Ab initio theoretical study of temperature and density dependence of molecular and thermodynamic properties of water in the entire fluid region: autoionization processes

The temperature and density dependence of the molecular and thermodynamic properties of water is investigated theoretically by means of the ab initio electronic structure theory combined with the reference interaction site model method, so-called RISM-SCF. We consider the autoionization process (H2O + H2O right harpoon over left harpoon H3O+ + OH-) by regarding H2O, H3O+, and OH- as "solute" molecules in an aqueous solution and evaluate molecular geometry, electronic structure, solvation structure, and the ionic product of water (pKw) of these species as functions of thermodynamic conditions. In our previous paper, we calculated these properties by using essentially the same method in a wide range of density values (0.6-1.4 g/cm3). However, the calculation was limited at rather higher density (>0.6 g/cm3) due to the difficulty of convergence, which is inherent to the hypernetted-chain (HNC) closure. The problem is overcome in this study by employing the Kovalenko-Hirata (KH) closure which hybridizes the HNC and the mean-spherical approximation (MSA). Here, we present the results for the thermodynamic range of densities from 0.025 to 1.0 g/cm3 and for temperatures from 300 to 800 K including the supercritical point.     

Understanding ionic liquids at the molecular level: facts, problems, and controversies

Ionic liquids (ILs) are organic salts with melting points near room temperature (or by convention below 100 degrees C). Recently, their unique materials and solvent properties and the growing interest in a sustainable, "green" chemistry has led to an amazing increase in interest in such salts. A huge number of potential cation and anion families and their many substitution patterns allows the desired properties for specific applications to be selected. Because it is impossible to experimentally investigate even a small fraction of the potential cation-anion combinations, a molecular-based understanding of their properties is crucial. However, the unusual complexity of their intermolecular interactions renders molecular-based interpretations difficult, and gives rise to many controversies, speculations, and even myths about the properties that ILs allegedly possess. Herein the current knowledge about the molecular foundations of IL behavior is discussed.     

Effect of sintering temperature on the magnetic properties and charge density distribution of nano-NiO

Phase pure nano nickel oxide was synthesized by the chemical precipitation method and sintered at 200°C, 400°C and 600°C, respectively, to study the effect of sintering on the charge distribution and magnetism. The samples were analyzed by X-ray diffraction for electron density distribution studies, vibrating sample magnetometry for magnetic behavior and by UV-VIS spectrophotometry for optical characteristics. Rearrangement of charge density distribution with respect to sintering temperature was analyzed through the maximum entropy method employed using powder X-ray diffraction data. The observed magnetic transition with respect to the temperature/size effect was analyzed and correlated with electron density distribution studies.     

Common features of orientational order at the temperature of maximum density for various water models: molecular dynamics study

Canonical ensembles for liquid water were obtained from molecular dynamics simulations at various temperatures using the TIP5P, TIP4P-FQ, TIP4P, and SPC/E water models at a fixed density of 1 g/cm3. From these ensembles, it was found that the distributions of the orientational order parameter q of these models showed similar patterns as temperature changed except that the distributions were shifted relative to each other by the difference of their temperature of maximum density (TMD). The four models exhibited similar distributions and average values of orientational order around their respective TMDs, and these common features were investigated in detail especially. The current study suggests that the unique microscopic configuration of water molecules cause TMD phenomenon in any reasonable water model. This finding provides a useful tool in the development of new water potentials by offering guidelines to predict the TMD, avoiding troublesome isothermal-isobaric ensemble simulations.     

Pt-Au合金团簇热力学性质分子动力学模拟研究

采用分子动力学方法和原子嵌入法模型势模拟了Pt原子和Au原子合金纳米团簇的熔化过程,研究了这些金属原子纳米团簇熔点与团簇组分的关系,发现不同组分纳米团簇的熔点不是单调变化的,同时均出现了负热容现象.通过对各种团簇溶化前后结构的比较研究,分析了导致这种现象的原因.     

Role of linear charge density and counterion quality in thermodynamic properties of strong acid type polyelectrolytes: divalent transition metal cations

Thermodynamic properties of aqueous solutions of poly[(vinyl alcohol)-co-(vinyl sulfate)] (PVAS) copolymer polyelectrolytes with divalent transition metal (Co(II), Ni(II), and Cu(II)) counterions have been determined by the gel deswelling method in the concentration range of 0.0005-0.12 mol of counterion/kg of water (0.09-9 w/w% of the polymer). The influence of the chemical nature of the counterion as well as the effect of the composition of the copolymer from small to medium linear charge density have been systematically studied. Solvent activity, reduced osmotic pressure, the Flory-Huggins pair interaction parameter, rational osmotic coefficients, and degrees of dissociation were calculated from the measured data. No difference could have been observed between the three counterions. Reduced osmotic pressure curves are found to be convex from above, as for Na+ counterions studied previously, which is contrary to the usual behavior of neutral polymers. Intercepts are increasing, and the calculated apparent molar masses and degrees of dissociation at infinite dilution are decreasing with increasing linear charge density of the polyelectrolytes. The pair interaction parameters show a considerable negative deviation from linearity, except for the high volume fraction region. From the differences, concentration dependence of degrees of dissociation could have been calculated. The values at infinite dilution are in good agreement with those obtained from the intercepts of the reduced osmotic pressure curves. Degrees of dissociation seem to decrease approximately linearly with increasing concentration and reach zero at finite concentrations. Rational osmotic coefficients have been calculated in three different ways, both regarding and neglecting the change in the degrees of dissociation.     

