Bulk Solution Concentration Research Articles

De Novo Design and Characterization of Amphiphilic Peptides with Basic Side Chains for Tailored Interfacial Chemistries.

Lysine-leucine (LK) peptides have been used as model systems and platforms for 2D material design for decades. LK peptides are amphiphilic sequences designed to bind and fold at hydrophobic surfaces through hydrophobic leucine side chains and hydrophilic lysine side chains extending into the aqueous subphase. The hydrophobic periodicity of the sequence dictates the secondary structure at the interface. This robust design makes them ideal candidates for controlling interfacial chemistry. This study presents the de novo design and characterization of two novel peptides: LRα14 and LHα14, which substitute lysine with arginine and histidine, respectively, in the helical LKα14 sequence. This modification is intended to expand the LK peptide platform to a new basic interfacial chemistry. We explore the stability of the new LRα14 and LHα14 designs with respect to changes in pH and salt concentration in bulk solution and at the interface using circular dichroism (UV-CD) and vibrational sum-frequency generation spectroscopy, respectively. Notably, the structural stability of the peptides remains unaffected across a wide range of pH and ionic strength values. At the same time, the variation of side-chain chemistry leads to a wide spectrum of interfacial water structures. By extension of the LK platform to include arginine and histidine, this study broadens the toolbox for designing tailored interfacial chemistries with applications in material and biomedical sciences.

Effect of dynamic strain ageing on flow stress and critical strain for jerky flow in Al-Mg alloys

A comprehensive approach addressing the flow behavior and the critical strain for the initiation of serrations in Al-Mg alloys is developed in the present work. The basic premise of the approach is that the solute atmosphere influences the friction as well as the strain hardening component of the flow stress. The friction effect of the solute cloud is modeled by considering the interplay between the characteristic solute migration time and the dislocation waiting time according to the cross-core diffusion mechanism. The impact on strain hardening is modeled by considering the apparent strengthening of the forest dislocations because of formation of solute aggregates near the vicinity of dislocation junctions. The apparent forest strengthening effect scales as the square root of the ratio of solute concentration in vicinity of the dislocation junctions and the bulk solute concentration. The modified constitutive model is validated against experimental flow curves obtained for strain rates varying over several orders of magnitude. It was observed that the modified constitutive model outperforms the standard constitutive model (considers only the friction effect of solute atmosphere) in predicting the flow curves in the dynamic strain aging domain. Furthermore, the modified constitutive model also accurately predicts the critical strain for the initiation of the jerky flow in both the normal and inverse regimes of the critical strain versus strain rate curve. Additional validation of the modified constitutive model is provided by dislocation character and density measurements via X-ray diffractograms, dislocation structure investigation via transmission electron microscopy along with fracture surface analysis.

Model-predicted effect of radial flux distribution on oxygen and glucose pericellular concentration in constructs cultured in axisymmetric radial-flow packed-bed bioreactors

Radial flow packed-bed bioreactors (rPBBs) overcome the transport limitations of static and axial-flow perfusion bioreactors and enable development of clinical-scale bioengineered tissues. We developed criteria to design rPBBs with uniform medium radial flux distribution along bioreactor length ensuring uniform construct perfusion. We report a model-based analysis of the effect of non-uniform axial distribution of medium radial flux on pericellular concentration of oxygen and glucose. Albeit pseudo-homogeneous, the model predicts how medium flux, solutes transport and cellular consumption interact and determine the pericellular oxygen and glucose concentrations in the presence of pore transport resistance to design optimal axisymmetric rPBBs and enable control of pericellular environment. Thus, oxygen and glucose supply may match cell requirements as tissue matures. Flow and solute transport in bioreactor empty spaces and construct was described with Navier-Stokes and Darcy-Brinkman equations, and with convection–diffusion and convection–diffusion-reaction equations, respectively. Solute transport in construct accounted for Michaelian cellular consumption and bulk medium-to-cell surface oxygen transport resistance in terms of a transport-equivalent bed of Raschig rings. The effect of relevant dimensionless groups on pericellular and bulk solute concentrations was predicted under typical tissue engineering operation and evaluated against literature data for bone tissue engineering. Axial distribution of medium radial flux influenced the distribution of pericellular solutes concentration, more so at high cell metabolic activity. Increasing medium feed flow rates relieved non-uniform solute concentration distribution and decayed at cell surface for metabolic consumption, also starting from axially non-uniform radial flux distribution. Model predictions were obtained in runtimes compatible with on-line control strategies.

