Concentration-dependent Diffusion Research Articles

Plasma Charge Carrier Attachment Induced Transport in Solid Ionic Materials

Lithium aluminum germanium phosphate of the stoichiometric formula Li1.5Al0.5Ge1.5(PO4)3 has been studied by means of the femtosecond laser plasma charge attachment induced transport (fs-plasma CAIT) [1]. The technique is based on attaching polarity selected charge carriers from a plasma to the front side of a sample, which induces transport of mobile charge carriers in the bulk of this sample. First, in a conductivity study, an activation energy of 0.73 eV for lithium ion transport has been measured by means of proton attachment. Second, a constant-voltage attachment experiment employing deuterium ions has been performed and subsequently analyzed by time-of-flight secondary ion mass spectrometry. The resulting concentration depth profile revealed a replacement of native lithium ions by deuterons in the first 350 nm of the sample. A theoretical analysis of the profile by means of the Nernst−Planck−Poisson equations provides access to the concentration dependent diffusion coefficient and the unique site energy distribution of the natively contained lithium ions. A full width at half-maximum of 113 meV for the SED of the Li+ ions is determined. The approach discussed can be applied to many more materials, where the potential energy landscape affects the function.More recent efforts in studying solid state ionic materials by means of the Plasma-CAIT technique will also be presented.[1] Jan L. Wiemer, Martin Schäfer, and Karl-Michael Weitzel, J. Phys. Chem. C, 125, 4977−4985, (2021) Figure 1

Control-Oriented Modeling of All-Solid-State Batteries Using Physics-Based Equivalent Circuits

Considered as one of the ultimate energy storage technologies for electrified transportation, the emerging all-solid-state batteries (ASSBs) have attracted immense attention due to their superior thermal stability, increased power and energy densities, and prolonged cycle life. To achieve the expected high performance, practical applications of ASSBs require accurate and computationally efficient models for the design and implementation of many onboard management algorithms, so that the ASSB safety, health, and cycling performance can be optimized under a wide range of operating conditions. A control-oriented modeling framework is thus established in this work by systematically simplifying a rigorous partial differential equation (PDE) based model of the ASSBs developed from underlying electrochemical principles. Specifically, partial fraction expansion and moment matching are used to obtain ordinary differential equation based reduced-order models (ROMs). By expressing the models in a canonical circuit form, excellent properties for control design such as structural simplicity and full observability are revealed. Compared to the original PDE model, the developed ROMs have demonstrated high fidelity at significantly improved computational efficiency. Extensive comparisons have also been conducted to verify its superiority to the prevailing models due to the consideration of concentration-dependent diffusion and migration. Such ROMs can thus be used for advanced control design in future intelligent management systems of ASSBs.

Spatiotemporal measurement of concentration-dependent diffusion coefficient

We introduce a method to measure the concentration-dependent diffusion coefficient from a sequence of images of molecules diffusing from a source toward a sink. Generally, approaches measuring the diffusion coefficient, such as fluorescence recovery after photobleaching (FRAP), assume that the diffusion coefficient is constant. Hence, these methods cannot capture the concentration dependence of the diffusion coefficient if present. Other approaches measure the concentration-dependent diffusion coefficient from an instantaneous concentration profile and lose the temporal information. These methods make unrealistic assumptions, are not robust, and lead to 100% error. We introduce an image analysis framework that utilizes spatial and temporal information in a sequence of concentration images and numerically solves the general form of Fick's second law using radial basis functions (RBF) to measure the concentration-dependent diffusion coefficient. We term this approach as concentration image diffusimetry (CID). Our method makes no assumptions about the sink and source size and the diffusion dependence on concentration. CID is superior to existing methods in estimating spatiotemporal changes and concentration-dependent diffusion. CID also provides a statistical uncertainty quantification on the measurements using a bootstrapping approach, improving the reliability of the diffusion measurement. We assessed CID's performance using synthetically generated images. Our analysis suggests that CID accurately measures the diffusion coefficient with less than 2% error for most cases. We validated CID with FRAP experimental images and showed that CID agrees with established FRAP algorithms for samples with a constant diffusion coefficient. Finally, we demonstrate the application of CID to experimental datasets of a concentration gradient-driven protein diffusion into a tissue replicate. In conclusion, this work presents an image-based methodology that uses the spatial and temporal changes of concentration fields to measure the concentration-dependent diffusion coefficient.

