Effect of water–methanol content on the structure of Nafion in the sandwich model and solvent dynamics in nano-channels; a molecular dynamics study

Continuing an ongoing study, molecular dynamics (MD) simulations were performed to investigate the effects of methanol concentration on Nafion morphology, such as the size of solvent cluster, solvent location, and polymer structure via the sandwich model. Our survey shows that high methanol concentrations resulted in increment of solvent cluster size in Nafion membrane. The sulfonic acid clusters also befall much in order as subsequent layers of such ionic clusters are formed. The number of neighbouring hydronium ions around a sulfur atom is independent of methanol concentration, but the first shell of hydronium and water around sulfonic acid clusters is broader. Although methanol would prefer to interact with water molecules rather than sulfonic acid groups, gathering of methanol molecules via hydrophobic self-aggregation is preferred. Methanol is located closer to the hydrophobic part of the polymer than water, while water is located closer to the hydrophilic part of the polymer. It was found that methanol distributes specifically more than water in nano-channels. Investigation of solvent dynamics in nano-channels shows that diffusion coefficients (D) of water, methanol, and hydronium decrease with increasing methanol concentration and they may be ordered as follows: D _Water?>?D _Methanol?>?D _Hydronium (D _Water?≈?1.6–2.0D _Methanol and D _Methanol?≈?2.1–3.0D _Hydronium).     

The equilibrium geometry of the SC3H radical: an ab initio study

The SC₃H radical is known by experiment to have a linear equilibrium structure, but even rather high-level ab initio computations give a bent equilibrium geometry. A theoretical study of the SCCCH radical has been carried out in order to analyse the influence of several factors in the computed equilibrium structure. Quadratic configuration interaction QCISD(T) and restricted coupled cluster RCCSD(T) computations have been performed in combination with large basis sets. Spin-orbit effects have been taken into account through the Breit-Pauli Hamiltonian using multi-configuration SCF and configuration interaction wavefunctions. Our final results indicate that the equilibrium structure must be linear, in agreement with the experimental studies [McCarthy, M. C., Vrtilek, J. M., Gottlieb, C. A., Wang, W., and Thaddeus, P., 1994, Astrophys. J., 431, L127; Hirahara, Y., Ohshima, Y, and Endo, Y, 1994, J. chem. Phys., 101, 7342]. Both spin-orbit and electron correlation effects appear to be of comparable importance, but an adequate computation of the correlation energy has been much more difficult and has ultimately required basis set extrapolations.     

Order-Disorder Transition in A(B'B')O3 Perovskite Type Ceramics with Second Nearest Neighbour Interactions

The approach based on an order-disorder transition model combined with the coulomb electrostatic interaction method allows investigating of the order-disorder phase transition in A(B′B′)O₃ complex perovskite type ceramics. An analytical analysis is given on how the second nearest neighbour interactions are connected to the quantities characterising the phase transition. While, the criterion proposed by Zhang et al. (Zhang, X.W., Wang, Q. and Gu, B.L. (1991). Study of the order-disorder transition in A(B′B′)O₃ perovskite type ceramics. J Am Ceram Soc., 74(11), 2846-2850.) in order to predict the phase transition and the behaviour of the curve which describes the quantity ?′ F ′₀ T corresponding to order parameter in function of the ionic radius variation and ionic radius ratio are discussed.     

Transport and relaxation properties of isotopomeric hydrogen–helium binary mixtures. II. HD,D2, T2–He mixtures

The comprehensive comparison between calculated bulk non-equilibrium properties of hydrogen–helium isotopomeric mixtures and experiment that has previously been carried out for H₂–helium mixtures [2004, Molec. Phys., submitted] has been extended to mixtures of HD, D₂ and T₂ with ³He and ⁴He. For HD–⁴He mixtures, comparison is also made, where possible, with previous calculations of Köhler and Schaefer [1983, Physica A, 120, 185]. The phenomena examined herein include low temperature interaction second virial coefficients, binary diffusion and thermal conductivity coefficients, rotational relaxation, transport property field effects and flow birefringence. Scattering calculations have been carried out for the HD–He PES of Schaefer and Köhler [1985, Physica A, 129, 469], and for both the Köhler–Schaefer and Tao [1994, J. chem. Phys., 100, 4947] potential surfaces for the D₂–He and T₂–He interactions. Comparisons between calculated and experimental results for HD, D₂, T₂–He mixtures confirm the conclusion, reached earlier from the H₂–He comparisons, that these potential surfaces are very close to the correct one for the hydrogen–helium interaction, and that the small differences between them cannot be distinguished readily by measurements of bulk gas phenomena unless the attendant experimental uncertainties are better than ±0.3%.     

Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling

ABSTRACT

Neutron diffraction measurements of H/D isotopic substitution have been performed for seven H/D substituted methanol-water mixtures of 0.3?mol fraction of methanol (x_M) under the supercritical (618?K, 100?MPa) and ambient (298?K, 0.1?MPa) conditions. The seven structure factors obtained were subjected to an empirical potential structure refinement (EPSR) modelling to derive all site-site pair correlation functions, coordination number distributions, spatial density functions, and cluster distributions. Water has a four coordinated structure in the first coordination shell under both ambient and supercritical conditions; however, the spatial density distribution of water molecules in the second coordination shell is delocalised under the supercritical condition. The mean coordination number of all atomic pairs with hydrophilic interactions decreases in the supercritical state. On the other hand, the mean coordination number of interactions between the hydrophobic part of methanol and water molecule is less sensitive to temperature. In the supercritical condition, water clusters with a wide size distribution are generated in a methanol-water mixture as well as in pure water. Since the critical temperature of a methanol-water mixture is lower than that of pure water, it can be concluded that the addition of methanol can generate fragment water clusters at a lower temperature.     

Effect of an ionic impurity on the caloric curves of water clusters

The equilibrium heat capacities of model pure and heterogeneous water clusters have been calculated using exchange Monte Carlo simulations. For the pure water cluster (H₂O)₂₀, microcanonical and canonical caloric curves obtained from various rigid intermolecular potentials indicate the onset of melting to lie in the range 140–180 K, in reasonable agreement with previous estimates. Clusters doped with a single hydronium or ammonium impurity show a significant shift of the melting point in the 20-molecule system, but a reduced effect when 50 molecules are reached.     

Spectroscopic Characterization of Eu(III) Complexes with New Monophosphorus Acid Derivatives of H<Subscript>4</Subscript>dota

The luminescence lifetimes of europium(III) complexes with new monophosphorus acid derivatives of H₄dota were measured by means of time-resolved laser-induced luminescence spectroscopy in H₂O and D₂O. The hydration numbers of these complexes were estimated using different empirical equations [Horrocks and Sudnick (1979) J. Am. Chem. Soc. 101 (1979) 334; Choppin and Barthelemy(1989) Inorg. Chem. 28, 3354–3357; Choppin and Bünzli Lanthanide probes in life, chemical and earth sciences. Theory and practice (1989); Kimura and Kato J. Alloys Comp. 275–277 (1998) 806; Parker (1999) J. Chem. Soc., Perkin Trans. 2, 493–503; Supkowski and Horroks (2002) Inorg. Chim. Acta. 340, 44–48]. It was shown that all the relationships gave similar results with a satisfactory precision. The hydration numbers of complexes of H₃do3a and H₄dota agreed with the literature values. One water molecule is coordinated in complexes of the new ligands. The results showed that the Choppin formula based on measurements only in H₂O can be satisfactorily used for estimation of the hydration numbers.     

Vibrational spectra of α-molybdic acid-MoO3.H2O

Their and Raman spectra of molybdenum trioxide—monohydrate are studied assuming an effective tetrahedral Mo-O coordination and isolated water molecules, although the crystallographic coordination is six-fold with two long Mo-O distances. Based onCt symmetry, the group theoretical analysis has been carried out and a vibrational assignment is proposed. The nature of hydrogen bonding and the librational modes of water molecules are discussed. The factor group splitting forv3, the asymmetric stretching mode of MoO₄ ⁼ ion, is large indicating strong interchain coupling.     

An accurate,global, ab initio potential energy surface for the H+ 3 molecule

A new global, ground-state, Born-Oppenheimer surface is presented for the H⁺ ₃ system. The energy switching approach has been used to combine different functional forms for three different regimes: a spectroscopic expansion at low energy, a Sorbie-Murrell function at high energy and known long-range terms combined with accurate diatomic potentials at large separations. At low energies we have used the ultra high accuracy ab initio data of Cencek et al. (1998, J. chem. Phys., 108, 2831). At intermediate energy we have calculated 134 new ab initio energies using a high accuracy, explicitly correlated procedure. The ab initio data of Schinke et al. (1980, J. chem. Phys., 72, 3909) has been used to constrain the high energy region. Two fits are presented which differ somewhat in their behaviour at energies over 45 000 cm^?1 above the H⁺ ₃ minimum. Below this energy, the fits reproduce each set of ab initio data close to their intrinsic accuracy. The ground state surface should provide a suitable starting point for renewed studies of the near-threshold photodissociation spectrum originally reported by Carrington et al. (1982, Molec. Phys., 45, 753).     

