Structure Of Liquid Surface Research Articles

Interfacial carbonyl groups of propylene carbonate facilitate the reversible binding of nitrogen dioxide.

The interaction of NO2 with organic interfaces is critical in the development of NO2 sensing and trapping technologies, and equally so to the atmospheric processing of marine and continental aerosol. Recent studies point to the importance of surface oxygen groups in these systems, however the role of specific functional groups on the microscopic level has yet to be fully established. In the present study, we aim to provide fundamental information on the interaction and potential binding of NO2 at atmospherically relevant organic interfaces that may also help inform innovation in NO2 sensing and trapping development. We then present an investigation into the structural changes induced by NO2 at the surface of propylene carbonate (PC), an environmentally relevant carbonate ester. Surface-sensitive vibrational spectra of the PC liquid surface are acquired before, during, and after exposure to NO2 using infrared reflection-absorption spectroscopy (IRRAS). Analysis of vibrational changes at the liquid surface reveal that NO2 preferentially interacts with the carbonyl of PC at the interface, forming a distribution of binding symmetries. At low ppm levels, NO2 saturates the PC surface within 10 minutes and the perturbations to the surface are constant over time during the flow of NO2. Upon removal of NO2 flow, and under atmospheric pressures, these interactions are reversible, and the liquid surface structure of PC recovers completely within 30 min.

Thermodynamics and structure of liquid surfaces investigated directly with surface analytical tools**

Measuring directly the composition, the distribution of constituents as function of the depth and the orientation of molecules at liquid surfaces is essential for determining physicochemical properties of liquid surfaces. While the experimental tools that have been developed for analyzing solid surfaces can in principal be applied to liquid surfaces, it turned out that they had to be adjusted to the particular challenges imposed by liquid samples, e.g. by the unavoidable vapor pressure and by the mobility of the constituting atoms/molecules. In the present work it is shown, how electron spectroscopy and ion scattering spectroscopy have been used for analyzing liquid surfaces. The emphasis of this review is on using the structural information gained for determining the physicochemical properties of liquid surfaces.

Influence of density ratio on the secondary atomization of liquid droplets under highly unstable conditions

In this paper, the influence of density ratio on the atomization of liquid droplets under highly unstable conditions is numerically investigated with a coupled volume-of-fluid and level-set method. By employing the adaptive mesh refinement technique, the computational cost is significantly reduced and good agreements with experimental measurements on both the morphology and trajectory of droplets are obtained. Specifically, at We=13, the present model correctly predicts bag breakup, which is consistent with earlier studies. Based on the present model, the effects of density ratio on the deformation and fragmentation of liquid droplets are investigated from the aspects of the time-resolved evolutions of gaseous Weber number, drag coefficient, liquid surface structure, as well as the morphology of liquid droplets, with special attention drawn to gain more in-depth insight into the dynamics of fragmentation under the highly unstable conditions (initial gaseous Weber number Weg,i=225, initial gaseous Reynolds number Reg,i=10062.5). The results indicate that the drag coefficient is determined by the recirculation region and therefore largely affected by the density ratio. A lower density ratio may lead to a higher deformation rate, while a higher density ratio results in more intensive fragmentation. Furthermore, the density ratio has a significant effect on the deformation and fragmentation in the investigated conditions when the density ratio exceeds 32 at a small Ohnesorge number.

Intrinsic profiles and the structure of liquid surfaces

We present a brief review of the advances made in the characterization of liquid surfaces over the last decade. We focus particularly on the links between the capillary wave theory, the density functional formalism and the direct evaluation of the intrinsic density profiles from computer simulations. A new perspective of the liquid surfaces is appearing, with a sharper view of their molecular structure, which opens new challenges for theoretical and experimental studies. Novel results on the intrinsic interfacial structure of molten salt liquid–vapor interfaces are presented.

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

We have recently measured core level and valence band XPS, UPS, and MIES spectra of two room temperature ionic liquids composed of bis(trifluoromethylsulfonyl)imide anions ([Tf(2)N](-)) and either 1-ethyl-3-methyl-imidazolium ([EMIm](+)) or 1-octyl-3-methyl-imidazolium cations ([OMIm](+)). [T. Ikari, A. Keppler, M. Reinmöller, W. J. D. Beenken, S. Krischok, M. Marschewski, W. Maus-Friedrichs, O. Höfft and F. Endres, e-J. Surf. Sci. Nanotechnol., 2010, 8, 241.] In the present work we analyze these spectra by means of partial density of states (pDOS) as calculated from a single ion pair of the respective ionic liquid using density functional theory (DFT). Subsequently we reconstruct the XPS and UPS spectra by considering photoemission cross sections and analyze the MIES spectra by pDOS, which provides us decisive hints to the ionic liquid surface structure.