Enhanced Neutral Exciton Diffusion in Monolayer WS2 by Exciton-Exciton Annihilation.

Dominant recombination pathways in monolayer transition metal dichalcogenides (TMDCs) depend primarily on background carrier concentration, generation rate, and applied strain. Charged excitons formed in the presence of background carriers mainly recombine nonradiatively. Neutral excitons recombine completely radiatively at low generation rates, but experience nonradiative exciton-exciton annihilation (EEA) at high generation rates. Strain can suppress EEA, resulting in near-unity photoluminescence quantum yield (PL QY) at all exciton densities. Although exciton diffusion is the primary channel of energy transport in excitonic materials and a critical optoelectronic design consideration, the combined effects of these factors on exciton diffusion are not clearly understood. In this work, we decouple the diffusion of neutral and charged excitons with chemical counterdoping and explore the effect of strain and generation rate on exciton diffusion. According to the standard semiconductor paradigm, a shorter carrier recombination lifetime should lead to a smaller diffusion length. Surprisingly, we find that increasing generation rate shortens the exciton lifetime but increases the diffusion length in unstrained monolayers of TMDCs. When we suppress EEA by strain, both lifetime and diffusion length become independent of generation rate. During EEA one exciton nonradiatively recombines and kinetically energizes another exciton, which then diffuses fast. Our results probe concentration-dependent diffusion of pure neutral excitons by counterdoping and elucidate how strain controls exciton transport and many-body interactions in TMDC monolayers.

Effect of interfacial kinetics on the settling of a drop in a viscous medium

Multiphase emulsions, such as drops in a continuous medium, tend to have surfactant-like impurities present at the interfaces, either naturally or introduced artificially for stability, which may influence the flow field and, hence, alter the motion of the drops through a host of different mechanisms. Here, we carry out a robust analysis to characterize multiple aspects of such interfacial phenomena by studying the settling of a drop in a quiescent viscous medium. The surface active agents are assumed to be bulk-insoluble and non-ideal, while the interface itself is assumed to have its own rheology, described by the Boussinesq–Scriven model. The diffusive fluxes of the surfactants are expressed in a thermodynamically consistent manner as proportional to the chemical potential gradient, which results in concentration dependent diffusivity. We subsequently derive semi-analytical solutions for approximately spherical drops without any other restrictions on the transport processes. Our results reveal that stresses originating from interfacial rheology tend to decrease the settling velocity and at the same time make the surfactant concentration uniform across the surface. Remarkably, this settling velocity is revealed to be independent of the choice of the free-energy isotherms and the extent of packing of the surfactants when a variable diffusivity is correctly accounted for. These insights will be helpful in better understanding of the underlying dynamics of surfactant-laden drops, having potential applications in microfluidic devices, food and pharmaceutical industries, and separation processes.

Calculation of gas concentration-dependent diffusion coefficient in coal particles: Influencing mechanism of gas pressure and desorption time on diffusion behavior

To investigate the effects of equilibrium pressure and desorption time on gas diffusion in coal particles, we described the time-varying diffusion process by introducing an indeterminate function of desorption time. Using gas desorption data, we established an analytical model for calculating the gas diffusion coefficient under the different desorption times. The analytical solution of the concentration-dependent diffusion model is equivalent to that of the unipore diffusion model at the initial desorption time. Still, the diffusion coefficient decreased with the decrease of gas equilibrium pressure and the extension of desorption time. Additionally, the numerical method compared adsorbed and free gas effects on gas desorption. The results demonstrated that the diffusion coefficient considered adsorbed gas was more significant than the analytical solution's. The diffusion coefficient in the free gas system was overestimated due to the change of diffusion distance and decreased to the actual value over desorption time. However, the attenuation rate of diffusion coefficients in the adsorbed gas system positively correlates with gas pressure. The concentration gradient of free gas is the driving force for gas diffusion. The simulation and experimental results show that the free to total gas ratio has a strong linear relationship with the diffusion coefficient, which explains the influencing mechanism of gas pressure and desorption time on the gas diffusion.

