Water modeled as an intermediate element between carbon and silicon

Water and silicon are chemically dissimilar substances with common physical properties. Their liquids display a temperature of maximum density, increased diffusivity on compression, and they form tetrahedral crystals and tetrahedral amorphous phases. The common feature to water, silicon, and carbon is the formation of tetrahedrally coordinated units. We exploit these similarities to develop a coarse-grained model of water (mW) that is essentially an atom with tetrahedrality intermediate between carbon and silicon. mW mimics the hydrogen-bonded structure of water through the introduction of a nonbond angular dependent term that encourages tetrahedral configurations. The model departs from the prevailing paradigm in water modeling: the use of long-ranged forces (electrostatics) to produce short-ranged (hydrogen-bonded) structure. mW has only short-range interactions yet it reproduces the energetics, density and structure of liquid water, and its anomalies and phase transitions with comparable or better accuracy than the most popular atomistic models of water, at less than 1% of the computational cost. We conclude that it is not the nature of the interactions but the connectivity of the molecules that determines the structural and thermodynamic behavior of water. The speedup in computing time provided by mW makes it particularly useful for the study of slow processes in deeply supercooled water, the mechanism of ice nucleation, wetting-drying transitions, and as a realistic water model for coarse-grained simulations of biomolecules and complex materials.     

Phase behavior and local structure of a binary mixture in pores: mean-field lattice model calculations for analyzing neutron scattering data

We investigate the phase behavior of an asymmetric binary liquid A-W mixture confined between two planar homogenous substrates (slit pore). Molecules of species W interact preferentially with the solid walls via a long-range potential. Assuming nearest-neighbor attractions between the liquid molecules, we employ a lattice-gas model and a mean-field approximation for the grand potential. Minimization of this potential yields the density profiles of thermodynamically stable phases for fixed temperature, chemical potentials of both species, pore width and strengths of attraction. This model is used to analyze experimental small-angle neutron-scattering (SANS) data on the microscopic structure of the binary system isobutyric acid (iBA)+heavy water (D2O) inside a mesoscopic porous matrix (controlled-pore glass of about 10 nm mean pore width). Confinement-independent model parameters are adjusted so that the theoretical liquid-liquid coexistence curve in the bulk matches its experimental counterpart. By choosing appropriate values of the pore width and the attraction strength between substrates and water we analyze the effect of confinement on the phase diagram. In addition to a depression of the liquid-liquid critical point we observe surface induced phase transitions as well as water-film adsorption near the walls. The temperature dependence of the structure of water-rich and iBA-rich phases of constant composition are discussed in detail. The theoretical predictions are consistent with results of the SANS study and assist their interpretation.     

The phase behavior, structure, and dynamics of rodlike mesogens with various flexibility using dissipative particle dynamics simulation

We present a systematic dissipative particle dynamics (DPD) study on the phase behavior, structure, and dynamics of rodlike mesogens. In addition to a rigid fused-bead-chain model with RATTLE constraint method, we also construct a semirigid model in which the flexibility is controlled by the bending constant of k(φ). Using this notation, the rigid model has an infinite bending constant of k(φ)=∞. Within the parameter space studied, both two kinds of models exhibit the nematic and smectic-A phases in addition to the isotropic and solid phases. All of the phase transitions are accompanied by the discontinuities in the thermodynamical, structural, and dynamical quantities and the hysteresis around the transition points, and are therefore first order. Note that the obtained solid state exhibits an in-layer tetragonal packing due to the high density. For the rigid model, the simulations show that the liquid crystal phases can be observed for mesogens with at least five beads and the nematic phase is the first one to appear. More importantly, the phase diagram of seven-bead-chain models is obtained as a function of k(φ) and temperature. It is found that decreasing the value of k(φ) reduces the anisotropy of molecular shape and the orientational ordering, and thereby shifts the liquid crystal phases to the lower temperature end of the phase diagram. Due to the different k(φ) dependence of phase transition temperatures, the nematic phase range exhibits a more marked narrowing than the smectic-A phase as k(φ) is reduced, implying that the flexibility has a destabilizing effect on the nematic and smectic-A phases. We also have investigated the anisotropic translational diffusion in liquid crystal phases and its temperature and flexibility dependence. In our study, we find that the phases formed, their statical and dynamic properties, as well as the transition properties are in close accord with those observations in real thermotropic liquid crystals. It is clear that both the rigid and semirigid models we used are valuable models with which to study the behavior of thermotropic liquid crystals using DPD algorithm.     