Utilizing machine learning to model interdependency of bulk molecular weight, solution concentration, and thickness of spin coated polystyrene thin films

Utilizing machine learning to model interdependency of bulk molecular weight, solution concentration, and thickness of spin coated polystyrene thin films

Properties of aqueous electrolyte solutions at carbon electrodes: effects of concentration and surface charge on solution structure, ion clustering and thermodynamics in the electric double layer.

Surfaces are able to control physical-chemical processes in multi-component solution systems and, as such, find application in a wide range of technological devices. Understanding the structure, dynamics and thermodynamics of non-ideal solutions at surfaces, however, is particularly challenging. Here, we use Constant Chemical Potential Molecular Dynamics (CμMD) simulations to gain insight into aqueous NaCl solutions in contact with graphite surfaces at high concentrations and under the effect of applied surface charges: conditions where mean-field theories describing interfaces cannot (typically) be reliably applied. We discover an asymmetric effect of surface charge on the electric double layer structure and resulting thermodynamic properties, which can be explained by considering the affinity of the surface for cations and anions and the cooperative adsorption of ions that occurs at higher concentrations. We characterise how the sign of the surface charge affects ion densities and water structure in the double layer and how the capacitance of the interface-a function of the electric potential drop across the double layer-is largely insensitive to the bulk solution concentration. Notably, we find that negatively charged graphite surfaces induce an increase in the size and concentration of extended liquid-like ion clusters confined to the double layer. Finally, we discuss how concentration and surface charge affect the activity coefficients of ions and water at the interface, demonstrating how electric fields in this region should be explicitly considered when characterising the thermodynamics of both solute and solvent at the solid/liquid interface.

Adsorption Kinetics of Poly(benzyl acrylate) Chains onto Alumina Interface during the Flow-Driven Translocation through Cylindrical Nanochannels.

In this work, the adsorption kinetics of the PBAN/AAO system under flushing condition has been investigated, where PBAN and AAO represent poly(benzyl acrylate) and anodic alumina oxide (AAO, average pore radius R0 ≈ 10 nm) nanochannel, respectively. Our specially designed double-pump flushing system is proved to eliminate the overshoot phenomenon and in situ monitor transmembrane pressure (ΔP) as a function of flushing time (t) and flow rate (Q), which gives the effective pore radius (R), cross-sectional coverage factor (χ = [1 - (R/R0)2]), and characteristic ratio (rc) of the increments of χ during each adsorption/desorption cycle at a given bulk solution concentration (Cbulk). Our findings include: (1) by gradient increasing Cbulk from 10 to 200 mg/L at Q = 10 mL/h, the shortest PBA40 displays a saturation adsorption behavior when Cbulk ≥ 80 mg/L and t ≥ 2000 s, which agrees well with the prediction of blob model, whereas for the longer PBAN chains, the chain length (N) and concentration-dependent adsorption tendency get stronger as N increases from 40 to 620 at t ≥ 2000 s, in particular, R/R0 ∼ N-0.20 is observed at Cbulk = 140 mg/L; (2) by focusing on the platform χ in the saturation adsorption regime (χsat), the longer PBAN displays a stronger adsorption trend with partially reversible feature at Q = 5.0 mL/h, namely, as N increases from 40 to 620, χsat increases from 0.15 to 0.83 at Cbulk = 100 mg/L, where rc changes from 0.25 ± 0.10 to 0.80 ± 0.10 as the adsorption/desorption flushing cycle increases from 1 to 8 at Cbulk = 100 mg/L; (3) by further assuming a solvent nonpenetrating and nondraining adsorption layer, χsat determined in the case of curved surface can be comparable to the physical meaning of adsorption thickness (Δad) in the case of flat-surface adsorption, and the fitting result indicates χsat ∼ Δad ∼ N0.58, falling between Δad ∼ N1/2 and Δad ∼ N1.0 predicted by the mean-field and scaling theories for real multichain adsorption, respectively. Overall, the present work not only clarifies some controversies but also provides unambiguous evidence supporting the existence of tightly adsorbed internal and loosely adsorbed external layers.

