Effective Diffusion Coefficient Deff Research Articles

Quantification of Blood-Brain Barrier Solute Permeability and Brain Transport by Multiphoton Microscopy

Development of an optimal systemic drug delivery strategy to the brain will require noninvasive or minimally invasive methods to quantify the permeability of the cerebral microvessel wall or blood-brain barrier (BBB) to various therapeutic agents and to measure their transport in the brain tissue. To address this problem, we used laser-scanning multiphoton microscopy to determine BBB permeability to solutes (P) and effective solute diffusion coefficients (Deff) in rat brain tissue 100-250 μm below the pia mater. The cerebral microcirculation was observed through a section of frontoparietal bone thinned with a microgrinder. Sodium fluorescein, fluorescein isothiocyanate (FITC)-dextrans, or Alexa Fluor 488-immunoglobulin G (IgG) in 1% bovine serum albumin (BSA) mammalian Ringer's solution was injected into the cerebral circulation via the ipsilateral carotid artery by a syringe pump at a constant rate of ∼3 ml/min. P and Deff were determined from the rate of tissue solute accumulation and the radial concentration gradient around individual microvessels in the brain tissue. The mean apparent permeability P values for sodium fluorescein (molecular weight (MW) 376 Da), dextran-4k, -20k, -40k, -70k, and IgG (MW ∼160 kDa) were 14.6, 6.2, 1.8, 1.4, 1.3, and 0.54 × 10-7 cm/s, respectively. These P values were not significantly different from those of rat pial microvessels for the same-sized solutes (Yuan et al., 2009, "Non-Invasive Measurement of Solute Permeability in Cerebral Microvessels of the Rat," Microvasc. Res., 77(2), pp. 166-73), except for the small solute sodium fluorescein, suggesting that pial microvessels can be a good model for studying BBB transport of relatively large solutes. The mean Deff values were 33.2, 4.4, 1.3, 0.89, 0.59, and 0.47 × 10-7 cm2/s, respectively, for sodium fluorescein, dextran-4k, -20k, -40k, -70k, and IgG. The corresponding mean ratio of Deff to the free diffusion coefficient Dfree, Deff/Dfree, were 0.46, 0.19, 0.12, 0.12, 0.11, and 0.11 for these solutes. While there is a significant difference in Deff/Dfree between small (e.g., sodium fluorescein) and larger solutes, there is no significant difference in Deff/Dfree between solutes with molecular weights from 20,000 to 160,000 Da, suggesting that the relative resistance of the brain tissue to macromolecular solutes is similar over a wide size range. The quantitative transport parameters measured from this study can be used to develop better strategies for brain drug delivery.

The model and mass transfer characteristics of convection drying of peach slices

The influences of drying temperature, hot air velocity and thickness of peach slice on the drying process were studied in a laboratory convective hot air dryer. The experimental data of moisture ratio of peach slices were used to fit the models and the effective moisture diffusion coefficients (Deff) were calculated. Moreover, the relative relations between the Deff and drying temperature was also related. The result showed that the Page model showed the best fit to experimental drying data for drying temperature, with the increase of temperature from 60 to 80°C, the Deff varied from 5.6083×10−10 to 1.3388×10−9m2·s−1, it fitted the Arrhenius equation, which the activation energy was 42.53kJ·mol−1; the Wang and Singh model was found to be the most satisfactory for air velocity, with velocity from 0.423 to 1.120m·s−1, the values of Deff varied from 6.6626×10−10 to 1.1350×10−9m2·s−1; the Logarithmic model was found to be the most satisfactory for the thickness of peach slice, with thickness from 0.002 to 0.004m, the values of Deff varied from 4.1543×10−10 to 1.1007×10−9m2·s−1.

Prediction of diffusivity and conversion of n-decane and CO in coated Pt/γ-Al2O3 catalyst depending on porous layer morphology

