Photochemistry of materials adsorbed on clay systems. Effect of the nature of the adsorption on the kinetic description of the reactions

Abstract

The fluorescent probe tris(2,2'-bipyridine)ruthenium(II) (RuII) has been used to investigate the nature of the surfaces of the following clays: laponite, which is a synthetic clay, and natural hectorite and montmorillonite. RuII is adsorbed completely by the clay by ion exchange and on excitation gives rise to a luminescence spectrum in the red part of the spectrum with a lifetime of about half a microsecond. The lifetime, quantum yield, and nature of the absorption spectrum are dependent on whether the RuII is adsorbed in layers as in the natural clays or whether it is adsorbed on the surface as with laponite. At low concentrations of laponite, RuII is adsorbed on outer layers and is in contact with the aqueous phase. However, at higher clay concentrations or in the presence of calcium chloride, layering of the clay occurs and the probe molecule is placed progressively between the layers where its photophysics is altered. The casting of a film from the laponite-RuII exhibits maximum spectral change as associated with maximum colloid layer formation. Such changes are not as readily observed with hectorite or montmorillonite, and this indicates that, for the most part, these systems exist as layered colloids and that RuII is already adsorbed between the layers. Other molecules such as Cu2+, dimethylaniline, and nitrobenzene react with excited RuII through electron-transfer reactions and are also adsorbed to varying extents on the clay surface. Cu2+ is adsorbed strongly and the kinetics are simplified due to the strong adsorption. Here, Stern-Volmer type kinetics are observed and a quenching rate constant is obtained which is lower than that in the aqueous solution, which gives an estimate of the degree of movement of cupric ions on the clay surface. Dimethylaniline and nitrobenzene are adsorbed weakly on the clay. However, the clay catalyzes the reaction of the RuII with these quenchers as both are adsorbed in a small volume, i.e., the clay surface. The kinetics that describe these latter reactions are of the Poisson form, and the kinetics indicate that the reactive quencher molecules are adsorbed around the RuII, in a zonelike effect, rather than being adsorbed randomly throughout the system. This tends to indicate that the sites of adsorption are not uniform on the clay surface but occur in regions.

Physical aspects of photochemistry and radiation chemistry of molecules adsorbed on silica, .gamma.-alumina, zeolites, and clays

“…Photochemistry can be carried out in these layers. [207][208] Several factors affect the photophysics and photochemistry on clays, the nature of adsorbed water210 on the surface and the spacing of the clay layers. The formation of pyrene cation radicals is shown to be a single photon ionization process.…”

Photoinduced Electron Transfer between Ruthenium-bipyridyl Complex and Methylviologen in Suspensions of Smectite Clays

“…Thus, the smectite-type clays provide characteristic microenvironments that are different from those provided by rigid inorganic porous materials such as zeolites. In fact, the interlayer spaces of the clays allow a variety of photochemical reactions in which specific behavior such as selective formation of unusual photoproducts has been discovered. − The colloidal suspensions of the clays mediate unusual photoprocesses such as controlled photoinduced electron/energy transfer among porphyrin molecules. ,− The smectite-type clays provide specific surface and interlayer environments also for the Ru(bpy) 3 2+ or MV 2+ molecules so that characteristic photochemical/photophysical behavior that is not observed in homogeneous solutions has been realized. − …”

Preparation of Smectite/Dodecyldimethylamine N-oxide Intercalation Compounds

“…Since the cationic guest species are strongly bound to the negatively charged silicate layers with electrostatic interactions and do not deintercalate upon the intercalation of C12AO, the possible effect of coadsorption of C12AO on the states of the preadsorbed [Ru(bpy) 3 ] 2+ was investigated. Photochemical and photophysical properties of [Ru(bpy) 3 ] 2+ on clay minerals and other layered materials have been investigated extensively. ,,− , The adsorbed states of [Ru(bpy) 3 ] 2+ on the surfaces of smectites have successfully been controlled by the coadsorption of [Zn(bpy) 3 ] 2+ , [Co(bpy) 3 ] 2+ , and poly(vinyl pyrrolidone). ,, In the present system, the adsorption of C12AO was expected to alter the states of the adsorbed [Ru(bpy) 3 ] 2+ on saponite.…”

“…In other words, one can modify the attractive properties of [Ru(bpy) 3 ] 2+ by introducing it into appropriate matrixes. Along this line, the photochemical and photophysical characteristics of [Ru(bpy) 3 ] 2+ adsorbed on smectites have been investigated so far. ,,− The spectroscopic characteristics of the adsorbed [Ru(bpy) 3 ] 2+ were expected to reflect the change in the microenvironments caused by the coadsorption of C12AO.…”