Self-diffusion of small molecules in colloidal systems

Abstract

Abstract:The self-diffusion of small molecules in colloidal systems is calculated using the cell model to describe the effect of varying concentration of colloidal particles. The relevant boundary conditions are found using arguments from the thermodynamics of irreversible processes. From a general description of the self-diffusion in systems with spherically symmetrical particles we derive expressions for the concentration dependence of the effective self-diffusion coefficient De~t for several cases of practical importance. It is shown that when the molecule studied is strongly attracted to the particle a minimum in D ~ff is expected around volume fraction 4~ = 0.35. It is also shown that the often made distinction between free and bound molecules is often problematic and a more general description is proposed. The obstruction effect generated by the excluded volume is discussed both for spherical and spheroidal systems. It is pointed out that the often used formula due to Wang ((1954) J Amer Chem Soc 76:4755) is incorrect for self-diffusion and for the obstruction factor for spheres we obtain (1 + 0.5 ~)-1. This expresion is tested both by experiments on water diffusion in systems containing latex particles and through computer simulations and it is found valid over a wide concentration range. For prolate ellipsoids the obstruction factor is not greatly different from that for spheres, while for oblate aggregates the limking obstruction factor of 2/3 can be obtained at low concentrations. It is demonstrated that this effect can be used to distinguish between different aggregate shapes. It is also shown that the disorder present in a solution of colloidal particles leads to a decrease in the obstruction effect.

On the dissolution of cellulose in tetrabutylammonium acetate/dimethyl sulfoxide: a frustrated solvent

“…(The hydroxide anion could not be observed separately as the signal from its exchangeable proton merges with the water signal.) What little dependence was observed, a few percent, could be accounted for by the excluded volume of the cellulose molecules (Jönsson et al 1986). Here, in contrast, the diffusion coefficient deviates from the cellulose-free value by as much as tens of percent, which is incompatible with the excluded volume explanation.…”

Macromolecular crowding studies of amino acids using NMR diffusion measurements and molecular dynamics simulations

“…[66], fit to the experimental data well. Also shown is the fitting of the isodesmic (i.e., Equation 11) aggregation model ( ) [65] to the experimental data. The measured diffusion coefficients are lower than those predicted by the obstruction models which imply that an additional process such as aggregation is likely to be present.…”

“…The obstruction factor depends on the shape, volume fraction, and spatial and orientation distribution of the obstructing molecules [48,[65][66][67]. The simple obstruction models derived to date assumed that all of the obstructing particles have the same size and shape (generally taken to be spheres) and are only valid at very low concentrations [48,[65][66][67][68][69][70][71].…”

“…A number of different obstruction models are used to simulate obstruction in glycine solution as a function of concentration in Figure 4. Jönsson et al [65] derived the obstruction factor for a system containing evenly spaced monodisperse spherical particles in a cell including the binding effects from solvent molecules [72]. Lekkerkerker and Dhont [71], on the other hand, provided the obstruction factor using the MSD of interacting Brownian particles.…”

Aplicação de modelos simples em fenômenos envolvendo monocamadas e micelas

“…Simulação computacional (Linse, 1989), dados de ressonância magnética nuclear (Wong et al, 1977) e reconstrução da curva de decaimento de fluorescência de sonda prototrópica (Kotlyar et al, 1994;Shimoni et al, 1993) indicam que o coeficiente de auto-difusão para íons em uma microcavidade deve ser menor do que em água bulk devido às restrições espaciais e orientação dos dipolos. Dentro do modelo apresentado, ainda é possível fazer uma estimativa de quanto o coeficiente de auto-difusão de uma espécie iônica diminui em função de W 0 via perturbação da equação de DebyeSmoluchowski em termos de um campo externo (Jönsson et al, 1986;Bell, 1964 O abaixamento da constante dielétrica faz a interação média partícula-micela reversa mais atrativa prendendo por mais tempo a espécie iônica na vizinhança da interface pool aquoso-AOT. Com o aumento de W 0 , aumenta a sensibilidade da razão entre coeficientes de auto-difusão com a mudança da constante dielétrica; é provável, porém, que seja somente uma característica do modelo de partícula puntiforme em dielétrico.…”