Adsorption of argon on mesoporous anodic alumina

We have studied the adsorption of Ar on regular, highly-ordered alumina membranes made by anodization. The straight, non-interconnected pores have nominal diameters of 31 and 83 nm, with a relative dispersion better than 5 % in the pore size. Adsorption isotherms taken on bare membranes with pores of 83 nm present two distinct hysteresis loops. This is found to be a consequence of the fabrication procedure that yields a central circular region formed by open pores surrounded by an outer ring with closed bottom pores of smaller size, about 40 nm. For the membrane with pores of 31 nm, the difference between these pores is much smaller, about 2 nm, and this explains why the isotherms on these membranes show a single hysteresis loop as expected. Detailed real space analysis of the membranes by electron microscopy confirms the adsorption conclusions.