Volume of supercooled water under pressure and the liquid-liquid critical point

The volume of water (H(2)O) was obtained at about 200-275 K and 40-400 MPa by using emulsified water. The plot of volume against temperature showed slightly concave-downward curvature at pressures higher than ≈200 MPa. This is compatible with the liquid-liquid critical-point hypothesis, but hardly with the singularity-free scenario. When the critical point is assumed to exist at ≈50 MPa and ≈223 K, the experimental volume and the derived compressibility are qualitatively described by the modified Fuentevilla-Anisimov scaling equation.     

Widom line and the liquid-liquid critical point for the TIP4P/2005 water model

The Widom line and the liquid-liquid critical point of water in the deeply supercooled region are investigated via computer simulation of the TIP4P/2005 model. The Widom line has been calculated as the locus of compressibility maxima. It is quite close to the experimental homogeneous nucleation line and, in the region studied, it is almost parallel to the curve of temperatures of maximum density at fixed pressure. The critical temperature is determined by examining which isotherm has a region with flat slope. An interpolation in the Widom line gives the rest of the critical parameters. The computed critical parameters are T(c)=193 K, p(c)=1350 bar, and ρ(c)=1.012 g/cm(3). Given the performance of the model for the anomalous properties of water and for the properties of ice phases, the calculated critical parameters are probably close to those of real water.     

Collective dynamics of supercooled water close to the liquid-liquid coexistence lines

We report molecular dynamics simulation results for the collective dynamical properties of supercooled bulk water at 180 K at three different densities, corresponding to different phases whose coexistence has recently been discovered in the supercooled regime. In this study, we focus on the behaviour of the longitudinal and transverse current correlation functions and their relative spectra, which we analyze in detail to understand the dynamical processes responsible for the main features observed. Despite the considerable differences in the structure and densities of the three thermodynamic states considered, the obtained current correlation functions show rather similar behaviour in every case. We show that the longitudinal spectra can only be described in terms of three Lorentzian lines, while the accurate reproduction of the transverse spectra requires at least four separate spectral lines. In fact, the behaviour of the peak frequency of the modes necessary to reproduce the spectra as a function of the wavevector indicates in a clear way the nature of the dynamical process. We demonstrate the presence of collective modes associated with the propagation of both longitudinal and transverse sound along with the important contribution of "optical-like" modes, which point out the relevance of localized motions for a right interpretation of the spectral line shape. The wavevector dependence of the relative contributions of the various modes to the total spectral area is also discussed.     

Heat capacity close to the unattainable critical layering point

An analysis of the literature data on the heat capacity of aqueous low-concentration solutions of tert-butanol at several temperatures showed that the heat capacity peak in this concentration region was largely related to critical concentration fluctuations close to the binodal, which appeared because of microlayering on the network of H-bonds in solutions of hydrophobic nonelectrolyte molecules.     

Thermal properties of ionic systems near the liquid-liquid critical point

Isobaric heat capacity per unit volume, C(p), and excess molar enthalpy, h(E), were determined in the vicinity of the critical point for a set of binary systems formed by an ionic liquid and a molecular solvent. Moreover, and, since critical composition had to be accurately determined, liquid-liquid equilibrium curves were also obtained using a calorimetric method. The systems were selected with a view on representing, near room temperature, examples from clearly solvophobic to clearly coulombic behavior, which traditionally was related with the electric permittivity of the solvent. The chosen molecular compounds are: ethanol, 1-butanol, 1-hexanol, 1,3-dichloropropane, and diethylcarbonate, whereas ionic liquids are formed by imidazolium-based cations and tetrafluoroborate or bis-(trifluromethylsulfonyl)amide anions. The results reveal that solvophobic critical behavior-systems with molecular solvents of high dielectric permittivity-is very similar to that found for molecular binary systems. However, coulombic systems-those with low permittivity molecular solvents-show strong deviations from the results usually found for these magnitudes near the liquid-liquid phase transition. They present an extremely small critical anomaly in C(p)-several orders of magnitude lower than those typically obtained for binary mixtures-and extremely low h(E)-for one system even negative, fact not observed, up to date, for any liquid-liquid transition in the nearness of an upper critical solution temperature.     