The effect of concentration-dependent diffusion on double-diffusive instability

The article studies the stability of a two-layer miscible system to the double-diffusive instability. The system is placed in a vertical Hele–Shaw cell and is composed of two homogeneous aqueous solutions initially separated by a narrow transient zone. We have restricted our consideration to the initially stable density stratification that precludes the Rayleigh–Taylor instability. The main objective of the study is to elucidate the effect of a concentration-dependent diffusion coefficient, which has been commonly ignored by researchers. Assuming linear dependence of the diffusion coefficient of each solute and using Picard's iteration scheme, we have derived a closed-form analytical expression for the time-dependent density profile. This permits the stability boundary to be established for a two-layer system with respect to the double-diffusive instability by taking into account the effect of a concentration-dependent diffusion coefficient. The obtained analytical result has been substantiated by the results of direct numerical simulation. The experiments have shown that a successive increase in the concentrations of both solutes, with their ratio remaining unchanged, can lead to opposite results. In the case of a NaNO3-H2SO4 pair, the two-layer system, being stable at low concentrations, becomes unstable as the concentrations proportionally increase, giving rise to convective motion in the form of salt fingers. On the contrary, a two-layer system consisting of LiCl and NaNO3 solutions is stabilized with increasing concentrations of dissolved substances. A further increase in the concentrations of these substances causes mechanical equilibrium breaking and subsequent formation of the so-called diffusive-layer convection. The experimental results are in good agreement with the theoretical predictions.

Diffusive fractionation of K isotopes in molten basalts

The 41K/39K isotope ratio profiles in diffusion couple experiments have been measured by Secondary Ion Mass Spectrometry (SIMS). One goal of this research is to push the use of SIMS in measuring non-traditional stable isotope ratios. The second, more important goal, is to quantify for the first time diffusive fractionation of K isotopes and its dependence on temperature and different counter-diffusion elements. The data show that the precision in a single day-night session of SIMS measurements can reach 0.2‰ (1σ hereafter) with effort, and the long-term accuracy without using any isotope ratio standard is about 2.5‰. At an initial concentration contrast (ratio of high concentration to low concentration) of about 70 in a diffusion couple, the total 41K/39K fractionation (maximum minus minimum) is about 10‰. The 41K/39K ratio profiles were initially fit by assuming constant effective binary diffusivity (D) of K2O, which led to minor misfits and more importantly, to large disagreement between diffusivities based on chemical diffusion and isotope diffusion profiles. It was found that D for K2O varies with its concentration. The profiles were then fit by assuming D for K2O increases exponentially with K2O concentration, which resolved the misfits and disagreements. That is, combining concentration and isotope ratio profiles enables distinguishing subtle concentration-dependent diffusivity. For the SiO2-K2O interdiffusion couples, the empirical diffusive isotope fractionation parameter β increases slightly with temperature from 0.104 ± 0.003 at 1260°C to 0.116 ± 0.003 at 1500°C. For the MgO-K2O interdiffusion couples, excluding an outlier point at 1260°C, β increases from 0.090 ± 0.005 at 1350°C to 0.100 ± 0.003 at 1500°C. These β values are roughly consistent with a diffusion mechanism of NaKO exchanging with SiO2 in the SiO2-K2O interdiffusion couples, or NaKO exchanging with MgO in the MgO-K2O interdiffusion couples. Applying the obtained β value to model diffusive isotope fractionation in nature, diffusive K isotope fractionation during magma mixing is expected to be large enough to be resolvable by SIMS. When collecting samples for K-Ar or K-Ca dating, it is important to correct for the effect of possible K isotope fractionation by measuring K isotope ratio in the sample. When diffusive and convective-diffusive modeling was applied to evaluate isotope fractionation during volatile loss through diffusion and evaporation, it is found that a loss of 80% potassium would lead to an increase of δ41K by 6.3‰ to 8.9‰, more than 10 times greater than the enrichment of δ41K in the Moon relative to the Earth. Hence, the depletion of K and the associated enrichment of δ41K in the Moon relative to the Earth are unlikely diffusion controlled.