Constant chemical potential-quantum mechanical-molecular dynamics simulations of the graphene-electrolyte double layer.

We present the coupling of two frameworks-the pseudo-open boundary simulation method known as constant potential molecular dynamics simulations (CμMD), combined with quantum mechanics/molecular dynamics (QMMD) calculations-to describe the properties of graphene electrodes in contact with electrolytes. The resulting CμQMMD model was then applied to three ionic solutions (LiCl, NaCl, and KCl in water) at bulk solution concentrations ranging from 0.5 M to 6 M in contact with a charged graphene electrode. The new approach we are describing here provides a simulation protocol to control the concentration of electrolyte solutions while including the effects of a fully polarizable electrode surface. Thanks to this coupling, we are able to accurately model both the electrode and solution side of the double layer and provide a thorough analysis of the properties of electrolytes at charged interfaces, such as the screening ability of the electrolyte and the electrostatic potential profile. We also report the calculation of the integral electrochemical double layer capacitance in the whole range of concentrations analyzed for each ionic species, while the quantum mechanical simulations provide access to the differential and integral quantum capacitance. We highlight how subtle features, such as the adsorption of potassium graphene or the tendency of the ions to form clusters contribute to the ability of graphene to store charge, and suggest implications for desalination.

Halogenate electroreduction from acidic solution at rotating disc electrode. Maximal steady-state convective-diffusion current for comparable concentrations of halogenate ions and protons

Process of electrochemically inactive halogenate-anion (XO3-, X = halogen) electroreduction at rotating disk electrode (RDE) under steady-state conditions via a mediator cycle: XO3- + 5 X- + 6H+→ 3 X2 + 3 H2O in solution (*), X2 + 2 e-⇄ 2 X- on electrode surface in acidic aqueous medium has been analyzed theoretically on the basis of a set of coupled convective-diffusional transport equations for predicting the dependence of its rate on numerous parameters of the system, without application of expensive commercial software for its numerical integration. Approximate solution of these equations for the concentration distributions of all reactive species (halogenate and halide anions, halogen molecules and protons), as well as for the maximal current, as functions of the RDE frequency, bulk-solution concentrations of XO3-, X2 and H+ species, diffusion coefficients of all reactive components, rate constant of comproportionation reaction (*), kinematic viscosity of solution, etc., is given as a simple analytical procedure which provides this solution in the numerical form for any set of values of these parameters.These characteristics of the process are determined primarily by the ratio of the diffusion (for species X2) and kinetic layer thicknesses: xdkC = zdC/ zk, which depends on the RDE frequency and the bulk-solution concentrations of species XO3- (Ao) and H+ (Ho), as well as on other parameters of the system. Weak-current regime takes place for its relatively small values: xdkC < 6, i.e. for sufficiently high RDE frequencies, where the maximal current is proportional to the bulk-solution concentration of X2 molecules, Co, being independent of the Ao and Ho concentrations. Within the opposite range of this parameter: xdkC>6, i.e. for sufficiently low RDE frequencies, the maximal current is comparable to the smaller of the diffusion-limited currents of XO3- and H+ species, jAlim and jHlim, being practically independent of the Co concentration. This strong-current regime which does not exist for the conventional EC-cat (or EC') mechanism is entirely due to the autocatalytic feature of the mediating redox cycle (EC-autocat mechanism) which may lead to enormous accumulation of the X2/X- redox couple components near electrode surface.

