Band 46, Heft 1-2

A method for the calculation of the thermodynamic functions in the three-dimensional using model near the phase transition is proposed. In the frame of a uniform approach, the critical exponents and the critical amplitudes of the main characteristics of the model (i.e. free energy, enthropy, specific heat, etc.) are obtained as functions of microscopic Hamiltonian parameters. It is shown that the correct sign and temperature behaviour of the entropy are provided due to the contribution of the long-wave spin moment modes. The short-wave ones are responsible for the values of the critical exponents.

An ion-dipole model of electrolyte solutions in the bulk case and near a charged or uncharged hard wall is considered. A method to derive the terms of optimized cluster expansions for the distribution functions of ions and dipoles which provides a set of approximations beyond the mean spherical approximation is given. The third cluster coefficient approximation is investigated

The analytic solution of the mean spherical approximation obtained by us previously for the ion-dipole system in a neutralizing background is applied to study the structural, thermodynamic and dielectric properties of sodium-ammonia solutions. It is shown that the structure of ammonia is closepacked and changes little with ion concentration. The shape of the ion-ion distribution function changes from solvation behaviour with Debye-like asymptotics at low ion concentration to density ordering in the metalic region.

The expansion of the entropy into one-body, two-body, three-body, etc. contributions was applied to estimate the excess entropy of the hard sphere fluid. Configuration samples provided by computer simulation were used to determine the two-particle and three-particle correlation functions. The results show that even at intermediate densities a non-negligible part of the structural information is represented by four-body and higher order correlations.

The derivation of an intermolecular potential function for the interaction of two hydroxylamine molecules, based on ab initio SCF-ECP calculations is reported. Ab initio values were compared for selected geometries with those obtained from calculations with much larger basis sets, inluding electron correlation effects by the CEPA method. A total 658 energy values were then fitted to an analytic sum of atom-atom isotropic pair potential functions, whose functional form was given a simple electrostatic interpretation. The major difficulties, arising from the relatively low values of the stabilisation energies of the system and the numerous possibilities to form hydrogen bonds, were overcome by a careful selection of sufficient points on the potential hypersurface, introduction of cut-offs and weight factors. The obtained function was seen to be able to give a good reproduction of the interaction energies and proved suitable in a preliminary MC simulation

(HDO) 2 can form two isomeric species, HOD.OHD and DOH.OHD. The species are studied in terms of the Bopp-Jancsö-Heinzinger potential. The deuterium-bonded structure is stabilized in zero-point vibrational energy by about 0.77 kJ mol -1 over the hydrogen-bonded one. In gas-phase the exactly equal stability of both structures would however be reached at a temperature of 638 K. The thermodynamic functions of HOD.OHD and DOH.OHD are different and, moreover, they essentially differ from the (overall) functions belonging to the equilibrium mixture of both structures (which is understood as one pseudospecies). For the heat capacity at constant pressure it is shown that a maximum of the difference between the pseudospecies' overall value and the HOD.OHD value appears at a temperature of about 39 K, amounting to some 4.6 J K -1 mol -1

With the low temperature matrix techique we have observed different isolated IR OH vibration bands caused by van der Waals forces of single atom groups of the matrix molecules. With a series of different perhalogenated matrix molecules these different OH band frequency shifts Δv OH can be correlated to OH interactions with C-F, C-Cl, C-Br resp. C-I groups. At higher temperatures T (solution spectra) the band positions of the different interactions become more similar because dv/d T increases with increasing interaction. This result clarifies why at room temperature band shoulders are observed and correlates these clearly to the different matrix bands and therefore to different interactions. These experiments demonstrate that intermolecular van der Waals potentials of asymmetric molecules cannot be approximated by one spheric potential but are angle dependent. This indicates the complexity of describe condensed media.

