Temperature Of Maximum Density Research Articles

Isotope effects on the structural transformation and relaxation of deeply supercooled water.

We have examined the structure of supercooled liquid D2O as a function of temperature between 185 and 255K using pulsed laser heating to rapidly heat and cool the sample on a nanosecond timescale. The liquid structure can be represented as a linear combination of two structural motifs, with a transition between them described by a logistic function centered at 218K with a width of 10K. The relaxation to a metastable state, which occurred prior to crystallization, exhibited nonexponential kinetics with a rate that was dependent on the initial structural configuration. When the temperature is scaled by the temperature of maximum density, which is an isostructural point of the isotopologues, the structural transition and the non-equilibrium relaxation kinetics of D2O agree remarkably well with those for H2O.

Numerical simulations of the three-dimensionalization of a shear flow in radiatively forced cold water below the density maximum

In cold water (temperatures between water's freezing point and the temperature of maximum density), near-surface heating (from the sun) generates dense water which in turn induces vertical currents. If there is a near-surface current, the resulting convective instabilities efficiently move momentum from the current to regions lower in the water column. Then, there is an induced momentum flux across the plume boundary leading to a complicated series of three-dimensional interactions resulting in turbulence. How might this process be affected by factors such as water clarity and current speed?

Thermobaric Processes Both Drive and Constrain Seasonal Ventilation in Deep Great Slave Lake, Canada

AbstractThe contrasting roles of seasonal stratification and seasonal ventilation are among the most important attributes of lake systems in defining their geochemistry, biology and response to climate forcing. The physical processes that regulate these processes are especially significant in very deep lakes, where the joint effect of temperature and pressure on water density becomes important. Here we report on a year‐long time series (2019–2020) of temperature from a mooring deployed in seasonally ice covered and dimictic Great Slave Lake (614 m depth). Because the temperature of maximum density (TMD) decreases with increasing pressure, once ∼4°C water begins to sink, it is no longer at its depth‐specific maximum density and additional processes are required to drive convection and convective mixing. The key physical mechanism governing deep‐water renewal is the conditional thermobaric instability. We show here that (a) different temperature dynamics control the quantity and timing of convective renewal in the upper ∼200 m versus deep‐water renewal below; (b) fall and spring deep‐water renewal events are asymmetric and linked to weather during the brief time of ice forming (melting) and surface temperatures cooling (warming) through 4°C; (c) short term variability (days to weeks) related to the length of time between surface waters passing through TMD and ice formation (fall) and melt (spring) shapes the subsequent thermal structure; and (d) the penetration of solar radiation through ice in early spring drives penetrative convection in the upper layer, deepening the mixed‐layer, and this likely affects the timing of spring phytoplankton production.

Development of overturning circulation in sloping waterbodies due to surface cooling

Cooling the surface of freshwater bodies, whose temperatures are above the temperature of maximum density, can generate differential cooling between shallow and deep regions. When surface cooling occurs over a long enough period, the thermally induced cross-shore pressure gradient may drive an overturning circulation, a phenomenon called ‘thermal siphon’. However, the conditions under which this process begins are not yet fully characterised. Here, we examine the development of thermal siphons driven by a uniform loss of heat at the air–water interface in sloping, stratified basins. For a two-dimensional framework, we derive theoretical time and velocity scales associated with the transition from Rayleigh–Bénard type convection to a horizontal overturning circulation across the shallower sloping basin. This transition is characterised by a three-way horizontal momentum balance, in which the cross-shore pressure gradient balances the inertial terms before reaching a quasi-steady regime. We performed numerical and field experiments to test and show the robustness of the analytical scaling, describe the convective regimes and quantify the cross-shore transport induced by thermal siphons. Our results are relevant for understanding the nearshore fluid dynamics induced by nighttime or seasonal surface cooling in lakes and reservoirs.

Characterizing the Seasonal Variability of Hypolimnetic Mixing in a Large, Deep Lake