Microstructure-based modelling of chloride diffusivity in non-saturated cement paste accounting for capillary and gel pores

Accurate prediction of chloride diffusivity in non-saturated cement paste is crucial for durability design of concrete. This paper presents an integrated framework for modelling chloride diffusivity in non-saturated cement paste considering 3D microstructure, water-gas distribution in pore network and electrical double layer effect. Results indicate that the chloride diffusivity in C-S-H pore solution is dominantly influenced by surface electric potential regardless of water saturation level, porosity of C-S-H and chloride concentration of bulk solution. With the decrease of water saturation level, the relative chloride diffusivity in cement paste experiences sharp, slow and slight decrease stages, followed by a non-diffusive stage, corresponding to connected capillary water, combination of capillary water and saturated C-S-H network and non-saturated C-S-H network as the main diffusion channel, and pore depercolation. The relative chloride diffusivity in cement paste at a given water content decreases with increasing water-to-cement ratio. The simulation results show good agreement with experimental data.

Characterization of Mass Transfer within the Crystal-Solution Boundary Layer of l-Alanine {120} Faces Using Laser Interferometry during Growth and Dissolution.

Crystallization and dissolution are important processes to consider in drug development as well as many other industrial processes. Many current growth and dissolution models are based on bulk solution properties and do not implicitly consider concentration variation close to the crystal surface-solution interface and how this is mediated by solute diffusive mass transfer. Solution boundary layer thickness and concentration distribution, for the {120} crystal habit face of single crystals of l-alanine in saturated aqueous solutions during both growth and dissolution processes, is measured as a function of super/undersaturation using a Mach-Zehnder optical interferometer system. Further analysis allows determination of the diffusion coefficient and mass flux within the boundary layer as well as whether the processes are controlled by mass transfer or crystal interfacial kinetics. The measurement of this study revealed that the {120} face was not saturated at its surface during growth or dissolution meaning both processes were somewhat limited by their crystal interfacial kinetics. Growth was limited by crystal interfacial kinetics at all supersaturations to the same degree, whereas dissolution had a mixed dependency on crystal interfacial kinetics and mass transfer at lower undersaturations becoming more limited by mass transfer at higher undersaturations. Boundary layer thickness increased with super/undersaturation but to a lesser degree than the increase in the concentration difference between the crystal surface and bulk solution leading to a higher mass flux of solute molecules through the boundary layer. At the same relative super/undersaturation mass flux of solute molecules was faster during dissolution which was concurrent with its increased surface to bulk solution concentration difference and boundary layer thickness.

PH Effects in a Model Electrocatalytic Reaction Disentangled.

Varying the solution pH not only changes the reactant concentrations in bulk solution but also the local reaction environment (LRE) that is shaped furthermore by macroscopic mass transport and microscopic electric double layer (EDL) effects. Understanding ubiquitous pH effects in electrocatalysis requires disentangling these interwoven factors, which is a difficult, if not impossible, task without physical modeling. Herein, we demonstrate how a hierarchical model that integrates microkinetics, double-layer charging, and macroscopic mass transport can help understand pH effects of the formic acid oxidation reaction (FAOR). In terms of the relation between the peak activity and the solution pH, intrinsic pH effects without consideration of changes in the LRE would lead to a bell-shaped curve with a peak at pH = 6. Adding only macroscopic mass transport, we can already reproduce qualitatively the experimentally observed trapezoidal shape with a plateau between pH 5 and 10 in perchlorate and sulfate solutions. A quantitative agreement with experimental data requires consideration of EDL effects beyond Frumkin correlations. Specifically, the peculiar nonmonotonic surface charging relation affects the free energies of adsorbed intermediates. We further discuss pH effects of FAOR in phosphate and chloride-containing solutions, for which anion adsorption becomes important. This study underpins the importance of a full consideration of multiple interrelated factors for the interpretation of pH effects in electrocatalysis.

Synergism in sequential inactivation of Cryptosporidium parvum with trypsin and UV irradiation.

