Unveiling the Role of Electrostatic Forces on Attraction between Opposing Polyelectrolyte Brushes

Electrostatic interaction and molecular excluded-volume effects are responsible for a plethora of nonintuitive phenomena in soft-matter systems, including local charge inversion and attraction between similar charges. In the current work, we study the surface forces and swelling behavior of opposing polyelectrolyte brushes using a classical density functional theory that accounts for electrostatic and excluded-volume correlations. We observe that the detachment pressure between similarly charged brushes is sensitive to salt concentration in both the osmotic and salted regimes and can be negative in the presence of multivalent counterions. A comparison of the theoretical results with the mean-field predictions unravels the role of correlation effects in determining the surface forces and brush structure. For systems containing multivalent counterions, the detachment pressure attains negative values at an intermediate brush–brush separation, and the attractive region in the pressure vs distance plot is magnified in terms of both the depth and width of attraction with increasing counterion valency. However, the interbrush attraction vanishes when the size-induced correlations are switched off. We also investigated the role of counterion size and polymer chain length on the detachment pressure. It is found that smaller counterions are more effective in neutralizing the polymer charge than bigger counterions, leading to a reduced interbrush repulsion and, in some cases, attraction between like-charged brushes at intermediate distances. Meanwhile, varying the chain length of the grafted polymers only shifts the location of the attraction basin, with little influence on the interaction strength. The theoretical predictions show qualitative agreement with experimental observations and offer valuable insights into the interaction between similarly charged polymer brushes in the presence of multivalent ions

Molecular Theory of Hydration at Different Temperatures

Solvation plays an important role in diverse chemical processes ranging from reaction kinetics to molecular recognition, solubility, and phase separations. Despite a long-history of theoretical exploration, quantitative prediction of solvation remains a theoretical challenge without relying on the macroscopic properties of the solvent as an input. Here we present a molecular density functional theory that provides a self-consistent description of the solvation structure and thermodynamic properties of small organic molecules in liquid water at different temperatures. Based on the solute configuration and force-field parameters generated from first-principles calculations, the theoretical predictions are found in good agreement with experimental data for the hydration free energies of 197 organic molecules in a temperature range from 0 to 40 °C. In addition to calibration with experimental results, the theoretical predictions are compared with recent molecular dynamics simulations for the hydration of five highly explosive nitrotoluenes. This work demonstrates the potential of the classical density functional theory for high-throughput prediction of solvation properties over a broad range of temperatures

Thermodynamic and Structural Evidence for Reduced Hydrogen Bonding among Water Molecules near Small Hydrophobic Solutes

The structure of water molecules near a hydrophobic solute remains elusive despite a long history of scrutiny. Here, we re-examine the subtle issue by a combination of thermodynamic analysis for Henry’s constants of several nonpolar gases over a broad range of temperatures and molecular dynamic simulations for the water structure in the hydration shell using several popular semiempirical models of liquid water. Both the structural and thermodynamic data indicate that hydrophobic hydration reduces the degree of the hydrogen bonding among water molecules, and the effect becomes more prominent at high temperatures. Hydrogen-bond formation is slightly hindered near a hydrophobic solute due to the restriction of the degree of freedom for water molecules in the solvation shell, and the confinement effect becomes more significant as temperature increases. Reduction in the extent of hydrogen bonding is fully consistent with a positive contribution of a small hydrophobic solute to the solution heat capacity. As predicted by the scaled-particle theory, both Henry’s constants and simulation results suggest that the hydration entropy is determined primarily by cavity formation in liquid water, with its magnitude rising with the solute size but declining with temperature

Separation of Carbon Isotopes in Methane with Nanoporous Materials

Traditional methods for carbon isotope separation are mostly based on macroscopic procedures such as cryogenic distillation and thermal diffusion of various gaseous compounds through porous membranes. Recent development in nanoporous materials renders opportunities for more effective fractionation of carbon isotopes by tailoring the pore size and the local chemical composition at the atomic scale. Herein we report a theoretical analysis of metal–organic frameworks (MOFs) for separation of carbon isotopes in methane over a broad range of conditions. Using the classical density functional theory in combination with the excess-entropy scaling method and the transition-state theory, we predict the adsorption isotherms, gas diffusivities, and isotopic selectivity corresponding to both adsorption- and membrane-based separation processes for a number of MOFs with large methane adsorption capacity. We find that nanoporous materials enable much more efficient separation of isotopic methanes than conventional methods and allow for operation at ambient thermodynamic conditions. MOFs promising for adsorption- and membrane-based separation processes have also been identified according to their theoretical selectivity for different pairs of carbon-isotopic methanes

A Site Density Functional Theory for Water: Application to Solvation of Amino Acid Side Chains

We report a site density functional theory (SDFT) based on the conventional atomistic models of water and the universality <i>ansatz</i> of the bridge functional. The excess Helmholtz energy functional is formulated in terms of a quadratic expansion with respect to the local density deviation from that of a uniform system and a universal functional for all higher-order terms approximated by that of a reference hard-sphere system. With the atomistic pair direct correlation functions of the uniform system calculated from MD simulation and an analytical expression for the bridge functional from the modified fundamental measure theory, the SDFT can be used to predict the structure and thermodynamic properties of water under inhomogeneous conditions with a computational cost negligible in comparison to that of brute-force simulations. The numerical performance of the SDFT has been demonstrated with the predictions of the solvation free energies of 15 molecular analogs of amino acid side chains in water represented by SPC/E, SPC, and TIP3P models. For theTIP3P model, a comparison of the theoretical predictions with MD simulation and experimental data shows agreement within 0.64 and 1.09 kcal/mol on average, respectively