AbstractIn this study, we report on turbulent mixing observed during the annual stratification cycle in the hypolimnetic waters of Lake Michigan (USA), highlighting stratified, convective, and transitional mixing periods. Measurements were collected using a combination of moored instruments and microstructure profiles. Observations during the stratified summer showed a shallow, wind‐driven surface mixed layer (SML) with locally elevated dissipation rates in the thermocline () potentially associated with internal wave shear. Below the thermocline, turbulence was weak () and buoyancy‐suppressed ( < 8.5), with low hypolimnetic mixing rates () limiting benthic particle delivery. During the convective winter period, a diurnal cycle of radiative convection was observed over each day of measurement, where temperature overturns were directly correlated with elevated turbulence levels throughout the water column (; ). A transitional mixing period was observed for spring conditions when surface temperatures were near the temperature of maximum density (TMD3.98) and the water column began to stably stratify. While small temperature gradients allowed strong mixing over the transitional period (), hypolimnetic velocity shear was overwhelmed by weakly stable stratification (; ), limiting the development of the SML. These results highlight the importance of radiative convection for breaking down weak hypolimnetic stratification and driving energetic, full water column mixing during a substantial portion of the year (>100 days at our sample site). Ongoing surface water warming in the Laurentian Great Lakes is significantly reducing the annual impact of convective mixing, with important consequences for nutrient cycling, primary production, and benthic‐pelagic coupling.

A new methodology for determining the temperature of maximum density against pressure. Application to 2-propanol and ethanol aqueous solutions

Aqueous solutions of 2-propanol and ethanol are experimentally studied in order to obtain the location of the maximum of density versus temperature at high pressure. To this aim, a new technique based on measuring the pressure against temperature for several quasi-isochores was developed. These data are combined with isothermal compressibility, experimentally obtained from high pressure density data, to determine the isobaric thermal expansivity in the quasi-isochores. The temperature of maximum density is obtained by detecting the temperature for which this magnitude becomes zero. These results are compared with those obtained using highly precise densitometry at atmospheric pressure and also with literature data. The differences between the temperature of maximum density for solutions and that of water are positive for very diluted solutions, but they become negative as concentration is raised, which constitutes a quite unusual behavior as compared with other systems. As pressure is raised, this anomalous behavior becomes much more pronounced with positive deviations that exceed 1.5 K at the highest pressures. The underlying microscopic picture that could explain the observed behavior is suggested.

Modeling the temperature of maximum density of aqueous tert-butanol solutions

Short-chain alcohols at high dilution are among the very few solutes that enhance the anomalous behavior of water, in particular the value of the temperature of maximum density. This peculiar feature, first discovered experimentally in the early twenties, has remained elusive to a full explanation in terms of atomistic models. In this paper, we first introduce a two-site model of tert-butanol in which the interactions involving hydrogen bonding are represented by a Stillinger–Weber potential, following the ideas established by Molinero and Moore (2009) for water. Our model parameters are fit so as to semi-quantitatively reproduce the experimental densities and vaporization enthalpies of previously proposed united atom and all atom OPLS models. Water is represented using the aforementioned potential model introduced by Molinero and Moore, with cross interaction parameters between water and tert-butanol optimized to yield a reasonable description of the experimental excess enthalpies and volumes over the whole composition range of the mixture. We will see that our simple model is able to reproduce the presence of a maximum in the change of the temperature of maximum density for very low alcohol mole fractions, followed by a considerable decrease until the density anomaly itself disappears. We have correlated this behavior with changes in the local structure of water and compared it with the results of all-atom simulations of water/tert-butanol mixtures.

Temperature of Maximum Density of Water-Electrolytes: A Review

Temperature of Maximum Density of Water-Electrolytes: A Review

Asymmetries in gravity currents attributed to the nonlinear equation of state

Abstract

Machine learning for reparameterization of four-site water models: TIP4P-BG and TIP4P-BGT.

Parameterizing an effective water model is a challenging issue because of the difficulty in maintaining a comprehensive balance among the diverse physical properties of water with a limited number of parameters. The advancement in machine learning provides a promising path to search for a reliable set of parameters. Based on the TIP4P water model, hence, about 6000 molecular dynamics (MD) simulations for pure water at 1 atm and in the range of 273-373 K are conducted here as the training data. The back-propagation (BP) neural network is then utilized to construct an efficient mapping between the model parameters and four crucial physical properties of water, including the density, vaporization enthalpy, self-diffusion coefficient and viscosity. Without additional time-consuming MD simulations, this mapping operation could result in sufficient and accurate data for high-population genetic algorithm (GA) to optimize the model parameters as much as possible. Based on the proposed parameterizing strategy, TIP4P-BG (a conventional four-site water model) and TIP4P-BGT (an advanced model with temperature-dependent parameters) are established. Both the water models exhibit excellent performance with a reasonable balance among the four crucial physical properties. The relevant mean absolute percentage errors are 3.53% and 3.08%, respectively. Further calculations on the temperature of maximum density, isothermal compressibility, thermal expansion coefficient, radial distribution function and surface tension are also performed and the resulting values are in good agreement with the experimental values. Through this water modeling example, the potential of the proposed data-driven machine learning procedure has been demonstrated for parameterizing a MD-based material model.

