Session J35: Liquids

Ion shape effect on dynamics of ionic liquids

Ionic liquids (ILs) are a group of salts composing of an organic cation and organic or inorganic anion with melting points below 100 $^{\circ}$C and have many suitable properties, such as negligible vapor pressure, low flammability, high ionic conductivity and high thermal stability for various applications. Moreover, a great number of ILs with a variety of physical and chemical properties can be synthesized from a combination of different cations (most differently substituted imidazolium, pyridinium, and quaternary ammonium or phosphonium ions) and anions. One can judiciously select from a multitude of ILs to suit a specific application, where the concept of designer solvent comes from. To expedite the development process of target ILs, it is crucial to understand the relationship between ion shape and dynamics of ILs. We studied a wide range of ILs with different ion shape pairings and found the planar-planar paired ILs have a better dynamics as a whole.   

Thermodynamic Phase Diagram of Hydrogen in Nano-porous Silica

One of the major challenges faced by inertial confinement fusion (ICF) is spherical uniformity of fuel capsules. Most current ignition target designs call for spherically uniform layers of deuterium-tritium (DT) mix in a solid phase. Smooth solid DT layers of high-quality single crystals are necessary, requiring stringent layering protocols. Liquid DT confined in a low-density nano-porous scaffold layer is a possible alternative target, having greatly relaxed layering requirements. Hydrogen in vycor, which has a high density and a relatively uniform pore size distribution, has previously been studied. The porous materials used for this study are silica aerogels. These have ultralow densities and broad pore size distributions. We discuss the thermodynamic phase diagram of hydrogen and deuterium condensed in silica aerogels studied using relaxation calorimetry. We find that crystallization temperatures of both isotopes are suppressed inside the aerogel; however, the freezing takes place over a relatively wide range of temperatures and non-trivially depends on the hydrogen filling fraction. We discuss the correlation of freezing temperatures with the pore size distribution. This work was performed under the auspices of the U.S. DOE my LLNL under Contract DE-AC52-07NA27344.   

Quasi-free Electron Energy in Near Critical Point He

We present the quasi-free electron energy $V_0(\rho)$, where $\rho$ is the density, in He from low density to the density of the gas/He-I/He-II triple point, at various noncritical temperatures and on an isotherm near the critical isotherm. These data, which were obtained using field enhanced photoemission, represent the first extended density overview of $V_0(\rho)$ for He, especially near the critical point. A novel critical point effect is observed and is accurately fitted to the local Wigner-Seitz model, thereby showing that this model, which was developed for attractive systems, can also be applied to repulsive systems.\\[4pt] The experimental measurements reported here were performed at the University of Wisconsin Synchrotron Radiation Center (NSF DMR-0537588). This work was supported by grants from the Petroleum Research Fund (45728-B6) and from the National Science Foundation (NSF CHE-0956719).   

COSMO-3D: Introducing Steric Effects into COSMO Solvation Models

We will present a modified COSMO-based thermodynamical model which can be used for the prediction of solvation in liquid mixtures. The method provides values for the surface-segment interactions and molecular contact areas as functions of the relative positions of all the segment pairings of the constituent molecules in a mixture. These expressions are obtained by placing the complete molecules into contact at each segment pair to determine if the molecules overlap, and using a probing particle to determine the total area of contact. The former expression can be used to discard terms from the COMSO-based segment sums which correspond to overlapping molecular configurations. The areas are used to determine the average contact energies based on Onsager's theory for dielectric screening. Our method allows us to remove three parameters from the original model without any loss of accuracy. We will show the results of our model as applied to binary mixture vapor-liquid equilibrium predictions.   

Charge induced hydrophobic/hydrophilic metallic surfaces

Understanding the interaction of water-metal systems in an atomic level is of fundamental importance in many areas, such as catalysis and materials science. We here present a detailed first-principles molecular dynamics study of bulk water molecules confined within Pd(111) surfaces. We show that there is a charge transfer between the substrate and the water inducing an asymmetry in the order of water molecules at Pd surfaces. Our results show that the hydrophobic/hydrophilic character of a metallic surface depend on its charge, which can be controlled by an applied voltage. We also propose a methodology to obtain the dipole moment of each water molecule and show how they are affected by the substrate induced polarization effects.   

Pulling adsorbed polymers at an angle: A low temperature theory

We consider several partially-directed walk models in two- and three-dimensions to study the problem of a homopolymer interacting with a surface while subject to a force at the terminal monomer. The force is applied with a component parallel to the surface as well as a component perpendicular to the surface. Depending on the relative values of the force in each direction, the force can either enhance the adsorption transition or lead to desorption in an adsorbed polymer. For each model, we determine the associated generating function and extract the phase diagram, identifying states where the polymer is thermally desorbed, adsorbed, and under the influence of the force. We note the different regimes that appear in the problem and provide a low temperature approximation to describe them. The approximation is exact at T=0 and models the exact results extremely well for small values of T. This work is an extension of a model considered by S. Whittington and E. Orlandini.   

Low-Energy Electron Induced Reactions in Condensed Methanol

We investigate the dynamics of low-energy electron-induced reactions in condensed thin films of methanol (CH$_3$OH) through electron stimulated desorption (ESD) and post-irradiation temperature programmed desorption (TPD) experiments. ESD experiments indicate that the anions which desorb from the methanol thin film during electron irradiation are predominantly formed through the dissociation of temporary negative ions formed by electron capture by methanol molecules, a process known as dissociative electron attachment (DEA). However, based on investigation of reaction products remaining in the methanol thin film post-irradiation through TPD experiments, DEA is not the obvious primary mechanism by which methoxymethanol (CH$_3$OCH$_2$OH) and ethylene glycol (HOCH$_2$CH$_2$OH) are formed. Evidence indicates formation of these molecules may be driven by both DEA and electron impact excitation.   

