Aggregation behavior of mono-endcapped hydrophobically modified poly(sodium acrylate)s in aqueous solution

Titration microcalorimetry and steady-state fluorescence spectroscopy have been used to study the aggregation of monoendcapped hydrophobically modifie d poly(sodium acrylate)s in aqueous solution. Polymers with molecular weigh ts varying between 800 and 31,700 were synthesized by radical polymerizatio n using an initiator and chain transfer agent. The resulting polymers form hydrophobic microdomains in aqueous solutions. The following conditions wer e applied: no salt and pH 5 and 9, respectively; 1 M sodium citrate and pH 9. At pH 5 the critical aggregation concentration (CAC, the concentration a t which microdomains are formed) increases with increasing molecular weight of the polymers, The concentration range for aggregation is about 0.2-2.4 mM. At pH 9 the carboxylic acid groups are deprotonated and electrostatic r epulsions are introduced; therefore the concentration for aggregation rises to about 80 mM, Interestingly, in case of polymers having M-n > 1400 the C AC decreases with increasing molecular weight due to a counterion-concentra tion gradient toward the hydrophobic microdomain. Near the microdomain the counterion binding is increased, reducing the electrostatic repulsions and allowing for lower aggregation concentrations. In the presence of 1 M sodiu m citrate this anomalous trend is suppressed to a large extent; since the o verall counterion binding is increased and the CAC is lower. The concentrat ion for aggregation is then in the same range as at pH 5 in the absence of salt. (C) 2000 Academic Press.