Increase In Attractive Interactions Research Articles

Polarizable force field for molecular dynamics simulations of silver nanoparticles

Contact of silver metal surfaces with water, ions and organic ligands experiences induced charges, leading to attractive polarization. These forces play an important role at inorganic/organic interfaces and complement other non-bonded surface interactions. Despite the importance of these interactions, it, however, remains difficult to implement polarization effects to classical molecular dynamics (MD) simulations. In this contribution, we first present an overview of two popular polarizable models, such as Drude oscillator and the rigid rod model, which are utilized to mimic the polarizability of bulk metals. Second, we implemented the rigid rod model to the polarizable force field (FF) for a silver atom, which was further adapted for atomistic MD simulations of silver nanoparticles (AgNPs) composed of 1397 atoms. In our model, induced charge polarization is represented by the displacement of a charge-carrying virtual site attached rigidly to an original Ag atom. To explore the role of polarization, we compared the performance of the classical nonpolarizable FF and the new polarizable model in the MD simulations of adsorption of water and ions onto quasi-spherical AgNP and the flat crystalline silver surface. The analysis of the radial distribution function of Ag-Ag atoms demonstrated that the introduction of the polarization effect had minor effects on face-centered cubic (fcc) packing of silver atoms of bare and water-solvated AgNPs. We found that the polarizable FF causes some increase in attractive interactions between the silver surface and water molecules and Na+ ions. As a crucial test of the developed polarizable model, the structure of adsorbed interfacial water molecules was analyzed. Our data suggest that the environment-induced polarization of the silver surface contributes significantly to the structure of adsorbed interfacial water layers and it also plays an important role in the adsorption of positive ions. However, it was also found out that the polarization effect has a rather short-range effect, so that a minor contribution of silver polarization was seen for adsorption of water molecules and ions from distant solvation shells.

Monte Carlo simulation of cylinders with short-range attractions.

Cylindrical or rod-like particles are promising materials for the applications of fillers in nanocomposite materials and additives to control rheological properties of colloidal suspensions. Recent advances in particle synthesis allows for cylinders to be manufactured with short-ranged attractions to study the gelation as a function of packing fraction, aspect ratio and attraction strength. In order to aid in the analysis of small-angle scattering experiments of rod-like particles, computer simulation methods were used to model these particles with specialized Monte Carlo algorithms and tabular superquadric potentials. The attractive interaction between neighboring rods increases with the amount of locally-accessible surface area, thus leading to patchy-like interactions. We characterize the clustering and percolation of cylinders as the attractive interaction increases from the homogenous fluid at relatively low attraction strength, for a variety of aspect ratios and packing fractions. Comparisons with the experimental scattering results are also presented, which are in agreement.

Modified interactions among globular proteins below isoelectric point in the presence of mono-, di- and tri-valent ions: A small angle neutron scattering study

Both short range attraction and long range electrostatic repulsion exist among globular protein Bovine Serum Albumin in solution below its isoelectric point (pI≈4.8). At pD≈4.0, below pI, protein has a net positive surface charge although local charge inhomogeneity presents. Small angle neutron scattering study reveals that in the presence of both mono-(Na+) and di-(Ni2+) valent ions attractive interaction increases and repulsive interaction decreases with the increase of salt concentration. However, for tri-valent (Fe3+) ions, both attractive and repulsive interaction increases with increasing salt concentration but the relative strength of repulsion is more than the attraction.

Intracellular pH modulates quinary structure.

NMR spectroscopy can provide information about proteins in living cells. pH is an important characteristic of the intracellular environment because it modulates key protein properties such as net charge and stability. Here, we show that pH modulates quinary interactions, the weak, ubiquitous interactions between proteins and other cellular macromolecules. We use the K10H variant of the B domain of protein G (GB1, 6.2 kDa) as a pH reporter in Escherichia coli cells. By controlling the intracellular pH, we show that quinary interactions influence the quality of in-cell (15) N-(1) H HSQC NMR spectra. At low pH, the quality is degraded because the increase in attractive interactions between E. coli proteins and GB1 slows GB1 tumbling and broadens its crosspeaks. The results demonstrate the importance of quinary interactions for furthering our understanding of protein chemistry in living cells.

Studies on interactions among lysozyme proteins in solution: Effects of concentration, pD, temperature and monovalent ions

Short-range attraction and long-range electrostatic repulsion among the lysozyme proteins in solution have been identified which are modified depending upon the protein concentration, solution pD, dissolved ions and solution temperature. Small angle neutron scattering study shows that for a particular pD, with increasing protein concentration, attractive interaction increases but repulsive interaction first increases and then decreases. With increasing solution pD, attractive interaction increases and repulsive interaction decreases, however this trend is not valid for very low and high protein concentrations. Presence of monovalent salt largely enhances the attractive interaction however the enhancement is low for the heat treated lysozyme solution.

