Correcting for solvent replacement effects in quartz crystal microbalance measurements

Jaishankar, Aditya; Jusufi, Arben; Vreeland, Jessica L.; Deighton, Shane; Schilowitz, Alan M.

doi:10.1016/j.sna.2018.04.030

Cited by 7 publications

(12 citation statements)

References 30 publications

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“…In Figure , we show adsorption isotherms measured using the QCM for stearic acid in heptane (a) and hexadecane (b). When the masses were calculated, the measured frequency shifts were corrected for solvent replacement effects (i.e., adsorbing stearic acid molecules replace solvent molecules initially adsorbed on the surface) according to the method outlined in a previous publication . The symbols are the measured data, and the lines are fits to eq .…”

Section: Resultsmentioning

confidence: 99%

“…Subsequently, a series of stearic acid in solvent solutions, each of different concentration, were injected for 2 h each, and the total frequency shift was measured. The adsorbed mass was calculated by removing the solvent contribution to the total frequency shift and the Sauerbrey equation using the method outlined in a previous publication . The experiments were done in a flow loop; the volume of solution looped through the flow cells was large enough to ensure that any stearic acid lost from the solution arising from surface adsorption made a negligible change in the bulk concentration.…”

Section: Methodsmentioning

confidence: 99%

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Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling

et al. 2019

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Improved friction performance is an important objective of equipment manufacturers for meeting improved energy efficiency demands. The addition of friction-reducing additives, or friction modifiers (FMs), to lubricants is a key part of the strategy. The performance of these additives is related to their surface activity and their ability to form adsorbed layers on the metal surface. However, the extent of surface coverage (mass per unit area) required for effective friction reduction is currently unknown. In this article, we show that full coverage is not necessary for significant friction reduction. We first highlight various features of surface adsorption that can influence the surface coverage, packing, and free energy of adsorption of organic FMs on iron oxide surfaces. Using stearic acid in heptane and hexadecane as model lubricant formulations, we employ a combination of experiments and molecular dynamics (MD) simulations to show how the dimerization of acid molecules in the bulk solvent and the crystallographic orientation of the surface modifies surface adsorption. In addition, we show that the solvent can strongly influence the adsorption kinetics, and MD simulations reveal that hexadecane tends to align on the surface, increasing the energy barrier for the adsorption of stearic acid to the surface. Furthermore, we present a combined approach using MD and molecular thermodynamic theory to calculate adsorption isotherms for stearic acid on iron oxide surfaces, which agrees well with experimental data obtained with a quartz crystal microbalance (QCM). Our results suggest that while the friction of systems lubricated with organic FMs decreases with increasing coverage, complete coverage of the surface is neither practically achievable nor necessary for effective friction reduction for the systems and conditions studied here.

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Section: Resultsmentioning

confidence: 99%

Section: Methodsmentioning

confidence: 99%

Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling

et al. 2019

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“…The importance of hydrogen bonding between the adsorbate and surface has been discussed in the literature for other systems ,− and can provide insights to understand the difference between adsorption of DGEBA on different functionalized iron oxide surfaces. The analysis we present uses a definition of a hydrogen bond as the distance between the acceptor and donor of less than 3.5 Å and an angle between D–H···A of more than 145°, where D–H···A indicates the hydrogen bond donor atom, hydrogen, and the acceptor atom, respectively.…”

Section: Resultsmentioning

confidence: 99%

Adsorption of Epoxy Oligomers on Iron Oxide Surfaces: The Importance of Surface Treatment and the Role of Entropy

2021

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Epoxy-based coatings are widely used in a range of industries as protective coatings. The performance of the final solid−polymer system is dependent on the physicochemical properties of the interface and the interaction between the polymer and the solid substrate. In this study, we perform atomistic molecular dynamics simulations to investigate the binding of a common component in epoxy resins, diglycidyl ether of bisphenol A (DGEBA), on two iron oxide surfaces, hematite (0001) and magnetite (100), and investigate the effect of surface hydroxylation on the binding energy. We show that adsorption of DGEBA on hematite is more favorable than on magnetite and that the adsorbed molecules are highly localized on the pristine hematite surface but mobile on highly hydroxylated hematite surfaces and magnetite surfaces irregardless of surface hydroxylation fraction. A high degree of hydroxylation significantly reduces the binding energy of DGEBA on hematite but not on magnetite. The free-energy calculations confirm the trends observed upon hydroxylation, but the magnitude of the potential of mean force is lower than the binding energy due to the entropic contributions. Therefore, it can be suggested that DGEBA will adsorb more strongly on a surface containing a higher content of hematite than magnetite and that the presence of hydroxyl groups will weaken this adsorption. The presence of hydroxyl groups increases mobility of the chains, which can affect the coating rigidity.

