High-density Liquid Research Articles

Liquid–liquid transition in supercooled water suggested by microsecond simulations

The putative liquid-liquid phase transition in supercooled water has been used to explain many anomalous behaviors of water. However, no direct experimental verification of such a phase transition has been accomplished, and theoretical studies from different simulations contradict each other. We investigated the putative liquid-liquid phase transition using the Water potential from Adaptive Force Matching for Ice and Liquid (WAIL). The simulation reveals a first-order phase transition in the supercooled regime with the critical point at ~207 K and 50 MPa. Normal water is high-density liquid (HDL). Low-density liquid (LDL) emerges at lower temperatures. The LDL phase has a density only slightly larger than that of the ice-Ih and shows more long-range order than HDL. However, the transformation from LDL to HDL is spontaneous across the first-order phase transition line, suggesting the LDL configuration is not poorly formed nanocrystalline ice. It has been demonstrated in the past that the WAIL potential provides reliable predictions of water properties such as melting temperature and temperature of maximum density. Compared with other simple water potentials, WAIL is not biased by fitting to experimental properties, and simulation with this potential reflects the prediction of a high-quality first-principle potential energy surface.

Conversion of Carbon Dioxide into Several Potential Chemical Commodities Following Different Pathways - A Review

This article reviews the literature related to the direct uses of CO2and its conversion into various value added chemicals including high energy density liquid fuels such as methanol. The increase in the direct uses of CO2and its conversion into potential chemical commodities is very important as it directly contributes to the mitigation of CO2related global warming problem. The method being followed at present in several countries to reduce the CO2associated global warming is capturing of CO2at its major outlets using monoethanolamine based solution absorption technique followed by storing it in safe places such as, oceans, depleted coal seams, etc., (i.e., carbon dioxide capturing and storing in safe places, CCS process). This is called as CO2sequestration. Although, the CCS process is the most understood and immediate option to mitigate the global warming problem, it is considerably expensive and has become a burden for those countries, which are practicing this process. The other alternative and most beneficial way of mitigating this global warming problem is to convert the captured CO2into certain value added bulk chemicals instead of disposing it. Conversion of CO2into methanol has been identified as one of such cost effective ways of mitigating global warming problem. Further, if H2is produced from exclusively water using only solar energy instead of any fossil fuel based energy, and is used to convert CO2into methanol there are three major benefits: i) it contributes greatly to the global warming mitigation problem, ii) it greatly saves fossil fuels as methanol production from CO2could be an excellent sustainable and renewable energy resource, and iii) as on today, there is no better process than this to store energy in a more convenient and highly usable form of high energy density liquid fuel. Not only methanol, several other potential chemicals and value added chemical intermediates can be produced from CO2. In this article, i) synthesis of several commodity chemicals including poly and cyclic-carbonates, sodium carbonate and dimethyl carbonate, carbamates, urea, vicinal diamines, 2-arylsuccinic acids, dimethyl ether, methanol, various hydrocarbons, acetic acid, formaldehyde, formic acid, lower alkanes, etc., from CO2, ii) the several direct uses of CO2, and iii) the importance of producing methanol from CO2using exclusively solar energy are presented, discussed and summarized by citing all the relevant and important references.

Finite-size scaling investigation of the liquid-liquid critical point in ST2 water and its stability with respect to crystallization

The liquid-liquid critical point scenario of water hypothesizes the existence of two metastable liquid phases--low-density liquid (LDL) and high-density liquid (HDL)--deep within the supercooled region. The hypothesis originates from computer simulations of the ST2 water model, but the stability of the LDL phase with respect to the crystal is still being debated. We simulate supercooled ST2 water at constant pressure, constant temperature, and constant number of molecules N for N ≤ 729 and times up to 1 μs. We observe clear differences between the two liquids, both structural and dynamical. Using several methods, including finite-size scaling, we confirm the presence of a liquid-liquid phase transition ending in a critical point. We find that the LDL is stable with respect to the crystal in 98% of our runs (we perform 372 runs for LDL or LDL-like states), and in 100% of our runs for the two largest system sizes (N = 512 and 729, for which we perform 136 runs for LDL or LDL-like states). In all these runs, tiny crystallites grow and then melt within 1 μs. Only for N ≤ 343 we observe six events (over 236 runs for LDL or LDL-like states) of spontaneous crystallization after crystallites reach an estimated critical size of about 70 ± 10 molecules.

