Liquid Structure Properties Research Articles

Evaluation of infrared spectroscopic methods for the study of structural properties of liquid alcohols

This work aims to evaluate the infrared spectroscopic methods for the study of the structural properties of liquid alcohols – specifically how many alcohol molecules and how many hydroxyl groups do not participate in hydrogen bonds as donors. They are known as monomer fractions and free hydroxyl group fractions, respectively, in the community of chemical engineering thermodynamics. The infrared spectra processing methodologies for the extraction of free hydroxyl groups/monomer fractions in 1-alcohol solutions at various temperatures and their assumptions are discussed extensively in a comprehensive manner. The application of these methods addresses the surprising conclusion of literature experimental data that indicated identical behavior of methanol and ethanol in terms of liquid structural properties over a wide range of temperatures. A discrepancy is observed in data regarding saturated liquid methanol and ethanol, which shows that methanol indeed exhibits a stronger association than ethanol, as expected, at higher temperatures. The difficulties in the interpretation of liquid alcohol infrared spectra are further demonstrated in the case of 1-octanol, where evidence indicates that an association scheme which considers branched clusters is more consistent, especially in the region of low free hydroxyl fractions.

Compositional effects in the liquid Fe–Ni–C system at high pressure

We performed molecular dynamics simulations based on density functional theory to systematically investigate the Fe–Ni–C system including (1) pure Fe and Ni; (2) binary Fe–Ni, Fe–C, and Ni–C; and (3) ternary Fe–Ni–C liquid compositions at 3000 K and three simulation volumes corresponding to pressure (P) up to 83 GPa. Liquid structural properties, including coordination numbers, are analyzed using partial radial distribution functions. Self-diffusion coefficients are determined based on the atomic trajectories and the asymptotic slope of the time-dependent mean-square displacement. The results indicate that the average interatomic distance between two Fe atoms (rFe–Fe) decreases with P and is sensitive to Ni (XNi) and C (XC) concentration, although the effects are opposite: rFe–Fe decreases with increasing XNi, but increases with increasing XC. Average rFe–C and rNi–C values also decrease with increasing XNi and generally remain constant between the two lowest P points, corresponding to a coordination change of carbon from ~ 6.8 to ~ 8.0, and then decrease with additional P once the coordination change is complete. Carbon clustering occurs in both binary (especially Ni–C) and ternary compositions with short-range rC-C values (~ 1.29 to ~ 1.57 Å), typical for rC-C in diamond and graphite. The self-diffusion results are generally consistent with high-P diffusion data extrapolated from experiments conducted at lower temperature (T). A subset of additional simulations was conducted at 1675 and 2350 K to estimate the effect of T on diffusion, yielding an activation enthalpy of ~ 53 kJ/mol and activation volume of ~ 0.5 cm3/mol.

The Mixing Enthalpy and Liquid Structural Properties of Ti–Al Alloys by ab inito Molecular Dynamics Simulation

The Mixing Enthalpy and Liquid Structural Properties of Ti–Al Alloys by ab inito Molecular Dynamics Simulation

Structural properties of liquids in extreme confinement.

We simulate a hard-sphere liquid in confined geometry where the separation of the two parallel, hard walls is smaller than two particle diameters. By systematically reducing the wall separation we analyze the behavior of structural and thermodynamic properties, such as inhomogeneous density profiles, structure factors, and compressibilities when approaching the two-dimensional limit. In agreement with asymptotic predictions, we find for quasi-two-dimensional fluids that the density profile becomes parabolic and the structure factor converges toward its two-dimensional counterpart. To extract the compressibility in polydisperse samples a perturbative expression is used which qualitatively influences the observed nonmonotonic dependence of the compressibility with wall separation. We also present theoretical calculations based on fundamental-measure theory and integral-equationtheory, which are in very good agreement with the simulation results.

