experimental solubility boundaries. The overall accuracy of our predictions is discussed in view of the well known limitations of the DLVO representation of protein solutions.

Equilibrium properties of charged hard spheres of different diameters in the electrolyte solution regime: Monte Carlo and integral equation results

Extensive Monte Carlo (MC) and integral equation calculations of thermodynamic and structural properties for the 1‐1 aqueous electrolyte regime of the primitive model are reported. The salt concentration covers the range 0.1–2 M. The ratio of the radii of the two ionic species α covers the range 0.1–0.8. The MC results are compared to the results of the HNC, MSA, and EXP approximations. The excess internal energies calculated in the HNC are found to be in good agreement with the MC values. In the MSA the energy turns out to be too high for large α and too small for small α. For α=0.4 the MSA value is accurate as compared to the simulation. Similar remarks hold for the osmotic coefficient. The radial distribution functions (RDF) calculated in the EXP approximation agree fairly well with the MC results at all molarities and radius ratios investigated, except for α=0.1, where the agreement is only qualitative. The MSA yields RDF which are in qualitative agreement with the MC only at high concentrations and f...

Structural properties and medium‐range order in calcium‐metasilicate (CaSiO3) glass: A molecular dynamics study

Structural properties and medium‐range order (MRO) in calcium metasilicate (CaSiO3) glass are investigated at different concentrations of calcium oxide through the molecular dynamics technique. Calculations are based on the Born–Mayer–Huggins potential, and show that the experimental Ca–Ca partial structure factor, which documents the existence in the glass of Ca MRO, can qualitatively be reproduced within such a model. The characteristics of Ca MRO are then examined in the context of the overall structure of the system. At equimolar concentrations of CaO and SiO2, the simulation evidentiates the formation in the glass of clusters of CaO6 octahedra in which Ca ions are roughly lying on a plane, in a configuration that closely resembles the one of crystalline CaSiO3. On the other hand, the ‘network’ structure and MRO of pure SiO2 glass appear sensibly affected by the presence of Ca ions, with considerable loose of connectivity between SiO4 tetrahedral units, and reduction of the first sharp diffraction pea...

Solution and thermodynamic consistency of the GMSA for hard sphere mixtures

The explicit numerical solution of the Generalized Mean Spherical Approximation (GMSA) for a two-component hard sphere mixture is obtained by employing an iterative procedure which starts from the known analytic Percus-Yevick (PY) result. The parameters which enter the theory are fitted to a set of thermodynamic data generated from parametrized forms of computer simulation results, so as to ensure the thermodynamic consistency of the theory. The radial distribution functions so obtained compare very favourably with the available Monte Carlo results, and systematically improve on the PY gij (r). A possible scheme for obtaining the thermodynamic consistency without having recourse to external data sources is also proposed. The q → 0 limit of the concentration-concentration structure factor S cc(q) is also investigated, this allowing a better understanding of the role played by the diameter ratio, R, and the concentration of the two species, in the enhancement or reduction of disorder in the mixture, especia...

Molecular-dynamics simulations of solid C60 and C70 through a spherical two-body potential

Abstract Molecular dynamics calculations of bulk properties of solid C 60 and C 70 , simulated in terms of a simple two-body potential, are presented. The bulk moduli and the room temperature equations of state are determined and compared with the available experimental data. The change in the lattice parameters as a function of pressure and temperature is also investigated.

Phase diagram of simple fluids: a comprehensive theoretical approach

The authors determine the phase-stability region of a Lennard-Jones fluid using an Ornstein-Zernike integral-equation approach. The theory, which is made self-consistent on the thermodynamic side, is complemented by an entropy-based criterion for the freezing of the liquid.

An improved closure for the Born–Green–Yvon equation for the electric double layer

An extensive series of calculations are made to compare an improved closure of the Born–Green–Yvon (BGY) equation for the electric double layer of primitive electrolytes to existing Monte Carlo simulation and other theories, such as the modified Poisson–Boltzmann (version 5) (MPB5) and the hypernetted chain/mean spherical approximation (HNC/MSA), and its recent improvements. In contrast to these theories the BGY equation satisfies the contact theorem always. Furthermore, the bulk pair correlation functions used are the most accurate available, namely, the HNC bulk results. The results show very good general agreement to the computer work for high densities and surface charges. In particular, we are able to go to higher densities than those hitherto published in the literature. For low density 2–2 and 2–1 electrolytes, the agreement is not as good as for the other theories.

Phase stability of dense charged hard sphere fluid mixtures

The mean spherical approximation (MSA), the hypernetted‐chain (HNC) approximation, and Monte Carlo (MC) calculations are used in order to investigate the phase stability of dense charged hard sphere fluid mixtures. According to the MSA, spinodal decomposition occurs in these systems in a certain range of the physical parameters. The HNC results indicate the existence of a spinodal line in qualitative agreement with the MSA prediction. Local compositions obtained by MC calculations show the expected behavior in a mixture approaching segregation. The concentrations of the coexisting phases are also calculated within the MSA, and the characteristics of the phase diagrams of different types of mixture are discussed. A detailed analysis is then made of the thermodynamic quantities which define the Gibbs free energy of mixing; this allows one to understand in more detail the segregation process. Finally, the applicability of the present approach for studying the phase stability conditions of molten salt mixture...