Time Scale Of Adsorption Research Articles

Interfacial Adsorption Kinetics of Methane in Microporous Kerogen.

Rapid declines in unconventional shale production arise from the poorly understood interplay between gas transport and adsorption processes in microporous organic rock. Here, we use high-fidelity molecular dynamics (MD) simulations to resolve the time-varying adsorption of methane gas in realistic organic rock samples, known as kerogen. The kerogen samples derive from various geological shale fields with porosities ranging between 20% and 50%. We propose a kinetics sorption model based on a generalized solution of diffusive transport inside a nanopore to describe the adsorption kinetics in kerogen, which gives excellent fits with all our MD results, and we demonstrate it scales with the square of the length of kerogen. The MD adsorption time constants for all samples are compared with a simplified theoretical model, which we derive from the Langmuir isotherm for adsorption capacitance and the free-volume theory for steady, highly confined bulk transport. While the agreement with the MD results is qualitatively very good, it reveals that, in the limit of low porosity, the diffusive transport term dominates the characteristic time scale of adsorption, while the adsorption capacitance becomes important for higher pressures. This work provides the first data set for adsorption kinetics of methane in kerogen, a validated model to accurately describe this process, and a qualitative model that links adsorption capacitance and transport with the adsorption kinetics. Furthermore, this work paves the way to upscale interfacial adsorption processes to the next scale of gas transport simulations in mesopores and macropores of shale reservoirs.

Effects of surfactant size and concentration on the internal flow fields of moving slug and Disk-like droplets via μ-PIV

• • A laser-based phase-locking system is developed and integrated with a Micro Particle Image Velocimetry system enabling quantitative measurement of the internal velocity of moving droplets. • • Effects of surfactant size on the flow field of moving droplets are investigated by choosing two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which have a large difference in adsorption timesale. • • Effects of surfactant concentration (zero to 10X) on the internal flow field of moving droplets are evaluated and discussed. • • Effects of droplet shape (slug and spherical) and droplet generation regimes on the internal flow field of moving droplets with surfactant are investigated and discussed. • • It is revealed that achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime. Understanding the effects of surfactant on the internal flow field of moving droplets is of fundamental and practical importance. A laser-based phase-locking system was developed and coupled with a micro particle image velocimetry system enabling quantitative measurement of the internal droplet flow filed. Five concentrations (zero to 10X critical micelle concentration) of each of the two surfactants, Sodium Dodecyl Sulphate (SDS) and Tween 20, which present a large difference in adsorption timescale, were evaluated. Slug- and disk-shaped droplets were considered because of their different contact area with channel walls and thus the resulted friction. Squeezing and transition droplet formation regimes which normally yield monodispersed droplets were evaluated using SDS. Flow retardation was observed at low surfactant concentrations for slug droplets, which was primarily attributed to the induced Marangoni flow opposing the internal flow. Achievement of complete remobilisation in the internal flow depends on surfactant type, concentration, and droplet operating regime.

Effect of Surfactant Dynamics on Flow Patterns Inside Drops Moving in Rectangular Microfluidic Channels

Drops contained in an immiscible liquid phase are attractive as microreactors, enabling sound statistical analysis of reactions performed on ensembles of samples in a microfluidic device. Many applications have specific requirements for the values of local shear stress inside the drops and, thus, knowledge of the flow field is required. This is complicated in commonly used rectangular channels by the flow of the continuous phase in the corners, which also affects the flow inside the drops. In addition, a number of chemical species are present inside the drops, of which some may be surface-active. This work presents a novel experimental study of the flow fields of drops moving in a rectangular microfluidic channel when a surfactant is added to the dispersed phase. Four surfactants with different surface activities are used. Flow fields are measured using Ghost Particle Velocimetry, carried out at different channel depths to account for the 3-D flow structure. It is shown that the effect of the surfactant depends on the characteristic adsorption time. For fast-equilibrating surfactants with a characteristic time scale of adsorption that is much smaller than the characteristic time of surface deformation, this effect is related only to the decrease in interfacial tension, and can be accounted for by the change in capillary number. For slowly equilibrating surfactants, Marangoni stresses accelerate the corner flow, which changes the flow patterns inside the drop considerably.

