Interaction of poly(N-isopropylacrylamide) with sodium n-alkyl sulfates in aqueous solution

Cloud point, calorimetric, and fluorescence probe methods have been combined to examine aqueous mixtures of poly(N-isopropylacrylamide) (PNIPAAM) and a series of sodium n-alkyl sulfates of alkyl chain lengths (n) in the range 1-16. Surfactants of chain lengths ≤ 4 depress the lower critical solution temperature (LCST) of PNIPAAM and exhibit no evidence of enhanced aggregation in PNIPAAM solutions. This pattern of behavior is characteristic of the simple salt Na_2SO_4 as well. Surfactants of intermediate chain lengths (n = 5-10) depress the LCST at low surfactant concentration but cause an increase in the LCST at concentrations that exceed the critical aggregation concentration (CAC). Sodium n-dodecyl sulfate (n = 12) elevates the LCST even at low concentrations and forms aggregates in PNIPAAM solutions at a concentration 10-fold lower than the critical micelle concentration (cmc). Sodium n-hexadecyl sulfate forms aggregates in PNIPAAM solutions at temperatures below its Krafft temperature in water. The formation of polymer-bound micelles below the cmc is reported by pyrene, 1-pyrenecarboxaldehyde, and sodium 2-(N-dodecylamino)naphthalene-6-sulfonate but not by 1-benzoylacetone. The theory of Nagarajan and Ruckenstein, and in particular the treatment of Ruckenstein and co-workers, serves to rationalize the observed aggregation behavior as a consequence of screening of surfactant-water interaction by PNIPAAM chain units at the micellar surface