Molecular dynamics simulations of aqueous solutions of short chain alcohols. Excess properties and the temperature of maximum density

The excess enthalpy hE and excess volume vE of the binary systems {methanol, ethanol, 1-propanol, 2-propanol, and tert-butanol + water} over the whole composition range at 298.15 K and the temperature of maximum density, TMD, in the alcohol diluted region were studied by MD simulations at atmospheric pressure. Our model systems were defined using the flexible version of the TIP4P/2005 model of water, the OPLS-AA for alcohols, and the geometric combining rule for all the Lennard-Jones cross interactions, with the exception of those between oxygen sites of water and alcohol molecules. These latter values were fitted to reproduce the experimental excess properties of the mixtures. This new parameterization allows for an appropriate description of the excess thermodynamics of these short-chain alcohol/water systems over the whole composition range. The changes in the temperature of maximum density of water induced by the addition of small amounts of alcohol concentration are, however, not adequately reproduced by our models. In all instances we observe a pronounced decrease of the simulated TMDs when alcohol concentration increases. This is in sharp contrast with the experimental behavior, which at very high dilution displays slight increases in the TMD right before the decreasing regime sets in for higher alcohol concentrations. For similar alcohol mole fractions, the simulated decrease of the TMDs grows in parallel with the size of the alkyl group of the alcohol. A similar situation is found experimentally for concentrations higher than those that increase the TMD. These differences disappear when the TMD change is expressed in terms of the mass concentration of alcohol. This leads to very similar Despretz constants for all solutions studied in this work.

Temperature, Pressure, and Concentration Derivatives of Nonpolar Gas Hydration: Impact on the Heat Capacity, Temperature of Maximum Density, and Speed of Sound of Aqueous Mixtures.

The hydrophobic effect is an umbrella term encompassing a number of solvation phenomena associated with solutions of nonpolar species in water, including the following: a meager solubility opposed by entropy at room temperature; large positive hydration heat capacities; positive shifts in the temperature of maximum density of aqueous mixtures; increases in the speed of sound of dilute aqueous mixtures; and negative volumes of association between interacting solutes. Here we present a molecular simulation study of nonpolar gas hydration over the temperature range 273.15-373.15 K and a pressure range -500 to 1000 bar to investigate the interrelationships between distinct hydrophobic phenomena. We develop a new free energy correlation for the solute chemical potentials founded on the Tait equation description of the equation-of-state of liquid water. This analytical correlation is shown to provide a quantitatively accurate description of nonpolar gas hydration over the entire range of thermodynamic state points simulated, with an error of ∼0.02 kBT or lower in the fitted chemical potentials. Our simulations and the correlation accurately reproduce many of the available experimental results for the hydration of the solutes examined here. Moreover, the correlation reproduces the characteristic entropies of hydration, temperature dependence of the hydration heat capacity, perturbations in the temperature of maximum density, and changes in the speed of sound. While negative volumes of association result from pairwise interactions in solution, beyond the limits of our simulations performed at infinite dilution, we discuss how our correlation could be supplemented with second virial coefficient information to expand to finite concentrations. In total, this work demonstrates that many distinct phenomena associated with the hydrophobic effect can be captured within a single thermodynamically consistent correlation for solute hydration free energies.

The temperature of maximum density for amino acid aqueous solutions. An experimental and molecular dynamics study

The temperature of maximum density (TMD) for aqueous solutions of seven amino acids has been experimentally determined by means of density measurements versus temperature. The selected amino acids have been arginine, cysteine, glutamic acid, glutamine, lysine, methionine, and threonine. The TMD dependence against composition has been obtained from the experimental data and characterized through the Despretz constant. All amino acids induce a depression in the TMD as compared with that of pure water. If the mole fraction is selected as composition variable, a clear dependence against amino acid molar mass is observed, which disappears when TMDs are represented versus mass fraction; almost all data shrink onto a single straight line. It must be pointed out that the TMD depressions for all studied amino acids are quite larger than those previously observed for proteins. This suggests that TMDs for proteins cannot be explained as a simple, additive result of its constituents, and, therefore complex cooperative phenomena seem to take place. The partial molar volume at infinite dilution has been obtained from density data, and a consistency test between its temperature dependence and that of temperature of maximum density versus composition has been performed, obtaining satisfactory results. A molecular dynamics study for all the studied systems has been also carried out. Amino acids have been modeled through the OPLS-AA force field, whereas the TIP4P/2005 model was used for water. The temperature of maximum density and partial molar volume have been calculated from the simulated density, and the results are compared with experimental data. Although the agreement is only fair, similar qualitative trends were obtained. The simulated Despretz constants are smaller than the experimental ones, a result that was already previously observed for methanol aqueous solutions. A structural analysis of water molecules in solution along the MD trajectories showed no enhancement of ice-like structures in complete agreement with the TMD decrease with concentration.

