Molecular dynamics simulation of nanostructural organization in ionic liquid/water mixtures

Molecular dynamics simulations have been carried out to investigate nanostructural organization in mixtures of 1-octyl-3-methylimidazolium nitrate ionic liquid and water at multiple water concentrations. Evolution of the polar network, water network, and micelle structures is visualized and analyzed via partial radial distribution functions. The calculated static partial structure factors show that within the range of water contents examined, polar networks, water networks, and micelles possess an approximately invariant characteristic length at around 20 A. Furthermore, the above calculations point out that, as the amount of water increases, the polar network is continuously broken up (screened) by the intruding water, while the structural organization of the water network and the micelle exhibits a turnover. At the turnover point, the most ordered micelle (cation-cation) structure and water (water-anion-water) network are formed. Thereafter, the structural organization abates drastically, and only loose micelle structure exists due to the dominant water-water interactions. The simulated turnover of structural organization agrees with the sharpest peak in the experimentally obtained structure factor in aqueous solutions of similar ionic liquids; the simulated water structure reveals that water can form liquidlike associated aggregates due to the planar symmetry and strong basicity of NO(3)-, in agreement with experiment. The turnover of structural organization of micelles results from the persistent competition between the hydrophobic interactions of the nonpolar groups and the breakup of the charged polar network with increasing water content, whereas the turnover of the water network results from the competition between the water-water and water-anion interactions.     

Water and polymer dynamics in chemically cross-linked hydrogels of poly(vinyl alcohol): a molecular dynamics simulation study

A topologically extended model of a chemically cross-linked hydrogel of poly(vinyl alcohol) (PVA) at high hydration degree has been developed for a molecular dynamics simulation with atomic detail at 323 K. The analysis of the 5 ns trajectory discloses structural and dynamic aspects of polymer solvation and elucidates the water hydrogen bonding and diffusion in the network. The features of local polymer dynamics indicate that PVA mobility is not affected by structural constraints of chemical junctions at the investigated cross-linking density, with a prevailing dumping effect due to water interaction. Simulation results are validated by a favorable comparison with findings of an incoherent quasi-elastic neutron scattering study of the same hydrogel system.     

Probing the dynamic nature of water molecules and their influences on ligand binding in a model binding site

The model binding site of the cytochrome c peroxidase (CCP) W191G mutant is used to investigate the structural and dynamic properties of the water network at the buried cavity using computational methods supported by crystallographic analysis. In particular, the differences of the hydration pattern between the uncomplexed state and various complexed forms are analyzed as well as the differences between five complexes of CCP W191G with structurally closely related ligands. The ability of docking programs to correctly handle the water molecules in these systems is studied in detail. It is found that fully automated prediction of water replacement or retention upon docking works well if some additional preselection is carried out but not necessarily if the entire water network in the cavity is used as input. On the other hand, molecular interaction fields for water calculated from static crystal structures and hydration density maps obtained from molecular dynamics simulations agree very well with crystallographically observed water positions. For one complex, the docking and MD results sensitively depend on the quality of the starting structure, and agreement is obtained only after redetermination of the crystal structure and refinement at higher resolution.     

Dynamics of a network of hydrogen bonds upon water electrocrystallization

Computer simulation is employed to study the dynamics of a network of hydrogen bonds and the structural properties of water placed between graphene layers. The presence of the graphene walls has been shown to substantially affect the water phase diagram. Glass transition processes are observed in the system, and liquid water completely passes to an amorphous state. Moreover, it has been established that the imposition of an external electric field with strength E ≥ 0.5 V/Å on the system subjected to increased pressure results in structural ordering of water. It has been found that water located between graphene layers is transformed into Ic cubic ice. The electrocrystallization of water has been shown to substantially change the dynamics of the network of hydrogen bonds.     

Dielectric studies of water clusters in cyclodextrins: Relevance to the transition between slow and fast forms of thrombin

Cyclodextrins are useful models in the study of hydrogen bonded water clusters. In alpha-cyclodextrin hexahydrate (alpha-CD.6H2O), water molecules are ordered and occupy well-defined positions whereas in the larger beta-cyclodextrin dodecahydrate (beta-CD.12H2O), there is considerable disorder with water molecules freely arranged over several possible sites. Here it is shown that beta-CD exhibits substantial structural flexibility and proton mobility compared with alpha-CD which is relatively very rigid and exhibits negligible short-range protonic conduction. These properties are directly controlled by the effective dielectric constant of the molecule, which is determined by the rotational freedom of water molecules in the hydrogen bond network. This model may be relevant to proteins where water clusters of this kind are found on the protein surface and occasionally in the protein interior. The case of thrombin, an allosteric enzyme incorporating a network of 20 internal hydrogen bonded water molecules, is discussed.     