Orientational dynamics for an amphiphilic-solvent solution

In this work, we performed Monte Carlo simulations on a lattice model for spontaneous amphiphilic aggregation, in order to study the orientational and hydrogen-bonding dynamics of water on different regions inside the micellar solution. We employed an associating lattice gas model that mimics the aqueous solvent, which presents a rich phase diagram with first- and second-order transition lines. Even though this is a simplified model, it makes possible to investigate the orientational dynamics of water in an equilibrium solution of amphiphiles, as well as the influence of the different phases of the solvent in the interfacial and bulk water dynamics. By means of extensive simulations, we showed that, at high temperatures, the behavior of the orientational relaxation and hydrogen bonding of water molecules in the bulk, first, and second hydration shells are considerable different. We observe the appearance of a very slow component for water molecules in the first hydration shell of micelles when the system reaches a high-density phase, consistent with previous theoretical and experimental studies concerning biological water. Also, at high temperatures, we find that water molecules in the second hydration shell of micelles have an orientational decay similar to that of bulk water, but with a generally slower dynamics. Otherwise, at low temperatures, we have two components for the orientational relaxation of bulk water in the low density liquid phase, and only a single component in the high density liquid (HDL) phase, which reflect the symmetry properties of the different phases of the solvent model. In the very dense region of water molecules in the first hydration shell of micelles at low temperatures, we find two components for the orientational relaxation on both liquid phases, one of them much slower than that in the single component of bulk water in the HDL phase. This happens even though our model does not present any hindrance to the water rotational freedom caused by the presence of the amphiphiles.     

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.     

Four phases of amorphous water: Simulations versus experiment

Multiplicity of the liquid-liquid phase transitions in supercooled water, first obtained in computer simulations [Brovchenko et al., J. Chem. Phys. 118, 9473 (2003)], has got strong support from the recent experimental observation of the two phase transitions between amorphous ices [Loerting et al., Phys. Rev. Lett. 96, 025702 (2006)]. These experimental results allow assignment of the four amorphous water phases (I-IV) obtained in simulations to the three kinds of amorphous ices. Water phase I (rho approximately 0.90 gcm(3)) corresponds to the low-density amorphous ice, phase III (rho approximately 1.10 gcm(3)) to the high-density amorphous ice, and phase IV (rho approximately 1.20 gcm(3)) to the very-high-density amorphous ice. Phase II of model water with density rho approximately 1.00 gcm(3) corresponds to the normal-density water. Such assignment is confirmed by the comparison of the structural functions of the amorphous phases of model water and real water. In phases I and II the first and second coordination shells are clearly divided. Phase I consists mainly of the four coordinated tetrahedrally ordered water molecules. Phase II is enriched with molecules, which have tetrahedrally ordered four nearest neighbors and up six molecules in the first coordination shell. Majority of the molecules in phase III still have tetrahedrally ordered four nearest neighbors. Transition from phase III to phase IV is characterized by a noticeable drop of tetrahedral order, and phase IV consists mainly of molecules with highly isotropic angular distribution of the nearest neighbors. Relation between the structures of amorphous water phases, crystalline ices, and liquid water is discussed.     

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.     

