Small Hydration Research Articles

Structure of Water Sorbed into Poly(MEA-co-HEMA) Films As Examined by ATR−IR Spectroscopy

The structure of water sorbed into poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and their copolymers (p(MEA/HEMA)) was investigated by attenuated total reflection infrared (ATR−IR) spectroscopy. The extinction coefficient of the OH stretching band of sorbed water (εOH) was calculated from the band area obtained by IR measurement and the amount of sorbed water obtained by thermogravimetric analysis. When the polymers contacted with water vapor (relative humidity = ∼55%), the εOH values were quite similar in all polymers. On the other hand, when the polymers contacted with liquid water, the εOH values were drastically changed by the content of 2-methoxyethyl acrylate (MEA). When the MEA content of the polymers was low (<60 mol %), the εOH value of the water sorbed into polymers in contact with liquid water was equal to that in contact with water vapor. In the higher MEA content (70−100 mol %), on the other hand, the εOH values of the water sorbed into polymers in contact with liquid water were 5−8 times larger than that in contact with water vapor. These results seemed to indicate that the interaction between the primary hydration water around the MEA-containing polymer chain and water molecules surrounding the primarily hydrating water is very weak. Such water with a large εOH value seemed to correspond to “cold-crystallizable” water, which has been observed by DSC as anomalous water other than intermediate and nonfreezable waters. Taking both the experimental results obtained in this work and those thermodynamically obtained previously into consideration, it was strongly suggested that the cold crystallization of water is generated by caging water molecules in a small space by the polymer chains with a small hydration region. The correlation between the εOH value and the blood compatibility of the copolymer was also discussed.

Efficient facilitated transport of lead, cadmium, zinc, and silver across a flat-sheet-supported liquid membrane mediated by lasalocid A

Flat-sheet-supported liquid membranes incorporating lasalocid A (a natural ionophore) were shown in previous works to achieve the transport of divalent transition metal cations such as Cd2+ and Zn2+ against a proton gradient, which is the driving force behind the process. This transport process has been extended to other metal species such as Pb2+ and Ag+ and also to the case where the two metal species compete for transport. A higher transport flux for Pb2+ as compared to Cd2+ and Zn2+ is observed, which is partly explained by a higher rate of interfacial complexation due to a smaller hydration shell of this species. This effect is confirmed by the data obtained with Ag(I).

Analysis of Atomic Force Curve Data for Mapping of Surface Properties in Water

This paper presents an analysis of atomic force versus distance curves for a silicon nitride probe and a silicon sample immersed in water. A custom-built atomic force microscope (AFM) was adapted for working in water by building a water cell from a liquid drop caught between a glass lamella fixed on the top of the cantilever base and the sample surface. An algorithm for processing of force curve data for long- and short-range forces is described. The force curve data taken for a sample consisting of a silicon wafer Si(111) patterned with V-shaped grooves and a silicon nitride cantilever in water were digitally acquired and automatically processed for mapping of surface properties. A weak repulsive double layer force with no relevant dependence on sample topography was observed on the force curves taken during approach movement of the cantilever. On the other hand, the attractive hydration force showed a strong dependence on the sample topography. Large hydration force values were noticed on the inclined faces of the V-shaped grooves while small hydration force values were noticed outside the grooves. The result was explained by the dependence of the tip curvature radius at the contact region on the tilt of the sample surface.

Pressure dependence of small-cage occupancy in the cyclopropane hydrate system

Laser Raman microspectroscopy and stability boundary observation for the cyclopropane hydrate system were investigated in a pressure range up to 460 MPa and temperature range from 278 to 320 K . The cyclopropane molecule is one of the largest guest species which build up the structure I hydrate lattice. It has been argued that the cyclopropane hydrate lattice would become stable in the presence of the perfectly vacant small cage. In the pressure range lower than 200 MPa , we can confirm the perfect vacancy of small hydrate cages. We have ascertained that the cyclopropane molecule is able to occupy the small hydrate cage in the pressure range higher than 200 MPa along with the three-phase coexisting curve. It becomes clear that the degree of small-cage occupancy increases in proportion to pressure without any phase transition in a pressure range up to 460 MPa .

Al 2O 3–ZrO 2 debris life cycle during wear: effects of the third body on wear and friction

Wear debris of alumina–zirconia materials generated during wear by abrasion and chipping (ring on disk) were analyzed in order to study their life cycle and their effects on wear and friction. When generated in the contact, wear debris contributed to redistribute the applied load on more contact points and absorbed a part of the energy provided to the system by microcracking and plastic deformation. The result was a decrease in the wear rate and an increase in the shear stress at the interface and, consequently, in the coefficient of friction. Inversely, removing debris from the contact did increase the wear. T.G.A., D.T.A., temperature measurements at the mid-thickness of the disk and debris morphology observations allowed us to detect some plastic deformation and a small hydration of the debris. Most of the plastic deformation occurred in zirconia particles since they became amorphous during wear by chipping. Hydration remains a minor phenomenon as the temperature is high at the interface.