Solvation and Dynamics of CO2 in Aqueous Alkanolamine Solutions

Carbon dioxide sequestration from flue gases by chemical absorption is the most versatile process. The molecular engineering of novel high-performance biogas upgrading alkanolamine compounds requires detailed information about their properties in mixed solution. The liquid structure properties of four representative alkanolamine molecules (monoethanolamine (MEA) as a reference and standard, 3-aminopropanol (MPA), 2-methylaminoethanol (MMEA), and 4-diethylamino-2-butanol (DEAB)) in the presence of CO2 were investigated over a wide range of solvent alkanolamine/water mixture compositions and temperature. In aqueous solution, for the alkanolamine molecules MEA, MPA, and MMEA, hydrogen bonding with solvent water molecules is dominating over CO2 interactions. Analysis of the liquid structure reveals that carbon dioxide shows no preference of approaching the alkanolamine but is rather displaced by water molecules as the water content increases. CO2 dissolved in aqueous DEAB, however, accumulates within clusters...

Molecular Dynamics Simulations of Lithium-Doped Ionic-Liquid Electrolytes.

Lithium bis(trifluoromethanesulfonyl)imide (LiNTf2) doped ionic liquids (ILs) are investigated herein, as potential electrolytes for lithium-ion batteries, via scaled-charge molecular dynamics simulations. Four model ILs based on the [NTf2]- anion and heterocyclic ammonium cations were studied with varying concentrations, ranging from 0 to 1 M solutions, of the dissolved lithium salt. The pyrrolidinium ([pyrHH]+), piperidinium ([pipHH]+), N-butyl-pyrrolidinium ([pyrH4]+), and N-butyl- N-methyl-pyrrolidinium ([pyr14]+) cations were considered to evaluate the combined effects of increased ring size, as well as the introduction of apolar groups on the nitrogen atom of the cations, on the liquid structure properties of the electrolytes. Among the investigated ILs, [pyr14][NTf2] is the only aprotic IL allowing for a comparison of protic and aprotic ILs. The lithium coordination shell is seen to be quite different in the various IL-based systems; networks of lithium ions bridged by [NTf2]- ions have interesting consequences on the solvation shells and coordination numbers. Aggregate existence and velocity autocorrelation functions are finally evaluated in order to characterize the caging effect of [NTf2]- ions around lithium ions. In conclusion, we find that the lithium mobility and transport are directly proportional to the strength of the interionic interactions within the liquids, whereas the ease of solvation shows opposite trends.

Structural analysis of zwitterionic liquids vs. homologous ionic liquids.

Zwitterionic liquids (Zw-ILs) have been developed that are homologous to monovalent ionic liquids (ILs) and show great promise for controlled dissolution of cellulosic biomass. Using both high energy X-ray scattering and atomistic molecular simulations, this article compares the bulk liquid structural properties for novel Zw-ILs with their homologous ILs. It is shown that the significant localization of the charges on Zw-ILs leads to charge ordering similar to that observed for conventional ionic liquids with monovalent anions and cations. A low-intensity first sharp diffraction peak in the liquid structure factor S(q) is observed for both the Zw-IL and the IL. This is unexpected since both the Zw-IL and IL have a 2-(2-methoxyethoxy)ethyl (diether) functional group on the cationic imidazolium ring and ether functional groups are known to suppress this peak. Detailed analyses show that this intermediate range order in the liquid structure arises for slightly different reasons in the Zw-IL vs. the IL. For the Zw-IL, the ether tails in the liquid are shown to aggregate into nanoscale domains.

3DRISM-HI-D2MSA: an improved analytic theory to compute solvent structure around hydrophobic solutes with proper treatment of solute–solvent electrostatic interactions

ABSTRACTThe 3D reference interaction site model (3DRISM) is a powerful tool to study the thermodynamic and structural properties of liquids. However, for hydrophobic solutes, the inhomogeneity of the solvent density around them poses a great challenge to the 3DRISM theory. To address this issue, we have previously introduced the hydrophobic-induced density inhomogeneity theory (HI) for purely hydrophobic solutes. To further consider the complex hydrophobic solutes containing partial charges, here we propose the D2MSA closure to incorporate the short-range and long-range interactions with the D2 closure and the mean spherical approximation, respectively. We demonstrate that our new theory can compute the solvent distributions around real hydrophobic solutes in water and complex organic solvents that agree well with the explicit solvent molecular dynamics simulations.

Direct links between dynamical, thermodynamic, and structural properties of liquids: Modeling results.

