Short-range Attractive Forces Research Articles

Aggregation in Quenched Systems Interacting through a Short-Range Attractive, Long-Range Repulsive Potential

The morphology and structure factor of a 2D system of particles interacting through a Lennard-Jones potential, modified to include a long-range repulsive component, are described. Of particular interest are those systems quenched from a stabilized colloidal state to temperatures that, in the pure Lennard-Jones system, correspond to the solid-vapor coexistence region. We find that competition between the short-range attractive forces—which cause aggregation—and the long-range repulsion—which acts to keep the particles apart—encourages the formation of space-filling networks. A characteristic peak in the structure factor is observed which moves toward lower wavevectors as the simulation evolves. However, unlike the pure Lennard-Jones system, which when given enough time will completely phase separate, this peak stops moving once a network has formed. The similarities between our simulations and gel formation are discussed.

Interactions in protein solutions near crystallisation: a colloid physics approach

I present a review of current ideas from colloid physics which might be relevant in order to understand the onset of crystallisation and amorphous aggregation processes in protein solution. In particular, the inadequacy of DLVO theory to account for all phenomenological aspects of crystallisation, such as salt-specificity, and for the basic features of the phase diagram, such as the presence of a metastable fluid–fluid separation, is discussed. The fundamental role of additional short-range attractive forces, microscopically orginating from the salting-out effect, is conversely stressed. In order to establish a simple model of protein interparticle interactions near crystallisation, I discuss some recent results obtained by our group for the osmotic compressibility of the metastable fluid phase of hen egg-white lysozyme. Light scattering measurements were performed in an extended volume fraction range at pH=4.7 as a function of temperature, adding NaC1 to screen the electrostatic interactions. The experimental compressibility up to particle volume fractions Φ≈0.23 is very successfully compared to the theoretical expression for a model of adhesive (“sticky”) hard spheres. This surprising quantitative agreement, obtained using a very simple form for the effective interparticle force, suggests that the thermodynamics of the system in the fluid phase is mainly determined by the very short-range nature of the interactions, and is rather insensitive to the detailed form of the potential.

Interaction between like-charged colloidal spheres in electrolyte solutions.

How colloidal particles interact with each other is one of the key issues that determines our ability to interpret experimental results for phase transitions in colloidal dispersions and our ability to apply colloid science to various industrial processes. The long-accepted theories for answering this question have been challenged by results from recent experiments. Herein we show from Monte-Carlo simulations that there is a short-range attractive force between identical macroions in electrolyte solutions containing divalent counterions. Complementing some recent and related results by others, we present strong evidence of attraction between a pair of spherical macroions in the presence of added salt ions for the conditions where the interacting macroion pair is not affected by any other macroions that may be in the solution. This attractive force follows from the internal-energy contribution of counterion mediation. Contrary to conventional expectations, for charged macroions in an electrolyte solution, the entropic force is repulsive at most solution conditions because of localization of small ions in the vicinity of macroions. Both Derjaguin-Landau-Verwey-Overbeek theory and Sogami-Ise theory fail to describe the attractive interactions found in our simulations; the former predicts only repulsive interaction and the latter predicts a long-range attraction that is too weak and occurs at macroion separations that are too large. Our simulations provide fundamental "data" toward an improved theory for the potential of mean force as required for optimum design of new materials including those containing nanoparticles.

Solid–fluid equilibrium for a molecular model with short ranged directional forces

The phase diagram of a system of hard spheres with short-range tetrahedral association has been determined by computer simulation and theory. The fluid phase and two solid phases were considered. One of these solid phases is a low-density solid closely related in structure to ice Ic and the other is a high-density solid closely related in structure to ice VII. At high temperatures freezing occurs into the high-density solid whereas at low temperatures freezing occurs into the low-density solid. At an intermediate temperature a triple point is found where the fluid coexists with the two solids simultaneously. Although the low-density solid melts to a high-density fluid, this transition is found to be metastable with respect to the transformation into a high-density solid. This is evidence that short-range tetrahedral attractive forces are not in and of themselves sufficient to explain the anomalous melting of water. Our results indicate that vapor–liquid equilibrium for the model is preempted by solidification. Monte Carlo simulation results for the fluid phase are described successfully by Wertheim’s theory whereas those of the solid phases are described qualitatively by the cell theory.

