INTERACTIONS OF SURFACTANTS WITH HYDROPHOBICALLY MODIFIED POLY(N-ISOPROPYLACRYLAMIDES) .1. FLUORESCENCE PROBE STUDIES

The interactions between surfactants and copolymers of N-isopropylacrylamide (NIPAM) and N-n-alkylacrylamides (n-alkyl = decyl, tetradecyl, and octadecyl groups; NIPAM to n-alkylacrylamide molar ratios, 100:1 and 200:1) have been examined by fluorescence measurements with two hydrophobic probes: pyrene and bis(1-pyrenylmethyl) ether (Dipyme). Strong association between the copolymers and the charged surfactants sodium dodecyl sulfate (SDS) and hexadecyltrimethylammonium chloride (HTAC) occurs by partition of the surfactants in a noncooperative mechanism. The interactions between the neutral surfactants n-octyl beta-D-glucopyranoside (OG) and n-octyl beta-D-thioglucopyranoside (OTG) take place by a cooperative mechanism at a critical aggregation concentration (cac) well below the critical micelle concentration (cmc) of the surfactants. The association results in the formation of mixed clusters consisting of the n-alkyl subsituents of the polymers surrounded by surfactant molecules (ca. 30 per n-alkyl substituent for the C8 and C-12 surfactants and ca. 15 for the C-16 surfactant). The values of the ratio I(E)/I(M) of pyrene monomer emission (intensity I(M)) to pyrene excimer emission (intensity I(E)) of Dipyme in aqueous solutions of copolymers, surfactants, and surfactant/ polymer mixtures indicate that the mixed clusters are more rigid than the corresponding surfactant micelles.