Solute–Solvent Interaction Effects on Protonation and Aggregation Constants of TTMAPP in Different Aqueous Solutions of Methanol

Abstract

The protonation constants of 5,10,15,20-tetrakis(4-trimethyl-ammonio-phenyl)-porphine tetratosylate (TTMAPP) were determined in water–methanol mixed solvents, using a combination of spectrophotometric and potentiometric methods at 25 °C in 0.1 mol·dm−3 sodium perchlorate. Two protonation constants, K 1 and K 2, were characterized and analyzed in various media in terms of the normalized polarity parameter ( $$ E_{\text{T}}^{\text{N}} $$ ). A linear correlation is observed when the experimental log10 K 1 and log10 K 2 values are plotted versus the calculated ones over the range of 40–90 % (v/v) methanol. The self aggregation of TTMAPP was observed from acidic media (pH ≅ 3) to alkaline pH, where it reached its highest intensity, when methanol is lower than 40 % in solution. The formation of aggregate species prevents a quantitative analysis of titration curves and thus, the determination of the protonation constants of TTMAPP. Therefore, to evaluate the protonation constants of TTMAPP in low or zero percent of methanol, the Yasuda–Shedlovsky extrapolation approach has been used.