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The diffusion coefficient of methane in water plays an important role in the formation and dissociation of methane hydrate. However, most of the previous studies on the diffusion coefficient of methane in brine are performed at room temperature and low pressures, which is quite different from the formation condition of methane hydrate. In this study, we measure the diffusion coefficient of methane in pure water and brine in capillary tube at 10.3 MPa and temperature ranging from 283.15 to 308.15 K. We use the Raman spectrum to measure the ratio of C-H bound signal of methane to the O-H bound signal of water, to estimate the concentration of methane dissolves in water/brine. The Raman spectrum is collected at different time and different positions away from the liquid-gas interface. Diffusion coefficient is determined by fitting the experimental data with the concentration profiles solved from Fick’s second law and semi-infinity boundary condition. By this method, we can evaluate the diffusion coefficient at different temperatures or salinities. The diffusion coefficient of methane in water/brine increases as the temperature increases. The diffusion coefficient of methane in brine is lower than that in pure water. Molecular dynamics (MD) simulation is also performed in this study to calculate the diffusion coefficient of methane in water/brine. The MD results can successfully predict the tendency of temperature effect and adding electrolyte.

The entropy of water molecules is calculated in the major and minor grooves of a B-DNA duplex and in the bulk using a statistical mechanical scheme developed by Goddard and co-workers. The calculations for the water under equilibrium conditions appropriately shows that the undoubted entropy gain time T is canceled by the energetic effects in the process of transferal of water from the grooves of DNA to the bulk.

The effect of the molecular architecture of a surfactant at the decane-water interface is investigated using atomistic MD simulations. The equilibrated model systems, which consist of decane, water bulk phases, plus a surfactant layer at the interface, are prepared.

The enthalpy, entropy, and free energy of water are important physical quantities for understanding many interesting phenomena in biological systems. However, conventional approaches require different treatments to incorporate quantum and diffusive effects of water in different states of matter. In this work, we demonstrate the use of the two-phase thermodynamic (2PT) model as a unified approach to obtain the properties of water over the whole phase region of water from short (~20 ps) classical molecular dynamics trajectories. The 2PT model provides an effective way to separate the diffusive modes (gas-like component) from the harmonic vibrational modes (solid-like component) in the vibrational density of states (DoS). Therefore, both diffusive and quantum effect can be properly accounted for water by applying suitable statistical mechanical weighting functions to the DoS components. We applied the 2PT model to systematically examine the enthalpy, entropy, and their temperature dependence of five commonly used rigid water models. The 2PT results are found to be consistent with those obtained from more sophisticated calculations. While the thermodynamic properties determined from different watermodels are largely similar, the phase boundary determined from the equality of free energy is very sensitive to the small inaccuracy in the values of enthalpy and absolute entropy. The enthalpy, entropy, and diffusivity of water are strongly interrelated, which challenge further improvement of rigid water model via parameter fitting. Our results show that the 2PT is an efficient method for studying the properties of water under various chemical and biological environments [ABSTRACT FROM AUTHOR]

The structure and dynamics of the dendrion formed by second-generation Frenchet polyaryl ethereal dendrimer as the hydrophilic component and linear PTFE is presented. The water molecules are incorporated into the dendimer block of the copolymer to form a nanophase-segregated structure.

A study was conducted, using the vibrational density of state (DoS) derived from classical molecular dynamic (MD) simulations, to investigate the origin of the generation-dependent stability of the hydrogen-bond-mediated self-assembly of supermolecular dendritic structures. The result confirms that circular and linear configurations may coexist at lower generations but that only circular forms are stable at higher generations.

The study of the possible packing of various lattices via molecular dynamics simulations at the atomistic level is presented. The relative stability of various lattices packed by the soft dendrimer balls, first synthesized and characterized with different packing methods is considered.

