Varying Interaction Strength Research Articles

Examining the Influence of Silver(I) Ion Coordination Environment in Ionic Liquids on Olefin-Paraffin Separations using Inverse Gas Chromatography.

Silver(I) ions (Ag+) undergo selective π-complexation with olefins and have been employed as separation media for the isolation of olefins from structurally similar paraffins. Ionic liquids (ILs) possess minimal vapor pressures, exceptional thermal stabilities, low melting points, as well as provide a favorable environment for π-complexation between Ag+ ions and olefins. The development of molecular drivers capable of highly selective olefin/paraffin separation systems with Ag+-containing ILs necessitates a comprehensive understanding of all factors that affect olefin solubility and selectivity. This study examines how coordinating different ligand species to Ag+ ions produces separation media with varying interaction strengths to olefins. Four coordination compounds, (4,4'-dimethyl-2,2'-bipyridine)silver(I) bis[(trifluoromethyl)sulfonyl]imide ([Ag+(DMBP)][NTf2-]), bis(pyridine)silver(I) [NTf2-] ([Ag+(Py)2][NTf2-]), bis(2,6-lutidine)silver(I) [NTf2-] ([Ag+(Lut)2][NTf2-]), and (triphenylphosphine)silver(I) [NTf2-] ([Ag+(PPh3)][NTf2-]) were dissolved in the 1-decyl-3-methylimidazolium [NTf2-] ([DMIM+][NTf2-]) IL and employed as stationary phases for inverse gas chromatography. Ligand coordination to the Ag+ ion was observed to modulate interactions of unsaturated hydrocarbons. The [Ag+(Py)2][NTf2-] complex offered the greatest olefin retention among the coordination complexes reaching 54% of the 1-octene retention factor of the uncoordinated [Ag+][NTf2-]. Hydrogen (H2) exposure studies showed ligand-dependent rates of reduction from Ag+ ion to elemental silver (Ag0). The [Ag+(PPh3)][NTf2-] complex exhibited superior stability, compared to the neat [Ag+][NTf2-] salt, reducing the retention factor of 1-octene by 15.3% and 19.4%, respectively, after 200 h of H2 exposure at 70 °C. The results from this study show that coordination complexes with Ag+ ions are useful in highly selective and efficient petroleum processing systems.

Balancing thermodynamic stability, dynamics, and kinetics in phase separation of intrinsically disordered proteins.

Intrinsically disordered proteins (IDPs) are prevalent participants in liquid-liquid phase separation due to their inherent potential for promoting multivalent binding. Understanding the underlying mechanisms of phase separation is challenging, as phase separation is a complex process, involving numerous molecules and various types of interactions. Here, we used a simplified coarse-grained model of IDPs to investigate the thermodynamic stability of the dense phase, conformational properties of IDPs, chain dynamics, and kinetic rates of forming condensates. We focused on the IDP system, in which the oppositely charged IDPs are maximally segregated, inherently possessing a high propensity for phase separation. By varying interaction strengths, salt concentrations, and temperatures, we observed that IDPs in the dense phase exhibited highly conserved conformational characteristics, which are more extended than those in the dilute phase. Although the chain motions and global conformational dynamics of IDPs in the condensates are slow due to the high viscosity, local chain flexibility at the short timescales is largely preserved with respect to that at the free state. Strikingly, we observed a non-monotonic relationship between interaction strengths and kinetic rates for forming condensates. As strong interactions of IDPs result in high stable condensates, our results suggest that the thermodynamics and kinetics of phase separation are decoupled and optimized by the speed-stability balance through underlying molecular interactions. Our findings contribute to the molecular-level understanding of phase separation and offer valuable insights into the developments of engineering strategies for precise regulation of biomolecular condensates.

Entanglement of edge modes in (very) strongly correlated topological insulators

Identifying topological phases for a strongly correlated theory remains a non-trivial task, as defining order parameters, such as Berry phases, is not straightforward. Quantum information theory is capable of identifying topological phases for a theory that exhibits quantum phase transition with a suitable definition of order parameters that are related to different entanglement measures for the system. In this work, we study entanglement entropy for a coupled SSH model, both in the presence and absence of Hubbard interaction and at varying interaction strengths. For the free theory, edge entanglement acts as an order parameter, which is supported by analytic calculations and numerical (DMRG) studies. We calculate the symmetry-resolved entanglement and demonstrate the equipartition of entanglement for this model which itself acts as an order parameter when calculated for the edge modes. As the DMRG calculation allows one to go beyond the free theory, we study the entanglement structure of the edge modes in the presence of on-site Hubbard interaction for the same model. A sudden reduction of edge entanglement is obtained as interaction is switched on. The explanation for this lies in the change in the size of the degenerate subspaces in the presence and absence of interaction. We also study the signature of entanglement when the interaction strength becomes extremely strong and demonstrate that the edge entanglement remains protected. In this limit, the energy eigenstates essentially become a tensor product state, implying zero entanglement. However, a remnant entropy survives in the non-trivial topological phase, which is exactly due to the entanglement of the edge modes.