The conversion in monolith reactors for automotive exhaust gas aftertreatment can be limited by diffusion in the catalytic layer. This is particularly important for monolith reactors with multiple coated layers. In this paper, we present detailed modeling methodology for prediction of effective diffusivity based on the actual structure of a porous coating (particle and pore size distributions). We demonstrate the approach on diffusion and oxidation of n-decane and CO in Pt/γ-Al2O3 layers typically used in diesel oxidation catalysts. To validate the model predictions experimentally, several layers were coated with uniform thickness on flat metal foils, and their macroporous structure was controlled by alumina particle size distribution, pore templates and compaction techniques. A multi-scale modelling approach was then applied to predict effective diffusivity and impact of the internal diffusion limitations on the achieved conversions. Diffusion of CO and n-decane was simulated on a micro-scale together with oxidation reactions in a 3D digitally reconstructed porous layer structure. The results were combined with a macroscopic 1D plug-flow model to calculate the reactor outlet conversions. Good agreement was achieved between the predicted and the measured conversions both for n-decane and CO oxidation. The predicted effective diffusion coefficients Deff through the tested Pt/γ-Al2O3 layers were 1.4, 3.6 and 6.4×10−6m2s−1 for CO at T=298K in compact, standard and macropore-templated sample, respectively. The corresponding diffusivities for n-decane were 0.53, 1.2 and 2.0×10−6m2s−1, respectively. The model quantified relative contributions of volume and Knudsen diffusion regimes to overall transport as well as temperature dependence of Deff.

Intrinsic coherence resonance in the chloride-induced temporal dynamics of the iron electrodissolution-passivation in sulfuric acid solutions

The existence of intrinsic coherence resonance (ICR) is demonstrated experimentally for the Fe|0.75M H2SO4 electrochemical system in which the presence of chlorides induces current oscillations. Chlorides, destroying locally the passive oxide film of Fe, lead to pitting corrosion. It is shown that the intensity of the system internal noise, stemming perhaps from fluctuations of ionic concentrations, may be regulated upon changing the applied at the Fe electrode potential (E). Within a certain chloride concentration range (20≤cCl-/mM<50), the ICR occurs within two oscillatory regions. One is located at lower potentials, where the passive-active state dissolution (early stages of pitting) occurs, while the other takes over at higher potentials where the electropolishing state dissolution (late stages of pitting) of the Fe electrode arises. At sufficiently high chloride concentrations (≥50mM) only the electropolishing state dissolution of Fe emerges out of its passive state. To quantify the ICR, the coefficient of variation R and the effective diffusion coefficient Deff were calculated as a function of E for the chloride-induced spiking dynamics of the current, which is the characteristic response of the passive-active state dissolution of Fe. The ICR for the electropolishing state dissolution of Fe was quantified by calculating the coherence factor β as a function of E for the low-amplitude current oscillations established over a limiting current region. The coherence of chloride-induced current oscillations becomes maximal at a certain value of the potential applied at the Fe electrode as a result of an optimal level of the internal noise.

Electron transport analysis in zinc oxide-based dye-sensitized solar cells: A review

Abstract This research review highlights the use of zinc oxide (ZnO) nanostructure as a photoanode in the fabrication of dye-sensitised solar cells (DSSC). ZnO nanostructure thin films offer large surface area, direct electron pathways and effective light scattering centre. The fabrication work can produce a variety of ZnO nanostructures, from nanotubes, nanoporous, nanosheets, nanoflowers, nanoflakes, nanobranches and nanolipsticks. The internal mechanism inside the cell gives useful information on the electron transport properties and efficiency of ZnO-based DSSC. The review ends with an outlook highlighting the electron transport parameters analysed by electrochemical impedance spectroscopy (EIS) unit. Such parameters including charge transport resistance (Rct), transport resistance (Rt), chemical capacitance (Cμ), effective electron lifetime (τeff), effective electron chemical diffusion coefficient (Deff), effective rate constant for recombination (keff), effective electron diffusion length of the photoanode (Ln) and finite Warburg impedance in the electrolyte (ZD). Monitoring the electron transport properties may improve the photovoltaic performances of the ZnO-based DSSC such as open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and power conversion efficiency (η).

Water Purification from Heavy Metal Ions in a Packed Column

The process of adsorption from water solutions containing ternary system of Cu(II), Zn(II) and Ni(II) ions in time-variable conditions in a packed column with lyophilized chitosan beads is considered. The system of partial differential equations describing the adsorption column, due to the assumption of a properly defined variable is transformed into a system of ordinary nonlinear equations which enables the identification of object parameters in simple experiments. Analytical solution of that system was possible due to the assumptions made regarding the equation describing sorption kinetics. Using experimental data, the effective diffusion coefficient Deff and the sorption kinetic constants K in the real environment were identified. The sorption capacity q* was obtained in separate sorption equilibrium experiments. Using a mathematical model the influence of initial concentration and solution flow rate on the breakthrough curve was determined. The approach which is presented provides a new method for modeling of sorption in a packed column.