We address the possible occurrence of a minimum extent of dissociation (α) of ionic liquid (IL) in IL-solvent mixtures. This phenomenon, known as the redissociation of IL, is responsible for many interesting composition-dependent properties in such mixtures. A thermodynamic model is developed to provide a semiquantitative prediction on the change of α with solvent concentration. It is found that the occurrence of minimum α coincides with the occurrence of a maximum in the mean activity coefficient of dissociated ions, indicating better solvation of free, dissociated ions both with decreasing and increasing solvent concentration. The favorable solvation of free ions is found to change from long-range ion-solvent dielectric polarization to ion-ion-pair dielectric polarization with decreasing solvent concentration. Therefore, the composition dependence of the IL solution dielectric constants, determined from that of the ion-pair and the solvent, is found to be the most important factor for the presence of redissociation in IL solutions. [ABSTRACT FROM AUTHOR]

The octanol-water partition coefficient (KOW) is a measure of the relative hydrophobicity and hydrophilicity of a chemical. Knowledge regarding how this value changes with the acidity (pH value) in the aqueous phase is important for understanding many biological activities. In this work, we develop a computational method to predict the effect of pH on the partition coefficient of ionizable drugs. In particular, three dissociation mechanisms were examined based on four relevant properties: the acid dissociation constant (pKa), and the octanol-water partition coefficient of the molecule in the neutral form (KOW,N), in the form of free ions (KOW,I) and in the form of an ion pair (KOW,IP). The values of KOW,N, KOW,I, and KOW,IP are predicted based on the COSMO-SAC activity coefficient model, and the value of pKa is determined using the SMD solvation model. The root-mean-square (RMS) error in the predicted log KOW,N, log KOW,IP, and pKa for 41 pharmaceuticals are found to be 0.636, 0.816, and 2.59, respectively. The predicted overall partition coefficient, as a function of pH, is also compared to experimental values whenever available. Our results show that the proposed method can provide satisfactory a priori prediction of pH dependency of KOW without input of any experimental data. This method can serve as a useful tool for modern drug delivery. [ABSTRACT FROM AUTHOR]

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We have studied two amphiphilic interfacial modifiers: low cost Cu phthalocyanine dye containing ether side chains (Cu–ph–ether dye) and a carboxylic acid- and bromine-terminated 3-hexyl thiophene oligomer (oligo-3HT-(Br)COOH, Mw∼ 5K) to enhance the interfacial interaction between poly(3-hexyl thiophene) (P3HT) and TiO2nanorods. A large improvement in the performance of fabricated solar cells was observed using these relatively large molecular modifiers when compared to pyridine-modified TiO2nanorods. UV-vis spectroscopy and X-ray photoelectron spectroscopy analyses reveal that the modifiers are adsorbed and chemically bonded to TiO2through unshared electrons associated with the modifiers. Furthermore, the new modifiers increased the hydrophobicity of TiO2with the order of oligo-3HT-(Br)COOH > Cu–ph–ether dye > pyridine. Synchrotron X-ray spectroscopy studies of the modified hybrid films indicate the crystallinity of P3HT is increased, following the same trend as the hydrophobicity, because the new modifiers function as plasticizers, increasing the flow characteristics of the film. Moreover, the same trend is also observed for the reduced recombination rate and increased lifetime of charge carriers in the device by transient photo-voltage measurement. Thus, the oligo-3HT-(Br)COOH outperforms the Cu–ph–ether dye and pyridine in enhancing the power conversion efficiency (PCE, η) of the solar cell. More than a two-fold improvement is shown compared to pyridine. The results are due to the large size, conductivity, and polar characteristics of the oligo-3HT-(Br)COOH unit, which facilitates both the crystallization of P3HT and the electron transport of the TiO2nanorods. This study provides a useful route for increasing the efficiency of hybrid solar cells viathe enhancement of interfacial interactions between organic donors and inorganic acceptor materials. [ABSTRACT FROM AUTHOR]

We present the prediction of vapor−liquid−liquid equilibrium (VLLE) from a newly developed PR+COSMOSAC equation of state (EOS). Unlike semiempirical thermodynamic models whose parameters vary with the data used in regression and reparameterization are usually necessary for describing VLLE, this theoretically sound model with its parameters determined from first-principles calculations is not biased against descriptions of either VLE or LLE and can thus be used to predict the occurrence conditions of VLLE. In this model, the molecular interaction and size parameters in the Peng−Robinson EOS are determined directly from a solvation model based on first principle solvation calculations. This new EOS contains neither species dependent parameter nor binary interaction parameters and can be used to predict vapor pressure, liquid density, critical properties of pure substances, and VLE and LLE of mixtures. Without adjustment to the model parameters, this method can predict VLLE with accuracy similar to that of the method modified UNIFAC model. This model is particular useful for the design of new processes involving chemicals whose interaction parameters are not available due to the lack of experimental data. [ABSTRACT FROM AUTHOR]