Effect of crystal seeding on the hydration of calcium phosphate cement.

In this paper, the effect of crystal seeding on the hydration of calcium phosphate cement (CPC) has been carefully investigated. The setting time of the CPC slurry not containing any crystal seeds was 150 min, while the setting time for the specimen containing 5 wt% low crystallinity hydroxyapatite used as a crystal seed was 7 min. This improvement in the setting time was due to HAP serving as a substrate for heterogeneous nucleation which accelerated nucleation. In addition, the compressive strength of the specimen containing the crystal seeding was deduced and we report values different from those previously reported in the literature. The calorimetric curve indicated that crystal seeding could reduce the induction period. A.c. impedance spectroscopy revealed that at the beginning of hydration, the rate of reaction increased and also that the mean diameter and porosity decreased as the seed content increased. At the end of the hydration reaction the situation was changed with the mean diameter and porosity in the sample without any seeds being a minimum, which indicated that the compressive strength was a maximum. This result could be explained by the dissolution and reprecipitation of small hydration products produced by the high rate of reaction produced by the introduction of the crystal seeds.

Origin of Entropy Convergence in Hydrophobic Hydration and Protein Folding.

An information theory model is used to construct a molecular explanation why hydrophobic solvation entropies measured in calorimetry of protein unfolding converge at a common temperature. The entropy convergence follows from the weak temperature dependence of occupancy fluctuations for molecular-scale volumes in water. The macroscopic expression of the contrasting entropic behavior between water and common organic solvents is the relative temperature insensitivity of the water isothermal compressibility. The information theory model provides a quantitative description of small molecule hydration and predicts a negative entropy at convergence. Interpretations of entropic contributions to protein folding should account for this result.

Unique carrier properties of neutral lanthanide complex for inorganic anions

Lipophilic lanthanide tris(fluorinated β-diketonates) exhibited interesting carrier properties for inorganic anions. Negative FABMS and 13C NMR studies revealed that they bound small Cl − anion with a large hydration energy more strongly than large Br − and I − anions with small hydration energies. The direct anion coordination features of lanthanide complexes offered unique carrier properties for inorganic anions in liquid membrane transport.

High-resolution neutron study of vitamin B12 coenzyme at 15 K: solvent structure

The solvent structure of crystalline vitamin B-12 coenzyme, determined from a high-resolution (0.9 Angstrom) neutron data set collected at 15 K, is presented. The study involved the identification of solvent peaks and the formulation of possible solvent networks. The solvent distribution within the crystal can be described in two regions, namely (a) a channel, comprizing statically disordered water molecules and (b) leading into this channel, a region of highly ordered water molecules. The identification of the disordered solvent peaks has enabled the formulation of two main solvent networks per asymmetric unit, with the assistance of criteria used in the analyses of solvent structures in crystal hydrates of small molecules. The two networks comprise 17 water molecules each. A comparison of these solvent networks is made with those identified in a previous study of a crystal of the coenzyme at 279 K. The covalent and hydrogen-bond geometries involving the water molecules of these networks have been analysed and agree well with those found in small molecular crystal hydrates. Furthermore, the analysis of the water structure around apolar groups of the B-12 coenzyme indicates the presence of clathrate-like water structures, as well as short distances which others have identified as C-H...O hydrogen bonds. Short range O...O non-hydrogen-bonded contacts obey known repulsive restriction rules formulated from small-molecule hydrate crystals.

Interactions of sorbed water with starch studied using proton nuclear magnetic resonance spectroscopy

Proton NMR measurements, made on hydrated starch, indicate that when small amounts of water (ca. 10% by weight) are sorbed by the polysaccharide, the water is highly mobile and reorients anisotropically. This may be compared with the dynamics of water in small crystalline organic hydrates, such as α-cyclodextrin hexahydrate, where the water motion is generally more constrained. Analysis of the wideline proton spectra indicates that an increase in sample crystallinity on addition of water is accompanied by an increase in the starch chain mobility. Cross-relaxation is shown to occur between the starch and the water protons via the secular ‘flip-flop’ process, rather than by proton chemical exchange. This process influences the proton spin–lattice relaxation and results in relatively fast communication between the different proton populations.