Marangoni spreading time evolution and synergism in binary surfactant mixtures

HypothesisMarangoni spreading driven by localized surfactant solution deposition previously has been studied only for single surfactant systems. For binary surfactant mixtures, interactions that generate surface tension synergism, a thermodynamic effect, may also synergistically enhance Marangoni spreading dynamics, introducing the concept of Marangoni synergism. Spreading dynamics and possible Marangoni synergism should depend not only on thermodynamic properties but also kinetic properties of the binary system. Experiments and modelingTracer experiments that capture post-deposition surfactant front motion were performed in parallel with computational modeling, using binary surfactant pairs with varying interaction strengths. The model coupled the Navier-Stokes and advective diffusion equations with a Frumkin-type binary adsorption model. FindingsWe confirm the existence of Marangoni synergism. Stronger binary surfactant attraction favors synergism in both surface tension reduction and Marangoni spreading. Binary composition ranges over which surface tension synergism occurs differ from those for Marangoni synergism, indicating that the origins of the two synergistic effects are not identical. Analysis of model spreading velocities show that the thermodynamic spreading parameter is the controlling factor at early times for both single and binary surfactant systems, while the intrinsic adsorption and desorption kinetics influence spreading velocities and thus the occurrence of Marangoni synergism at later times.

Monte Carlo investigation of phase changes and the order of transition of Ising modeled single-walled nanotube

We present the Monte-Carlo analysis report for the magnetic response of the single-walled nanotube using the Metropolis and Wang Landau algorithms. Our nanotube architecture uses the spin half Ising model with nearest neighbors interaction. We find the phase changes from ferromagnetic/anti ferromagnetic to paramagnetic by modifying the system interaction parameters. We determine the transition temperatures for varying interaction strength in the absence of the magnetic field. We find the transition temperature changes with the external magnetic field for a fixed interaction strength. Our study confirmed that the transition from ferromagnetic/anti ferromagnetic to paramagnetic is second order.

Design and optimization of metformin hydrophobic ion pairs for efficient encapsulation in polymeric drug carriers

Loading small molecular weight hydrophilic drugs into polymeric carriers is a challenging task. Metformin hydrochloride (MET) is a highly soluble oral antidiabetic drug of small size and high cationic charge. Hydrophobic ion pairing (HIP) is an approach for reversible modulation of solubility and hydrophilicity of water-soluble drugs via complexation with oppositely charged molecules. Herein, we prepared MET ion pairs and carefully studied and characterized MET interaction with different ligands, with the aim of increasing MET lipophilicity and loading efficiency. HIP was successful using three hydrophilic anionic ligands; sodium dodecyl sulphate (SDS) Carbopol (CB) and tannic acid (TA). Electrostatic interaction and hydrogen bonding drove the complexation per spectroscopic and thermal studies. Complexation efficiency depended on ligand type and charge ratio. While complexes had varying interaction strengths, the excessive stability of TA/MET resulted in unfavorable poor MET dissociation. Notably, HIP imparted a 450 and tenfold lipophilicity increase for SDS/MET and CB/MET, respectively. The latter showed favorable controlled, yet complete release of MET at pH 6.8 and was loaded into alginate beads. Complex bulkiness and decreased lipophilicity resulted in a dramatic 88% increase of MET loading, demonstrating the success of HIP as a simple, efficient and applicable approach for modulating drug’s properties.

Inferring species interactions from ecological survey data: A mechanistic approach to predict quantitative food webs of seed feeding by carabid beetles.

