Critical adsorption of SF6 on a finely divided graphite substrate

The adsorption behavior of a near‐critical fluid (SF6) on a finely dispersed graphitic adsorbent (graphitized carbon black; Vulcan 3‐G) has been studied. The surface excess concentration γ was measured by approaching the critical point along near‐critical isochores using a volumetric technique. From the scaling theory of critical adsorption by Fisher and de Gennes it is expected that γ diverges at the critical point. At temperatures well above criticality of the fluid (T–Tc≥ 3 K) we find the expected increase of Γ with decreasing T; however, closer to Tc the adsorption does not increase further but decreases sharply for T → Tc. This unexpected behavior was also found when the experiment was repeated under microgravity conditions on the space platform EURECA I. A similar maximum in Γ(T) was also observed for SF6 adsorbed in a mesoporous controlled‐pore glass (CPG‐10) with 31 nm mean pore diameter.