Evaluation of moisture diffusion characteristics and the effect of moisture treatment on flexural properties of expanded perlite-based building material

The expanded perlite-based building material for drywall application consisting of sodium silicate solution as a binder was manufactured by varying the degree of compaction and sodium silicate content to investigate moisture diffusion behavior and the effect of moisture treatment on flexural properties of the composites. Moisture treatment was conducted on specimens in a climatic chamber at a temperature of 37°C and a relative humidity of 90% until saturation. Results show that moisture absorption decreased with increasing compaction ratio for a constant sodium silicate content in binder and increased with increasing sodium silicate content in binder for a constant compaction ratio. A range of volume fractions of solid sodium silicate in the foam is identified, in which the fully Fickian diffusion gradually transformed to non-Fickian diffusion as sodium silicate content in foam increased. The concentration-dependent diffusion method was found to be suitable to explain this behavior. The moisture diffusion below this transition range showed an entirely Fickian diffusion and changed to concentration-dependent diffusion above the range. As a result of moisture treatment, the flexural strength of medium density foams was decreased but the lowest- and highest-density foams were not affected while the flexural modulus was increased only for the highest density foam and no significant effects were seen in other cases. The bending failure mechanism of the composite was not affected by the moisture treatment.

Preparation of infrared axial gradient refractive index lens based on powder stacking and the sintering thermal diffusion method

The gradient refractive index (GRIN) lens is widely used in the visible band, but it is still elusive in the infrared band. In this paper, we propose a new method of fabricating chalcogenide GRIN by spark plasma sintering (SPS) technology based on powder stacking and sintering thermal diffusion. We replaced Se in Ge11.5As24Se64.5 glass with S and prepared several Ge11.5As24Se(64.5-x)Sx glasses as infrared transmission GRIN materials. The maximum refractive index difference (Δn) of the matrix glass is 0.18. The effects of heat treatment temperature and time on diffusion depth and concentration-dependent thermal diffusion coefficient were investigated. The diffusion depth of 100 µm was demonstrated under the condition of 400 °C-48 h by this method. The thickness of the glass layer can be well controlled by powder stacking. The obtained GRIN glass is highly transparent in the near- and mid-infrared wavelength region.

Convective instability in multicomponent mixtures with Soret effect

We study the two-dimensional modes of gravity-dependent convective instability arising in a ternary mixture in a plane horizontal layer. The gravity force is oriented perpendicular to the layer heated from below. It is assumed that the system is in the state of mechanical equilibrium of a mixture considered as the base state of the system, which can become unstable under certain conditions. The mathematical model uses two-dimensional Navier-Stokes equations, transfer equations for describing mixture components with the Soret effect, and a heat transfer equation. Under the influence of a concentration gradient, the heat transfer symmetric to the Soret effect is neglected because it is usually small in water-based mixtures. The possible concentration-dependent diffusion and cross-diffusion of dissolved components are also neglected. We investigate a convective stability using linear approximation and under finite-amplitude convection regimes. The stability analysis of the base state after linearization of the governing equations with respect to the state of mechanical equilibrium predicts various convection modes. For each case, a neutral curve and the dependences of the disturbance growth decrements on both the Rayleigh number and the wave number are plotted. We show that in the case of the layer being heated from below, there are ranges of governing parameters where both long-wave and short-wave modes of oscillatory instability occur. The numerical analysis of a nonlinear coupling between these modes has revealed that such instabilities develop in the ternary mixtures whose components are arranged (due to the Soret effect) in different directions with respect to the temperature gradient. The effect of the Rayleigh number on the flow structure and heat mass transfer is demonstrated.