We develop an approach to liquid thermodynamics based on collective modes. We perform extensive molecular-dynamics simulations of noble, molecular, and metallic liquids, and we provide direct evidence that liquid energy and specific heat are well-described by the temperature dependence of the Frenkel (hopping) frequency. The agreement between predicted and calculated thermodynamic properties is seen in the notably wide range of temperature spanning tens of thousands of Kelvin. The range includes both subcritical liquids and supercritical fluids. We discuss the structural crossover and interrelationships between the structure, dynamics, and thermodynamics of liquids and supercritical fluids.

In situ characterization of liquid network structures at high pressure and temperature using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press

ABSTRACTWe review recent progress in studying structural properties of liquids using X-ray absorption spectroscopy coupled with the Paris-Edinburgh press at third-generation synchrotron facilities. This experimental method allows for detecting subtle changes in atomic arrangements of melts over a wide pressure–temperature range. It has been also employed to monitor variations of the local coordination environment of diluted species contained in glasses, liquids and crystalline phases as a function of the pressure and temperature. Such information is of great importance for gaining deeper insights into the physico-chemical properties of liquids at extreme condition, including the understanding of such phenomena as liquid–liquid phase transitions, viscosity drops and various transport properties of geological melts. Here, we describe the experimental approach and discuss its potential in structural characterization on selected scientific highlights. Finally, the current ongoing instrumental developments and future scientific opportunities are discussed.

Understanding the Solubility of Acetaminophen in 1-n-Alkyl-3-methylimidazolium-Based Ionic Liquids Using Molecular Simulation.

During the manufacturing of pharmaceutical compounds, solvent mixtures are commonly used, where the addition of a cosolvent allows for the tuning of the intermolecular interactions present in the system. Here we demonstrate how a similar effect can be accomplished using a room temperature ionic liquid. The pharmaceutical compound acetaminophen is studied in 21 common ionic liquids composed of a 1-n-alkyl-3-methylimidazolium cation with 1 of 7 anions. Using the acetate anion, we predict a large enhancement in solubility of acetaminophen relative to water. We show how this is caused by a synergistic effect of favorable interactions between the ionic liquid and the phenyl, hydroxyl and amide groups of acetaminophen, demonstrating how the ionic liquid cation and anion may be chosen to preferentially solvate different functional groups of complex pharmaceutical compounds. Additionally, while the use of charge scaling in ionic liquid force fields has previously been found to have a minute effect on ionic liquid structural properties, we find it appreciably affects the computed solvation free energy of acetaminophen, which in turn affects the predicted solubility.

Density measurements and structural properties of liquid and amorphous metals under high pressure

We have implemented an in situ X-ray diffraction analysis method suitable for the determination of pressure–volume–temperature equations of state in the critical case of liquid and amorphous materials over an extended thermodynamic range (T>2000 K and P>40 GPa). This method is versatile, it can be applied to data obtained using various angle-dispersive X-ray diffraction high pressure apparatus and, contrary to in situ X-ray absorption techniques, is independent from the sample geometry. Further advantage is the fast data acquisition (between 10 and 300 s integration time). Information on macroscopic bulk properties (density) and local atomic arrangement (pair distribution function g(r)) can be gathered in parallel. To illustrate the method, we present studies on liquid Fe–S alloys in the Paris Edinburgh press and in laser-heated diamond anvil cell (DAC), and measurements on Ce glass in DAC at room temperature.

Structural properties in liquid Si during rapid cooling processes

The solidification processes of liquid Si are studied by molecular dynamics simulation using the Stillinger–Weber potential. The structural properties of liquid and amorphous silicon are analyzed by means of several structural analysis methods. Amorphous silicon network is formed during the solidification processes of liquid Si. The simulation results are in good agreement with the experimental ones. The structural changes of Si during solidification are described in detail by the radial distribution function, coordination number and the visualization technology. It reveals that the cavities are the important structural characters of liquid silicon. And the tetrahedron as the key structural unit are distributed in amorphous silicon randomly, and connected with other atoms together to form network ring structures finally.

