Title: Aggregation and Adsorption Behavior of Organic Corrosion Inhibitors studied using Molecular Simulations
We have performed all-atom classical molecular dynamics simulations of aggregation and adsorption of different corrosion inhibitor molecules on metal surfaces. We report free energies of aggregation and adsorption of imidazolinium-type (henceforth referred to as imid) and quaternary ammonium-type (referred to as quat) corrosion inhibitors of different alkyl tail lengths. Corrosion inhibitor molecules show a strong tendency to adsorb onto metal surfaces in the unaggregated state. Inhibitor micelles, on the other hand, experience a free energy barrier to adsorption. The quat micelles are found to be thermodynamically stable in the adsorbed state whereas the imid micelles are only metastable in the adsorbed state. Quat micelles deform and partially disintegrate upon adsorption, which renders stability, while the imid micelles do not deform. The inhibitor molecules demonstrate a strong tendency to aggregate into micelles in the aqueous phase. The micellization free energy is found to be ~68 kBT for a micelle comprising of 18 molecules of imid molecules. more »« less
While both field experience and laboratory experiments have shown that the efficiency of adsorbed corrosion inhibitor films improves upon exposure of the aqueous solution to a hydrocarbon phase, a credible explanation of these results is lacking. Using a combination of experiments and molecular simulations, this study examines how exposure to oil molecules affects the nature of adsorbed corrosion inhibitor films on metal surfaces. It is found that oil molecules get coadsorbed in the corrosion inhibitor films, making them more hydrophobic, structurally more ordered, and well packed. Corrosion inhibitor molecules with a bulky polar head adsorb in nonplanar, cylinder-like morphologies. Coadsorption of oil molecules changes the morphology of these films to a planar self-assembled monolayer.
In spite of their high surface charge (zeta potential ζ = +34 mV), aqueous suspensions of portlandite (calcium hydroxide: Ca(OH) 2 ) exhibit a strong tendency to aggregate, and thereby present unstable suspensions. While a variety of commercial dispersants seek to modify the suspension stability and rheology ( e.g. , yield stress, viscosity), it remains unclear how the performance of electrostatically and/or electrosterically based additives is affected in aqueous environments having either a high ionic strength and/or a pH close to the particle's isoelectric point (IEP). We show that the high native ionic strength (pH ≈ 12.6, IEP: pH ≈ 13) of saturated portlandite suspensions strongly screens electrostatic forces (Debye length: κ −1 = 1.2 nm). As a result, coulombic repulsion alone is insufficient to mitigate particle aggregation and affect rheology. However, a longer-range geometrical particle–particle exclusion that arises from electrosteric hindrance caused by the introduction of comb polyelectrolyte dispersants is very effective at altering the rheological properties and fractal structuring of suspensions. As a result, comb-like dispersants that stretch into the solvent reduce the suspension's yield stress by 5× at similar levels of adsorption as compared to linear dispersants, thus enhancing the critical solid loading ( i.e. , at which jamming occurs) by 1.4×. Significantly, the behavior of diverse dispersants is found to be inherently related to the thickness of the adsorbed polymer layer on particle surfaces. These outcomes inform the design of dispersants for concentrated suspensions that present strong charge screening behavior.
Electrochemical atomic force microscopy (EC-AFM) experiments, including simultaneous linear polarization resistance (LPR) tests and in situ AFM imaging, under a CO2 atmosphere, were performed to investigate the adsorption characteristics and inhibition effects of a tetradecyldimethylbenzylammonium corrosion inhibitor model compound. When the inhibitor bulk concentration was at 0.5 critical micelle concentration (CMC), in situ AFM results indicated nonuniform tilted monolayer formation on the mica surface and EC-AFM results indicated partial corrosion of the UNS G10180 steel surface. At 2 CMC, a uniform tilted bilayer or perpendicular monolayer was detected on mica, and corrosion with UNS G10180 steel was uniformly retarded. Consistently, simultaneous LPR tests showed that corrosion rates decreased as the inhibitor concentration increased until it reached the surface saturation value (1 and 2 CMC). Molecular simulations have been performed to study the formation of the inhibitor layer and its molecular-level structure. Simulation results showed that at the initiation of the adsorption process, islands of adsorbed inhibitor molecules appear on the surface. These islands grow and coalesce to become a complete self-assembled layer.
