Gas-like State Research Articles

Stable interface dynamics for copper electrodeposition in the presence of organic additives: Evidence for a gas-like surface state of the organic adsorbate

The interface dynamics of metal electrodeposits in the presence of organic additives is followed in two-dimensional glass cells. Stable Cu interfaces are formed at intermediate thiourea concentrations when a critical, but small surface coverage of thiourea (\ensuremath{\theta}) is reached. For \ensuremath{\theta} values corresponding to a close-packed adsorbate, the interface again becomes unstable because Cu growth takes place at defective sites in the adsorbed layer. The ability of additive molecules to suppress unstable growth is related to the formation of a diluted two-dimensional gas-like surface state preferentially adsorbed at protrusions. Protrusions are more efficient for capturing the additive than valleys, as additive molecules arrive at the growing deposit under diffusion control from the solution.

Intermingled basins due to finite accuracy

We investigate numerically first a chaotic map interrupted by two small neighborhoods, each containing an attracting point, and secondly a periodically tilted box within which disorderly colliding disks can reach different attracting configurations, due to dissipation. For finite, arbitrarily small accuracy, both systems have basins of attraction that are indistinguishable from intermingled basins: any neighborhood of a point in phase space leading to one attractor contains points leading to the other attractor. A bifurcation destabilizing the fixed points or the disk configurations causes on-off intermittency; the disks then alternate between a "frozen" and a gaslike state.

Parametric instability of dust lattice waves in a turbulent plasma sheath

It is shown that dust lattice waves (DLWs) become parametrically unstable in a turbulent dusty plasma sheath that contains random-phase ion plasma waves. Physically, the parametric instability arises because of the nonlinear coupling between ion plasma waves and DLWs. The parametric instability is identified as the possible reason for the observed "sublimation" transition of the plasma crystal from a solid to a gaslike state.

Dust–dust and dust-plasma interactions of monolayer plasma crystals

The horizontal and vertical oscillation modes of monolayer plasma crystal are investigated and are related to dust–dust and dust-plasma interactions. It is found that the dust particles in the plasma sheath excited by a biased wire show parametric resonances. This parametric resonance is identified as the reason for the observed “sublimation” transition of the plasma crystal from a solid to a gaslike state.

Molecular mechanism of capillary condensation of acetonitrile vapor on MCM-41 with the aid of a time-correlation function analysis of IR spectroscopy

The adsorption isotherm and IR spectra of acetonitrile adsorbed on MCM-41 (pore-width=3.2 nm) were measured at 303 K. The adsorption isotherm had a sharp jump at P/ P 0=0.3 without adsorption hysteresis. The CN stretching ν 2-band of adsorbed acetonitrile had two-component α- and β-bands at 2263 and 2254 cm −1, respectively, assigned to hydrogen-bonded molecules on surface hydroxyls of MCM-41 and physisorbed molecules in mesopores whose walls are coated with hydrogen-bonded molecules, respectively. The β-band was analyzed with a time correlation function, giving a reorientation time τ. The τ value of the adsorbed molecule before capillary condensation was smaller than that of the bulk liquid, indicating a gas-like state. After capillary condensation, τ was the same as for the bulk liquid.

Experimental investigation of the melting transition of the plasma crystal.

Measurements of the melting transition of a Coulomb crystal consisting of dust particles immersed in an rf parallel plate discharge in helium were performed. The dust crystal is shown to be solid at higher gas pressure (120 Pa) and low discharge power (10--20 W). Reducing the gas pressure or increasing the discharge power leads to fluid states of the dust ensemble. Even gaslike states are observed at low pressures of about 40 Pa. The transition is attributed to an increasing effective particle temperature. The phase transition is compared with one-component-plasma and Yukawa models, and with basic predictions of theories for two-dimensional melting. \textcopyright{} 1996 The American Physical Society.

Mechanics of collisional motion of granular materials. Part 2. Wave propagation through vibrofluidized granular layers

According to numerous experimental observations and theoretical models vibrated layers composed of large granules behave like a solid plastic body. In contrast, in this study experimental data are presented that reveal that, for constant vibration amplitudes A ≥ 1 cm with the frequency ω increasing from zero, all layers pass through three vibrational states, with the respective behaviours being as of (i) a solid plastic body, (ii) a liquid, (iii) a gas. In the liquid-like vibrational state transverse waves propagating along the layer width were observed. These waves were shown to be gravitational resonance waves, with the corresponding frequencies well correlated by the known formula for incompressible liquids. In the gas-like vibrational state compression (shock) and expansion waves propagating across the layer height, were observed.A theoretical model for time-periodic collisional vibrational regimes was developed on the basis of the Euler-like equations of a granular gas composed of inelastic spheres. The model shows that the vibrational granular state (bed porosity, shock wave speed, granular pressure and kinetic energy) is inter alia governed by the dimensionless parameter V = (Aω)/(hmg)1/2, with g, hm being the gravitational acceleration and the height of the resting layer, respectively. This is in contrast with the previous studies, where the behaviour of vibrated granular layers was interpreted in terms of the dimensionless acceleration Δ = (Aω2)/g. The proposed model was tested by processing the data obtained from photographs of the particle distribution within vibrated layers. Theoretical predictions of the particle average concentration compared favourably with the experimental data.Other phenomena observed in vibrated granular layers include the formation of caverns, circulatory motion of granules and synchronized periodic motion of two adjacent vibrated layers of different widths. The importance of the observed phenomena in relation to various technological processes involving bulk materials (vibromixing, vibroseparation, etc.) is discussed.