Structural Properties of Liquid SiC during Rapid Solidification

The rapid solidification of liquid silicon carbide (SiC) is studied by molecular dynamic simulation using the Tersoff potential. The structural properties of liquid and amorphous SiC are analyzed by the radial distribution function, angular distribution function, coordination number, and visualization technology. Results show that both heteronuclear and homonuclear bonds exist and no atomic segregation occurs during solidification. The bond angles of silicon and carbon atoms are distributed at around 109° and 120°, respectively, and the average coordination number is <4. Threefold carbon atoms and fourfold silicon atoms are linked together by six typical structures and ultimately form a random network of amorphous structure. The simulated results help understand the structural properties of liquid and amorphous SiC, as well as other similar semiconductor alloys.

Liquid theory with high accuracy and broad applicability: Coupling parameter series expansion and non hard sphere perturbation strategy

Thermodynamic and structural properties of liquids are of fundamental interest in physics, chemistry, and biology, and perturbation approach has been fundamental to liquid theoretical approaches since the dawn of modern statistical mechanics and remains so to this day. Although thermodynamic perturbation theory (TPT) is widely used in the chemical physics community, one of the most popular versions of the TPT, i.e. Zwanzig (Zwanzig, R. W. J. Chem. Phys. 1954, 22, 1420-1426) 1st-order high temperature series expansion (HTSE) TPT and its 2nd-order counterpart under a macroscopic compressibility approximation of Barker-Henderson (Barker, J. A.; Henderson, D. J. Chem. Phys. 1967, 47, 2856-2861), have some serious shortcomings: (i) the nth-order term of the HTSE is involved with reference fluid distribution functions of order up to 2n, and the higher-order terms hence progressively become more complicated and numerically inaccessible; (ii) the performance of the HTSE rapidly deteriorates and the calculated results become even qualitatively incorrect as the temperature of interest decreases. This account deals with the developments that we have made over the last five years or so to advance a coupling parameter series expansion (CPSE) and a non hard sphere (HS) perturbation strategy that has scored some of its greatest successes in overcoming the above-mentioned difficulties. In this account (i) we expatiate on implementation details of our schemes: how input information indispensable to high-order truncation of the CPSE in both the HS and non HS perturbation schemes is calculated by an Ornstein-Zernike integral equation theory; how high-order thermodynamic quantities, such as critical parameters and excess constant volume heat capacity, are extracted from the resulting excess Helmholtz free energy with irregular and inevitable numerical errors; how to select reference potential in the non HS perturbation scheme. (ii) We give a quantitative analysis on why convergence speed of the CPSE in both the HS and non HS perturbation schemes is certainly faster than that of the HTSE and the HS perturbation scheme. (iii) We illustrate applications of the CPSE TPT in both the HS and non HS perturbation schemes in calculating thermodynamic properties of various coarse-grained potential function models and as input information of other liquid state theories such as a classical density functional theory (DFT), and also discuss, in the framework of classical DFT, the potential of our CPSE scheme in several typical problems of chemical physics interest. (iv) Finally, we consider several topics which are possibly expected to be settled in the immediate future and possible integration with other liquid state theory frameworks aiming to solve problems in complex fluids in both bulk and inhomogeneous states.

Improved modeling of liquidGeSe2: Impact of the exchange-correlation functional

The structural properties of liquid ${\text{GeSe}}_{2}$ are studied by using first-principles molecular dynamics in conjunction with the Becke, Lee, Yang, and Parr (BLYP) generalized gradient approximation for the exchange and correlation energy. The results on partial pair-correlation functions, coordination numbers, bond-angle distributions, and partial-structure factors are compared with available experimental data and with previous first-principles molecular-dynamics results obtained within the Perdew and Wang (PW) generalized gradient approximation for the exchange and correlation energy. We found that the BLYP approach substantially improves upon the PW one in the case of the short-range properties. In particular, the Ge-Ge pair-correlation function takes a more structured profile that includes a marked first peak due to homopolar bonds, a first maximum exhibiting a clear shoulder, and a deep minimum, all these features being absent in the previous PW results. Overall, the amount of tetrahedral order is significantly increased in spite of a larger number of Ge-Ge homopolar connections. Due to the smaller number of miscoordinations, diffusion coefficients obtained by the present BLYP calculation are smaller by at least one order of magnitude than in the PW case.

