Solubilization of p-nitrophenol in aggregates formed by hydrophobically modified polyelectrolytes

Abstract

The solubilization of p-nitrophenol into the hydrophobic microdomains provided by polyelectrolytes carrying alkyl side chains of different length has been investigated in aqueous solutions of pH 5.0 and 8.0. Under these pH conditions p-nitrophenol is predominantly present in its neutral and ionic forms, respectively. Potassium salts of poly(maleic acid-co-1-olefins), PA- nK 2 with n=12,14,16,18, were synthesized, and the pseudo-phase model was used to determine the distribution coefficient K S, and the standard free energy of transfer Δ μ 0 t of p-nitrophenol between water and polymer aggregates. The results indicate that at both pH's the solubilization of p-nitrophenol increases with increasing size of the side alkyl chain; i.e., the values of K S follow the order PA-18K 2 > PA-16K 2 > PA-14K 2 > PA-12K 2. The free energies, Δ μ 0 t , were plotted as a function of the number of carbon atoms in the side alkyl chain and a linear relation was found. From these plots contributions of −0.324 and −0.676 kJ mol −1 per methylene group were determined at pH 5.0 and 8.0, respectively. The effect of aggregate size on the solubility of phenol is attributed to the hydrophobic contribution per CH 2 group to the free energy of transfer. The hydrophobic nature of the CH 2 group is suggested to derive largely from the enthalpic contribution.