Entry of gas molecules into liquids

Heterogeneous gas–liquid interactions play a fundamental role in many atmospheric processes. Pivotal for the understanding of such processes is the rate of gas uptake by the relevant liquid. Accurate experimental techniques have been developed to study uptake of gases by liquids. Gas–liquid interactions have already been investigated for more than thirty gas-phase species encompassing two classes of molecules: (1) those that do not react in the liquid but are readily solvated by it and (2) those that have low solubility in the liquid but react in the liquid rapidly to form highly soluble species. Uptake studies of non-reactive gas molecules in water have been explained by a model which treats the water surface as a narrow region of a dense gas-like state within which trace gas–solvent collisions occur. Gas accommodation occurs via nucleation of solvent molecules. Uptake studies for reactive gas molecules indicate that, for some species, reaction rates are much more rapid at the liquid interface than in the bulk liquid. The results of non-reactive and reactive uptake experiments are brought together here. The uptake model is briefly described, and the implication of the model for the kinetic nature of liquid surfaces is examined. Reaction-driven uptake studies which provide clear evidence for reactions at the gas/liquid interface are presented. The nature of interfacial reactions and their connection to the uptake model are examined.

Grain formation, stringlike collective motion, and the onset of diffusion in disordered two-dimensional crystals.

Two-dimensional crystals in random potentials are studied by molecular-dynamics methods. The ground state of the system is characterized by a transition of the random-potential-induced defect configurations, which evolve from a dilute gaslike state at weak random potentials to a grain-boundary state at strong random potentials. The dynamics of the system is strongly affected by the defect structure. Stringlike collective hopping of particles along the grain boundaries greatly assists the onset of diffusion in the system at low temperatures.

Melting of colloidal crystals: A Monte Carlo study

Electrostatically stabilized colloidal crystals show phase transitions into liquid and gaslike states as the ionic impurity concentration increases. Using Monte Carlo simulations we theoretically investigate the melting of four colloidal crystals (two fcc crystals and two bcc crystals) which have also been examined experimentally. We calculate the pair correlation function g(r), the total potential energy Ut, and the mean square displacement of a particle 〈u2〉 for the colloidal suspensions at various ionic impurity concentrations ni. We calculate the structure factor S(Q) by Fourier transforming g(r). We find that the parameters gmax [the maximum of the first peak in g(r)], Smax [the maximum of the first peak in S(Q)], Δr [the half width at half maximum of the first peak in g(r)], and Ut (the total potential energy) all show discontinuous behavior on melting. We relate the calculated values of Smax, gmax, and the mean square displacement at the point of melting of our colloidal crystals to that of atomic crystals. We find that the ratio of the rate of change of the Wendt–Abraham parameter, gmin/gmax [gmin : the minimum value of g(r) after the first peak], with respect to ni, in colloidal crystal to that in liquid is constant but specific to the crystal structure (bcc or fcc). We calculate the latent heat of melting of colloidal crystals.

Mechanism of formation of FAB spectra

Recent experimental and interpretative work by the authors on the mechanism of formation of FAB spectra is reviewed and a physical picture of the desorption ionization event is presented. The collision cascade causes extensive ionization in the matrix and induces a transition from the liquid to a gas-like state. The extensive ion/molecule reactions that occur in this “gas” are governed by gas phase and not liquid phase energetics. As expected for a gas, extensive positive ion/negative ion recombination takes place, and only a small fraction of the ions survives into the vacuum. FAB spectra of diethanolamine (DEA) solutions of various concentrations in glycerol are successfully reproduced with a kinetic model based on high pressure ion/molecule reactions. There is a wide distribution of “residence times” for the ions in the “gas”, and ions with longer residence times have lower internal excitation energies. A model for the desorption process, the Phase Explosion model, that uses classical thermodynamics as a guide to the desorption process offers explanations for most of the experimental observations.

