Solvent effects on model telechelic polymers

A theory to predict the competition between intermolecular and intramolecular hydrogen bonding is extended to mixtures and applied to a model telechelic mixture. The theory is tested by comparing with simulation results for a mixture of fully flexible linear chains that can associate in a hydrogen bonding solvent. The simulation model for the telechelic is a flexible linear tetramer hard sphere chain with two hydrogen bonding sites, one on each terminal segment. The solvent is modelled as a hard sphere with four tetrahedrally arranged hydrogen bonding sites. The solvent is seen to affect the ability of the solute to bond intermolecularly and intramolecularly. The extent of hydrogen bonding and thermodynamic properties of the system were studied using Monte Carlo simulation and compared with predictions from a new statistical mechanics based theory for mixtures. Agreement of simulation and theory is good over the range of densities, temperatures and compositions studied.     

Can we understand (and model) aqueous solutions without any long range electrostatic interactions?

A computer simulation experiment has been conducted to study the extent to which long range Coulombic interactions are indispensable when modelling aqueous solutions of electrolytes. A simple molecular model, which accounts explicitly for the molecular structure of water but which does not incorporate any long range Coulombic interactions is employed. The solvent is primitive water (EPM5-4 model) and the solute molecules are hard spheres interacting with the interaction sites of the water molecule by means of either repulsive (like-charge interaction) or attractive (unlike-charge interaction) short range triangular-well tails. The structural changes (hydrophobic ordering, structure breaking, and structure enhancement) which take place in an infinitely dilute solution upon ‘charging’ the solute were studied, in terms of the correlation functions and of the orientational distribution functions and of the average binding energy of the water molecules around the solute in terms of their dependence on the solute-water oxygen distance. The main thermodynamic property reflecting these changes is the residual entropy. This quantity is found to exhibit an asymmetric double maximum, in agreement with the findings for a realistic counterpart of this simple model that employs long range Coulombic interactions.     

Structure of polymer solutions at interfaces: a Monte Carlo simulation study

The canonical ensemble Monte Carlo method is applied to study the structure of polymer solutions confined between surfaces. The polymer molecules are modeled as fused-sphere freely rotating chains with fixed bond length and bond angles and the solvent as hard spheres. The simulation results for the configurational and conformational properties of the chains are presented with varying interfacial distances, chain concentrations, and chain lengths. The chains are depleted at the wall at lower density, which, however, becomes less at higher density. With an increase in the interfacial distance, the enhancement/depletion of the chains at the wall becomes more marked. At all interfacial distances and chain lengths, increasing the concentration of the solvent makes the oscillation in the density profile of the chains more pronounced. Conformational properties provide important indications regarding the behaviour of chains as they approach surfaces.     

Modified thermodynamic perturbation theory for fused–sphere dimer fluids

Wertheim's thermodynamic perturbation theory of first order (TPT1) is based on the approximation that the monomer–monomer distribution functions can be approximated by the reference fluid distribution functions regardless of the amount of bonding. This is remarkably accurate for chains formed by tangent spheres, but no longer valid for chains of fused spheres. This constitutes the reason for the inadequacy of TPT1 for fused sphere chains. We present a systematic modification of TPT1, the path integral perturbation method, that takes into account the variations of the distribution functions with extent of bonding. We demonstrate the accuracy of the theory for mixtures of hard spheres and diatomics over a range of extent of bonding (pure monomers to pure dimers) and degree of fusion (bond length 0–1). We found that the choice of reference fluid was decisive for the accuracy of the model's predictions. The proposed theory can accurately predict the properties of mixtures of hard spheres and diatomics, and of the pure fused diatomic fluids. The results from the path integral theory are in excellent agreement with simulation results, and compare favourably with the results from the Tildesley–Streett and the Boublík–Nezbeda equations of state.     

Equation of state of long chain molecules and rings

An equation of state for long chain molecules has been proposed using statistical associating fluid theory (SAFT). The formalism derived here is based on the assumption that the chain is formed by the pairs of trimers. The equations of state for 48-mers and 192-mers are formulated and compared with Monte Carlo results. The theory has been developed to treat hard sphere molecules with two attraction sites to form a ring molecule. The equations of state for trimer, hexamer and 12-mer ring molecules have been formulated. There is excellent agreement with available Monte Carlo results. Second virial coefficients of tangent chain molecules and ring molecules have been determined numerically and compared with simulation results.     

