Fickian and thermal diffusion coefficients of binary mixtures of isobutylbenzene and n-alkanes at different concentrations from the optical beam deflection technique.

We report the Fickian diffusion (D12), thermal diffusion (DT), and Soret (ST) coefficients of 4 binary mixtures of isobutylbenzene (IBB) and n-alkanes (n-hexane, n-octane, n-decane, and n-dodecane) at 298.15 K and atmospheric pressure. The concentration is varied in the whole range. The Optical Beam Deflection technique is used in the measurements. We first verify our measurements with published data. The concepts of molecular similarity and mobility are invoked to investigate D12 and DT dependency on molecular weight and concentration. Our analysis reveals a combined effect of molecular mobility and similarity dependency of DT on concentration and molecular weight of the n-alkanes. The mobility of individual molecules describes the D12 dependency on concentration and molecular weight of alkanes. The dependency of D12 on concentration weakens as the n-alkane molecular weight increases. DT increases with IBB concentration for nC6 and nC8 and decreases with IBB concentration for nC10 and nC12. In this work, we demonstrate that the temperature contrast factors can be accurately estimated without the use of an interferometer.

Guest diffusion in interpenetrating networks of micro- and mesopores.

Pulsed field gradient NMR is applied for monitoring the diffusion properties of guest molecules in hierarchical pore systems after pressure variation in the external atmosphere. Following previous studies with purely mesoporous solids, also in the material containing both micro- and mesopores (activated carbon MA2), the diffusivity of the guest molecules (cyclohexane) is found to be most decisively determined by the sample "history": at a given external pressure, diffusivities are always found to be larger if they are measured after pressure decrease (i.e., on the "desorption" branch) rather than after pressure increase (adsorption branch). Simple model consideration reproduces the order of magnitude of the measured diffusivities as well as the tendencies in their relation to each other and their concentration dependence.

Tracing pore-space heterogeneities in X-type zeolites by diffusion studies.

Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.

Paramagnetic relaxation enhancement (PRE) as a tool for probing diffusion in environmentally relevant porous media.

The transport diffusivity of the paramagnetic molecule 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) was measured by monitoring its influence on the NMR transverse relaxation time (T2) on surrounding water protons - also known as paramagnetic relaxation enhancement (PRE). Due to the nature of the PRE effect, few paramagnetic molecules are able to simultaneously reduce the T2 of many NMR active nuclei, which represents a significant gain in sensitivity. In an aqueous solution, the minimal detectable TEMPO concentration was around 70 ppm. The value of the diffusivity was estimated by fitting the relaxation data, collected as a function of time, with the appropriate solutions of the second Fick's law in respect to the corresponding sample geometry and dimensions. Considering the experimentally determined TEMPO relaxivity in water ("TEMPO-water relaxivity"; R(TEMPO) = (1.05 ± 0.12) × 10⁻³ ppm⁻¹ s⁻¹), the obtained diffusion coefficients (D) of TEMPO in homogeneous solution and in a water saturated sand column (D(bulk) = (6.7 ± 0.4) × 10⁻¹° m² s⁻¹ and D(sand) = (1.4 ± 0.5) × 10⁻¹° m² s⁻¹, respectively) are in good agreement with the expected values (literature values: D(bulk) = 6.6 × 10⁻¹° m² s⁻¹, 1.3 × 10⁻¹° m² s⁻¹ < D(sand) < 2.3 × 10⁻¹° m² s⁻¹). This new approach enables one to determine the diffusivity of paramagnetic molecules in homogeneous (aqueous solution) and porous media with basic NMR equipment, at low concentrations and in a noninvasive manner.