Heat capacity of Ar monolayer adsorbed on basal plane graphite

The specific heat of Ar monolayer film adsorbed on basal planes of graphite has been measured for film coverage ranging from 0.31 to 0.97 monolayers for 4.2 < T < 68 K. It is found that a broad specific heat peak centered around 50 K exists over most of the submonolayer coverages until reaching monolayer completion. A comparison with recent neutron scattering and LEED studies relates the anomaly to a possible higher order phase transition between 2D solid and 2D fluid.     

On the orientational alignment of neon adsorbed on basal plane graphite

The Novaco-McTague theory of orientational alignment is discussed for neon adsorbed on basal plane graphite. A degenerate case of the theory which occurs for a √7 superlattice is evaluated and is shown to lead to a novel registry energy.     

Melting of the first monolayer of Ar adsorbed on the basal face of graphite

We have measured adsorption isotherms of Ar on graphite from 67 to 77 K in the range of coverages at which the monolayer freezes. The transition appears continuous and shows some similarity with a λ transition. The large increase of the isothermal compressibility at freezing seems to indicate that it cannot be accounted for by the Kosterlitz-Thouless-Halperin-Nelson-Young theory.     

Fluid-registered solid transition in a monolayer of carbon monoxide adsorbed on the basal face of graphite

Adsorption isotherms of CO on the basal face of graphite have been measured, showing a first order transition within the dense monolayer. There is high a priori probability that this transition is the analogue of the fluid √3 × √3 R 30° transition of the isoelectronic molecule N₂ adsorbed on the same substrate. The comparison between the thermodynamic properties of both transitions, using in particular a corresponding states law scaling, very strongly supports this conjecture.     

Atom-solid scattering: He + graphite basal plane

Theoretical studies of scattering of atoms by solid surfaces and calculations of atomic band structure of adsorbed atoms are presented. Inelastic effects on the intensities of diffracted beams are considered within the framework of the optical model potential. The atom-solid potential used for carrying out the numerical calculation is the sum of Lennard-Jones 12-6 pair potentials for the He-graphite system. Resonances with bound states of adsorbed atoms are shown to enhance the diffraction into some of the open channels and inelastic scattering. The degree of enhancement for a certain process depends on the coupling between that process and the resonant bound state and the coupling between the bound state and the incident beam. For first order bound state resonances, minima in the specular intensity may result from a large increase in some of the diffracted intensities or enhancement of inelastic scattering.     

Adsorption of krypton on the basal plane of graphite: LEED and Auger measurements

The adsorption of krypton on (0001) graphite has been studied by LEED and Auger. Stepwise isotherms are observed and thermodynamic quantities such as the latent heat of two dimensional adsorption and the binding energy of a krypton atom are determined. The mean free path of electrons in krypton is measured. The LEED pattern of the krypton layer shows a √3 × √3 superstructure. Some vibrational properties are examined by LEED and possible implications of the experimental findings are discussed. The potentials and limitations of the techniques used in the present work are critically examined with respect to other techniques.     

Effect of the combined thermal and mass concentration gradients on the stability of a chemically reacting fluid in a porous medium

Summary The onset of instability, due to the combined effects of thermal and mass concentration gradients, is investigated in the hydrodynamic stability regime. It is found that the critical Rayleigh number, which determines the onset of instability, increases as the chemical reaction rate constant increases hyperbolically over a wide range of values at both moderate and high permeabilities. In addition, the instability grows with increase in porosity. Previous results show that the critical Rayleigh number rises linearly when only the mass concentration gradient is considered.     

Effect of structural misalignments on the temperature variation of basal plane electrical conductivity of graphite

The amount and extent of structural misaligments in natural graphite crystals have been determined, and the temperature variation of the basal plane electrical conductivity (σ_⊥) of naturally occurring graphite has also been studied from 300 to 90 K. The conductivity (σ_⊥) has been found to obey a law σ_⊥α($\text{1}\text{T}$) down to a certain temperature θ (θ varying from sample to sample), below which the variation deviates from linearity towards lower values of σ_⊥. This behaviour, which was earlier thought to be a characteristic of graphite and whose origin could not be traced, has been shown to be an effect of the structural misalignments usually present in natural samples of graphite.     

Selective adsorption of 3He and 4He on the basal plane surface of graphite

Atomic beam scattering was used to measure the binding energies of selectively adsorbed states of two isotopes of helium on the basal plane of graphite. The results are used to determine semi-empirical potential parameters, and the ground state energies are compared with values derived from adsorption studies for both isotopes.     

A methane molecule adsorbed on a graphite surface

In this paper we present a molecular dynamics study of a methane molecule adsorbed on a graphite surface. The methane is modelled using five interaction sites and the surface is an infinite hexagonal array of carbon atoms. The potential parameters are fitted using static calculations to the rotational barrier heights and using molecular dynamics to the isoteric enthalpy at zero coverage. The effective pairwise potential predicts the height of the monolayer above the surface and the vibrational frequency against the surface. We have examined the translational and rotational behaviour of a single methane molecule across the surface.     

