Thermodynamic characteristics of small water clusters

Physicotechnical Institute of Low Temperatures, Academy of Sciences of the Ukrainian SSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 29, No. 5, pp. 89–94, September–October, 1988.     

Thermodynamic anomalies in a lattice model of water: solvation properties

We investigate a lattice-fluid model of water, defined on a three-dimensional body-centered-cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified by removing distinction between "donors" and "acceptors." We focus on the solvation properties, mainly as far as an ideally inert (hydrophobic) solute is concerned. As in our previous analysis, devoted to neat water [J. Chem. Phys. 121, 11856 (2004)], we make use of a generalized first-order approximation on a tetrahedral cluster. We show that the model exhibits quite a coherent picture of water thermodynamics, reproducing qualitatively several anomalous properties observed both in pure water and in solutions of hydrophobic solutes. As far as supercooled liquid water is concerned, the model is consistent with the second critical-point scenario.     

Thermodynamic properties of internal water molecules in the hydrophobic cavity around the catalytic center of cytochrome c oxidase

Cytochrome c oxidase is a redox-driven proton pump that creates a membrane proton gradient responsible for driving ATP synthesis in aerobic cells. The crystal structure of the enzyme has been recently solved; however, the details of the mechanism of its proton pumping remain unknown. The enzyme internal water molecules play a key role in proton translocation through the enzyme. Here, we examine the thermodynamic properties of internal water in a hydrophobic cavity around the catalytic center of the enzyme. The crystal structure does not show any water molecules in this region; it is believed, however, that, since protons are delivered to the catalytic center, where the reduction of molecular oxygen occurs, at least some water molecules must be present there. The goal of the present study was to examine how many water molecules are present in the catalytic center cavity and why these water molecules are not observed in the crystal structure of the enzyme. The behavior of water molecules is discussed in the context of redox-coupled proton translocation in the enzyme.     

Investigation of properties of polymeric composition materials around a heterogeneous matrix

The results of microstructure analysis of polymeric material around an epoxy oligomer cured by diaminodiphenylsulfone, as well as of thermoplastic polymers like polysulfone and polyarylene ester ketone, are presented. Calculation of the operating life of items produced from this material during multicycle loading is carried out.     

Thermodynamic properties of N-octyl- and N-dodecylnicotinamide chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl- and N-dodecylnicotinamide chlorides in water over an extended concentration region. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived as a function of the surfactant concentration. For both surfactants, plots of volumes, enthalpies and free energies vs. concentration are regular whereas those of heat capacities and entropies present anomalies at about 0.8 and 0.1m for the octyl and dodecyl compounds, respectively. Changes in the slope of a plot of osmotic coefficients times molality vs. molality were also observed at these same concentrations. These peculiarities are ascribed to micelle structural transitions. The nonideal free energies do not seem to depend on the alkyl chain length when they are plotted vs. m/C _cmc . Also, a plot of the nonideal free energy vs. logm/C _cmc is roughly independent of the nature of the surfactant because of the constant activity of surfactants in micellar solutions. Nonideal free energies, enthalpies and entropies have been calculated at 15 and 35°C. At each concentration the nonideal free energy is temperature independent as a result of a compensatory effect between enthalpy and entropy. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data suggest that the nicotinamide group possesses less hydrophilic character than the ammonium group.     

Thermodynamic properties and conductivities of some dodecylsurfactants in water

Densities, heat capacities, enthalpies of dilution, osmotic coefficients and conductivities are reported for dodecylamine hydrochloride, dodecyldimethylammonium and dodecyltrimethylammonium chloride in water over a wide range of concentration. The last two properties were also measured for dodecyltrimethylammonium bromide. From the thermodynamic data partial molar volumes, heat capacities and relative enthalpies and nonideal free energies and entropies were derived as a function of the surfactant concentration. The cmc's and degree of counterion dissociation were also calculated from the transport properties. It is shown that the trends of volumes, enthalpies, free energies and entropies are quite regular whereas heat capacities present maxima and minima at concentrations which depend on the nature of surfactants. Corresponding changes were observed in the osmotic coefficients and specific conductivities. The thermodynamic functions of micellization were evaluated on the basis of the pseudo-phase transition model. Finally, the effects of the introduction of methyl groups in the hydrophilic moiety of the surfactant and of the nature of the counterion on the thermodynamic properties of monomers and micelles are examined.     

