Evidence of Two Kinds of Two-Dimensional Adsorbed Fluids by Mobility Measurements

Abstract

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 ...

Mobility measurements of two kinds of two-dimensional fluids. Methane adsorbed on graphite

“…Preliminary reports were published some time ago [16,17]. In order to deal with this problem, we measured the mobility of a submonolayer of methane physisorbed on graphite and its variation as a function of coverage and temperature.…”

“…Moreover this system is approachable through different complementary techniques : adsorption isotherms [1, 35,36], NMR [18,37], QENS [16,17,38,39] and neutron diffraction of the deuterated compound [13,40,41]. Its thermodynamic properties are fairly well known [1] above 77 K. The 2D-critical temperature is estimated around 75 K, while neutron diffraction yields a triple point at 56 K. The liquid-gas coexistence domain spreads out within 20 K, so it allows an easy characterization of its properties.…”

“…This industrial product is suitable for neutron scattering studies on two-dimensional phases. Although it had been intensively used [13,16,17], it remained incompletely characterized. Up to the present it has been assumed that the orientation distribution of papyex is the same as that of grafoil, although no characterization had been made of papyex.…”

Melting mechanism of monolayers adsorbed in cylindrical pores: The influence of the pore wall roughness

“…Our experimental results can be compared with neutron scattering studies of methane on graphite 18 bundles. 20 On graphite, partial monolayers melt, or show translational dynamics, above 56 K, although a complete monolayer melts at ϳ10 K above the melting point of bulk methane.…”

“…17͔͒, a progressive onset of liquidlike translational dynamics being observed at temperatures as low as 60 K. 18 For comparison, the complete monolayer melts abruptly at 100 K that is 10 K above the melting point of bulk methane. Of relevance to this work, the melting of partial monolayers ͑patches͒ of methane on graphite has been measured on a time-of-flight spectrometer ͓IN5 at ILL ͑Ref.…”