Crystalline Liquid and Rubber-Like Behavior in Cu Nanowires

Via in situ TEM tensile tests on single crystalline copper nanowires with an advanced tensile device, we report here a crystalline-liquid-rubber-like (CRYS-LIQUE-R) behavior in fracturing crystalline metallic nanowires. A retractable strain of the fractured crystalline Cu nanowires can approach over 35%. This astonishing CRYS-LIQUE-R behavior of the fracturing highly strained single crystalline Cu nanowires originates from an instant release of the stored ultralarge elastic energy in the crystalline nanowires. The release of the ultralarge elastic energy was estimated to generate a huge reverse stress as high as ~10 GPa. The effective diffusion coefficient (D(eff)) increased sharply due to the consequent pressure gradient. In addition, due to the release of ultrahigh elastic energy, the estimated concomitant temperature increase was estimated as high as 0.6 Tm (Tm is the melting point of nanocrystalline Cu) on the fractured tip of the nanowires. These factors greatly enhanced the atomic diffusion process. Molecular dynamic simulations revealed that the very high reverse stress triggered dislocation nucleation and exhaustion.

Curing and diffusion coefficient study in pastırma, a Turkish traditional meat product

Changes in water activity (aw), moisture and salt contents and salt effective diffusion coefficients (Deff) of pastırma samples during the curing process were determined. At the end of the curing stage, aw values decreased to 0.942. The average initial moisture content of the samples decreased from 74.56% to 66.64%, depending on the curing time and the average salt content increased to 15.65g NaCl/100g dry matter at the end of the 48-hour curing process. Pastırma samples were assumed the geometry of endless slices, and the analytical solution of Fick's second equation was used for determination of salt Deff values. Salt Deff values were found to vary between 1.49×10−9–4.08×10−9m2/s.

Analysis and comparison of mass transfer phenomena related to Cu2+ sorption by hydroxyapatite and zeolite

Zeolite and hydroxyapatite (HAP) are well-known sorbent materials, with high capacities for removal of toxic heavy metals. In this study, the influence of process parameters onto kinetics of Cu2+ sorption by zeolite and HAP was analyzed and compared. The effects of Cu2+ concentration, sorbent mass and agitation speed were explored for both materials, as well as the effect of zeolite particle size. Using single resistance mass transfer models, the values of volumetric mass transfer coefficient (kfa) and effective diffusion coefficient (Deff) were defined. The obtained results showed that the sorption onto HAP was governed only by pore diffusion, since the film diffusion and surface saturation occurred within the first few minutes of the reaction. Using zeolite, sorption was governed by film diffusion during the first 10min, followed by diffusion inside the pores. The resistance of releasing cations counter diffusion was negligible in the fluid film, but it was considerable in sorbent pores.

Influnce of the external force on the directed transport of feedback-coupled Brownian ratchet

The directed transport performance of two coupled Brownian particles in the double-well ratchet potential under the external force has been studied in this paper. Langevin equation in the overdamped regime is solved numerically. The influence of the external force, the thermal noise, and the asymmetric parameter of the potential on the transport properties of the coupled Brownian particles including the average velocity, the effective diffusion coefficient Deff , and the Pe number, is discussed in detail. It is found that the average velocity changes periodically under the external force. Meanwhile, there is an optimal value of the intensity of the thermal noise at which the current reaches the maximum. It is worthwhile to point out that the enhancement of the current can be achieved by changing the structure of the ratchet potential.

Real-time measurement and modeling of intraparticle pH gradient formation in immobilized cephalosporin C amidase

A new and generic approach of dynamic reaction–diffusion modeling was applied to analyze the catalytic performance of an industrially used cephalosporin C amidase immobilized on a series of insoluble carriers (epoxy-activated Sepabeads) that differed in particle or pore diameter. Enzymatic conversion of cephalosporin C into 7-amino cephalosporanic acid and d-α-amino adipic acid (DAAA) results in net proton release, and this was shown previously to bring about significant internal acidification of the enzyme immobilizates during reaction. In this study, we demonstrate first time model parameter estimation to in situ measured real-time pH data, recorded both inside the enzyme carrier as well as externally in the liquid bulk of a stirred tank reactor. An essential and novel feature of the model was applicability to intraparticle pH data derived from space-averaged opto-chemical measurements. Using model-based estimation, we determined the maximal intrinsic reaction rate of the immobilized amidase completely unmasked from diffusional and carrier acidification effects (vmax,specific) as well as the effective proton diffusion coefficient (Deff, H+). Neither parameter is accessible directly from the experiment. The resulting vmax,specific estimates showed that carrier-bound amidase had lost between 14% and 55% of the free enzyme's activity due to immobilization, depending on the carrier type used. The Deff, H+ estimate (≈1×10−10m2/s) was about one order of magnitude lower than expected for proton diffusion in such porous structures, suggesting a role for co-diffusion of the proton and the corresponding DAAA anion in decreasing the proton diffusivity. The model was validated against independent time-course data sets, and it was shown to give useful prediction of immobilized enzyme effectiveness factors for the applied range of carrier characteristics and enzyme loadings. Simulation was used to characterize the influence of geometrical features of the carrier on distribution of substrate and products in the carrier pore, and it is demonstrated how these intraparticle concentration gradients affect enzyme reactor performance under different process conditions.

