Equation of state and liquid-vapor equilibria of one- and two-Yukawa hard-sphere chain fluids: theory and simulation

The accuracy of several theories for the thermodynamic properties of the Yukawa hard-sphere chain fluid are studied. In particular, we consider the polymer mean spherical approximation (PMSA), the dimer version of thermodynamic perturbation theory (TPTD), and the statistical associating fluid theory for potentials of variable attractive range (SAFT-VR). Since the original version of SAFT-VR for Yukawa fluids is restricted to the case of one-Yukawa tail, we have extended SAFT-VR to treat chain fluids with two-Yukawa tails. The predictions of these theories are compared with Monte Carlo (MC) simulation data for the pressure and phase behavior of the chain fluid of different length with one- and two-Yukawa tails. We find that overall the PMSA and TPTD give more accurate predictions than SAFT-VR, and that the PMSA is slightly more accurate than TPTD.     

Cluster kinetics of pressure-induced glass formation

A prior correlation model for glass formation based on cluster-size distribution kinetics is here extended to account for pressure effects as well as temperature effects. The model describes how rapidly cooling or compressing a liquid or colloid leads to structural arrest and a consequent sharp rise in viscosity or dielectric relaxation time. In addition to activation energies, we include activation volumes in the rate coefficients for monomer-cluster addition and dissociation and cluster aggregation and breakage. The approach leads to scaled pressure correlations and plots for viscosity that reveal strong and fragile glass behavior, and agree with experimental data. A simple relationship among viscosity, attractive interparticle energy, and particle volume fraction displays how hard spheres with attractive forces can vitrify at small particle densities.     

Molecular kinetics of cluster formation in the dense fluids

In this paper we report on molecular dynamics (MD) simulations of the cluster formation kinetics in dense systems. We suggest a cluster identification procedure for dense systems that takes into account the interactions between particles. Problems of cluster existence in unsaturated compressed gas are considered and cluster formation in saturated compressed gas is studied. Molecular dynamics models of adsorbed gas condensation on the surface and the kinetics of the adsorption layer formation have been considered.     

Cluster pair correlation function of simple fluids: energetic connectivity criteria

We consider the clustering of Lennard-Jones particles by using an energetic connectivity criterion proposed long ago by Hill [J. Chem. Phys. 32, 617 (1955)] for the bond between pairs of particles. The criterion establishes that two particles are bonded (directly connected) if their relative kinetic energy is less than minus their relative potential energy. Thus, in general, it depends on the direction as well as on the magnitude of the velocities and positions of the particles. An integral equation for the pair connectedness function, proposed by two of the authors [Phys. Rev. E 61, R6067 (2000)], is solved for this criterion and the results are compared with those obtained from molecular dynamics simulations and from a connectedness Percus-Yevick-type integral equation for a velocity-averaged version of Hill's energetic criterion.     

Cluster ion formation in laser mass spectrometry of substituted pyridines

Emission of cluster ions occurs during laser irradiation of substituted pyridines even at threshold laser power densities. The clusters generated include dimers and trimers, and appear in both positive-ion and negative-ion laser mass spectra. Fragments of cluster ions are observed and can be rationlized as losses of neutral molecules from (nM ± H)^±. Dissociation of clusters occurs primarily from substituents on the pyridine ring. Laser mass spectrometry of pyridoxine hydrochloride and pyridoxamine-dihydrochloride resulted in the emission of clusters analogous to those observed for nicotinic acid. In contrast to these results, secondary-ion and field-desorption mass spectra of salts contain the ions C_nA_n?1⁺ and C_nA_n+1^?, that were not detected in the laser mass spectra.     

Cluster formation and rheology of photoreactive nanoparticle dispersions

We show how photocrosslinking of small nanoparticles within a very dilute colloidal dispersion leads to the formation of large fractal particle clusters, which have a strong influence on the viscosity of the dispersion although the overall solid content is well below 5 wt %. Furthermore, the solvent plays an important role because of its function as an optical filter, for example, in toluene only photocrosslinking but no photocleavage takes place. Therefore, a diffusion-controlled cluster growth mechanism, leading to clusters with low fractal dimension, is expected; on the other hand, in tetrahydrofuran the photoreaction is partially reversible. Therefore, the cluster growth in this case is reaction controlled, leading to more compact clusters with higher fractal dimension, which therefore only have a negligible effect on the rheological properties of the solvent. In this context, we will briefly discuss the possibility to use our nanoparticle system as opto-rheological switch.     

