Liquid-vacuum Interface Research Articles

Orientation and mutual location of ions at the surface of ionic liquids

The structure of the liquid-vacuum interface in room temperature ionic liquids (ILs) is investigated using angle-resolved X-ray photoelectron spectroscopy (ARXPS) and synchrotron X-ray photoelectron spectroscopy (SXPS). By varying the polar angle and comparing the results for the chosen ionic liquids, we identify the presence of a surface layer that is chemically different to the bulk. In particular, this layer: (i) is enriched by aliphatic carbon atoms from the saturated carbon chains of the anions and cations, and (ii) contains an unequal distribution of cations and anions in a direction normal to the surface. This unequal distribution creates a potential gradient which extends from the surface into the liquid. We show unequivocally that this layer is not due to the presence of impurities.

On the origin of the electrostatic potential difference at a liquid-vacuum interface

The microscopic origin of the interface potential calculated from computer simulations is elucidated by considering a simple model of molecules near an interface. The model posits that molecules are isotropically oriented and their charge density is Gaussian distributed. Molecules that have a charge density that is more negative toward their interior tend to give rise to a negative interface potential relative to the gaseous phase, while charge densities more positive toward their interior give rise to a positive interface potential. The interface potential for the model is compared to the interface potential computed from molecular dynamics simulations of the nonpolar vacuum-methane system and the polar vacuum-water interface system. The computed vacuum-methane interface potential from a molecular dynamics simulation (-220 mV) is captured with quantitative precision by the model. For the vacuum-water interface system, the model predicts a potential of -400 mV compared to -510 mV, calculated from a molecular dynamics simulation. The physical implications of this isotropic contribution to the interface potential is examined using the example of ion solvation in liquid methane.

Computation of methodology-independent ionic solvation free energies from molecular simulations. II. The hydration free energy of the sodium cation

The raw ionic solvation free energies computed from atomistic (explicit-solvent) simulations are extremely sensitive to the boundary conditions (finite or periodic system, system shape, and size) and treatment of electrostatic interactions (Coulombic, lattice sum, or cutoff based) used during these simulations. In the present article, it is shown that correction terms can be derived for the effect of (A) an incorrect solvent polarization around the ion due to the use of an approximate (not strictly Coulombic) electrostatic scheme; (B) the finite size or artificial periodicity of the simulated system; (C) an improper summation scheme to evaluate the potential at the ion site and the possible presence of a liquid-vacuum interface in the simulated system. Taking the hydration free energy of the sodium cation as a test case, it is shown that the raw solvation free energies obtained using seven different types of boundary conditions and electrostatic schemes commonly used in explicit-solvent simulations (for a total of 72 simulations differing in the corresponding simulation parameters) can be corrected so as to obtain a consistent value for this quantity.

Molecular Dynamics Study to Identify the Reactive Sites of a Liquid Squalane Surface

Molecular dynamics simulations of liquid squalane, C30H62, were performed, focusing in particular on the liquid-vacuum interface. These theoretical studies were aimed at identifying potentially reactive sites on the surface, knowledge of which is important for a number of inelastic and reactive scattering experiments. A united atom force field (Martin, M. G.; Siepmann, J. I. J. Phys. Chem. B 1999, 103, 4508-4517) was used, and the simulations were analyzed with respect to their interfacial properties. A modest but clearly identifiable preference for methyl groups to protrude into the vacuum has been found at lower temperatures. This effect decreases when going to higher temperatures. Additional simulations tracking the flight paths of projectiles directed at a number of randomly chosen surfaces extracted from the molecular dynamics simulations were performed. The geometrical parameters for these calculations were chosen to imitate a typical abstraction reaction, such as the reaction between ground-state oxygen atoms and hydrocarbons. Despite the preference for methyl groups to protrude further into the vacuum, Monte Carlo tracking simulations suggest, on geometric grounds, that primary and secondary hydrogen atoms are roughly equally likely to react with incoming gas-phase atoms. These geometric simulations also indicate that a substantial fraction of the scattered products is likely to undergo at least one secondary collision with hydrocarbon side chains. These results help to interpret the outcome of previous measurements of the internal and external energy distribution of the gas-phase OH products of the interfacial reaction between oxygen atoms and liquid squalane.

