Two-Phase Stopped-Flow Measurement of the Protonation of Tetraphenylporphyrin at the Liquid−Liquid Interface

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Abstract
The formation rate of the protonated form of tetraphenylporphyrin (TPP) in a dispersed two-phase system composed of dodecane and aqueous trichloroacetic acid (TCA) was studied by means of a stopped-flow method. The protonation reaction took place at the liquid−liquid interface, and the diprotonated TPP (H2TPP2+) formed was adsorbed there. In order to determine the rate-determining process, changes in absorbance at the absorption maximum wavelengths of TPP and H2TPP2+ were analyzed. The obtained rate constant for the decrease of TPP in the organic phase, 21 ± 2 s-1, was in agreement with that for the increase of diprotonated TPP at the interface, 20 ± 3 s-1. The observed rate constants did not show any dependence on concentrations of both TPP and the acid. The experimental results suggested the rate-determining step to be the molecular diffusion process of TPP in the stagnant layer in the organic phase side at the liquid−liquid interface, and the thickness of the stagnant layer was estimated as 1.4 × 10-4 cm.