Monotonicity properties of the atomic charge density function

The present knowledge of the monotonicity properties of the spherically averaged electron density ρ(r) and its derivatives, which comes mostly from Roothan-Hartree-Fock calculations, is reviewed and extended to all Hartree-Fock ground-state atoms from hydrogen (Z = 1) to uranium (Z = 92). In looking for electron functions with universal (i.e., valid in the whole periodic table) monotonicity properties, it is found that there exist positive values of α so that the function g_o(r; α) = ρ(r)/r^α is convex, and g₁(r;α) = −ρ′(r)/r^α is not only monotonically decreasing from the origin but also convex. This is, however, not the case for the function g₂(r; α) = ρ′(r)/r^α. Additionally, the conditions which specify values for β such that the function g_n(r; β) = (−1) ′ρ⁽ⁿ⁾(r)/r^β is logarithmically convex are obtained and numerically calculated for n = 0,1 in all neutral atoms below uranium. The last property is used to obtain inequalities of general validity involving three radial expectation values which generalize all the similar ones known to date, as well as other relationships among these quantities and the values of the electron density and its derivatives at the nucleus. © 1996 John Wiley & Sons, Inc.     

Photosensitizing properties of a boronated phthalocyanine: studies at the molecular and cellular level.

A synthetic procedure has been developed for the preparation of a Zn-phthalocyanine peripherally substituted with a dodecaborane. The absorption spectrum of the derivative is typical of the phthalocyanine chromophore. Moreover, the boronated phthalocyanine exhibits a high photosensitizing efficiency against a model biological substrate, such as N-acetyl-L-tryptophanamide, and a singlet oxygen quantum yield of 0.53 in dimethylformamide. Even though the presence of the dodecaborane moiety appears to decrease the affinity of the phthalocyanine for HT-1080 transformed human fibroblasts, the boronated phthalocyanine causes an essentially complete loss of cell viability upon irradiation with 600-700 nm light under mild conditions (1 microM concentration, 5-min irradiation at 10 mW/cm(2)).     

Theoretical investigation on the structural and thermodynamic properties of FeSe at high pressure and high temperature

A theoretical investigation on structural and thermodynamic properties of 11-type iron-based superconductor FeSe at high pressure and high temperature was performed by employing the first-principles method based on the density functional theory. Some structural parameters of FeSe in both tetragonal and hexagonal phases are reported. According to the fourth-order Birch-Murnaghan equation of states, the transition pressure P(t) of FeSe from the PbO-type phase to the NiAs-type phase was determined. The calculated results are found to be in good agreement with the available experimental data. Based on the quasi-harmonic Debye model, the pressure and temperature dependence of the thermodynamic properties for hexagonal phase FeSe were investigated. Our theoretical calculations suggest that the pressure and temperature have significant effects on the heat capacity, vibrational internal energy, vibrational entropy, vibrational Helmholtz free energy, thermal expansion coefficient and Debye temperature. Even though few theoretical reports on the structural properties of FeSe are found in the current literature, to our knowledge, this is a novel theoretical investigation on the structural and thermodynamic properties of FeSe at high temperature. We hope that the theoretical results reported here can give more insight into the structural and thermodynamic properties of other iron-based superconductors at high temperature.     

Lubrication properties of bottle-brush polyelectrolytes: an AFM study on the effect of side chain and charge density

The effect of side chain to charge ratio on the frictional properties of adsorbed layers formed by bottle-brush polyelectrolytes with poly(ethylene oxide) side chains has been investigated. The brush polyelectrolytes were preadsorbed from 0.1 mM NaNO(3) solutions onto mica and silica surfaces; the interfacial friction was then measured in polyelectrolyte-free solutions via AFM (with the silica surface acting as the colloidal probe). It was concluded that the decisive factor for achieving favorable lubrication properties is the concentration of nonadsorbing poly(ethylene oxide) side chains in the interfacial region. However, contrary to what may be expected, the results showed that an ideal brush layer structure with the adsorbed polymers adopting comb-like conformation is not necessary for achieving a low coefficient of friction in the asymmetric mica-silica system. In fact, the lowest coefficient of friction (<0.01) under applied pressures as high as 30 MPa was observed for a system with a side chain to charge ratio of 9:1, incapable of forming brush-like layers.     