Anharmonic onsets in polypeptides revealed by neutron scattering: Experimental evidences and quantitative description of energy resolution dependence

Neutron scattering measurements on protein powders reveal two deviations from harmonic dynamics at low temperature, whose molecular origin, physical nature and biological relevance are still matter of discussion. In this study we present a new experimental and theoretical approach to evidence the resolution dependence of anharmonic onsets: the use of strategically selected homomeric polypeptides allows revealing the exact resolution dependence; a two-site energy landscape model, where resolution effects are explicitly taken into account, is able to interpret quantitatively the experimental data in terms of energy landscape parameters. The energetic description provided by this analysis, together with recent experimental evidences obtained on chemically and structurally different peptide systems, allows us to connect the protein/water energy landscape structure with the two-wells water interaction potential proposed to explain the low-density→high-density liquid–liquid transition observed in supercooled water.

Cooperative Molecular Motions in Water

We discuss evidence relating to the hypothesis that, in addition to the known critical point in water (below which two fluid phases—a lower-density gas and a higher-density liquid—coexist), there exists a “second” critical point at low temperatures (below which two liquid phases—a higher-density liquid and a lower-density liquid — can coexist).

Phase-coexistence simulations of fluid mixtures by the Markov Chain Monte Carlo method using single-particle models

We present a single-particle Lennard–Jones (L-J) model for CO2 and N2. Simplified L-J models for other small polyatomic molecules can be obtained following the methodology described herein. The phase-coexistence diagrams of single-component systems computed using the proposed single-particle models for CO2 and N2 agree well with experimental data over a wide range of temperatures. These diagrams are computed using the Markov Chain Monte Carlo method based on the Gibbs-NVT ensemble. This good agreement validates the proposed simplified models. That is, with properly selected parameters, the single-particle models have similar accuracy in predicting gas-phase properties as more complex, state-of-the-art molecular models. To further test these single-particle models, three binary mixtures of CH4, CO2 and N2 are studied using a Gibbs-NPT ensemble. These results are compared against experimental data over a wide range of pressures. The single-particle model has similar accuracy in the gas phase as traditional models although its deviation in the liquid phase is greater. Since the single-particle model reduces the particle number and avoids the time-consuming Ewald summation used to evaluate Coulomb interactions, the proposed model improves the computational efficiency significantly, particularly in the case of high liquid density where the acceptance rate of the particle-swap trial move increases. We compare, at constant temperature and pressure, the Gibbs-NPT and Gibbs-NVT ensembles to analyze their performance differences and results consistency. As theoretically predicted, the agreement between the simulations implies that Gibbs-NVT can be used to validate Gibbs-NPT predictions when experimental data is not available.

Synthesis and pyrolysis studies of bis(nitratomethyl)-1,3-bishomocubane—A high-energy high-density liquid

The synthesis and thermal decomposition of the energetic material, bis(nitratomethyl)-1,3-bishomocubane (DNMBHC) were undertaken. The optimized structure of DNMBHC was obtained by ab initio methods and was utilized to calculate the heat of formation, heat of combustion and detonation properties. The slow thermal decomposition of DNMBHC was investigated by thermogravimetric analysis (TGA) at heating rates of 10°C/min, while fast pyrolysis was investigated utilizing sub-milligram quantities at heating rates of 100°C/s. The gaseous products from fast pyrolysis were detected by rapidscan FTIR spectroscopy to postulate probable decomposition pathways. The primary decomposition pathway was found to involve ONO2 bond scissions to form NO2, subsequently leading to several secondary reactions. A secondary pathway involving detachment of the nitrate ester group from the ring to form MeONO2 was also predicted. The products involving the bishomocubane ring were found to be relegated to the condensed phase.

Analysis of a novel gravity driven organic Rankine cycle for small-scale cogeneration applications

A naturally-controlled organic Rankine cycle (ORC) without negative work is proposed. Compared with conventional ORCs, the novel system uses gravity instead of a pump to pressurize the working fluid, and hence the problems associated with the pump can be avoided. A possible application of the gravity driven ORC for combined heat and power generation is presented to outline the feasibility. Mathematical models are built. The required height for pressurization and the cycle efficiency varying with the condensation and evaporation temperatures are analyzed on using R123, R245ca, R113, PF5050 and PF5060. The results reveal that on the same condition of operating temperature, the required height for pressurization may vary drastically when the working fluid is different. Fluid of high liquid density and low saturation pressure offers a low height. The power efficiency of the novel ORC is adequate for low temperature applications. With an expander efficiency of 75%, evaporation temperature of 100°C and condensation temperature of 40°C, the highest efficiency is about 10.2% with fluid of R123, which is 0.9% higher than that of the pump driven ORC. And the lowest required height is only about 20.9m with fluid of PF5060. The proposed ORC has both cost and efficiency advantages.