A new study of associating inhomogeneous fluids with classical density functional theory

ABSTRACT

A new density functional for the study of associating inhomogeneous fluids based on Wertheim's first-order thermodynamic perturbation theory is presented and compared to the most currently used associating density functionals. This functional is developed using the weighted density approximation in the range of association of hard spheres. We implement this functional within the framework of classical density functional theory together with modified fundamental measure theory to account for volume exclusion of hard spheres. This approach is tested against molecular simulations from literature of pure associating hard spheres and mixtures of non-associationg and associating hard spheres with different number of bonding sites close to a hard uniform wall. Furthermore, we compare and review our results with the performance of associating functionals from literature, one based on fundamental measure theory and the inhomogeneous version of Wertheim's perturbation theory. Results obtained with classical DFT and the three functionals show excellent agreement with molecular simulations in systems with one hard wall. For the cases of small pores where only one or two layers of fluid are allowed discrepancies between results with classical DFT and molecular simulations were found.     

A new equation of state for associating Lennard–Jones fluids with two sites: small bond angle

ABSTRACT

We developed a new equation of state for Lennard–Jones spheres with two short-ranged, directional association sites. The theory is novel in that association is dependent on bond angle between the two sites, and formation of self-assembled linear and cyclic clusters is predicted in the model. The competition between ring and chain formations at various densities and temperatures is predicted by the theory and verified by Monte Carlo simulations. It was found that closed-loop structures become important at low temperatures and densities. Also, at fixed association energy, intensifying the Lennard–Jones energy reduces the extent of association in the fluid. The theory and simulation are in excellent agreement.     

Density functional theory for molecular orientation of hard rod fluids in hard slits

A density functional theory (DFT) is used to investigate molecular orientation of hard rod fluids in a hard slit. The DFT approach combines a modified fundamental measure theory (MFMT) for excluded-volume effect with the first order thermodynamics perturbation theory for chain connectivity. In the DFT approach, the intra-molecular bonding orientation function is introduced. We consider the effects of molecular length (i.e. aspect ratio of rod) and packing fraction on the orientations of hard rod fluids and flexible chains. For the flexible chains, the chain length has no significant effect while the packing fraction shows slight effect on the molecular orientation distribution. In contrast, for the hard rod fluids, the chain length determines the molecular orientation distribution, while the packing fraction has no significant effect on the molecular orientation distribution. By making a comparison between molecular orientations of the flexible chain and the hard rod fluid, we find that the molecular stiffness distinctly affects the molecular orientation. In addition, partitioning coefficient indicates that the longer rodlike molecule is more difficult to enter the confined phase, especially at low bulk packing fractions.     

Associating fluids with four bonding sites against a hard wall: density functional theory

We present two new perturbation density functional theories to investigate non-uniform fluids of associating molecules. Each fluid molecule is modelled as a spherical hard core with four highly anisotropic square well sites placed in tetrahedral symmetry on the hard core surface. In one theory we apply the weighting from Tarazona's hard sphere density functional theory to Wertheim's bulk first-order perturbation theory. The other theory uses the inhomogeneous form of Wertheim's theory as a perturbation to Tarazona's hard-sphere density functional theory. Each theory approaches Tarazona's theory in the limit of zero association. We compare results from theory and simulation for density profiles, fraction of monomers, and adsorption of an associating fluid against a hard, smooth wall over a range of temperatures and densities. The non-uniform fluid theory which uses Tarazona's weighting of Wertheim's theory in the bulk is in good agreement with computer simulation results.     

Continuum percolation of the four-bonding-site associating fluids

Wertheim’s integral equation theory for associating fluids is reformulated for the study of the connectedness properties of associating hard spheres with four bonding sites. The association interaction is described as a square-well saturable attraction between these sites. The connectedness version of the Ornstein-Zernike (OZ) integral equation is supplemented by the PY-like closure relation and solved analytically within an ideal network approximation in which the network is represented as resulting from the crossing of ideal polymer chains. The pair connectedness functions and the mean cluster size are calculated and discussed. The condition for the percolation transition and the analytical form of the percolation threshold are derived. The connection of the percolation with the gas-liquid phase transition is discussed.     

Zeta potential of colloidal particle in solvent primitive model electrolyte solution: a density functional theory study

A systematic study of zeta potential for a spherical double layer (SDL) around a colloidal particle in electrolyte solutions, is performed using density functional theory and Monte Carlo simulation. The usual recipe under the solvent primitive model is employed to model the system, where macroion, counterions, and coions are represented by charged hard spheres of uniform charge density and the presence of solvent is taken into account by modelling it as neutral hard spheres. All the components of the system are embedded in a dielectric continuum in order to consider the electrostatic effect of the solvent. The density functional theory employs a suitable weighted density approximation to calculate the hard-sphere contribution, whereas the residual electrostatic interactions are calculated as a small perturbation around the uniform fluid. The zeta potential profiles of a SDL in the presence of a number of electrolytes have been calculated and are found to be considerably influenced in the presence of solvent with an increase in the concentration of the electrolyte. The theory successfully predicts the maxima and sign reversal of the zeta potential profiles at high macroion surface charge density and in the presence of multivalent counterions, as obtained from the Monte Carlo simulation.     