Electron Paramagnetic Resonance Measurements of Four Nitroxide Probes in Supercooled Water Explained by Molecular Dynamics Simulations.

Electron paramagnetic resonance (EPR) measurements of the rotational diffusion of small nitroxide probes have been demonstrated to be a powerful technique for experimentally investigating the properties of supercooled liquids, such as water. However, since only the rotational diffusion of the probe molecules is measured and EPR measurements are indirect, it is not clear what the relationship between the behavior of water and the probe molecule is. To address this, we have performed molecular dynamics simulations of four nitroxide probes in TIP4P-Ew and OPC water models to directly compare with EPR experiments and to determine the behavior of the water and the underlying microscopic coupling between the water and the probes. In all, 200 ns simulations were run for 23 temperatures between 253 and 283 K for all four probes with each water model for an aggregate of 36.8 μs of simulation time. Simulations for both water models systematically underestimated the rotational diffusion coefficients for both water and probes, though OPC simulations were generally in better agreement with the experiments than TIP4P-Ew simulations. Despite this, when the temperature dependence of the data was fit to a power law, fit parameters for TIP4P-Ew were generally in better agreement with the experiments than OPC. For probe molecules, the singular temperature was found to be T0 = 226.5 ± 0.4 K from experiments, T0 = 208 ± 2 K for OPC water, and T0 = 215 ± 2 K for TIP4P-Ew water. While for water molecules, the singular temperature was found to be T0 = 220.3 ± 0.2 K from experiments, T0 = 208 ± 2 K for OPC water, and T0 = 220 ± 1 K for TIP4P-Ew water. Systematic underestimation of the rotational diffusion coefficients was most pronounced at lower temperatures and was clearly observed in changes to the Arrhenius activation energy. Above the maximum density temperature of Tρmax = 277 K, an activation energy of EA ≈ 16.7 kJ/mol was observed for the probes from experiments, while OPC had EA ≈ 15.2 kJ/mol and TIP4P-Ew had EA ≈ 14.6 kJ/mol. Below the maximum density temperature, the activation energy jumped to EA ≈ 32.5 kJ/mol for experiments but only EA ≈ 23 kJ/mol for OPC and EA ≈ 22 kJ/mol for TIP4P-Ew. In all cases, we saw good agreement between the behavior of the probe molecules and water. To understand why, we calculated the average number of hydrogen bonds between the probe molecules and water. From this, we were able to explain the rotational diffusion times for all of the probes. These results show that current molecular models are sufficient to capture physical phenomena observed with EPR and to help elucidate why the probes provide accurate insights into the behavior of supercooled water.

The progression of thermodynamic anomalies in MX2 networks with local tetrahedral geometries

Key thermodynamic anomalies in density and compressibility, as well as the related stability limits, are determined using an ionic model for BeF2 which includes many-body polarization terms. BeF2 is chosen as an example of an archetypal network-forming system whose structure can be rationalised in terms of connected local tetrahedral coordination polyhedra. The anion dipole polarizability (which effectively controls the bond angles linking neighbouring tetrahedra) is used as a single free parameter in order to help rationalise the changes in the anomaly locations in phase space, whilst all other potential parameters remain fixed. The anomalies and stability limits systematically shift to lower temperature and higher pressure as the anion polarizability is increased. At high dipole polarizabilities the temperature of maximum density anomaly locus becomes suppressed into the supercooled regime of the phase space. The movements of the anomaly loci are analysed in terms of the network structure and the correlation with the inter-tetrahedral bond angles is considered. The high sensitivity of the anomalies to the details of the potential models applied is discussed with reference to previous works on related systems. The relationship to analogous studies on Stillinger–Weber liquids is discussed.

Eu3+ luminescent ions detect water density anomaly

It is well known that water density varies anomalously with temperature. However, nowadays there is still a lack of agreement on the causes of this phenomenon. In this work, we use Eu3+ luminescent ion as a probe to investigate the changes in the arrangement of water molecules around the temperature of maximum density. The slight changes in the arrangement of water molecules in the first coordination sphere of Eu3+ ions due to the water density anomaly affect the variation with temperature of Eu3+ ions emission spectra and intensity decay time. This is the first time, to the best of our knowledge, that the effect of the anomaly in the density of water is detected by a luminescent material. From these experimental results we can infer that possibly the anomaly could be caused by variation in the length of hydrogen bonds between molecules with temperature for both water and heavy water.