First-order liquid-liquid phase transition in cerium.

We report the first experimental observation of a liquid-liquid phase transition in the monatomic liquid metal cerium, by means of in situ high-pressure high-temperature x-ray diffraction experiments. At 13 GPa, upon increasing temperature from 1550 to 1900 K high-density liquid transforms to a low-density liquid, with a density difference of 14%. Theoretic models based on ab initio calculations are built to investigate the observed phase behavior of the liquids at various pressures. The results suggest that the transition primarily originates from the delocalization of f electrons and is deemed to be of the first order that terminates at a critical point.

Studies on ultrathin films of tricycloquinazoline (TCQ) based discotic liquid crystal molecules

The assembly of disk-shaped molecules on surfaces has drawn considerable attention because of their unique electro-optical properties. We have studied the monolayer of the tricycloquinazoline based disk-shaped (TCQCB) molecules at air-water and air-solid interfaces. The TCQCB molecules form a stable Langmuir monolayer at the air-water interface. The monolayer exhibits gas, low density liquid (L1), and high density liquid (L2) phases. In L1 and L2 phases, the molecules prefer edge-on conformation. The atomic force microscope study on the Langmuir-Blodgett films of the molecules reveals a structural transformation. The elongated domains in the film deposited in L1 phase transformed to a compact grainy texture in the film deposited in the L2 phase.

Glass and liquid phase diagram of a polyamorphic monatomic system

We perform out-of-equilibrium molecular dynamics (MD) simulations of a monatomic system with Fermi-Jagla (FJ) pair potential interactions. This model system exhibits polyamorphism both in the liquid and glass state. The two liquids, low-density (LDL) and high-density liquid (HDL), are accessible in equilibrium MD simulations and can form two glasses, low-density (LDA) and high-density amorphous (HDA) solid, upon isobaric cooling. The FJ model exhibits many of the anomalous properties observed in water and other polyamorphic liquids and thus, it is an excellent model system to explore qualitatively the thermodynamic properties of such substances. The liquid phase behavior of the FJ model system has been previously characterized. In this work, we focus on the glass behavior of the FJ system. Specifically, we perform systematic isothermal compression and decompression simulations of LDA and HDA at different temperatures and determine "phase diagrams" for the glass state; these phase diagrams varying with the compression/decompression rate used. We obtain the LDA-to-HDA and HDA-to-LDA transition pressure loci, P(LDA-HDA)(T) and P(HDA-LDA)(T), respectively. In addition, the compression-induced amorphization line, at which the low-pressure crystal (LPC) transforms to HDA, P(LPC-HDA)(T), is determined. As originally proposed by Poole et al. [Phys. Rev. E 48, 4605 (1993)] simulations suggest that the P(LDA-HDA)(T) and P(HDA-LDA)(T) loci are extensions of the LDL-to-HDL and HDL-to-LDL spinodal lines into the glass domain. Interestingly, our simulations indicate that the P(LPC-HDA)(T) locus is an extension, into the glass domain, of the LPC metastability limit relative to the liquid. We discuss the effects of compression/decompression rates on the behavior of the P(LDA-HDA)(T), P(HDA-LDA)(T), P(LPC-HDA)(T) loci. The competition between glass polyamorphism and crystallization is also addressed. At our "fast rate," crystallization can be partially suppressed and the glass phase diagram can be related directly with the liquid phase diagram. However, at our "slow rate," crystallization cannot be prevented at intermediate temperatures, within the glass region. In these cases, multiple crystal-crystal transformations are found upon compression/decompression (polymorphism).

Free energy surface of ST2 water near the liquid-liquid phase transition

We carry out umbrella sampling Monte Carlo simulations to evaluate the free energy surface of the ST2 model of water as a function of two order parameters, the density and a bond-orientational order parameter. We approximate the long-range electrostatic interactions of the ST2 model using the reaction-field method. We focus on state points in the vicinity of the liquid-liquid critical point proposed for this model in earlier work. At temperatures below the predicted critical temperature we find two basins in the free energy surface, both of which have liquid-like bond orientational order, but differing in density. The pressure and temperature dependence of the shape of the free energy surface is consistent with the assignment of these two basins to the distinct low density and high density liquid phases previously predicted to occur in ST2 water.

