Ethyl(hydroxyethyl)cellulose and Some Model Uncharged Polymers: A Calorimetric Study of Their Interaction with Surfactants in Aqueous Solution

The interactions between uncharged polymers, e.g. ethyl(hydroxylethyl)cellulose (EHEC) and some model polymers, and ionic surfactants in aqueous solution has been studied by using isothermal titration and differential scanning calorimetry. We found that titration calorimetry could give quite detailed information about polymer - surfactant interactions. Values for the critical aggregation concentration cac and the saturation concentration C2 can be derived from the calorimetric titration curves. Surfactant aggregation in polymer solution shows clear similarities of the solubilization of small uncharged molecules (e.g. n-alkanols) in ionic micelles. Both anionic and cationic surfactants interact with EHEC (especially the lower cloud point ones) and PPO while the alkyltrimethylammonium halides are indifferent (or almost) to PEO and PVP which interact strongly with SDS. Preaggregation between surfactant monomers and, particularly, the more hydrophobic polymers can be significant as indicated by substantial endothermic enthalpy contributions before the cac from hydrophobic pair-wise interaction. In some cases, there is a non-cooperative binding of surfactant monomers to existing hydrophobic microdomains or simply to the hydrophobic backbone of the polymer in the form of pair-wise interaction. Such kind of non-cooperative binding can easily be observed on addition of surfactants to solutions of hydrophobically modified polymer. The polymer hydrophobic tails act as nucleation sites and the surfactants bind to them to form mixed micelles. If the surfactant interacts with the parent polymer (unmodified one), coperative aggregation will take place above the cac. Differential scanning calorimetry combined with investigation of phase behavior, gave information about the formation of thermal reversible gels in systems of EHEC and the ionic surfactants SDS and CTAC. The endothermic peaks in the DSC traces of pure aqueous solution of gelling EHEC indicate that temperature-induced polymer - polymer association through hydrophobic interaction precedes phase separation. Added surfactant will preferably bind in hydrophobic microdomains developed through polymer association. At low surfactant concentration crosslinks are formed and a polymer network develops upon warming. The electrostatic repulsion between the surfactant headgroups arrests the phase separation and a gel is formed. Addition of 4 mol/kg NaCl drastically reduces the clouding temperature at low surfactant concentration but the region for gel formation is almost unchanged. DSC traces of samples with and without NaCl look about the same. Only the temperature-induced association of the polymer needed for gel formation is disclosed by DSC. The temperature dependent association of the polymer thus seems to be needed for gelation, but not for phase separation and these two phenomena could be separated by the addition of electrolyte