The change in the frequency of the CH stretching vibration and in its anharmonicity constant, quadratic, cubic and quartic force constants and the equilibrium CH bond distance, on condensation, were investigated by performing model calculations for liquid chloroform and fluoroform. The interactions between the hydrogen atom and the CH oscillator, represented by a Morse potential, and the halogen atoms of its neighbor molecules were described by the sum of Lennard- Jones and Coulomb potential functions employed in molecular dynamics simulation. The calculations were carried out for different molecular arrangements in the liquid and have shown that mainly the size of the halogen atom, consequently the intermolecular distances, govern the direction of the CH vibrational frequency shift and of the change in the anharmonicity constant on the vapor-liquid transition. The pressure and temperature dependence of the CH stretching vibration was also studied for liquid chloroform. While the calculated pressure dependence is in good agreement with that observed, only the direction of the temperature dependence is in accordance with experimental observation.

The liquid-vapor isotope fractionation factors, LVIFF, α = CD 3 X/CH 3 X) 1 /(CD 3 X/CH 3 X) v , of CD 3 H-CH 4 and CD 3 F CH 3 F mixtures have been measured from 102 160 K and 182 254 K, respectively. The results are in good agreement with previous vapor pressure ratios, VPIE, which cover the ranges 90 112 K and 132 211 K, respectively. Over the temperature ranges of the LVIFF measurements, ln α is negative for the CD 3 H-CH 4 mixture and positive for the CD 3 F-CH 3 F mixture. There is significant curvature in a plot of T 2 In α vs. T for the CD 3 H-CH 4 mixture. The curvature is related to the densities of the liquid and vapor phases. It is shown that an expansion of the pair correlation function up to powers of the square of the densities of the liquid and vapor phases accounts for the T 2 In α vs. T curvature and leads to In a equal to zero at the critical temperature. Correction of the LVIFF values of T 2 In a for the density effect gives values for the contributions of translation, rotation and internal vibration in good agreement with previous cell model calculations. Similar treatment of the LVIFF data on the CD 3 F-CH 3 F mixture leads to the conclusion that the internal zero point energy effect on the differences in the heats of vaporization of CH 3 F and CD 3 F makes a small contribution to the overall difference in free energies of vaporization.

The use of Reverse Monte Carlo simulation, a novel method of structural modelling, looks very promising for the case of metallic glasses. In this paper initial results are shown for glassy Ni 2 B, using experimental radial distribution functions as input information.

Neutron diffraction measurements on aqueous 2-14 molal urea solutions have been made using nitrogen isotope substitution on the urea and hydrogen isotope substitution on the water. In addition, the hydration region of the tetramethylammonium ion in 2.0 m urea solution has been investigated by nitrogen isotope substitution on the TMA at 0.5 m and 2.0 m concentration of TMAC1. The N-water correlation functions are very similar across the concentration range, and the H-H correlation function in concentrated urea solution is little changed from that of pure water. In the TM AC1 solutions the hydration peak is unchanged by the addition of 2.0 m urea. Thus urea appears to fit into the water network without causing significant perturbation of the average water structure or of the hydration region of another solute. In addition to the study of urea we have also made preliminary measurements to test the feasibility of carbon isotope substitution in liquid methanol.

Pulsed neutron diffraction measurements are reported on 2 molar aqueous solutions of Pr 3 + , Nd 3 + , Tb 3 + , Dy 3 + , Tm 3 + , and Yb 3+ in D 2 O at room temperature. The first order difference technique on isomorphous ions is used to derive the metal ion pair correlation functions in these systems. The metal-oxygen and metal-deuterium distances and the hydration numbers are determined. The data indicate that the hydration number changes from ~ 10 for the lighter ions to ~8 for the heavier ones in the series. The water dipole is orientated on the average at an angle of 10 ~ 20 degrees to the metal-oxygen vector; these values are significantly smaller than those found for mono- and di-valent ions. The present results are compared with other results from neutron diffraction with isotope substitution, X-ray diffraction, and extended X-ray absorption fine structure.