The putative liquid-liquid transition is a liquid-solid transition in atomistic models of water

We use numerical simulation to examine the possibility of a reversible liquid-liquid transition in supercooled water and related systems. In particular, for two atomistic models of water, we have computed free energies as functions of multiple order parameters, where one is density and another distinguishes crystal from liquid. For a range of temperatures and pressures, separate free energy basins for liquid and crystal are found, conditions of phase coexistence between these phases are demonstrated, and time scales for equilibration are determined. We find that at no range of temperatures and pressures is there more than a single liquid basin, even at conditions where amorphous behavior is unstable with respect to the crystal. We find a similar result for a related model of silicon. This result excludes the possibility of the proposed liquid-liquid critical point for the models we have studied. Further, we argue that behaviors others have attributed to a liquid-liquid transition in water and related systems are in fact reflections of transitions between liquid and crystal.     

To be or not to be – nematic liquid crystals from shape-persistent V-shaped nematogens with the ‘magic angle’

The tetrahedral bending angle in V-shaped nematogens was claimed to be the optimum for finding a biaxial nematic liquid crystal phase. The benzo[1,2-b:4,3-b’]dithiophene core, recently successfully applied as a tetrahedral bending unit in mesogens with lateral flexible chains, is here embedded in a scaffold with only terminal chains, which conventionally promotes the formation of nematic phases at low temperature. A series of new mesogens has been successfully prepared, realising hockey-stick, hockey-stick dimer and V-shaped molecular topologies. Only the hockey-stick mesogens assemble in uniaxial nematic phases over a broad temperature range. Single crystal structure analysis of a hockey-stick and V-shaped compound reveal remarkable similarities with the benzodithiophene core wrapped by aliphatic chains. A model explaining the absence of nematic mesophases in the family of V-shaped, shape-persistent mesogens with terminal aliphatic chains is presented and results in the proposal of a new design for biaxial nematogens.     

Density depletion at solid-liquid interfaces: a neutron reflectivity study

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.     

Dynamic light scattering study of the dynamics of a gelled polymeric micellar system

The dynamics of the E(92)B(18)/water system are studied by dynamic light scattering (DLS) in the liquid, soft gel, and hard gel phases. Both the liquid and the soft gel phases are micellar phases, although the structural order is higher in the soft gel phase than in the liquid phase. The hard gel phase corresponds to a face-centered cubic arrangement of micelles. DLS results show that the dilute liquid phase is characterized by a single characteristic time tau(1) associated with the diffusion of the micelles. In addition, a second characteristic time tau(2) associated with the presence of micellar clusters in the system is identified in the concentrated liquid and in the soft gel phases. According to these results, DLS suggests that the structure of the soft gel phase comprises micellar clusters coexisting with micellar fluid, in good agreement with hypotheses from our previous work. The dynamics of the system slows down as the hard gel phase is approached and a plateau is observed in the DLS correlation function. The structure of the hard gel is "softened" upon increasing temperature and/or decreasing concentration.     

Liquid-liquid phase transitions in supercooled water studied by computer simulations of various water models

Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by Monte Carlo (MC) simulations of isotherms of density fluctuation-restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P, and SPCE models and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well-known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids are discussed.     

Consequences of chain networks on thermodynamic, dielectric and structural properties for liquid water

A vast array of experimental data on water provides a global view of the liquid that implicates its tetrahedral hydrogen-bonding network as the unifying molecular connection to its observed structural, thermodynamic, and dielectric property trends with temperature. Recently the classification of water as a tetrahedral liquid has been challenged based on X-ray absorption (XAS) experiments on liquid water (Ph. Wernet et al., Science, 2004, 304, 995), which have been interpreted to show a hydrogen-bonding network that replaces tetrahedral structure with chains or large rings of water molecules. We examine the consequences of tetrahedral vs. chain networks using three different modified water models that exhibit a local hydrogen bonding environment of two hydrogen bonds (2HB) and therefore networks of chains. Using these very differently parameterized models we evaluate their bulk densities, enthalpies of vaporization, heat capacities, isothermal compressibilities, thermal expansion coefficients, and dielectric constants, over the temperature range of 235-323 K. We also evaluate the entropy of the 2HB models at room temperature and whether such models support an ice I(h) structure. All show poor agreement with experimentally measured thermodynamic and dielectric properties over the same temperature range, and behave similarly in most respects to normal liquids.     