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“…Two OFMs were adsorbed on the hematite surface and the projection onto the x-y plane of the distance between the headgroups was used as colvar. Subsequently, the cross-sectional area occupied by each OFM on the hematite surface was estimated using the position of the PMF minimum, following the approach used by Jaishankar et al 79 for SA at the n-heptane-iron oxide interface and Sresht et al 36 for polyethylene glycol at the air-water interface. The results obtained using this first method can be found in the Supporting Information.…”

Section: Hard-disk Areamentioning

confidence: 99%

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

Acero

Mohr

Bernabei

et al. 2021

Preprint

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The performance of lubricant additives, such as organic friction modifiers (OFMs), depends critically on their ability to adsorb onto the surfaces of moving components and form protective self-assembled layers (SAMs). Therefore, understanding the relationship between the concentration of the additive in the base oil and the resulting surface coverage is extremely important for lubricant formulations, as well as many other surfactant applications. Here, we use molecular dynamics (MD) simulations to study the adsorption isotherms of three different OFMs, stearic acid (SA), glycerol monoostearate (GMS), and glycerol monooleate (GMO), onto a hematite surface from hydrocarbon solvents, n-hexadecane and poly-α-olefin (PAO). First, we calculate the potential of mean force (PMF) of the adsorption process using MD simulations with the adaptive biasing force (ABF) algorithm. Our MD simulations show that SA has the weakest adsorption energy on hematite, followed by GMS, and finally GMO, due to the increasing number of functional groups available to bind to the surface. We also estimate the area occupied by each OFM molecule on the surface in the high-coverage limit using MD simulations of the annealing of OFM films with different initial surface coverages. We obtain a similar hard-disk area for GMS and GMO, but a lower value for SA, which is due to its smaller headgroup size. Based on the adsorption energy and surface area, we determine the corresponding adsorption isotherms using the molecular thermodynamic theory (MTT), which agree well with one available experimental data-set for SA. Two other experimental data-sets for SA require lateral interactions between surfactant molecules to be accounted for. SA forms monolayers with lower surface coverage than GMO and GMS at low concentrations (due to a smaller adsorption energy), but also has the highest plateau coverage (due to a smaller hard-disk area). We validate the adsorption energies from the MD simulations using high frequency reciprocating rig (HFRR) friction experiments with different concentrations of the OFMs in PAO. We use the Jahanmir and Beltzer model to estimate the surface coverage at each concentration and the adsorption energy of each OFM from the HFRR friction data. For OFMs with saturated tailgroups (SA and GMS), we obtain good agreement between the predictions made by the simulations and the experiments. The MD simulation and experimental results deviate for OFMs containing Z-unsaturated tailgroups (GMO), with the former suggesting stronger adsorption for GMO than GMS, while the latter predicts the opposite trend. We suggest that this is can be attributed to the higher steric barrier of adsorption of the OFMs with kinked Z-unsaturated tailgroup through a partially formed monolayer, an aspect which was not captured in the current simulations. This study demonstrates that MD simulations with the ABF algorithm, alongside MTT, are an accurate and efficient tool to predict adsorption isotherms at solid-liquid interfaces.

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Correcting for solvent replacement effects in quartz crystal microbalance measurements

Cited by 7 publications

References 30 publications

Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling

Adsorption of Stearic Acid at the Iron Oxide/Oil Interface: Theory, Experiments, and Modeling

Adsorption of Epoxy Oligomers on Iron Oxide Surfaces: The Importance of Surface Treatment and the Role of Entropy

Molecular Simulations of Surfactant Adsorption on Iron Oxide from Hydrocarbon Solvents

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