The formation and kinetics of Ionized Cluster Beams

The formation and kinetics of large vapourized-material cluster beams (large size metal clusters) are discussed. The clusters are formed by injecting the vapour of solid state materials into a high vacuum region through a nozzle of a heated crucible. The conditions under which metal clusters form are analysed using nucleation theory. Computer simulation by combining the nucleation and flow equations has also been made. The results show that the theory can be useful in predicting qualitative dependences of metal cluster formation on operation conditions. Several experimental results are also presented, which support the finding that a large size metal cluster is formed by homogeneous nucleation and growth. The advantageous characteristics of ionized cluster beam for thin film formation are also discussed.     

The structural properties of a two-Yukawa fluid: Simulation and analytical results

Standard Monte Carlo simulations are carried out to assess the accuracy of theoretical predictions for the structural properties of a model fluid interacting through a hard-core two-Yukawa potential composed of a short-range attractive well next to a hard repulsive core, followed by a smooth, long-range repulsive tail. Theoretical calculations are performed in the framework provided by the Ornstein-Zernike equation, solved either analytically with the mean spherical approximation (MSA) or iteratively with the hypernetted-chain (HNC) closure. Our analysis shows that both theories are generally accurate in a thermodynamic region corresponding to a dense vapor phase around the critical point. For a suitable choice of potential parameters, namely, when the attractive well is deep and/or large enough, the static structure factor displays a secondary low-Q peak. In this case HNC predictions closely follow the simulation results, whereas MSA results progressively worsen the more pronounced this low-Q peak is. We discuss the appearance of such a peak, also experimentally observed in colloidal suspensions and protein solutions, in terms of the formation of equilibrium clusters in the homogeneous fluid.     

Liquid-vapor phase diagram and cluster formation of two-dimensional ionic fluids

Direct molecular dynamics simulations on interfaces at constant temperature are performed to obtain the liquid-vapor phase diagram of the two-dimensional soft primitive model, an equimolar mixture of equal size spheres carrying opposite charges. Constant temperature and pressure simulations are also carried out to check consistency with interface simulations results. In addition, an analysis of the cluster formation of mixtures of particles with charge asymmetry in the range 1:1 to 1:36 at low and high densities is performed. The number of free ions, when plotted as a function of the positive ion charge, Z(+), has an oscillatory behavior and is independent of the density. The formation of aggregates is analyzed in terms of the attraction and repulsion between ions.     

Communication: thermodynamic signatures of cluster formation in fluids with competing interactions

Convergent theoretical evidence, based on self-consistent integral equations for the pair structure and on Monte Carlo simulations, is presented for the existence of small simultaneous jump discontinuities of several thermodynamic and structural properties of systems of colloidal particles with competing short-range attractive and long-range repulsive interactions, under physical conditions close to the onset of particle clustering. The discontinuities thus provide a signature of the transition from a homogeneous fluid phase to a locally inhomogeneous cluster phase.     

Cluster conformations and multipole distributions in ionic fluids. I. Two-dimensional systems of mobile ions

A new association-biased Monte Carlo (MC) method is proposed for efficient simulation of association and dissociation of ions in an ionic fluid. The method is then utilized to carry out extensive MC simulations, in order to study the properties of ionic fluids in two-dimensional systems that consist of mobile ions. The size distributions of the ionic clusters, their conformations, as well as the clusters' multipole distributions are computed over wide ranges of temperature T and ions' density rho. At any given T, bonded dipolar pairs are dominant in the insulating phase, but larger clusters with an even number of ions are also present. In the conducting phase at the same T, however, single (free) ions are abundant, while clusters of larger sizes are also present. As for the conformations of the clusters, at any T, perturbed folded structures are dominant in the insulating phase, whereas perturbed linear chains are the dominant conformation in the conducting phase at the same T. Moreover, ionic clusters with closed loops are rarely formed, if at all, over the range of T that we study. As T decreases, more clusters with symmetrical conformations are formed. The multipole distributions are shown to be accurate indicators for the various types of conformations of the ionic clusters. They are also shown to be accurate means of differentiating the conformations of ionic clusters that may appear to be only slightly different, and may be difficult to distinguish otherwise, as the multipoles are sensitive to the details of the conformations. Some exact results are presented for the dipoles and quadrupoles of several types of cluster conformations. These results give rise, for the first time, to a numerical "spectroscopy" of ionic fluids, whereby each conformation is associated with distinct values of the dipole and quadrupole of the ionic cluster. We also suggest a new method of locating the critical locus T(c)(rho) that separates the conducting and insulating phases-the Kosterlitz-Thouless transition-based only on the size distribution of the ionic clusters and its dependence on the ions' density.     

