Effects of substrate heterogeneity on adsorption isotherms near a two-dimensional gas-liquid critical point

We calculate adsorption isotherms for a model of a two-dimensional (2D) fluid system obeying Griffiths's 2D Ising scaling equation of state, in equilibrium with a 3D ideal gas. We model the effects of an imperfect substrate using the theory of Dash and Puff. We compare the Ising-model results with experimental isotherms of Kr adsorbed on MgO smoke. The model isotherms are used to evaluate empirical methods for determining the critical temperature and the calculation of the critical exponents {γ} and {β} from experimental isotherms. We find that current methods overestimate Tc, presently available substrates preclude a direct measurement of {γ}, and the blunt shape of the phase boundary makes the precise determination of {β} difficult.