Examining the Role of Different Molecular Interactions on Activation Energies and Activation Volumes in Liquid Water

A structure-dynamics relationship enables prediction of the water hydrogen bond exchange activation energy from experimental data

It has long been understood that the structural features of water are determined by hydrogen bonding (H-bonding) and that the exchange of, or "jumps" between, H-bond partners underlies many of the dynamical processes in water. Despite the importance of H-bond exchanges there is, as yet, no direct method for experimentally measuring the timescale of the process or its associated activation energy. Here, we identify and exploit relationships between water's structural and dynamical properties that provide an indirect route for determining the H-bond exchange activation energy from experimental data. Specifically, we show that the enthalpy and entropy determining the radial distribution function in liquid water are linearly correlated with the activation energies for H-bond jumps, OH reorientation, and diffusion. Using temperature-dependent measurements of the radial distribution function from the literature, we demonstrate how these correlations allow us to infer the value of the jump activation energy, Ea,0, from experimental results. This analysis gives Ea,0 = 3.43 kcal mol-1, which is in good agreement with that predicted by the TIP4P/2005 water model. We also illustrate other approaches for estimating this activation energy consistent with these estimates.

Computation of X-ray and Neutron Scattering Patterns to Benchmark Atomistic Simulations against Experiments

Molecular Dynamics simulations study material structure and dynamics at the atomic level. X-ray and neutron scattering experiments probe exactly the same time- and length scales as the simulations. In order to benchmark simulations against measured scattering data, a program is required that computes scattering patterns from simulations with good single-core performance and support for parallelization. In this work, the existing program Sassena is used as a potent solution to this requirement for a range of scattering methods, covering pico- to nanosecond dynamics, as well as the structure from some Ångströms to hundreds of nanometers. In the case of nanometer-level structures, the finite size of the simulation box, which is referred to as the finite size effect, has to be factored into the computations for which a method is described and implemented into Sassena. Additionally, the single-core and parallelization performance of Sassena is investigated, and several improvements are introduced.

Effects of polarizability and charge transfer on water dynamics and the underlying activation energies

A large number of force fields have been proposed for describing the behavior of liquid water within classical atomistic simulations, particularly molecular dynamics. In the past two decades, models that incorporate molecular polarizability and even charge transfer have become more prevalent, in attempts to develop more accurate descriptions. These are frequently parameterized to reproduce the measured thermodynamics, phase behavior, and structure of water. On the other hand, the dynamics of water is rarely considered in the construction of these models, despite its importance in their ultimate applications. In this paper, we explore the structure and dynamics of polarizable and charge-transfer water models, with a focus on timescales that directly or indirectly relate to hydrogen bond (H-bond) making and breaking. Moreover, we use the recently developed fluctuation theory for dynamics to determine the temperature dependence of these properties to shed light on the driving forces. This approach provides key insight into the timescale activation energies through a rigorous decomposition into contributions from the different interactions, including polarization and charge transfer. The results show that charge transfer effects have a negligible effect on the activation energies. Furthermore, the same tension between electrostatic and van der Waals interactions that is found in fixed-charge water models also governs the behavior of polarizable models. The models are found to involve significant energy-entropy compensation, pointing to the importance of developing water models that accurately describe the temperature dependence of water structure and dynamics.

A Maxwell relation for dynamical timescales with application to the pressure and temperature dependence of water self-diffusion and shear viscosity

A Maxwell relation for a reaction rate constant (or other dynamical timescale) obtained under constant pressure, p, and temperature, T, is introduced and discussed. Examination of this relationship in the context of fluctuation theory provides insight into the p and T dependence of the timescale and the underlying molecular origins. This Maxwell relation motivates a suggestion for the general form of the timescale as a function of pressure and temperature. This is illustrated by accurately fitting simulation results and existing experimental data on the self-diffusion coefficient and shear viscosity of liquid water. A key advantage of this approach is that each fitting parameter is physically meaningful.

Statistical prediction and sensitivity analysis of kinetic rate constants for efficient thermal valorization of plastic waste into combustible oil and gases

Sensitivity analyses of rate constants for chemical kinetics of the pyrolysis reaction are essential for the efficient valorization of plastic waste into combustible liquids and gases. Finding the role of individual rate constants can provide important information on the process conditions, quality, and quantity of the pyrolysis products. The reaction temperature and time can also be reduced through these analyses. For sensitivity analysis, one possible approach is to estimate the kinetic parameters using MLRM (multiple linear regression model) in SPSS. To date, no research reports on this research gap are documented in the published literature. In this study, MLRM is applied to kinetic rate constants, which slightly differ from experimental data. The experimental and statistically predicted rate constants varied up to 200% from their original values to perform sensitivity analysis using MATLAB software. The product yield was examined after 60 min of thermal pyrolysis at a fixed temperature of 420 °C. The predicted rate constant "k(8)" with a slight difference of 0.02 and 0.04 from the experiment revealed 85% oil yield and 40% light wax after 60 min of operation. The heavy wax was missing from the products under these conditions. This rate constant can be utilized to maximize the commercial-scale extraction of liquids and light waxes from thermal pyrolysis of plastics.

