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2021 ◽  
Author(s):  
Mahmoud Elsayed ◽  
Hyung Kwak ◽  
Ammar El-Husseiny ◽  
Mohamed Mahmoud

Abstract Tortuosity, in general characterizes the geometric complexity of porous media. It is considered as one of the key factors in characterizing the heterogonous structure of porous media and has significant implications for macroscopic transport flow properties. There are four widely used definitions of tortuosity, that are relevant to different fields from hydrology to chemical and petroleum engineering, which are: geometric, hydraulic, electrical, and diffusional. Recent work showed that hydraulic, electrical and diffusional tortuosity values are roughly equal to each other in glass beads. Nevertheless, the relationship between the different definitions of Tortuosity in natural rocks is not well understood yet. Understanding the relationship between the different Tortuosity definitions in rocks can help to establish a workflow that allows us to estimate other types from the available technique. Therefore, the objective of this study is to investigate the relationship between the different tortuosity definitions in natural rocks. A major focus of this work is to utilize Nuclear Magnetic Resonance (NMR) technology to estimate Tortuosity. Such technique has been traditionally used to obtain diffusional tortuosity which can be defined as the ratio of the free fluid self-diffusion coefficient to the restricted fluid self-diffusion coefficient inside the porous media. In this study, the following techniques were used to quantify hydraulic, electrical, and diffusional tortuosity respectively on the same rock sample: (1) Microcomputed Tomography 3D imaging (2) Four-Electrodes resistivity measurements (3) Pulsed-Field Gradient Nuclear Magnetic Resonance (PFG NMR). PFG NMR is very powerful, non-invasive technique employed to measure the self-diffusion coefficient for free and confined fluids. The measurements were done based on two carbonate rock core plugs characterized by variable porosity, permeability and texture complexity. Results show that PFG NMR can be applied directionally to quantify the pore network anisotropy created by fractures. For both samples, hydraulic tortuosity was found to have the lowest magnitude compared to geometric, electrical and diffusional tortuosity. This could be explained by the more heterogeneous microstructure of carbonate rocks. NMR technique has however advantages over the other electrical and imaging techniques for tortuosity characterization: it is faster, non-destructive and can be applied in well bore environment (in situ). We therefore conclude that NMR can provide a tool for estimating not only diffusional tortuosity but also for indirectly obtaining hydraulic and electrical tortuosity.


2021 ◽  
Author(s):  
Daniel Bellaire ◽  
Oliver Großmann ◽  
Kerstin Münnemann ◽  
Hans Hasse

Diffusion coefficients at infinite dilution are important basic data for all processes involving mass transfer. They can be obtained from studying samplesin equilibrium using nuclear magnetic resonance spectroscopy with pulsed field gradients (PFG-NMR), a technique which is widely used in chemistry but isonly rarely applied in engineering studies. This advantageous technique was employed here to measure the self-diffusion coefficients of diluted solutions ofcarbon dioxide and methane in the pure solvents water, ethanol, cyclohexane, toluene, methanol, and acetone at 298.15 K. For the systems (carbon dioxide +water) and (carbon dioxide + ethanol), measurements were also carried out at 308.15 K, 318.15 K and 333.15 K. Except for (methane + water) and (methane +toluene), no literature data for the methane-containing systems were previously available. At the studied solute concentrations, there is practically no differencebetween the self-diffusion coefficient and the mutual diffusion coefficient. The experimental results are compared to experimental literature data as well as toresults from semi-empirical methods for the prediction of diffusion coefficients at infinite dilution. Furthermore, molecular dynamics simulations were carried outfor all systems to determine the diffusion coefficient at infinite dilution based on force fields that were taken from the literature, and the results are compared tothe experimental data and those from the classical prediction methods.


