Strength Of Attractive Interaction Research Articles

Selection rules and superconducting correlations in carbon nanotubes

We carry out a detailed analysis of the effective interaction arising from electron-phonon scattering and its capability to produce the superconducting correlations observed in the carbon nanotubes. It is shown that certain selection rules prevent the exchange of phonons in some of the interaction channels, depending on the geometry of the nanotubes and on whether they are doped or not. In addition, we discuss the mechanism working in nanotube ropes by which the electrostatic coupling among a large number of metallic nanotubes leads to a substantial reduction in the strength of the Coulomb interaction. The scaling equation for the superconducting response function is then improved nonperturbatively, by including the exact contribution from forward-scattering processes. This allows us to estimate the boundary between superconducting and nonsuperconducting phases in the ropes, as well as to constrain the actual values of the strength of the effective attractive interaction.

Computer simulation on the self-assembly of associating polymers

We present the results of Monte Carlo simulations of self-assembly in melt of pseudo-block copolymer formed through the association of associating polymer chains with attractive end-groups (sticker sites). Complexities of the phase structures result from these pseudo-block copolymers strongly depend on parameters, such as attractive interaction strength between two stickers and chain length of associating polymers. The weak, intermediate and strong interactions between two stickers lead, respectively, to the macro-separated, disordered and ordered lamellar phase structures. The ordered lamellar phases formed by these pseudo-block copolymers are more stable than that formed by pure diblock copolymers. Pseudo-block copolymers with shorter associating polymer chain prefer to form ordered lamellar phase and that with longer associating chain tend to form disordered phase.

Bose-Einstein condensation of cesium.

Bose-Einstein condensation of cesium atoms is achieved by evaporative cooling using optical trapping techniques. The ability to tune the interactions between the ultracold atoms by an external magnetic field is crucial to obtain the condensate and offers intriguing features for potential applications. We explore various regimes of condensate self-interaction (attractive, repulsive, and null interaction strength) and demonstrate properties of imploding, exploding, and non-interacting quantum matter.

Density functional model of surfactant mesostructures

We present a theoretical study of the effects of amphiphile microscopic properties on mesoscopic properties of their solutions. These results were obtained using an off lattice density functional method. We consider fused hard-sphere amphiphiles with a single hard sphere solvent. Mean field attractions are accounted for with a 1/R6 dispersion interaction. Mesoscopic properties, such as the bending rigidity, saddle splay modulus, and surface tension, are presented as a function of the amphiphile hard sphere radii and attractive interaction strength. Detailed analysis of the work of amphiphile cluster formation with cluster size is presented.

Analysis of Light Scattering Data on the Calcium Ion Sensitivity of Caseinate Solution Thermodynamics: Relationship to Emulsion Flocculation

We describe the quantitative interrelation between the thermodynamic parameters of caseinate submicelles in the presence of calcium ions (0–14 mM) in aqueous medium and the capacity of the protein to induce depletion flocculation in oil-in-water emulsions at pH 7.0 and ionic strength 0.05 mol dm−3. Measurements have been made by static and dynamic multiangle laser light scattering of the weight-average molecular weight, the radius of gyration, the hydrodynamic radius, and the second virial coefficient of caseinate submicelles in aqueous solution. Successive thermodynamic approximations with and without consideration of correlations between caseinate submicelles have been used to calculate the osmotic pressure in caseinate aqueous solutions and the free energy of the depletion interaction between droplets in oil-in-water emulsions stabilized by caseinate. Numerical results from both thermodynamic approximations are in reasonably good agreement with experiment, predicting a pronounced decrease in the strength of the depletion attraction at concentrations of Ca2+ in the range 4–8 mM (with a minimum value at 8 mM). This correlates well with the great enhancement of stability of these emulsions with respect to flocculation in comparison with systems having no added ionic calcium and emulsions with lower (2 mM) or higher (10 mM) Ca2+ contents. Nevertheless, the allowance for interactive correlations between caseinate submicelles seems to lead to a better prediction of emulsion flocculation on a qualitative level over the whole range of Ca2+ concentrations studied (2–14 mM). The calculated pronounced decrease in depletion interaction strength is attributable to marked changes in weight-average molecular weight and mean size of aggregates, and to more positive values of the second virial coefficient of caseinate submicelles with increasing Ca2+ content. Finally, we discuss the part played by the electrical charge on the protein in determining the overall strength of the flocculation-inducing attractive interactions between droplets.