Shear-induced structure in a colloidal suspension: II. Light scattering

We study, by light diffusion, the deformation of the pair distribution function of a colloidal suspension in a cylindrical Couette flow. The suspension is made of monodisperse latex particles ( d 0 = 750 A ̊ ) at a volume fraction Φ = 1.3%. At equilibrium, the suspension has a liquid-like structure. The structure factor S(k) is recorded for different orientations of the diffusion wave vector in the plane of shear for Peclet numbers ( Pe = αa 2 D 0 ) between 10 −2 and 10 −1. In order to get rid of different spurious causes of scattering the spectra are normalized by the same suspension in a gas-like state, obtained by the addition of salt. The structure is shown to deform asymmetrically when we increase the shear rate: it increases slowly with a small linear shift of the maximum on the compression side whereas the intensity falls rapidly on the dilatation side. These experimental results are discussed with the help of numerical simulations on a similar system.

Collapse transition of the isolated polymer chain

Abstract A mean field theory of chain dimensions is formulated which is very similar to the van der Waals theory of a simple fluid. In the limit of infinite chain length, the chain undergoes a second-order phase transition. At low temperatures, the chain is in a condensed or globular state, and the mean square gyration radius varies as m2/3 where m is the molecular weight. At high temperatures, the chain is in a gaslike or coil state where varies as m6/5. In the globular state, fluctuations in are very small, whereas they are very large in the coiled state. A characteristic feature of the theory is that ternary and higher order intramolecular interactions are approximated. At high temperatures, only binary interactions are important, but at low temperatures, many of the higher order terms contribute. An important conclusion of this study is that a polymer chain does not obey ideal chain statistics at Θ temperature. Although the second virial coefficient vanishes at Θ, the third virial coeff...

Simulation of a lipid monolayer using molecular dynamics

Numerical simulation is often a useful tool f or investigating the behaviour of complex systems with many degrees of freedom. Of the two major methods in this field, the Monte Carlo method and the molecular dynamics method, only the first has been applied to realistic models of lipid monolayers1–5. The term lipid monolayer is used here to describe a class of systems consisting of chain molecules on a liquid substrate, the characteristic properties of which can be summarized as follows. (1) The constituent molecules are amphipathic, that is they consist of a hydrophilic (polar) head group and one or more hydrophobic hydrocarbon chains. (2) Due to the amphipathic character of the molecules, the head groups are constrained to the plane of the substrate, whereas the tails are directed outwards from this plane. (3) The collective properties of the molecules are determined by their short-range repulsive and long-range attractive interactions and by the steric repulsion of the tails. We now present what we believe to be the first molecular dynamics simulation of a realistic model of a lipid monolayer. The model system, which has all three properties enumerated above, shows a first order phase transition from an ordered fluid-like state to a disordered, gas-like state.

Is there an isomorphous phase transition in cubic nickel iodine boracite at 120K?

It is suggested that there is a phase transition in nickel iodine boracite at approximately 120K, which is associated with no change of symmetry-an isomorphous transition. The cause of this transition is a condensation of OI atoms from a gas-like state to a four-state disorder.

Condensation of a chain polymer molecule

A new expression for the expansion coefficient of a flexible chain polymer molecule can be given in such a way that its first term corresponds to the case with only a hard-sphere repulsion and the other terms represent the effects of attraction between the segments. For low and high temperatures, more convenient expressions are used to find a possible condensation into a liquid-like condensed state from a gas-like state. It is found that in the condensed state, the square average end-to-end distance varies as N ϵ, where N is the number of segments and the exponent ϵ is around 0.6 in contrast to 1.2 for the high temperature case.

Diffusion and flow of gases and vapours through micropores III. Surface diffusion coefficients and activation energies

When less than a complete monolayer is present, surface-diffusion coefficients decrease rapidly with decreasing coverage. This is attributed to variation in heats of adsorption over the adsorbent surface. Diffusion is due to the most loosely bound molecules, and these tend to disappear first as coverage decreases. Only a limited range of coverage could be followed in a given system and, within this, activa­tion energies showed no marked variation. This should follow if molecules which require high activation energies play relatively little part in surface diffusion. In no system so far studied have activation energies been small enough to support the existence of ‘freely mobile’ or ‘gas-like’ adsorbed layers. A gas-like state is not approached at low coverages, since molecules adsorbed under these conditions are practically immobile. Surface diffusion coefficients compared at constant temperature and constant coverage differ for various molecules according to differences in molecular size, shape and polar character. When the monolayer is complete and multilayers are formed, diffusion coefficients are little dependent on coverage, and, though perhaps higher than for diffusion in the liquid state, are of a similar order of magnitude. Capillary condensation, in at least the initial stages, produces a rapid rise in surface-diffusion coefficients. Measurements in this region have only limited reproducibility. The importance of pore structure on the shape of adsorption isotherms is stressed. Its effect on surface-diffusion coefficients is indicated, but has not been followed in detail.