Interaction between pentaethylene glycol n-octyl ether and low-molecular-weight poly(acrylic acid)

The interaction between pentaethylene glycol n-octyl ether (C8E5) and low-molecular-weight poly(acrylic acid) (PAA, M_w = 2000) in aqueous solution has been investigated by various experimental techniques at constant polymer concentration (0.1% w/w) with varying surfactant molality. Spectrofluorimetry, using pyrene as molecular probe, shows (i) the formation of surfactant-polymer aggregates at a surfactant molality (T1) lower than the critical micelle concentration (cmc) of C₈E₅ in water and (ii) the formation of free micelles at a surfactant molality (T2) slightly higher than the cmc. Fluorescence quenching measurements indicate that the presence of PAA induces a lowering of the C₈E₅ aggregation number. Calorimetry confirms spectrofluorimetric evidence; in addition, it shows the presence of weak interactions below T₁ between monomeric surfactant molecules and the polymer chains. Tensiometry shows that, above T₁, only a low fraction of surfactant molecules interact with the polymer and that free micelle formation occurs before polymer saturation. The peculiarities of the interaction between surfactants and low-molecular-weight polymers have been discussed.