Excited state proton transfer as a probe for polymer-surfactant interaction

Abstract

Interaction between the polymer, poly(vinylpyrrolidone) (PVP) and an anionic surfactant (sodium dodecyl sulfate, SDS) has been studied using excited-state proton transfer (ESPT) of 1-naphthol as a probe. In aqueous solution, the ESPT process of 1-naphthol is very fast and occurs in ~35 ps. The kinetics of the ESPT process remains unchanged on addition of 4 mg PVP per mL. However, on addition of SDS, in the presence of PVP, the ESPT process of 1-naphthol is retarded dramatically. Compared to ordinary water, the intensity of the emission of the neutral form of 1-naphthol (at 360 nm) is enhanced ~200 times in the presence of PVP (4 mg per mL) and 15 mM SDS. In the PVP-SDS complex, the anion emission of 1-naphthol shows a rise time of 1.6 ns and a decay of lifetime 13 ns, while the neutral emission (at 360 nm) exhibits a biexponential decay having components of 1.6 and 5.3 ns. This indicates the existence of two kinds of environment in the SDS-PVP aggregates. In one of them, ESPT of 1-naphthol is totally suppressed leading to a long lifetime (5.3 ns) of the neutral emission. In the other environment, ESPT occurs on a 1.6 ns time scale. The critical association concentration (CAC) of SDS for the PVP-SDS system is 10 times lower than the CMC of SDS.