Modification of interactions among proteins with the lowering of solution pD toward the isoelectric point in presence of different valent ions

Bovine serum albumins show a short-range attraction and a long-range electrostatic repulsion among them and these interactions are modified depending upon the solution pD and different dissolved counterions in the solution. Small angle neutron scattering study shows that for equal mono-valent (Na+) and di-valent (Ni2+) ion concentrations, both the attractive and repulsive interaction decreases with lowering the solution pD toward the protein isoelectric point, however for the tri-valent (Fe3+) ion attractive interaction increases and repulsive interaction decreases. Interaction variation for the equal ionic strength of the three different valent ions becomes prominent as the pD decreases toward the isoelectric point.

Inhomogeneous BCS-BEC crossover for trapped cold atoms in optical lattices

The BCS-BEC crossover in a lattice is a powerful paradigm to understand how a superconductor deviates from the Bardeen-Cooper-Schrieffer physics as the attractive interaction increases. Optical lattices loaded with binary mixtures of cold atoms allow to address it experimentally in a clean and controlled way. We show that, however, the possibility to study this phenomenon in actual cold-atoms experiments is limited by the effect of the trapping potential. Real-space Dynamical Mean-Field Theory calculations show indeed that interactions and the confining potential conspire to pack the fermions in the center of the trap, which approaches a band insulator when the attraction become sizeable. We show how this physics reflects in several observables, and we propose an alternative strategy to disentangle the effect of the harmonic potential and measure the intrinsic properties resulting from the interaction strength.

Unconventional superconductivity on honeycomb lattice: Theory of Kekule order parameter

A spatially non-uniform superconducting phase is proposed as the electronic variational ground state for the attractive interactions between nearest neighbors on graphene's honeycomb lattice, close to and right at the filling one half. The state spontaneously breaks the translational invariance of the lattice into the Kekule pattern of bond order parameters, and it is gapped, spin triplet, and odd under the sublattice exchange. With the increase of attractive interactions we first find the transition from the semimetallic phase into the p-Kekule superconductor, defined as being odd under the exchange of Dirac points, with the additional discontinuous superconductor-superconductor transition into the even s-Kekule state, deep within the superconducting phase. Topological excitations of the Kekule superconductor and its competition with other superconducting states on the honeycomb lattice are discussed.

Translocation dynamics with attractive nanopore-polymer interactions

Using Langevin dynamics simulations, we investigate the influence of polymer-pore interactions on the dynamics of biopolymer translocation through nanopores. We find that an attractive interaction can significantly change the translocation dynamics. This can be understood by examining the three components of the total translocation time tau approximately tau1+tau2+tau3 corresponding to the initial filling of the pore, transfer of polymer from the cis side to the trans side, and emptying of the pore, respectively. We find that the dynamics for the last process of emptying of the pore changes from nonactivated to activated in nature as the strength of the attractive interaction increases, and tau3 becomes the dominant contribution to the total translocation time for strong attraction. This leads to nonuniversal dependence of tau as a function of driving force and chain length. Our results are in good agreement with recent experimental findings, and provide a plausible explanation for the different scaling behavior observed in solid state nanopores vs that for the natural alpha-hemolysin channel.

Chemical Bonding States at Copper/Graphite Interfaces with Additional Elements

Chemical bonding states of Cu/graphite interfaces were investigated by using the first-principles molecular orbital method with model clusters. It was found that Cu atoms in the interface layer undergo electronic overlap with C atoms at the first and second neighboring sites. Chemical bonding strength across the interface is strongly affected by the presence of Ti or Zr in the interface layer. The interfacial attractive interaction increases by doping the larger-sized Ti or Zr atom, although the interlayer distances between Cu and graphite become larger. Therefore, the interface strength of Cu/graphite can be improved by the additional elements at the interface.

Microphase Separation within a Comb Copolymer with Attractive Side Chains: A Computer Simulation Study

Computer simulation modelling of a flexible comb copolymer with attractive interactions between the monomer units of the side chains is performed. The conditions for the coil-globule transition, induced by the increase of attractive interaction, e, between side chain monomer units, are analysed for different values of the number of monomer units in the backbone, N, in the side chains, n, and between successive grafting points, m. It is shown that the coil-globule transition of such a copolymer corresponds to a first-order phase transition. The energy of attraction (e) required for the realisation of the coil-globule transition decreases with increasing n and decreasing m. The coil-globule transition is accompanied by significant aggregation of side chain units. The resulting globule has a complex structure. In the case of a relatively short backbone (small value of N), the globule consists of a spherical core formed by side chains and an enveloping shell formed by the monomer units of the backbone. In the case of long copolymers (large value of N), the side chains form several spherical micelles while the backbone is wrapped on the surfaces of these micelles and between them.

Simultaneous imaging of tunneling current and damping energy by noncontact-AFM in ultra-high vacuum

We have simultaneously observed the images of tunneling current and damping energy with the nc-AFM (noncontact atomic force microscopy) image of Si(111)7×7. When inverted contrast is observed in the constant frequency shift nc-AFM image, the current image is not inverted, and vice versa. On the other hand, the damping images show a contrast similar to that of the nc-AFM images; the damping decreases at a narrow separation between the tip and the sample. This possibly indicates that the damping decreases as the attractive interaction increases under a constant oscillation amplitude of the cantilever. To keep the oscillation amplitude constant under an attractive interaction between a tip and a sample, the total energy of the cantilever oscillation is reduced and the oscillation velocity of the cantilever decreases. An explanation is proposed that the change in energy dissipation occurs in the cantilever oscillation, depending on the oscillation velocity of the cantilever, and the value is estimated from a simple model.