Anomalous small angle x-ray scattering determination of ion distribution around a polyelectrolyte biopolymer in salt solution

The distribution of counterions in solutions of high molecular mass hyaluronic acid, in near-physiological conditions where mono- and divalent ions are simultaneously present, is studied by small angle neutron scattering and anomalous small angle x-ray scattering. The solutions contain either sodium or rubidium chloride together with varying concentrations of calcium or strontium chloride. The effects of monovalent-divalent ion exchange dominate the amplitude and the form of the counterion cloud. In the absence of divalent ions, the shape of the anomalous scattering signal from the monovalent ions is consistent with the distribution calculated from the Poisson-Boltzmann equation, as found by other workers. In mixtures of monovalent and divalent ions, however, as the divalent ion concentration increases, both the diameter and the amplitude of the monovalent ion cloud decrease. The divalent counterions always occupy the immediate neighborhood of the charged polyanion. Above a given concentration their anomalous scattering signal saturates. Even in a large excess of divalent ions, ion exchange is incomplete.     

Thermodynamic properties and structure of water films in flotation processes

The thermodynamic properties of water layers on barite and marble samples covered with various amounts of tetradecylammonium chloride (TDAHCl) after flotation processes were investigated by thermal analysis. The relation between the activation energy, the enthalpy and the entropy of the bonded water and the amount of TDAHCl previously deposited on the bare mineral surfaces was determined. An interpretation of the changes in these parameters, the water structure and the flotability is described.

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Zusammenfassung Mittels Thermoanalyse wurden nach Schwimmaufbereitungsverfahren die thermodynamischen Eigenschaften der Wasserschichten über Baryt- und Marmorproben mit verschiedenem Gehalt an Tetradecylammoniumchlorid (TDACl) untersucht. Es wurde die Beziehung zwischen der Aktivierungsenergie, der Enthalpie und der Entropie des gebundenen Wassers und der Menge an zuvor an der Oberfläche des bloßen Minerals festgesetzen TDACl festgestellt. Die Veränderlichkeit dieser Parameter sowie die bindung des Wassers und di Flotierbarkeit werden interpretiert.

     

The water structure around the chloride ion in aqueous polyethylene oxide solutions

Neutron diffraction and nuclear magnetic relaxation yield complementary information on the interaction of ions and water molecules in solution. In the present study the two techniques are used to investigate the influence of poly (ethyleneoxide) on the Cl⁻ hydration sphere. It is concluded that the important effect of PEO on the ³⁵Cl relaxation rate is due to the occurrence of a long correlation time rather than structural changes.     

Dynamic properties of water around a protein-DNA complex from molecular dynamics simulations

Formation of protein-DNA complex is an important step in regulation of genes in living organisms. One important issue in this problem is the role played by water in mediating the protein-DNA interactions. In this work, we have carried out atomistic molecular dynamics simulations to explore the heterogeneous dynamics of water molecules present in different regions around a complex formed between the DNA binding domain of human TRF1 protein and a telomeric DNA. It is demonstrated that such heterogeneous water motions around the complex are correlated with the relaxation time scales of hydrogen bonds formed by those water molecules with the protein and DNA. The calculations reveal the existence of a fraction of extraordinarily restricted water molecules forming a highly rigid thin layer in between the binding motifs of the protein and DNA. It is further proved that higher rigidity of water layers around the complex originates from more frequent reformations of broken water-water hydrogen bonds. Importantly, it is found that the formation of the complex affects the transverse and longitudinal degrees of freedom of surrounding water molecules in a nonuniform manner.     

Thermodynamic properties of N-octyl-, N-decyl- and N-dodecylpyridinium chlorides in water

Densities, heat capacities and enthalpies of dilution at 25°C and osmotic coefficients at 37°C were measured for N-octyl-, N-decyl- and N-dodecyl-pyridinium chlorides in water over a wide concentration region. Conductivity measurements were performed in order to evaluate the cmc and the degree of counterion dissociation. Partial molar volumes, heat capacities, relative enthalpies and nonideal free energies and entropies at 25°C were derived from the experimental data as functions of the surfactant concentration. The changes with concentration of these properties are quite regular with the exception of the heat capacities which display anomalies at about 0.9, 0.25 and 0.12 mol-kg^–1 for the octyl, decyl and dodecyl compounds, respectively. At these concentrations there were also changes in the slopes of the specific conductivity and of the product of the osmotic coefficients and the molality vs. concentration. These peculiarities can be ascribed to micelle structural transitions. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. These data have been compared to those for alkyltrimethylammonium bromides and alkylnicotinamide chlorides. It is shown that the introduction of the hydrophilic CONH₂ group lowers the hydrophilic character of the pyridinium ring.     

