title: Solvent dynamical behavior in an organogel phase as studied by NMR relaxation and diffusion experiments.
abstract: An organogelation process depends on the gelator-solvent pair. This study deals with the solvent dynamics once the gelation process is completed. The first approach used is relaxometry, i.e., the measurement of toluene proton longitudinal relaxation time T(1) as a function of the proton NMR resonance frequency (here in the 5 kHz to 400 MHz range). Pure toluene exhibits an unexpected T(1) variation, which has been identified as paramagnetic relaxation resulting from an interaction of toluene with dissolved oxygen. In the gel phase, this contribution is retrieved with, in addition, a strong decay at low frequencies assigned to toluene molecules within the gel fibers. Comparison of dispersion curves of pure toluene and toluene in the gel phase leads to an estimate of the proportion of toluene embedded within the organogel (found around 40%). The second approach is based on carbon-13 T(1) and nuclear Overhauser effect measurements, the combination of these two parameters providing direct information about the reorientation of C-H bonds. It appears clearly that reorientation of toluene is the same in pure liquid and in the gel phase. The only noticeable changes in carbon-13 longitudinal relaxation times are due to the so-called chemical shift anisotropy (csa) mechanism and reflect slight modifications of the toluene electronic distribution in the gel phase. NMR diffusion measurements by the pulse gradient spin-echo (PGSE) method allow us to determine the diffusion coefficient of toluene inside the organogel. It is roughly two-thirds of the one in pure toluene, thus indicating that self-diffusion is the only dynamical parameter to be slightly affected when the solvent is inside the gel structure. The whole set of experimental observations leads to the conclusion that, once the gel is formed, the solvent becomes essentially passive, although an important fraction is located within the gel structure.
authors.affiliation: Méthodologie RMN, CRM2 (UMR 7036, UHP-CNRS), Faculté des Sciences et Techniques, B.P. 239, 54506 Vandœuvre-lès-Nancy (Cedex), France.
authors.forename: Mehdi
authors.initials: M
authors.lastname: Yemloul
authors.forename: Emilie
authors.initials: E
authors.lastname: Steiner
authors.forename: Anthony
authors.initials: A
authors.lastname: Robert
authors.forename: Sabine
authors.initials: S
authors.lastname: Bouguet-Bonnet
authors.forename: Florent
authors.initials: F
authors.lastname: Allix
authors.forename: Brigitte
authors.initials: B
authors.lastname: Jamart-Grégoire
authors.forename: Daniel
authors.initials: D
authors.lastname: Canet