  We present 77 K isotherms of krypton adsorption on bundles of closed
highly-pure HiPco single-walled carbon nanotubes (SWCNTs). Two volumetric
adsorption protocols were used, one with an increasing Kr dose per injection
(IAD), one with a constant dose (CAD). Detailed microstructural examination
showed that the SWCNTs combine into small bundles (of 25-30 SWCNTs) which are
heterogeneous in diameter with a consequential range of interstitial channel
(IC) shapes and sizes. The IC-sites are the subnanoscaled pores with
alternating enlargements and constrictions along the tube axes. This results in
adsorption dosing (AD) dependent characteristics of the low-pressure region of
the isotherm. In the IAD protocol the switch-back behavior of the isotherm
stemmed from metastable adsorption. Using the CAD protocol, different branches
are observed. Well-pronounced substeps were established which we interpret as
corresponding to the formation of various phases of confined Kr with different
atoms arrangement. The height of a given substep obtained in different
measurements depends on the AD value which can strongly influence the
population of the site. Some substeps existing only for certain values of AD
suggests the existence of a certain selectivity or of a preferential phase
formation according to this value.