Crystallization of tetrahedral patchy particles in silico

We investigate the competition between glass formation and crystallization of open tetrahedral structures for particles with tetrahedral patchy interactions. We analyze the outcome of such competition as a function of the potential parameters. Specifically, we focus on the separate roles played by the interaction range and the angular width of the patches, and show that open crystal structures (cubic and hexagonal diamond and their stacking hybrids) spontaneously form when the angular width is smaller than about 30°. Evaluating the temperature and density dependence of the chemical potential of the fluid and of the crystal phases, we find that adjusting the patch width affects the fluid and crystal in different ways. As a result of the different scaling, the driving force for spontaneous self-assembly rapidly grows as the fluid is undercooled for small-width patches, while it only grows slowly for large-width patches, in which case crystallization is pre-empted by dynamic arrest into a network glass.     

Density, DSC, X-ray and NMR measurements through the gel and lamellar phase transitions of 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC): observation of slow relaxation processes and mechanisms of phase transitions

Dialkyl lecithin dispersions in water exhibit two phase transitions upon cooling from the lamellar phase (L(α)). At the main transition (T(M)) the L(α) phase changes to a ripple (gel) phase (P(β')) which then transforms to a second gel phase (L(β')) at the "pretransition" (T(P)). We have made accurate density measurements through the various phases for two lecithins having unequal chains: 1-myristoyl-2-stearoyl-sn-glycero-3-phosphatidylcholine (MSPC) and 1-stearoyl-2-myristoyl-sn-glycero-3-phosphatidylcholine (SMPC). The measurements were carried out over five heat/cool cycles from 5 to 55 °C, followed by cooling back to 5 °C. The samples were then held at 50 °C for 24 hours, followed by a further three cool/heat cycles. For SMPC we observe an increase in density of the gel phases over the first 5 cycles, followed by much smaller changes after incubation at 50 °C. The lamellar phase also shows an increase in density, albeit much smaller. This parallels the behaviour of 1,2-di-palmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) and 1,2-di-myristoyl-sn-glycero-3-phosphatidylcholine (DMPC) reported earlier (Jones et al., Liquid Crystals 32, 1465 (2005)). For MSPC we observe a decrease in density within the gel phases while T(P) almost disappears after the first cycle. The lamellar phase shows little evidence of any change with each cycle. Within the lamellar phases there is a marked reduction in density on approaching T(M), which is attributed to the formation of transitory gel phase domains. Additional measurements by DSC and X-ray diffraction show that the changes in densities are not accompanied by large changes in transition enthalpies or phase structures. NMR data indicate that the pretransitional event within the L(α) phase is accompanied by ordering of the alkyl chains. The results indicate that the exact nature of the lipid alkyl chains could play a key role in the formation of gel phase patches within membrane bilayers. Their detailed chemical structures merit more attention than by simply assuming a uniform "bending energy" to describe the behaviour.     

Formation of ice-like water structure on the surface of an antifreeze protein

Antifreeze proteins (AFPs) are found in different species from polar, alpine, and subarctic regions where they serve to inhibit ice crystal growth by adsorption to ice surfaces. Computational methods have the power to investigate the antifreeze mechanism in atomic detail. Molecular dynamics simulations of water under different conditions have been carried out to test our water model for simulations of biological macromolecules in extreme conditions: very low temperatures (200 K) and at the ice/liquid water interface. We show that the flexible F3C water model reproduces properties of water in the solid phase (ice I(h)), the supercooled liquid phase, and at the ice/liquid water interface. Additionally, the hydration of the type III AFP from ocean pout was studied as a function of temperature. Hydration waters on the ice-binding surface of the AFP were less distorted and more tetrahedral than elsewhere on the surface. More ice-like hydrating water structures formed on the ice-binding surface of the protein such that it created an ice-like structure in water within its first hydration layer but not beyond, suggesting that this portion of the protein has high affinity for ice surfaces.     

