Local fluctuations in solution mixtures

An extension of the traditional Kirkwood-Buff (KB) theory of solutions is outlined which provides additional fluctuating quantities that can be used to characterize and probe the behavior of solution mixtures. Particle-energy and energy-energy fluctuations for local regions of any multicomponent solution are expressed in terms of experimentally obtainable quantities, thereby supplementing the usual particle-particle fluctuations provided by the established KB inversion approach. The expressions are then used to analyze experimental data for pure water over a range of temperatures and pressures, a variety of pure liquids, and three binary solution mixtures - methanol and water, benzene and methanol, and aqueous sodium chloride. In addition to providing information on local properties of solutions it is argued that the particle-energy and energy-energy fluctuations can also be used to test and refine solute and solvent force fields for use in computer simulation studies.     

The cluster structure of dilute aqueous-alcoholic solutions and molecular light scattering in them

The structures, equations of state, and character of fluctuations of dilute water-glycerol solutions are discussed. Two or three glycerol and about ten water molecules were found to form a fairly stable molecular complex. We call this complex elementary cluster (pseudoparticle). In a certain region of state parameters, the system could be considered a solution of pseudoparticles (clusters). Its properties were modeled by the van der Waals equation. The character of interactions between clusters was analyzed. An anomalous increase in concentration and molecular light scattering fluctuations was caused by the approach to the solution “pseudospinodal.” The experimental data were found to be in quite satisfactory agreement with theoretical estimates.     

Photodegradation of natural organic matter exposed to fluctuating levels of solar radiation

Irradiation of natural water samples with natural or artificial UVR typically results in a progressive loss of color and decreased absorbance; a process often referred to as photobleaching. In a typical photobleaching experiment, samples are exposed to a relatively constant level of artificial or natural UVR. However, under most natural situations, the vertical mixing of the water within the upper mixed layer results in strong and periodic fluctuations in UV irradiance. In this paper, we present the results of an experiment in which natural lake water was exposed to solar radiation in quartz tubes that were incubated either at fixed depths or rotating within the water column. We found differences between rotating and fixed samples in (i) photobleaching, (ii) nutrient release, and (iii) subsequent use by algae and bacteria. The evidence presented in this study demonstrated that photochemical processes might be affected by vertical water motion. The reasons for such differences remain largely unknown. Although we offer a potential explanation for such differences, our proposed mechanism is based on a post-hoc analysis of the data and should be taken solely as a working hypothesis for future research.     

Molecular dynamics simulation of cellobiose in water

The conformational behavior of cellobiose was studied by molecular dynamics simulation in a periodic box of waters. Several different initial conformations were used and the results compared with equivalent vacuum simulations. The average positions and rms fluctuations within single torsional conformations of cellobiose were affected only slightly by the solvent. However, water damped local torsional librations and transitions. The conformational energies of the solute and their fluctuations were also sensitive to the presence of solvent. Intramolecular hydrogen bonding was weakened relative to that observed in vacuo due to competition with solvating waters. All cellobiose hydroxyl groups participated in intermolecular hydrogen bonds with water, with approximately eight hydrogen bonds formed per glucose ring. The hydrogen bonding was predominantly between water hydrogens and solute hydroxyl oxygens. Intermolecular hydrogen bonding to ring and bridge oxygens was seldom present. The diffusion coefficients of both water and solute agree closely with experimental values. Water interchanged rapidly between the solvating first shell and the bulk on the picosecond time scale. © 1993 John Wiley & Sons, Inc.     