Oxygen-17 enriched methanol CD 3 17 OH was synthesized and 1 H as well as 17 O nmr relaxation rates were measured in the temperature range 180-310 K. By varying the 17 O-content different contributions to the proton relaxation rate could be separated and from the 17 O-H dipolar interaction contribution the correlation time of the OH bond was determined. These results are compared to recent computer simulation data. Additional deuteron relaxation measurements on CH 3 OD yielded the quadrupole coupling constant which increases with falling temperature. The 17 O quadrupole coupling constant shows a similar trend with temperature but not as pronounced.

Molecular dynamics simulations with pair interactions reproduce experimental excess properties of methanol-water mixtures. Water molecules lose, and methanol molecules gain neighbours in the mixtures as compared to the solvents. The water-methanol mixture with 0.25 mole fraction of methanol, resulting in extreme values for different excess properties, is characterized by the highest number of molecules with maximal number of H-bonded neighbours.

This review compares results of neutron and X-ray diffraction experiments with computer simulation and theoretical calculation for aqueous electrolyte solutions at the atomic level in terms of the partial radial distribution functions of several ionic solutions, and includes results for the ion-water and ion-ion structure of systems containing alkali ions, alkaline earth ions, transition metal cations and a few anions

A Monte Carlo simulation of ammonium aqueous solution at infinite dilution shows that this ion is on the average rather loosely bonded to three of the fourteen water molecules present in its first hydration shell. This result agrees with conclusions suggested by recent experiments on the rotational mobility of ammonium in liquid water.

A Monte Carlo simulation has been performed for a magnesium ion in 18.45 mol% aqueous ammonia solution at 20 °C, using additive pairwise potential functions obtained from ab-initio calculations. The structural arrangement of the solvent molecules in the vicinity of the ion is discussed in terms of radial distribution functions and average molecular orientations. It was found that the first solvation shell of the ion consists of four water and three ammonia molecules, and the second shell of nine water and five ammonia molecules.

Hydrated lanthanide(III) ions Ln 3 + (H 2 O) 8 with Ln 3 + = La 3 + to Yb 3+ have been studied using ab-initio methods. The binding energy as a function of ion-water distance and position of the ion in the rare earth series is discussed. The calculations are related to recent experimental results elucidating the hydration behaviour of lanthanide(III) ions. Since no S-shape behaviour is found for complex binding energies or metal-oxygen distance, the calculations indicate that observed thermodynamic anomalities should come from hydration number changes due to the different ionic radii within the series. The non-additivity of the ion-water interactions has been calculated and it is found that its relative importance increases from 26% for La 3+ to 32% for Ln 3 +

The conductance of Nal and Et 4 NI in methanol-acetonitrile and methanol-water mixtures was measured at 25 ± 0.005 °C for the whole range of the solvent compositions, the salt molarity ranging from 5 • 10 -5 up to 1 • 10 -2 . Several equations describing the influence of the salt concentration on the equivalent conductance are examined and the Fuoss-Hsia equation with the Fernandez-Prini parameters is found to be the most appropriate one for systems with weak ionic association. Variations with the solvent composition of the limiting equivalent conductance, the distance between ions forming ion pairs and the association constant are discussed. Nonmonotonous changes of the association constant are concluded to be a feature of microheterogenous systems.

For heavy water solutions of LiCl, NaCl, KCl and CsCl at 25 and 40 C and KI at 25 °C the values of B(M + ) and B(X - ) in the equation ]η/η 0 = 1 + A √c + [B(M + ) + B(X - )] c were determined under the conventional assumption B(K + ) = ß(Cl - ). Here η 0 is the viscosity of the solvent and c the molarity. The solvent isotope effect on the B values was found to be small. The B values for the medium-sized ions K + , Cs + , Br - , and I - were systematically more negative in heavy water than in light water. This is consistent with the idea that negative B values indicate the ability of the ion to break the structure of the water, the structure of heavy water being more developed than that of light water. For the tetraalkylammonium ions, Me 4 N + , Pr 4 N + , and Bu 4 N + , on the other hand, the solvent isotope effect was small and random irrespective of the ion size. The temperature coefficients of B for the monatomic ions in heavy water were the same as for those in light water.