Molecular dynamics simulation of supersaturated vapor nucleation in slit pore. II. Thermostatted atomic-wall model

Molecular dynamics simulations of nucleation of a supersaturated Lennard-Jones vapor in slit nanopores are carried out. In this study we extend a previous work [K. Yasuoka, G. T. Gao, and X. C. Zeng, J. Chem. Phys. 112, 4279 (2000)] in that the walls of the slit are treated as actual atomic walls serving as both the confining solid surfaces and a thermostat. The walls are fixed in place in a fcc lattice structure and wall atoms are subjected to a stiff biharmonic potential thereby bounded to lattice sites. The two walls of the slit have an identical surface [fcc (100)], but different strength of attractive interaction with the vapor particles—one is strongly adsorbing and another is weakly adsorbing. Heterogeneous nucleation of the supersaturated vapor in the slit is investigated and events of nucleus formation are monitored in real time. A comparison with the previous simulation (using rigid structureless walls) leads to useful insight into the influence of the wall model to the nucleus formation. In particular, it is found that although the adsorbed particles on the structureless wall diffuse faster than those on the atomic wall, the rate of nucleus formation on the structureless wall is actually about one order of magnitude lower. A detailed analysis of particle and cluster-formation flux indicates that the rate of nucleus formation on the wall is more sensitive to the kinetics of adsorption of gas particles onto the wall than the diffusion rate of adsorbed particles. The higher flux of cluster formation on the atomic wall is apparently due to the higher rate of deposition of monomers onto the wall.

Controlled collapse of a Bose-Einstein condensate.

The point of instability of a Bose-Einstein condensate (BEC) due to attractive interactions was studied. Stable 85Rb BECs were created and then caused to collapse by slowly changing the atom-atom interaction from repulsive to attractive using a Feshbach resonance. At a critical value, an abrupt transition was observed in which atoms were ejected from the condensate. By measuring the onset of this transition as a function of number and attractive interaction strength, we determined the stability condition to be N(absolute value of a) / a(ho) = 0.459+/-0.012+/-0.054, slightly lower than the predicted value of 0.574.

Effects of excluded surface area and adsorbate clustering on surface adsorption of proteins: I. Equilibrium models

Statistical–thermodynamic models for the equilibrium adsorption of proteins onto homogeneous, locally planar surfaces are presented. An extension of earlier work [R.C. Chatelier, A.P. Minton, Biophys. J. 71 (1996) 2367], the models presented here allow for the formation of a broadly heterogeneous population of adsorbate clusters in addition to excluded volume interactions between all adsorbate species. Calculations are carried out for three simple models for the structure of adsorbate, illustrating similarities and differences in the equilibrium properties of maximally compact clusters, minimally compact clusters and isomerizing clusters. Depending upon the strength of attractive interactions between adsorbate molecules, the resulting equilibrium isotherms may exhibit negative cooperativity, positive cooperativity, essentially no apparent cooperativity, or a mixture of positive cooperativity at low surface density and negative cooperativity at high surface density of adsorbate. The condition of apparent lack of cooperativity, which might naively be interpreted as evidence of a lack of interaction between adsorbate molecules, actually conceals a balance between attractive and repulsive interactions and extensive clustering of adsorbate.

Interaction between solute molecules in medium density solvents

Solvation properties of solutes in supercritical, medium density solvents have been analysed using hypernetted-chain theory with the emphasis on the solvent-mediated interaction between solute molecules. The solvent and solute molecules are Lennard-Jones particles, and the solute is present at infinite dilution. Also a pair of solute molecules separated by different distances has been considered using reference interaction site model theory. Mainly, solvents at two typical densities (1.09pc and 2.91pc; pc is the critical density) that are in medium and high density regions, respectively, are treated. The temperature is set at 1.04TC (Tc is the critical temperature). When the solute size is larger than the solvent size and the strength of the solute-solvent attractive interaction is greater than that of the solvent-solvent in the medium density region, the solvent structure confined between a pair of solute molecules is largely different from that near a single solute molecule. The confined solvent becomes denser and more stabilized as the distance between the solute molecules decreases, and an attractive interaction is induced between them. The interaction becomes even more attractive as the strength of the solute-solvent attractive interaction increases. The observations are qualitatively different from those in the high density region. Another high density region, which is well below the critical temperature, has been considered, but the behaviour observed is similar to that in the high density region above the critical temperature.