Temperature and scattering contrast dependencies of thickness fluctuations in surfactant membranes

Temperature and scattering contrast dependencies of thickness fluctuations have been investigated using neutron spin echo spectroscopy in a swollen lamellar phase composed of nonionic surfactant, water, and oil. In the present study, two contrast conditions are examined; one is the bulk contrast, which probes two surfactant monolayers with an oil layer as a membrane, and the other is the film contrast, which emphasizes an individual surfactant monolayer. The thickness fluctuations enhance dynamics from the bending fluctuations, and are observed in a similar manner in both contrast conditions. Thickness fluctuations can be investigated regardless of the scattering contrast, though film contrasts are better to be employed in terms of the data quality. The thickness fluctuation amplitude is constant over the measured temperature range, including in the vicinity of the phase boundary between the lamellar and micellar phases at low temperature and the boundary between the lamellar and bicontinuous phases at high temperature. The damping frequency of the thickness fluctuations is well scaled using viscosity within the membranes at low temperature, which indicates the thickness fluctuations are predominantly controlled by the viscosity within the membrane. On the other hand, in the vicinity of the phase boundary at high temperature, thickness fluctuations become faster without changing the mode amplitude.     

Assessment of Temporal Variability Trends for Inorganic Parameters in Ground Water

Abstract

The passage of environmental legislation in the United States has dramatically increased ground-water monitoring in the vicinity of point sources such as abandoned waste disposal sites, operational waste disposal sites, and municipal landfills. Even though these programs require sufficient sampling to define background conditions as part of the site characterization process, there is still a general absence of quantitative information on the magnitude and periodicity of temporal fluctuations for inorganic constituents in ground water. This paper presents an approach that has been used to develop an initial characterization of these temporal trends.

A search if on-going site investigation reports identified 18 facilities across the United States that had monthly monitoring data at a frequency of at least monthly for a period of one and a half years or longer (15 RCRA-C hazardous waste disposal facilities with monthly data for a period of 2–3 years, 2 research monitoring locations with biweekly monitoring data for a period of one and a half years, and a precious metal mining operation with daily monitoring data for a limited number of parameters for a period of one and a half years). The data from these site investigations were used to describe the temporal variability of several ground-water constituents including pH, specific conductance, sulfate, sodium, chloride, alkalinity, silica, iron, and manganese. An assessment of these data suggests that the magnitude of temporal ground-water fluctuations are on the order of 20 percent of the average concentration for chloride, 10 percent of the average concentration for sodium, manganese and specific conductance, 5 percent of the average concentration for alkalinity and pH, and essentially zero for silica. The apparent periodicities of these temporal fluctuations ranged from 40 weeks to approximately 2 years. The magnitude and periodicities in ground water are substantially smaller than those that have been reported and documented for the same constituents in surface waters. These differences are due to the fact that sunlight and wind, two energy factors that drive temporal cycles in surface water, do not exert a similar influence on the environmental chemistry of ground-water constituents.     

Enhanced small-angle scattering connected to the Widom line in simulations of supercooled water

We present extensive simulations on the TIP4P∕2005 water model showing significantly enhanced small-angle scattering (SAS) in the supercooled regime. The SAS is related to the presence of a Widom line (T(W)) characterized by maxima in thermodynamic response functions and Ornstein-Zernike correlation length. Recent experimental small-angle x-ray scattering data [Huang et al., J. Chem. Phys. 133, 134504 (2010)] are excellently reproduced, albeit with an increasing temperature offset at lower temperatures. Assuming the same origin of the SAS in experiment and model this suggests the existence of a Widom line also in real supercooled water. Simulations performed at 1000 bar show an increased abruptness of a crossover from dominating high-density (HDL) to dominating low-density (LDL) liquid and strongly enhanced SAS associated with crossing T(W), consistent with a recent determination of the critical pressure of TIP4P∕2005 at 1350 bar. Furthermore, good agreement with experimental isothermal compressibilities at 1000, 1500, and 2000 bar shows that the high pressure supercooled thermodynamic behavior of water is well described by TIP4P∕2005. Analysis of the tetrahedrality parameter Q reveals that the HDL-LDL structural transition is very sharp at 1000 bar, and that structural fluctuations become strongly coupled to density fluctuations upon approaching T(W). Furthermore, the tetrahedrality distribution becomes bimodal at ambient temperatures, an observation that possibly provides a link between HDL-LDL fluctuations and the structural bimodality in liquid water indicated by x-ray spectroscopic techniques. Computed x-ray absorption spectra are indeed found to show sensitivity to the tetrahedrality parameter.     