Ecological networks are valuable for ecosystem analysis but their use is often limited by a lack of data because many types of ecological interaction, for example, predation, are short‐lived and difficult to observe or detect. While there are different methods for inferring the presence of interactions, they have rarely been used to predict the interaction strengths that are required to construct weighted, or quantitative, ecological networks.Here, we develop a trait‐based approach suitable for inferring weighted networks, that is, with varying interaction strengths. We developed the method for seed‐feeding carabid ground beetles (Coleoptera: Carabidae) although the principles can be applied to other species and types of interaction.Using existing literature data from experimental seed‐feeding trials, we predicted a per‐individual interaction cost index based on carabid and seed size. This was scaled up to the population level to create inferred weighted networks using the abundance of carabids and seeds from empirical samples and energetic intake rates of carabids from the literature. From these weighted networks, we also derived a novel measure of expected predation pressure per seed type per network.This method was applied to existing ecological survey data from 255 arable fields with carabid data from pitfall traps and plant seeds from seed rain traps. Analysis of these inferred networks led to testable hypotheses about how network structure and predation pressure varied among fields.Inferred networks are valuable because (a) they provide null models for the structuring of food webs to test against empirical species interaction data, for example, DNA analysis of carabid gut regurgitates and (b) they allow weighted networks to be constructed whenever we can estimate interactions between species and have ecological census data available. This permits ecological network analysis even at times and in places when interactions were not directly assessed.

XY model with antinematic interaction.

We consider the XY model with ferromagnetic (FM) and antinematic (AN) nearest-neighbor interactions on a square lattice for a varying interaction strength ratio. Besides the expected FM and AN quasi-long-range order (QLRO) phases we identify at low temperatures another peculiar canted ferromagnetic (CFM) QLRO phase, resulting from the competition between the collinear FM and noncollinear AN ordering tendencies. In the CFM phase neighboring spins that belong to different sublattices are canted by a nonuniversal (dependent on the interaction strength ratio) angle and the ordering is characterized by a fast-decaying power-law intrasublattice correlation function. Compared to the FM phase, in the CFM phase correlations are significantly diminished by the presence of zero-energy domain walls due to the inherent degeneracy caused by the AN interactions. We present the phase diagram as a function of the interaction strength ratio and discuss the character of the respective phases as well as the transitions between them.

Concentration Dependence of Ion Pairing in Imidazolium-Based Ionic Liquid Solutions.

Infrared vibrational spectroscopy was used to probe concentration-dependent ion pair dissociation of imidazolium-based ionic liquids with three different halide anions (I- , Br- , and Cl- ) in deuterated chloroform. Dissociation of the ion pairs at low concentrations of ionic liquids was found to be the easiest for ionic liquid with Cl- anion, the most electronegative anion among the three investigated. This anomalous trend of ion pair dissociation was explained in terms of varying interaction strength between the solvent (CDCl3 ) and the anions investigated.

Bunching-antibunching crossover in harmonically trapped few-body Bose-Fermi mixtures

We investigate the ground state of a few-body Bose-Fermi mixture in a one-dimensional harmonic trap with varying interaction strengths and mass ratio. A bunching-antibunching crossover of the bosonic species for increasing interspecies' repulsion is observed within our fully correlated \textit{ab~initio} studies. Interestingly, this crossover is suppressed if the bosonic repulsion exceeds a critical value which strongly depends on the mass ratio. In order to unveil the physical origin of this crossover, we employ different levels of approximations: while a species mean-field approach can account for the antibunching, only the inclusion of the interspecies correlations can lead to the bunching. We show that these correlations effectively create an induced bosonic interaction, which in turn elucidates the occurrence of the bosonic bunching. Finally, we derive a two-site extended Bose-Hubbard model which reveals the low-energy physics of the bosons for the case of much heavier fermions.

Benchmark of Excitation Energy Shifts from Frozen-Density Embedding Theory: Introduction of a Density-Overlap-Based Applicability Threshold.

We present a thorough investigation of the errors in results obtained with the combination of frozen-density embedding theory and the algebraic diagrammatic construction scheme for the polarization propagator of second order (FDE-ADC(2)). The study was carried out on a set of 52 intermolecular complexes with varying interaction strength, each consisting of a chromophore of fundamental interest and a few small molecules in its environment. The errors emerging in frozen-density embedding theory-based methods originate from (a) the solver of the quantum many-body problem used to obtain the embedded wave function (ΨAemb), (b) the approximation for the explicit density functional for the embedding potential, and (c) the choice of the density representing the environment (ρB( r⃗)). The present work provides a comprehensive analysis of the errors in the excitation energies based on the last two factors. Furthermore, a density-overlap-based parameter is proposed to be used as an a priori criterion of applicability.

Ring and Linear Copolymer Blends under Confinement.

