Characterizing Surfaces of Garnet and Steel, and Adsorption of

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage

Characterising Surfaces of Garnet and Steel, and Adsorption of Organic Additives Jeffrey Poon, David C Madden, Mary H. Wood, and Stuart M. Clarke Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01405 • Publication Date (Web): 11 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018

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Characterising Surfaces of Garnet and Steel, and Adsorption of Organic Additives Jeffrey Poon 1, David C. Madden 1, Mary H. Wood 1, and Stuart M. Clarke*1 1

BP Institute and Department of Chemistry, University of Cambridge, Cambridge UK, CB3 0EZ

*Corresponding author Key Words: steel, garnet, additives, adsorption, surface , abrasive blasting, adsorption, isotherm Abstract This work reports that abrasive-blasting of a structural steel results in significant retention of garnet abrasive residues. A comparative study of the adsorption behaviour of a number of organic species, relevant to paint components and additives, onto the surfaces of garnet and S355 steel from nonaqueous solutions is also presented. Areas per adsorbed molecule, estimated from the isotherm data, suggest a range of molecular orientations on the surfaces. Pronounced differences in the adsorption strength to the garnet and steel were observed, particularly that most additives bind more strongly to steel than to garnet. Surface characterisation data from acid-base titrations, photoelectron spectroscopy, and backscattered electron diffraction were used to rationalise the adsorption data obtained. The ramifications of these findings for particular industrial processes, with regards the strength of paint adhesion and paint additive formulations, are highlighted. Introduction The adsorption of additives to solid surfaces from solutions underpins many commercial processes, from mineral separation by floatation to lubrication of engines. Painting of steel surfaces is used to delay corrosive species progressing to the steel, which can result in structural degradation. These paints are generally organic polymer coatings with a number of additives, some of which are believed to act at the paint/metal interface. This work explores a range of relevant chemical functionalities of such additives, used to enhance the performance and longevity of coatings. Although a large body of work exists on adsorption to iron and steel surfaces,1–10 there are rather few reports concerning the behaviour of mineral substrates. Adsorption to minerals is important in many areas, including flotation,11 enhanced oil recovery12,13 and overbasing in engine oils.14 Almandine garnet is used in the abrasive blasting of steel, prior to painting. In this work, a study of the adsorption behaviour of particular chemical functionalities from non-aqueous solvents to garnet and S355 carbon steel is presented. This work also reports structural and chemical characterisation of the steel surface and evidence that indicates a significant retention of the blasting material in the ‘cleaned’ steel. This is intended to provide some insight into the chemical functionality of the garnet surface and its similarities and differences to steel, but also to inform applications where both steel and garnet surfaces are present together and preferential adsorption of organic additives could be significant at the steel-paint interface. Almandine garnet is a naturally occurring iron aluminium silicate mineral with the composition Fe3Al2(SiO4)3 and a cubic crystal structure, illustrated in Figure SI1.15,16 From the crystal structure, 1 ACS Paragon Plus Environment

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surface groups of the type Al-OH, Si-OH and/or Fe-OH may be expected when the mineral surface is exposed to water. These groups may give rise to a pH-dependent surface charge as they can deprotonate to give negative surface sites or protonate to give positive sites. The isoelectric points of these sites differ significantly: that of the silanol (Si-OH) group is approximately pH 217,18 and those of aluminium hydroxyl (Al-OH) and iron hydroxyl (Fe-OH) are both approximately pH 8,19–23 although there is significant variation between samples and measurement methods. The isoelectric point of garnet has been reported to be pH 5,24 intermediate between these two classes, which suggests that there is a significant contribution from both Si-OH groups as well as Al-OH/Fe-OH at the exposed surfaces. The surfaces of iron and steels under ambient conditions have been shown to be covered by layers of oxides.25–28 The surface chemistry of the oxides is complex and depends on the crystal form of the oxide present, with the nature and number of surface chemical sites varying for different phases.29 In this work, both the steel and garnet have been exposed to air and surface hydroxides may have formed. In non-aqueous systems, significant protonation and deprotonation is less likely than in aqueous systems, due to the low dielectric constant of the solvents, which greatly increases the energetic cost of separating charged species.30 However, there is evidence of some dissociation in non-polar solvents,31,32 particularly for large, low-valent ions such as ‘weakly associating’ ions, and solutions used for supercapacitors.33–36 The adsorbate species of interest here were selected to capture key functionalities relevant to binding to iron and steel, such as components of epoxy paints. The molecular structures of these species are shown in Figure 1. The solvents used were dodecane and toluene, representing a simple alkane and a simple aromatic material.