Concentration Depth Profiles of Inorganic Ions at Liquid Surfaces

For a decade, it has been discussed whether or not inorganic ions like iodide or bromide adsorb preferentially at liquid surfaces. Thermodynamic relationships cannot be used to derive detailed information about the molecular structure of surfaces. Thus, it is import to investigate the structure of liquid surfaces directly. Neutral Impact Collision Ion Scattering Spectroscopy is one of the very few techniques that can be used to determine directly the molecular structure of liquid surfaces.

Comparative study of the surface layer density of liquid surfaces

Capillary wave fluctuations blur the inherent structure of liquid surfaces in computer simulations. The intrinsic sampling method subtracts capillary wave fluctuations and yields the intrinsic surface structure, leading to a generic picture of the liquid surface. The most relevant magnitude of the method is the surface layer density ${n}_{\text{s}}$ that may be consistently determined from different properties: the layering structure of the intrinsic density profiles, the turnover rate for surface layer particles, and the hydrodynamic damping rate of capillary waves. The good agreement among these procedures provides evidence for the physical consistency of the surface layering hypothesis, as an inherent physical property of the liquid surfaces. The dependence of the surface compactness, roughness, and exchange rate with temperature is analyzed for several molecular interaction models.

The intrinsic structure of the water surface

An operational procedure to obtain the intrinsic structure of liquid surfaces is applied here to a molecular dynamics simulation of water, with a model of point charges for the molecular interactions. The method, which had been recently proposed and used for simple fluids, is successfully extended to a molecular liquid with the complex bond structure of water. The elimination of the capillary wave fluctuations, in the intrinsic density and orientation profiles, gives a new overall view of the water surface, at the sharpest molecular level, and without the size-dependent broadening observed in the mean profiles. The molecules belonging to the outer liquid layer are clearly identified, and we find that only these molecules exhibit a clear preferential orientation to lie flat on the surface. Moreover, there is a strong correlation between the dipolar structure and the local curvatures of the intrinsic surface, so that at the extrusions of the intrinsic surface the molecular dipoles point preferentially toward the vapor side of the interface. Finally, we have found an intrinsic density layering structure, although the inner structure is strongly damped beyond the second layer.

Determination of a solute electron energy spectrum not accessible experimentally by means of Singular Value Decomposition

Due to its extreme surface sensitivity Metastable Induced Electron Spectroscopy (MIES) can be applied to study the structure of liquid surfaces. In the case of surface active solutes like tetrabutylammonium iodide (TBAI) in formamide (FA) and hydroxypropionitrile (HPN) the segregation process can be followed. The electron energy spectrum of the pure solute can not be measured directly, the salt has no liquid phase accessible by the experiment. Quantitative analysis is therefore based on the changes of the pure solvent spectra by added salt. The salt spectrum given as Ssalt=Smixture−a×Ssolvent often contains statistical uncertainties. Therefore, a different approach is described to gain the electron spectrum of the pure TBAI: the Singular Value Decomposition (SVD) treats each spectrum as an element of a vector space spanned by a certain number of reference data. For the mixtures of TBAI+FA and TABI+HPN we assume the dimension of that vector space to equal 3. Two references can be identified with the pure solvent spectra, the third is attributed to the pure salt TBAI. It is outlined how the solute spectrum not accessible for the experiment is evaluated numerically, restrictions and needed assumptions are discussed.

Reduction in the surface energy of liquid interfaces at short length scales

Liquid-vapour interfaces, particularly those involving water, are common in both natural and artificial environments. They were first described as regions of continuous variation of density, caused by density fluctuations within the bulk phases. In contrast, the more recent capillary-wave models assumes a step-like local density profile across the liquid-vapour interface, whose width is the result of the propagation of thermally excited capillary waves. The model has been validated for length scales of tenths of micrometres and larger, but the structure of liquid surfaces on submicrometre length scales--where the capillary theory is expected to break down--remains poorly understood. Here we report grazing-incidence X-ray scattering experiments that allow for a complete determination of the free surface structure and surface energy for water and a range of organic liquids. We observe a large decrease of up to 75% in the surface energy of submicrometre waves that cannot be explained by capillary theory, but is in accord with the effects arising from the non-locality of attractive intermolecule interactions as predicted by a recent density functional theory. Our data, and the results of comparable measurements on liquid solutions, metallic alloys, surfactants, lipids and wetting films should thus provide a stringent test for any new theories that attempt to describe the structure of liquid interfaces with nanometre-scale resolution.