Adsorption trajectories of nonspherical particles at liquid interfaces.

The adsorption of colloidal particles at liquid interfaces is of great importance scientifically and industrially, but the dynamics of the adsorption process is still poorly understood. In this paper we use a Langevin model to study the adsorption dynamics of ellipsoidal colloids at a liquid interface. Interfacial deformations are included by coupling our Langevin dynamics to a finite element model while transient contact line pinning due to nanoscale defects on the particle surface is encoded into our model by renormalizing particle friction coefficients and using dynamic contact angles relevant to the adsorption timescale. Our simple model reproduces the monotonic variation of particle orientation with time that is observed experimentally and is also able to quantitatively model the adsorption dynamics for some experimental ellipsoidal systems but not others. However, even for the latter case, our model accurately captures the adsorption trajectory (i.e., particle orientation versus height) of the particles. Our study clarifies the subtle interplay between capillary, viscous, and contact line forces in determining the wetting dynamics of micron-scale objects, allowing us to design more efficient assembly processes for complex particles at liquid interfaces.

Adsorption time scales of cluster-forming systems.

A microscopic model of adsorption in cluster forming systems with competing interaction is considered. The adsorption process is described by the master equation and modelled by a kinetic Monte Carlo method. The evolution of the particle concentration and interaction energy during the adsorption of particles on a plane triangular lattice is investigated. The simulation results show a diverse behavior of the system time evolution depending on the temperature and chemical potential and finally on the formation of clusters in the system. The characteristic relaxation times of adsorption vary in several orders of magnitude depending on the thermodynamic parameters of the final equilibrium state of the adsorbate. A very fast adsorption of particles is observed for highly ordered adsorbate equilibrium states.

Adsorption selectivity of CO2 over CH4, N2 and H2 in melamine\u2013resorcinol\u2013formaldehyde xerogels

Sorptive selectivity of melamine–resorcinol–formaldehyde xerogels, towards CO2, CH4, N2 and H2, is reported, where all systems demonstrate potential for selective adsorption of CO2 from corresponding binary gas mixtures. Selected gas mixtures represent important gas separation applications found in industry, i.e. CO2 removal from power plant flue gases (CO2–N2), sour gas sweetening (CO2–CH4), and separation of species in the water–gas shift reaction (CO2–H2). All materials tested exhibit microporous character, enhancing adsorption of small molecules, however, it is the inclusion of a nitrogen-rich material into the gel matrix that results in enhanced selectivities for these systems. Despite the porous character of the gels, under the test conditions used to simulate industrial parameters, all three balance gases, i.e. H2, N2 and CH4, showed low affinities for the xerogels, while CO2 adsorption was notably higher and increased with the inclusion and increased concentration of melamine. Ideal adsorbed solution theory was used to demonstrate significant differences in adsorption uptake, especially for CO2–CH4, and high selectivities for CO2 over N2. In all cases, selected xerogels exhibited industrially relevant adsorption timescales for CO2 over competitor gases, demonstrating the potential of these materials for the selective adsorption of CO2 from process streams.

Development of a 3D-printed modified Scheludko-cell: Potential application for adsorption and thin liquid film study

A systematic study on the formation of thin liquid films (TLFs) of n-dodecane, as an intervening liquid phase in aqueous surfactant solution, was performed using a modified 3D-printed Scheludko-cell (MSC). The formation and stability of TLFs are electrically detected using the current density-voltage (J-Ψ) characteristics of the system, and it is confirmed by in-situ visualization of TLFs. The conductivity of the TLFs is measured using the J-Ψ curve and resistors in a series model of the system. The induced conductivity of the film is measured for different concentrations of potassium chloride (KCl) and sodium dodecyl sulfate (SDS) aqueous solutions. We monitored an increase in TLF conductivity due to the accumulation of inorganic hydrophilic ions like (K+ or Cl−) and adsorption of amphiphilic anion of dodecyl sulfate to the interface. The higher rate of increase in the TLF conductivity in the case of SDS is attributed to the ability of amphiphilic ions to pass through a so-called unscreened potential in the vicinity of oil-aqueous solution interface leading to an unscreenable potential at the boundary of the TLF. The predicted adsorption time-scale of surfactants using the MSC cell in ionic liquid (IL) phase, at O/W interface, is found to be in good agreement with the dynamic interfacial tension test results.

