Sodium 2-(N-dodecylamino)naphthalene-6-sulfonate as a probe of polymer-surfactant interaction

Abstract

Sodium 2-(N-dodecylamino)naphthalene-6-sulfonate (C_(12)NS) was used as an intrinsic fluorescence probe in investigations of the association of poly(N-isopropylacrylamide) (PNIPAAM) with surfactants in aqueous solution. The wavelength of maximum fluorescence emission from C_(12)NS shifts from 430 nm in water to 408 nm in 1-butanol; thus, the fluorophore may be used as a reporter of environmental polarity. C_(12)NS does not form micelles in water at room temperature but aggregates in PNIPAAM solutions at C_(12)NS concentrations ≥ 20 µM. The behavior of C_(12)NS is in this respect analogous to that of sodium n-hexadecyl sulfate, which exhibits a Krafft temperature in water of 31 °C. Aggregation of C_(12)NS in PNIPAAM solution is accompanied by a blue shift in the emission maximum to 424 nm, a shift similar to that observed when submicromolar concentrations of C_(12)NS are solubilized in sodium n-dodecyl sulfate micelles. Above the lower critical solution temperature (LCST) of PNIPAAM, the polymer-rich phase serves to solubilize even submicromolar C_(12)NS. At a surfactant concentration of 0.4 µM, the emission is shifted to 415 nm at temperatures above the LCST. This emission is assigned to monomeric surfactant bound to PNIPAAM, since in this concentration range the polymer is in molar excess. At C_(12)NS concentrations ≥ 10 µM, the emission maximum appears at 424 nm, identical with that observed at 10-fold higher C_(12)NS concentrations at room temperature. The 424-nm emission is assigned to polymer-bound C_(12)NS micelles, which form at lower surfactant concentrations above the LCST than below. Addition of increasing amounts of C_(12)NS elevates the LCST of PNIPAAM and causes broadening of the calorimetric endotherm associated with separation of the polymer-rich phase. C_(12)NS thus shows striking parallels to the sodium n-alkyl sulfates, but the behavior of C_(12)NS can be probed in substantially greater detail through examination of its fluorescence emission.