Computation of methodology-independent ionic solvation free energies from molecular simulations. I. The electrostatic potential in molecular liquids

The computation of ionic solvation free energies from atomistic simulations is a surprisingly difficult problem that has found no satisfactory solution for more than 15 years. The reason is that the charging free energies evaluated from such simulations are affected by very large errors. One of these is related to the choice of a specific convention for summing up the contributions of solvent charges to the electrostatic potential in the ionic cavity, namely, on the basis of point charges within entire solvent molecules (M scheme) or on the basis of individual point charges (P scheme). The use of an inappropriate convention may lead to a charge-independent offset in the calculated potential, which depends on the details of the summation scheme, on the quadrupole-moment trace of the solvent molecule, and on the approximate form used to represent electrostatic interactions in the system. However, whether the M or P scheme (if any) represents the appropriate convention is still a matter of on-going debate. The goal of the present article is to settle this long-standing controversy by carefully analyzing (both analytically and numerically) the properties of the electrostatic potential in molecular liquids (and inside cavities within them). Restricting the discussion to real liquids of "spherical" solvent molecules (represented by a classical solvent model with a single van der Waals interaction site), it is concluded that (i) for Coulombic (or straight-cutoff truncated) electrostatic interactions, the M scheme is the appropriate way of calculating the electrostatic potential; (ii) for non-Coulombic interactions deriving from a continuously differentiable function, both M and P schemes generally deliver an incorrect result (for which an analytical correction must be applied); and (iii) finite-temperature effects, including intermolecular orientation correlations and a preferential orientational structure in the neighborhood of a liquid-vacuum interface, must be taken into account. Applications of these results to the computation methodology-independent ionic solvation free energies from molecular simulations will be the scope of a forthcoming article.

Field extraction of ions from liquid solutions with the use of polymer track membranes

The general principles of the electric field assisted ion evaporation in the membrane ion source are considered. In the ion source, the liquid sample under investigation is placed in narrow channels of a polymer track membrane, which separates the liquid sample at atmospheric pressure from the vacuum chamber. Stability of the liquid at strong electric fields is provided by a choice of the diameters of channels and the liquid–polymer contact angle. The electric charge on the vacuum–polymer interface is of great importance for creation of the strong electric field near the liquid–vacuum interface. Such a conclusion is made from the computations of the electric field in the framework of the model developed. The mechanism of the electric field assisted evaporation of ions is discussed to explain the observed mass spectra for the ions extracted from liquid.

Detection of a single electron produced inside bulk superfluid helium

The HERON project is an effort to develop a detector for low-energy solar neutrinos in real time by observing their elastic scattering from electrons using superfluid helium as the target material. By applying appropriate electric fields, the recoil electron can be separated from the positive ion, drifted upward to the liquid–vacuum interface, transmitted through the surface with the aid of a vortex ring, and detected using a calorimeter. By studying the correlation of the 16eV photon signal produced by scintillation and the single-electron signal, we can locate a neutrino event in a large detector and distinguish it from the background events involving multiple Compton scattering.

Generation of electrohydrodynamic waves at the liquid-vacuum interface

The linear electrodynamic problem of the stability of a viscous liquid surface at the liquid-air interface is studied in view of the surface charge kinetics. The electric field critical intensities which break the stability of the plane interface are determined. Conditions for generating sustained waves at the interface are found.

A free-surface lattice Boltzmann method for modelling the filling of expanding cavities by Bingham fluids

The filling process of viscoplastic metal alloys and plastics in expanding cavities is modelled using the lattice Boltzmann method in two and three dimensions. These models combine the regularized Bingham model for viscoplastic fluids with a free-interface algorithm. The latter is based on a modified immiscible lattice Boltzmann model in which one species is the fluid and the other one is considered to be a vacuum. The boundary conditions at the curved liquid-vacuum interface are met without any geometrical front reconstruction from a first-order Chapman-Enskog expansion. The numerical results obtained with these models are found in good agreement with available theoretical and numerical analysis.

Positive and negative ion emission from perfluorinated poly-ethers

Positive and negative ion ejection in the mass range below 200 Dalton is observed for perfluorinated poly-ether surfaces subjected to electron- and ion-beam irradiation. The exciting projectiles are Ar + ions of 300 and 4000 eV and electrons of 3000 eV. The main positive ions found are fragments of CF 3 + ions. Larger ions are found with lower probability. The main negative ion observed is F. The results show that the liquid–vacuum interface is formed by CF 3 groups from the polymer.

The absolute magnetization of 3He at zero pressure

We have measured the absolute magnetization of liquid 3He at saturated vapor pressure over the temperature range 0.3–500 mK. The technique compares the magnetization of the liquid to that of vacuum by passing the liquid–vacuum interface through a SQUID coil. We find that, contrary to expectations, the electronic diamagnetism of the normal liquid is approximately 6% less than the calculated atomic value. This appears to be the first measurement of the diamagnetic susceptibility of any noble gas in a condensed state.