Equilibrium study of the adsorption of molybdenum(VI) on activated carbon

The adsorption of molybdenum(VI) onto activated carbon from aqueous solution has been investigated in the pHc range 1.0–6.5 at 25° in 1.0 mol dm−3 sodium chloride. The molybdenum concentration was varied from 5 × 10−4 to 2 × 10−2mol dm−3. Computer treatment of the distribution data in which all the relevant protonation and condensation equilibria of molybdate were taken into account, led to an adsorption model comprising the three species [HMo2O7]−, Mo(OH)6 and [HMoO4]−, of which [HMo2O7]− predominates by far. The very strong adsorption of [HMo2O7]− is attributed to its small hydration energy and an increase in the coordination number of molybdenum(VI) by bonding to basic surface oxygen atoms analogous to complex formation with e.g. oxalate.

Ammonium ion representation in Monte Carlo simulations of biomolecular solutions

Monte Carlo computer simulation techniques may be used to predict structural properties of solvent networks in helical fragments of nucleic acids, provided that suitable potential functions are available to describe the interactions between nucleic acid atoms, water and counterions. Previous studies have shown that simple non-bonded and point charge parameters are adequate for mononuclear ions such as sodium and calcium. In this study a model interaction potential for the polynuclear ammonium ion is evaluated. The parameters used take account of the distribution of charge over the constituent atoms in the ion. Simulations are carried out on the ammonium salt of a small nucleic acid crystal hydrate and a comparison is made between the predicted and experimental results. It is shown that the simulated structure is in reasonable agreement with experiment. It is therefore feasible to use this potential in studies of ammonium-containing bimolecular systems.

Monte carlo simulations of nucleotide crystal hydrates and their counter-ions

A knowledge of structural and energetic aspects of water- and ion-nucleic acid interactions is essential for the understanding of the role of solvent and counterions in stabilising the various helical forms of nucleic acids. In this study, Monte Carlo computer simulation techniques have been used to predict structural properties of solvent networks in small nucleic acid crystal hydrates containing the ions sodium, ammonium and calcium. Appropriate parameters to describe the interaction potentials of the ions are added to those previously developed for water and nucleic acid atoms. A comparison is made between the predicted and experimental results and it is concluded that the potential functions used lead to simulated solvent structure in reasonable agreement with experimental data, at least in the cases of sodium and calcium. It is now feasible to use these functions in studies of hydration of larger helical fragments of nucleic acids of more direct biological interest.

Clathrate formation in water-tetraalkyl ammonium iodide systems at high pressure

The phase diagrams of aqueous binary systems with PrBu3NI(I), Bu4NI(II), i-AmBu3NI(III) and i-Am4NI(IV) were studied at atmospheric and high pressures by DTA method. In systems III-H2O and IV-H2O at atmospheric pressure we observed polyhydrates melting incongruently at 7.1 and 14.7, correspondingly. In systems I-H2O and II-H2O hydrates form at higher pressure only, there are PrBu3NI (15–25) H2O at P≧0.13 kbar, Bu4NI (25–35) H2O at P≧0.4 kbar. In water systems with II–IV at pressure 1.2, 1.4 and 0.26 kbar correspondingly polyhydrates with smaller hydrate number form. Formation of hydrates in solution in the II-H2O system does not occur at pressure greater than 7 kbar. The summarized P, T, X-phase diagram is discussed.

Water structure in vitamin B12 coenzyme crystals. I. Analysis of the neutron and x-ray solvent densities

The disordered solvent distribution in crystals of vitamin B12 coenzyme was examined using the methods of high-resolution neutron and x-ray diffraction. One set of neutron (0.95 A) and two sets of x-ray (0.94 and 1.1 A) data were collected and the resulting models were extensively refined using least-squares and Fourier syntheses. The solvent regions were analyzed in two stages: first, main sites were assigned to the well defined regions of solvent density and refined using least squares; second, continuous sites were assigned representing the more disordered diffuse and elongated regions of solvent density. During the analysis an acetone molecule was also located. Water networks were formulated from the assigned sites in the above models and also from those assigned in the original structure determination (Lenhert, 1968), using criteria that included hydrogen bonding (derived from small crystal hydrates), van der Waals contact distances, side-chain disorder, water molecule orientations, and the presence or absence of foreign solvent. The well established networks extend throughout all the solvent regions of the crystal with interesting orientational arrangements of the individual waters around both polar and apolar groups of the coenzyme molecule. The networks were seen to be consistent among each of the four models in terms of occupying relatively similar positions. However, the occupancy values of the individual networks varied between the models; some networks were clearly visible in one but attenuated in another. The specific details of the water structure (bonding geometries, short-range nonbonded contacts, orientations of the waters, polar and apolar interactions, etc.) are described in the following paper.