Phototransformation rate constants of PAHs associated with soot particles

Photodegradation is a key process governing the residence time and fate of polycyclic aromatic hydrocarbons (PAHs) in particles, both in the atmosphere and after deposition. We have measured photodegradation rate constants of PAHs in bulk deposits of soot particles illuminated with simulated sunlight. The photodegradation rate constants at the surface (kp0), the effective diffusion coefficients (Deff), and the light penetration depths (z0.5) for PAHs on soot layers of variable thickness were determined by fitting experimental data with a model of coupled photolysis and diffusion. The overall disappearance rates of irradiated low molecular weight PAHs (with 2–3 rings) on soot particles were influenced by fast photodegradation and fast diffusion kinetics, while those of high molecular weight PAHs (with 4 or more rings) were apparently controlled by either the combination of slow photodegradation and slow diffusion kinetics or by very slow diffusion kinetics alone. The value of z0.5 is more sensitive to the soot layer thickness than the kp0 value. As the thickness of the soot layer increases, the z0.5 values increase, but the kp0 values are almost constant. The effective diffusion coefficients calculated from dark experiments are generally higher than those from the model fitting method for illumination experiments. Due to the correlation between kp0 and z0.5 in thinner layers, Deff should be estimated by an independent method for better accuracy. Despite some limitations of the model used in this study, the fitted parameters were useful for describing empirical results of photodegradation of soot-associated PAHs.

Measurement of diffusion coefficients in dry anaerobic digestion media

Dry anaerobic digestion of solid wastes is now commonly used at the industrial scale. Dry solid content in waste material is usually higher than 15%. w/w in these processes most industrial technologies in dry modes are not mixed, or sometimes intermittently mixed. In these situations convective transfer is negligible and diffusive transfer predominates and controls the mobility of soluble substrates within the digestion media. However, no data specifically related to diffusive phenomena in dry anaerobic digestion media (digestate) can be found in the literature. In the present study, an innovative experimental method was developed to determine diffusion coefficients experimentally. This method was based on the use of a specifically designed diffusion cell where iodide was used as a soluble tracer. The influence of total solid (TS) content on diffusion coefficients in digestate material was then investigated. Experimental results showed a strong influence of TS content. It was found that the effective diffusion coefficient (Deff) decreased drastically when TS content increased, with numerical values 50 to 185 times smaller at 8% TS and 25% TS, respectively than the reference value in water. This observation was probably due to the huge increase of viscosity within this range of dry solid content.

Implications of pore space characteristics on diffusive transport in basalts and granites

A detailed characterization of the pore space is crucial for understanding of transport and element transfer in rocks. Here, the effect of differences in texture and content of secondary minerals on transport in pore systems was determined for two rocks of widespread occurrence, mid-ocean ridge basalts (MORB) and granites. Pore space characteristics were analyzed by Hg-porosimetry, intrusion of a molten alloy, and synchrotron-based X-ray tomographic microscopy. For evaluating the role of pore space characteristics for the prediction of diffusive transport, data on porosity, and the effective diffusion coefficient (D eff) were compared. Extended connective pore systems due to cracks and mineral dissolution are present in samples of both rocks, indicating high internal specific surface area. Uneven pore size distributions in altered MORB samples can be assigned to secondary minerals. Pore spaces determined by X-ray tomography, used to determine main direction of pores in the 3-D orthogonal system, suggest a slight anisotropy. In log–log plots, both rocks show roughly a linear dependence of D eff for H2O and compounds with comparable diffusivities (D2O, monovalent cations, and anions) on porosity, but at same porosity D eff is clearly higher in granitic than in basaltic samples. This difference is increasing with decreasing porosity, indicating that at low porosities the efficiency of element transport in basaltic samples is diminished, mainly inherited by the presence of small pores slowing down diffusion. The fact that diffusive transport in basaltic rocks is stronger dependent on porosity than in granitic rocks shows that also other rock characteristics such as pore size distribution and tortuosity of the pore network, highly affected by the alteration degree, can markedly affect transport and reactivity of pore solution.

