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Nanometer size pores in ionic liquid loaded silica gel characterized by positron lifetime spectroscopy

We present studies on the free volume of silica gel dependent on water absorption and on loading with a different amount of the ionic liquid (IL) [C2 MIM][NTf2] using positron lifetime spectroscopy (PLS). The found two long lifetime components ( ≈ 2 and 10–20 ns) are attributed to the annihilation of ortho‐positronium (o‐Ps) in micropores of different size. The application of a semi‐empirical quantum mechanical model allowed us to correlate the measured o‐Ps lifetimes to the absolute mean size of the micropores by approximation with a spherical shape. The mean diameter of the pores in the pure silica gel could, hence, be determined to 1.72(6) nm. IL loading with up to 70 vol% lead to the formation of smaller voids with a mean diameter of 1.28(4) nm according to a reduction of the mean pore volume by about 60%. In addition, the accumulation of H2 O molecules from the surrounding air inside the pores could be observed in‐situ by time dependent PLS.