Prediction of collective diffusion coefficient of bovine serum albumin in aqueous electrolyte solution with hard-core two-Yukawa potential

A new method to predict concentration dependence of collective diffusion coefficient of bovine serum albumin (BSA) in aqueous electrolyte solution is developed based on the generalized Stokes-Einstein equation which relates the diffusion coefficient to the osmotic pressure. The concentration dependence of osmotic pressure is evaluated using the solution of the mean spherical approximation for the two-Yukawa model fluid. The two empirical correlations of sedimentation coefficient are tested in this work. One is for a disordered suspension of hard spheres, and another is for an ordered suspension of hard spheres. The concentration dependence of the collective diffusion coefficient of BSA under different solution conditions, such as pH and ionic strength is predicted. From the comparison between the predicted and experimental values we found that the sedimentation coefficient for the disordered suspension of hard spheres is more suitable for the prediction of the collective diffusion coefficients of charged BSA in aqueous electrolyte solution. The theoretical predictions from the hard-core two-Yukawa model coupled with the sedimentation coefficient for a suspension of hard spheres are in good agreement with available experimental data, while the hard sphere model is unable to describe the behavior of diffusion due to its neglect of the double-layer repulsive charge-charge interaction between BSA molecules.     

Gas-hydrate formation, agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs, which make the drilling unsafe. Some oil-based drilling fluids (OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation, agglomeration and inhibition by an experimental system under the temperature of 4 ℃ and pressure of 20 MPa, which would be similar to the case of 2000 m water depth. The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF. The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles. The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later. Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Gas-hydrate formation,agglomeration and inhibition in oil-based drilling fluids for deep-water drilling

One of the main challenges in deep-water drilling is gas-hydrate plugs,which make the drilling unsafe.Some oil-based drilling fluids(OBDF) that would be used for deep-water drilling in the South China Sea were tested to investigate the characteristics of gas-hydrate formation,agglomeration and inhibition by an experimental system under the temperature of 4 ?C and pressure of 20 MPa,which would be similar to the case of 2000 m water depth.The results validate the hydrate shell formation model and show that the water cut can greatly influence hydrate formation and agglomeration behaviors in the OBDF.The oleophobic effect enhanced by hydrate shell formation which weakens or destroys the interfacial films effect and the hydrophilic effect are the dominant agglomeration mechanism of hydrate particles.The formation of gas hydrates in OBDF is easier and quicker than in water-based drilling fluids in deep-water conditions of low temperature and high pressure because the former is a W/O dispersive emulsion which means much more gas-water interfaces and nucleation sites than the later.Higher ethylene glycol concentrations can inhibit the formation of gas hydrates and to some extent also act as an anti-agglomerant to inhibit hydrates agglomeration in the OBDF.     

Influence of coagulant and free stabilizer on formation of aggregates in magnetic fluids

The influence of coagulant (isopropanol) and free oleic acid on the formation of quasi-spherical aggregates in a colloidal magnetite solution in kerosene is studied experimentally. It is revealed that the mean sizes of these aggregates fit the 60–90-nm range and are independent of the concentrations of coagulant and free oleic acid provided that these concentrations do not exceed 10–12 vol %. The independence of the mean sizes of aggregates on temperature and disperse composition of particles is considered as a supplementary argument in favor of hypothesis that the main reason for their formation are the defects of protective layers, whereas magnetodipole interactions play a secondary role. It is shown that an excess of oleic acid causes a disproportional decrease in the initial susceptibility of magnetic fluid that is interpreted as a result of the formation of droplet aggregates with characteristic sizes of a few micrometers or more. Apparently, isopropanol leads to an analogous effect, but only at low temperatures.     

Spectrophotometric determination of diphenhydramine hydrochloride in pharmaceutical preparations and biological fluids via ion-pair formation

A simple, sensitive and accurate spectrophotometric method has been described for the assay of diphenhydramine hydrochloride (DPH) in raw material and in biological samples. The method is based on extraction of DPH into dichloromethane as ion-pair complexes with patent blue (PB), eriochrome black T (EBT), methyl orange (MO) and bromocresol purple (BCP) in acidic medium. The coloured species exhibited absorption maxima at 632, 514, 428 and 414 nm for PB, EBT, MO and BCP, with molar absorptivity values of 1.32 × 10⁵, 2.36 × 10⁴, 3.68 × 10⁴ and 3.07 × 10⁴ l mol^?1 cm^?1, respectively. The reaction conditions were optimized to obtain the maximum colour intensity. Beer’s law was obeyed with a good correlation coefficient (0.9982–0.9993) in the concentration ranges 0.5–3, 2.0–16, 2.0–10 and 1.0–10 μg ml^?1 for PB, EBT, MO and BCP methods, respectively. The composition ratio of the ion-association complexes was found to be 1:1 in all cases as established by Job’s method. The conditional stability constant (K_f) and the free energy changes (ΔG°) were determined for all complexes formed. The proposed method was successfully applied for the determination of DPH in tablets and human urine with good accuracy and precision. Statistical comparison of the results with those obtained by the official method showed good agreement and indicated no significant difference in accuracy and precision.