To the theory of homogeneous nucleation: The nonisothermality of the Szilard model

The kinetics of clusters in a nucleating system was considered. The inapplicability in principle of the Gauss (symmetrical) approximation to the distribution of clusters in a system with temperature fluctuations was shown. The appearance of “nonisothermality” was demonstrated by consistently using the exact (nonsymmetrical) representation of fluctuations. The nonisothermality under consideration was shown to arise even at saturation S = 1. The results that describe the degree of nonisothermality of the distribution of water clusters were obtained. These results showed that the nonisothermality effect weakly depended on the rate of heat transfer between clusters and the environment, which was in agreement with the experimental data on nucleation in various gases at various pressures.     

First-principles simulations of the electrode|electrolyte interface

We review a direct dynamics method for the simulation of metal|water interfaces. The occupancy of on-top binding sites for water in this model as applied to a (100) surface of ‘copper' is very sensitive to potential. We suggest that this may account for some previously unexplained features of X-ray data on water structure and noble metal|water interfaces. We discuss the problem of statistical fluctuations on the occupancy of such tightly bound water molecules in such simulations. Though the problem is not too serious for charged interfaces, the problem of accounting for fluctuations at zero charge can be quite formidable, as we illustrate for the (100) surface of copper.     

Evaluation and application of internal standardization in atomic absorption spectrometry with electrothermal atomization

A new dual-channel system developed for use in atomic absorption spectrometry is used to assess the internal standard technique for electrothermal atomization. Cobalt was found to be a suitable internal standard for iron determinations, and is used for determination of 7—330 ng Fe ml^-1 in water samples. With use of the internal standard technique, fluctuations caused by atomizer variables are reduced and interferences from many cations are also decreased.     

Rotational fluctuations of water confined to layered oxide materials: nonmonotonous temperature dependence of relaxation times

The rotational molecular dynamics of water confined to layered oxide materials with brucite structure was studied by dielectric spectroscopy in the frequency range from 10(-2) to 10(7) Hz and in a broad temperature interval. The layered double hydroxide samples show one relaxation process, which was assigned to fluctuations of water molecules forming a layer, strongly adsorbed to the oxide surface. The temperature dependence of the relaxation rates has an unusual saddlelike shape characterized by a maximum. The model of Ryabov et al. (J. Phys. Chem. B 2001, 105, 1845) recently applied to describe the dynamics of water molecules in porous glasses is employed also for the layered materials. This model assumes two competing effects: rotational fluctuations of water molecules that take place simultaneously with defect formation, allowing the creation of free volume necessary for reorientation. The activation energy of rotational fluctuations, the energy of defect formation, a pre-exponential factor, and the defect concentration are obtained as main parameters from a fit of this model to the data. The values of these parameters were compared with those found for water confined to nanoporous molecular sieves, porous glasses, or bulk ice. Several correlations were discussed in detail, such as the lower the value of the energy of defect formation, the higher the number of defects. The pre-exponential factor increases with increasing activation energy, as an expression of the compensation law, and indicates the cooperative nature of the motional process. The involvement of the surface OH groups and of the oxygen atoms of the interlayer anions in the formation of hydrogen bonds was further discussed. For the birnessite sample, the relaxation processes are probably overlaid by a dominating conductivity contribution, which is analyzed in its frequency and temperature dependence. It is found that the conductivity of birnessite obeys the characteristics of semiconducting disordered materials. Especially the Barton/Nakajima/Namikawa relationship is fulfilled. Analyzing the temperature dependence of the direct current (dc) conductivity sigma0 in detail gives some hint that sigma0(T) has also an unusual saddlelike form.     

