Microscopic evidence of "necklace and bead"-like morphology of polymer-surfactant complexes: a comparative study on poly(vinylpyrrolidone)-sodium dodecyl sulfate and poly(diallyldimethylammonium chloride)-sodium dodecyl sulfate systems.

Here, we report the microscopic evidence of "necklace and bead"-like morphology, which has long been the most widely accepted model for polymer-surfactant complexes. The lack of microscopic evidence of the initial complexation between surfactant and polymer has resulted in many contradictory reports in the literature. In this paper, we visualized these initial complexes formed between negatively charged surfactant sodium dodecyl sulfate (SDS) with neutral poly(vinylpyrrolidone) (PVP) and cationic poly(diallyldimethylammonium chloride) (PDADMAC) polymer through photoluminescence (PL) microscopy and atomic force microscopy (AFM) using silicon quantum dot (Si QD) as an external PL marker. It is observed that, for the PVP-SDS system, SDS molecules bind at the hydrophobic sites on the random-coiled PVP chain through their hydrocarbon tails, while for the PDADMAC-SDS system, SDS head groups are associated with the positively charged nitrogen centers of the polymer, where the polymer chain wraps around the surfactant head groups.