On the role played by hydrogen bonding between water soluble polyacids and surfactants on their micellization as a function of pH

Hydrogen bonds between carboxylic groups of hydrophilic polyacids and anionic moieties in surfactant headgroups may reduce the critical concentration above which surfactants aggregate provided the interaction is sufficiently strong. As undissociated acid groups are needed, the reduction should depend on the solution pH and acid concentration. Pyrene fluorescence was employed to investigate the impact that polyacrylic acid has on the critical concentration of two sulfonate containing surfactants, sodium dodecylbenzene sulfonate and lauryl sulfobetaine, as a function of pH and polyacid concentration. Non-ionic Tween80 was instead used to gauge if polyacid hydrophobicity plays any role. Monte Carlo simulations were also run to better understand the impact of changing composition variables. No change in Tween80 critical concentration was detected, suggesting that polyacrylic acid hydrophobicity has a negligible impact on aggregation modalities. A marked reduction of the critical aggregation concentration of the ionic surfactants was instead measured titrating solutions containing 0.05 and 0.2% (w/w) of polyacrylic acid, its impact found to be pH and concentration dependent. Albeit qualitatively agreeing with experiments and rationalizing the lowering of sodium dodecylbenzene sulfonate critical concentration at high pH, Monte Carlo results indicated the need for a model re-parameterization.