Fig. 6b shows the CO2 adsorption isotherms for all silica membrane
materials. As it can observed, all isotherms exhibit Langmuir-
type with approximately linear region [54]. The adsorbate–
adsorbent interactions are dependent on the polarity of the interacting
functional groups, the adsorption temperature and the particular
surface composition and/or the pore structure [53]. As it can
be observed in Fig. 6b, samples B–Si and Co–Si exhibit a higher
adsorption capacity and a higher adsorption potential for CO2 compared
to samples Hyd–Si and Hyd–Co–Si. This observation must be
related to the presence of a highly developed narrow microporous
structure (larger values of Vn) together with the presence of a larger
proportion of hydroxyl groups on the surface of hydrophilic
samples. Hence the adsorption occurs with these active sites at
the surface at the range of very low relative pressure resulting enhanced
micropore filling. On the contrary, for samples Hyd–Si and
Hyd–Co–Si, due to the presence of CH3 groups on the surface, the
interaction of the CO2 molecule with the surface is weaker and occurs
at the range of higher relative pressure. A similar effect of the
probe molecule has been reported in the literature for N2 and Ar
(kinetic diameter is 0.34 nm) adsorption on samples B–Si and
Hyd–Si, where the shape of the isotherms highly depended on
the specific surface interaction and the size of the sorbent gas [16].