Figure 1. Schematic illustrations of the adsorbates in this work: (a) 2,4,6Tris(dimethylaminomethyl)phenol (DMP-30), (b) Bisphenol A (BPA), (c) 4-Mercaptophenol (HT), (d) Bis(2-ethylhexyl)phosphate (BEHP), and (e) Palmitic acid (PA). Experimental

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Chemicals and reagents 2,4,6-Tris(dimethylaminomethyl)phenol (DMP-30, 95%), 4-Mercaptophenol (HT, 97%), Bis(2ethylhexyl)phosphate (BEHP, 97%), and Palmitic acid (PA, ≥99%), Bisphenol A (BPA, ≥99%) were provided by Sigma-Aldrich. Adsorption isotherms were mostly conducted from dodecane ((≥99%) supplied by Merck, although particular cases used toluene (≥99.85%) from Fisher Scientific. All reagents were used without further purification. The S355 steel powder adsorbent was supplied by Sandvik Osprey. The composition of the bulk steel is given in Table 1. S355 steel coupons from Parker Steel were polished using 320 grade silicon carbide paper until no visible major scratches were observable. The substrates were then polished using successively finer grades of diamond paste from Kemet. The samples were first polished with diamond grit of 25 µm for 30 min, then 14 µm, 6 µm, and 1 µm for 10 min each. The polished substrates were washed in 2%/wt neutracon® solution, Decon Laboratories Limited and sonicated using a Fisher Bioblock Scientific 750 W Sonicator, Fisher Scientific for 1 min to remove diamond paste residues. The coupons were rinsed quickly ten times, with 50 mL of ultrapure water, (18.2 MΩ cm). The samples were then sonicated in ultrapure water for 1 min, and quickly rinsed ten times again. Finally, the samples were blown dry with a jet of dry nitrogen.

C Si Mn P S Cr 0.200% 0.500% 0.900-1.650% 0.035% 0.030% 0.30% Al Cu N Nb Ti V 0.020% 0.35% 0.015% 0.060% 0.030% 0.12% Table 1: Elemental composition of S355 steel, as given by the manufacturer

Mo 0.10% Fe balance

Ni 0.015%

The S355 steel powder was characterised using electron microscopy, shown in Figure SI2a (see Supporting Information section). The images and Energy Dispersive X-ray Spectroscopy (EDX) data were collected using a JEOL Model JSM 6360LV scanning electron microscope, JEOL (UK) Ltd. As shown in Figure SI2b, the average diameter of steel powder particles was determined to be 5.53 ± 1.72 µm, leading to an approximate specific surface area of 0.14 m2 g-1. This is in broad agreement with the Brunauer–Emmett–Teller (BET) isotherm of nitrogen (determined at the Department of Materials, Cambridge - TriStar 3000, Micromeritics), which gave a specific surface area of 0.30 ± 0.06 m2 g-1. The EDX analysis spectrum is shown in Figure SI2c and the atomic composition displayed in Figure SI2d. The results indicated that the S355 powder elemental composition was in reasonable agreement to the composition listed in Table 1 for S355 steel, albeit with a larger oxygen signature, attributed to a larger surface contribution from the S355 powder with its surface oxide. Almandine garnet powder was supplied by GMA Garnet. Almandine garnet (‘garnet’) is a natural mixture containing other trace minerals. The mineral composition from the supplier is given in Table 2. To give a finer particle size (and hence enhance the surface area and extent of adsorption) the received 80 Mesh (177 µm) garnet was milled with a Planetary Ball Mill PM 100, Retsch GmbH. The mineral was milled in 60 g batches, within a 125 mL tungsten carbide grinding jar with seven 20 mm tungsten carbide milling balls. Each batch was milled for 20 min at 500 rpm, switching milling direction at ten-minute intervals. The resultant milled garnet had its size reduced to below 10 µm, as demonstrated in Figure SI3a. The characterisation of the garnet will be discussed further in the Results and Discussion section.


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Mineral Name Formula Percentage Composition Almandine garnet Fe3Al2(SiO4)3 > 97% Ilmenite FeTiO3 < 1.5% Calcium Carbonate CaCO3 < 1.5 % Zircon ZrSiO4 < 0.2 % Quartz SiO2 < 0.5 % Table 2: Quoted composition of the almandine garnet (‘garnet’) abrasive To remove the calcium carbonate impurity, the garnet was acid washed using 10%/wt nitric acid 69%/wt nitric acid diluted with ultrapure water. 24 g of milled garnet powder was washed with 30 mL 10%/wt nitric acid, tumbled and equilibrated for 24 hrs. Initially, evolution of gas, presumably carbon dioxide from calcium carbonate neutralisation, was observed when acid was added. This acidic suspension, at pH 1, was then continuously stirred until no visible effervescence was observed, before being left to equilibrate. The suspension was centrifuged using a Heraus Multifuge 1 S-R, Thermo Scientific, under a centrifugal force of 15000 g for 30 minutes. The acidic supernatant was discarded and the powder washed five times with ultrapure water. Each washing step involved the re-suspension of solids, equilibration for 1 hour, and re-centrifugation. Washed solids were then spread on a glass petri dish and dried under reduced pressure at 100 oC for 6 hours. The acid-washed garnet has a nitrogen BET specific surface area of 5.54 ± 0.03 m2 g-1. This was a slight reduction from the 6.38 ± 0.01 m2 g-1 when the mineral was first milled, attributed to the loss of the fines in the acid washing process. Solution depletion: The adsorption behaviour of the additives was investigated at room temperature and pressure using the solution depletion method. 2.0 g of acid-washed garnet powder or 20 g S355 steel powder, unless stated otherwise, was placed in polypropylene centrifuge tubes (50 mL, Falcon® Brand, Corning Inc., USA). Solutions of known concentrations of an additive were added and the final solution was made up to 20 mL, and the samples were tumbled for 24 hours, to equilibrate, using a Rotator Drive STR4, Stuart Scientific. For samples using toluene as a solvent, equilibration was conducted in 14 ml glass vials, so all component quantities were halved to maintain the ratio between solids and liquid. The solids were separated by centrifuging the suspension for 30 minutes at a centrifugal force of 15000 g. The supernatant was withdrawn carefully with a pipette and the change in concentration was determined with ultraviolet-visible, infrared, or nuclear magnetic resonance spectroscopy and used to infer the adsorption of the additive on the substrate, relative to calibration measurements. Scanning Electron Microscopy (SEM): Energy Dispersive X-ray Spectroscopy (EDX) and elemental mapping, and Electron Backscattered Diffraction (EBSD) experiments, apart from steel powder characterisation, were performed using a FEI QEMSCAN 650F, Thermo Fisher Scientific, at the Department of Earth Sciences, University of Cambridge. Steel powders were characterised using a JEOL Model JSM 6360LV scanning electron microscope, JEOL (UK) Ltd., at the Department of Chemistry, University of Cambridge X-ray Photoelectron Spectroscopy (XPS) experiments were conducted using a K-Alpha X-ray Photoelectron Spectrometer, Thermo Scientific at the NEXUS XPS service, University of Newcastle. Depth profiling was done using a 4 kV argon monoatomic ion beam.


