SODIUM 2-(N-DODECYLAMINO)NAPHTHALENE-6-SULFONATE AS A PROBE OF POLYMER SURFACTANT INTERACTION

Sodium 2-(N-dodecylamino)naphthalene-6-sulfonate (C12NS) 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 C12NS shifts from 430 nm in water to 408 nm in 1-butanol; thus, the fluorophore may be used as a reporter of environmental polarity. C12NS does not form micelles in water at room temperature but aggregates in PNIPAAM solutions at C12NS concentrations >20 MM. The behavior of C12NS is in this respect analogous to that of sodium rc-hexa-decyl sulfate, which exhibits a Krafft temperature in water of 31°C. Aggregation of C12NS 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 C12NS are solubilized in sodium n-dodecyl sulfate micelles. Above the lower critical solution temperature (LOST) of PNIPAAM, the polymer-rich phase serves to solubilize even submicromolar C12NS. 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 C12NS concentrations ≥10 μM, the emission maximum appears at 424 nm, identical with that observed at 10-fold higher C12NS concentrations at room temperature. The 424-nm emission is assigned to polymer-bound C12NS micelles, which form at lower surfactant concentrations above the LCST than below. Addition of increasing amounts of C12NS elevates the LCST of PNIPAAM and causes broadening of the calorimetric en-dotherm associated with separation of the polymer-rich phase. C12NS thus shows striking parallels to the sodium n-alkyl sulfates, but the behavior of C12NS can be probed in substantially greater detail through examination of its fluorescence emission. © 1990, American Chemical Society. All rights reserved.