Probing interactions within complex colloidal systems using PGSE‐NMR

Abstract

Pulsed-gradient spin-echo NMR (PGSE-NMR) has evolved into a powerful technique for probing colloidal structure, particularly that of complex mixtures. Indeed, with the exception perhaps of neutron scattering, no single technique has such universal applicability. The parameter of interest, the (self-) diffusion coefficient, is of fundamental importance as it is determined by both the chemistry of the system -the size and shape of the individual molecular species -and its physical environment/state -dissolved, dispersed, aggregated, constrained within boundaries, etc. Understanding such behaviour is of central importance in the biological and colloidal fields. The basis of using diffusion as a probe of such systems relies on the fact that each species in a mixture can be separated by differences in their diffusion coefficient and/or by the chemical specificity of NMR. Binding, aggregation and structure elucidation studies have undergone a renaissance in the last decade owing to the increased commercial availability of hardware and improved data processing. This paper highlights just a few of the many and diverse applications to which PGSE-NMR can be put.

Cationic Porphyrin–Anionic Surfactant Mixtures for the Promotion of Self‐Organized 1:4 Ion Pairs in Water with Strong Aggregation Properties

“…Figure 5s hows the 1 HNMR spectra of the surfactant (Figure 5A), the "free" H 2 TMPyPCl 4 porphyrin ( Figure 5B), and the mixture of porphyrin with 5mol equivalents of surfactant (Figure 5C) xperiments were performed to measuret he diffusion coefficient of the objects present in solution and to estimate their hydrodynamic radius. [48] In the case of H 2 T(C18) 5 (porphyrin associated with 5mol equivalents of surfactant) in D 2 O, the diffusionc oefficient was 6.0 AE 0.3 10 À11 m 2 s À1 .T his diffusion coefficient was the same when calculated from the signals of the porphyrin or from those of the surfactant. Thisv alue was significantly lower than that calculated for the free porphyrin (2.4 AE 0.3 10 À10 m 2 s À1 ;F igure 5B)a nd the free surfactant (3.4 AE 0.3 10 À10 m 2 s À1 ,m easured at 50 8C; Figure 5A), thus supporting the presence of as trong association of both compounds forming the ionp air in the aggregates.T he calculation of the hydrodynamic diameter from the diffusion coefficient by using the Stockes-Einstein relation, [49] resulted in ad iameter of 8nm, which was much smaller than the mean diameter of aggregates measured by DLS (d = 170 nm at 100 mm).…”

Micelle–Vesicle Transition of Fatty Acid Based Ion‐Pair Surfactants: Interfacial Evidence and Influence of the Ammonium Counterion Structure

“…To confirm, in the case of NbC14, the formation of micelles followed by a lyotropic transition to another kind of objects in the bulk, we measured the self-diffusion coefficient by pulsegradient spin-echo NMR spectroscopy experiments (PGSE-NMR). This technique has been widely employed for surfactant studies [28][29][30] and allows measurements at very low concentrations. Nevertheless, even with a cryoprobe, concentrations lower than 10 À5 m remained unreachable.…”

Probing interactions by means of pulsed field gradient nuclear magnetic resonance spectroscopy

“…Griffiths et al also investigated the interactions between SDS and a self-assembling copolymer of vinyl-pyrollidone (VP) and vinyl-acetate (VA). [59] Diffusion coefficient of SDS monotonically decreases as SDS concentration increases, while no significant variations were observed for the D of copolymer. Consequently either the polymer and polymer-surfactant complexes have a similar dimension or the binding of SDS does not change the polymer's structure.…”