Hydrophobically modified sodium polyacrylates in aqueous solutions: Association mechanism and characterization of the aggregates by fluorescence probing

The association mechanism of several hydrophobically modified sodium polyacrylates (HMPA) in aqueous solutions has been investigated by fluorescence probing with pyrene as the fluorescent probe, by electrical conductivity, and by C-13 NMR. The investigated HMPAs differed by the molecular weight of the poly(acrylic acid) precursor, the degree of hydrophobe substitution, and the carbon number of the hydrophobe (alkyl chain). The results showed that the association of HMPAs is less cooperative than the micellization of conventional surfactants and that there is a concentration below which no micelle-like aggregates are present in the systems. C-13 NMR studies showed that only a fraction F-aggr, of hydrophobes is involved in the formation of hydrophobic microdomains, and that F-aggr increases with the hydrophobe length and degree of substitution. The combination of the values of F-aggr with the hydrophobic microdomain concentration, obtained from steady state and time-resolved fluorescence quenching measurements, allowed the determination of the average number of alkyl chains making up a hydrophobic microdomain. The polydispersity of the aggregates was estimated from time-resolved fluorescence quenching measurements. These characteristics of the HMPA microdomains were compared to those of micelles of conventional surfactants and of microdomains present in systems of other amphiphilic polymers.