2014 Le coefficient de diffusion d'une couche de CH4 adsorbée sur la face (0001) du graphite est mesuré par diffusion quasi élastique de neutrons, en fonction du degré de recouvrement et de la temperature. Les données expérimentales permettent de différencier nettement deux types de fluides adsorbés : un liquide bidimensionnel coexistant avec un gaz et un fluide hypercritique dont la densité varie continuement avec le degré de recouvrement. La mobilité disparaît à la solidification qui peut être obtenue par diminution de la température à degré de recouvrement constant ou par augmentation de la concentration à constante température. La coexistence du solide et du liquide bidimensionnels a aussi été observée. Enfin, tous ces résultats sont utilisés pour tracer un diagramme de phase de la première couche de méthane adsorbée sur le graphite. Abstract. 2014 The diffusion coefficient of a CH4 submonolayer adsorbed on (0001) graphite is measured as a function of coverage and temperature by quasi-elastic neutron scattering. The experimental data permit us to clearly distinguish two kinds of adsorbed fluids, i.e. a 2D liquid coexisting with a 2D gas and a 2D hypercritical fluid whose density varies continuously with coverage. The mobility disappears at solidification which can be obtained either by decreasing temperature at constant coverage or by increasing coverage at constant temperature. The coexistence of 2D solid and 2D liquid is also observed. Finally a phase diagram of the first layer of methane adsorbed on graphite is proposed.
The diffusion coefficient of a mobile CH 4 submonolayer adsorbed on graphite (0001) is systematically measured by quasielastic neutron scattering. Its behavior with coverage is used to distinguish clearly two kinds of adsorbed fluids; a two-dimensional liquid and a two-dimensional liquid an a two-dimensional hypercritical fluid.The existence of phase transitions on physisorbed monolayers was observed about ten years ago by Thorny and Duval 1 and the names "two-dimensional (2D) gas," "2D liquid/' and "2D solid" were justified by analogy with the the three-dimensional phases in interpreting entropy determination obtained from adsorption isotherm measurements. More recently microscopic probes have been used in order to observe the actual mobility of these different phases, especially to distinguish between a solidlike and a fluidlike behavior. Different techniques, including NMR, 2 " 4 Mossbauer resonance, 5 or neutron scattering, 6 have been used to detect 2D melting by measuring an enhancement in the mobility of the adsorbed phase. The large mobility observed in these experiments gives no information about the nature of the 2D fluid 0 It is not easy to make a precise distinction between a hypercritical fluid with a high compressibility, occupying the whole surface which is available for it, and a liquid whose domains are limited by a well-defined boundary.We believe the results reported here answer the question in the case of a submonolayer of methane adsorbed on the basal plane of graphite. We measured the diffusion coefficient of the 2D CH 4 fluid by incoherent quasielastic neutron scattering at constant temperature as a function of coverage, and in addition we have shown that the type of variation of the 2D diffusion coefficient with coverage enables us to distinguish between two kinds of mobile phases, 2D liquid and 2D hypercritical fluid.The CH 4 -graphite (0001) system has several advantages: The (0001) graphite surface is highly uniform; methane has a strong incoherent neutron cross section and can be considered as a spherical molecule at the temperature at which the experiment was performed; its rotation frequency is very large (co r^1 0 13 s) and does not result in a quasielastic effect in the range where the translational diffusion is observed,, Furthermore, the phase diagram of the CH 4 adsorbed layer is fairly well knowa, 1 The condensation occurs in a layer-by-layer mode and the first layer undergoes two first-order phase transitions within a triple point and a critical temperature range« The 2D critical temperature (~ 72 K) is determined by adsorption isotherm measurements and the triple point is about 56 K as found by neutron scattering. 697 A graphite "powder" called Papyex 8 is used to increase the surface per unit volume and obtain a measurable neutron signal. This powder is a recompressed exfoliated graphite having almost nothing but uniform (0001) surfaces with a preferential orientation. This product is very similar to Grafoil, which has been extensively used in surface phase studies. 9 The ...
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