Effective viscosity of a dilute emulsion of spherical drops containing soluble surfactant

The effective viscosity of a dilute emulsion of spherical drops containing a soluble surfactant is calculated under a linear creeping flow. It is assumed that convection of surfactant is small relative to diffusion, and thus the Peclet number, Pe, is small. We calculate the effective viscosity of the emulsion to $\mathcal {O}(Pe\phi \mu )$ , where ϕ is the small volume fraction of the dispersed drops and μ is the viscosity of the surfactant-free suspending fluid. This $\mathcal {O}(Pe\phi \mu )$ contribution is a sensitive function of the bulk and interfacial surfactant transport. Specifically, soluble surfactant molecules diffuse from the bulk to the interface and then adsorb to the interface. The ratio of the time scale for bulk diffusion to the time scale for adsorption to the interface is quantified by a Damkohler number, Da. The adsorption of surfactant to the interface may cause a significant decrease in the bulk concentration, which is known as depletion. The impact of depletion is characterized by two parameters: h, which is a dimensionless depletion depth; and k, which is the ratio of the desorption time scale to the adsorption time scale. We analytically determine how the $\mathcal {O}(Pe\phi \mu )$ contribution to the effective viscosity depends on h, k, and Da. Surprisingly, for certain regimes in the h − k − Da parameter space, we predict the effective viscosity of the emulsion to be greater than Einstein’s result for the viscosity of a suspension of rigid spheres. Large Marangoni stresses driven by convective transport of soluble surfactant molecules are responsible for this result.

Adsorption of Ellipsoidal Particles at Liquid-Liquid Interfaces.

The adsorption of particles at liquid-liquid interfaces is of great scientific and technological importance. In particular, for nonspherical particles, the capillary forces that drive adsorption vary with position and orientation, and complex adsorption pathways have been predicted by simulations. On the basis of the latter, it has been suggested that the timescales of adsorption are determined by a balance between capillary and viscous forces. However, several recent experimental results point out the role of contact line pinning in the adsorption of particles to interfaces and even suggest that the adsorption dynamics and pathways are completely determined by the latter, with the timescales of adsorption being determined solely by particle characteristics. In the present work, the adsorption trajectories of model ellipsoidal particles are investigated experimentally using cryo-SEM and by monitoring the altitudinal orientation angle using high-speed confocal microscopy. By varying the viscosity and the viscosity jump across the interfaces, we specifically interrogate the role of viscous forces.

Numerical Study of Impurity-Induced Growth Hysteresis on a Growing Crystal Surface

Growth hysteresis is one of the remarkable phenomena of crystal growth induced by impurities; namely, the growth rates of crystal take different values between when supersaturation is decreased and is increased. The appearance of hysteresis has been explained on the basis of the mean field theory, in which the physical quantities such as step velocity, step interval, and density of impurities adsorbed on the crystal surface are averaged over space and time. Here, we propose a new method for the numerical simulation of step dynamics with impurity adsorption–desorption kinetics on a growing crystal surface. We numerically reproduce the growth hysteresis in a more realistic situation, in which the physical quantities fluctuate. The calculated hysteresis agrees with the prediction of the mean field theory. We have also found that the supersaturation should be changed within a period that is similar to the impurity adsorption time scale or slightly longer in order to observe obvious growth hysteresis. Measurem...

Effect of Nanoparticle Surface Chemistry on Adsorption and Fluid Phase Partitioning in Aqueous/Toluene and Cellular Systems.