2021 ◽  
Author(s):  
◽  
Nelly Malassagne-Bulgarelli

<p>Emulsions are kinetically stabilised mixtures of two immiscible fluids (e.g. oil and water). They are encountered in many industrial applications including cosmetics, food, road, drug delivery and paint technology. Despite their wide spread use, the formulation of emulsions remains largely empirical. The nature of the relationships between ingredients, composition, emulsification method and energy input, defining the microstructure (e.g. droplet size distribution and surfactant packing at the oil/water interface), the dynamics (e.g. interdroplet exchange) and the lifetime of emulsions, is still poorly understood. In particular, little work has focused on the mutual interactions between emulsifier and oil molecules and how these affect the properties of the interfacial domain and emulsion dynamics. The emulsion system oil/Triton X-100/water was investigated, where Triton X-100 is a commercially available non ionic surfactant and the oil is one of toluene, p-xylene or octane. The microstructure and the dynamics of these oil/Triton X-100/water emulsions were monitored upon varying oil type, oil concentration, emulsion age and ionic strength while maintaining the oil-to-surfactant weight ratio, temperature, energy input and emulsification method constant. For this purpose, laser scanning confocal microscopy, cryo scanning electron microscopy (cryo-SEM), pulsed field gradient NMR (PFG-NMR), macroscopic phase separation and light scattering techniques were used as experimental techniques. The occurrence of an oil exchange between oil droplets that is not coupled to droplet growth and emulsion destabilization is reported for the three oil systems: toluene, p-xylene or octane. The mixture of two separately stained emulsions, using green and red fluorescing dye molecules, leads to all droplets emitting yellow fluorescence under the confocal microscope within ∼10 min of mixing due to the interdroplet exchange of the two water insoluble dyes. Furthermore, the PFG-NMR data for both toluene and p-xylene systems indicate that, for long observation times, Δ, the echo attenuation of the oil signal decays as a single exponential upon increasing the diffusion parameters. In other words the individual motions of the droplets and oil molecules are described by a unique diffusion coefficient belying the system polydispersity and indicative of a dynamic process occurring on a time scale faster than the observation time. One way to explain this outcome is to consider a motional averaging of the oil diffusion arising from either oil permeation upon droplet collision or reversible coalescence of the droplets. These two mechanisms are supported by the extensive droplet contact observed by cryo-SEM. Such an oil transfer occurring in three distinct oil systems, independently of emulsion destabilization, has not been reported previously. Upon decreasing the NMR observation time below a specific value, Δswitch, a switch of the echo attenuation data was detected between a single exponential and a multiexponential decay, the latter indicative of the emulsion droplet size distribution. The time scale of the oil transfer, Δswitch, was probed upon varying oil type, oil concentration, emulsion age and ionic strength. In particular, the time scale of the oil exchange is an increasing function, spanning from ~300 ms to ~3 s, of droplet concentration in toluene emulsions despite the concomitant increase of the droplet collision frequency. Upon increasing the toluene content and decreasing the mean interdroplet spacing, the oil droplets are kinetically stabilized by the enhancement of the surfactant packing at the oil/water interface. In addition to the surfactant packing at the surface of the oil droplets, ionic strength and droplet size, the rate of oil exchange is controlled by the mutual interactions between oil and Triton X-100 molecules. The rate of oil transfer is a decreasing function from toluene to p-xylene to octane. The increase of the mean droplet size in the same order cannot solely account for the observed slowdown of the oil exchange. The macroscopic phase separation data indicate that the Triton X-100 layer is increasingly robust with respect to oil transfer from toluene to p-xylene to octane. This can be compared with the oil exchange process and explained in terms of oil penetration effects into the surfactant layer and energy cost for hole nucleation.</p>


2021 ◽  
Author(s):  
◽  
Nelly Malassagne-Bulgarelli

<p>Emulsions are kinetically stabilised mixtures of two immiscible fluids (e.g. oil and water). They are encountered in many industrial applications including cosmetics, food, road, drug delivery and paint technology. Despite their wide spread use, the formulation of emulsions remains largely empirical. The nature of the relationships between ingredients, composition, emulsification method and energy input, defining the microstructure (e.g. droplet size distribution and surfactant packing at the oil/water interface), the dynamics (e.g. interdroplet exchange) and the lifetime of emulsions, is still poorly understood. In particular, little work has focused on the mutual interactions between emulsifier and oil molecules and how these affect the properties of the interfacial domain and emulsion dynamics. The emulsion system oil/Triton X-100/water was investigated, where Triton X-100 is a commercially available non ionic surfactant and the oil is one of toluene, p-xylene or octane. The microstructure and the dynamics of these oil/Triton X-100/water emulsions were monitored upon varying oil type, oil concentration, emulsion age and ionic strength while maintaining the oil-to-surfactant weight ratio, temperature, energy input and emulsification method constant. For this purpose, laser scanning confocal microscopy, cryo scanning electron microscopy (cryo-SEM), pulsed field gradient NMR (PFG-NMR), macroscopic phase separation and light scattering techniques were used as experimental techniques. The occurrence of an oil exchange between oil droplets that is not coupled to droplet growth and emulsion destabilization is reported for the three oil systems: toluene, p-xylene or octane. The mixture of two separately stained emulsions, using green and red fluorescing dye molecules, leads to all droplets emitting yellow fluorescence under the confocal microscope within ∼10 min of mixing due to the interdroplet exchange of the two water insoluble dyes. Furthermore, the PFG-NMR data for both toluene and p-xylene systems indicate that, for long observation times, Δ, the echo attenuation of the oil signal decays as a single exponential upon increasing the diffusion parameters. In other words the individual motions of the droplets and oil molecules are described by a unique diffusion coefficient belying the system polydispersity and indicative of a dynamic process occurring on a time scale faster than the observation time. One way to explain this outcome is to consider a motional averaging of the oil diffusion arising from either oil permeation upon droplet collision or reversible coalescence of the droplets. These two mechanisms are supported by the extensive droplet contact observed by cryo-SEM. Such an oil transfer occurring in three distinct oil systems, independently of emulsion destabilization, has not been reported previously. Upon decreasing the NMR observation time below a specific value, Δswitch, a switch of the echo attenuation data was detected between a single exponential and a multiexponential decay, the latter indicative of the emulsion droplet size distribution. The time scale of the oil transfer, Δswitch, was probed upon varying oil type, oil concentration, emulsion age and ionic strength. In particular, the time scale of the oil exchange is an increasing function, spanning from ~300 ms to ~3 s, of droplet concentration in toluene emulsions despite the concomitant increase of the droplet collision frequency. Upon increasing the toluene content and decreasing the mean interdroplet spacing, the oil droplets are kinetically stabilized by the enhancement of the surfactant packing at the oil/water interface. In addition to the surfactant packing at the surface of the oil droplets, ionic strength and droplet size, the rate of oil exchange is controlled by the mutual interactions between oil and Triton X-100 molecules. The rate of oil transfer is a decreasing function from toluene to p-xylene to octane. The increase of the mean droplet size in the same order cannot solely account for the observed slowdown of the oil exchange. The macroscopic phase separation data indicate that the Triton X-100 layer is increasingly robust with respect to oil transfer from toluene to p-xylene to octane. This can be compared with the oil exchange process and explained in terms of oil penetration effects into the surfactant layer and energy cost for hole nucleation.</p>