A Two‐Dimensional Polymer Synthesized at the Air/Water Interface

A trifunctional, partially fluorinated anthracene‐substituted triptycene monomer was spread at an air/water interface into a monolayer, which was transformed into a long‐range‐ordered 2D polymer by irradiation with a standard UV lamp. The polymer was analyzed by Brewster angle microscopy, scanning tunneling microscopy measurements, and non‐contact atomic force microscopy, which confirmed the generation of a network structure with lattice parameters that are virtually identical to a structural model network based on X‐ray diffractometry of a closely related 2D polymer. The nc‐AFM images highlight the long‐range order over areas of at least 300×300 nm². As required for a 2D polymer, the pore sizes are monodisperse, except for the regions where the network is somewhat stretched because it spans over protrusions. Together with a previous report on the nature of the cross‐links in this network, the structural information provided herein leaves no doubt that a 2D polymer has been synthesized under ambient conditions at an air/water interface.     

Structure Formation and Molecular Mobility in Water and in Aqueous Solutions

The study of the structure of water and of aqueous solutions has recently received new impetus from the efforts at commercial desalineation of sea water and from developments in molecular biology. The current view that, apart from single molecules, water contains only one type of structural element, namely “flickering” network structures with tetrahedrally hydrogen-bonded water molecules (two-states model) is proving inadequate in the interpretation of new experimental data and in the calculation of thermodynamic functions. After a critical discussion of the basis of this model and of the concept of hydrogen bonds, a second kind of structural element, i.e. a third state, is suggested: small aggregates of molecules containing mainly non-tetrahedral hydrogen bonds as well as some tetrahedral ones, and packed more densely than allowed by the lattice-like structure. These aggregates – dimers to hexamers – can be regarded as the primary products of disruption of the network structures, and displace the latter as structural components in water with increasing temperature or concentration of solutes. This “combined” model allows a consistent interpretation of the properties of water and of the various effects of dissolved substances.     

Short-cut structural design of reverse osmosis desalination plants

A design method for reverse osmosis desalination plants has been developed. It incorporates rigorous mathematical models for the prediction of the performance of various process units (reverse osmosis modules, pumps, energy recovery turbines) employed in the flowsheet and taken into account the network structure using an appropriate superstructure, which represents various reverse osmosis networks. Cost equations relating the capital and operating cost to the design variables, as well as the structural variables of the designed network have been introduced in the objective function. The total cost of the plant is minimized in order to determine the optimal operating and structural variables. The model is accurate enough to describe the process and yet simple enough to be used for design purposes. During the simulation and optimization studies, several structures for multistage reverse osmosis systems have been found. Results concerning the economics of the process are presented. Optimal results have also been used for the derivation of design curves concerning the effect of quality and quantity of produced water to the total annualized cost of the plant for various types of membrane modules.     

Structural and thermodynamic properties of different phases of supercooled liquid water

Computer simulation results are reported for a realistic polarizable potential model of water in the supercooled region. Three states, corresponding to the low density amorphous ice, high density amorphous ice, and very high density amorphous ice phases are chosen for the analyses. These states are located close to the liquid-liquid coexistence lines already shown to exist for the considered model. Thermodynamic and structural quantities are calculated, in order to characterize the properties of the three phases. The results point out the increasing relevance of the interstitial neighbors, which clearly appear in going from the low to the very high density amorphous phases. The interstitial neighbors are found to be, at the same time, also distant neighbors along the hydrogen bonded network of the molecules. The role of these interstitial neighbors has been discussed in connection with the interpretation of recent neutron scattering measurements. The structural properties of the systems are characterized by looking at the angular distribution of neighboring molecules, volume and face area distribution of the Voronoi polyhedra, and order parameters. The cumulative analysis of all the corresponding results confirms the assumption that a close similarity between the structural arrangement of molecules in the three explored amorphous phases and that of the ice polymorphs I(h), III, and VI exists.     

Slow dynamics in a primitive tetrahedral network model

We report extensive Monte Carlo and event-driven molecular dynamics simulations of the fluid and liquid phase of a primitive model for silica recently introduced by Ford et al. [J. Chem. Phys. 121, 8415 (2004)]. We evaluate the isodiffusivity lines in the temperature-density plane to provide an indication of the shape of the glass transition line. Except for large densities, arrest is driven by the onset of the tetrahedral bonding pattern and the resulting dynamics is strong in Angell's classification scheme [J. Non-Cryst. Solids 131-133, 13 (1991)]. We compare structural and dynamic properties with corresponding results of two recently studied primitive models of network forming liquids-a primitive model for water and an angular-constraint-free model of four-coordinated particles-to pin down the role of the geometric constraints associated with bonding. Eventually we discuss the similarities between "glass" formation in network forming liquids and "gel" formation in colloidal dispersions of patchy particles.     