Copolymers of di(ethylene glycol) methyl ether methacrylate (x = MeO2MA) and oligo(ethylene glycol) methyl ether methacrylate (y = OEGMA) display lower critical solution phenomena in aqueous systems that are tunable by the copolymer ratio (x:y), ionic strength, and temperature. These properties enable tuning the hydrophobicity of macromolecular systems by variation of (x:y). For nanoparticles stabilized with these macromolecules, this provides a systematic approach to understanding the impact of surface chemistry, specifically hydrophobicity, on the equilibrium and transport properties of nanomaterials in biphasic systems. We synthesized a homologous series of gold nanoparticles capped by these copolymers, Au@(MeO2MA(x)-co-OEGMA(y)). By varying the copolymer 95:5 < (x:y) < 80:20 ratio, we studied the effect of surface hydrophobicity on the nanoparticle equilibrium adsorption isotherm and phase transfer at the aqueous-toluene interface. The increase in hydrophobicity from (x:y) = 80:20 to (x:y) = 95:5 is accompanied by an increase in the fractional coverage of the aqueous-toluene interface from f = 0.3 to f > 1, or multilayer adsorption and an increase in the characteristic adsorption timescale from τ(D) = 31 to τ(D) = 450 seconds. The equilibrium partition coefficient for the aqueous/toluene systems, K(T/W) is also a strong function of (x:y), increasing from K(T/W) (80:20) = 0.7 to K(T/W) (95:5) = 9.8. We also observed an increase in cellular uptake for increasing (x:y) suggesting that surface chemistry alone plays a significant role in intercellular transport processes.

A sub-pore model for multi-scale reaction–diffusion problems in porous media

Applications of reaction–diffusion systems in porous media pose a challenging problem for computational modeling approaches due to their multi-physics and multi-scale nature. The length scales usually span 3–4 orders of magnitude while physical phenomena involved include heat and mass transfer processes, and surface reactions. In this paper, a novel methodology that accounts for all the length scales and physical phenomena involved in a single framework is described. A length scale based dual approach is proposed – the larger pore channels (macro-pores) are resolved using conventional numerical techniques and a novel ‘sub-pore’ model is used to account for the unresolved pore channels (sub-pores) and the important physics therein. The porous network in the sub-pore system is composed of a fractal-like hierarchical system of straight cylindrical pores. Simplified governing equations for mass and energy transport are solved within the sub-pore system along with a reaction kinetics model to account for surface adsorption. An implicit coupling strategy is used to couple the macro-pore and the sub-pore systems so as to ensure conservation. The developed methodology is then applied to a few test cases and it is established that the proposed framework is necessary for problems where the adsorption time scale is much smaller than (diffusion-limited) or comparable to the diffusion time scale. It is also demonstrated that the framework can be potentially used to model the network of porous channels in its entirety thus significantly reducing computational costs.

Size and surface chemistry of nanoparticles lead to a variant behavior in the unfolding dynamics of human carbonic anhydrase.

The adsorption induced conformational changes of human carbonic anhydrase I (HCAi) and pseudo wild type human carbonic anhydrase II truncated at the 17th residue at the N-terminus (trHCAii) were studied in presence of nanoparticles of different sizes and polarities. Isothermal titration calorimetry (ITC) studies showed that the binding to apolar surfaces is affected by the nanoparticle size in combination with the inherent protein stability. 8-Anilino-1-naphthalenesulfonic acid (ANS) fluorescence revealed that HCAs adsorb to both hydrophilic and hydrophobic surfaces, however the dynamics of the unfolding at the nanoparticle surfaces drastically vary with the polarity. The size of the nanoparticles has opposite effects depending on the polarity of the nanoparticle surface. The apolar nanoparticles induce seconds timescale structural rearrangements whereas polar nanoparticles induce hours timescale structural rearrangements on the same charged HCA variant. Here, a simple model is proposed where the difference in the timescales of adsorption is correlated with the energy barriers for initial docking and structural rearrangements which are firmly regulated by the surface polarity. Near-UV circular dichorism (CD) further supports that both protein variants undergo structural rearrangements at the nanoparticle surfaces regardless of being "hard" or "soft". However, the conformational changes induced by the apolar surfaces differ for each HCA isoform and diverge from the previously reported effect of silica nanoparticles.