Seasonal stratification of a deep, high-altitude, dimictic lake: Nam Co, Tibetan Plateau

We investigate the seasonal evolution of stratification in a deep (100 m), high-altitude (4,730 m a.s.l.) dimictic lake on the Tibetan Plateau using three years of observation of Nam Co. The lake is situated at relatively low latitude (30 °N) where it receives high solar radiative forcing (observed maximum daily average 400 W m−2), yet the annual average air temperature is close to 0 °C at this high altitude. These features make Nam Co distinct from most well-documented dimictic lakes, which are usually located at higher latitude and lower altitude. We classify seasonal stratification into six phases based on strength of stratification, surface temperature relative to the temperature of maximum density, ice-cover, and heat and mixing dynamics, which we use to compare Nam Co with better-documented, higher-latitude dimictic lakes. While the three warm stratification phases (i.e. when water is warmer than the temperature of maximum density) in Nam Co are relatively cold, they are otherwise similar to that observed in high-latitude dimictic lakes. Conversely, two of Nam Co’s cold stratification phases are distinct from that reported in high-latitude lakes. These two phases are characterized by the interplay between the relatively strong radiative forcing and surface heat flux, and include the following: 1) during fall turnover, persistent winds aided by radiatively driven convection prolong vertical mixing (i.e. turnover) and surface heat loss such that the entire water column cools well below the temperature of maximum density (as low as 1 °C); and 2) in contrast, under ice-cover with relatively little snow, the entire water column of the lake warms continuously due to through-ice solar radiative flux. The intense cooling and heating during these two phases counteract each other such that hypolimnetic temperature at spring turnover is similar to that observed in high latitude lakes. Our observations highlight the relative importance of radiatively driven convection on the seasonal stratification dynamics of Nam Co, and underscore that these dynamics must be considered when attempting to predict climate change impacts on high-altitude, low-latitude lakes, including the >1100, largely unstudied, lakes on the Tibetan Plateau.

Waterlike anomalies in hard core–soft shell nanoparticles using an effective potential approach: Pinned vs adsorbed polymers

In this work, a two dimensional system of polymer-grafted nanoparticles is analyzed using large-scale Langevin dynamics simulations. Effective core-softened potentials were obtained for two cases: one where the polymers are free to rotate around the nanoparticle core and a second where the polymers are fixed, with a 45° angle between them. The use of effective core-softened potentials allows us to explore the complete system phase space. In this way, the PT, Tρ, and Pρ phase diagrams for each potential were obtained, with all fluid and solid phases. The phase boundaries were defined analyzing the specific heat at constant pressure, system mean square displacement, radial distribution function, and discontinuities in the density–pressure phase diagram. Also, due to the competition in the system, we have observed the presence of waterlike anomalies, such as the temperature of maximum density (TMD)—in addition with a tendency of the TMD to move to lower temperatures (negative slope)—and the diffusion anomaly. Different morphologies (stripes, honeycomb, and amorphous) for each nanoparticle were observed. We observed that for the fixed polymer case, the waterlike anomalies are originated from the competition between the potential characteristic length scales, while for the free to rotate case, the anomalies arise due to a smaller region of stability in the phase diagram, and no competition between the scales was observed.

Temperature of maximum density of proteins in water: α-Chymotrypsin and bovine serum albumin

Density as a function of temperature is determined for a set of α-chymotrypsin and bovine serum albumin solutions in pure water both in the native and the degraded state, the latter achieved by heating them above the denaturation temperature. This allows the temperature of maximum density (TMD) to be obtained against composition. Partial molar volume at infinite dilution of the protein is also calculated. Thermodynamic consistency between composition dependence of TMD and temperature dependence of partial volume is checked, obtaining satisfactory results. TMD behaviour against mass fraction for both native proteins is very similar, but important differences between native and degraded states, and also between the degraded forms of both proteins are found. These results show, for the first time, the strong effects that protein degradation have over the TMD of aqueous solutions, and therefore, over the microscopic structure of water.

Temperature of Maximum Density for Binary Aqueous Solutions of Five Amino Acids

Density in the temperature interval of 273.65–283.15 K has been experimentally determined for aqueous solutions of l-tyrosine, l-tryptophan, l-histidine, l-phenylalanine, and l-proline. These data ...