In situ determination of Fe–Fe 3S phase diagram and liquid structural properties up to 65 GPa

Lighter elements than iron such as sulphur are required in the Earth's core to account of the core density deficit. Accurate determination of the evolution of the Fe–FeS phase diagram at high pressure is essential to determine sulphur amount in the Earth's core. Ab initio calculations predict extensive solubility of S in solid Fe at core pressures of 330 GPa, whereas multi anvil quench analysis exhibits deep eutectic system at moderate pressure of 21 GPa. In this study, we investigated the Fe-rich part of Fe–FeS phase diagram up to 65 GPa and 2200 K using in situ angle dispersive X-ray diffraction. We report a uniform increase with pressure of the eutectic temperatures ( T Eut), of about 15 K/GPa. Above 50 GPa, we evidence a decrease of S content in eutectic liquid with increasing pressure. Extrapolating this trend to inner core boundary pressures suggests that S cannot account for the 10 wt.% outer core density deficit and that other light elements, such as Si and O, are needed. Diffraction pattern recorded at 42 GPa and 2150 K was selected for structural investigations of the Fe–S liquid. By applying liquid structure simulation based on Gaussian distribution of atoms around crystalline positions, a good agreement has been found with hcp Fe model-structure, rather than with Fe 3S structure. It suggests that S acts as an interstitial impurity in the liquid state. Therefore, S could have a relatively minor effect on sound velocities in liquid outer core.

EVOLUTION OF STRUCTURE OF LIQUID AND AMORPHOUS Al2O3·2SiO2 NANOPARTICLES UPON COOLING FROM THE MELTS

Structural properties of liquid and amorphous Al 2 O 3·2 SiO 2 nanoparticles have been investigated by molecular dynamics (MD) method. The simulations were done in a spherical model with different sizes of 2 nm, 3 nm and 4 nm under nonperiodic boundary conditions. We use the Born–Mayer type interatomic potentials. We determined the changes in mean interatomic distances, coordination number distributions, partial radial distribution functions (PRDFs) and bond-angle distributions of Al 2 O 3·2 SiO 2 nanoparticles upon cooling towards glassy state. Calculations show that liquid and amorphous Al 2 O 3·2 SiO 2 nanoparticles have a slightly distorted tetrahedral network structure with the mean coordination number Z Al - O ≈ 4.0 and Z Si - O ≈ 4.0, which is contrary to that of the liquid and amorphous bulk. Temperature dependence of structural defects in nanoparticles was found and discussed. In addition, we found size effects on glass transition temperature of Al 2 O 3·2 SiO 2 nanoparticles in that it decreases with decreasing size.

Structural properties of simulated liquid and amorphous aluminium silicates

The structural properties of liquid and amorphous aluminium silicates have been studied using the molecular dynamics (MD) method. Simulations were done in the basic cube under periodic boundary containing 3024, 3025 and 3029 ions for 2Al2O3. SiO2 (A2S), Al2O3. SiO2 (AS) and Al2O3.2SiO2 (AS2) models, respectively. The composition dependence of the structural properties of the systems was found through interatomic distances, coordination number distribution and defect distribution. The evolution of structure upon cooling from the melt of the AS was observed and discussed. Calculations show that the calculated data agree well with the experimental ones and with those obtained previously in other simulation works. Moreover, we found clear evidence of the existence of triclusters and the composition dependence of triclusters was studied for the amorphous state at a temperature of 350 K.

Structure and dynamics of liquid and amorphous Al2O3.2SiO2

Structural properties of liquid and amorphous Al 2 O 3 .2SiO 2 (denoted as AS2) have been studied in a model containing 3025 particles under periodic boundary conditions with the Born-Mayer type pair potentials. Models have been obtained by cooling from the melt via molecular dynamics (MD) simulation. Structural properties of an amorphous model obtained at the temperature of 350 K have been analyzed in details through the partial radial distribution functions (PRDF), coordination number distribution, bond-angle distributions and interatomic distances. Calculations show that calculated data agree well with the experimental ones and with those obtained previously in other simulation works. The evolution of structure upon cooling from the melt was observed and discussed. We found a clear evidence of the existence of triclusters in the system. Diffusion constant of particles has been calculated and discussed. Calculations presented that the temperature dependence of diffusion constant D of components in the system shows an Arrhenius law at temperatures ranged from 2450 K to 4200 K and it shows a power law, $D\propto (T-T_C )^\gamma$ , at higher temperatures.