Direct calculation of the temperature dependence of 2D-IR spectra: Urea in water

A method for directly calculating the temperature derivative of two-dimensional infrared (2D-IR) spectra from simulations at a single temperature is presented. The approach is demonstrated by application to the OD stretching spectrum of isotopically dilute aqueous (HOD in H₂O) solutions of urea as a function of concentration. Urea is an important osmolyte because of its ability to denature proteins, which has motivated significant interest in its effect on the structure and dynamics of water. The present results show that the temperature dependence of both the linear IR and 2D-IR spectra, which report on the underlying energetic driving forces, is more sensitive to urea concentration than the spectra themselves. Additional physical insight is provided by calculation of the contributions to the temperature derivative from different interactions, e.g., water-water, water-urea, and urea-urea, present in the system. Finally, it is demonstrated how 2D-IR spectra at other temperatures can be obtained from only room temperature simulations.

Shining (Infrared) Light on the Hofmeister Series: Driving Forces for Changes in the Water Vibrational Spectra in Alkali-Halide Salt Solutions

The Hofmeister series is frequently used to rank ions based on their behavior from chaotropes ("structure breakers"), which weaken the surrounding hydrogen-bond network, to kosmotropes ("structure makers"), which enhance it. Here, we use fluctuation theory to investigate the energetic and entropic driving forces underlying the Hofmeister series for aqueous alkali-halide solutions. Specifically, we exploit the OH stretch infrared (IR) spectrum in isotopically dilute HOD/D₂O solutions as a probe of the effect of the salt on the water properties for different concentrations and choice of halide anion. Fluctuation theory is used to calculate the temperature derivative of these IR spectra, including decomposition of the derivative into different energetic contributions. These contributions are used to determine the thermodynamic driving forces in terms of effective internal energy and entropic contributions. This analysis implicates entropic contributions as the key factor in the Hofmeister series behavior of the OH stretch IR spectra, while the effective internal energy is nearly ion-independent.

Water Diffusion Proceeds via a Hydrogen-Bond Jump Exchange Mechanism

The self-diffusion of water molecules plays a key part in a broad range of essential processes in biochemistry, medical imaging, material science, and engineering. However, its molecular mechanism and the role played by the water hydrogen-bond network rearrangements are not known. Here we combine molecular dynamics simulations and analytic modeling to determine the molecular mechanism of water diffusion. We establish a quantitative connection between the water diffusion coefficient and hydrogen-bond jump exchanges, and identify the features that determine the underlying energetic barrier. We thus provide a unified framework to understand the coupling between translational, rotational, and hydrogen-bond dynamics in liquid water. It explains why these different dynamics do not necessarily exhibit identical temperature dependences although they all result from the same hydrogen-bond exchange events. The consequences for the understanding of water diffusion in supercooled conditions and for water transport in complex aqueous systems, including ionic, biological, and confined solutions, are discussed.

Using Activation Energies to Elucidate Mechanisms of Water Dynamics

Recent advances in the calculation of activation energies are shedding new light on the dynamical time scales of liquid water. In this Perspective, we examine how activation energies elucidate the central, but not singular, role of the exchange of hydrogen-bond (H-bond) partners that rearrange the H-bond network of water. The contributions of other motions to dynamical time scales and their associated activation energies are discussed along with one case, vibrational spectral diffusion, where H-bond exchanges are not mechanistically significant. Nascent progress on outstanding challenges, including descriptions of non-Arrhenius effects and activation volumes, are detailed along with some directions for future investigations.

Molecular Simulations of Phase Equilibria and Transport Properties in a Model CO2-Expanded Lithium Perchlorate Electrolyte

Carbon-dioxide (CO₂)-expanded liquids, in which a significant mole fraction of CO₂ is dissolved into an organic solvent, have been of significant interest, especially as catalytic support media. Because the CO₂ mole fraction and density can be controlled over a significant range by changing the CO₂ partial pressure, the transport properties of these solvents are highly tunable. Recently, these liquids have garnered interest as potential electrolyte solutions for catalytic electrochemistry; however, little is currently known about the influence of the electrolyte on CO₂ expansion. In the present work, we use molecular-dynamics simulations to study diffusion and viscosity in a model lithium perchlorate electrolyte in CO₂-expanded acetonitrile and demonstrate that these properties are highly dependent on the concentration of the electrolyte. Our present results indicate that the electrolyte slows down diffusion of both CO₂ and MeCN, and that the slowed diffusion in the former is driven by changes in the activation entropy, whereas slowed diffusion in the latter is driven by changes in the activation energy.