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

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-nK2 with n = 12, 14, 16, 18, were synthesized, and the pseudo-phase model was used to determine the distribution coefficient KS, and the standard free energy of transfer Deltamut0 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 KS follow the order PA-18K2 > PA-16K2 > PA-14K2 > PA-12K2. The free energies, Deltamut0, 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 CH2 group to the free energy of transfer. The hydrophobic nature of the CH2 group is suggested to derive largely from the enthalpic contribution.