THIONINE IN THE CAGE OF ZEOLITE-L

The room temperature exchange isotherm of thionine (TH+) in aqueous potassium zeolite L dispersions shows that TH+ can exchange up to 7.5% of the cations belonging to the main channel from which an average thionine-thionine center to center distance of 27 angstrom is calculated. This corresponds to a 0.2 M TH+ concentration with respect to the zeolite L volume. We conclude that the maximum number of TH+ that can be incorporated is limited by the space available in the channel. Even at these high concentrations the electronic spectrum is that of a monomer. It is more structured than in diluted aqueous or alcoholic solutions, thus indicating the restricted freedom in the channel. The spectrum observed just after mixing the dye with a zeolite L dispersion is that of aggregates, easily recognized by the sudden color change from blue to purple. With time the short-wavelength absorption decreases and the monomer spectrum develops. Intercalation kinetics is slow at room temperature in the case of a potassium zeolite L and takes about 2 weeks to be completed. It is much faster at 70-degrees-C where it takes about 10 h for a potassium zeolite and only about 1 h for a proton zeolite. We have observed that the acid-base behavior of thionine in the channels of a proton zeolite L is comparable to that otherwise observed in 2.5 M hydrochloric acid. Methylene blue and ethylene blue being slightly larger than thionine do not show any cage effect since they do not go into the main channel of zeolite L. Their electronic spectra observed even after 2 weeks are those of aggregates. We conclude that they remain adsorbed at the outside of the zeolite particles because the geometry of a zeolite L does not allow the formation of parallel dimers inside the main channel.