Solid-liquid phase transition in small water clusters: a molecular dynamics simulation study

Water clusters, (H₂O) _n , of varying sizes (n = 8, 12, 16, 20, 24, 28, 32, 36, and 40) have been studied at different temperatures from 0 to 200 K using molecular dynamics simulations. Transitions between solid and liquid phases were investigated to estimate the melting temperature of the clusters. Although the melting temperatures showed non-monotonic behaviour as a function of cluster size, their general tendency follows the classical relationship T _m ∝ n ^?1/3 to the cluster size n. Moreover, it was observed that the liquid-solid surface tension decreased with the cluster size in a similar way to the liquid-vapour surface tension in bulk water. Upon cooling, ice-like crystals were formed from the smaller clusters with n up to 20, while the larger clusters were transformed to glassy structures. The decrease in the glass transition temperature with the cluster size was observed to be much less than the corresponding melting temperature. The mutual order of the melting and glass-transition temperatures were found to be reversed compared with that observed for bulk water.     

Equation of state for systems of soft diatomic molecules

A new potential that is a modification of the BBL (Bratko, D.,Blum, L., and Luzar, A.,1985, J. chem. Phys., 83, 6367; Blum, L., Vericat, F., and Bratko, D., 1995, J. chem. Phys., 102, 1461) potential and of the one recently solved analytically by Blum and Vericat (BV) (1995, Molec. Phys., 86, 809; 1996, J. phys. Chem., 100, 1197) is studied by Monte Carlo simulation. The main feature of this potential is that it can be solved using only a small number of parameters (3 in the case treated by BV), and therefore produces a substantial simplification of earlier work. The new potential has an orientational octupole–octupole interaction term which is found necessary to reproduce the broad peak of the oxygen–oxygen structure function due to the tetrahedral position of the second nearest neighbour water molecule. This important feature was absent in the original BBL potential. This model agrees also with the experimental pair correlation functions for oxygen–hydrogen and hydrogen–hydrogen, and yields 42·6 kJ mol^-1 for the internal energy of water, also in agreement with experiment. The hard core central repulsion causes the sharpness of the first peaks in all three correlation functions. This is not necessary but convenient for an analytical solution.     

Anomalous Ortho-to-Para Ratio of Nuclear Spin Isomers of H2O at Low Temperatures

A theoretical model proposed for nuclear spin isomers of H₂O molecules located inside the C₆₀ fullerene explains an anomalously high stability of ortho-H₂O isomers detected in the experiments reported in [B. Meier et al., Nature Commun. 6, 8112 (2015)] at a temperature of T = 5 K.

     

Absolute line intensities in the ν1 + 3ν3 band of 12C2H2 by laser photoacoustic spectroscopy and Fourier transform spectroscopy

Line intensities and self-broadening coefficients in the ν₁ + 3ν₃ band of ¹²C₂H₂ near 0.8 μm at room temperature were measured by means of both laser photoacoustic and Fourier transform spectroscopy. An experimental protocol has been developed to obtain absolute intensities from the photoacoustic measurements. Namely, the spectrometer was calibrated using water vapour line intensities available in Hitran 1996 [L. S. Rothman et al. (1998) J. quant. Spectrosc. Radiat. Transfer, 60, 665–710]. These photoacoustic line intensities were found to be on average 5% higher than corresponding measurements performed using Fourier transform spectroscopy, the accuracy of the latter being estimated to better than 4%. The accuracy of the photoacoustic intensities is discussed. Previous results from the literature [F. Herregodts, D. Hurtmans, J. Vander Auwera, and M. Herman (1999) J. chem. Phys., 111, 7954—7960] are revised.     