A coupled density functional-molecular mechanics Monte Carlo simulation method: The water molecule in liquid water

A theoretical model to investigate chemical processes in solution is described. It is based on the use of a coupled density functional/molecular mechanics Hamiltonian. The most interesting feature of the method is that it allows a detailed study of the solute's electronic distribution and of its fluctuations. We present the results for isothermal-isobaric constant-NPT Monte Carlo simulation of a water molecule in liquid water. The quantum subsystem is described using a double-zeta quality basis set with polarization orbitals and nonlocal exchange-correlation corrections. The classical system is constituted by 128 classical TIP3P or Simple Point Charge (SPC) water molecules. The atom-atom radial distribution functions present a good agreement with the experimental curves. Differences with respect to the classical simulation are discussed. The instantaneous and the averaged polarization of the quantum molecule are also analyzed. © 1996 by John Wiley & Sons, Inc.     

The Structural and Dynamical Properties of the Hydration of SNase Based on a Molecular Dynamics Simulation

The dynamics of protein–water fluctuations are of biological significance. Molecular dynamics simulations were performed in order to explore the hydration dynamics of staphylococcal nuclease (SNase) at different temperatures and mutation levels. A dynamical transition in hydration water (at ~210 K) can trigger larger-amplitude fluctuations of protein. The protein–water hydrogen bonds lost about 40% in the total change from 150 K to 210 K, while the Mean Square Displacement increased by little. The protein was activated when the hydration water in local had a comparable trend in making hydrogen bonds with protein– and other waters. The mutations changed the local chemical properties and the hydration exhibited a biphasic distribution, with two time scales. Hydrogen bonding relaxation governed the local protein fluctuations on the picosecond time scale, with the fastest time (24.9 ps) at the hydrophobic site and slowest time (40.4 ps) in the charged environment. The protein dynamic was related to the water’s translational diffusion via the relaxation of the protein–water’s H-bonding. The structural and dynamical properties of protein–water at the molecular level are fundamental to the physiological and functional mechanisms of SNase.     

Electrical fluctuations on the surfaces of proteins from hydrodynamic data

We calculate the electrical capacitance on the surfaces of protein molecules from hydrodynamic data of the proteins. Then we estimate the electrical fluctuations (charge, voltage) through the fluctuation-dissipation theorem, which links the electrical capacitance of the system with these fluctuations. From the intrinsic viscosity of the proteins, we estimate the polarizability, which leads to knowledge of the field and dipole fluctuations. From the fitting of the capacitance, polarizability, and electrical fluctuations as a function of the molecular weight of the proteins, we report numerical equations that make it possible to estimate these physical magnitudes for a given protein, knowing the molecular weight. Charge fluctuations are in fractions of unit charge range, voltage fluctuations are in the three-mV-digit range, field fluctuations are in the two-digit mV/nm (10(6) V/m) range, and the dipole moment fluctuations range from two to three digits, times the dipole moment of the water molecule. These surface properties of proteins have not been reported before.     

Fluorescence study of the sol-gel process in hybrid precursors: evidence of concentration fluctuations at the local scale

Hybrid sol-gel materials have been prepared by hydrolytic polycondensation of an alkoxysilane. The sol-gel process of methacryloxypropyltrimethoxysilane (MAPTMS) has been followed by fluorescence spectroscopy with 2-naphthol as a probe. The experimental results showed that this photoprobe was dramatically sensitive to the microenvironment polarity. Spectroscopic studies revealed fluctuations of the maximum emission intensity and wavelength as a function of time. These fluctuations were attributed to the amphiphilic behavior of the hydrolyzed precursor. The maximum emission wavelength of the probe corresponding to its protonated form was higher than in pure water. All the results suggest that the presence of water molecules, tightly bonded to the polar head of the silanols, increased locally the sol polarity and induced a red-shifted emission. Fluorescence spectroscopy emphasized the reversibility of monomeric silanol aggregates and the changes in hydroxy group number of the silica network during the sol maturation. The behavior of this system upon shaking confirmed this statement.     