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Time-of-Flight Secondary Ion Mass Spectrometry (TOF-SIMS) and depth-profiling data were collected using a IONTOF ToF-SIMS 5, IONTOF, with a 25 keV bismuth ion beam in high current bunched mode, at Imperial College, London. Powder X-ray Diffraction (PXRD) data was collected on a D8 Advance Powder Diffractometer, Bruker, using a copper source, with a wavelength of approximately 1.54 Å, at the Department of Earth Sciences, University of Cambridge. Acid-base titrations were performed using a Metrohom 809 Titrando, Metrohm with the pH monitored using an Unitrode Pt100. Transmission Infra-red (IR) Spectroscopy was conducted on a Perkin Elmer Spectrum 100 FTIR Spectrometer at the Department of Chemistry, University of Cambridge. The samples were analysed using a liquid cell, through caesium chloride windows and a 2 mm Teflon spacer. Inductive Coupled Plasma-Optical Emission Spectrometry (ICP-OES) was performed using a PerkinElmer Optima 8000 ICP-OES Spectrometer, at the Department of Geography, University of Cambridge. Ultraviolet-Visible Spectroscopy (UV-Vis) was done using an Agilent 8453 UV-Visible Spectrophotometer, Agilent Technologies at the Department of Chemistry, University of Cambridge. Hellma absorption cuvettes of 3 ml volume with a pathlength of 10 mm, made of Suprasil® quartz, were used. Abrasive blasting experiments were performed using a Guyson Model Formula F1200 Bench-Top Blast Cleaning/Finishing System, Guyson International Limited, with a 6.4 mm ceramic blasting nozzle and blasting 80 mesh garnet particles. Results and Discussions Steel Characterisation XPS: Elemental analysis and depth profiling of polished steel samples is shown in Figure 2a. Iron levels rise with increasing depth as the native oxide is penetrated, reaching a plateau atomic percentage of approximately 90%. Carbon levels fall from approximately 20% towards zero with increasing depth, indicative of high levels of adventitious carbon on the exposed surface. Manganese levels remain at approximately 1%, consistent with the specifications provided in Table 1. Significantly, there is an initially high oxygen atomic percentage (55%) which falls with increasing depth, although after 90 s of argon etching, the oxygen level is still 10%. These results support the intuitive hypothesis that a native iron oxide layer forms on the steel surface after polishing and washing with ultrapure water. The binding energy of the XPS peak can also be used to infer the oxidation state and chemical environment of the iron species surface groups. Fe 2p peak positions are referenced to those given by the XPS database of National Institute of Standards and Technology (NIST), USA: wustite (FeO) at 709.6 eV,37 magnetite (Fe3O4) at 710.7 eV,38 hematite (Fe2O3) at 711.4 eV,39 lepidocrocite and goethite (FeOOH) at 711.5 eV.40 Examples of typical XPS peak fits are illustrated in Figure SI4. Peak fitting data may be used to infer the mineral phase, but ideally with further fitting constrains from 5 ACS Paragon Plus Environment

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surface crystallographic data. Backscattered electron diffraction (EBSD) is therefore used as a complementary technique. EBSD: EBSD patterns are used to characterise the crystallographic mineral phases present at the polished S355 steel surface, by comparison to the known patterns of iron oxides and related species. As shown in Figure 2b, the composition of different oxides determined from EBSD agrees well with that determined by XPS.

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Figure 2. a) XPS depth profile of elements present in a polished S355 steel sample. b) Comparison of results between XPS and EBSD for surface iron oxides present on the polished steel sample. Black: magnetite, red: hematite, blue: goethite, magenta: wustite, green: lepidocrocite. Based on the XPS and EBSD data we conclude that the S355 steel surface is mainly magnetite, with a significant contribution of wustite, although more oxidised forms such as hematite, lepidocrocite, and goethite are also present. These oxides have well-characterised surface chemistries.29 Hence the surface chemistry of S355 steel may be inferred based on its mineral composition. For example, the densities of surface hydroxyl groups, which are the functional groups often reported to be responsible for the surface chemistry of iron oxides,9,21 are estimated in Table. 3. The average iron surface oxidation state is determined to be 2.59 ± 0.02, and the average site density to be 5.24 ± 0.10 sites nm-2. The mixture of FeII and FeIII oxides indicated by these results is in good agreement with the XPS (Figure SI4a), which demonstrates the presence of several iron species. However, these values should only be taken as very approximate, given the simple approach and that calculated site densities vary significantly depending on the characterisation technique.29

Crystal

Formula

Percentage/%

Average hydroxyl group density29/nm-2 5.2 4.4 6.0 8.0 5.0

Number of groups/nm-2

Magnetite Fe3O4 47.7 2.48 Haematite Fe2O3 15.5 0.68 Goethite FeO(OH) 5.8 0.35 Lepidocrocite FeO(OH) 6.1 0.49 Wustite FeO 25.0 1.25 Total 5.24 Table 3. Summary of surface iron oxide composition (based on EBSD) and surface chemistry inferred from literature values.