Surfactant-induced rigidity of interfaces: a unified approach to free and dip-coated films.

The behavior of thin liquid films is known to be strongly affected by the presence of surfactants at the interfaces. The detailed mechanism by which the latter enhance film stability is still a matter of debate, in particular concerning the influence of surface elastic effects on the hydrodynamic boundary condition at the liquid/air interfaces. In the present work, "twin" hydrodynamic models neglecting surfactant transport to the interfaces are proposed to describe the coating of films onto a solid plate (Landau-Levich-Derjaguin configuration) as well as soap film pulling (Frankel configuration). Experimental data on the entrained film thickness in both configurations can be fitted very well using a single value of the surface elasticity, which is in good agreement with independent measurements by mean of surface expansion experiments in a Langmuir through. The analysis thus shows that soap films or dip coating experiments may be used to measure the surface elasticity of surfactant solutions in the insoluble limit, namely as long as the film generation dynamics is fast compared to the surfactant adsorption timescale.

Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species.

Three primary amine materials functionalized onto mesoporous silica with low, medium, and high surface amine coverages are prepared and evaluated for binary CO2/H2O adsorption under dilute conditions. Enhancement of amine efficiency due to humid adsorption is most pronounced for low surface amine coverage materials. In situ FT-IR spectra of adsorbed CO2 on these materials suggest this enhancement may be associated with the formation of bicarbonate species during adsorption on materials with low surface amine coverage, though such species are not observed on high surface coverage materials. On the materials with the lowest amine loading, bicarbonate is observed on longer time scales of adsorption, but only after spectral contributions from rapidly forming alkylammonium carbamate species are removed. This is the first time that direct evidence for bicarbonate formation, which is known to occur in liquid aqueous amine solutions, has been presented for CO2 adsorption on solid amine adsorbents.

Nanoplasmonic biosensing for soft matter adsorption: kinetics of lipid vesicle attachment and shape deformation.

An indirect nanoplasmonic sensing platform is reported for investigating the kinetics of attachment and shape deformation associated with lipid vesicle adsorption onto a titanium oxide-coated substrate. The localized surface plasmon resonance (LSPR) originates from embedded gold nanodisks and is highly sensitive to the local lipid environment. To interpret the corresponding results, we have extended treatments of diffusion-limited adsorption kinetics and adsorbate-related LSPR physics, identified the expected scaling laws for the LSPR-tracked kinetics measured at different lipid concentrations and/or nanometer-scale vesicle sizes in the case when vesicle deformation is negligible, and scrutinized experimental deviations accordingly. After adsorption, the smallest 58 nm diameter vesicles were found to maintain shape on the time scale of adsorption at high lipid concentrations in solution, and shape deformation became more appreciable at lower lipid concentrations. Higher saturation coverage was observed with increasing lipid concentration, which is attributed to the difference in relative time scales of vesicle attachment and deformation. For larger vesicles between 80 and 160 nm diameter, deviations associated with their shape deformation and correlations with the location of gold nanodisks became more apparent at moderate and high coverages. Taken together, the results obtained support that the quantitative measurement capabilities of nanoplasmonic biosensing should be considered for applications demanding highly surface-sensitive characterization of soft matter adsorption and related phenomena at liquid-solid interfaces.

Glassy dynamics of soft matter under 1D confinement: How irreversible adsorption affects molecular packing, mobility gradients and orientational polarization in thin films

