  A microscopic mechanism of low-pressure insertion and separation of
chain-like molecules in host matrices is proposed. It is shown that the
intramolecular correlations combined to appropriate host activities are
responsible for a low-pressure condensation of chain molecules. This allows
recover a fine structure of the isotherms and to explain recent experiments on
the insertion of $C_2H_2$ and $CO_2$ guest species. We argue that the mechanism
should be dominant in low-dimensional host geometries, where the entropic
effects are strongly suppressed and the major factors are the chain
connectivity and packing.