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.

Quantum transition to supersolid phase

We study a possible coexistence of superconducting state and charge density waves which, in a broad sense, might be called a supersolid phase. We investigate the infinite dimensional (d=∞) attractive Hubbard model by applying a sublattice dependent Gutzwiller wave function \(g_{\text{A}}^{D_{\text{A}} } g_{\text{B}}^{D_{\text{B}} } |{\text{BCS}}\)|BCS〉 as a variational wave function describing the ground state. One may naively expect that the BCS superconducting state evolves continuously to the Bose–Einstein condensed state of bipolarons as the attractive interaction increases, as far as the system is dilute. However, we show that our variational wave function has lower energy than the simple BCS wave function for all electron densities and the interaction strengths. Our variational parameters increase (gA,B→∞) as we increase the interaction strength (U→∞). The energy gap turns out to be a mixture of s and extended-s waves. In the vicinity of half-filling, we find a quantum transition from a simple superconducting phase to a supersolid phase with increase of the electron density and/or the interaction strength.

Vibrational Line Profiles as a Probe of Molecular Interactions

A Raman experiment which yields basic insights into molecular interactions via vibrational line shape analysis is presented. Specifically, line shifts and changes in profile of the acetone C-C and C=O vibrations during dilution with methanol and CCl4 are examined. The spectra during methanol dilution reveal an increase in attractive interactions, while those resulting from dilution with CCl4 reveal increasing repulsive interactions.

A model for the temperature-dependent interactions in uncharged droplet microemulsions

Microemulsions consisting of uncharged droplets generally behave as hard spheres at the solubilisation phase boundary (SPB) where the actual droplet size is close to the preferred droplet size. As the temperature is changed to move inside the one-phase microemulsion region, experimental data suggest that the droplets may either grow or cluster together due to increasing interdroplet attraction. We present a model in which it is assumed that droplet growth is absent and that the temperature dependence of the interdroplet interactions arises primarily from the energy associated with bending the stabilising surfactant monolayer. At temperatures close to the SPB, the actual curvature of the monolayer coating the droplets is close to its preferred value and the energy required to deform the interface is high, with the result that the droplets behave as hard spheres. Away from the SPB, where the actual monolayer curvature becomes higher than the preferred value, the energy required for deformation decreases and the droplets behave as ‘soft’ spheres. This results in an apparent increase in attractive interactions between the droplets. The model shows reasonable agreement with experimental turbidity data for uncharged microemulsions.

Torsional potential energy surface of acetone in the S 1 (n, [formula omitted]) state

Examination has been made of the torsional potential energy surface of acetone in the S 1 (n, π ∗ ) state in order to clarify methyl-methyl interaction, based on CASSCF calculations for two models (fully-relaxed and partially-relaxed). Skeletal changes during methyl torsion motion are 7 ° for the CO out-of-plane wagging angle, 0.003 Å for the CO bond length, 2.7 ° for the CCC bond angle, and 0.005 Å for the CC bond length. Skeletal flexing introduces the terms sin 3( θ 1 − θ 2) and sin 3 θ 1 − sin 3 θ 2 in the torsional potential function, where θ 1 and θ 2 are the torsional angles of the two methyl tops. Change in the CO wagging angle during torsion motion is shown to be a geometric relaxation with a decrease in steric hindrance rather than an increase in attractive interaction. The large change in the CO out-of-plane wagging angle indicates that the torsional mode is coupled with the CO out-of-plane wagging mode. Calculated frequencies for low-lying levels on the torsional potential are shown to be in good agreement with those observed.

Intermicellar interactions and micelle size distribution in aqueous solutions of polyoxyethylene surfactants

Mutual diffusion coefficients (D) and self-diffusion coefficients (D+) in D2O solution of n-dodecylpolyoxyethylene surfactants [C12H25(OC2H4)mOH, m= 5 and 8] have been measured by dynamic light scattering and pulsed-gradient F.t.n.m.r., respectively. On the basis of the ratio D/D+, micelle size distribution and intermicellar interactions have been discussed. The results have been compared with those for the C12E6 system reported previously. Below ca.Tc– 30 °C (Tc is the lower critical solution temperature) size distribution is narrow and intermicellar interactions are weak; this is consistent with the existence of spherical micelles. In the temperature range Tc–30–Tc–15 °C, the excluded volume effects are dominant and increase in magnitude with increasing temperature. At the same time, the size distribution becomes broad. These results originate from elongation of micelles. Above ca.Tc–15 °C, the results can be explained by the decrease in the excluded volume effects and/or rapid increase in attractive interactions with increasing temperature. At still higher temperatures (Tc–4 °C–Tc), attractive interactions become dominant. Proton n.m.r. linewidths and viscosities have also been measured.