Conductance measurements of citric acid and neutral citrates (tri-lithium citrate, tri-sodium citrate and tri-potassium citrate) were performed in water at 278.15 to 308.15 K. The equilibrium constants for the primary and secondary steps of dissociation, Kx and K2, and the limiting conductances, λ 0 (H 2 Cit - ), λ°(1/2 HCit2 - ), and λ 0 (1/3 Cit3 - ) are reported as a function of temperature. They are obtained by application of the Quint and Viallard conductance equation. The enthalpies of dissociation are estimated.

The experimental values of density, viscosity, electric conductivity and diffusion coefficients of trace ions in aqueous solutions of lithium chloride in wide ranges of concentration and temperature are collected and discussed. The thermal expansivity and the activation energies of viscous flow, electric conductance and diffusion of trace ions show significant changes at a molality around 12 mol/kg, beyond which the hydration requirement is not satisfied

In molecular dynamics simulations of molten LiCl and LiBr at about 1000 K and various pressures the self-exchange velocity of unlike ion pairs divided by the molar volume increases with increasing pressure up to ca. 1000 MPa, reflecting the trend experimentally found for the electric conductivity up to 100 MPa. While the coordination of unlike ions is tetrahedral at low pressures, it is nearly octahedral at 2000 MPa.

The structural and dynamic properties of molten lithium iodide are investigated at two pressures (334 MPa at 784 K and 1054 MPa at 915 K), using molecular dynamics simulations with Born- Mayer-Huggins type pair potentials. On increasing the pressure, the local packing of the ions changes from tetrahedral to octahedral, the self-exchange velocity in the coordination shells decreases by a factor of 0.031, D Li by a factor of 0.033 and D 1 by a factor of 0.021.

The question of the structure of the electrical double-layer at electrodes in contact with electrolyte solutions has several aspects in common with the long and short-range structure of electrolyte solutions. Measurements of the capacitance of the double-layer have traditionally provided information on this question for the case of dilute electrolyte solutions. With proton hydrates from H 3 O + through H 9 O 4 + to 1 M aqueous H + solution made by stoichiometric addition of water to the acid CF 3 SO 3 H, the transition between "molten-salt" (H 3 O + CF 3 SO 3 above 307 K) to "dilute-solution" behaviour of the interphasial double-layer at Hg and Au electrodes can be investigated. The H 3 O + CF 3 SO 3 is a highly conducting molten salt above 307 K. The results indicate that the double-layer capacitance at potentials negative to the RHE, both at Hg and Au, in the H 3 O + molten salt has values surprisingly similar to that in 1 M aq. H + solution. Intermediate states of hydration of the proton (H 5 O 2 + , H 7 O 3 , H 9 O 4 + ) have higher values, especially at Au. However, the capacitance behaviour is quite different from that observed in high-temperature, alkali-metal molten salts at Pb. Models are proposed for the structures of the double-layers in these proton-hydrate systems, and the problem of accounting for a "normal" value (ca. 18 μF cm -2 ) of capacitance at Hg in the H 3 O + melt is examined by means of some model calculations. Comparative electrode-kinetic measurements on proton discharge add further information on the behaviour of the proton hydrates.

