Inclusion phenomena in a system designed for exclusion—A cavitary problem and a model

The Sephadex gels were introduced for their molecular-sieve-like properties. For some types of solute, however, they exhibit affinity for which the presence of water seems mandatory. Partitioning in these gels has been attributed to steric exclusion, hydrophobic interactions, dispersion interactions, etc. but may be due to a common structural feature, possibly a cavitary structure. Structural and partitioning similarities between the Sephadex gels and the cycloamyloses support this concept. A simple cavitary model vicinal water shell (CVS) is proposed in which the selectivity of the gel depends essentially on internal partitioning between a two phase system consisting of a vicinal-matrix-perturbed water shell lining the cavity and a core of normal (bulk) water. The CVS model cannot be used as yet to predict K_d values from the physico-chemical properties of the gels alone. The starting point is a known K_d value in one gel and the K_d value of the same solute in another gel is then calculated using the physical-chemical properties. The model was tested on several solutes and yielded promising results. Its implications are discussed.