Molecular dynamics of fluid phase change

We have applied molecular dynamics simulation for various fluid systems to investigate microscopic mechanisms of phase change. The paper reviews the works recently done in our group. (1) Evaporation–condensation dynamics of pure fluids under equilibrium condition: Besides the difference of condensation coefficient among species, its strong temperature dependence is found, which can be explained from the viewpoint of the structure of liquid surface and the molecular exchange. (2) Evaporation–condensation dynamics of pure fluids under non-equilibrium condition: The condensation behavior seems strongly dependent on the temperature and the density of vapor. This kind of molecular simulation will be useful for longstanding questions such as how fast hot vapor condenses on cool liquid surface, or what the evaporation coefficient means. (3) Gas absorption dynamics: CO 2 gas absorption mechanism on water surface is similarly analyzed, from the view point of adsorption–desorption dynamics.

X-ray beam sheet deflection for liquid surface spectrometry

An x-ray beam incident on a liquid surface at glancing angles may be specularly reflected (XR), diffracted (GID), or yield fluorescent radiation (GIF). The three mechanisms, encompassing liquid surface x-ray spectrometry, give complementary information about the liquid surface structure. In common is the incident monochromatic beam: It has the shape of a downward sloping thin sheet with the sheet normal and the propagation direction ideally spanning a vertical plane. Methods to produce such a beam from a horizontal, polychromatic synchrotron radiation beam are discussed. For a high brilliance undulator source the simplest solution turns out to be Bragg reflection from a single tilted monochromator crystal in Laue geometry.

Viscoelastic surface waves and the surface structure of liquids

The dispersion relations for surface waves on a planar liquid surface are obtained from first principles. The high-frequency elastic, low-frequency hydrodynamic and intermediate viscoelastic regimes are investigated separately. It is shown that the structure of the liquid surface modifies the expression for Rayleigh's elastic surface waves and Kelvin's capillary waves and allows for the existence of new surface waves besides those of the phenomenological theories. For example, an hydrodynamic counterpart to the Rayleigh waves and an elastic counterpart to the capillary waves are found. The experimental observation of these surface waves could in principle yield direct access to the structure of the liquid surface.

Elastic surface waves and the structure of liquid surfaces

The high-frequency elastic behavior of fluid surfaces is studied from first principles. The general dispersion equation for elastic surface waves thus obtained reveals the influence of the surface structure on Rayleigh's surface waves. New types of surface waves are also found. Their existence depends crucially on the presence of nonvanishing surface excess elastic moduli and hence these waves are inaccessible to the macroscopic theory of elasticity. The experimental observation of these elastic surface waves could provide new information about the structure of the liquid surface.

Density-functional theory of the surface tension of simple liquid metals

A general density-functional formalism for an inhomogeneous liquid metal is described. The free energy is expressed in terms of two densities: electronic and ionic. In the limit of small spatial density variations of arbitrary wave number, the electron density can be eliminated exactly and the free energy expressed as a functional of the ionic density alone, although the electronic degrees of freedom are still implicitly present. This formalism is applied within the gradient approximation to calculate the surface tension and surface widths of a number of simple liquid metals. The free-energy density is evaluated within the structural expansion in conjunction with a hard-sphere variational approximation to treat the liquid structure, and the gradient coefficient is calculated using a mean-spherical-like approximation to estimate the direct correlation function. This first-principles calculation thus has as input only a parameter describing the pseudopotential and the bulk liquid density. Results for the surface tension of the alkali metals are in excellent agreement with experiment. The temperature derivative of the surface tension is calculated to within a factor of 3 of experiment and probably within experimental error bars. Results for Al and Zn lead to a surface tension which is considerably larger than experiment. Surface widths are in all instances computed to be quite narrow but in reasonable agreement with available experiment. It is argued that discrepancies for the polyvalent metals arise not from the theory itself but rather from difficulty in calculating parameters of the theory from first principles. To verify this, a simple scaling form for surface tension is proposed, motivated by the density-functional theory but in which the relevant parameters are estimated in terms of the liquid density and melting temperature. Agreement with experiment for the scaling expression is at least as good as existing empirical expressions. Finally, a formalism is described which permits, in principle, calculation of both the electronic and ionic singlet density near the liquid surface. Both one-body and two-body forces are obtained, although no numerical evaluations of these are presented. The possible relevance of these to liquid surface structure is briefly discussed.