Study of a model polyelectrolyte solution with directional attractive forces between the macroions

A modification of the primitive model of a polyelectrolyte solution, containing macroions and counterions, is presented. The macroions are assumed to carry 20 (or in some calculations 10) negative charges and the counterions are monovalent. Asymmetry in size between the two species is 15:2. In addition to the Coulombic forces, the macroions also interact via a short-range directional attractive force. The parameters of this attraction are chosen to result in the formation of dimers, but no trimers or higher clusters can be formed. This model of a polyelectrolyte solution is studied for a range of concentrations and for several values of the depth of the potential well, using the statistical-mechanical theory based on the two-density formalism for associating fluids. The results indicate that the short-range attraction between macroions and consequently the formation of dimers, yields a significant decrease of the osmotic coefficient for the model polyelectrolyte solution. Part of the effect is due to the strong attraction between the doubly charged aggregates and counterions in the solution. The effect of dimerization is also reflected in the excess internal energies and in all three pair distribution functions.

Molecular Mechanisms Controlling the Self-Assembly Process of Polyelectrolyte Multilayers

The distance dependent interaction between polyelectrolyte-covered mica surfaces in aqueous solution was investigated with the surface forces apparatus. We find the following: (i) The surface charge changes sign, when an oppositely charged polyelectrolyte from a concentrated polyelectrolyte solution is adsorbed. (ii) Tails and loops of the adsorbed polyions dangle into the bulk phase, inducing a small steric force. If polycations and poyanions are adsorbed on top of each other, a strong short range attractive force is seen due to ion-pair formation after crossing a large repulsive electrostatic/steric barrier. (iii) Obviously, after polyelectrolyte adsorption, there are still nonoccupied binding places (point charges) on the substrate. We show that these adsorption properties regulate the build-up of polyelectrolyte multilayers: Ion pairs between oppositely charged polyion segments and the substrate are formed, until the surface charge is inversed. The electrostatic barrier limits the adsorbed amount, guarantees the equal thickness of consecutively adsorbed layers, and furthermore causes a strong adsorption hysteresis, which leads to conveniently stable polyelectrolyte multilayers in various environmental conditions.

Static and dynamic structure of liquid metals: Role of the different parts of the interaction potential

The influence of different parts of the interaction potential on the microscopic behavior of simple liquid metals is investigated by molecular dynamics simulation. The role of the soft-core repulsive, short-range attractive, and long-range oscillatory forces on the properties of liquid lithium close to the triple point is analyzed by comparing the results from simulations of identical systems but truncating the potential at different distances. Special attention is paid to dynamic collective properties such as the dynamic structure factors, transverse current correlation functions, and transport coefficients. It is observed that, in general, the effects of the short-range attractive forces are important. On the contrary, the influence of the oscillatory long-range interactions is considerably less, being the most pronounced for the dynamic structure factor at long wavelengths. The results of this work suggest that the influence of the attractive forces becomes less significant when temperature and density increase. @S1063-651X~97!13108-7#

Monte Carlo simulation of pore systems: primitive model of water in slit-like pores

A primitive model of associating fluids due to Nezbeda and Smith has been applied to water confined to planar slits with hard walls. It describes the molecules by hard bodies with additional short-range off-center attractive forces mimicking hydrogen bonding. Monte Carlo simulations have been performed on this model. Both the density profile across the slit and the pressure normal to the walls have been estimated for slits of different width. A significant change in structure and a decrease in pressure at the wall have been found in comparison to the underlying pure hard-sphere fluid. For comparison, some simulation results for confined square-well fluids have been included.