The structural dynamics of polymers and simple liquids confined at the nanometer scale has been intensively investigated in the last two decades in order to test the validity of theories on the glass transition predicting a characteristic length scale of a few nanometers. Although this goal has not yet been reached, the anomalous behavior displayed by some systems--e.g. thin films of polystyrene exhibit reductions of Tg exceeding 70 K and a tremendous increase in the elastic modulus--has attracted a broad community of researchers, and provided astonishing advancement of both theoretical and experimental soft matter physics. 1D confinement is achieved in thin films, which are commonly treated as systems at thermodynamic equilibrium where free surfaces and solid interfaces introduce monotonous mobility gradients, extending for several molecular sizes. Limiting the discussion to finite-size and interfacial effects implies that film thickness and surface interactions should be sufficient to univocally determine the deviation from bulk behavior. On the contrary, such an oversimplified picture, although intuitive, cannot explain phenomena like the enhancement of segmental mobility in proximity of an adsorbing interface, or the presence of long-lasting metastable states in the liquid state. Based on our recent work, we propose a new picture on the dynamics of soft matter confined in ultrathin films, focusing on non-equilibrium and on the impact of irreversibly chain adsorption on the structural relaxation. We describe the enhancement of dynamics in terms of the excess in interfacial free volume, originating from packing frustration in the adsorbed layer (Guiselin brush) at t(*) ≪ 1, where t(*) is the ratio between the annealing time and the time scale of adsorption. Prolonged annealing at times exceeding the reptation time (usually t(*) ≫ 1 induces densification, and thus reduces the deviation from bulk behavior. In this Colloquium, after reviewing the experimental approaches permitting to investigate the structural relaxation of films with one, two or no free surfaces by means of dielectric spectroscopy, we propose several methods to determine gradients of mobility in thin films, and then discuss on the unexploited potential of analyses based on the time, temperature and thickness dependence of the orientational polarization via the dielectric strength.

Contaminant Transport in Partially Saturated Porous Media

We discuss the numerical modelling of unsaturated flow in porous media with contaminant transport, dispersion and adsorption. The mathematical model for unsaturated flow is based on Richard's nonlinear and degenerate equation. The model of contaminant transport is based on Darcy's law and mass balance equation. We present the operator splitting method for this problem. In nature all of the physical processes are realized simultaneously and the time scale for adsorption differs from diffusion and convection significantly. In our numerical approximation we consider three sub-problems - first one is the problem of unsaturated flow, second one the problem of transport and dispersion and third one the adsorption problem. Our numerical solution is based on implicit time discretization and space discretization. To achieve more correct numerical approximation we treat each of the sub-problems with another time scale and the data computed from one sub-problem, is the input data for the next sub-problem in each time interval. The convergence to the weak solution of the original problem is also discussed. Finally we show a couple of practical applications of the method and numerical experiments for direct problems.

Lattice–Boltzmann simulations of dynamic interfacial tension due to soluble amphiphilic surfactant

An amphiphilic Lattice–Boltzmann approach is adopted to model dynamic interfacial tension due to non-ionic surfactant. In the current system, the surfactant adsorption kinetics is diffusion dominated and the interface separates two immiscible fluids. A rotational relaxation time and a diffusive/viscous relaxation time are associated with the surfactant. The model results are compared with experimental data for the dynamic interfacial tension of a pendant oil droplet in water, with oil soluble surfactant. We demonstrate how to adapt and calibrate the model to capture the adsorption timescale of the surfactant and the magnitude of interfacial tension reduction due to surfactant. A scheme to overcome numerical instabilities due to the relatively low surfactant concentration, is devised. We are able to qualitatively match the Frumkin equation of state for the interfacial tension.

Importance of adsorption for CCN activity and hygroscopic properties of mineral dust aerosol

This study uses published data on dust‐water interactions to examine the importance of including water adsorption effects when describing the hygroscopic and cloud condensation nuclei (CCN) behavior of mineral dust aerosol. Adsorption activation theory (AT) better represents fresh dust‐water interactions than Köhler theory (KT), as i) a consistent set of adsorption parameters can describe the hygroscopic behavior of dust (under both sub and supersaturated conditions), and ii) the dependence of critical supersaturation, sc, with particle dry diameter, Ddry, is closer to observations. The long adsorption timescale could also contribute to the large differences observed between dry and wet generated dust hygroscopicity. If KT and AT are consistently applied to the same dust size distribution, KT predicts up to tenfold higher CCN and 40% higher droplet number concentration than AT. This profoundly different behavior between the theories suggests that both may be required for a comprehensive description of atmospheric dust CCN activity.