Cryptosporidium, a protozoan parasite, in wastewater presents a major public health concern for water safety. However, bactericidal efficiencies of conventional disinfection methods towards Cryptosporidium oocysts are still hampered owing to the presence of their thick outer wall. In this study, we present a novel UV inactivation process where the efficiency has been significantly enhanced by addition of a trypsin pretreatment stage. Notably, inactivation (log-reduction) of oocysts was noted to be 73.75-294.72% higher than that obtained by UV irradiation alone, under identical conditions. Experimental observations and supporting mechanistic analyses suggest that trypsin led to cleavage of the protein layers on the oocyst wall, facilitating penetration of UV radiation into the oocysts leading to degradation of their genomic DNA (gDNA). The dissociative effect of trypsin on the oocyst wall was indicated by the fact that 64.50% of oocysts displayed early apoptosis after trypsinization. Imaging by scanning electron microscopy indicated that this combined treatment led to substantial disruption of the oocyst coat, deforming their shape. This resulted in the release of cellular proteins and gDNA, their concentrations in bulk solution increasing by 1.22-8.60 times. As UV irradiation time was prolonged, gDNA was degraded into smaller fragments with lower molecular masses. Both laddering and diffuse smear patterns in gel analysis indicated significantly detrimental effects on gDNA and viability of oocysts. Overall, this study demonstrated enhancement of UV inactivation of Cryptosporidium oocysts by trypsin and explored the underlying mechanisms for the process.

Halate electroreduction from acidic solution at rotating disk electrode: Theoretical study of the steady-state convective-migration-diffusion transport for comparable concentrations of halate ions and protons

Theoretical analysis of electrochemically inactive halate-anion (XO3−, X = halogen) electroreduction at rotating disk electrode (RDE) under steady-state conditions via autocatalytic mediator cycle based on X2/X− redox couple in acidic aqueous medium has been performed in relation to prospects to use this reaction as cathodic process of high energy density flow batteries. For the first time, full set of coupled diffusion-migration-convection transport equations for the components of the system, including kinetic terms due to the comproportionation reaction between solute species, has been solved via numerical and analytical tools. All principal characteristics of the system have been found: distributions of concentrations of all components inside the diffusion layer as well as of induced electric field and of the chemical reaction rate, diffusion-layer thicknesses for all components as functions of the principal dimensionless parameters: passing current, J, ratios of the diffusion-limited currents for species H (protons) and A (XO3−), JHA, or for species C (X2) and A, JCA, ratio of the diffusion and kinetic layer thicknesses for species A, xdkA = zdA / zk, as well as ratios of the diffusion coefficients of components, DiA = Di / DA (i = other components). The strongest dimensionless current, Jmax, which can pass across the system, can also be calculated for any set of particular values of the dimensionless parameters: JHA, JCA, xdkA and DiA. General features of the dependence, Jmax vs. xdkA, have been analyzed for various values of JHA (i.e. of the ratio of their bulk-solution concentrations, Ho/Ao) while the value of JCA (and consequently of the concentrations' ratio, Co/Ao) is very small. It has been demonstrated that within the range of sufficiently low rotation frequencies the maximal dimensional current density, jmax, is comparable with the combined diffusion-limited current of species A (XO3−) and H (H+), even for extremely low bulk-solution concentration of catalytic species C (X2), Co, i.e. passing current can reach enormous values, despite the electrochemical inactivity of species A and H. This feature which is highly beneficial for the application of this reaction in power sources is a direct consequence of the autocatalytic character of the redox-mediator cycle based on reactions of species XO3−, X− and X2 in acidic medium.

A Model for the Soil Freezing Characteristic Curve That Represents the Dominant Role of Salt Exclusion

AbstractThe phenomenon of freezing point depression in frozen soils results in the co‐existence of ice and liquid water in soil pores at temperatures below 273.15 K (0°C), and is thought to have two causes: (a) capillary and adsorption effects, where the phase transition relationship is modified due to soil‐air‐water‐ice interactions, and (b) solute effects, where the presence of salts lowers the freezing temperature. The soil freezing characteristic curve (SFC) characterizes the relationship between liquid water content and temperature in frozen soils. Most hydrological models represent the SFC using only capillary and adsorption effects with a relationship known as the Generalized Clapeyron Equation (GCE). In this study, we develop and test a salt exclusion model for characterizing the SFC, comparing this with the GCE‐based model and a combined salt‐GCE effect model. We test these models against measured SFCs in laboratory and field experiments with diverse soil textures and salinities. We consistently found that the GCE‐based models under‐predicted freezing‐point depression. We were able to match the observations with the salt exclusion model and the combined model, suggesting that salinity is a dominant control on the SFC in real soils that always contain solutes. In modeling applications where the salinity is unknown, the soil bulk solute concentration can be treated as a single fitting parameter. Improved characterization of the SFC may result in improvements in coupled mass‐heat transport models for simulating hydrological processes in cold regions, particularly the hydraulic properties of frozen soils and the hydraulic head in frozen soils that drives cryosuction.