Surface Bose–Einstein condensate: clusters and free surface of 4He

The role of surface fluctuations on the local Bose–Einstein condensate (BEC) in a self-bounded system like 4He is investigated at T=0 K. We find that when the wave function (wf ) properly includes these fluctuations the local BEC only reaches about 50% and not 100% as found with a standard wf. Further, out of the surface the condensate decreases and correspondingly there is an enhanced population of small momentum states. We find this behaviour both for the liquid–vacuum interface and for clusters larger than about 200 particles. Our results are based on a novel shadow wave function which describes the self-binding of 4He only via interparticle correlations.

Multichannel extraction of charged species from liquid with use of track membranes

An extraction of negative charged species under electric field from liquid solutions into vacuum by use of poly(ethylene terephialate) track membranes has been investigated. The measurements of a total current at different voltages, applied between a liquid sample and a electrode in vacuum, has been carried out. In addition, an influence of an electrolyte concentration on the total current for the KI solution in glycerol has been studied. It is shown that a special feature of extraction of charged species from the channels of dielectric membranes is connected largely with on accumulation of charges on the vacuum surface of the polymer membrane. So, the current from dielectric membrane is controlled by several processes including formation of strong electric field near the liquid-vacuum interface, transport of ions along the channels to the interface and the field-assisted transition of charged species into vacuum.

Correlations in the liquid interfaces of simple liquids

From molecular-dynamics simulation of liquid–liquid interface at low temperatures between two simple liquids, we determine two point correlation functions, also for the liquid–vacuum interface. True Fourier vector in-plane dependence, correlations parallel and perpendicular to the interface, susceptibilities, and surface scattering factor (in the Born approximation) are discussed. The Bhatia–Thornton transformation is generalized to inhomogeneous systems and applied. Effects of capillary waves, surface reconstruction, surface layering, and prominence of bulk structures are found and discussed.

Scattering of water from the glycerol liquid-vacuum interface

Molecular dynamics calculations of the scattering of D2O from the glycerol surface at different collision energies are reported. The results for the trapping probabilities and energy transfer are in good agreement with experiments. The calculations demonstrate that the strong attractive forces between these two strongly hydrogen bonding molecules have only a minor effect on the initial collision dynamics. The trapping probability is influenced to a significant extent by the repulsive hard sphere-like initial encounter with the corrugated surface and, only at a later stage, by the efficiency of energy flow in the multiple interactions between the water and the surface molecules.

Effects of matrix viscosity on FAB spectra

The influence on fast atom bombardment (FAB) spectra of the viscosity of the matrix was studied. The viscosity was varied in two different ways: (1) by changing the matrix temperature and (2) by adding glucose to the matrix. With both methods the viscosity was varied in the 10 3 to 10 6 centiPoise range. It was found that the spectra degraded severely with increasing viscosity and that the extent of degradation was similar with the two methods. It is concluded that the temperature, per se, has only a small influence on FAB spectra. The very strong dependence on viscosity is explained as a result of changes in the transport processes at the liquid—vacuum interface and of a gradual shift in the desorption mechanism.

A mass spectrometer to measure transepithelial unidirectional labeled water fluxes.

A sealed quartz micropipette forms a liquid-vacuum interface that together with a uranium reduction element and a mass spectrometer functions as an in vitro real-time labeled water-selective microelectrode. Its performance is demonstrated by monitoring unidirectional fluxes across toad urinary bladder. This instrument has direct applicability to studies of water flux across epithelial membranes and could also be useful in monitoring water fluxes across such preparations as isolated tubules or capillaries.

A Scanning Micropipette Molecule Microscope

A movable quartz micropipette, whose tip is sealed with a polymer plug, is used as a liquid-vacuum interface to a mass spectrometer. A light microscope allows observation of, and positioning of, the micropipette tip on the surface of a sample mounted in a perfusion chamber. This forms the basis of an instrument which enables one to study, in vitro, the localization of transepithelial transport of water and other molecules. Some preliminary results from the use of this instrument are presented.

Studies of Liquid Metal Surfaces Using Auger Spectroscopy

ABSTRACTThe surface composition of liquid gallium-tin alloys has been studied in an Auger electron spectrometer (AES) as a function of bulk composition and temperature. The sessile drop samples were cleaned by argon ion bombardment sputtering of the liquid. This technique produced surfaces that were entirely free of impurities within the sensitivity of AES and remained so for many days. Tin was found to be strongly adsorbed at the liquid-vacuum interface. Surface concentrations based on Auger measurements are in reasonably good agreement with values calculated from surface tension measurements interpreted in terms of a monolayer depth distribution model for the adsorbed tin.