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Garnet Characterisation The milling and washing processes are assessed by determination of the amounts of aluminium and calcium in the washings using ICP-OES, as illustrated in Figure SI5. As expected, the calcium level is found to fall with increasing water washings until it is undetectable after the sixth washing, indicating that all Ca has been removed. Similarly, in acidic conditions (pH ≈ 1), any dissolved aluminium is effectively washed away by the sixth water wash. Electron Microscopy: Figure SI3a indicates that the majority of particles in the garnet are smaller than 10 µm diameter, a significant reduction from the as-received powder. Calcium is still observable in the EDX spectra of the garnet particles (Figure SI3b and SI3c), suggesting that calcium is contained within the acid-washed garnet particles themselves, even though it cannot be dissolved away. However, the surfaces of interest should not be significantly contaminated. PXRD: Figure SI3d shows that the mineral is predominantly almandine garnet, with excellent agreement to the expected diffraction pattern.16 It is concluded that the acid-washed garnet has no significant surface-level calcium carbonate that might interfere with any adsorption study. Surface titrations: Figure 3 shows a titration curve of the acid-washed garnet in an inert supporting electrolyte, sodium nitrate (100 mM). The curve shows three equivalence points and two equilibrium constant points; the acid dissociation constants, pKa, characteristic of the surface groups are found at points of gradient minima as 4.57 ± 0.17 and 6.05 ± 0.07.41 It is useful to compare the experimental pKa values to those of the possible surface groups (Al-OH, Fe-OH, and Si-OH). For aluminum oxide surfaces such as gibbsites, the pKa value is around 10,42 whilst common iron oxides such as hematite have a pKa values of approximately 8,21 and silica has pKa values ranging from 4 to 9, specifically dependent on the site coordination.43–46 The pKa values from the titration of garnet do not correspond well with any of the literature proposed hydroxyl groups alone. This may indicate that the sites present at the almandine garnet surface are either hydroxyl sites with a mixture of specific coordination environments, plane versus edge for example, or hydroxyl sites with a mixture of different coordinating elements, as previously theoretically proposed.47 The density of surface binding sites can be estimated from the volume of acid/base added between the equivalence points, points of maximum gradient, as 1.05 ± 0.08 sites nm-2. The mathematical details of the calculation are given in the Supporting Information. The site density is relatively low in comparison to typical precipitated iron, aluminum, or silicon oxides, which are typically upwards of 2 sites nm-2,17,20,21,29,42,44 and somewhat lower than that observed using the mineral analysis above, although this approach is expected to be very approximate. However, it is generally difficult to determine surface site densities and reports vary very significantly in the literature, ranging from 0.35 to 10 sites nm-2.20,21,48 In this context, the value obtained is not unreasonable.


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Figure 3. A typical acid-base titration curve of 100 g L-1 acid-washed almandine garnet suspension in 100 mM sodium nitrate solution. Equivalence points are labelled in red, and equilibrium points are labelled in blue. Titration experiments have been conducted in different supporting electrolyte concentrations to determine the Point of Zero Charge (PZC) for non-specifically adsorbing electrolytes (Figure SI6). This PZC is an important physical parameter of the surface and the experiment is done to ensure this has not changed on acid treatment. The surface charge curves cross at a common intersection point at the PZC of pH 5, in good agreement with literature values.24 In contrast, with specifically adsorbing ions there is no common intersection point as additional ions in solution change the surface charge.49 Abrasive-blasted steel characterisation S355 steel coupons are first blasted with almandine garnet (non-acid washed) for 30 seconds at 40 psi. Mill scales and paint residues are visibly rapidly removed and a metallic grey surface resulted. EDX: EDX is a convenient approach to assess the extent of abrasive blasting material incorporation at the steel surface, for different blasting durations and pressures. An example of the elemental mapping of SEM-EDX is illustrated in Figure 4. Regions of garnet are associated with silicon, calcium, titanium and magnesium in contrast to the iron background (from the steel). By appropriate data analysis, the iron contribution can be removed to give a map of garnet-associated elements on the steel surface, which can also be used to quantify the coverage of garnet. Details are given in the Supporting Information.


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Figure 4. a) An EDX element map of the steel surface abrasive blasted with garnet for 10 s at 40 psi. b) Image after subtraction of the iron signal. The non-black area of the image is used to estimate the surface coverage of abrasive. The variation of garnet coverage with (a) blasting duration and (b) gun pressure is illustrated in Figure 5. Figure 5a shows that the surface coverage of garnet rapidly reaches a value of approximately 30 -35%, which is then essentially independent of duration. A model proposed for this behaviour is that in the blasting process, although new material is arriving, it dislodges material present on the surface and hence achieves a dynamic steady state. Figure 5b shows that there appears to be no obvious pressure dependence on the surface coverage of garnet for samples abrasive blasted for 30 s.