Abstract Americium is a highly radioactive actinide element found in used nuclear fuel. Its adsorption on aluminum (hydr)oxide minerals is important to study for at least two reasons: (i) aluminum (hydr)oxide minerals are ubiquitous in the subsurface environment and (ii) bentonite clays, which are proposed engineered barriers for the geologic disposal of used nuclear fuel, have the same ≡AlOH sites as aluminum (hydr)oxide minerals. Surface complexation modeling is widely used to interpret the adsorption behavior of heavy metals on mineral surfaces. While americium sorption is understudied, multiple adsorption studies for europium, a chemical analog, are available. In this study we compiled data describing Eu(III) adsorption on three aluminum (hydr)oxide minerals—corundum (α-Al 2 O 3 ), γ-alumina (γ-Al 2 O 3 ) and gibbsite (γ-Al(OH) 3 )—and developed surface complexation models for Eu(III) adsorption on these minerals by employing diffuse double layer (DDL) and charge distribution multisite complexation (CD-MUSIC) electrostatic frameworks. We also developed surface complexation models for Am(III) adsorption on corundum (α-Al 2 O 3 ) and γ-alumina (γ-Al 2 O 3 ) by employing a limited number of Am(III) adsorption data sourced from literature. For corundum and γ-alumina, two different adsorbed Eu(III) species, one each for strong and weak sites, were found to be important regardless of which electrostatic framework was used. The formation constant of the weak site species was almost 10,000 times weaker than the formation constant for the corresponding strong site species. For gibbsite, two different adsorbed Eu(III) species formed on the single available site type and were important for the DDL model, whereas the best-fit CD-MUSIC model for Eu(III)-gibbsite system required only one Eu(III) surface species. The Am(III)-corundum model based on the CD-MUSIC framework had the same set of surface species as the Eu(III)-corundum model. However, the log K values of the surface reactions were different. The best-fit Am(III)-corundum model based on the DDL framework had only one site type. Both the CD-MUSIC and the DDL model developed for Am(III)-γ-alumina system only comprised of one site type and the formation constant of the corresponding surface species was ~ 500 times stronger and ~ 700 times weaker than the corresponding Eu(III) species on the weak and the strong sites, respectively. The CD-MUSIC model for corundum and both the DDL and the CD-MUSIC models for γ-alumina predicted the Am(III) adsorption data very well, whereas the DDL model for corundum overpredicted the Am(III) adsorption data. The root mean square of errors of the DDL and CD-MUSIC models developed in this study were smaller than those of two previously-published models describing Am(III)-γ-alumina system, indicating the better predictive capacity of our models. Overall, our results suggest that using Eu(III) as an analog for Am(III) is practical approach for predicting Am(III) adsorption onto well-characterized minerals. Graphical Abstract
Lu, Tieyi; Guo, Wen; Datar, Prathamesh M.; Xin, Yue; Marsh, E. Neil; Chen, Zhan(
, Chemical Science)
Protein adsorption on surfaces greatly impacts many applications such as biomedical materials, anti-biofouling coatings, bio-separation membranes, biosensors, antibody protein drugs etc. For example, protein drug adsorption on the widely used lubricant silicone oil surface may induce protein aggregation and thus affect the protein drug efficacy. It is therefore important to investigate the molecular behavior of proteins at the silicone oil/solution interface. Such an interfacial study is challenging because the targeted interface is buried. By using sum frequency generation vibrational spectroscopy (SFG) with Hamiltonian local mode approximation method analysis, we studied protein adsorption at the silicone oil/protein solution interface in situ in real time, using bovine serum albumin (BSA) as a model. The results showed that the interface was mainly covered by BSA dimers. The deduced BSA dimer orientation on the silicone oil surface from the SFG study can be explained by the surface distribution of certain amino acids. To confirm the BSA dimer adsorption, we treated adsorbed BSA dimer molecules with dithiothreitol (DTT) to dissociate these dimers. SFG studies on adsorbed BSA after the DTT treatment indicated that the silicone oil surface is covered by BSA dimers and BSA monomers in an approximate 6 : 4 ratio. That is to say, about 25% of the adsorbed BSA dimers were converted to monomers after the DTT treatment. Extensive research has been reported in the literature to determine adsorbed protein dimer formation using ex situ experiments, e.g. , by washing off the adsorbed proteins from the surface then analyzing the washed-off proteins, which may induce substantial errors in the washing process. Dimerization is a crucial initial step for protein aggregation. This research developed a new methodology to investigate protein aggregation at a solid/liquid (or liquid/liquid) interface in situ in real time using BSA dimer as an example, which will greatly impact many research fields and applications involving interfacial biological molecules.
Himanshu Singh, Yathish Kurapati.
"Aggregation and Adsorption Behavior of Organic Corrosion Inhibitors studied using Molecular Simulations". NACE Corrosion conference (). Country unknown/Code not available. https://par.nsf.gov/biblio/10099463.
@article{osti_10099463,
place = {Country unknown/Code not available},
title = {Aggregation and Adsorption Behavior of Organic Corrosion Inhibitors studied using Molecular Simulations},
url = {https://par.nsf.gov/biblio/10099463},
abstractNote = {We have performed all-atom classical molecular dynamics simulations of aggregation and adsorption of different corrosion inhibitor molecules on metal surfaces. We report free energies of aggregation and adsorption of imidazolinium-type (henceforth referred to as imid) and quaternary ammonium-type (referred to as quat) corrosion inhibitors of different alkyl tail lengths. Corrosion inhibitor molecules show a strong tendency to adsorb onto metal surfaces in the unaggregated state. Inhibitor micelles, on the other hand, experience a free energy barrier to adsorption. The quat micelles are found to be thermodynamically stable in the adsorbed state whereas the imid micelles are only metastable in the adsorbed state. Quat micelles deform and partially disintegrate upon adsorption, which renders stability, while the imid micelles do not deform. The inhibitor molecules demonstrate a strong tendency to aggregate into micelles in the aqueous phase. The micellization free energy is found to be ~68 kBT for a micelle comprising of 18 molecules of imid molecules.},
journal = {NACE Corrosion conference},
author = {Himanshu Singh, Yathish Kurapati},
}
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