QXP: powerful, rapid computer algorithms for structure-based drug design.

New methods for docking, template fitting and building pseudo-receptors are described. Full conformational searches are carried out for flexible cyclic and acyclic molecules. QXP (quick explore) search algorithms are derived from the method of Monte Carlo perturbation with energy minimization in Cartesian space. An additional fast search step is introduced between the initial perturbation and energy minimization. The fast search produces approximate low-energy structures, which are likely to minimize to a low energy. For template fitting, QXP uses a superposition force field which automatically assigns short-range attractive forces to similar atoms in different molecules. The docking algorithms were evaluated using X-ray data for 12 protein-ligand complexes. The ligands had up to 24 rotatable bonds and ranged from highly polar to mostly nonpolar. Docking searches of the randomly disordered ligands gave rms differences between the lowest energy docked structure and the energy-minimized X-ray structure, of less than 0.76 A for 10 of the ligands. For all the ligands, the rms difference between the energy-minimized X-ray structure and the closest docked structure was less than 0.4 A, when parts of one of the molecules which are in the solvent were excluded from the rms calculation. Template fitting was tested using four ACE inhibitors. Three ACE templates have been previously published. A single run using QXP generated a series of templates which contained examples of each of the three. A pseudo-receptor, complementary to an ACE template, was built out of small molecules, such as pyrrole, cyclopentanone and propane. When individually energy minimized in the pseudo-receptor, each of the four ACE inhibitors moved with an rms of less than 0.25 A. After random perturbation, the inhibitors were docked into the pseudo-receptor. Each lowest energy docked structure matched the energy-minimized geometry with an rms of less than 0.08 A. Thus, the pseudo-receptor shows steric and chemical complementarity to all four molecules. The QXP program is reliable, easy to use and sufficiently rapid for routine application in structure-based drug design.

Effect of Short-Range Interactions on Spreading

Two decades ago, the author concluded that, even if the macroscopic spreading condition is satisfied, the droplet may not spread completely over the entire surface of the solid (assumed large enough) as single atoms, but may form a planar droplet of multimolecular thickness, which has a rapid variation of angle near the leading edge. In the present paper, a simple and approximate expression for the free energy of a thin planar film which accounts for the attractive dispersion interactions and the repulsive Born interactions is used to identify the conditions under which spreading as a planar droplet or spreading as single atoms occurs. In addition, the case of very small planar droplets, assumed to have the shape of a disc, is treated. The radius and height of the disc depend upon the volume of the droplet when the latter is of the order of 10−18cm3. Such small volumes are encountered in supported metal catalysts. If the macroscopic spreading condition is not satisfied, but sufficiently strong short-range attractive forces are acting near the leading edge, the angle at the leading edge may become zero and spreading of a monolayer, bilayer, or multilayer from the droplet will occur. At equilibrium, the droplet coexists with the spread layer, and the angle at the leading edge between the droplet and the covered solid surface is finite. Explanations are suggested for (i) the absence of spreading (autophobicity) over a surface covered with a monolayer of molecules of the liquid of the droplet, and (ii) the superspreading of droplets of aqueous solutions of trisiloxane oligo (ethylene oxide) surfactants over a hydrophobic surface.

Influence of Polymer Architecture and Polymer−Surface Interaction on the Elution Chromatography of Macromolecules through a Microporous Media

