NMR investigation of the micellar properties of monoalkylphoshpates

The micellization of several monoalkylphoshpates with chain lenghts from 6 to 10 carbons have been investigated by³¹P and¹³C NMR spectroscopy with some additional electrical conductivity, density and small angle neutron scattering measurements.The major part of this study was performed on sodium monooctylphosphate. The critical concentration of micellization is 0.14 M, the partial molar volume is 170 cm³/mole in the micellar state. Small angle neutron scattering measurements on a 0.5 molar solution lead to an aggregation number of 43. NMR studies over the whole concentration range show that the pseudophase model holds. The geometry around the polar head is found almost unchanged upon micellization. The frequency denpendence of the³¹P longitudinal relaxation is interpreted by an isotropic fas motion ( ^R=6.5 × 10^–11 for the free monomers and by a strongly anisotropic motion withD /D 10 and ₀=(6D)^–1 =1.25×10^–9 s in the micellar state.