NMR Study of Molecular Motions in the Mesophases of Potassium Laurate-D2O System

In the Presence of water amphiphilic molecules display a large number of phases. As shown by X-ray studies the structures of these phases are characterized by a large scale periodic organization oa aqueous and paraffinic media. Moreover, in the mesophases, these same X-ray diagrams show that his long range order coexists with a short range disorder in the paraffinic chains moiety. Previous studies of the NMR line shape of paraffinic protons had suggested this disorder to be dynamical. We present and discuss here pulsed NNR measurements, performed on the paraffinic protons of potassium laurate-D2O samples, in the lamellar mesophase, in order to gain a better knowledge of the details of the motions involved. The complex shape of the free precession decay can approximately be analysed into “solid-like” (gaussian) and “liquid-like” (exponential) components. This suggests that the motion is not uniform along the paraffinic chain: protons close to the polar head are less mobile than those at the end of the chain. The first experience residual dipolar interactions (measured by the second moments of the gaussian signals) while these are averaged out for the second which exhibits a transverse relaxation time T2 (measured by the time constant of the exponential signal). Despite the complexity of the free precession decay the measured value of relaxation time T1 is the same for all the protons and is frequency independent. The complex shape of the free precession decay can approximately be analysed into “solid-like” (gaussian) and “liquid-like” (exponential) components. This suggests that the motion is not uniform along the paraffinic chain: protons close to the polar head are less mobile than those a t the end of the chain. The first experience residual dipolar interactions (measured by the second moments of the gaussian signals) while these are averaged out for the second which exhibits a transverse relaxation time Ta (measured by the time constant of the exponential signal). Despite the complexity of the free precession decay the measured value of relaxation time T1 is the same for all the protons and is frequency independent. Second moments and relaxation data indicate that the surfactant molecules undergo rapid deformations (τR ∼ 10−9 sec.) about the C-C bonds which (modulating intramolecular interactions) are responsible for T1 and that a slow molecular diffusion (τS, ∼ 10−6 sec.) over macroscopic distances is responsible for T2, averaging out intermolecular interactions.