The elution chromatography of flexible polymer molecules flowing through a microporous particle media is described by a combination of the Casassa model of flow segregation and the Di Marzio−Rubin lattice method for calculating the partition function of confined polymers. This combination of models allows for the treatment of polymer-surface interactions so that polymer chromatography in the exclusion and adsorption regimes can be described within a unified framework. The compensation point where repulsive polymer-surface excluded volume forces and short-range polymer-surface attractive forces counterbalance each other offers opportunities for separating complex molecules. For example, calculations for a diblock copolymer where one of the components is at the compensation point (“adsorption ϑ point”) indicate that only the remaining block influences the elution of the block copolymer as a whole. This theoretical result accords with experiments on block copolymers. This singular observation provides support for the Casassa viewpoint of molecular partitioning dominated polymer elution. The chromatography of triblock copolymers, stars, and combs is also examined to determine the selectivity of elution chromatography for separating these molecular architectures. The theoretical development in the present paper should lead to improved methods for the characterization of polymers with different molecular architectures. These developments also suggest new tools for studying polymer adsorption from dilute solution.

Fluctuations of a flux driven interface

We investigate a low temperature lattice gas with short range attractive forces. Boundary chemical potentials are imposed in such a way that a sharp interface separates low and high density domains and such that there is a constant mass flux perpendicular to this interface. From a continuum theory (fluctuating hydrodynamics) we determine the interface fluctuations in this nonequilibrium steady state. They turn out to be reduced as compared to thermal equilibrium.

Structures and phase transitions in colloidal dispersions from theory and simulation

Equilibrium properties of uncharged colloidal dispersions with particles interacting via harsh short-ranged repulsive forces and strong short-ranged attractive forces are determined by means of an approach in which colloids are treated as supramolecular versions of simple fluids. The interparticle interactions are represented by short-ranged square-well potentials. Structure factors and phase diagrams for the dispersions are determined analytically by mapping the square-well potential onto the adhesive-hard-sphere model by equating the respective second virial coefficients. Theoretical predictions are compared with molecular simulation results for the conditions relevant to real colloidal dispersions, Our results show that the ad lies adhesive-hard-sphere model can describe accurately structure factors and phase diagrams of nonionic microemulsions and inverted micellar systems containing .AOT surfactant dispersed in oil. Both upper-critical and lower-critical consolute points in colloidal dispersions dominated by short-ranged attractive forces can be described using the same formalism.

Knotting and supercoiling in circular DNA: A model incorporating the effect of added salt.

We consider a model of a circular polyelectrolyte, such as DNA, in which the molecule is represented by a polygon in the three-dimensional simple cubic lattice. A short-range attractive force between nonbonded monomers is included (to account for solvent quality) together with a screened Coulomb potential (to account for the effect of added salt). We compute the probability that the ring is knotted as a function of the number of monomers in the ring, and of the ionic strength of the solution. The results show the same general behavior as recent experimental results by Shaw and Wang [Science 260, 533 (1993)] and by Rybenkov, Cozzarelli, and Vologodskii [Proc. Natl. Acad. Sci. U.S 90, 5307 (1993)] on the knot probability in circular DNA as a function of added salt. In addition, we compute the writhe of the polygon and show that this also increases as the ionic strength increases. The writhe computations model the conformational behavior of nicked circular duplex DNA molecules in salt solution.

Adhesive-hard-sphere approximation for “vapor/liquid” transitions in colloidal dispersions

The accuracy of the adhesive-hard-sphere model for describing the phase stability of unchanged colloidal dispersions in which particles interact via harsh short-range repulsive forces and strong short-range attractive forces is examined. Dispersions for which the interparticle potential can be written as the square-well or the Yukawa potentials are considered. Phase diagrams for the dispersions are determined analytically by mapping the square-well and the Yukawa potentials onto the adhesive-hard-sphere model using their second virial coefficients and are compared with Gibbsensemble Monte Carlo results. The theoretical predictions show that the adhesive-hard-sphere model can describe the phase diagrams adequately when the range of the square-well attraction is less than about one-fifth of the hard-sphere diameter. Similar results for the phase diagrams are obtained for short-range Yukawa models as well. It is shown that the adhesive-hard-sphere model can describe both upper-critical and lower-critical consolute points in colloidal dispersions dominated by short-range attractive forces.