Comparative Study between Hot Air and Infrared Drying of Parboiled Rice: Kinetics and Qualities Aspects

The objectives of this study were to analyze the effects of drying employing three different heat sources on drying kinetics and to evaluate qualities of parboiled rice after drying. Drying temperature was varied between 60 and 100C. Power of infrared (IR) heat source was fixed at 1,000 and 1,500 W and air flow rate was fixed at 1.0 ± 0.2 m/s. The three drying strategies composed of hot air (HA), IR and combined HA + IR drying. The experimental results were simulated using various equilibrium moisture content models and the mathematical drying model for prediction of drying kinetics and evaluation of effective diffusion coefficient (Deff) followed by Fick's law of diffusion. The results revealed that Deff values of HA and IR drying were in the range of 10−12–10−11 m2/s and relatively depended on temperature. For quality evaluation, conclusions reached that head rice yield using HA + IR drying had the highest value, while yellowness and whiteness of the parboiled rice are significantly affected by drying condition (P > 0.05). Practical Applications These results suggested that combined hot air and infrared (HA + IR) drying offers a great potential for preserving Leb Nok Pattani parboiled rice. Drying kinetics and quality aspects determined that IR drying is more efficient than the other drying method. However, the color degradation in the grain kernel is an issue for quality measurement.

Mass Transfer Model for Coated Urea Particle in Controlling-Release Process

Based on mass conservation and Fick’s diffusion law, a model for describing the nutrient release from coated urea particles was proposed. This model was verified by the release data of three novel paraffin- rosin coated urea in distilled water. The results indicate that the model is effective in describing and predicting the delivery behaviors for a diffusion-controlled coated- urea and their correlation coefficients R2 are 0.9976, 0.9973 and 0.9984, respectively. The model shows that the nutrient release rate for coated urea is direct proportion to effective diffusion coefficient Deff, inverse proportion to diameter’s square of urea granule and inverse proportion to the coating thickness.

Diffusion measurements of isopentane, 1‐hexene, cyclohexane in polyethylene particles by the intelligent gravimetric analyzer

AbstractThe diffusion coefficients of isopentane (i‐C5H12), 1‐hexene (C6H12), cyclohexane (C6H14) in polyethylene particles were measured by a highly sensible microbalance—intelligent gravimetric analyzer, using the desorption method with N2 as a bypassing gas. Both the effective diffusion coefficients (Deff) and dilute diffusion coefficients ( $D_{{\rm{eff}}}^\sim $) were measured simultaneously. It was found that $D_{{\rm{eff}}}^\sim $ was about one magnitude smaller than Deff and the logarithmic form of $D_{{\rm{eff}}}^\sim $ was linear with that of Deff. In addition, the effects of particle size and distribution, polymer properties (e.g., the crystallinity and melt index), and operating conditions (e.g., temperature and pressure) on Deff were systematically investigated. The results showed that Deff increased linearly with the increase of the square of particle size. The diffusion behaviors of hydrocarbons in polyethylene are of great importance in the polymerization and purging processes. © 2010 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

Pore‐scale simulation of mixing‐induced calcium carbonate precipitation and dissolution in a microfluidic pore network

We develop a 2‐D pore scale model of coupled fluid flow, reactive transport, and calcium carbonate (CaCO3) precipitation and dissolution. The model is used to simulate transient experimental results of CaCO3 precipitation and dissolution under supersaturated conditions in a microfluidic pore network (i.e., micromodel) in order to improve understanding of coupled reactive transport systems perturbed by geological CO2 injection. In the micromodel, precipitation is induced by transverse mixing along the centerline in pore bodies. The reactive transport model includes the impact of pH upon carbonate speciation and a CaCO3 reaction rate constant, the effect of changing reactive surface area upon the reaction, and the impact of pore blockage from CaCO3 precipitation on diffusion and flow. Overall, the pore scale model qualitatively captured the precipitate morphology, precipitation rate, and maximum precipitation area using parameter values from the literature. In particular, we found that proper estimation of the effective diffusion coefficient (Deff) and the reactive surface area is necessary to adequately simulate precipitation and dissolution rates. In order to match the initial phase of fast precipitation, it was necessary to consider the top and bottom of the micromodel as additional reactive surfaces. In order to match a later phase when dissolution occurred, it was necessary to increase the dissolution rate compared to the precipitation rate, but the simulated precipitate area was still higher than the experimental results after ∼30 min, highlighting the need for future study. The model presented here allows us to simulate and mechanistically evaluate precipitation and dissolution of CaCO3 observed in a micromodel pore network. This study leads to improved understanding of the fundamental physicochemical processes of CaCO3precipitation and dissolution under far‐from‐equilibrium conditions.

