Kinetics and mechanism of the adsorption of methylene blue from aqueous solution on the surface of a quartz cuvette by on-line UV–Vis spectrophotometry

• Characterization of the adsorption of methylene blue on a quartz cuvette.
• High quality kinetic data on the absorption with a time resolution of 1 s.
• Spectroscopic and kinetic evidence suggesting the adsorption of dye monomers.
• An elaborate kinetic model describing the mechanism of adsorption.

It is often challenging to follow fast surface adsorption processes with sufficient time resolution. Here we report a convenient UV–Vis spectrophotometry based method to study the fast kinetics of an archetypical surface process: the adsorption of MB on quartz. The test object was a commercial quartz cuvette. About 3.3% of the dye was adsorbed from a 34 μM solution at pH 7 in 10 min. Biphasic kinetics was observed for the depletion of MB under continuous stirring at 1000 rpm. A kinetic model was postulated for the interpretation of time resolved spectral changes. This model assumes the reversible adsorption of MB on the surface which is followed by the isomerization of the binding mode. The overall process is kinetically first order with respect to MB, thus dimer formation, which was previously proposed to be favored on quartz, can be ruled out. The experimental adsorption isotherm is consistent with the formation of an adsorption monolayer from single molecules. The UV–vis absorbance spectrum of adsorbed MB does not show any blue shift. With the simplest experimental setup, it is shown that the dimerization of MB is not exceptionally favored on quartz surface.

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