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Figure 5. Surface coverage studies of abrasive blasting: a) variable duration at a constant pressure of 40 psi; b) variable pressure at a constant duration of 30 s. Several approaches were considered to assess how easily the garnet could be removed. Figure 6 shows a range of cleaning methods in order of increasing vigour, in comparison to a control that has undergone 30 s of abrasive blasting. The methods range from a gentle cleaning of the blasted steel surface with a strong nitrogen jet for 30 s, to rinsing under ultrapure water for 30 s then blowing dry with a nitrogen jet for 30 s, to sonicating in ultrapure water for 30 s then blowing dry with a nitrogen 9 ACS Paragon Plus Environment

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jet for 30 s. The figure shows that despite forceful cleaning with sonication, approximately 20% of the steel surface remains covered by garnet blasting residue.

Figure 6. Surface coverage of garnet abrasive material residue on the steel surface for different cleaning methods. The SEM-EDX surface-coverage data in Figure SI7 indicates there is significant calcium on the surface after abrasive blasting, not evident in the initial coupons. This suggests that the minor impurity of calcium carbonate in the garnet also accumulates on the surface, up to 10% coverage. Interestingly, estimates of coverage suggest that the calcium carbonate is enriched at the surface relative to the initial garnet composition shown in Table 2. XPS: Figure 7a shows a comparison of XPS spectra from a polished S355 steel coupon and one that has been blasted for 30 s. Figure 7b outlines the atomic percentages determined from the spectra. The polished steel surface has only iron, carbon, oxygen, and manganese present. As discussed above, the carbon content is likely to be mainly of adventitious origin, and the strong oxide peak indicates the existence of a native surface iron oxide layer. After blasting, the garnet-associated elements (iron, aluminium, and silicon from almandine garnet, calcium from calcium carbonate, and titanium from ilmenite) are present at the steel surface. These contribute up to 10 % of the surface atoms present. The prominent carbon peak on the blasted steel can now be attributed both to adventitious carbon and calcium carbonate residues. Similarly, for the oxygen peak, contributions come from iron oxides, almandine garnet, and calcium carbonate. Therefore, the XPS spectra also suggest that significant abrasive residues remain on the posttreatment steel surface, in good agreement with the SEM-EDX analysis.


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b)

Figure 7. a) Surface X-ray photoelectron spectra comparison between S355 steel after 30 s abrasive blasting with garnet (green) and polished S355 steel (blue). The most intense peaks of various elements are labelled in red. b) Surface elemental composition determined by XPS for polished S355 steel (blue) and after 30 s of abrasive blasting with garnet (green). All error bars are standard deviations. TOF-SIMS: Depth profiling results, shown in Figure SI8, are presented for two samples, abrasiveblasted for different durations. A strong initial calcium signal is seen for both samples. As expected, the amount of calcium falls while the amount of iron increases with increasing depth. With increased blasting time, it takes longer for the surface layer containing calcium to be etched through and the signal to reach that of bulk steel. Hence, with longer blasting duration, there appears to be a larger build-up of calcium carbonate on the steel surface, consistent with the EDX data shown in Figure SI7. Interestingly, both the aluminium and silicon signals remain largely constant with deeper etching. This indicates that the almandine garnet is penetrating the steel to a significant extent. This contrasts with the calcium carbonate residues, which remain at the surface. The amount of calcium carbonate at the steel surface is, perhaps surprisingly, higher than that present in the blasting material. This can be attributed to the relative difference in hardness of the blasting material components: Almandine garnet (seven on Mohs scale of hardness), is harder than steel (four) and so cleans the steel,50,51 but also embeds in the surface. In contrast, calcite (three) is softer than the steel, and so deforms and ‘splats’ on the surface.52 In this way, the calcite content is increased at the surface. This is schematically illustrated in Figure 8. A steady state is reached at longer blasting times, where new material displaces material already on the surface.


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Figure 8. A schematic illustration of the proposed model for the attachment of abrasive material at the steel surface. Samples from offshore facilities from the Brent oil site in the North Sea have also been analysed. The samples were previously blasted, then painted over with multiple coatings. In this work the coatings have been removed by soaking the painted steel samples in N-methyl-2-pyrrolidone, and then gently peeling, separating the paint from the coupon. The EDX elemental mapping data in Figure 9 again shows clear evidence of elements associated with the blasting material (aluminium, silicon, calcium, and titanium) at the surface, indicating that garnet abrasive is retained in the field as well as in laboratory studies.

a)

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Figure 9. Representative EDX elemental mapping of samples sent from North Sea Brent offshore facilities, with a) the steel surface, and b) the underside of the gently peeled paint, both analysed. To summarise, SEM-EDX elemental mapping, XPS, and TOF-SIMS data all clearly indicate the presence of significant amounts of abrasive blasting materials at notionally ‘cleaned’ steel surfaces. The behaviour of these alternative substrates interacting with relevant paint components is therefore of interest.