Modulation of intra- and inter-sheet interactions in short peptide self-assembly by acetonitrile in aqueous solution

Besides our previous experimental discovery(Zhao Y R, et al. 2015 Langmuir, 31, 12975) that acetonitrile(ACN)can tune the morphological features of nanostructures self-assembled by short peptides KIIIIK(KI4K) in aqueous solution,further experiments reported in this work demonstrate that ACN can also tune the mass of the self-assembled nanostructures.To understand the microscopic mechanism how ACN molecules interfere peptide self-assembly process, we conducted a series of molecular dynamics simulations on a monomer, a cross-β sheet structure, and a proto-fibril of KI4 K in pure water, pure ACN, and ACN-water mixtures, respectively. The simulation results indicate that ACN enhances the intra-sheet interaction dominated by the hydrogen bonding(H-bonding) interactions between peptide backbones, but weakens the inter-sheet interaction dominated by the interactions between hydrophobic side chains. Through analyzing the correlations between different groups of solvent and peptides and the solvent behaviors around the proto-fibril, we have found that both the polar and nonpolar groups of ACN play significant roles in causing the opposite effects on intermolecular interactions among peptides. The weaker correlation of the polar group of ACN than water molecule with the peptide backbone enhances H-bonding interactions between peptides in the proto-fibril. The stronger correlation of the nonpolar group of ACN than water molecule with the peptide side chain leads to the accumulation of ACN molecules around the proto-fibril with their hydrophilic groups exposed to water, which in turn allows more water molecules close to the proto-fibril surface and weakens the inter-sheet interactions. The two opposite effects caused by ACN form a microscopic mechanism clearly explaining our experimental observations.     

用激光拉曼光谱研究液态乙醇的水合作用过程

为研究室温下乙醇-水二元混合物内部的分子间缔合情形,测得了不同体积配比溶液的拉曼光谱,发现位于2 800~3 050 cm-1波数区间的C—H伸缩振动频率随乙醇中加入水量的增加整体呈现蓝移趋势,而位于1 048 cm-1附近的CO伸缩振动频率的变化规律却与此相反。分析认为,这种现象主要由溶液内部分子间发生的不同水合作用所致,并据此阐明了液态乙醇的水合作用过程:水分子首先与纯乙醇中的自缔合短链发生氢键缔合作用,形成了含有较多乙醇分子数的乙醇水合团簇,直到溶液中水的体积含量达到50%时,乙醇的水合作用达到暂时饱和;而当水的体积含量继续增加到70%以后,水分子致使原有乙醇水合团簇解离形成较小尺寸的团簇,并与解离点位上的乙醇分子羟基发生进一步水合作用;而后,当水体积含量增至一定程度后,还会导致乙醇分子疏水基CH基团与水分子间形成弱氢键C—H…O。     

氢键对2-(N-甲基)氨基-5-硝基吡啶分子非线性光学性质的影响

本文在杂化密度泛函理论水平上研究了溶剂对2-(N-甲基)氨基-5-硝基吡啶分子非线性光学性质的影响.在溶剂中,构造了包括氢键作用的超分子体系,在优化结构的基础上分别研究了由极化连续模型模拟的溶剂与该分子的长程相互作用、溶剂与该分子的氢键相互作用以及溶剂与包括氢键作用的超分子体系整体的相互作用对分子的几何结构、非线性光学性质、紫外吸收光谱和电荷分布等特性的影响.结果表明,溶剂中分子电偶极矩、线性极化率和第一超极化率都增大,而溶剂与溶质分子通过氢键形成的超分子结构与单体有着明显区别.因此,氢键对分子结构和性质的影响较大,从而将明显的影响该类分子的非线性光学性质.     

Network Forming Fluids: Yukawa Square-Well <Emphasis Type="Italic">m</Emphasis>-Point Model

Thermal and connectivity properties of the Yukawa square-well m-point (YSWmP) model of the network forming fluid are studied using solution of the multidensity Ornstein-Zernike and connectedness Ornstein-Zernike equations supplemented by the associative mean spherical approximation (AMSA). The model is represented by the multicomponent mixture of Yukawa hard spheres with m_s^am_{s}^{a} square-well sites, located on the surface of each hard sphere. To validate the accuracy of the theory, computer simulation is used to calculate the structure, thermodynamic and connectivity properties of the one-component YSW4P version of the model which is compared against corresponding theoretical data. In addition, connectivity properties of the model were studied using Flory-Stockmayer (FS) theory. Predictions of the AMSA for the thermal properties of the model (radial distribution functions (RDF), internal energy, pressure, fractions of the particles in different bonding states) are in good agreement with computer simulation predictions. Similarly, good agreement was found for the connectedness RDF (CRDF), except for the statepoints located close to the percolation threshold, where the theory fails to reproduce the long-range behavior of the CRDF. Results of both theories (AMSA and FS) for the mean cluster size are reasonably accurate only at low degrees of association. Predictions of the FS theory for the percolation lines are in a good agreement with computer simulation predictions. AMSA predictions of percolation are much less accurate, where corresponding percolation lines are located at a temperatures approximately 25% lower then those calculated using computer simulation.     

