Choudhury Rudra Prosad, Schönhoff Monika
Institut für Physikalische Chemie, Wesfälische Wilhelms-Universität Münster, D-48149 Münster, Germany.
J Chem Phys. 2007 Dec 21;127(23):234702. doi: 10.1063/1.2807239.
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.
采用脉冲场梯度核磁共振（PFG-NMR）技术研究了亚微米级空心聚合物胶囊胶体分散体中苯酚分子的分布和交换动力学。通过在二氧化硅颗粒上逐层组装聚电解质多层膜，随后溶解二氧化硅核来制备胶囊。在胶囊分散体中，苯酚的（1）H PFG回波衰减为单指数形式，这意味着苯酚在自由位点和胶囊结合位点之间快速交换。然而，从回波衰减中提取的表观扩散系数取决于扩散时间，这在快速交换极限情况下通常并非如此。我们将此归因于一种特殊情况，即观察到的表观扩散系数仅来自自由苯酚的信号，但受到与结合在胶囊上的分子交换的影响，这些分子表现出非常快的自旋弛豫。实际上，在存在胶囊的情况下，苯酚的弛豫速率显著增强，这表明苯酚与胶囊壁结合而非封装在内部。我们根据组合的扩散 - 弛豫模型进行了定量分析，即使在结合位点充当弛豫汇的这种特殊情况下，也可以通过扩散和自旋弛豫实验确定交换时间。分析结果得出了自由苯酚与结合在胶囊壁上的苯酚之间的交换时间，约为30毫秒，因此比扩散控制极限慢。从结合分数和自由分数中提取了苯酚在胶囊壁上的吸附等温线。讨论了结合机制和交换机制。本文介绍的扩散以及弛豫回波衰减的全局分析对于胶体系统或其他系统中的吸附动力学具有重要意义，在这些系统中，标准的扩散回波衰减分析因快速弛豫的边界条件而变得复杂。
