Cooperative binding of surfactant ions by small oligomers of opposite charge

The binding of cationic surfactant, dodecylpyridinium ion (DP+) by anionic oligomers, i.e., citrate (with three negative charges), triphosphate (five negative charges), and tetraphosphate (six negative charges) has been investigated in the presence of NaCl by using potentiometric techniques based on a surfactant-ion-selective solid-state membrane electrode. The binding isotherms by triphosphate and tetraphosphate both exhibit the typical feature of sigmoidal shape, indicating the occurrence of cooperative binding in these two cases. On the contrary, the obvious cooperative behavior is not observed for the case of citrate, although a nonspecific or gradual binding process is recognizable. The critical binding concentration (cbc) for citrate gives a different added-salt concentration dependency from two other oligomers. For triphosphate and tetraphosphate, cbc: increases with an increase of added salt concentration, whereas for citrate it decreases as the critical micelle concentration (CMC) does. In summary, our results show that the cooperative binding of surfactant ions by oppositely charged polyion takes place when the polyion has at least five charged binding sites.