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Organic isotherms Recent work has reported the adsorption behaviour of a number of organic components to iron oxide and related surfaces.9,10,53,54 The adsorption of particular organics to the surface of calcite has also been presented.55,56 Here, the adsorption of key organic groups to garnet has been determined using the solution depletion method. The results are compared to adsorption on S355 steel surfaces. The organic molecules have been chosen for their importance as paint components and with key chemical functionality: a phenol-thiol, a phosphate, 2,4,6-tris(dimethylaminomethyl)phenol (DMP30), a phenol, and a carboxylic acid to compare the affinity of these species to the steel and garnet substrates. The adsorption behaviour can be used to tailor adhesion promotion for the substrates and/ or related surface treatments. In this non-aqueous environment, no evidence has been found for corrosion-related problems over the timescale of the experiments.

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Figure 10. Adsorption isotherms of a range of organic molecules, conducted in organic solvents: a) 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) in dodecane, b) bisphenol A in toluene, c) 4mercaptophenol in toluene, d) bis(2-ethylhexyl)phosphate (BEHP) in dodecane and e) palmitic acid in dodecane. Black circles: steel, green triangles: acid-washed garnet. a) Tertiary amine: 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30) in dodecane 2,4,6-Tris(dimethylaminomethyl)phenol, DMP-30, is used as a hardener for epoxy paints.57 The adsorption isotherm of DMP-30 onto steel and acid-washed garnet from dodecane, determined using UV-Vis spectroscopy (with characteristic absorbance at 285 nm) is shown in Figure 10a. The amount adsorbed initially rises rapidly with increasing DMP-30 concentration, and then reaches a plateau, which is taken to indicate the completion of an adsorbed layer. The data can be reasonably well fitted to a Langmuir-type adsorption model, Equation 1, where q is the adsorbed amount of DMP-30 in equilibrium with a concentration c of DMP-30 in the solution, Kads the adsorption equilibrium constant, and Q the amount of DMP-30 adsorbed in the limit of high concentration. ொ௄

௖

‫ = ݍ‬ଵା௄ೌ೏ೞ ௖

(1)

ೌ೏ೞ

The Langmuir adsorption equilibrium constant for DMP-30 on acid-washed garnet is determined to be 2.09 ± 0.36 × 105 M-1. The amount adsorbed at the plateau can also be used to infer the area per molecule on the surface as approximately 168.1 Å2. Based on simple molecular geometry arguments, this value is consistent with the DMP-30 molecules lying flat on the surface, an orientation that appears reasonable as it is the only arrangement that would allow all three nitrogen groups, the phenol hydroxyl group, and the aromatic groups to interact with the surface. It is also entropically favoured as one DMP-30 molecule can adsorb and release several solvent molecules. DMP-30 shows similar behaviour on steel, with an adsorption equilibrium constant of 1.07 ± 0.19 × 105 M-1 and molecular area of 168.4 Å2. The excellent agreement of the molecular areas on steel and acid-washed garnet suggests that the DMP-30 determines the area per molecule, rather than sitespecific binding. The different binding constants suggest that DMP-30 binds more strongly on steel than on garnet. b) Phenol: Bisphenol A (BPA) in toluene Bisphenol A (BPA) is a very common component for high performance, high durability coatings, either as an integral paint component or as an additive.58–61 Its adsorption isotherms onto steel and acid-washed garnet from dodecane are shown in Figure 10b, where the concentration is monitored using UV-Vis spectroscopy at 287 nm. The adsorption of BPA follows Langmuir-type behaviour on both steel and acid-washed garnet: there is increased adsorption with increasing equilibrium solution concentration, before reaching a plateau. The adsorption equilibrium constant on acid-washed garnet is 2.04 ± 0.32 × 103 M-1. This indicates a slightly weaker adsorption than on steel, where the equilibrium constant is 1.18 ± 0.18 × 104 M-1. Both values are significantly smaller than those for the more strongly adsorbed DMP-30 above. On garnet, the adsorption plateau indicates that each molecule occupies an area of 145.7 Å2; for steel, the area per molecule is 63.2 Å2. These estimates indicate that the BPA molecule is more densely packed on the steel surface than on garnet. Each molecule has an area of approximately 100 Å2 14 ACS Paragon Plus Environment