Protective Effects of Amino Acids on Plasmid DNA Damage Induced by Therapeutic Carbon Ions.

Radioprotectors with few side effects are useful for carbon-ion therapy, which directly induces clustering damage in DNA. With the aim of finding the most effective radioprotector, we investigated the effects of selected amino acids which might have chemical DNA-repair functions against therapeutic carbon ions. In the current study, we employed five amino acids: tryptophan (Trp), cysteine (Cys), methionine (Met), valine (Val) and alanine (Ala). Samples of supercoiled pBR322 plasmid DNA with a 17 mM amino acid were prepared in TE buffer (10 mM Tris, 1 mM ethylenediaminetetraacetic acid, pH 7.5). Phosphate buffered saline (PBS) was also used in assays of the 0.17 mM amino acid. The samples were irradiated with carbon-ion beams (290 MeV/u) on 6 cm spread-out Bragg peak at the National Institute of Radiological Sciences and Heavy Ion Medical Accelerator in Chiba, Japan. Breaks in the DNA were detected as changes in the plasmids and quantified by subsequent electrophoresis on agarose gels. DNA damage yields and protection factors for each amino acid were calculated as ratios relative to reagent-free controls. Trp and Cys showed radioprotective effects against plasmid DNA damage induced by carbon-ion beam, both in PBS and TE buffer, comparable to those of Met. The double-strand break (DSB) yields and protective effects of Trp were comparable to those of Cys. The yields of both single-strand breaks and DSBs correlated with the scavenging capacity of hydroxyl radicals (rate constant for scavenging hydroxyl radicals multiplied by the amino acid concentration) in bulk solution. These data indicate that the radioprotective effects of amino acids against plasmid DNA damage induced by carbon ions could be explained primarily by the scavenging capacity of hydroxyl radicals. These findings suggest that some amino acids, such as Trp, Cys and Met, have good potential as radioprotectors for preventing DNA damage in normal tissues in carbon-ion therapy.

PH Overpotential for Unveiling the pH Gradient Effect of H + /OH − Transport in Electrode Reaction Kinetics

pH Overpotential for Unveiling the pH Gradient Effect of H ⁺ /OH ⁻ Transport in Electrode Reaction Kinetics

Grain boundary co-segregation in magnesium alloys with multiple substitutional elements

Alloying additions in magnesium can modify common basal textures during recrystallization based on their solid solubility and precipitation behavior. With a variety of tenable theories in the literature, a far greater understanding is required of how restricted growth due to drag effects and annealing texture formation are linked. In the current work, a complex magnesium alloy Mg-3Al-1Zn-0.3Ca (wt.%) with multiple substitutional elements was subjected to a combination of deformation and annealing treatments in order to examine how a variation of bulk solute concentrations would influence its segregation and precipitation behavior. The work focuses in particular on solute segregation to grain boundaries and demonstrates that the type and level of segregation play a key role in controlling the growth behavior in such a way that restricts preferential growth of grains with a basal texture. Deeper knowledge in this area can be expected to advance current alloy design strategies by tweaking the solute concentration in the solid solution through targeted heat treatments to result in the required amounts of second phase precipitation and grain boundary segregation.

Multiple objects interacting with a solidification front

The interaction of objects suspended in a liquid melt with an advancing solidification front is of special interest in nature and engineering sciences. The front can either engulf the object into the growing crystal or repel it. Therefore, the object-front confrontation can have a strong influence on the microstructure and mechanical or functional properties of the solidified material. The past theoretical models and experimental studies have mostly investigated the interaction of isolated, spherical, and hard objects in pure melts. However, the outcome of object-front interactions in complex (more realistic) systems, where multiple objects and solutes are present, is still poorly understood. Here we show the interaction of multiple oil droplets with an ice-water front in the absence and presence of solute effects using in situ cryo-confocal microscopy. We report on how the object size, number of objects, and bulk solute concentration influence the the object-front interaction and the front morphology, as well as the subsequent object spatial distribution. We suggest that the volume fraction of objects suspended in a liquid melt in conjunction with the amount of bulk solute concentration are two important parameters to be incorporated in the development of object-front interaction models.