Collapse of stiff conjugated polymers with chemical defects into ordered, cylindrical conformations

The optical, electronic and mechanical properties of synthetic and biological materials consisting of polymer chains depend sensitively on the conformation adopted by these chains. The range of conformations available to such systems has accordingly been of intense fundamental as well as practical interest, and distinct conformational classes have been predicted, depending on the stiffness of the polymer chains and the strength of attractive interactions between segments within a chain. For example, flexible polymers should adopt highly disordered conformations resembling either a random coil or, in the presence of strong intrachain attractions, a so-called 'molten globule'. Stiff polymers with strong intrachain interactions, in contrast, are expected to collapse into conformations with long-range order, in the shape of toroids or rod-like structures. Here we use computer simulations to show that the anisotropy distribution obtained from polarization spectroscopy measurements on individual poly[2-methoxy-5-(2'-ethylhexyl)oxy-1,4-phenylenevinylene] polymer molecules is consistent with this prototypical stiff conjugated polymer adopting a highly ordered, collapsed conformation that cannot be correlated with ideal toroid or rod structures. We find that the presence of so-called 'tetrahedral chemical defects', where conjugated carbon-carbon links are replaced by tetrahedral links, divides the polymer chain into structurally identifiable quasi-straight segments that allow the molecule to adopt cylindrical conformations. Indeed, highly ordered, cylindrical conformations may be a critical factor in dictating the extraordinary photophysical properties of conjugated polymers, including highly efficient intramolecular energy transfer and significant local optical anisotropy in thin films.

Energy disposal in collisions of nitric oxide with molecular adlayers on transition metal single crystal surfaces: Translational energy disposal

The dynamics of the scattering of nitric oxide (NO) from well-characterized and ordered layers of carbon monoxide (CO) and ethylidyne (CCH3) adsorbed on a Pt(111) substrate have been investigated by time-of-flight methods. Time-of-flight profiles for scattered NO are reported for a range of incident NO translational energies between 10 and 50 kJ mol−1 from scattering substrates at 108 and 300 K. The measurements reveal that scattering occurs both into a trapping-desorption channel and a direct inelastic channel. The latter is accompanied by a significant (>60%) loss in the NO translational energy. This is true for both substrates and contrasts markedly with the many observations made of atomic and molecular scattering from clean metal single crystal planes. Hard cube model calculations are presented as a basis for the interpretation of the direct inelastic behavior that point toward a surface collision partner mass of the same magnitude as the incident molecule and toward differences in the strength of the NO-adlayer attractive interaction as the origin of subtle changes in the scattering behavior from the low temperature substrates as a function of the incident translational energy.

Protein interactions in concentrated ribonuclease solutions

To investigate the protein interactions involved in the crystallization process of ribonuclease A, dynamic light scattering (DLS) and small angle X-ray scattering experiments (SAXS) were performed on concentrated solutions. Whereas the translational diffusion coefficient obtained from DLS is sensitive to thermodynamic and hydrodynamic interactions and permits to calculate an interaction parameter, the shape of the SAXS curves is related to the type of interaction (attractive or repulsive). We compared the effect of pH on protein interactions in the case of two types of crystallizing agents: a mixture of salts (3 M sodium chloride plus 0.2 M ammonium sulfate) and an organic solvent (ethanol). The results show that in the presence of ethanol, as in low salt, protein interactions become more attractive as the pH increases from 4 to 8 and approaches the isoelectric point. In contrast, a reverse effect is observed in high salt conditions: the strength of attractive interactions decreases as the pH increases. The range of the pH effect can be related to ionization of histidine residues, particularly those located in the active site of the protein. The present observations point out the important role played by localized charges in crystallization conditions, whatever the precipitating agent.