Molecular statistical model for ferroelectric - antiferroelectric transition in chiral smectic liqitid crystals

Abstract We discuss ferroelectric-antiferroelectric transition in chiral smectic liquid crystmais with molecular theorp. The pair interaction between model molecules consist's of repulsive hard core potential aid short range attractive force. We take a distinction of the head and tail of the molecule into consideration as an asymmery of the short range attractive part of the pair interaction. Using this model, we show that the head-tail asymmetry contributes to the stability of the ferrielectric phase.

Exploring the interactions between glycolipid bilayers

Lipid bilayers may be deposited onto molecularly smooth (mica) surfaces by the Langmuir-Blodgett deposition technique and the interbilayer force laws (force vs. distance) measured with the surface forces apparatus. Certain plasma membrane lipids (phospholipids, glycolipids and their mixtures) have been studied in this way in order to investigate the effect of carbohydrate moieties on the close approach and adhesion properties of the bilayer surfaces. Results indicate that long-range repulsive forces may be explained in terms of electrostatic (double-layer) interactions, short-range repulsions in terms of steric/hydration forces and short-range attractive forces may be ascribed to van der Waals interactions. However, the adhesive contacts obtained for the lipid mixtures are not simply a product of the characteristics of their constituent lipids, thus implying a certain synergistic glycolipid/phospholipid interaction.

Three-point correlation functions in uniformly and randomly driven diffusive systems.

Driven far away from equilibrium by both uniform and random external fields, a system of diffusing particles with short-range attractive forces displays many singular thermodynamic properties. Surprisingly, measuring pair correlations in lattice-gas models with saturation drives, we find little difference between the uniform and random cases, even though the underlying symmetries are quite distinct. Motivated by this puzzle, we study three-point correlations using both field-theoretic and simulation techniques. The continuum theory predicts the following: (a) The three-point function is nonzero only for the uniformly driven system; (b) it is odd under a parity transformation; and (c) there exists an infinite discontinuity singularity at the origin in momentum space. Simulation results are clearly consistent with these predictions. Based on these findings, we suggest several avenues for future investigations.

Novel Template Guided Synthesis of Poly Aniline

In acidic medium, polyaniline has positive charges on its molecular backbone. These charges come from either protonation at the nitrogen atom sites or polarons in its β-electronic system [1]. Although polyaniline, due to its positive charges, is a polycation, yet its physical properties are quite different from the classical polyelectrolytes. For example, the polycation vinylbenzyltrimethylammonium chloride is soluble in water but polyaniline molecules (and other conducting polymers too) tends to aggregate into intractable solid. Conducting polymers aggregate into intractable solid because of their stiff molecular backbone and the strong short range attractive forces between βclouds of adjacent polyaniline chains. The insolubility of polyaniline poses problem for solution processing of polymers. There have been a heavy effort towards the resolution of this problem [2].

Fluorescence microscopy of phospholipid monolayer phase transitions

Over many years, a detailed picture of the phase transitions in phospholipid monolayers at the air-water interface has been constructed from extensive studies of the force-area, viscoelastic and surface potential properties of phospholipid monolayers, yet the microscopic nature of the transitions has remaned obscure. Recent investigations have focused specifically on these aspects. Through the use of fluorescence microscopy, electron diffraction and X-ray scattering experiments, in combination with data obtained by classical methods, a wealth of new information regarding the properties of monolayers undergoing phase transistions has been genrated. Direct observation of fluid-solid phase coexistence at the air-water interface has been achieved with fluorescence microscopy and on solid supports with electron miscroscopy. The fluid-solid coexistence region has been studied most thoroughly to date, but regions of gas-fluid and fluid-fluid phase coexistence have also been detected. Numerous factors govern the properties of the coexistence region; however, the promiment features can be explained in terms of a competition between forces: long-range electrostatic forces and short-range attractive forces. In this review these recent experimental findings and theoretical interpretations are summarized.