Effect of molecular weight of poly(acrylic acid) on the interaction of oppositely charged ionic surfactant-polyelectrolyte mixtures

The isothermal titration calorimetry was applied to investigate the effect of molecular weight (MW) of poly(acrylic acid) (PAA) on the interaction of tetradecyltrimethyl ammonium bromide (C(14)TAB) with PAA in the bulk phase. The initial formation of PAA/C(14)TAB complexes was driven by electrostatic interactions between the cationic head groups of C(14)TAB and the oppositely charged PAA chains, resulting in enhanced hydrophobicity of the complexes. The further addition of C(14)TAB would induce either complex-C(14)TAB interaction or complex-complex interaction due to hydrophobic effect. Increasing the MW of PAA would enhance the tendency of complex-complex interaction to form aggregation and even precipitation. For PM MW >= 130,000, significant precipitation of interpolymer complexes was consistently observed and all the obtained critical micelle concentration, phase separation concentration, heat of complexation, and heat of micellization were independent of MW. For PAA MW of 25,000, the complexes formed visible millimetric aggregates at phase separation concentration. With further addition of C(14)TAB, these aggregates gradually dissolved, and the solution became clear again. For PAA at an MW of <= 5000, turbidity was observed in a wide range of concentrations, but no macroscopic precipitation was observed and the solutions were always homogeneous. (C) 2018 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.