INTERACTION OF SURFACTANTS WITH HYDROPHOBICALLY MODIFIED POLY(N-ISOPROPYLACRYLAMIDES) .2. FLUORESCENCE LABEL STUDIES

The interactions between surfactants and pyrene-labeled derivatives of poly(N-isopropylacrylamide) (PNIPAM) have been examined by fluorescence measurements. The polymers are copolymers of N-isopropylacrylamide (NIPAM) and N-[14-(1-pyrenyl)butyl]-N-n-octadecylacrylamide (molar ratios 200:1 and 400:1, PNIPAM-C18Py/200 and PNIPAM-C18Py/400, respectively) or N-[4-(1-pyrenyl)butyl]acrylamide (molar ratio 200:1, PNIPAM-Py/200). Changes in the ratio of the intensity of the pyrene excimer emission (I(E)) to the intensity of the pyrene monomer emission (I(M)) were monitored in aqueous polymeric solutions as a function of surfactant concentration. In the case of PNIPAM-Py/200, association with surfactants occurred by a cooperative mechanism at a critical surfactant concentration lower than the critical micelle concentration (cmc) of the surfactants for each of the three surfactants investigated: sodium dodecyl sulfate (SDS), hexadecyltrimethylammonium chloride (HTAC), and n-octyl beta-D-thioglucopyranoside (OTG). Association of SDS and HTAC with the hydrophobically modified copolymers PNIPAM-C18Py/200 and PNIPAM-C18Py/400 took place by a noncooperative mechanism. Binding of surfactants to these polymers was detected at surfactant concentrations as low as 10(-5) mol L-1 and saturation occurred well below the cmc. On the other hand, binding of OTG to PNIPAM-C18Py/200 and PNIPAM-C18Py/400 occurred by a cooperative mechanism. The behaviors toward surfactants of PNIPAM-C18Py/200 and PNIPAM-Py/200 are compared to those of the corresponding unlabeled polymers, PNIPAM-C18/200 and PNIPAM, respectively, in terms of binding mechanism and structure of the polymer/surfactant aggregates.