Structure of Liquid Surface and Mechanics of Surface Tension

The structure of the surface of a liquid is described by introducing a super-liquid covering the whole surface. Using this condition together with the conditions that the Gibbs-Helmholtz equation applies and that the entropy sum is constant, the thicknesses of the layer of super-liquid and the tension layer have been calculated. In the case of water, the thickness of the layer of super-water is 660 Å, while the thickness of the tension layer is monoatomic in all materials. The cause of surface tension is an electromagnetic force exerted by the super-liquid on the interfacial normal liquid; high-frequency attractive and repulsive forces make the interfacial atoms approach each other, forming a two-dimensional ordered structure. The restoring force of the constituent particles in the tension layer produces the surface tension.

Simple liquid surface structure and surface tension

Simple liquid surface structure and surface tension

Ion hydration near air/water interfaces and the structure of liquid surfaces

Ion hydration near air/water interfaces and the structure of liquid surfaces

Ion hydration near air/water interfaces and the structure of liquid surfaces

Negative adsorption of ions, commonly observed at air-water interfaces, is examined in terms of models of restricted polarization of the solvent by ions at the interface and the structure of the liquid interface. The Born and other models of ionic hydration are applied to evaluate the self-energy of the ion arising in the region of solvent near its interface and in the vacuum or vapour beyond. The adsorption energy of an ion varies substantially with distance from the liquid interface so that a distribution of ions arises as a function of distance from the interface. Integration of this distribution gives an expression, and results, for the ionic surface excess. The diffuse-layer potential, which an unequal distribution of cations and anions give rise to, gives a contribution to the surface potential of the electrolyte solution at finite concentrations. Structural aspects of the liquid interface at which ions are negatively adsorbed are discussed in terms of Stefan's ratio and the superficial excess entropies of various liquid surfaces. These entropies are related to the cohesive energy densities of the bulk liquids. Ion solvent-structure co-sphere interactions with structured interfaces will lead to specificity of negative adsorption of ions.

Evaporation and surface structure of liquids

The condensation coefficient (c. c.) of a liquid is defined as the ratio of the observed rate of evaporation under given conditions to the ‘expected’ rate calculated from the known saturation vapour pressure, for the same conditions. Values of the c. c. less than 1 have been reported in the past, but the accuracy of the experiments has been criticized on various grounds. Experimental work is described which establishes that the c. c. of glycerol at 18°C is 0⋅052 ± 0⋅005. Previous experimental determinations are reviewed in the light of this result, and it is found that for various liquids the c. c. is nearly equal to the free-angle ratio defined by Kincaid and Eyring on the basis of a simple model of liquid structure. This equality is not found for chloroform. The theory of evaporation is discussed on a model of the liquid surface consisting of a compact ordered layer with a mobile adsorbed layer of molecules above it. On the assumption that evaporating and condensing molecules pass through the adsorbed condition, it is shown that the c. c. is determined by the rates of exchange between substrate and adsorbed layer, and adsorbed layer and vapour, respectively. In particular, a c. c. near unity results if the rate of exchange between substrate and mobile layer is very much greater than that between mobile layer and vapour. The above assumption does not give a satisfactory explanation of the equality of c. c. and free-angle ratio for alcohol molecules. It is shown that for polar molecules which form an oriented surface layer in the liquid state it is to be expected that evaporation will take place mainly by direct exchange of molecules between the ordered layer and the vapour, although an adsorbed layer will be present. In this case an argument given by Herzfeld shows the condition for equality of c. c. and free-angle ratio to be that there should be no exchange of energy between rotational and translational co-ordinates in the process of evaporation. It is further concluded from a rather qualitative discussion that a major contribution to c. c. and free-angle ratio for the simple alcohols arises from restriction of rotation of the hydroxyl group about the carbon-oxygen bond.