Pulsed field gradient NMR study of phenol binding and exchange in dispersions of hollow polyelectrolyte capsules

Abstract

The distribution and exchange dynamics of phenol molecules in colloidal dispersions of submicron hollow polymeric capsules is investigated by pulsed field gradient NMR (PFG-NMR). The capsules are prepared by layer-by-layer assembly of polyelectrolyte multilayers on silica particles, followed by dissolution of the silica core. In capsule dispersion, (1)H PFG echo decays of phenol are single exponentials, implying fast exchange of phenol between a free site and a capsule-bound site. However, apparent diffusion coefficients extracted from the echo decays depend on the diffusion time, which is typically not the case for the fast exchange limit. We attribute this to a particular regime, where apparent diffusion coefficients are observed, which arise from the signal of free phenol only but are influenced by exchange with molecules bound to the capsule, which exhibit a very fast spin relaxation. Indeed, relaxation rates of phenol are strongly enhanced in the presence of capsules, indicating binding to the capsule wall rather than encapsulation in the interior. We present a quantitative analysis in terms of a combined diffusion-relaxation model, where exchange times can be determined from diffusion and spin relaxation experiments even in this particular regime, where the bound site acts as a relaxation sink. The result of the analysis yields exchange times between free phenol and phenol bound to the capsule wall, which are on the order of 30 ms and thus slower than the diffusion controlled limit. From bound and free fractions an adsorption isotherm of phenol to the capsule wall is extracted. The binding mechanism and the exchange mechanism are discussed. The introduction of the global analysis of diffusion as well as relaxation echo decays presented here is of large relevance for adsorption dynamics in colloidal systems or other systems, where the standard diffusion echo decay analysis is complicated by rapidly relaxing boundary conditions.

The Stejskal–Tanner equation generalized for any gradient shape—an overview of most pulse sequences measuring free diffusion

“…[20], these can be combined into a single differential equation. During a pulse sequence, rf-pulses are applied to convert one type of magnetization into another.…”

“…In the case of chemical exchange between two or more states that is fast compared to the time over which diffusion is measured, an averaged value for the diffusion coefficient will be measured. Finally, in the case of intermediate exchange, the multiexponential decay will, besides the populations, provide information concerning the rate of the exchange process (19)(20)(21). Typical examples are the determination of the critical micellar concentration (CMC) of surfactants (11) or the binding of small molecules with micelles or vesicles (12)(13)(14).…”

“…Diffusion NMR has also been applied to the study of the kinetics of chemical reactions that occur slowly relative to the diffusion measurement (17,18). Finally, in the case of intermediate exchange, the multiexponential decay will, besides the populations, provide information concerning the rate of the exchange process (19)(20)(21).…”

Partitioning and Localization of Fragrances in Surfactant Mixed Micelles

“…PFG-NMR has already been used as a powerful tool for studies of binding and dynamics in various examples of complex fluids. These include surfactant solutions and microemulsions [26,27], liquid crystals and membranes [28], associating polymers [29], or colloidal encapsulation systems [30][31][32][33][34]. The approach to employ diffusion studies in solubilization problems was pioneered by Stilbs [35][36][37].…”

Suppressing magnetization exchange effects in stimulated-echo diffusion experiments

“…(1). Sometimes, this added complexity in diffusion NMR experiments is exploited as a valuable source of information, for example about the rate of chemical exchange [15][16][17][18][19][20][21][22][23][24][25][26][27]. If, however, the main interest is in obtaining accurate self-diffusion coefficients the effect is unwanted and appears as a source of errors.…”