Substrate dimensional changes upon adsorption: Methane, xenon and krypton adsorbed on graphite

An improved strain gauge has been developed for studying the deformation of a graphite substrate occurring upon adsorption of methane, krypton and xenon around 100 K in the monolayer coverage region. Two regimes of substrate dimensional changes have been found. At low coverage, the graphite substrate contracts, showing that the adsorbate exerts a negative pressure, whereas at higher coverage (≳0.5 of a monolayer) the substrate expands with a relative dimensional change Δll proportional to the two-dimensional Gibbs pressure of the film. 2D fluid 2D solid phase transitions result in a sharp break in the curve Δll versus coverage. The negative deformation of the graphite is analysed in terms of adsorbate-substrate interactions and substrate mechanical properties (Poisson ratio). Experimental results bear out the proposed theory.