Liquid Water Interface Research Articles

Calculation of the cadmium–water interface

This paper presents the results of a very nearly ab initio calculation of the molecular and electronic distributions at the interface of hcp cadmium and liquid water. The calculated results were obtained by combining a molecular dynamics simulation for the H2O molecular distributions with a self-consistent density functional calculation for the valence electron density of the cadmium surface. The method employs a three-dimensionally periodic unit cell, uses standard molecular dynamics potentials for H2O–H2O interactions, and calculates a self-consistent electronic structure for the metal at each time step of the molecular dynamics simulation, with the electron–H2O and electron–Cd interactions represented by pseudopotentials. The electronic and H2O distributions of the Cd–water interface are obtained both for zero applied potential, and for a range of electric potentials applied across the interface. Results are given for the equilibrium structure of interface, and its response to applied potentials. An analysis of the electrostatics of the interface yields a prediction for the compact capacitance of the cadmium–electrolyte interface, which is compared to experiment.

N-Alkyl alcohols at the water/vapour and water/benzene interfaces: a study on phase transfer energies

A preliminary study on the behaviour of molecular solutes at the interface of liquid water with other media is reported. The study is based on an extension of the polarizable continuum model (PCM) which gives solvation free energy as the difference between the quantum mechanically computed free energy of the molecule inserted in the medium and that of the isolated molecule. The PCM computational scheme is modified to take into account the position and orientation of the molecule with respect to the phase separation boundary. The energetic quantity object of this study is thus a position and orientation dependent free energy function. The other features of standard PCM, in particular the use of continuum solvent response functions, are maintained. The comparison with molecular dynamics simulations shows that this model gives reasonable results, also in the presence of a phase separation. The decomposition of the free energy values along the best energetic passage from one phase to another shows that the minimum and the maximum present in the free energy profile mainly depend on non-electrostatic dispersion and steric terms.

Study of the pseudo‐steady‐state kinetics of CO2 hydrate formation and stability

AbstractThis article describes a dynamic model for formation and stability of CO2‐hydrate on the interface of liquid CO2(LCO2) and ocean water at large depths. Experimental results indicate that a thin film of hydrate naturally forms on the interfaces between LCO2 and water, and inhibits diffusion between the two phases. Experiments further shows that the flux of CO2 through the hydrate film is dependent of the CO2‐concentration in the ambient sea water. The model proposed here explains these phenomena by introducing four major mechanisms; diffusion of water to the LCO2‐phase, formation of hydrate in the LCO2‐hydrate interface, decay of hydrate in the water‐hydrate interface, and diffusion of CO2 through the water phase. The model explains the CO2 flux not by diffusion through the hydrate film, but suggest a mechanism of continuous hydrate formation and decay. The overall effect is a “moving,” pseudo‐steady‐state hydrate film due to transport of CO2 through the film. The film velocity is dependent of liquid‐liquid diffusivity parameters and reaction constant, and lacking experimental values of these parameters, an order–of‐magnitude analysis is done by fitting the model to experimentally obtained data for the overall film velocity. The motivation for this work is to elucidate options for CO2 depositions in deep oceans, of which liquid CO2 sequestration is believed to be one of the most feasible. Spreading of CO2 from a liquid CO2‐lake and associated lowering of pH in the ecosystem surrounding the lake is of large concern. The work presented here concludes that diffusion of CO2 in the ocean is largely reduced by the hydrate film and suggests that hydrate formation may alleviate some of the environmental concerns regarding deep ocean sequestration of liquid CO2. © 1994 John Wiley & Sons, Inc.

Surface diffusion of silver at the silver (111)/liquid-water interface from electrocrystallization measurements

The authors observed rhythmic Ag cluster nucleation that was coupled to surface mass transport, and we used this phenomenon to measure D[sub s] for silver adatoms. Low coverages ([theta] [approx] 0.002) of Ag(ad) were generated on a surface that was initially adatom-free by reduction of Ag[sup +](aq) at sparse, redox-active sites. The adatoms diffused from the generation sites across step-free Ag(111) surfaces of known geometry. Diffusing adatoms were trapped at redox-silent sites that were potent catalysts for the nucleation of 2D clusters. The first supercritical cluster to form at a nucleation site swept rapidly over the Ag(111) surface by catalytic reduction of Ag[sup +](aq) at the reactive step edge. The expansion of the monatomic step from the nucleation site yielded a brief charge pulse, a fresh, adatom-free surface, and renewal of the cycle. Regular trains of charge pulses were observed in steady-state experiments. Analysis yielded the locations of the nucleation sites and the following values of D[sub s]: (1.4 [+-] 0.2) [times] 10[sup [minus]6] cm[sup 2]s at 20.0[degrees]C, (3.3 [+-] 1.0) [times] 10[sup [minus]6] cm[sup 2]/s at 35.0[degrees]C, and (4.0 [+-] 0.9) [times] 10[sup [minus]6] cm[sup 2]/s at 50.0[degrees]C ([+-]2[sigma]). These are the same as values predicted by molecular dynamicsmore » simulation for Ag(ad) diffusion on Ag(111) in ultrahigh vacuum, implying that there is little net effect of the interfacial water on Ag(ad) diffusion rates. The mean residence time for Ag(ad) species at the Ag(111)/water interface is > 4 s at these temperatures, corresponding to a standard exchange rate I[degrees] < 40 nA/cm[sup 2] between Ag(ad) and Ag[sup +](aq). In extreme contrast, the authors measure I[degrees] [approx] 1,300 A/cm[sup 2], more than 10 orders of magnitude larger, for the same exchange reaction at the edge of an atomic step on Ag(111). 96 refs., 8 figs.« less

Ice cold energy storage using direct contact phase change heat transfer between refrigerant and PCM in an enclosure.

A cold energy storage method having excellent heat transfer characteristics was proposed, utilizing natural circulation of a refrigerant. Three layers, i. e., a refrigerant vapor(R 12 or R 114) layer, a water layer and a refrigerant liquid layer stratified in this order from the top were kept in an enclosure composed of a condenser, an evaporator and a condensate receiver-and-return tube. Direct contact heat transfers of a refrigerant with phase change were applied both for the cold energy storage and release processes. In the storage process, the refrigerant liquid vaporizes at the refrigerant liquid-water interface with the formation of ice, the vapor condenses at the condensing surface and the condensate returns to the refrigerant liquid layer simultaneously. In the release process, the refrigerant liquid vaporizes at the vaporizing surface, the vapor condenses on the ice with melting, and the condensate and melted water returns to the original layers simultaneously. It was confirmed by the measurements that rapid ice formation and melting were realized.

Molecular orientation and surface potential at the liquid water interface

Preliminary results are reported on the molecular orientation and surface potential of liquid water at 4°C.