Polymers ◽  
2021 ◽  
Vol 13 (19) ◽  
pp. 3265
Author(s):  
Guilherme A. Ferreira ◽  
Watson Loh ◽  
Daniel Topgaard ◽  
Olle Söderman ◽  
Lennart Piculell

Internally structured block copolymer-surfactant particles are formed when the complex salts of ionic-neutral block copolymers neutralized by surfactant counterions are dispersed in aqueous media. Here, we report the 1H NMR signal intensities and self-diffusion coefficients (D, from pulsed field gradient nuclear magnetic resonance, PFG NMR) of trimethyl alkylammonium surfactant ions and the poly(acrylamide)-block-poly(acrylate) (PAAm-b-PA) polyions forming such particles. The results reveal the presence of an “NMR-invisible” (slowly exchanging) fraction of aggregated surfactant ions in the particle core and an “NMR-visible” fraction consisting of surface surfactant ions in rapid exchange with the surfactant ions dissociated into the aqueous domain. They also confirm that the neutral PAAm blocks are exposed to water at the particle surface, while the PA blocks are buried in the particle core. The self-diffusion of the polyions closely agree with the self-diffusion of a hydrophobic probe molecule solubilized in the particles, showing that essentially all copolymer chains are incorporated in the aggregates. Through centrifugation, we prepared macroscopically phase-separated systems with a phase concentrated in particles separated from a clear dilute phase. D values for the surfactant and block copolymer indicated that the dilute phase contained small aggregates (ca. 5 nm) of surfactant ions and a few anionic-neutral block copolymer chains. Regardless of the overall concentration of the sample, the fraction of block copolymer found in the dilute phase was nearly constant. This indicates that the dilute fraction represented a tail of small particles created by the dispersion process rather than a true thermodynamic solubility of the complex salts.


2021 ◽  
Author(s):  
Tian Huang ◽  
Bo Li ◽  
Huan Wang ◽  
Steve Granick

Bipolar reactions have been provoked by reports of boosted diffusion during chemical and enzymatic reactions. To some, it is intuitively reasonable that relaxation to truly Brownian motion after passing an activation barrier can be slow, but to others the notion is so intuitively unphysical that they suspect the supporting experiments to be artifact. Here we study a chemical reaction according to whose mechanism some intermediate species should speed up while others slow down in predictable ways, if the boosted diffusion interpretation holds. Experimental artifacts would do not know organic chemistry mechanism, however. Accordingly, we scrutinize the absolute diffusion coefficient (D) during intermediate stages of the CuAAC reaction (coppercatalyzed azide-alkyne cycloaddition click reaction), using proton pulsed field-gradient nuclear magnetic resonance (PFG-NMR) to discriminate between the diffusion of various reaction intermediates. For the azide reactant, its D increases during reaction, peaks at the same time as peak reaction rate, then returns to its initial value. For the alkyne reagent, its D decreases consistent with presence of the intermediate large complexes formed from copper catalyst and its ligand, except for the 2Cu-alk complex whose more rapid D may signify that this species is the real reactive complex. For the product of this reaction, its D increases slowly as it detaches from the triazolide catalyst complex. These examples of enhanced diffusion for some molecular species and depressed diffusion for others causes us to conclude that diffusion coefficients during these elementary reactions are influenced by two components: hydrodynamic radius increase from complex formation, which slows diffusion, and energy release rate during the chemical reaction, which speeds it up. We discuss possible mechanisms and highlight that too little is yet understood about slow solvent reorganization during chemical reactions.<br>


