Results from Brownian dynamics simulations of hard sphere diffusion in polymer networks of wormlike chains are presented. The influence of the size of the diffusing particles, the polymer concentration, the polymer radius, and the persistence length of the chains were investigated. The diffusion coefficients obtained in systems of stiff chains were compared to a theory [Macromolecules 24, 6024 (1991)]. The agreement between the simulation data and the theory was satisfactory, except for dense systems where the theory predicted somewhat too high diffusion coefficients. To account for the diffusion in systems of more flexible polymers, we derived a semiphenomenological theory based on ideas from the concept of fractals. By this approach we could show how the available volume fraction for particles in polymer networks scales with the particle radii. The scaling is not a power law, but instead is a stretched exponential, and is related to the local structure of the polymer chains. Finally, the inherent assumptions in the theoretical descriptions, e.g., the negligence of the hydrodynamic interactions, and the applicability of the theory to real polymer systems, are thoroughly discussed.

The effects of polydispersity on fluorescence quenching in micelle systems are discussed theoretically. Predictions are tested on computer generated fluorescence decay data for quenching in polydisperse micelle systems. It is shown that the estimated parameters, i.e, aggregation numbers and rate constants, are highly dependent on the micelle size distribution, and on the dynamic properties of the solubilizate–micelle system. Two extreme cases are considered: a static one, where no size changes of the micelles occur during the residence time, and a dynamic one where very large size variations take place. In the static case, the estimated aggregation number will decrease with quencher concentration from the weight average aggregation number in the limit of zero quencher concentration. In the dynamic case, the number average aggregation number is obtained, independent of quencher concentration. The initial decay constant contains information about the aggregation numbers, and in the generated data, this could be used to indicate the presence of polydispersity. Data from a number of experiments with the single photon counting technique are presented and analyzed according to the presented theory.

New data on the permeabilities of hydrophilic markers in two commonly used in vitro models, i.e., excised intestinal segments from the rat and monolayers of Caco-2 cells, are presented. The results are compared to human in vivo data. Two groups of hydrophilic marker molecules were tested: (1) monodisperse polyethylene glycols of molecular weights ranging from 194 to 502 g/mol and (2) a heterogeneous group of molecules consisting of urea, creatinine, erythritol, and mannitol (60-182 g/mol). The permeabilities of the marker molecules showed a nonlinear dependence on the molecular weight and decreased in the order rat ileum > rat colon > Caco-2 cells. Surprisingly, the polyethylene glycols permeated more easily than the other marker molecules, indicating that characteristics other than molecular weight, e.g., the flexibility of the structure, may also be important for permeation through the membrane. Comparisons with the published permeability profiles of polyethylene glycols in human intestinal segments in vivo (i.e., calculated permeability coefficients as a function of molecular weight) indicate that the human intestine is more permeable than the in vitro models. However, the permeability profiles of the corresponding segments in the human intestine and the in vitro models were comparable. Thus, good correlations were established between permeabilities of the human ileum and rat ileum and between those of human colon, rat colon, and the Caco-2 cells. We conclude that the paracellular absorption in humans can be studied mechanistically in these in vitro models.

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