Nucleation of self-associating fluids: free versus activated association

We use the density functional theory of statistical mechanics in a square gradient approximation to analyze the structure, size, and work of formation of critical nuclei in self-associating fluids where association reduces the strength of the interactions between bonded particles. This effect is expected in systems of strongly dipolar particles that associate into chains. In this work we analyze the nucleation behavior of two types of self-associating fluids: a system comprised of particles that can freely associate, and a system in which the association process involves a thermally activated initiation step. For the first case, we explore the properties of critical nuclei in fluids that exhibit a metastable critical point between a vapor phase and a highly associated liquid phase. In fluids where the association dynamics involves an initiation step, we investigate the nucleation behavior in the vicinity of the polymerization transition. In both cases critical nuclei undergo a structural transition that shares many of the features of the coil-globule transition reported in Monte Carlo simulations of strongly dipolar Stockmayer fluids. Our results suggest that the sharp structural transition observed in these simulations is evidence of the existence of a second-order or nearly second-order association transition in these model fluids.     

Phase behavior and particle size cutoff effects in polydisperse fluids

We report a joint simulation and theoretical study of the liquid-vapor phase behavior of a fluid in which polydispersity in the particle size couples to the strength of the interparticle interactions. Attention is focused on the case in which the particle diameters are distributed according to a fixed Schulz form with degree of polydispersity delta = 14%. The coexistence properties of this model are studied using grand canonical ensemble Monte Carlo simulations and moment free energy calculations. We obtain the cloud and shadow curves as well as the daughter phase density distributions and fractional volumes along selected isothermal dilution lines. In contrast to the case of size-independent interaction [N. B. Wilding et al., J. Chem. Phys. 121, 6887 (2004)], the cloud and shadow curves are found to be well separated, with the critical point lying significantly below the cloud curve maximum. For densities below the critical value, we observe that the phase behavior is highly sensitive to the choice of upper cutoff on the particle size distribution. We elucidate the origins of this effect in terms of extremely pronounced fractionation effects and discuss the likely appearance of new phases in the limit of very large values of the cutoff.     

Phase behavior of colloidal suspensions with critical solvents in terms of effective interactions

We study the phase behavior of colloidal suspensions the solvents of which are considered to be binary liquid mixtures undergoing phase segregation. We focus on the thermodynamic region close to the critical point of the accompanying miscibility gap. There, due to the colloidal particles acting as cavities in the critical medium, the spatial confinements of the critical fluctuations of the corresponding order parameter result in the effective, so-called critical Casimir forces between the colloids. Employing an approach in terms of effective, one-component colloidal systems, we explore the possibility of phase coexistence between two phases of colloidal suspensions, one being rich and the other being poor in colloidal particles. The reliability of this effective approach is discussed.     

Phase behavior of model confined fluids. Influence of substrate-fluid interaction strength

We examine the relationship between the macroscopic phase behavior of nanoconfined fluids and the nature of microscopic interactions between a confining substrate and fluid. Two model slit-pore systems are explored using grand canonical transition-matrix Monte Carlo simulation. One system consists of a square-well fluid interacting with a square-well substrate, and the other contains an embedded point charge model of lysozyme interacting with a mica surface. Fluid phase diagrams are constructed for a broad range of substrate conditions. Our results indicate that one observes a maximum in the critical temperature of the fluid phase envelope upon variation of substrate strength for a given slit width. Both systems studied exhibit such maxima at intermediate wall strength. The physical rationale for this observation suggests that this behavior should be generally expected. We introduce two metrics that enable one to predict conditions that produce maxima in critical temperature. The first is related to the contact angle a fluid develops at a single confining substrate. The second is based upon virial coefficient information and requires knowledge of the substrate-fluid and fluid-fluid interaction potentials only.     