Probing the microstructure of nonionic microemulsions with ethyl oleate by viscosity, ROESY, DLS, SANS, and cyclic voltammetry

Microemulsions are important formulations in cosmetics and pharmaceutics and one peculiarity lies in the so-called "phase inversion" that takes place at a given water-to-oil concentration ratio and where the average curvature of the surfactant film is zero. In that context, we investigated the structural transitions occurring in Brij 96-based microemulsions with the cosmetic oil ethyl oleate and studied the influence of the short chain alcohol butanol on their structure and properties as a function of water addition. The characterization has been carried out by means of transport properties, spectroscopy, DLS, SANS, and electrochemical methods. The results confirm that the nonionic Brij 96 in combination with butanol as cosurfactant forms a U-type microemulsion that upon addition of water undergoes a continuous transition from swollen reverse micelles to oil-in-water (O/W) microemulsion via a bicontinuous region. After determining the structural transition through viscosity and surface tension, the 2D-ROESY studies give an insight into the microstructure, i.e., the oil component ethyl oleate mainly is located at the hydrophobic tails of surfactant while butanol molecules reside preferentially in the interface. SANS experiments show a continuous increase of the size of the structural units with increasing water content. The DLS results are more complex and show the presence of two relaxation modes in these microemulsions for low water content and a single diffusive mode only for the O/W microemulsion droplets. The fast relaxation reflects the size of the structural units while the slower one is attributed to the formation of a network of percolated microemulsion aggregates. Electrochemical studies using ferrocene have been carried out and successfully elucidated the structural transformations with the help of diffusion coefficients. An unusual behavior of ferrocene has been observed in the present microheterogeneous medium, giving a deeper insight into ferrocene electrochemistry. NMR-ROESY experiments give information regarding the internal organization of the microemulsion droplets. In general, one finds a continuous structural transition from a W/O over a bicontinuous to an O/W microemulsion, however with a peculiar network formation over an extended concentration range, which is attributed to the somewhat amphiphilic oil ethyl oleate. The detailed knowledge of the structural behavior of this type of system might be important for their future applications.     

A polarizable force field for water using an artificial neural network

A force field for liquid water including polarization effects has been constructed using an artificial neural network (ANN). It is essential to include a many-body polarization effect explicitly into a potential energy function in order to treat liquid water which is dense and highly polar. The new potential energy function is a combination of empirical and nonempirical potentials. The TIP4P model was used for the empirical part of the potential. For the nonempirical part, an ANN with a back-propagation of error algorithm (BPNN) was introduced to reproduce the complicated many-body interaction energy surface from ab initio quantum mechanical calculations. BPNN, described in terms of a matrix, provides enough flexibility to describe the complex potential energy surface (PES). The structural and thermodynamic properties, calculated by isobaric-isothermal (constant-NPT) Monte Carlo simulations with the new polarizable force field for water, are compatible with experimental results. Thus, the simulation establishes the validity of using our estimated PES with a polarization effect for accurate predictions of liquid state properties. Applications of this approach are simple and systematic so that it can easily be applied to the development of other force fields besides the water-water system.     

Interaction of water with the model ionic liquid [bmim][BF4]: molecular dynamics simulations and comparison with NMR data

We report on molecular dynamics simulations of the ionic liquid [bmim][BF 4] and its mixtures with water, from zero up to 0.5 mol fraction of water. All of the simulations are carried out with two published force fields. The results are compared with each other and with published as well as new NMR data on the same mixtures, whenever possible. We perform extensive analyses of structural quantities, such as pair correlation functions, nearest-neighbor analysis and size distribution of the water clusters formed at higher concentrations. We show that the water clusters are formed almost exclusively by linear chains of hydrogen-bonded molecules. There is a nanoscale structuring of the mixtures but no macroscopic phase separation among the components, in agreement with experiment. Roughly, we identify two solvation regimes. At low water content, the ions are selectively coordinated by individual water molecules, but their ionic network is largely unperturbed. At high water content, the ionic network is somewhat disrupted or swollen in a nonspecific way by the water clusters.     

