Interactions in Water of Alkyl and Perfluoroalkyl Surfactants with Fluorocarbon- and Hydrocarbon-Modified Poly(N-isopropylacrylamides)

摘要

Fluorescence spectroscopy and isothermal titration calorimetry (ITC) have been used to study the interactions in water at 25 deg C of two anionic surfactants-sodium dodecyl sulfate (SDS) and sodium perfluorononanoate (SPFN)-with various pyrene-labeled hydrophobically modified poly(N-isopropyl-acrylamides) (HM-PNIPAM) grafted at random with small amounts of fluorocarbon chains (~1H,~1H-perfluorooctyl,CH_2C_7F_(15));(PNIPAM-F),or (n-octadecyl,C_(18)H_(37)) (PNIPAM-HPy) or both (PNIPAM-F/ HPy).In aqueous solution,the copolymers form micellar structures consisting of a loose corona of hydrated poly(N-isopropylacrylamide) chains and a hydrophobic core rich in hydrocarbon or fluorocarbon groups.From fluorescence studies based on changes in the ratio of pyrene excimer to monomer emission intensity,it has been established (1) that mixed SDS/C_(18)H_(37) clusters form along the polymer chain upon addition of SDS to either PNIPAM-HPy or PNIPAM-F/HPy and (2) that SPFN does not interact with the hydrocarbon-rich microdomains of the polymeric micelles.The conclusions were corroborated by ITC experiments,which yield the overall enthalpy change associated with polymer/surfactant interactions.They provided strong evidence (1) that SDS molecules adsorb along the PNIPAM main chain but do not mix with the fluorocarbon-rich microdomains of PNIPAM-F or PNIPAM-F/HPy and (2) that SPFN associates with the perfluorocarbon substituents of PNIPAM-F and PNIPAM-F/HPy but has a poor affinity for the polymer chain.