(Invited) Investigation of Corrosion Inhibitor Adsorption on Mild Steel By Electrochemical Atomic Force Microscopy

Mild steel has wide application in oil and gas industry due to its excellent mechanical properties and low costs. However, it is susceptible to corrosion attack in a typical water-containing service environment. Organic corrosion inhibitors can adsorb on steel surfaces to form protective layers even when added in a small amount. As conceptualized in the literature, the decrease in corrosion rates due to addition of inhibitor should be directly related to their surface coverage. However, studies have so far not provided any direct evidence of such a link, primarily due to limitations of conventional electrochemical analysis that have been used to investigate them. In the present work, a specially designed holder for electrochemical atomic force microscopy (EC-AFM) has been used to study the influence of a tetra-decyl-dimethyl-benzyl-ammonium inhibitor model compound on mitigation of corrosion. AFM results shows that when the bulk solution inhibitor concentration was at 0.5 CMC, partial coverage of the surface by the inhibitor film and patchy corrosion (which could lead to localized attack) of the UNS G1018 steel surface were observed by contact mode AFM imaging. When inhibitor bulk concentration was above the surface saturation value (1 CMC), full surface protection was indicated by the almost unchanged AFM topography images from 10 minutes to 4 hours exposure to the inhibited solution. The LPR results obtained with EC-AFM showed that the corrosion rates of this steel decreased as the bulk solution concentration of inhibitor in separate tests was increased from 0, 0.5 CMC to 1 CMC, while further increase of concentrations (from 1 CMC to 2 CMC) didn’t lead to any apparent reduction of general corrosion rate. Although the LPR corrosion rates at 1 CMC and 2 CMC were extremely low (0.05 - 0.1 mm/year), the values didn’t reduce to 0 under these full coverage conditions. Additional AFM tapping mode imaging results on mica indicated the formation of a porous film (a layer with ‘holes’) which is used to explain the types of adsorption layers possible and their probable influence on corrosion.

Kinetic and equilibrium adsorption parameters estimation based on a heterogeneous intraparticle diffusion model

In this work, a commercial resin with a well-developed internal pore structure was chosen to adsorb four parabens used as probe molecules. The main novelty was to propose and validate a phenomenological transient adsorption model based on conservation law in both phases coupled with Langmuir’s equilibrium law and Fick’s mass transfer rate law inside the pores. With such an aim, a heterogeneous three-parameter intraparticle diffusion model, IPDM, was formulated, and its numerical solution was fitted to time-dependent concentration data by minimizing the sum of squared residuals. Equilibrium constants were also predicted by fitting Langmuir isotherm to equilibrium data. A monolayer capacity of 0.81 mmol/g was calculated for the four parabens regardless of the number of carbons in the ester group. With the optimal parameters values from the IPDM fitting process, a system of ODEs comprising local sensitivity coefficients as dependent variables was solved to compute the parameters’ variance-covariance matrix and infer their ranges for a 95% marginal confidence interval. In order to test the validity of the proposed model, an attempt to crosscheck between the parameters obtained by the estimation of the equilibrium related parameter, κ, and the modified capacity parameter, ξp′, and the ones obtained by fitting the Langmuir’s isotherm to equilibrium data was carried out. As far as equilibrium related parameters concern, there is a relative agreement inside the limits of the confidence range between the estimated values of the amount adsorbed in equilibrium with initial bulk solution concentration, q0, and Langmuir’s equilibrium constant, K, adjusted to kinetic and equilibrium data, independently. Additionally, the order of magnitude of pore diffusivity obtained in this work is in accordance with the one predicted by Wilke-Chang correlation and is inversely proportional to the van der Waals volume raised to the power 0.53 in close agreement with the literature.