Critical adsorption of SF₆ on a finely divided graphite substrate

Abstract

The adsorption behavior of a near-critical fluid (SF,) on a finely dispersed graphitic adsorbent (graphitized carbon black; Vulcan 3-G) has been studied. The surface excess concentration r 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 r diverges at the critical point. At temperatures well above criticality of the fluid (T-Tc>3 K) we find the expected increase of r w i t h decreasing T; however, closer to T, the adsorption does not increase further but decreases sharply for T-tT,. This unexpected behavior was also found when the experiment was repeated under microgravity conditions on the space platform EURECA I. A similar maximum in T ( T ) was also observed for SF, adsorbed in a mesoporous controlled-pore glass with 31 nm mean pore diameter.

Reliable measurement of near-critical adsorption by gravimetric method

“…An exception to the general trend is observed near the critical temperature and the critical density. At reduced density values between 0.9 and 1.2, the measured values of the excess amount adsorbed at 31.4 • C are in fact smaller than or equal to those at 36.3 • C. Similar effects have been observed previously for other experimental systems, and have been called "critical depletion" (Machin , 1999;Rajendran et al, 2002;Thommes et al, 1994Thommes et al, , 1995. Such phenomenon has always eluded a convincing theoretical explanation.…”

“…They found that at a temperature of 32 • C the isotherm was exhibiting a "bump" around the critical density, differently from the isotherms measured at higher temperatures, where an almost linear behavior was found in the same density range. Another phenomenon, called critical depletion, was presented for the first time by Findenegg and coworkers for the case of SF 6 on graphitized carbon black using a volumetric adsorption apparatus (Thommes et al, 1994). The phenomenon of critical depletion was then further confirmed by adsorption measurements of the same adsorbate on controlled pore glasses CPG-350 and CPG-100 (Machin , 1999).…”

Effects of confinement on critical adsorption: Absence of critical depletion for fluids in slit pores

“…Although Γ increased as τ was reduced, consistent with (1.2), close to T c (τ ∼ 5 × 10 −3 ) the adsorption reached a maximum and decreased sharply taking on negative values very close to T c . Microgravity experiments by the same group confirmed these results [1,2]. The critical depletion of Γ was attributed to the confining effects of colloidal particles on the near critical fluid and new experiments, designed to test this conjecture, were performed on the sorption of SF 6 in a mesoporous glass CPG-350, which comprises a rigid interconnected system of mesopores with a nominal pore diameter of 31nm.…”

“…The term 'critical depletion' was introduced in connection with experiments designed to study the phenomenon of critical adsorption for a pure fluid at a solid substrate [1,2]. When a fluid is brought to its bulk critical point in the presence of an attracting external wall or substrate, for example along a critical isochore, the amount adsorbed (adsorption) diverges as τ ≡ (T − T c )/T c → 0.…”

“…In Ref. [1] measurements were performed for SF 6 adsorbed on a finely divided (colloidal) graphite adsorbent (Vulcan 3G). The volumetric method employed in these experiments measured Γ directly on approaching the critical point along the critical or near-critical isochores.…”