Tracer diffusion in proton-exchanged congruent LiNbO3 crystals as a function of hydrogen content.

The proton-exchange process is an effective method of fabricating low-loss waveguides based on LiNbO3 crystals. During proton-exchange, lithium is replaced by hydrogen and Li1-xHxNbO3 is formed. Currently, mechanisms and kinetics of the proton-exchange process are unclear, primarily due to a lack in reliable tracer diffusion data. We studied lithium and hydrogen tracer diffusion in proton-exchanged congruent LiNbO3 single crystals in the temperature range between 130-230 °C. Proton-exchange was done in benzoic acid with 0, 1, 2, or 3.6 mol% lithium benzoate added, resulting in micrometre thick surface layers where Li is substituted by H with relative fractions between x = 0.45 and 0.85 as determined by Nuclear Reaction Analysis. For the diffusion experiments, ion-beam sputtered isotope enriched 6LiNbO3 was used as a Li tracer source and deuterated benzoic acid as a H tracer source. Isotope depth profile analysis was carried out by secondary ion mass spectrometry. From the experimental results, effective diffusivities governing the lithium/hydrogen exchange as well as individual hydrogen and lithium tracer diffusivities are extracted. All three types of diffusivities can be described by the Arrhenius law with an activation enthalpy of about 1.0-1.2 eV and increase as a function of hydrogen content nearly independent of temperature. The effective diffusivities and the lithium tracer diffusivities are identical within a factor of two to five, while the hydrogen diffusivities are higher by three orders of magnitude. The results show that the diffusion of Li is the rate determining step governing the proton-exchange process. Exponential dependencies between diffusivities and hydrogen concentrations are determined. The observed increase of Li tracer diffusivities and effective diffusivities as a function of hydrogen concentration is attributed to a continuous reduction of the migration enthalpy of diffusion by a maximum factor of about 0.2 eV. Simulations based on the determined diffusivities can reproduce the step-like profile of hydrogen penetration during proton-exchange.

Investigation on desorption process in fixed bed for lithium recovery

• Desorption process of Li + in the fixed bed is studied with lithium-aluminum hydroxides adsorbent. • Basic mass transfer equations for Li + desorption process are established and solved. • The observed Li + elution curves in fixed bed columns were well described by the mathematical model. With the development of lithium-ion batteries, the demand for lithium increase rapidly. Adsorption method is one of the most promising method to extract lithium from salt lake brine. In this work, desorption process for lithium recovery from simulated salt lake brine was systematically investigated under various operational conditions in fixed bed columns packed with lithium-aluminum hydroxides adsorbent, including the effects of washing volume, temperature and flowrate on the washing performance, as well as the effects of elution flowrate and volume on elution rate of Li + . The optional washing temperature is 283 K, and washing flowrate is 45 mL/min. The desorption rate of lithium is only affected by elution volume, and flow rate may not affect the Li desorption significantly under the temperature of 333 K. The reason might be that the diffusion rate is fast enough and the process is dominated by adsorption equilibrium. Thus, temperature may be a key factor to the performance of fixed bed desorption process. The concentration-dependent Homogeneous Surface Diffusion Model (HSDM) was developed to fit and simulate the dynamic elution performance of lithium in fixed bed column. The overall pattern of the elution process can be represented by the HSDM. These results provided significant data for the design, optimization and scale-up of the column desorption process for brine lithium recovery.

