INTERACTION OF POLY(N-ISOPROPYLACRYLAMIDE) WITH SODIUM NORMAL-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 less-than-or-equal-to 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 Na2SO4 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.