2021 ◽  
Author(s):  
Tian Huang ◽  
Bo Li ◽  
Huan Wang ◽  
Steve Granick

Bipolar reactions have been provoked by reports of boosted diffusion during chemical and enzymatic reactions. To some, it is intuitively reasonable that relaxation to truly Brownian motion after passing an activation barrier can be slow, but to others the notion is so intuitively unphysical that they suspect the supporting experiments to be artifact. Here we study a chemical reaction according to whose mechanism some intermediate species should speed up while others slow down in predictable ways, if the boosted diffusion interpretation holds. Experimental artifacts would do not know organic chemistry mechanism, however. Accordingly, we scrutinize the absolute diffusion coefficient (D) during intermediate stages of the CuAAC reaction (coppercatalyzed azide-alkyne cycloaddition click reaction), using proton pulsed field-gradient nuclear magnetic resonance (PFG-NMR) to discriminate between the diffusion of various reaction intermediates. For the azide reactant, its D increases during reaction, peaks at the same time as peak reaction rate, then returns to its initial value. For the alkyne reagent, its D decreases consistent with presence of the intermediate large complexes formed from copper catalyst and its ligand, except for the 2Cu-alk complex whose more rapid D may signify that this species is the real reactive complex. For the product of this reaction, its D increases slowly as it detaches from the triazolide catalyst complex. These examples of enhanced diffusion for some molecular species and depressed diffusion for others causes us to conclude that diffusion coefficients during these elementary reactions are influenced by two components: hydrodynamic radius increase from complex formation, which slows diffusion, and energy release rate during the chemical reaction, which speeds it up. We discuss possible mechanisms and highlight that too little is yet understood about slow solvent reorganization during chemical reactions.<br>


Membranes ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 385
Author(s):  
Vitaliy I. Volkov ◽  
Alexander V. Chernyak ◽  
Irina A. Avilova ◽  
Nikita A. Slesarenko ◽  
Daria L. Melnikova ◽  
...  

The results of NMR, and especially pulsed field gradient NMR (PFG NMR) investigations, are summarized. Pulsed field gradient NMR technique makes it possible to investigate directly the partial self-diffusion processes in spatial scales from tenth micron to millimeters. Modern NMR spectrometer diffusive units enable to measure self-diffusion coefficients from 10−13 m2/sec to 10−8 m2/sec in different materials on 1 H, 2 H, 7 Li, 13 C, 19 F, 23 Na, 31 P, 133 Cs nuclei. PFG NMR became the method of choice for reveals of transport mechanism in polymeric electrolytes for lithium batteries and fuel cells. Second wide field of application this technique is the exchange processes and lateral diffusion in biological cells as well as molecular association of proteins. In this case a permeability, cell size, and associate lifetime could be estimated. The authors have presented the review of their research carried out in Karpov Institute of Physical Chemistry, Moscow, Russia; Institute of Problems of Chemical Physics RAS, Chernogolovka, Russia; Kazan Federal University, Kazan, Russia; Korea University, Seoul, South Korea; Yokohama National University, Yokohama, Japan. The results of water molecule and Li+, Na+, Cs+ cation self-diffusion in Nafion membranes and membranes based on sulfonated polystyrene, water (and water soluble) fullerene derivative permeability in RBC, casein molecule association have being discussed.


2021 ◽  
Author(s):  
Daniel Bellaire ◽  
Hendrik Kiepfer ◽  
Kerstin Münnemann ◽  
Hans Hasse

Recently, Guevara-Carrion et al. published a comprehensive molecular dynamics (MD) study on the thermodynamic properties of binary mixtures containing methanol, ethanol, acetone, benzene, cyclohexane, toluene, and carbon tetrachloride which also includes results on self-diffusion coefficients. However, for the mixtures acetone/cyclohexane, acetone/ethanol, acetone/toluene, cyclohexane/ethanol, and toluene/ethanol, no experimental data on self-diffusion coefficients were available for comparison. Therefore, in the present work, self-diffusion coefficients in these mixtures were measured by 1H NMR spectroscopy using pulsed field gradients (PFGs) at 298.15 K and ambient pressure. The experimental data were compared to the simulations of Guevara-Carrion et al. Good agreement was observed for all mixtures that do not contain ethanol, whereas, for ethanol-containing mixtures, the deviations were larger. This finding is attributed to deficiencies of the molecular model in describing the hydrogen-bonding of ethanol. Furthermore, self-diffusion data for the ternary mixture acetone/toluene/cyclohexane were measured and compared to molecular simulation data from the present work. Good agreement was observed.


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