Empirical Model and PSO-Based Algorithm for Efficient Measurement of Gas Permeation Through High Barrier

Efficient measurement of gas permeation through high-barrier materials requires effective theoretical and numerical tools for data reduction and parameter extraction, to reduce the measurement period within error tolerance. An empirical permeation model was developed in this regard, by comprehensively considering gas–surface interactions as well as time and concentration-dependent diffusivity of the permeant particle. An algorithm was developed based on particle swarm optimization (PSO), to numerically fit the experimental data to the permeation model and to extract the characteristic permeation parameters. Compared with the traditional models, the comprehensive model is more accurate to predict the actual permeation phenomena, allowing 75% reduction of the measurement period within 10% tolerance on the extracted steady-state permeability. In addition, the PSO algorithm demonstrates extended applications in other non-steady-state transport problems such as convective–diffusive heat transfer.

First-principles studies of the concentration-dependent tritium diffusion in the zirconium hydrides with and without Sn impurity

We performed first-principles calculations to study the 3H diffusion in the zirconium hydrides with or without Sn impurity. Our results show that the formation of Zr3Hx becomes preferable as the 3H concentration increases. Based on the most stable configurations of Zr3Hx (x = 0.5, 1.0, 1.5, 2.0), we studied the interstitial 3H diffusion with or without Sn impurity. The results show that the interstitial 3H diffusion becomes less probable in the hydrides with a higher 3H concentration. When introducing a substitutional Sn impurity, the diffusion barriers increase. Therefore, the substitutional Sn in the diffusion pathway hampers the 3H diffusion in Zr hydrides.

Physics-based modeling of sodium-ion batteries part II. Model and validation

Sodium-ion batteries (SIBs) have recently been proclaimed as the frontrunner 'post lithium' energy storage technology. This is because SIBs share similar performance metrics with lithium-ion batteries, and sodium is 1000 times more abundant than lithium. In order to understand the electrochemical characteristics of SIBs and improve present-day designs, physics-based models are necessary. Herein, a physics-based, pseudo-two-dimensional (P2D) model is introduced for SIBs for the first time. The P2D SIB model is based on Na3V2(PO4)2F3 (NVPF) and hard carbon (HC) as positive and negative electrodes, respectively. Charge transfer in the NVPF and HC electrodes is described by concentration-dependent diffusion coefficients and kinetic rate constants. Parametrization of the model is based on experimental data and genetic algorithm optimization. It is shown that the model is highly accurate in predicting the discharge profiles of full cell HC//NVPF SIBs. In addition, internal battery states, such as the individual electrode potentials and concentrations, can be obtained from the model at applied currents. Several key challenges in both electrodes and the electrolyte are herein unraveled, and useful design considerations to improve the performance of SIBs are highlighted.

Non-Fickian diffusion in viscous aerosol particles

Slow condensed phase diffusion in organic aerosol particles can impede many chemical and physical processes associated with atmospheric aerosol (e.g., gas-particle equilibrium partitioning). The characteristic times associated with these high viscosity particles are typically modelled using a concentration-dependent diffusivity within a purely Fickian framework. In that model, the medium in which diffusion is taking place is treated as being inviscid as far as mass transport is concerned. In this report, we investigate the validity of assuming that the viscosity is equal to zero by using a transport model that includes viscous pressure gradients. It is found that the effect of viscosity is negligible for particles with radii that are larger than 100 nm, but below that radius, it can delay water uptake and loss by orders of magnitude for physically realistic viscosities. However, if the Stokes–Einstein relation is obeyed, then viscosity can be ignored, even for nanosized particles. In addition to numerical calculations, a dimensionless Deborah number is defined that indicates the significance of Fickian diffusion compared with the rheological response during water transport.