Exploring the space of protein folding Hamiltonians: The balance of forces in a minimalist β-barrel model

A rapid and effective method for obtaining thermodynamic quantities for Hamiltonians whose configurational space has not been examined through a direct simulation has been developed. This approach extends the scope of the weighted histogram analysis method and is applied to the exploration of the balance of forces within the off-lattice Honeycutt–Thirumalai 46-mer beta-barrel model. Specificity is introduced into the long range hydrophobic interactions by scaling back the non-native attractive component of the hydrophobic interactions through a scaling factor λ (0<λ<1). Thermodynamic properties for incremental values of λ are extrapolated from the sampling of the original (λ=1) Hamiltonian. The results were found to be in good agreement with the thermodynamic signatures obtained by direct simulations. Decreasing the strength of the non-native attractive hydrophobic interactions leads to a more cooperative folding with the folding and collapse temperatures nearly coinciding at λ=0.0. The free energy surfaces were also seen to become progressively smoother while retaining a pronounced native well. Thus, this methodology may be used in the development, refinement, and exploration of folding for off-lattice protein models.

Molecular dynamics simulation of solute diffusion in Lennard-Jones fluids

We performed a molecular dynamics (MD) simulation for a system of 5 solute molecules in 495 solvent molecules interacting through the Lennard-Jones (LJ) 12-6 potential, in order to study solvent density effects on the diffusion coefficients in supercritical fluids. The effects of the size of the solute and the strength of the solute-solvent attractive interaction on the diffusion coefficient of the solute were examined. The diffusion coefficients of the solute molecules were calculated at T = 1.5 (in the LJ reduced unit), slightly above the critical temperature, from ρ = 0.1 to ρ = 0.95, where ρ is the number density in the LJ reduced unit. The memory function in the generalized Langevin equation was calculated, in order to know the molecular origin of the friction on a solute. The memory function is separated into fast and slow components. The former arises from the solute-solvent repulsive interaction, and is interpreted as collisional Enskog-like friction. The interaction strength dependence of the collisional friction is larger in the low- and medium-density regions, which is consistent with the ‘clustering’ picture, i.e., the local density enhancement due to the solute-solvent attractive interaction. However, the slow component of the memory function suppresses the effect of the local density on the diffusion coefficients, and as a result the effect of the attractive interaction is smaller on the diffusion coefficients than on the local density. Nonetheless, the solvent density dependence of the effect of the attraction on the diffusion coefficient varies with the local density, and it is concluded that the local density is the principal factor that determines the interaction strength dependence of the diffusion coefficient in the low- and medium-density regions (ρ < 0.6).

Integral equation and Monte Carlo simulation studies of clusters in infinitely dilute supercritical solutions

We present a systematic study of both the size and attractive interaction strength effects on solvent clustering in infinitely dilute supercritical solutions. Both integral equation and computer simulation methods are brought to bear. We argue that monitoring the net local solvent density change due to the presence of the solute as well as residual solute chemical potential offers a predictive way of identifying clustering or declustering in infinitely dilute supercritical solutions. Phase diagram-like figures can be obtained, wherein the ‘phase boundaries’ divide the solvent enrichment (or effective attraction) from the solvent depletion (or effective repulsion) region. In conjunction with the corresponding states analyses these diagrams can provide a useful semiquantitative means of predicting the nature of solvation in many supercritical fluid systems.

On the solid–fluid interface of adhesive spheres

The adhesive-sphere interaction potential provides a good model system to study the influence of the attractive well depth on phase behavior and interfacial phenomena. We investigate the solid–fluid phase behavior of adhesive spheres with the modified weighted density approximation (MWDA) of Denton and Ashcroft. We then apply a planar-averaged density functional approach (PWDA) to determine interfacial properties. We find both a narrowing of the interface between fluid and coexisting fcc solid and an increase in the interfacial energy with increasing attractive interaction strength in accord with the empirical relation γ≊ 0.47ΔHρ2/3s. In addition, we investigate metastable solid nucleation through calculation of metastable bcc solid–fluid interfacial tensions and find results suggesting the possibility of such a route to stable solid formation.

