NMR and ESR Studies of Dibutylphosphate Micellar Aggregates

Abstract

Sodium dibutylphosphate micellar solutions were investigated by ³¹P, ¹³C and ²³Na NMR as well as by ESR, using VO²⁺ as paramagnetic probe, the CMC being determined by conductometry. The aggregation number of DBP⁻ was estimated from the reorientation correlation time of the vanadyl probe bound to the surface of micelles. The value of N = 11 ± 2 at 300 K was confirmed by the concen-tration dependence of the ³¹P longitudinal relaxation at 36.5 and 111.7 MHz. The temperature dependence of the CMC and of the aggregation number were interpreted in terms of size distribution and area per polar head assuming a spherical or cylindrical shape of the micelles. The conformations and dynamics of DBP⁻ molecules in the aggregates were investigated by ³¹P and ¹³C relaxation in diamagnetic solutions or in the presence of paramagnetic Ni²⁺ ions bound to the micellar surface. This procedure allows the determination of the trans and gauche populations of the oxyalkyl chains as well as the reorientation correlation time of DBP⁻ molecules and the rates of segmental motions in the non polar core of the micelles.