NMR probing of structural peculiarities in ionic solutions close to critical point

(1)H, (23)Na, (35)Cl, (79)Br, and (81)Br NMR chemical shifts (delta) and signal half widths (Delta(12)) have been measured in aqueous electrolyte mixtures [tetrahydrofuran/H(2)ONaCl and 3-methylpyridine (3MP)H(2)ONaBr] at different mass fractions of salt (X) in the one-phase region, close to their lower critical solution points (T(CL)). Discontinuous changes in slope of delta=f(X) and Delta(12)=f(X) have been found in (23)Na and (81)Br NMR spectra of 3MP/water/NaBr solution at X approximately 0.1 and T=301 K. The dependency of (1)H NMR signals of 3MP is continuous over the whole investigated range of X=0.002-0.2, whereas changes in the slope of H(2)O chemical shifts are hardly noticeable. In the two-phase region, i.e., at T>T(CL), a doubling of all NMR signals has been observed. The sensitivity of NMR parameters depends more on composition of solution for anions (Cl(-) and Br(-)) than for cations (Na(+)). A very strong relaxation effect for (81)Br nuclei with relaxation rates reaching 14 000 s(-1) was observed. The results are interpreted in terms of ion-molecular clustering and changes in coherency of dipole configurations of water molecules during supramolecular restructuring of solutions.     

Heat capacity of the liquid-liquid mixture nitrobenzene and dodecane near the critical point

The heat capacity of the liquid-liquid mixture nitrobenzene-dodecane has been measured for the first time near its upper critical consolute point using an adiabatic calorimeter. The theoretical expression for the heat capacity near the critical point was applied to our combined data runs. The critical exponent alpha was determined to be 0.124+/-0.006, which was consistent with theoretical predictions. When alpha was fixed at its theoretical value of 0.11, our value for the amplitude ratio A(+)A(-)=0.58+/-0.02 was consistent with experimental determinations and theoretical predictions. However, the two-scale-factor universality ratio X, now consistent among experiments and theories with a value between 0.019 and 0.020, was violated in this system when using a previously published value for the correlation length.     

Modelling aqueous solutions near the critical point of water

We review the thermodynamic properties of dilute solution near the critical point of the solvent. Two examples are discussed, a solution of a non-electrolyte and a solution of an electrolyte. The limiting behavior of the electrolyte solutions is modelled with a Debye-Huckel term in the Helmholtz free energy. The partial molar properties, in particular the volume and isobaric thermal expansion are examined in detail. The derivation of these properties is introduced by considering the geometry of the thermodynamic surfaces near to and far from the critical point of the solvent. We conclude that the properties of solutions near the solvent critical point are dominated by that feature; solution properties cannot be adequately modelled without including the functional forms associated with the critical point.     

Liquid-vapor and liquid-liquid phase equilibria of the Brodholt-Sampoli-Vallauri polarizable water model

Liquid-vapor and liquid-liquid phase equilibria of the polarizable Brodholt-Sampoli-Vallauri water model have been investigated by Gibbs ensemble Monte Carlo computer simulations. The coexisting liquid and vapor densities and energy of vaporization of the model is found to be in a reasonable agreement with experimental data in the entire temperature range of liquid-vapor coexistence. The critical temperature and density of the model are found to be 615 K and 0.278 gcm(3), respectively, close to the experimental values of 647.1 K and 0.322 gcm(3). In the supercooled state two distinct liquid-liquid coexistence regions are observed. The existence of liquid-liquid phase separation of a polarizable water model is demonstrated for the first time.     

The critical behavior of the refractive index near liquid-liquid critical points

The nature of the critical behavior in the refractive index n is revisited in the framework of the complete scaling formulation. A comparison is made with the critical behavior of n as derived from the Lorentz-Lorenz equation. Analogue anomalies to those predicted for the dielectric constant ε, namely, a leading |t|(2β) singularity in the coexistence-curve diameter in the two-phase region and a |t|(1-α) along the critical isopleth in the one phase region, are expected in both cases. However, significant differences as regards the amplitudes of both singularities are obtained from the two approaches. Analysis of some literature data along coexistence in the two-phase region and along the critical isopleth in the one-phase region provide evidence of an intrinsic effect, independent of the density, in the critical anomalies of n. This effect is governed by the shift of the critical temperature with an electric field, which is supposed to take smaller values at optical frequencies than at low frequencies in the Hz to MHz range.     

Kinetics of two redox reactions in 2-butoxyethanol and water near its critical point of solution

The rate constants of two redox reactions and in the critical solution of 2-butoxyethanol and water have been measured by using the UV spectrophotometry at the initial reaction stage. It was found that the rate constants at various temperatures for two reactions were well described by the Arrhenius equation in the noncritical region. The critical slowing down effect was detected in the critical region. The critical slowing down exponents were determined to be 0.044 ± 0.004 and 0.046 ± 0.005 for reactions and , respectively. The values of the critical slowing down exponents showed that only dynamic critical slowing down effect, and no thermodynamic singularity could be observed for the two reactions.     