A neutron investigation of krypton adsorbed on a graphite

Neutron diffraction exhibits a superstructure peak of krypton adsorbed on graphite, which shifts with coverage. Nevertheless variations in the intensities of the substrate peaks indicates registry. A model is given, grounded on polycrystallinity (antiphases) of krypton mono and bilayer still registered on graphite. The registered phase persists in our sample up to the filling of the bilayer contrary to other results. We think of it as a pinned phase.     

NMR study of n-dodecane adsorbed on graphite

In this brief contribution we demonstrate that 1H and 2H NMR spectroscopy can be an effective method of investigating adsorption from liquids at the solid-liquid interface. The method is illustrated here with the adsorption of a simple alkane adsorbed on graphite, in particular the system n-dodecane and graphite at coverages of 1 and 5 monolayers. Static single-pulse proton nuclear magnetic resonance and static quadrupolar echo deuterium nuclear magnetic resonance spectra were recorded for both coverages. The experimental NMR results presented here show features clearly consistent with earlier calorimetric and neutron scattering work and demonstrate the formation of solid adsorbed layers that coexist with the bulk adsorbate with both isotopes. This ability to probe both deuterated and protonated materials simultaneously illustrates that this experimental approach can be readily extended to investigate the adsorption behaviour of multicomponent mixtures.     

X-ray diffraction on chlorine adsorbed on graphite

Cl₂ layers adsorbed on Grafoil have been investigated by X-ray diffraction between 78 and 285 K. Mono- and submonolayers crystallize in a parallelogram lauttce, incommensurate with the (001) graphite surfaces, the molecules oriented almost perpendicular to the surface. The structure is closely related to that of the (001) planes of three-dimensional solid chlorine. For coverages above the monolayer completion bulk crystalization sets in.     

Debye-Waller factor of molecules adsorbed on graphite

The Mössbauer effect in¹¹⁹Sn was employed to study the dynamics of molecules of Sn(CH₃)₄, SnCl₄ and SnI₄ adsorbed on graphite. The mean squared displacement normal to the basal plane was obtained from the temperature dependence of the recoilless fraction. These results were used to estimate the binding energy of the adsorbed molecules.     

X-ray study of molecular oxygen adsorbed on graphite

We have performed a detailed X-ray diffraction study of O₂ adsorbed on UCAR-ZYX and Le Carbon Lorraine vermicular exfoliated graphite between 15 and 50 K. At least four phases of physisorbed oxygen are found. The monolayer δ phase consists of a centered parallelogram lattice, with the molecular axes parallel to the graphite surface. The data are consistent with a triple point at 26 K. The melting transition at a coverage of one monolayer appears to be first order. At higher coverages the molecules undergo a lying-down to standing-up transition; the higher coverage ζ phase froms an approximately triangular lattice with the molecular axes perpendicular to the graphite surface. Satellite peaks around the (1, 0) Bragg peak indicate, however, that this cannot be a simple triangular lattice; possible explanations include successively incommensurate layers or a sinusoidal density modulation. For coverages in the two-layer region the ζ phase modulation peaks disappear at 37 K, and at 40 K the adsorbed oxygen appears to undergo a first order melting transition into a fluid phase. With increasing coverage, the 2D X-ray diffraction profiles and phase boundaries do not connect smoothly onto those of the 3D α and β phases. At low temperatures (T < 30 K) the ζ phase always coexists with bulk crystallites; for temperatures near the 2D melting transition the 3D peaks are not observable. These data, taken together with the heat capacity results, suggest a wetting transition with only the bilayer lamellar phase or bulk O₂ being stable at low temperatures.     

A LEED study of adsorbed neon on graphite

Neon monolayers on graphite have been investigated by high resolution LEED in the range 14.5 < T < 7.5 K and 10^?8 < p < 10^?4 Torr. The fluid-solid transition line has the form ln$\text{(}\text{p}\text{Torr}\text{)= ?}\text{A}\text{T + B}\text{,}\text{with}\text{A = 740 ± 45}\text{K and}\text{B = 31.0 ± 2.8}$. The solid is an incommensurate rotated phase whose lattice parameter decreases and rotation angle increases away from the transition line. Comparison is made with other thermodynamic and diffraction studies, and a preliminary phase diagram is constructed. Extrapolation of these data to higher temperatures and pressure suggests that this rotated solid monolayer is stable up to 23–25 K (P = 3?10 Torr) at coverage x = 1, and is stable over the range 0.88 < x < 1.0 at T = 16 K (2 × 10^?7 < p 1.5 × 10^?2 Torr). Extrapolation to lower temperatures gives the 2D triple point pressure in the range p = (0.3?3) × 10^?10 Torr. The steep slope of the fluid-solid transition line is consistent with the fluid phase having a high density (x?0.80) in the temperature range studied.     

Layer-by-layer growth of neon adsorbed on exfoliated graphite

Neon isotherms adsorbed on exfoliated graphite were measured in the temperature interval of 93.5 to 24 K up to about eleven layers, using the volumetric method. Layer-by-layer growth is clearly observed up to the fourth layer. From there on, the growth mode shows that the saturation vapor pressure is approached asymptotically. Analysis of the data indicates that in the studied temperature interval the adsorbed neon behaves as type 1 film, that is, complete wetting occurs for these temperatures.