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 inves tigated by surface tension and viscometry. The first method allowed de termination 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 concentra tion 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 surfac tants obeys the same rule as the cmc does. The energy change in transf erring one methylene unit from micellar to water environment was nearl y the same for micelles and complex and proves that both phenomena hav e similar driving forces. Viscometric data evidenced in turn the T-1 a nd 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 wa s a little higher than T-v by viscometry. The longer the alkyl chain o f the surfactant, the lower was T-v. The composition of complexes at T -v was nearly constant and suggested that a considerable number of eth ylene oxide groups do not participate in complex formation. The effect of surfactants on PAA at T-v was compared to that of inorganic electr olytes (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 PA A-nonionic surfactant interaction, and the data were discussed in ligh t of the latest experimental and theoretical achievements about the me chanism of complex formation.