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.

“Relaxometry” experiments and analysis of dispersion curves: An illustrative example of toluene in liquid and in organogel phases

“…This appears to be reasonable for such organogels (12). 2 and 4), the plateau value for ''in'' toluene being assumed to be the one of pure toluene multiplied by (1 À p).…”

“…In a previous study (12), we reached the conclusion that toluene could be inside the gel structure (this will be denoted ''in'' toluene) or in the isotropic phase, that is the liquid phase which coexists with the gel structure (this will be denoted ''out'' toluene). p will denote the proportion of ''in'' toluene.…”

Bile acid alkylamide derivatives as low molecular weight organogelators: Systematic gelation studies and qualitative structural analysis of the systems

“…6). Even though there are studies showing that the interactions between the solvent and the gelator are practically non-existent when the solvent is included in the xerogel solid structure [90], the chlorobenzene molecules may affect on or direct the packing of 1 in the process of formation of the microstructure [91]. As can be seen, the composition or structure of a xerogel or a crystalline sample is not straightforward and oversimplifications should be avoided.…”

Gels with sense: supramolecular materials that respond to heat, light and sound

“…[134][135][136] Supramolecular gels may also exhibit swelling phenomena. 134 However, gel-solvent interactions are commonly (with some exceptions 137 ) important for the stability of the fibrous network, 76 so removal of even a fraction of the solvent may lead to destruction of gel-like properties. For example, Kiyonaka et al found that heating a hydrogel of 7a to 65 o C produces reversible shrinkage with visible expulsion of water, but heating to 69 o C results in the complete loss of solvent to give a white precipitate (Fig.…”