Effect of retardation in the dispersion forces between spherical particles

A model potential for intermolecular attractive forces, which takes into account the retardation effect at large distances, is proposed. This potential is shown to be capable of approximating arbitrary empirical potentials of intermolecular interactions. For this model potential, the generalization of the known Hamaker formula for the attractive energy of macroscopic particles is derived. A comparison of the laws of attraction of particles of spherical shapes, when the retardation effect is taken into account as realized in the model discussed, with the classical Hamaker result neglecting such dependence is carried out.     

Phase behavior of amphiphilic polymers: A dissipative particles dynamics study

Dissipative particle dynamics is a mesoscopic simulation method which allows one to predict the self-assembly of amphiphilic polymers and surfactants. It was possible to reproduce the formation of microemulsions of the oil/water (o/w), water/oil (w/o), and L₃-type of C10E4 in water and n-decane with excellent accuracy. We are able to predict the experimentally not investigated emulsion formation of a poly(ethylene butylene)–poly(ethylene oxide) in water and methylcyclohexane.     

Electrostatic interactions between amphoteric latex particles and proteins

The electrostatic interactions between amphoteric polymethyl methacrylate latex particles and proteins with different pI values were investigated. These latex particles possess a net positive charge at low pH, but they become negatively charged at high pH. The nature and degree of interactions between these polymer particles and proteins are primarily controlled by the electrostatic characteristics of the particles and proteins under the experimental conditions. The self-promoting adsorption process from the charge neutralization of latex particles by the proteins, which have the opposite net charge to that of the particles, leads to a rapid reduction in the zeta potential of the particles (in other words colloidal stability), and so strong flocculation occurs. On the other hand, the electrostatic repulsion forces between similarly charged latex particles and the proteins retard the adsorption of protein molecules onto the surfaces of the particles. Therefore, latex particles exhibit excellent colloidal stability over a wide range of protein concentrations. A transition from net negative charge to net positive charge, and vice versa (charge reversal), was observed when the particle surface charge density was not high enough to be predominant in the protein adsorption process.     

Effect of stress-jump condition on electrophoretic behavior of a spherical dispersion of soft particles

The electrophoresis of a concentrated dispersion of soft particles, where a particle comprises a rigid core and an ion-penetrable membrane layer, is modeled theoretically, taking the effect of double-layer polarization into account. In particular, the influence of a stress-jump condition of the flow field at the membrane layer-liquid interface on the electrophoretic mobility of a particle is investigated. The type of particles considered mimic biocolloids, such as cells and microorganisms, and inorganic colloids covered by an artificial polymer layer such as surfactant molecules. A unit cell model is adopted to simulate the present spherical dispersion, and the governing equations and the associated boundary conditions are solved by a pseudo-spectral method based on Chebyshev polynomials. We show that while the stress-jump condition, characterized by a stress-jump coefficient, can have a significant influence on the mobility of a particle, the associated flow field is not influenced appreciably. Also, the influence of the stress-jump condition on the mobility of a particle depends largely on the nature of the membrane layer, characterized by its friction coefficient.     

Relationships between three-body and two-body interactions in fluids and solids

Molecular dynamics data are reported for two-body and three-body interactions in noble gases at densities covering the gas, liquid, and solid phases. The data indicate that simple relationships exist between three- and two-body interactions in both fluid and solid phases. The relationship for liquids has a simple density dependence with only one external parameter. In contrast, the solid phase relationship depends both on density and on the square of density and requires the evaluation of two parameters. The relationships are tested for both system-size and temperature dependences. The values of the relationship parameters are only sensitive to system size when a small number of atoms are involved. For 500 or more atoms, they remain nearly constant. The relationships are valid for both subcritical and slightly supercritical temperatures. A practical benefit of the relationships is that they enable the use of two-body intermolecular potentials for the prediction of the properties of real systems without the computational expense of three-body calculations.     