A simple method is proposed to improve the accuracy in prediction of drug solubility using the COSMO-SAC model. In this method, the data of solubility of drug in a pure solvent is used to determine any error in the COSMO-SAC model for the drug−solvent interactions. These values are then used to correct for the solubility in the mixture of solvents. We have examined this method for the solubility of 33 drug compounds in 127 different solvent mixtures (235 systems, 1955 data points) over a temperature range of 0−50 °C and a wide solubility range of 10−1−10−6(mole fraction scale). The error in the predicted solubility reduces significantly from 442% (original COSMO-SAC) to only 88% (COSMO-SAC with corrections), making it a practical approach for drug development. The accuracy of this method found to be comparable to another predictive method, NRTL-SAC. [ABSTRACT FROM AUTHOR]

Presented here is the two-phase thermodynamic (2PT) model for the calculation of energy and entropy of molecular fluids from the trajectory of molecular dynamics (MD) simulations. In this method, the density of state (DoS) functions (including the normal modes of translation, rotation, and intramolecular vibration motions) are determined from the Fourier transform of the corresponding velocity autocorrelation functions. A fluidicity parameter (f), extracted from the thermodynamic state of the system derived from the same MD, is used to partition the translation and rotation modes into a diffusive, gas-like component (with 3Nfdegrees of freedom) and a nondiffusive, solid-like component. The thermodynamic properties, including the absolute value of entropy, are then obtained by applying quantum statistics to the solid component and applying hard sphere/rigid rotor thermodynamics to the gas component. The 2PT method produces exact thermodynamic properties of the system in two limiting states: the nondiffusive solid state (where the fluidicity is zero) and the ideal gas state (where the fluidicity becomes unity). We examine the 2PT entropy for various water models (F3C, SPC, SPC/E, TIP3P, and TIP4P-Ew) at ambient conditions and find good agreement with literature results obtained based on other simulation techniques. We also validate the entropy of water in the liquid and vapor phases along the vapor−liquid equilibrium curve from the triple point to the critical point. We show that this method produces converged liquid phase entropy in tens of picoseconds, making it an efficient means for extracting thermodynamic properties from MD simulations. [ABSTRACT FROM AUTHOR]

A novel approach combining first-principles solvation calculations and cubic equations of state is proposed for the prediction of phase equilibria of both pure and mixture fluids. The temperature and composition dependence of the energy parameter, a(T,x), in the EOS is determined from the attractive contribution to the solvation free energy. The volume parameter, b(x), is estimated to be the mole-fraction-weighted average volume of the molecular solvation cavity. This approach does not require the input of any experimental data (e.g., critical properties or acentric factor) for pure components and does not presume any composition dependence of the energy parameter. The Peng−Robinson EOS combined with a solvation model based on COSMO-SAC calculations, denoted as PR+COSMOSAC, is used to illustrate the applicability of this approach. It is found that the relative error from PR+COSMOSAC is 48% in vapor pressure, 21% in liquid density at the normal boiling point, 10% in critical pressure, 4% in critical temperature, and 5% in critical volume for 1295 pure substances and 27.56% in pressure and 5.18% in vapor-phase composition for 116 binary mixtures in vapor−liquid equilibrium. The errors in binary mixtures can be reduced significantly to 6.24% and 2.25% if experimental vapor pressures are used to correct for any errors in the calculated charging free energies of pure species. [ABSTRACT FROM AUTHOR]

The article presents a method for estimating cubic equation parameters of state based on ab initio solvation calculations. The two parameters, temperature-dependent interaction and volume, are calculated by looking at the solvation free energy and solvation cavities. The only parameters necessary for using this technique are the atomic radius, the dispersion coefficient, and electrostatic and hydrogen-bonding interactions. The parameters calculated in this manner, called equations of state (EOS), can express the entire fluid phase diagram. The technique is useful for any chemical about which experimental data has not been published.