The behavior of dense mixtures of two topologically different diblock copolymer (CP) chains, viz., linear (L)-CP and ring (R)-CP of the same molecular weight, which form lamellae is studied under confinement by two nonselective substrates. The effect of varying interaction strength between L-CP and R-CP from purely repulsive (demixed state) to weakly attractive (mixed state) on the morphology, domain size, chain conformations, and distribution of chains in the film is investigated. In the demixed state, collective structure factor S(q) shows a split of the predominant peak indicating the presence of two dominant length scales, while there is only one predominant peak in the mixed state, and hence a lamellar structure with single domain size. We show that the peak position q* of S(q) can be varied with the L/R interaction strength and thus allows one to control domain size by tuning L/R interaction strength without altering the chain size. We further characterize the chain size and illustrate that this domain size variation is a consequence of the variation in the size of L-CPs. Furthermore, results on the average instantaneous shape of R/L-CP reveal that their shapes are very different both in bulk and near the substrate, and R-CP assumes an oblate shape near the substrate. This shape/size difference leads to the segregation of R-CPs near the polymer-substrate interface and hence a relatively higher density of R-CPs at the interface.

The nature of frontier orbitals under systematic ligand exchange in (pseudo-)octahedral Fe(ii) complexes.

Understanding and controlling properties of transition metal complexes is a crucial step towards tailoring materials for sustainable energy applications. In a systematic approach, we use resonant inelastic X-ray scattering to study the influence of ligand substitution on the valence electronic structure around an aqueous iron(ii) center. Exchanging cyanide with 2-2'-bipyridine ligands reshapes frontier orbitals in a way that reduces metal 3d charge delocalization onto the ligands. This net decrease of metal-ligand covalency results in lower metal-centered excited state energies in agreement with previously reported excited state dynamics. Furthermore, traces of solvent-effects were found indicating a varying interaction strength of the solvent with ligands of different character. Our results demonstrate how ligand exchange can be exploited to shape frontier orbitals of transition metal complexes in solution-phase chemistry; insights upon which future efforts can built when tailoring the functionality of photoactive systems for light-harvesting applications.

Bosonic quantum dynamics following a linear interaction quench in finite optical lattices of unit filling

The nonequilibrium ultracold bosonic quantum dynamics in finite optical lattices of unit filling following a linear interaction quench from a superfluid to a Mott insulator state and vice versa is investigated. The resulting dynamical response consists of various inter and intraband tunneling modes. We find that the competition between the quench rate and the interparticle repulsion leads to a resonant dynamical response, at moderate ramp times, being related to avoided crossings in the many-body eigenspectrum with varying interaction strength. Crossing the regime of weak to strong interactions several transport pathways are excited. The higher-band excitation dynamics is shown to obey an exponential decay possessing two distinct time scales with varying ramp time. Studying the crossover from shallow to deep lattices we find that for a diabatic quench the excited band fraction decreases, while approaching the adiabatic limit it exhibits a non-linear behavior for increasing height of the potential barrier. The inverse ramping process from strong to weak interactions leads to a melting of the Mott insulator and possesses negligible higher-band excitations which follow an exponential decay for decreasing quench rate. Finally, independently of the direction that the phase boundary is crossed, we observe a significant enhancement of the excited to higher-band fraction for increasing system size.

Comparison of metal-binding strength between methionine and cysteine residues: Implications for the design of metal-binding motifs in proteins

Metals play vital role in various physiological processes and are bound to biomolecules. Although cysteine sulfur is more frequently found as metal-binding ligand, methionine prefers to occur in copper-binding motifs of some proteins. To address methionine's lower preference in copper-binding sites in comparison to cysteine, we have considered copper-binding motifs (His-Cys-His-Met) from seven different high-resolution protein structures. We performed quantum chemical calculations to find out the strength of interactions between sulfur and metal ion in both Met and Cys residues. In the case of Cys, both neutral (CysH) and the deprotonated form (Cys−) were considered. We used two different levels of theory (B3LYP and M06-2X) and the model compounds methyl propyl sulfide, ethanethiol and ethanethiolate were used to represent Met, CysH and Cys− respectively. To compare the metal-binding strength, we mutated Met in silico to CysH/Cys− and performed the calculations. We also carried out calculations with wild-type Cys present in the same metal-binding motif. On average, interactions of Met with copper ion are stronger by 13–35kcal/mol compared to CysH. However, Cys− interactions with copper is stronger than that of Met by ~250kcal/mol. We then considered the entire metal-binding motif with four residues and calculated the interaction energies with the copper ion. We also considered Met→Cys− mutation in the motif and repeated the calculations. Interaction of the wild-type motif with the copper ion is ~160kcal/mol weaker than that of mutated motif. Our studies suggest the factors that could explain why Met is not as frequently observed as Cys in the metal-binding motifs. Results of these studies will help in designing metal-binding motifs in proteins with varying interaction strengths.

