On the interaction of sodium dodecyl sulfate with oligomers of poly(ethylene glycol) in aqueous solution

Isothermal calorimetric titrations of aqueous solutions of poly(ethylene glycol) (PEG) with sodium dodecyl sulfate (SDS) are known to exhibit a peculiar trend consisting of endothermic and exothermic effects. This behavior was explained with the formation of two different mixed micellar aggregates, one characterized by hydrophobic interactions and the second by ion-dipole association. Present NMR measurements on C-13, H-1, and Na-23 nuclei do not support the formation of a number of PEG-SDS aggregates characterized by interactions of different nature. Our data are rather in accordance with the initial formation, at low surfactant concentration, of a polymer-surfactant aggregate in which the polymeric chain assumes a strained conformation in order to bind a small micellar cluster. The subsequent growing of the aggregate with increasing surfactant concentration allows the polymer to relax to a more, expanded, energetically favored, conformation. Further calorimetric titrations with a set of PEG samples of different molecular weight (200 to 20000 Daltons) allowed to establish a few points so far unclear. The minimum molecular weights necessary for observing the onset and the settling, respectively, of polymer-surfactant interaction were identified and the characteristic multiple peak curve of the titration of the polymer with molecular weight of 8000 Dalton was found related to the discrete binding of two successive SDS micellar clusters on the same polymeric chain.