Correlation between the photocatalytic degradability over TiO2 in water of meta and para substituted methoxybenzenes and their electron density, hydrophobicity and polarizability properties

In an earlier study [Pichat et al. (1993) Photocatalytic Purification and Treatment of Water and Air, pp. 207–223. Elsevier, Amsterdam] the photocatalytic degradability over TiO2 of some chlorophenols in water was correlated to the 1-octanol-water partition coefficient (Kow) and to the Hammett constant (σ) of these compounds. To determine whether a similar correlation can be found for less closely structurally related aromatic pollutants, the case of methoxybenzenes (anisoles) substituted at the meta or para positions by F, Cl, NO2, OH, NH2 groups was investigated. The Hammett constant, σ, the Brown constant σ+ and the ionization potential were successively used as descriptors for the electron availability of these compounds. The best correlation was log kapp=0.20 log Kow−0.20σ+−0.033 MR−−0.43 (r=0.903) where kapp (in min−1) is the pseudo-first order rate constant of the pollutant disappearance in TiO2 aqueous suspensions and MR is the molar refractivity of the pollutant. With respect to σ, σ+ has the interest of introducing a greater difference between the meta and para derivatives. MR, which is proportional to the polarizability, is thought to reflect the pollutant-pollutant interaction and to amend the effect of Kow in accounting for the distribution of the pollutant molecules in the TiO2-water system. This descriptor induces much smaller differences between the various anisoles than Kow and, above all, σ+. The validity of methods to evaluate Kow when experimental values are not available or MR when the refractive index is unknown, is also discussed.