We report computer simulations of the adsorption process of water on a platinum (100) surface. At very low impact energies the sticking coefficient approaches unity in agreement with experimental evidence. The sticking coefficient for the bare surface decreases strongly with increasing impact energy already in the energy range <0.5 eV and also with increasing surface temperature. At all but the lowest impact energies the sticking coefficient increases drastically if the impact zone of the approaching molecule is in the vicinity of preadsorbed water molecule(s). The reason for this phenomenon is efficient energy transfer into water-water degrees of freedom and/or into the metal O "bond" of the preadsorbed water molecules. In the present study, where surface diffusion does not play an important role because the impact zone is chosen to be close to the adsorbed molecule or cluster, hydrogen bonds are formed between incoming and preadsorbed molecules during approach and impact. Excess energy is transferred into the lattice within a few picoseconds leading to a two-dimensional hydrogen bonded water cluster.

The adsorption of 1-hydroxyethylidene-1,1 -diphosphonic acid (HEDP) on an iron electrode in neutral aqueous solution has been examined by the radiotracer method. The role of calcium and zinc cations in the adsorption mechanism has also been studied in order to understand the synergistic effect when HEDP is applied together with these cations Summary The results obtained by 14 C labelled HEDP, 65 Zn and 45 Ca isotopes have shown that HEDP by itself forms a loosely bound adsorption layer on iron oxide surface, and that the presence of zinc or calcium ions in solution leads to an increased adsorption of HEDP molecules. The maximum value in the efficiency of HEDP-Zn mixtures is achieved at about Zn/HEDP = 2 in the solution. For higher values of this ratio the ratio in the protective film remains about the same. The HEDP adsorption is monotonously enhanced with increasing amount of Ca 2+ ions in the solution, Ca/HEDP on the metal surface being unity. These results indicate a precipitation mechanism for inhibition in both cases. Different weakly soluble complex compounds with zinc or calcium ions which precipitates on the metal surface, providing a protective layer may be formed by HEDP.

The thermodynamic theory for a fluid mixture in a gravitational field under a given pressure is developed. The local density and the local molfraction are varied to minimize the Gibbs free energy of the system. With the approximation of local thermodynamics, a set of coupled linear differential equations of first order is obtained. The non-local van der Waals-Landau contributions, leading to a set of linear differential equations of second order, are discussed for the two component lattice gas. Experimental methods allowing to determine concentration and density gradients are discussed also.

The dissolution of cubic crystals of NaF, KF, CsF, LiCl, NaCl, and KCl consisting of 32 cations and 32 anions in an isolated box containing 216 water molecules was studied at 298 K by molecular dynamics simulations. The ion-ion, ion-water and water-water interactions were described in terms of the Tosi-Fumi, Kistenmacher-Popkie-Clementi, and Matsuoka-Clementi-Yoshimine potentials, respectively. During the simulation periods of 12 ps for NaF, CsF and LiCl and 20 ps for KF, NaCl and KCl cations did not dissolve, while anions dissolved from the CsF, LiCl and NaCl crystals but not from the NaF, KF and KCl crystals. The mass effect in the dissolution of CsF was examined by giving the ceasium ions the atomic weight of the fluoride ion (18.998). In case of the "light" caesium ions in the crystal fluctuated less far and again fluoride ions but no caesium ions were dissolved.

Non-equilibrium crystallization was simulated on a two dimensional square lattice. Several clusters were grown simultaneously by using the model of diffusion limited aggregation. The growing process was reversible, i.e. dissolution of particles from the boundary of any cluster was made possible. The rate of growth and dissolution was determined by a stochastic method. The simulation resulted in an aggregate pattern having a few large and several small clusters. The fractal dimensions of the large clusters were found in the range of D = 1.62-1.72.

The electric conductivities of molten (Na, Ag)Cl, (Rb.Ag)Cl, (Na,Ag)Br, (Rb. Ag)Br, and (Cs, Ag)Br are determined as functions of composition and temperature from the melting point to 1100 K. The relation of the temperature coefficients to the phase diagram of the systems (K.Ag)Cl, (Rb. Ag)Cl, (Cs, Ag)Cl, and (Cs, Ag)I near and at the phase transition (liquid-solid) is studied. The molar conductivities and mobilities of the cations with reference to the common anion are evaluated.