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when flat on the surface based on simple geometrical arguments; for comparison, a benzene molecule occupies approximately 40 Å2 when lying flat and 18 Å2 when vertical. This may signal that one of the benzene rings of the BPA molecule lies flat on the steel surface, whilst the other is relatively upright, constrained by the steric conditions of the two methyl groups at the centre. On garnet, BPA sparsely populates the surface, possibly lying as flat as possible and not closely packed. c) Phenol-thiol: 4-mercaptophenol in toluene The adsorption of 4-mercaptophenol, HT, onto acid-washed garnet and S355 steel is performed in toluene, due to the poor solubility of HT in simple alkanes such as dodecane. Many coatings also use toluene-based solvents. The concentration of HT in equilibrated samples is measured using UV-Vis spectroscopy at a wavelength of 290 nm. The adsorption isotherm of HT on S355 steel, Figure 10c, clearly shows typical Langmuir-type behaviour, similar to that seen for DMP-30 and BPA. The adsorption equilibrium constant is 6.73 ± 1.49 × 104 M-1, slightly lower than that of DMP-30, suggesting weaker adsorption. The plateau suggests an area per molecule in the complete monolayer of 44.5 Å2, similar to the ‘flat’ molecular area of approximately 40.5 Å2. From simple geometrical calculations, a molecular area of approximately 18 Å2 would be expected if the molecule is upright, and therefore these results are consistent with the molecule lying flat on the steel surface. Interestingly and somewhat unexpectedly, in contrast, there is no observable adsorption of HT onto the acid-washed garnet. From the previous characterisation of the garnet, surface sites of silicon, aluminium, or iron hydroxyl groups, or a mixture of the three, are expected to be present. However, similar adsorption behaviour to the surface iron hydroxyl groups on S355 steel is not observed, despite the possibility of iron hydroxyl groups on the garnet surface. From these results, it can be concluded that competitive adsorption of HT in toluene-based solvents onto garnet is very unlikely, when steel is present. This may present an opportunity for phenol-thiolbased additives to be used when specific targeting of the steel surface is desired. Alternatively, this highlights a potential failure mechanism when garnet remains on the steel surface, because the paint may not bind as strongly as it would to a clean steel surface. d) Phosphate: Bis(2-ethylhexyl)phosphate (BEHP) in dodecane Bis(2-ethylhexyl)phosphate (BEHP) has been reported to form self-assembled films on iron surfaces in aqueous solution62 and used to selectively extract ferric ions into the organic phase from aqueous ferric/ferrous ion mixtures.63 These characteristics make it an interesting candidate as an anticorrosion additive, with other evidence from recent studies that shows the phosphate sodium salt adsorbing onto minerals.64 Here we report the adsorption of BEHP on garnet. Steel reacting with BEHP have been observed to form reaction products not dissimilar to the phosphating of steel surfaces used in industry.65 However, these results will be discussed in a future publication. Figure 10d illustrates that the adsorption of BEHP on garnet from dodecane displays a Langmuir-type relationship, with an adsorption equilibrium constant of 6.02 ± 2.94 × 104 M-1. At full coverage, each BEHP molecule occupies 49.3 Å2, indicating that the molecule is likely to be adsorbed mainly vertically with its aliphatic chains pointing away from the acid-washed garnet surface. This is a marked difference to the DMP-30, which lies flat on the surfaces of both steel and garnet. However, 15 ACS Paragon Plus Environment

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this upright orientation is more likely with the amphiphilic BEHP molecule, because the polar phosphate ‘head’ is much more likely to want to bind to any surface hydroxyl groups, than is the non-polar aliphatic ‘tail’. Close-packed alkyl tails occupy approximately 20 Å2, therefore 50 Å2 for a branched BEHP di-chain appears reasonable. It is expected that only a monolayer would form instead of a bilayer due to the hydrophobic nature of the continuous phase. e) Carboxylic acid: Palmitic acid (PA) in dodecane Previous literature indicate that palmitic acid, PA, and other carboxylic acids, saturated and unsaturated, adsorb strongly at the iron-iron oxide/liquid interface.9,10,53,66–70 Here, the adsorption of PA from dodecane onto garnet and S355 steel is investigated. The concentration of palmitic acid in solution is determined using transmission IR spectroscopy by monitoring the peaks at 1714 and 1766 cm-1. 53 In Figure 10e, Langmuir-like adsorption behaviour is seen for PA on both acid-washed garnet and S355 steel. Interestingly, each PA molecule occupies 179.6 Å2 on acid-washed garnet, but only 35.7 Å2 on steel. This clearly indicates very different molecular orientations of the PA on these surfaces: upright on steel and flat on garnet. On garnet, the equilibrium adsorption constant for PA is 3.02 ± 0.83 × 103 M-1, whereas on steel the value is 1.82 ± 0.18 × 104 M-1, in line with previous literature results for carboxylic acid adsorption on steels.10 These results indicate the preference of the carboxylic acid functionality to adsorb on steel rather than garnet, should competitive adsorption occur.

Adsorbate

Garnet: log Kads/M-1

Garnet: Garnet: S355 steel: area per Molecules log Kads/M-1 molecule per site /Å2 DMP-30 5.32 ± 0.07 168.1 0.6 5.03 ± 0.08 BPA 3.31 ± 0.07 145.7 0.6 4.07 ± 0.06 HT / / / 4.83 ± 0.10 BEHP 3.78 ± 0.21 44.3 2.1 / PA 3.48 ± 0.12 179.6 0.5 4.26 ± 0.04 Table 4: Summary of organic adsorption isotherm values

Steel: area per molecule/ Å2 168.4 63.2 44.5 / 35.7

Steel: Molecules per site 0.1 0.3 0.4 / 0.5

A summary of the adsorption data is shown in Table 4. These data provide a ready comparison between the affinities of steel and acid-washed garnet for the various organic species and the corresponding molecular orientations. All molecules prefer adsorbing onto the S355 steel, rather than on the acid-washed garnet. The numbers of adsorbed molecules per surface site are calculated based on the site densities obtained from acid-base titrations for acid-washed garnet or the crystal structures determined from XPS-EBSD for S355 steel; these are also given in Table 4. From the ‘molecules per site’ column in Table 4, it is worth noting that the ratios between molecules and surface sites are not one-to-one, despite the adsorption isotherms indicating that saturation coverages have been reached for the organic molecules mentioned. This indicates that binding sites are not directly determined by the periodicity of substrate sites. Instead, the saturation coverage is determined by the molecular adsorbed overlayer, with the different organic species considered to adopt different conformations, to pack as closely as the experimental conditions would allow on the substrate surface. This phenomenon is not uncommon for adsorbed layers.71,72 16 ACS Paragon Plus Environment