Endovascular Embolization by Parent Artery Reconstruction of a Symptomatic Fusiform Posterior Cerebral Artery Aneurysm Using Onyx HD-500: A Neurointerventional Report

Posterior cerebral artery aneurysms are treatment challenge for the neurosurgeon. Parent artery occlusion, trapping and bypass have been the classic treatment options for aneurysms in this location. With the introduction of newer embolic agents such as Onyx®, endovascular intervention is now a viable therapy for these aneurysms. We report the case of a 60-year-old man who presented with a symptomatic, though unruptured, fusiform left posterior cerebral artery aneurysm. Given the distal location of this dominant sided aneurysm, post-operative visual deficits and aphasia were a concern if parent vessel occlusion were to be performed. Therefore, an endovascular reconstruction using Onyx HD-500 and two closed-cell stents was performed. This report illustrates the ability of a high-density liquid embolic agent to provide immediate reconstruction of a fusiform aneurysm in a distal location.

Liquid-liquid transition in ST2 water

We use the weighted histogram analysis method [S. Kumar, D. Bouzida, R. H. Swendsen, P. A. Kollman, and J. M. Rosenberg, J. Comput. Chem. 13, 1011 (1992)] to calculate the free energy surface of the ST2 model of water as a function of density and bond-orientational order. We perform our calculations at deeply supercooled conditions (T = 228.6 K, P = 2.2 kbar; T = 235 K, P = 2.2 kbar) and focus our attention on the region of bond-orientational order that is relevant to disordered phases. We find a first-order transition between a low-density liquid (LDL, ρ ≈ 0.9 g/cc) and a high-density liquid (HDL, ρ ≈ 1.15 g/cc), confirming our earlier sampling of the free energy surface of this model as a function of density [Y. Liu, A. Z. Panagiotopoulos, and P. G. Debenedetti, J. Chem. Phys. 131, 104508 (2009)]. We demonstrate the disappearance of the LDL basin at high pressure and of the HDL basin at low pressure, in agreement with independent simulations of the system's equation of state. Consistency between directly computed and reweighted free energies, as well as between free energy surfaces computed using different thermodynamic starting conditions, confirms proper equilibrium sampling. Diffusion and structural relaxation calculations demonstrate that equilibration of the LDL phase, which exhibits slow dynamics, is attained in the course of the simulations. Repeated flipping between the LDL and HDL phases in the course of long molecular dynamics runs provides further evidence of a phase transition. We use the Ewald summation with vacuum boundary conditions to calculate long-ranged Coulombic interactions and show that conducting boundary conditions lead to unphysical behavior at low temperatures.

Mode coupling and fragile to strong transition in supercooled TIP4P water

We consider one of the most used model for water, the rigid four site TIP4P potential, and we study by molecular dynamics simulation the dynamical properties of the liquid upon supercooling. In the previous studies of the thermodynamics of the TIP4P model a liquid-liquid critical point (LLCP) located at the end of the coexistence between the low density liquid (LDL) and the high density liquid (HDL) of water was found. We present here the analysis of the self intermediate scattering functions in a large range of temperatures and densities and we show that the structural relaxation in the region of mild supercooling is in agreement with the predictions of the mode coupling theory. In the more deep supercooled region we observe that the α-relaxation time deviates from the mode coupling theory (MCT) trend and a crossover takes place from a fragile to a strong behavior upon crossing the Widom line emanating from the LLCP. The HDL and the LDL phases are associated with the fragile and the strong behavior, respectively.

Nonmonotonic Dependence of the Absolute Entropy on Temperature in Supercooled Stillinger-Weber Silicon

Using a recently developed thermodynamic integration method, we compute the precise values of the excess Gibbs free energy (G e ) of the high density liquid (HDL) phase with respect to the crystalline phase at different temperatures (T) in the supercooled region of the Stillinger-Weber (SW) silicon (Stillinger and Weber in Phys. Rev. B 31:5262–5271, 1985). Based on the slope of G e with respect to T, we find that the absolute entropy of the HDL phase increases as its enthalpy changes from the equilibrium value at T≥1065 K to the value corresponding to a non-equilibrium state at 1060 K. We find that the volume distribution in the equilibrium HDL phases become progressively broader as the temperature is reduced to 1060 K, exhibiting van-der-Waals (VDW) loop in the pressure-volume curves. Our results provides insight into the thermodynamic cause of the transition from the HDL phase to the low density phases in SW silicon, observed in earlier studies near 1060 K at zero pressure.