Structure of liquid water with ST2 model potentials in the temperature interval 273–373 K

Ufa Aviation Institute. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 4, pp. 69–73, July–August 1990.     

Tracking ultrafast excited-state bond-twisting motion in solution close to the Franck-Condon point

Applying optimal control to photoinduced trans-cis isomerization in condensed phase, the dynamics of bond-twisting motion of 1,1'-diethyl-4,4'-cyanine in methanol and propanol is revealed. The shape of the optimized pulse resulting from minimization of the photoisomer formation can be directly related to the initial excited-state dynamics in close proximity to the Franck-Condon point. The solvent viscosity-dependent ultrafast wavepacket motion is reflected in the prominent down-chirp of the optimized pulses and reveals a detailed picture of the control mechanism: The reduction of the isomer production is achieved by most efficient dumping of excited population back to the trans ground state. In the higher-viscosity solvent, propanol, wavelength-dependent oscillatory features are superimposed to the overall chirp structure pointing to the importance of excited-state vibrational coherences for the dumping process.     

Salting out near the critical point

The salting out of solid solutes near the critical point of the solvent is investigated using the results of a previous paper (J. Phys. Chem. B 2006, 110, 24077) on the fluctuation theory of salting out. It is found that the salting out coefficient at infinite dilution of cosolvent is approximately proportional to the compressibility of the solvent and is consequently quite large near the critical point. No estimate is given for the range of cosolvent concentrations over which the infinite dilution slope might be a good approximation. Far from the critical point, it is known to be a good approximation over a considerable cosolvent concentration range.     

Coexistence curve of 2,6-lutidine + water,near its lower critical point

Measurements of the refractive index of both phases of the mixture 2,6-lutidina + water, near the lower critical point, are reported. For T-T_c < 0.5 K, a simple power law, Δn = (0.0471±0.001)(T-T_c)^{(0.338±0.003)}, describes the data. For 0.5 K <T-T_c < 15 K, a decorated lattice model better describes the data.     

Core-softened fluids, water-like anomalies, and the liquid-liquid critical points

Molecular dynamics simulations are used to examine the relationship between water-like anomalies and the liquid-liquid critical point in a family of model fluids with multi-Gaussian, core-softened pair interactions. The core-softened pair interactions have two length scales, such that the longer length scale associated with a shallow, attractive well is kept constant while the shorter length scale associated with the repulsive shoulder is varied from an inflection point to a minimum of progressively increasing depth. The maximum depth of the shoulder well is chosen so that the resulting potential reproduces the oxygen-oxygen radial distribution function of the ST4 model of water. As the shoulder well depth increases, the pressure required to form the high density liquid decreases and the temperature up to which the high-density liquid is stable increases, resulting in the shift of the liquid-liquid critical point to much lower pressures and higher temperatures. To understand the entropic effects associated with the changes in the interaction potential, the pair correlation entropy is computed to show that the excess entropy anomaly diminishes when the shoulder well depth increases. Excess entropy scaling of diffusivity in this class of fluids is demonstrated, showing that decreasing strength of the excess entropy anomaly with increasing shoulder depth results in the progressive loss of water-like thermodynamic, structural and transport anomalies. Instantaneous normal mode analysis was used to index the overall curvature distribution of the fluid and the fraction of imaginary frequency modes was shown to correlate well with the anomalous behavior of the diffusivity and the pair correlation entropy. The results suggest in the case of core-softened potentials, in addition to the presence of two length scales, energetic, and entropic effects associated with local minima and curvatures of the pair interaction play an important role in determining the presence of water-like anomalies and the liquid-liquid phase transition.     

Review of properties of the system 2-butoxyethanol/water in the vicinity of its lower critical point relevant to an experiment to be carried out under reduced gravity

The system 2-butoxyethanol/water is chosen as a transparent model system to study the physics behind the unexpectedly massive separation of coexisting phases in monotectic alloys under microgravity conditions. For that purpose the experiment NUGRO was proposed. It will be carried out during the German Spacelab Mission D-2 with the aim of studying the kinetics of liquid/liquid phase separation in the system 2-butoxyethanol/water. The results obtained in the ground-based research program of NUGRO were used in the planning of the space experiment and will be needed to understand its physics. A review of the ground-based research is given.