Characterization of C18-bonded liquid chromatographic stationary phases by Raman spectroscopy: the effect of temperature

This study represents the first time that both the mobile phase composition and the temperature are simultaneously controlled to examine silica-bonded octadecylsilyl (C18) ligands spectroscopically at typical liquid chromatographic (LC) mobile phase flow-rates and back-pressures. Raman spectroscopy is used to characterize the behavior of the C18 bonded ligands equilibrated at temperatures from 45 to 2 degrees C in neat, single-component, mobile phase solvents including: water, acetonitrile, methanol, and chloroform. In addition, the effect of stationary phase ligand bonding density is examined by using two different monomeric reversed-phase liquid chromatographic (RPLC) stationary phases, a 2.34 and a 3.52 micromol m(-2) Microporasil C18 stationary phase, under identical conditions. The direct, on-column, spectroscopic analysis used in this study allows direct evaluation of the temperature-dependent behavior of the bonded C18 ligands. The temperature-dependent ordering of the stationary phase ligands is examined to determine if the ligands undergo a phase transition from a less-ordered "liquid-like" state at higher temperatures to a more-ordered "solid-like" state at lower temperatures. A discrete phase transition was not observed, but rather a continual ordering as temperature was lowered.     

Quantitative SAXS analysis of the P123/water/ethanol ternary phase diagram

The ternary phase diagram of the amphiphilic triblock copolymer PEO-PPO-PEO ((EO)(20)(PO)(70)(EO)(20) commercialized under the generic name P123), water, and ethanol has been investigated at constant temperature (T = 23 degrees C) by small-angle X-ray scattering (SAXS). The microstructure resulting from the self-assembly of the PEO-PPO-PEO block copolymer varies from micelles in solution to various types of liquid crystalline phases such as cubic, 3D hexagonal close packed spheres (HCPS), 2D hexagonal, and lamellar when the concentration of the polymer is increased. In the isotropic liquid phase, the micellar structural parameters are obtained as a function of the water-ethanol ratio and block copolymer concentration by fitting the scattering data to a model involving core-shell form factor and a hard sphere structure factor of interaction. The micellar core, the aggregation number, and the hard sphere interaction radius decrease when increasing the ethanol/water ratio in the mixed solvent. We show that the fraction of ethanol present in the core is responsible for the swelling of the PPO blocks. In the different liquid crystalline phases, structural parameters such as lattice spacing, interfacial area of PEO block, and aggregation number are also evaluated. In addition to classical phases such as lamellar, 2D hexagonal, and liquid isotropic phases, we have observed a two-phase region in which cubic Fm3m and P6(3)mmc (hexagonally close packing of spheres (HCPS)) phases coexist. This appears at 30% (w/w) of P123 in pure water and with 5% (w/w) of ethanol. At 10% (w/w) ethanol, only the HCPS phase remains present.     

Computational investigation of order, structure, and dynamics in modified water models

Model liquids have been constructed to study the role of local structure in the anomalous properties of liquid water. The intermolecular potentials were modified by increasing the weight of the Lennard-Jones term relative to the electrostatic term in the SPC/E model for water. The resulting family of liquids varies from SPC/E water to a Lennard-Jones-like liquid. Properties were measured as a function of density and temperature. The local structure was described by two order parameters, one measuring the tetrahedral order and the other measuring the translational order. The translational order parameter was found to be large for both tetrahedral and Lennard-Jones liquids, but to go through a minimum as the potentials were modified, demonstrating that the two types of structure are incompatible. Just as in water several properties (e.g., the translational diffusion coefficient, entropy) exhibit anomalous density dependence as a result of the breakdown of local tetrahedrality, we observed nonmonotonic behavior of the translational diffusion constant and reorientational relaxation rate as the fluids were transformed from tetrahedral to Lennard-Jones-like. This is also an indication of the incompatibility between Lennard-Jones and water-like structure.