Two-phase stopped flow measurement of the protonation of tetraphenylporphyrin at the liquid-liquid interface

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-now method. The protonation reaction took place at the liquid-liquid interface, and the diprotonated TPP (H2TP2+) 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 x 10(-4) cm.