On a mechanism low-pressure insertion of chain molecules into crystalline matrices

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.