Correlation between the photocatalytic degradability over TiO2 in water of meta and para substituted methoxybenzenes and their electron density, hydrophobicity and polarizability properties
Abstract 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 TiO 2 of some chlorophenols in water was correlated to the 1-octanol-water partition coefficient ( K ow ) 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, NO 2 , OH, NH 2 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 k app =0. 0.20 log K ow −0.20 σ + −0.033 MR−−0.43 ( r = 0.903) where k app (in min −1 ) is the pseudo-first order rate constant of the pollutant disappearance in TiO 2 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 K ow in accounting for the distribution of the pollutant molecules in the TiO 2 -water system. This descriptor induces much smaller differences between the various anisoles than K ow and, above all, σ + . The validity of methods to evaluate K ow when experimental values are not available or MR when the refractive index is unknown, is also discussed.
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