Studies on Langmuir monolayer of tricycloquinazoline based disk-shaped liquid crystal molecules

The assembly of disk-shaped molecules on surfaces has drawn considerable attention because of their unique electronic and optical properties. We have studied the monolayer of the amphiphilic tricycloquinazoline (AmTCQ) based disk-shaped molecules at the air–water and air–solid interfaces. The AmTCQ molecules form a stable Langmuir monolayer at the air–water interface. The monolayer exhibits gas phase, low density liquid (L1) phase, and high density liquid (L12) phase. In L1 phase, the molecules prefer face-on conformation with their flexible side-chain being interdigitated with the neighboring molecules. Interestingly, the molecular conformations in L12 phase were found to be both the face-on and edge-on. The structure of the aggregates of the molecules on a surface can be influenced by the intermolecular and molecule–substrate interactions. The deficiency in π-electrons of the AmTCQ molecule may lead to a weak intermolecular interaction. The observation of both the conformations of the molecules in L12 phase can be attributed to the weak intermolecular interaction.

Phase diagrams of particles with dissimilar patches: X-junctions and Y-junctions

We use Wertheim’s first-order perturbation theory to investigate the phase behaviour and the structure of coexisting fluid phases for a model of patchy particles with dissimilar patches (two patches of type A and fB patches of type B). A patch of type α = {A,B} can bond to a patch of type β = {A,B} in a volume vαβ, thereby decreasing the internal energy by ϵαβ. We analyse the range of model parameters where AB bonds, or Y-junctions, are energetically disfavoured (ϵAB < ϵAA/2) but entropically favoured (vAB ≫ vαα), and BB bonds, or X-junctions, are energetically favoured (ϵBB > 0). We show that, for low values of ϵBB/ϵAA, the phase diagram has three different regions: (i) close to the critical temperature a low-density liquid composed of long chains and rich in Y-junctions coexists with a vapour of chains; (ii) at intermediate temperatures there is coexistence between a vapour of short chains and a liquid of very long chains with X- and Y-junctions; (iii) at low temperatures an ideal gas coexists with a high-density liquid with all possible AA and BB bonds formed. It is also shown that in region (i) the liquid binodal is reentrant (its density decreases with decreasing temperature) for the lower values of ϵBB/ϵAA. The existence of these three regions is a consequence of the competition between the formation of X- and Y-junctions: X-junctions are energetically favoured and thus dominate at low temperatures, whereas Y-junctions are entropically favoured and dominate at higher temperatures.

Economic analysis of novel synergistic biofuel (H2Bioil) processes

Fast-pyrolysis based processes can be built on small-scale and have higher process carbon and energy efficiency as compared to other options. H2Bioil is a novel process based on biomass fast-hydropyrolysis and subsequent hydrodeoxygenation (HDO) and can potentially provide high yields of high energy density liquid fuel at relatively low hydrogen consumption. This paper contains a comprehensive financial analysis of the H2Bioil process with hydrogen derived from different sources. Three different carbon tax scenarios are analyzed: no carbon tax, $55/metric ton carbon tax and $110/metric ton carbon tax. The break-even crude oil price for a delivered biomass cost of $94/metric ton when hydrogen is derived from coal, natural gas or nuclear energy ranges from $103 to $116/bbl for no carbon tax and even lower ($99–$111/bbl) for the carbon tax scenarios. This break-even crude oil price compares favorably with the literature estimated prices of fuels from alternate biochemical and thermochemical routes. The impact of the chosen carbon tax is found to be limited relative to the impact of the H2 source on the H2Bioil break-even price. The economic robustness of the processes for hydrogen derived from coal, natural gas, or nuclear energy is seen by an estimated break-even crude oil price of $114–$126/bbl when biomass cost is increased to $121/metric ton.

Differences in first neighbor orientation behind the anomalies in the low and high density trans-1,2-dichloroethene liquid

Trans-1,2-dichloroethene (HClC=CClH) has several structural and dynamic anomalies between its low- and high-density liquid, previously found through neutron scattering experiments. To explain the microscopic origin of the differences found in those experiments, a series of molecular dynamics simulations were performed. The analysis of molecular short-range order shows that the number of molecules in the first neighbor shell is 12 for the high-density liquid and 11 for the low-density one. It also shows that the angular position of the center of mass of the first neighbor is roughly the same although the molecular orientation is not. In both liquids the first neighbor and its reference molecule arrange mainly in two configurations, each being the most probable in one of the liquids. First neighbors in the configuration that predominates in the high-density liquid tend to locate themselves closer to the reference molecule, an evidence that they are more strongly bonded. This arrangement facilitates a better packing of the rest of molecules in the first neighbor shell so that on average an additional molecule can be included, and is proposed to be the key in the explanation of all the observed anomalies in the characteristics of both liquids.