Influence of mechanical properties of pectin films on charge density and charge density distribution in pectin macromolecule

The hydration and mechanical properties of citrus pectin films were examined in conditions relevant to those in the plant cell wall. The pectins used for this study varied in the degree of esterification (DE) (high or low) and charge distribution on the backbone (random or block). The hydration of the films was controlled in an osmotic pressure experiment using polyethylene glycol solutions (PEG 20000). Hysteresis tests at constant deformation rate (stress vs deformation) were used for investigating the mechanical behaviour of films. Mechanical and hydration properties of pectin films were examined as a function of charge density, charge density distribution and counterion environment—K⁺, Ca²⁺, Mg²⁺. Swelling decreased with increasing counterion concentration. The effect is stronger in the case of Ca²⁺ and Mg²⁺ for low esterified pectins and therefore crosslinks from divalent ions could be assumed. The crosslink effect is confirmed in mechanical experiments where an increase in the film tensile modulus is observed with increasing counterion concentration. It is shown for the first time that in case of highly concentrated pectin solutions Mg²⁺ cations also act as a crosslinker for pectin macromolecules.     

Determination of ceiling temperature and thermodynamic properties of low ceiling temperature polyaldehydes

Knowledge of the ceiling temperature and thermodynamic variables for low ceiling temperature polymers is critical to understanding the material's synthesis and use. Synthesis of the polymer below its ceiling temperature is the routine polymerization route. In situ ¹H NMR of the equilibrium polymerization reaction can provide critical information for determining the enthalpy and entropy of polymer formation. Three polyaldehydes were synthesized with in situ ¹H NMR, and their energies of formation were determined for the linear region of ceiling temperature. Insights into the mechanism of polymerization were also found using this method. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 221–228     

Understanding the optical spectroscopy of amphiphilic molecular rectifiers: a density functional approach

We present results of first principles density functional theory calculations of the electronic and atomic structural properties of model Z-type Langmuir-Blodgett (LB) layers comprising amphiphilic quinolinium tricyanoquinodimethanide (Q3CNQ) chromophores. We find that the chromophore electronic ground state is not as clearly "zwitterionic" as required by models to explain electrical rectification purportedly seen in such systems. The computed visible region transitions are not what have been assumed to be the intervalence charge transfer bands seen in the visible region of molecules in Z-type LB films. Our own LB deposition and spectroscopic studies suggest that almost all visible region features previously seen may be ascribed to aggregates. The calculated lowest energy electronic excitation between HOMO and LUMO levels, which is located in the near infrared region, has a transition moment aligned approximately 9° off the molecular long axis, and has a normalized oscillator strength of 1 order of magnitude higher than those of the visible region transitions. This most dominant feature has been neglected from discussions of Langmuir-Blodgett layer rectification but our own deposition studies show no sign of this feature, indicating that the structure of the modeled system differs from that of typical experimental structures. The model indicates that such idealized LB layer structures cannot confidently be invoked to explain their experimental optical or electrical properties.     

Experimental and theoretical charge density study of the neurotransmitter taurine

The experimental electron density distribution in taurine, 2-aminoethane sulfonic acid, 1, has been determined from high-resolution X-ray diffraction data collected at a temperature of 100 K. Taurine crystallizes as a zwitterion in the monoclinic space group P2(1)/c. Topological analysis of the experimental electron density and a comparison with high-level theoretical gas-phase calculations show that the crystal environment has a significant influence on the electronic configuration of the sulfonate moiety in 1, which in the crystal is more delocalized than in the gas phase. This crystal effect is mainly due to hydrogen bonding.     

Effect of molecular topology on the transport properties of dendrimers in dilute solution at Theta temperature: a Brownian dynamics study

Structure and transport properties of dendrimers in dilute solution are studied with the aid of Brownian dynamics simulations. To investigate the effect of molecular topology on the properties, linear chain, star, and dendrimer molecules of comparable molecular weights are studied. A bead-spring chain model with finitely extensible springs and fluctuating hydrodynamic interactions is used to represent polymer molecules under Theta conditions. Structural properties as well as the diffusivity and zero-shear-rate intrinsic viscosity of polymers with varied degrees of branching are analyzed. Results for the free-draining case are compared to and found in very good agreement with the Rouse model predictions. Translational diffusivity is evaluated and the difference between the short-time and long-time behavior due to dynamic correlations is observed. Incorporation of hydrodynamic interactions is found to be sufficient to reproduce the maximum in the intrinsic viscosity versus molecular weight observed experimentally for dendrimers. Results of the nonequilibrium Brownian dynamics simulations of dendrimers and linear chain polymers subjected to a planar shear flow in a wide range of strain rates are also reported. The flow-induced molecular deformation of molecules is found to decrease hydrodynamic interactions and lead to the appearance of shear thickening. Further, branching is found to suppress flow-induced molecular alignment and deformation.     

Benchmark thermodynamic properties of alkanediamines: Experimental and theoretical study

Vapor pressures of (dl)-1,2-propanediamine and 2-methyl-1,2-propanediamine were measured using the transpiration method. Molar enthalpies of vaporization were derived from the vapor pressure temperature dependence. Thermodynamic data on alkanediamines available in the literature were collected and treated uniformly. Consistency of the experimental data set for alkanediamines was evaluated with group-contribution and quantum-chemical methods.The standard molar entropy of formation and the standard molar Gibbs function of formation have been calculated. Vaporization and formation enthalpies of alkanediamines of benchmark quality are recommended for practical thermochemical calculations and validation of empirical and theoretical methods.