The role of solvent fluctuations in hydrophobic assembly

We use a coarse-grained solvent model to study the self-assembly of two nanoscale hydrophobic particles in water. We show how solvent degrees of freedom are involved in the process. By using tools of transition path sampling, we elucidate the reaction coordinates describing the assembly. In accord with earlier expectations, we find that fluctuations of the liquid-vapor-like interface surrounding the solutes are significant, in this case leading to the formation of a vapor tunnel between the two solute particles. This tunnel accelerates assembly. While considering this specific model system, the approach we use illustrates a methodology that is broadly applicable.     

Ultrafast anisotropy dynamics of water molecules dissolved in acetonitrile

Infrared pump-probe experiments are performed on isolated H(2)O molecules diluted in acetonitrile in the spectral region of the OH stretching vibration. The large separation between water molecules excludes intermolecular interactions, while acetonitrile as a solvent provides substantial hydrogen bonding. Intramolecular coupling between symmetric and asymmetric modes results in the anisotropy decay to the frequency-dependent values of approximately 0-0.2 with a 0.2 ps time constant. The experimental data are consistent with a theoretical model that includes intramolecular coupling, anharmonicity, and environmental fluctuations. Our results demonstrate that intramolecular processes are essential for the H(2)O stretching mode relaxation and therefore can compete with the intermolecular energy transfer in bulk water.     

Dielectric dispersion interpretation of single enzyme dynamic disorder, spectral diffusion, and radiative fluorescence lifetime

A formulation based on measurable dielectric dispersion of enzymes is developed to estimate fluctuations in electrostatic interaction energy on time scales as long as milliseconds to seconds at a local site in enzymes. Several single molecule experimental obsevations occur on this time scale, currently unreachable by real time computational trajectory simulations. We compare the experimental results on the autocorrelation function of the fluctuations of catalysis rate with the calculations using the dielectric dispersion formulation. We also discuss the autocorrelation functions of the fluorescence lifetime and of spectral diffusion. We use a previously derived relation between the observables and the electric field fluctuations and calculate the latter using dielectric dispersion data for the proteins and the Onsager regression hypothesis.     

Statistical mechanics of hydrated electron recombination in liquid and supercritical water

The photochemical yield of hydrated electrons as a function of temperature in liquid and supercritical water is treated in terms of energy fluctuations of the medium. The geminate pair, consisting of a positive ion and a hydrated electron, is regarded as a H-like atom embedded in a completely relaxed dielectric continuum. If the local medium energy is larger than the ionization energy of this atom, the electron escapes its geminate partner. By making use of the classical theory of energy fluctuations, escape probability is described by a simple explicit function, the variable of which is a combination of temperature, relative permittivity, and specific heat. First our earlier calculations on the recombination of solvated electrons, produced by ionizing radiation in a number of polar liquids, are improved and then the theory is compared with the experimental results on temperature dependent electron survival by Kratz et al. [S. Kratz, J. Torres-Alcan, J. Urbanek, J. Lindner, and P. Vo?hringer, Phys. Chem. Chem. Phys. 12, 12169 (2010)]. Two adjustable parameters are needed to achieve reasonable quantitative agreement.     

Time-resolved microscopy of chromatin in vitro and in vivo

In eukaryotic cell nuclei, double-stranded DNA is found in the form of chromatin, a large fiber made up of DNA complexed to histone proteins. In this article, recent studies using fluorescence techniques to look at the dynamics of chromatin, both in vivo and in vitro, are reviewed. Two-photon counterpropagating fluorescence recovery after patterned photobleaching is used to examine chromatin fluctuations on lengthscales ranging from less than 100 nm to microns. By combining in vivo studies with data on isolated nuclei and by measuring how these fluctuations depend on variables like ionic strength and photochemical cross-linking, it is demonstrated that the relatively large-scale motions of chromatin observed in vivo are consistent with smaller scale modifications of the histone-DNA interaction. This connection may provide a means to use conformational dynamics as an in vivo probe of the biochemical events involved in gene expression.