Surfactant binding to oppositely charged polyions

Binding of undecyl-, tridecyl-, tetradecyl-, and pentadecyl-pyridinium cations to poly(styrenesulfonate) anion, PSS-, in aqueous 0.01 and 0.1 m NaBr has been investigated by means of potentiometry based on the surfactant cation-selective membrane electrodes and flow microcalorimetry. It has been observed that the cations of each surfactant start binding strongly to PSS- polyion at a total surfactant concentration well below its critical micelle concentration, cmc. Above this law limiting concentration the measured exothermic enthalpies of binding, Delta H-b, expressed per mole of the bound surfactant become independent of the surfactant concentration. They do, however, increase with the increasing length of the surfactant hydrocarbon chain. These observations are in close agreement with the results of earlier studies on similar systems according to which strong surfactant binding to the oppositely charged polyions results primarily from the aggregation of the hydrocarbon chains of the bound surfactant ions into small aggregates. Comparison of the measured Delta H-b values with the corresponding, recently determined enthalpies of micellization, Delta H-mic, shows that the Delta H-b values are by about 9 kJ/mole more exothermic than the Delta H-mic values, regardless of the surfactant chain length or the simple salt concentration. This suggests that besides the micellization of the bound surfactant ions the polyion-counterion-solvent interactions taking place in the immediate vicinity of the polyion in a local environment that consists of surfactant hydrocarbon tails and water molecules map also play an important role in determining the binding of surfactant ions to the charged macromolecules.