Structure and dynamics of water in mixed solutions including laponite and PEO

To investigate the structure and dynamics of water in mixed solutions including laponite clay particles and poly(ethylene oxide) (PEO), we measured the Raman spectra of the mixed solutions in the temperature range 283-313 K. The results show that the vibrational energies of the O-H stretching modes in the mixed solutions depend on the water content and temperature. The energy shifts of the O-H stretching modes are attributed to changes in the water structure. By applying a structural model of bulk water to the spectra in the O-H stretching region, the local structures of water in the solutions were analyzed. The result shows that the formation probability of hydrogen bonds in the solutions decreases as the water content decreases. Laponite and PEO have effects to disrupt the network structure of hydrogen bonds between water molecules. Further, it was found that laponite and PEO cause increase in the strength of hydrogen bonds of surrounding water,although the strength of the hydrogen bonds increases with the order water-laponite < water-water < water-PEO. It is concluded that water in laponite-PEO mixed solutions has a less-networked structure with strong hydrogen bonds compared with bulk water.     

Computational study of structural and dynamical properties of formamide-water mixtures

A molecular dynamics simulation study of structural and dynamical properties in liquid mixtures of formamide and water is presented. Site-site radial pair distribution functions, local mole fractions, pair energy distributions, and tetrahedral orientational order are the quantities analyzed to investigate the local structure in the simulated mixtures, along with a review of the intermolecular structure in terms of the distribution of hydrogen bonds. Our results indicate that there is a substitution of formamide molecules by water in the hydrogen bonds and a formation of a common hydrogen bond network. By analyzing the extent of tetrahedral order in the liquid as a function of composition, it is observed that whereas the tetrahedral network of liquid water is progressively lost by increasing the formamide concentration, the water structure within the first coordination shell is preserved and somewhat enhanced. The hydrogen-bond mean lifetimes were estimated by performing a time integration of the autocorrelation functions of bond occupation numbers. The lifetimes associated with hydrogen bonds between water, formamide, and interspecies pairs are found to increase with increasing formamide concentration. The lifetimes of the water hydrogen bonds show the largest variations, supporting the picture of an enhancement of the water structure among the nearest neighbors within the first coordination shell. We have used two different force field models for water, SPC/E [J. C. Berendsen et al., J. Phys. Chem. 91, 6269 (1987)] and TIP4P/2005 [J. L. F. Abascal and C. Vega, J. Chem. Phys. 123, 234505 (2005)]. Our results for structural and dynamical properties yield very small differences between those models, the TIP4P/2005 predicting a slightly more structured liquid and, consequently, exhibiting a slightly slower translational and librational dynamics.     

Prediction of extraction efficiency in supported liquid membrane with a stagnant acceptor phase by means of artificial neural network

An artificial neural network model of supported liquid membrane extraction process with a stagnant acceptor phase is proposed. Triazine herbicides and phenolic compounds were used as model compounds. The model is able to predict the compound extraction efficiency within the same family based on the octanol–water partition coefficient, water solubility, molecular mass and ionisation constant of the compound. The network uses the back‐propagation algorithm for evaluating the connection strengths representing the correlations between inputs (octanol–water partition coefficients logP, acid dissociation constant pK_a, water solubility and molecular weight) and outputs (extraction efficiency in dihexyl ether and undecane as organic solvents). The model predicted results in good agreement with the experimental data and the average deviations for all the cases are found to be smaller than ±3%. Moreover, standard statistical methods were applied for exploration of relationships between studied parameters.     

Model study of the effect of pore structure and condensation on multilayer adsorbate transport in porous media

A single-pore model approach to multilayer adsorbate transport in mesoporous media, previously shown to be capable of interpreting the observed behavior of relative vapor permeability PHs/PHL (or of the corresponding surface diffusion coefficient Ds), has been incorporated in a model pore network. The resulting more sophisticated model can simulate realistically the effect on PHs/PHL or Ds (i) of salient structural features of the porous medium (notably pore size dispersion and network connectivity) and (ii) of vapor condensation, which inevitably accompanies multilayer adsorbate transport in reality. An extensive generic parametric study of these effects has been performed on this basis. The results indicate that the aforementioned effects are ordinarily unlikely to induce substantial deviations of observed PHs/PHL behavior from the single-pore model benchmark. Thus, the utility of this simple model as a good basis for data analysis is confirmed, while the network model offers the possibility of exploiting external structural and other available information for a more refined interpretation of PHs/PHL behavior in particular mesoporous solid-vapor systems.     

The Wolf method applied to the liquid-vapor interface of water

The Wolf method for the calculation of electrostatic interactions is applied in a liquid phase and at the liquid-vapor interface of water and its results are compared with those from the Ewald sums method. Molecular dynamics simulations are performed to calculate the radial distribution functions at room temperature. The interface simulations are used to obtain the coexisting densities and surface tension along the coexistence curve. The water model is a flexible version of the extended simple point charge model. The Wolf method gives good structural results, fair coexistence densities, and poor surface tensions as compared with those obtained using the Ewald sums method.