Limits to Causal Inference with State-Space Reconstruction for Infectious Disease.

Infectious diseases are notorious for their complex dynamics, which make it difficult to fit models to test hypotheses. Methods based on state-space reconstruction have been proposed to infer causal interactions in noisy, nonlinear dynamical systems. These “model-free” methods are collectively known as convergent cross-mapping (CCM). Although CCM has theoretical support, natural systems routinely violate its assumptions. To identify the practical limits of causal inference under CCM, we simulated the dynamics of two pathogen strains with varying interaction strengths. The original method of CCM is extremely sensitive to periodic fluctuations, inferring interactions between independent strains that oscillate with similar frequencies. This sensitivity vanishes with alternative criteria for inferring causality. However, CCM remains sensitive to high levels of process noise and changes to the deterministic attractor. This sensitivity is problematic because it remains challenging to gauge noise and dynamical changes in natural systems, including the quality of reconstructed attractors that underlie cross-mapping. We illustrate these challenges by analyzing time series of reportable childhood infections in New York City and Chicago during the pre-vaccine era. We comment on the statistical and conceptual challenges that currently limit the use of state-space reconstruction in causal inference.

Effects of short-range electron-electron interactions in doped graphene

We study theoretically the effects of short-range electron-electron interactions on the electronic structure of graphene, in the presence of single substitutional impurities. Our computational approach is based on the $\pi$ orbital tight-binding approximation for graphene, with the electron-electron interactions treated self-consistently at the level of the mean-field Hubbard model. We compare explicitly non-interacting and interacting cases with varying interaction strength and impurity potential strength. We focus in particular on the interaction-induced modifications in the local density of states around the impurity, which is a quantity that can be directly probed by scanning tunneling spectroscopy of doped graphene. We find that the resonant character of the impurity states near the Fermi level is enhanced by the interactions. Furthermore, the size of the energy gap, which opens at high-symmetry points of the Brillouin zone of the supercell upon doping, is significantly affected by the interactions. The details of this effect depend subtly on the supercell geometry. We use a perturbative model to explain these features and find quantitative agreement with numerical results.

Pair condensation of polarized fermions in the BCS–BEC crossover

We investigate a two-component Fermi gas with unequal spin populations along the BCS–BEC crossover. By using the extended BCS equations and the concept of off-diagonal-long-range-order we derive a formula for the condensate number of Cooper pairs as a function of energy gap, average chemical potential, imbalance chemical potential and temperature. Then we study the zero-temperature condensate fraction of Cooper pairs by varying interaction strength and polarization, finding a depletion of the condensate fraction by increasing the population imbalance. We also consider explicitly the presence of an external harmonic confinement and we study, within the local-density approximation, the phase separation between superfluid (SF) and normal phase regions of the polarized fermionic cloud. In particular, we calculate both condensate density profiles and total density profiles from the inner SF core to the normal region passing for the interface, where a finite jump in the density is a clear manifestation of this phase-separated regime. Finally, we compare our theoretical results with the available experimental data on the condensate fraction of polarized 6Li atoms (Zwierlein et al 2006 Science 311 492). These experimental data are in reasonable agreement with our predictions in a suitable range of polarizations, but only in the BCS side of the crossover up to unitarity.

Graphene buckles under stress: An x-ray standing wave and scanning tunneling microscopy study

Graphene on Ir(111) is studied using scanning tunneling microscopy (STM) and x-ray standing waves (XSW). The graphene layer has a corrugated shape due to the moir\'e pattern formed as a result of a spatially varying interaction strength between graphene and its substrate. The coherent fraction ${F}^{(111)}$ determined in XSW allows us to determine the amplitude of this intrinsic corrugation when the incoherent contributions of wrinkles, step edges, graphene edges, and contaminations are taken into account. The corrugation is found to depend on the stress state of the carbon sheet. Using density functional theory (DFT) calculations, we show that graphene can compensate stress by increasing its corrugation rather than by a reduction of the C-C bond length.

Formation of “Quasi” Contact or Solvent‐separated Ion Pairs in the Local Environment of Probe Molecules Dissolved in Ionic Liquids

The change from "quasi" contact to "quasi" solvent-separated ion-pair configuration in the local environment of a probe molecule in ionic liquids depends on the varying interaction strength of the chosen anions. The ion speciation in these Coulomb fluids could be shown by combining infrared spectroscopy, density functional theory calculations, and natural bond orbital analysis using a low-self-clustering probe molecule.