Impact Factors to Regulate Mass Transfer Characteristics of Stable Alginate Membrane Performed Superior Sensitivity on Various Organic Chemicals

Calcium alginate membrane has great potential for membrane separation technology. The polymer frameworks of the membrane were successfully regulated by the mass fraction of homopolymeric blocks of α-L-guluronic acid ( FGG ) in the entire molecular chain of alginate and the additive CaCl2 as a cross-linker. The mechanical strength can be controlled by regulated the FGG and CaCl2 concentration. Selected mass transfer of saccharides (Glucose,G, 180 Da; Maltose,M, 324 Da; Raffinose,R, 504 Da) was achieved. The mass transfer flux of saccharides was strongly changed and inversely proportional to the 4.4 th power of molecular volume. The mass transfer flux of saccharides was inversely proportional to the 2 nd power of FGG at prepared 1 M CaCl2. The mass transfer flux of urea (60 Da) was clearly decreased with increasing CaCl2 concentration. Especially, in higher FGG range, effect of CaCl2 concentration remarkably appeared on its effective diffusion coefficient ( Deff ). The volumetric water fraction with calcium concentration was very slight, regardless of FGG . The tortuosity increased linearly with increasing additive CaCl2 concentration. In higher CaCl2 concentration, the effect of FGG on the tortuosity remarkably appeared. Obtained empirical equation was anticipated as a guideline to estimate the effective diffusion coefficient of the alginate membrane. High permeability was performed by low FGG (0.18) membrane performed high permeability than high FGG (0.56) membrane. The water permeation mechanism was obeyed by Hagen-Poiseuille flow regardless of FGG .

The Effect of Surface Charge and Wettability on H2O Self Diffusion in Compacted Clays

AbstractOrgano-clays are of interest in the modification of anion transport properties in engineered barriers. In the present study, surface charge and wettability were assessed for tracking changes in the effective diffusion coefficient (Deff) by the formation or suppression of bound H2O layers on the external surfaces of clays. Bentonite samples modified with three different organic cations in amounts of 0 to 400% of the cation exchange capacity were used. Diffusive transport was determined in H2O→D2O exchange experiments in a newly constructed cell adapted to the attenuated total reflectance (ATR) accessory of a Fourier-transform infrared (FTIR) spectrometer at two different dry bulk densities and various degrees of water saturation.All organo-clay combinations showed changes in surface charge after the addition of organic cations, from a negative value of 99 mmolc/kg for the original bentonite to a maximum positive value of 230.5 mmolc/kg for hexadecylpyridinium (HDPy)-montmorillonite. The positive charge resulted from adsorption of the organic cation in excess of the CEC. Hydrophobic surface properties with contact angles &gt;90° were obtained for HDPy-montmorillonite samples with monolayers of organic cations on the external surfaces only. Here, where hydrophobicity suppressed the formation of bound H2O layers, the largest Deff o f 2.7×10−10 m2/s was observed in the high dry bulk density range (1.0–1.5 g/cm3) under water-saturated conditions. In the low dry bulk density range (0.6–0.9 g/cm3) this effect was weakened significantly because, with increasing pore size, the effect of bound H2O layers was reduced. In the high dry bulk density range at partial water saturation (40%), diffusive transport was hindered by the small water volume. Previous work found that, in the high dry bulk density range and water-saturated state, Deff was 2.4×10−11 m2/s for the original bentonite. Deff for all hydrophilic organo-clay samples was ⩽2.1×10−11 m2/s, somewhat less than for the hydrophobic sample. In hydrophilic organo-clay samples, retardation factors that retard the value for Deff, up toa magnitude of 0.5, include an increase in dry bulk density and a decrease in water saturation. In the water-saturated state at high dry bulk densities, hydrophobic surface properties suppressing the formation of bound H2O layers can increase Deff by one order of magnitude.