The Widom line and noise power spectral analysis of a supercritical fluid

In a typical pressure-temperature phase diagram for a fluid, there is a first order phase transition line between the liquid and the vapor phases that terminates at a critical point. Beyond this critical point lies the supercritical regime where one can go continuously between the liquid and vapor phases. In the supercritical region, there is a line of specific heat maxima, called the Widom line, which is often regarded as an extension of the liquid-vapor coexistence line. Using molecular dynamics simulations of a Lennard-Jones fluid, we find that the noise power spectrum of the density fluctuations on the Widom line of the liquid-vapor transition can be divided into 3 frequency regions. The intermediate frequency region with 1/f noise appears as the temperature approaches the Widom temperature from above or below. Furthermore, we find that the power spectra of both the density and potential energy fluctuations at low frequency have a maximum on the Widom line, suggesting that the noise power can provide an alternative signature of the Widom line.   

Disappearance of Widom Line for Liquid-Liquid Phase Transition with Horizontal Coexistence Line

The study of spherically symmetric two-scale Jagla model with both repulsive and attractive ramps has been very successful in demonstrating the anomalous behavior of liquids (especially water) and its relation with respect to the existence of a liquid-liquid (LL) critical point. However, the co-existence line of Jagla model shows a positive slope, which is opposite to what has been found in the simulations of water. To more convincingly link the result of the study on Jagla model with that of water, we applied discrete molecular dynamics to Gibson and Wilding's modified Jagla model and found that by shrinking both the attractive and repulsive ramps, the slope of the coexistence line can be reduced to zero. However, at these values of the parameters, the LL critical point becomes completely unstable with respect to crystal and glass. We further studied the Widom line, defined as extreme of response functions and also continuation of the coexistence line into one phase region, and found Widom line disappeared in the case of zero slope of the coexistence line, due to the equal enthalpy of low-density liquid (LDL) and high-density liquid (HDL).   

Effects of pressure on transitions between disordered phases in supercooled liquid Si

In the last 20 years, there has been an increasing interest in liquid-liquid transitions. Substances that exhibit negative melting slopes are considered to undergo such transitions. In this work, we study the pressure effects on the transitions between the disordered phases in supercooled liquid Si through Monte Carlo simulations and efficient methods to compute free energies. Our calculations, using a realistic interatomic potential for Si, indicate that at zero pressure the liquid-liquid phase transition, between the high density liquid and the low density liquid, occurs at a temperature 325 K below melting. We found that the liquid-liquid transition temperature decreases with increasing pressure, following the liquid-solid coexistence curve. As pressure increases, the liquid-liquid coexistence curve approaches the region where the glass transition between the low density liquid and the low density amorphous takes place. Above 5 GPa, our calculations show that the liquid-liquid transition is suppressed by the glassy dynamics of the system. We also found that above 5 GPa, the glass transition temperature is lower than that at lower pressures, suggesting that under these conditions the glass transition occurs between the high density liquid and the high density amorphous.   

Thermodynamics of Supercooled Water

We present the currently available experimental information for the thermodynamic properties of supercooled ordinary and heavy water and the possibility of modeling these thermodynamic properties on a theoretical basis. Part of the interest into the thermodynamic behavior of supercooled water is caused by an anomalous temperature dependence of the heat capacity, the compressibility and the thermal expansivity. We show that by assuming the existence of a virtual liquid--liquid critical point in supercooled water, the theory of critical phenomena can give an accurate account of the experimental thermodynamic-property data up to a pressure of 150~MPa. In addition, we show that a semi-empirical extension of the theoretical model can account for all currently available experimental data in the supercooled region, up to 400~MPa. Critical-point thermodynamics describes the available thermodynamic data on supercooled water within experimental accuracy, thus establishing a benchmark for any further developments in this area.   

Diffusivity crossover and liquid-liquid transition in solutions of hard spheres in Jagla particles

We study by discrete molecular dynamics simulations the relation between the thermodynamics and the diffusive behavior in solutions of hard spheres in Jagla particles, close to their liquid-liquid critical point. For comparison, we also show the same properties in the bulk Jagla particles system. The hard spheres and the Jagla model are used as spherically symmetric potentials for small hydrophobic solutes in water. We find that the fragile to strong dynamic transition observed when studying the diffusive behavior is always coupled to the low density to high density liquid transition. Above the liquid-liquid critical pressure the diffusivity crossover happens exactly at the Widom line of the systems, where the thermodynamic response functions show maxima. Below the liquid-liquid critical pressure, the diffusivity crossover corresponds to the crossing of the limit of mechanical stability lines and it shows hysteresis when going from high to low temperatures or vice versa. These findings prove that the strong connection between dynamics and thermodynamics found for bulk Jagla particles persists in hydrophobic solutions for concentrations from low to moderate.   

Small-to-Large Polaron Transition in Water

\noindent Heavier charged particles, such as protons, dissolved in water form a hydration shell, which partially shields the charge and significantly increases their effective mass. On the other hand, electrons do not form a hydration shells in water. Leggett's path-integral formalism can be combined with Feynman's polaron theory to construct a theory for the effective mass of a charged particle in water in terms of the charge structure factor, $S({\bf q},\omega)$ of water, or alternatively, the frequency and wavefunction dependent dielectric function of water. Measurements of $S({\bf q},\omega)$ for density fluctuations indicate that water has a soft mode with a wave number in the range of an inverse Angstrom. By combining these experiments with analytical and numerical models of the dielectric function of water, we discuss the small-to-large polaron transition in water.