Fragmentation and reliable size distributions of large ammonia and water clusters

The interaction of large ammonia and water clusters in the size range from <n> = 10 to 3 400 with electrons is investigated in a reflectron time-of-flight mass spectrometer. The clusters are generated in adiabatic expansions through conical nozzles and are nearly fragmentation free detected by single photon ionization after they have been doped by one sodium atom. For ammonia also the (1+1) resonance enhanced two photon ionization through the state with v = 6 operates similarly. In this way reliable size distributions of the neutral clusters are obtained which are analyzed in terms of a modified scaling law of the Hagena type [Surf. Sci. 106, 101 (1981)]. In contrast, using electron impact ionization, the clusters are strongly fragmented when varying the electron energy between 150 and 1 500 eV. The number of evaporated molecules depends on the cluster size and the energy dependence follows that of the stopping power of the solid material. Therefore we attribute the operating mechanism to that which is also responsible for the electronic sputtering of solid matter. The yields, however, are orders of magnitude larger for clusters than for the solid. This result is a consequence of the finite dimensions of the clusters which cannot accommodate the released energy. Received 21 November 2001     

Water-silica interaction in clusters

The interaction of water with SiO ₂ is an important problem in geophysics, materials physics, and environmental science. In this paper, we present recent results on studies of H ₂ O-silica clusters from first-principles Born-Oppenheimer molecular dynamics calculations. Bond strength and chemical stability are investigated as a function of cluster size and chemical composition. Both physisorption and chemisorption of water molecules on the clusters are discussed via analysis of energetics. Calculations of clusters are compared with the results from extended surfaces. The validity of clusters as models of surfaces is discussed.     

A computer study of the absorption spectra of the water-carbon monoxide disperse system

The IR absorption, emission, and reflectance spectra of (CO) _i (H₂O)₂₀, 1 ≤ i ≤ 10, clusters were calculated using the molecular dynamics model. After the adsorption of CO molecules by clusters, IR radiation absorption and reflection by the system composed of them weakened, whereas thermal radiation power increased. We tracked changes in spectral characteristics as the number of molecules in clusters increased. Growing water clusters decreased absorption and gradually increased IR emission power. The growth of a water cluster with the addition of CO molecules to it as a rule caused the opposite effect. On the whole, clusterization and the entrapment of CO molecules by clusters had an antigreenhouse effect.     

Constrained reaction coordinate dynamics for systems with constraints

In the context of molecular dynamics simulations of rare events, the application of constraints on a suitable reaction coordinate has often been found useful for sampling of the free energy barrier. The efficiency of these calculations is hampered by geometrical difficulties, related to the metric factor and inertial forces. Some years ago Mulders et al. [1996, J. chem. Phys., 104, 48691 suggested a way to simplify the approach. Their idea was demonstrated shortly afterwards by Sprik and Ciccotti [1998, J. chem. Phys., 109, 77371. The present paper extends these results to vector reaction coordinate and molecular systems modelled with holonomic constraints.     

Cooperative binding of drugs on human serum albumin

In order to explain the adsorption isotherms of the amphiphilic penicillins nafcillin and cloxacillin onto human serum albumin (HSA), a cooperative multilayer adsorption model is introduced, combining the Brunauer–Emmet–Teller (BET) adsorption isotherm with an amphiphilic ionic adsorbate, whose chemical potential is derived from Guggenheim's theory. The non-cooperative model has been previously proved to qualitatively predict the measured adsorption maxima of these drugs [Varela, L. M., García, M., Pérez-Rodríguez, M., Taboada, P., Ruso, J. M., and Mosquera, V., 2001, J. chem. Phys., 114, 7682]. The surface interactions among adsorbed drug molecules are modelled in a mean-field fashion, so the chemical potential of the adsorbate is assumed to include a term proportional to the surface coverage, the constant of proportionality being the lateral interaction energy between bound molecules. The interaction energies obtained from the empirical binding isotherms are of the order of tenths of the thermal energy, therefore suggesting the principal role of van der Waals forces in the binding process.     

The Structure of Water Clusters Interacting with Gaseous Acetylene

The interaction of water clusters with acetylene molecules at T = 230 K was studied by the molecular dynamics method. The structure of clusters was analyzed by constructing Voronoi polyhedra. Water clusters interacting with C₂H₂ molecules are characterized by a diversity of H-bond orientations, a more uniform distribution of H-bonds over the cluster volume, a larger number of bonds per atom, and smaller bond lengths. The spectrum of bond lengths broadens as the number of acetylene molecules interacting with the water cluster increases. C₂H₂ molecules have a pressing action on water clusters.