Solvent interactions in nucleic acid crystal hydrates

A detailed knowledge of structural and energetic aspects of water-nucleic acid interactions is essential for understanding the role of solvent in stabilizing the various helical forms of nucleic acids. In this study, computer simulation techniques have been used to predict structural properties of solvent networks in small nucleic acid crystal hydrates. A detailed comparison of predicted and experimental results on the structure of the solvent networks is presented and includes an analysis of both the local environment and hydrogen bond pattern of each water molecule. A correlation between the environment of each unique water molecule and its energetic properties (such a dipole moment and binding energy) is seen. As in the previous studies on small amino acid hydrate crystals, non-pair additive (cooperative) effects are found to be non-negligible. It is concluded that the potential functions used in this initial study lead to simulated solvent networks in reasonable agreement with experimental data. Thus, it is now feasible to use them in studies of hydration of larger helical fragments of nucleic acids of more direct biological interest.

Influence of charge density on the acid–base equilibrium of ethylenediamine in montmorillonites

The ethylenediammonium (enH22+)–Ca ion-exchange reaction has been studied at three temperatures on four montmorillonites of different charge densities. The exchange reaction is exergonic and exothermic at all charge densities because of the higher thermodynamic functions of hydration for Ca. Both ΔG⊖ and ΔH⊖ of exchange decrease with decreasing charge density and tend to vanish at zero charge, an observation which is explained by the increasing interlamellar hydration as evidenced by the concomitant increase in the d001 spacing of the homoionic end members Ca and EDAH22+. The protonation constants in the interface, as obtained from the enH22+–H+ exchange constants, exceed the value in bulk solution and decrease to the latter at zero charge density. The extra stability gained by the enH22+ ion upon adsorption in the interface is ascribed to its higher polarizability (compared with the proton) and to its smaller hydration state in the interlamellar phase, analogous to the case of transition metal ion complexes.

Monte Carlo simulation of small hydrate clusters of NO−2

The ground-state Hartree–Fock (HF) potential for the NO−2:H2O dimer has been computed for 102 different intermolecular geometrical configurations and has been expressed in a computationally convenient analytical form. The main conclusion drawn from these calculations is that the ion–solvent attraction is mainly electrostatic for intermolecular distances between 6.0 and 7.0 bohr (N-to-O distance). Keeping the dipole vector of the H2O molecule oriented toward the NO−2 ion yields energetically favorable conformations. Rotations of the H2O molecule which do not change the dipole orientation of the H2O have been found to have small barriers (∼4 kcal/mole), whereas those that destroy proper dipole alignment encounter large (∼30 kcal/mole) barriers. The use of such ion–H2O intermolecular potentials together with the H2O:H2O pair potential of Clementi permits Monte Carlo techniques to be used to examine the nature of the inner hydration shells of NO−2. The results of Monte Carlo simulations of NO−2(H2O)n1⩽n⩽15 are discussed in some detail.

Kinetics of the dissolution of calcite and its applications to karstification

Using Plummer et al.'s rate equations on dissolution of CaCO 3 in stirred H 2OCO 2 systems, dissolution rates are calculated as functions of the initial state of the pure H 2OCO 2 system and the Ca 2+ concentration already dissolved. This is done for all cases of geological interest, such as systems open to a CO 2 atmosphere at constant pressure, systems in equilibrium with CO 2 initially and then closed to further CO 2 absorption and for the case of mixing of two saturated waters, i.e. mixing corrosion. An important result is, that the solution rates are controlled by the ratio of the volume of the solution to the surface of the solid CaCO 3. For large ratios the rate is surface controlled and for small values hydration of CO 2 determines the rates. These results are applied to undersaturated water flowing in flat or circular conduits of limestone and widening them by solution. The values thus obtained agree well with observations reported in the literature. Finally, the solution at a crossing between two water-leading flat conduits by mixing—corrosion is considered. At the crossing a circular tube develops and the time dependence of its diameter is calculated. Using realistic data for the width of the flat conduits, i.e. joints and partings, we find that cave development to passage diameters of 1 m is possible in a few million years. This sheds a new light on the theory of the origin of caves and gives a comprehensive picture of the development of underground drainage and the formation of caves.

Polarization model for water and its ionic dissociation products

In order to achieve a simple description of aggregates of deformable water molecules, a new model has been constructed which treats H+ and O2− particles as the basic dynamical and structural elements. The H+ units are bare protons, while the O2− units possess a form of nonlocal polarizability consistent with their electronic structure. The model yields water molecules which have the correct geometry and dipole moment, and which engage in hydrogen bonding to one another. Minimum-energy structures have been determined for the water dimer and trimer and for small hydrate clusters of H+ and OH−; comparison with relevant experiments and quantum–mechanical calculations is satisfactory.