Activation energy of silicon diffusion in gallium oxide: Roles of the mediating defects charge states and phase modification

Silicon (Si) is an efficient n-type dopant in gallium oxide (Ga2O3)—an ultra-wide bandgap semiconductor promising in a number of applications. However, in spite of the technological importance for device fabrication, the activation energy for Si diffusion in Ga2O3 is missing in the literature. In the present work, we do such measurements in ion implanted monoclinic β-Ga2O3 samples employing anneals in air ambient, also admitting the influence of potential ion beam induced phase modifications on diffusion. Importantly, we show that Si diffusion in β-Ga2O3 fits with the concentration dependent diffusion model, involving neutral and single negatively charged point defects to mediate the process; so that we assumed gallium vacancies in the corresponding charge states to assist Si diffusion in β-Ga2O3 with activation energies of 3.2 ± 0.3 and 5.4 ± 0.4 eV, respectively. Moreover, we also found that a preexisting phase modified surface layer efficiently suppressed Si diffusion in β-Ga2O3 for temperatures up to 1000 °C.

Physical Origin of the Differential Voltage Minimum Associated With Lithium Plating in Li-Ion Batteries

The main barrier to fast charging of Li-ion batteries at low temperatures is the risk of short-circuiting due to lithium plating. In-situ detection of Li plating is highly sought after in order to develop fast charging strategies that avoid plating. It is widely believed [1,2,3] that Li plating after a single fast charge can be detected and quantified by using a minimum in the differential voltage (DV) signal during the subsequent discharge, which indicates how much lithium has been stripped. In this work, [4] a pseudo-2D physics-based model is used to investigate the effect on Li plating and stripping of concentration-dependent diffusion coefficients [5] in the active electrode materials. A new modelling protocol is also proposed, in order to distinguish the effects of fast charging, slow charging and Li plating/stripping. The model predicts that the DV minimum associated with Li stripping is in fact a shifted and more abrupt version of a minimum caused by the stage II-stage III transition in the graphite negative electrode. Therefore, the minimum cannot be used to quantify stripping. Using concentration-dependent diffusion coefficients yields qualitatively different results to previous work [2,3]. This knowledge casts doubt on the utility of DV analysis for detecting Li plating.[1] Mathias Petzl and Michael A. Danzer, J.Power Sources vol. 254, pp. 80-87, 2014.[2] Xiao-Guang Yang et al., J. Power Sources vol. 395, pp. 251-261, 2018.[3] Dongsheng Ren et al., J. Electrochem. Soc. vol. 165 (10), pp. A2167-A2178, 2018.[4] Simon E. J. O’Kane et al., J. Electrochem. Soc. vol. 167, pp. 090540, 2020.[5] Madeleine Ecker et al., J. Electrochem. Soc. vol. 162 (9), pp. A1836-A1848, 2015.[6] Ian D. Campbell et al., J. Electrochem. Soc. vol. 166 (4), pp. A725-A739, 2019. Figure 1

Protein Transport upon Advection at the Air/Water Interface: When Charge Matters.

The formation of dense protein interfacial layers at a free air-water interface is known to result from both diffusion and advection. Furthermore, protein interactions in concentrated phases are strongly dependent on their overall positive or negative net charge, which is controlled by the solution pH. As a consequence, an interesting question is whether the presence of an advection flow of water toward the interface during protein adsorption produces different kinetics and interfacial structure of the adsorbed layer, depending on the net charge of the involved proteins and, possibly, on the sign of this charge. Here we test a combination of the following parameters using ovalbumin and lysozyme as model proteins: positive or negative net charge and the presence or absence of advection flow. The formation and the organization of the interfacial layers are studied by neutron reflectivity and null-ellipsometry measurements. We show that the combined effect of a positive charge of lysozyme and ovalbumin and the presence of advection flow does induce the formation of interfacial multilayers. Conversely, negatively charged ovalbumin forms monolayers, whether advection flow is present or not. We show that an advection/diffusion model cannot correctly describe the adsorption kinetics of multilayers, even in the hypothesis of a concentration-dependent diffusion coefficient as in colloidal filtration, for instance. Still, it is clear that advection is a necessary condition for making multilayers through a mechanism that remains to be determined, which paves the way for future research.