An NMR study of macromolecular aggregation in a model polymer-surfactant solution

Abstract

A model complex-forming nonionic polymer-anionic surfactant system in aqueous solution has been studied at different surfactant concentrations. Using pulsed-field-gradient diffusion NMR spectroscopy, we obtain the self-diffusion coefficients of poly͑ethylene glycol͒ ͑PEO͒ and sodium dodecyl sulfate ͑SDS͒ simultaneously and as a function of SDS concentration. In addition, we obtain NMR relaxation rates and chemical shifts as a function of SDS concentration. Within the context of a simple model, our experimental results yield the onset of aggregation of SDS on PEO chains ͑CAC= 3.5 mM͒, a crossover concentration ͑C 2 =60 mM͒ which signals a sharp change in relaxation behavior, as well as an increase in free surfactant concentration and a critical concentration ͑C m = 145 mM͒ which signals a distinct change in diffusion behavior and a crossover to a solution containing free micelles. C m also marks the concentration above which obstruction effects are definitely important. In addition, we obtain the concentration of SDS in monomeric form and in the form of free micelles, as well as the average number of SDS molecules in a PEO-SDS aggregate ͑N Aggr ͒. Taken together, our results suggests continuous changes in the aggregation phenomenon over much of the concentration but with three distinct concentrations that signal changes in the nature of the aggregates.

NMR investigations of self-aggregation characteristics of SDS in a model assembled tri-block copolymer solution

“…The a-CH 2 group is more susceptible to the extramicellar environment than CH 3 group, which resides in the core of SDS micelle. Hence, the chemical shift difference (Dd) between a-CH 2 and CH 3 is an indication of a change in the local environment of the SDS molecule [34]. The chemical shift difference between a-CH 2 and CH 3 of pure SDS is denoted by Dd 1 , whereas such difference in the presence of mixture-I and mixture-II are represented by Dd 2 and Dd 3 , respectively, and are plotted against SDS concentration depicted in Fig.…”

“…The diffusion coefficients thus obtained are translated into hydrodynamic radii and the corresponding R H 's for pure SDS are found to be 3.9 Å (unimers) and 44 Å (micelles). The former value (3.9 Å) corresponds to the hydrodynamic radius of all-trans N = 12 carbon chain of SDS molecule [34], whereas the latter (44 Å) represents the size of the SDS micelle. However, SDS exists in the form of unimers, mixed aggregates/micelles and free micelles in the presence of the polymer.…”

“…However, at higher concentrations of SDS, the polymer is saturated by SDS and the excess SDS monomers facilitate the secondary micelle formation at secondary micelle concentration (C 2 or cmc-II) [26,30,31]. There are a few combined NMR chemical shift, self-diffusion and spin-relaxation investigations on SDS-PEO systems, in addressing such polymersurfactant interactions [32][33][34]. The interpretation of such NMR data has been made using two-site exchange model [32], which reasonably provides the quantitative aspects of both unimer and micelle features.…”

Characterization of dynamics and internal structure of a mixed-surfactant wormlike micellar system using NMR and rheometry

“…In our previous work, 38 we have shown that mixed-species (polymer-surfactant) aggregates are easier to study than singlespecies (surfactant) aggregates because the dynamics of each species can be independently and simultaneously measured via complementary NMR experiments. This allows models of aggregate structure to be sufficiently constrained.…”

“…It has been shown to be a reasonable approximation to assume that the surface-associated water is essentially stationary in comparison to the bulk water. 38,47 Thus, water diffusion coefficient obtained as a function of surfactant packing fraction report directly on the fraction of surface associated water. Fig.…”