Titration calorimetric study of the interaction between ionic surfactants and uncharged polymers in aqueous solution

The interaction between uncharged polymers (PEO, PPO, PVP, and EHEC) and ionic surfactants in dilute aqueous solution has been studied using isothermal titration microcalorimetry. The surfactants studied were lithium, sodium, and magnesium dodecyl sulfate, alkyltrimethylammonium halides (RTA(+)X(-) with R equal to C-12, C-14, and C-16 and X either Br or Cl), and dodecylammonium chloride (DoAC). The critical aggregation concentration cac, the saturation concentration C-2 and the amount of polymer-bound surfactant were derived from the calorimetric titration curves. The anionic surfactants interacted strongly with PEO while RTABr were indifferent. However, changing the counterion to chloride or modifying the headgroup to the unsubstituted ammonium ion gave observable interaction, The presence of extra salt lowered the cac and increased the C-2 with a more pronounced effect for PPO and EHEC. An increase in temperature had no noticeable effect on cac and C-2 although the shape of the calorimetric titration curves changed drastically. PPO 1000 showed both small molecule behavior and polymer character in the interaction with SDS. Due to the hydrophobicity of the backbone, PVP showed significant interaction with SDS well below the cac. PVP showed no affinity to the cationic RTAX surfactants.