Thermodynamic properties of some N-alkyl-N-methylpiperidinium chlorides and N-alkylpiperidine hydrochlorides in water

Densities and heat capacities at 25°C were measured for N-octyl-, N-decyl- and N-dodecyl-N-methylpiperidinium chlorides and for N-octyl- and N-dodecylpiperidine hydrochlorides in water as functions of concentration. Enthalpies of dilution at 25°C and osmotic coefficients at 37°C of the N-methyl-N-alkylpiperidinium chlorides were also measured as functions of concentration. The partial molar volumes, heat capacities, relative enthalpies, nonideal Gibbs energies and entropies at 25°C were derived as functions of the surfactant concentration. By increasing the alkyl chain length of the surfactant, both the apparent molar volume vs. concentration curves are shifted toward greater values while the corresponding ones for the heat capacity are moved toward more negative values. These results are consistent with the higher hydrophobicity the longer the alkyl chain of the surfactant is. In the micellar region, the entropy and enthalpy vs. log m/m _cmc curves increase in a parallel manner by decreasing the alkyl chain length of the surfactant. Consequently, the negligible effect of the hydrophobicity of the surfactant on the Gibbs energy vs. log m/m _cmc trends is due to the enthalpy-entropy compensative effect. The thermodynamic functions of micellization were graphically evaluated on the basis of the pseudo-phase transition model. The absolute values of both the volume and heat capacity of micellization increase with an increasing number of carbon atoms in the alkyl chain (n _c ). The enthalpy and entropy of micellization vs. n _c are convex curves. Comparisons are also made between the present data and those of some alkylpyridinium chlorides reported elsewhere.     

Thermodynamic anomalies in a lattice model of water

We investigate a lattice-fluid model of water, defined on a three-dimensional body centered cubic lattice. Model molecules possess a tetrahedral symmetry, with four equivalent bonding arms, aiming to mimic the formation of hydrogen bonds. The model is similar to the one proposed by Roberts and Debenedetti [J. Chem. Phys. 105, 658 (1996)], simplified in that no distinction between bond "donors" and "acceptors" is imposed. Bond formation depends both on orientation and local density. In the ground state, we show that two different ordered (ice) phases are allowed. At finite temperature, we analyze homogeneous phases only, working out phase diagram, response functions, the temperature of maximum density locus, and the Kauzmann line. We make use of a generalized first-order approximation on a tetrahedral cluster. In the liquid phase, the model exhibits several anomalous properties observed in real water. In the low temperature region (supercooled liquid), there are evidences of a second critical point and, for some range of parameter values, this scenario is compatible with the existence of a reentrant spinodal.     

Thermodynamic properties of a hindered rotator

It is demonstrated that a finite dimensional representation of the hamiltonian of a hindered rotator can be used to get reasonably accurate values of thermodynamic properties, e.g., the specific heat of an assembly of such rotators. It is known that there is a large difference between the classical values and quantum statistical values when the height of the hindering potential is small.     

Study on ion formation in electrospray droplet impact secondary ion mass spectrometry

A new type of cluster secondary ion mass spectrometry (SIMS), named electrospray droplet impact (EDI), has been developed in our laboratory. In general, rather strong negative ions as well as positive ions can be generated by EDI compared with conventional SIMS. In this work, various aspects of ion formation in EDI are investigated. The Brønsted bases (proton acceptor) and acids (proton donor) mixed in the analyte samples enhanced the signal intensities of deprotonated molecules (negative ions) and protonated molecules (positive ions), respectively, for analytes. This suggests the occurrence of heterogeneous proton transfer reactions (i.e. M + M′ → [M+H]⁺ + [M′? H]^?) in the shockwave‐heated selvedge of the colliding interface between the water droplet and the solid sample deposited on the metal substrate. EDI‐SIMS shows a remarkable tolerance to the large excess of salts present in samples. The mechanism for desorption/ionization in EDI is much simpler than those for MALDI and SIMS because only very thin sample layers take part in the shockwave‐heated selvedge and complicated higher‐order reactions are largely suppressed. Copyright © 2007 John Wiley & Sons, Ltd.     

L-alanine in a droplet of water: a density-functional molecular dynamics study

We report the results of a Born-Oppenheimer molecular dynamics study on an L-alanine amino acid in neutral aqueous solution. The whole system, the L-alanine zwitterion and 50 water molecules, was treated quantum mechanically. We found that the hydrophobic side chain (R = CH3) defines the trajectory path of the molecule. Initially fully hydrated in an isolated droplet of water, the amino acid moves to the droplet's surface, exposing its hydrophobic methyl group and alpha-hydrogen out of the water. The structure of an L-alanine with the methyl group exposed to the water surface was found to be energetically favorable compared to a fully hydrated molecule. The dynamic behavior of the system suggests that the first hydration shell of the amino acid is localized around carboxylate (CO2-) and ammonium (NH3+) functional groups; it is highly ordered and quite rigid. In contrast, the hydration shell around the side chain is much less structured, suggesting a modest influence of the methyl group on the structure of water. The number of water molecules in the first hydration shell of an alanine molecule is constantly changing; the average number was found to equal 7. The molecular dynamics results show that L-alanine in water does not have a preferred conformation, as all three of the molecule's functional sites (i.e., CH3, NH3+, CO2-) perform rotational movements around the C(alpha)-site bond.