Interaction between poly(acrylic acid) and nonionic surfactants with the same poly(ethylene oxide) but different hydrophobic moieties

The effect of homogeneous nonionic surfactants (CnE8 with n = 10, 12, and 14) on the solution behavior of poly(acrylic acid) (PAA) was investigated by surface tension and viscometry. The first method allowed determination of the critical micelle concentration (cmc), the critical aggregation concentration (cac or T-1), the saturation of the polymer and the onset of free micelles into solution (T-2) and an intermediate concentration (T-2') that was defined as the stoichiometric concentration for binding. The T-1 points were lower than cmc's, and the change of T-1 vs the number of carbon atoms in the alkyl chain of the surfactants obeys the same rule as the cmc does. The energy change in transferring one methylene unit from micellar to water environment was nearly the same for micelles and complex and proves that both phenomena have similar driving forces. Viscometric data evidenced in turn the T-1 and T-v points. T-v is the minimum point that appears in the viscosity curve of PAA-nonionic surfactant systems and T-2 by surface tension was a little higher than T-v by viscometry. The longer the alkyl chain of the surfactant, the lower was T-v. The composition of complexes at T-v was nearly constant and suggested that a considerable number of ethylene oxide groups do not participate in complex formation. The effect of surfactants on PAA at T-v was compared to that of inorganic electrolytes (i.e., HCl and NaCl), and the following order was established: surfactant < NaCl less than or equal to HCl. The results revealed the important role played by hydrogen bonding and hydrophobic forces in PAA-nonionic surfactant interaction, and the data were discussed in light of the latest experimental and theoretical achievements about the mechanism of complex formation.