Effects of organic salts on polymer-surfactant interactions: Roles of Bu₄NBr and Pr₄NBr in PVP-SDS complexation

Pyrene solubulization and NMR spectroscopy were employed to study the competition between two surfactant aggregation processes in aqueous polymer/organic salt/surfactant systems. Two organic salts used in the study are tetrabutylammonium bromide (Bu₄NBr) and tetrapropylammonium bromide (Pr₄NBr). The anionic surfactant sodium dodecyl sulfate (SDS) is known to have moderate interactions with poly(N-vinylpyrrolidone) (PVP), which results in the formation of a PVP-SDS aggregation complex. On the other hand, SDS also can associate with tetraalkylammonium bromides (TAABs) to form mixed micelles by a strong electrostatic attraction. The pyrene solubilization experiments reveal that the dominant aggregation in PVP/TAAB/SDS systems is determined by the [SDS]/[TAAB] ratio, and complexation of PVP with SDS does not occur until the [SDS]/[TAAB] ratio is larger than a specific ratio. For example, when [SDS]/[Bu₄NBr] is less than 1.1, only Bu₄N₊-SDS mixed micelles are formed, and the mixed micelles do not associate with PVP. When [SDS]/[Bu₄NBr] is higher than 1.1, polymer-induced SDS aggregation is detected along with the mixed micellization between SDS and Bu₄NBr. More interestingly, the Bu₄N₊ ions do incorporate with the PVP-bound SDS aggregate to form a PVP-(Bu₄N₊-SDS) complex. By means of 2D NOESY and PFG NMR experiments, both structural and dynamic aspects of the aggregate species formed in the PVP/TAAB/SDS solution were studied. An interaction model was proposed to manifest the role of the two TAAB salts for PVP-SDS complexation in the PVP/TAAB/SDS systems. (Figure Presented).