A statistical model for the simulation of adsorption from a mixture of deformable particles

Competitive and cooperative adsorption from a mixture of deformable particles has been investigated by Monte Carlo simulation on a two-dimensional square lattice. Compact macromolecules (globular proteins) are modeled as soft particles made from connected subunits whose average configuration is determined by a potential that is coupled to the particle perimeter length and controls its deformability. Highly deformable macromolecules (disordered proteins and polysaccharides) are modeled as so-called fractal polymers of high conformational entropy. Numerical results show the dependence of the surface coverage and composition on the relative adsorption energies of the species and the strength of attractive interparticle subunit interactions. The importance of particle deformability and macromolecular interactions in relation to the structure of adsorbed layers is demonstrated by this model.

Theory of the pressure effect on the curvature and phase behavior of AOT/propane/brine water-in-oil microemulsions

Pressure effects on both the curvature and phase behavior of water-in-oil microemulsions (swollen reverse micelles) are predicted with a unified classical and molecular thermodynamic theory developed by Peck et al. (this issue). The theory is used to identify quantitatively the roles of the intramicellar interfacial interactions and micelle-micelle interactions. A supplementary molecular model is used to calculate the strength of attractive intermicellar interactions over a wide range of conditions, based on previous small-angle neutron-scattering data. An important distinction is made between systems with a small water-to-oil ratio and those where the water-to-oil ratio is much larger, on the order of unity. In the latter the micelle radius is controlled primarily by intramicellar interfacial interactions, specifically the enthalpic propane-surfactant tail interactions. For a small water-to-oil ratio, the micelle radius is limited by attractive micelle-micelle interactions. As pressure increases, the radius increases but eventually reaches a maximum governed by the intramicellar interfacial interactions. There is good agreement between the predictions and experiments over a wide range of water-to-oil ratios.

Molecular dynamics simulations of wetting and drying in LJ models of solid-fluid interfaces in the presence of liquid-vapour coexistence

The structure and phase behaviour of adsorbed films is modelled by cut and shifted LJ fluid in the presence of cut and shifted 9-3 walls. At fixed N, it is straightforward to use molecular dynamics simulation procedures to study planar adsorbed films in the presence of ‘bulk’ liquid-vapour coexistence. A series of five isotherms are investigated in detail, ranging from temperatures close to the bulk triple point to a little over half way to the critical point. As a by-product, accurate data are obtained for liquid-vapour coexistence properties and surface tensions of cut and shifted LJ fluid. Wall-fluid excess grand potentials are calculated and compared with the predictions of weighted density functional theory. Isotherms of contact angle against wall-fluid attractive interaction strength are given, each covering the entire region from complete drying to complete wetting. The dynamics of wetting transitions is not accessible in our geometry. In contrast, the presence of a suitable collective mode enables transitions to complete drying to be directly observed by molecular dynamics simulation in planar symmetry. Overall, we find a remarkable similarity with the results of previous projects based on square-well models of atomic matter.

Mean-field and Monte Carlo simulation studies of the lateral distribution of proteins in membranes

Monte Carlo simulations and mean-field calculations have been applied to a statistical mechanical lattice model of lipid-protein interactions in membranes in order to investigate the phase equilibria as well as the state of aggregation of small integral membrane proteins in dipalmitoyl phosphatidylcholine bilayers. The model, which provides a detailed description of the pure lipid bilayer phase transition, incorporates hydrophobic matching between the lipid and protein hydrophobic thicknesses as a major contribution to the lipid-protein interactions. The model is analyzed in the regime of low protein concentration. It is found that a large mismatch between the lipid and protein hydrophobic thicknesses does not guarantee protein aggregation even though it strongly affects the phase behaviour. This result is consistent with experimental work (Lewis and Engelman 1983) considering the effect of lipid acyl-chain length on the planar organization of bacteriorhodopsin in fluid phospholipid bilayers. The model calculations predict that the lipid-mediated formation of protein aggregates in the membrane plane is mainly controlled by the strength of the direct lipid-protein hydrophobic attractive interaction but that direct protein-protein interactions are needed to induce substantial aggregation.