Electro-optical properties of dilute solutions of flexible molecules ; the wormlike chain model

A theory of propagation and scattering of light in dilute solutions of flexible chain molecules is developed. The solvent is represented as a continuous isotropic dielectric medium. The polymer molecules are modelled as dielectric objects which may have an anisotropic dielectric tensor. The theory, applied to the wormlike chain model, represents the polymer molecules as flexible cylinders of constant cross section and with an isotropic internal dielectric constant. The increment in the refractive index of the solution and the isotropic and anisotropic light scattering intensities due to the presence of the chain molecules are derived. The numerical results of our model for the depolarization ratio as a function of the contour length and the persistence length of the polymer and of the refractive index of the solvent are presented and compared with the corresponding results for the popular bond additive approximation (BAA). It is found that the differences between our model and this approximation are due to the neglect of intramolecular dipole-induced dipole interactions in the BAA.     

Intramolecular bonding in a statistical associating fluid theory of ring aggregates

The first-order thermodynamic perturbation theory of Wertheim (TPT1) is extended to treat ring aggregates, formed by inter- and intramolecular association. The expression for the residual association contribution to the Helmholtz free energy for ring aggregates, incorporating the appropriate terms in Wertheim's fundamental graph sum of the TPT1 density expansion, is derived to calculate the distribution of the molecular bonding states. This requires the introduction of two new parameters to characterise each possible ring type: the ring size τ, which is equal to one in the case of intramolecular association, and a parameter W that captures the likelihood of two ring-forming sites bonding. The resulting framework can be incorporated in equations of state that account for the residual association contribution to the free energy, such as the statistical associating fluid theory (SAFT) family, or the cubic plus association (CPA) equation of state. This extends the applicability of these equations of state to mixtures with an arbitrary number of association sites capable of hydrogen bonding to form intramolecular and intermolecular rings. The formalism is implemented within SAFT-VR Mie to calculate the fluid-phase equilibria of model chain-like molecules containing two associating sites A and B, allowing for the formation of open-chain aggregates and intramolecular bonds. The effect of adding a second component that competes for the association sites that mediate intramolecular association in the chain is also examined. Accounting for intramolecular bonding is shown to have a significant impact on the phase equilibria of such systems.     

Evaluating virial coefficients for multicomponent mixtures: hard sphere mixtures and flexible chains

A new algorithm to compute the virial coefficients of multicomponent mixtures is proposed. The number of graphs that must be evaluated increases dramatically in a multicomponent mixture so that it becomes difficult to enumerate and compute all possible graphs. However, once all of them are known and evaluated, the virial coefficient of the mixture can be evaluated for any composition. If one is interested in the virial coefficient of a mixture of a certain composition, then a simpler approach can be followed. Starting from the graphs of a pure fluid, we assign a random chemical identity to each of the molecules of the graph. The probability of assigning a given chemical identity is taken from the composition of the mixture. In this way composition is treated as a random variable within the Monte Carlo procedure which determines the virial coefficient. The algorithm is checked by comparison with the virial coefficients of binary hard spheres mixtures which are well known. Good agreement is found. The procedure is then extended to multicomponent mixtures of hard spheres. Finally the procedure is applied to the determination of the virial coefficients of a flexible molecule. For flexible molecules the possible configurations of the molecules are treated as different components of the mixture. In this way we present what appears to be the first determination of the third and fourth virial coefficients of polymers in the continuum.     

A quantitative mean-field theory of the hydrophobic effect of neutral and charged molecules of arbitrary geometry

A self-consistent two-length scale theory of the interaction between a hydrophobic molecule and a water environment is considered. This theory allows the width of the hydrophobic layer to be calculated for molecules of arbitrary geometry by explicitly taking into account the water structure through the correlation function of a pure liquid. This approach is used to calculate the density profile ρ(r) around a molecule of arbitrary geometry and the solvation free energy ΔG(R) related to the transport of the molecule from a vacuum to a liquid. The model parameters are adjusted by comparing the results of numerical Monte Carlo simulations taken from the literature with predictions of the model for molecules of spherical geometry. The free energy of the interaction Δ G(D) between two spheres of radius R separated by distance D is also determined using the developed approach. The model is generalized to electrostatic interactions within the framework of a self-consistent scheme in which water is modeled by a gas of point dipoles. Analysis of the derived equations shows that this theory coincides with the electrostatic theory of a continuous medium with an effective permittivity in the limit of weak electric fields.