Structure and flow behavior of crosslinked ethyl acrylate–methacrylic acid copolymer dispersion particles

 Changes in viscosity, pH and static light scattering behavior on alkali addition of model dispersions of statistical copolymers of ethyl acrylate with 15 and 50 wt% of methacrylic acid prepared by semicontinuous emuslion copolymerization and crosslinked with various amount of N,N′-methylenebisacrylamide were investigated. It was found that about 1% of crosslinking agent was sufficient to prevent unpredictable disintegration of dispersion particles and gauranteed reproducible flow behavior of alkalinized dispersions. The viscosity of alkalinized dispersions of crosslinked particles at low concentration is controlled by the effective hydrodynamic volume of swelled particles which decreases with the crosslinking degree of copolymer. On the other hand, at higher particle concentration close to critical concentration at which the space is filled up by swelled particles the influence of particle interaction prevails and due to lower deformability of crosslinked particles viscosity increases with a crosslinking degree. In comparison with similar dispersions prepared without a crosslinking agent these results indicated much uniform structure of dispersion particles and suppression of influence of random crosslinking due to chain transfer reactions. The study shows that dispersions of crosslinked copolymers of this type could be considered as thickners of latex binders with stable and reproducible thickening properties. Received: 23 February 1998 Accepted: 11 June 1998     

Extended DLVO interactions between spherical particles and rough surfaces

An "extended DLVO" approach that includes Lifshitz-van der Waals, Lewis acid-base, and electrostatic double layer interactions is used to describe interaction energies between spherical particles and rough surfaces. Favorable, unfavorable, and intermediate deposition conditions are simulated using surface properties representing common aquatic colloids and polymeric membranes. The surface element integration (SEI) technique and Derjaguin's integration method are employed to calculate interaction energy. Numerical simulations using SEI demonstrate that nanometer scale surface roughness features can produce a distribution of interaction energy profiles. Local interaction energies are statistically analyzed to define representative interaction energy profiles-minimum, average, and maximum-for various combinations of simulated particles and surfaces. In all cases, the magnitude of the average interaction energy profile is reduced, but the reduction of energy depends on particle size, asperity size, and density of asperities. In some cases, a surface that is on average unfavorable for deposition (repulsive) may possess locally favorable (attractive) sites solely due to nanoscale surface roughness. A weighted average of the analytical sphere-sphere and sphere-plate expressions of Derjaguin reasonably approximates the average interaction energy profiles predicted by the SEI model, where the weighting factor is based on the fraction of interactions involving asperities.     

Many-body interactions between particles in a polydisperse polymer fluid

We use a continuum chain model and develop an analytical theory for the interaction between many spheres immersed in a fluid of ideal polydisperse polymers. Assuming local spherical symmetry of the polymer field about each particle, combined with a local approximation, compact expressions are derived for the many-body interaction between the spheres. We use a mean-field approximation to investigate the fluid-fluid phase diagram for the mixture.     

Long-range interactions between soft colloidal particles in slit-pore geometries

Combining theoretical and experimental techniques, we investigate the structure formation of charged colloidal suspensions of silica particles in bulk and in spatial confinement (slit-pore geometry). Our focus is to identify characteristic length scales determining typical quantities, such as the position of the main peak of the bulk structure factor and the period of the oscillatory force profile in the slitpore. We obtain these quantities from integral equations/SANS experiments (bulk) and Monte Carlo simulations/colloidal probe-AFM measurements (confinement), in which the theoretical calculations are based on the Derjaguin-Landau-Verwey-Overbeck (DLVO) potential. Both in bulk and in the slitpore, we find excellent qualitative and quantitative agreement between theory and experiment as long as the ionic strength chosen in the DLVO potential is sufficiently low (implying a relatively long-ranged interaction). In particular, the bulk properties of these systems obey the widely accepted density scaling of xi proportional to phi(-1/3). On the other hand, systems with larger ionic strengths and, consequently, more short-ranged interactions do not obey such power law behavior and rather resemble an uncharged hard-sphere fluid, in which the relevant length scale is the particle diameter.     