Impurity effects on ionic-liquid-based supercapacitors

<p>Small amounts of an impurity may affect the key properties of an ionic liquid and such effects can be dramatically amplified when the electrolyte is under confinement. Here the classical density functional theory is employed to investigate the impurity effects on the microscopic structure and the performance of ionic-liquid-based electrical double-layer capacitors, also known as supercapacitors. Using a primitive model for ionic species, we study the effects of an impurity on the double layer structure and the integral capacitance of a room temperature ionic liquid in model electrode pores and find that an impurity strongly binding to the surface of a porous electrode can significantly alter the electric double layer structure and dampen the oscillatory dependence of the capacitance with the pore size of the electrode. Meanwhile, a strong affinity of the impurity with the ionic species affects the dependence of the integral capacitance on the pore size. Up to 30% increase in the integral capacitance can be achieved even at a very low impurity bulk concentration. By comparing with an ionic liquid mixture containing modified ionic species, we find that the cooperative effect of the bounded impurities is mainly responsible for the significant enhancement of the supercapacitor performance.</p

Spreading of a Unilamellar Liposome on Charged Substrates: A Coarse-Grained Molecular Simulation

Supported lipid bilayers (SLBs) are able to accommodate membrane proteins useful for diverse biomimetic applications. Although liposome spreading represents a common procedure for preparation of SLBs, the underlying mechanism is not yet fully understood, particularly from a molecular perspective. The present study examines the effects of the substrate charge on unilamellar liposome spreading on the basis of molecular dynamics simulations for a coarse-grained model of the solvent and lipid molecules. Liposome transformation into a lipid bilayer of different microscopic structures suggests three types of kinetic pathways depending on the substrate charge density, that is, top-receding, parachute, and parachute with wormholes. Each pathway leads to a unique distribution of the lipid molecules and thereby distinctive properties of SLBs. An increase of the substrate charge density results in a magnified asymmetry of the SLBs in terms of the ratio of charged lipids, parallel surface movements, and the distribution of lipid molecules. While the lipid mobility in the proximal layer is strongly correlated with the substrate potential, the dynamics of lipid molecules in the distal monolayer is similar to that of a freestanding lipid bilayer. For liposome spreading on a highly charged surface, wormhole formation promotes lipid exchange between the SLB monolayers thus reduces the asymmetry on the number density of lipid molecules, the lipid order parameter, and the monolayer thickness. The simulation results reveal the important regulatory role of electrostatic interactions on liposome spreading and the properties of SLBs

Solvation Structure of Surface-Supported Amine Fragments: A Molecular Dynamics Study

Amine-grafted silica gel is an efficient heterogeneous catalyst for the Knoevenagel condensation and draws much attention in green chemistry for applications like heavy metal adsorption and CO₂ fixation. Despite its successful usage in diverse areas, fundamental questions remain on how the silica substrate affects the local chemical environment of the tethered amines. In this work, we use all-atom molecular dynamics simulation to investigate the solvation structures of two primary amines tethered onto a silica surface at different pHs of aqueous solutions. The atomic density profiles in the solvation shell are analyzed with a spherical harmonics expansion method for both isolated and silica-supported amines in different aqueous environments. The simulation results provide direct evidence for the strong influence of the silica surface on the hydration structure that is often ignored in the theoretical analysis of surface reactions. The surface effect becomes less prominent on the tethered amine as the alkyl chain length increases

Kinetic Charging Inversion in Ionic Liquid Electric Double Layers

The charging kinetics of electric double layers (EDLs) has a pivotal role in the performance of a wide variety of nanostructured devices. Despite the prevalent use of ionic liquids as the electrolyte, relatively little is known on the charging behavior from a microscopic perspective. Here, we study the charging kinetics of ionic liquid EDLs using a classical time-dependent density functional theory that captures the molecular excluded volume effects and electrostatic correlations. By examining variations of the ionic density profiles and the charging density in response to an electrode voltage, we find that at certain conditions, the electrode charge shows a rapid surge in its initial response, rises quickly to the maximum, and then slowly decays toward equilibrium. The electrode charge and voltage may have opposite signs when the cell width is commensurate with the layer-by-layer ionic distributions. This unusual charging behavior can be explained in terms of the oscillatory structure of ionic liquids near the electrodes

Solvent Effect on the Pore-Size Dependence of an Organic Electrolyte Supercapacitor

Organic electrolytes such as tetraethylammonium tetrafluoroborate dissolved in acetonitrile (TEA-BF₄/ACN) are widely used in commercial supercapacitors and academic research, but conflicting experimental results have been reported regarding the dependence of surface-area-normalized capacitance on the pore size. Here we show from a classical density functional theory the dependence of capacitance on the pore size from 0.5 to 3.0 nm for a model TEA-BF₄/ACN electrolyte. We find that the capacitance–pore size curve becomes roughly flat after the first peak around the ion diameter, and the peak capacitance is not significantly higher than the large-pore average. We attribute the invariance of capacitance with the pore size to the formation of an electric double-layer structure that consists of counterions and highly organized solvent molecules. This work highlights the role of the solvent molecules in modulating the capacitance and reconciles apparently conflicting experimental reports