Fluorine magnetic resonance and equilibrium dialysis study of the binding of sodium 12,12,12-trifluorododecylsulfate by polyethylene glycol

Abstract

Fluorine chemical shifts have been measured for sets of solutions containing fixed amounts of polyethylene glycol and variable concentrations of the anionic surfactant sodium 12,12,12trifluorododecylsulfate. Equilibrium dialysis experiments with similar solutions were used to determine the binding isotherm. Polymer samples with nominal average molecular weights of 7000 and 20 000 exhibit identical binding behavior, but there is little or no binding when the molecular weight is 1500. For the samples of higher molecular weight, binding occurs only when the surfactant concentration exceeds 9.6 m M, or about two-thirds of the critical micelle concentration, 14.6 m M. The number of moles of surfactant bound per gram of polymer reaches a maximum near 7.7, but the data suggest that this is only about 90% of the amount required to saturate the polymer. Micellization then intervenes to block the further increase in the free detergent concentration that would be needed to produce an increase in the amount bound. The chemical shift of the trifluoromethyl groups of the bound anions is independent of the amount bound and is essentially equal to the shift of micellized anions. The results may be rationalized using a model involving the binding of approximately 15 anions in one step to form micelle-like clusters each stabilized by an “effective segment” of the polymer chain with a molecular weight of about 1800. The unitary free energies of binding and micellization, computed with similar simplifying assumptions, are −5.07 and −4.94 kcal/mole of detergent ions, respectively.