Phase behavior of the restricted primitive model of ionic fluids with association in slitlike pores. Density-functional approach

We present results of investigations of condensation of restricted primitive model of electrolyte solutions with association between oppositely charged ions confined to slitlike pores. The associative interaction leads to the formation of ionic pairs. It is accounted for by incorporating the first-order thermodynamic perturbation theory into the free energy functional. In order to elucidate the role of association, the phase diagrams are compared with those obtained by us recently [O. Pizio et al., J. Chem. Phys. 121, 11957 (2004)] for the restricted primitive model. The inclusion of the association into the theory leads to lowering the critical temperature for the fluid confined to pores with uncharged and with charged walls. We have observed that the average fraction of bonded ions is high along the coexistence envelope.     

Colloidal interactions between Langmuir-Blodgett bitumen films and fine solid particles

In oil sand processing, accumulation of surface-active compounds at various interfaces imposes a significant impact on bitumen recovery and bitumen froth cleaning (i.e., froth treatment) by altering the interfacial properties and colloidal interactions among various oil sand components. In the present study, bitumen films were prepared at toluene/water interfaces using a Langmuir-Blodgett (LB) upstroke deposition technique. The surface of the prepared LB bitumen films was found to be hydrophobic, comprised of wormlike aggregates containing a relatively high content of oxygen, sulfur, and nitrogen, indicating an accumulation of surface-active compounds in the films. Using an atomic force microscope, colloidal interactions between the LB bitumen films and fine solids (model silica particles and clay particles chosen directly from an oil sand tailing stream) were measured in industrial plant process water and compared with those measured in simple electrolyte solutions of controlled pH and divalent cation concentrations. The results show a stronger long-range repulsive force and weaker adhesion force in solutions of higher pH and lower divalent cation concentration. In plant process water, a moderate long-range repulsive force and weak adhesion were measured despite its high electrolyte content. These findings provide more insight into the mechanisms of bitumen extraction and froth treatment.     

Phase behavior of attractive and repulsive ramp fluids: integral equation and computer simulation studies

Using computer simulations and a thermodynamically self-consistent integral equation we investigate the phase behavior and thermodynamic anomalies of a fluid composed of spherical particles interacting via a two-scale ramp potential (a hard core plus a repulsive and an attractive ramp) and the corresponding purely repulsive model. Both simulation and integral equation results predict a liquid-liquid demixing when attractive forces are present, in addition to a gas-liquid transition. Furthermore, a fluid-solid transition emerges in the neighborhood of the liquid-liquid transition region, leading to a phase diagram with a somewhat complicated topology. This solidification at moderate densities is also present in the repulsive ramp fluid, but in this case inhibits the fluid-fluid separation.     

Towards a mesoscopic model of water-like fluids with hydrodynamic interactions

We present a mesoscopic lattice model for non-ideal fluid flows with directional interactions, mimicking the effects of hydrogen bonds in water. The model supports a rich and complex structural dynamics of the orientational order parameter, and exhibits the formation of disordered domains whose size and shape depend on the relative strength of directional order and thermal diffusivity. By letting the directional forces carry an inverse density dependence, the model is able to display a correlation between ordered domains and low density regions, reflecting the idea of water as a denser liquid in the disordered state than in the ordered one.     

Alignment of particles in sheared viscoelastic fluids

We investigate the shear-induced structure formation of colloidal particles dissolved in non-Newtonian fluids by means of computer simulations. The two investigated visco-elastic fluids are a semi-dilute polymer solution and a worm-like micellar solution. Both shear-thinning fluids contain long flexible chains whose entanglements appear and disappear continually as a result of Brownian motion and the applied shear flow. To reach sufficiently large time and length scales in three-dimensional simulations with up to 96 spherical colloids, we employ the responsive particle dynamics simulation method of modeling each chain as a single soft Brownian particle with slowly evolving inter-particle degrees of freedom accounting for the entanglements. Parameters in the model are chosen such that the simulated rheological properties of the fluids, i.e., the storage and loss moduli and the shear viscosities, are in reasonable agreement with experimental values. Spherical colloids dispersed in both quiescent fluids mix homogeneously. Under shear flow, however, the colloids in the micellar solution align to form strings in the flow direction, whereas the colloids in the polymer solution remain randomly distributed. These observations agree with recent experimental studies of colloids in the bulk of these two liquids.