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Preliminary sum frequency generation spectroscopy data, obtained from polished garnet surfaces dosed with DMP-30, BPA, BEHP, and PA, has indicated the presence of adsorbed layers, with molecular conformations consistent with the areas per molecule calculated from the adsorption isotherms on garnet powder described herein. Further studies are planned to investigate the effect of the polishing process on the nature of the garnet surface, and to determine the conformations of the different functional groups in the adsorbed molecules. Conclusions In this work, the surfaces of S355 steel, almandine garnet, and abrasive-blasted S355 steel have been characterised. Estimates of the binding constants of the surface sites, their number density and surface oxidation states of almandine garnet are proposed. Abrasive blasting has been shown to leave residues that cover approximately a third of the treated S355 steel surface. Hence, when considering the chemistry of surface organic coatings, both the steel surface and also the garnet should be included. A series of organic species with particular coating-relevant functionalities have been adsorbed onto S355 steel and garnet. The adsorption constants, reflecting the strength of binding, are obtained, along with estimates of the areas occupied per molecule. The binding is found to be generally stronger on steel than garnet. This body of data suggests that abrasive-blasting contaminants can provide a weak point for coating systems to de-adhere. Hence it would be useful to identify new species that bind more strongly to garnet, than the species studied here.

Supporting Information Further information relating to almandine garnet crystal structure (SI1), S355 powder characterisation (SI2), acid-washed garnet characterisation (SI3), polished steel surface species deconvolution (SI4), acid treatment of almandine garnet (SI5), surface charge titration of acidwashed almandine garnet (SI5), post-blasting steel calcium surface coverage (SI6), TOF-SIMS spectra of blasted steel (SI7) can be found in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding The funding of this research is provided by the Royal Dutch Shell Company. Acknowledgements


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The authors thank the Royal Dutch Shell Company for the funding and technical support, and in particular to Dr. Ron van Tol and Dr. Lene Hviid of Shell Global Research centre, Amsterdam, The Netherlands. The authors thank Dr. Giulio I. Lampronti at the Department of Earth Sciences, University of Cambridge for technical help with EBSD, the NEXUS XPS service at Newcastle University, Dr. Chris Rolfe at the Department of Geography, University of Cambridge for his assistance in ICP-OES analysis, and Dr. Mike Casford at the Department of Chemistry, University of Cambridge for his contribution to the infrared spectroscopy work. Abbreviations DMP-30, 2,4,6-Tris(dimethylaminomethyl)phenol; HT, 4-Mercaptophenol; BEHP, Bis(2ethylhexyl)phosphate; PA, Palmitic acid; BPA, Bisphenol A; SEM, Scanning Electron Microscopy; EDX, Energy Dispersive X-Ray Spectroscopy; BET, Brunauer–Emmett–Teller; XPS, X-ray Photoelectron Spectroscopy; TOF-SIMS, Time of Flight-Secondary Ion Mass Spectrometry; PXRD, Powder X-Ray Diffraction; FTIR, Fourier Transformed Infrared Spectroscopy; ICP-OES, Inductive Coupled Plasma-Optical Emission Spectrometry; UV-Vis, Ultraviolet-Visible Spectrscopy; EBSD, Electron Backscattered Diffraction.


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Welbourn, R. J. L.; Lee, S. Y.; Gutfreund, P.; Hughes, A.; Zarbakhsh, A.; Clarke, S. M. Neutron Reflection Study of the Adsorption of the Phosphate Surfactant NaDEHP onto Alumina from Water. Langmuir 2015, 31 (11), 3377–3384.

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Sankara Narayanan, T. S. N. SURFACE PRETREATMENT BY PHOSPHATE CONVERSION COATINGS – A REVIEW. Rev.Adv.Mater.Sci 2005, 9, 130–177.

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Taheri, P.; Wielant, J.; Hauffman, T.; Flores, J. R.; Hannour, F.; de Wit, J. H. W.; Mol, J. M. C.; Terryn, H. A Comparison of the Interfacial Bonding Properties of Carboxylic Acid Functional Groups on Zinc and Iron Substrates. Electrochim. Acta 2011, 56 (4), 1904–1911.

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Kern, P.; Landolt, D. Adsorption of an Organic Corrosion Inhibitor on Iron and Gold Studied with a Rotating EQCM. J. Electrochem. Soc. 2001, 148 (6), B228. 22 ACS Paragon Plus Environment

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Frey, M.; Harris, S. G.; Holmes, J. M.; Nation, D. A.; Parsons, S.; Tasker, P. A.; Winpenny, R. E. P. Elucidating the Mode of Action of a Corrosion Inhibitor for Iron. Chem. - A Eur. J. 2000, 6 (8), 1407–1415.

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Götzen, J.; Käfer, D.; Wöll, C.; Witte, G. Growth and Structure of Pentacene Films on Graphite: Weak Adhesion as a Key for Epitaxial Film Growth. Phys. Rev. B 2010, 81 (8), 85440.


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Table of Contents Graphic


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Figure 1. Schematic illustrations of the adsorbates in this work: (a) 2,4,6-Tris(dimethylaminomethyl)phenol (DMP-30), (b) Bisphenol A (BPA), (c) 4-Mercaptophenol (HT), (d) Bis(2-ethylhexyl)phosphate (BEHP), and (e) Palmitic acid (PA). 51x47mm (150 x 150 DPI)

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Figure 2. a) XPS depth profile of elements present in a polished S355 steel sample. b) Comparison of results between XPS and EBSD for surface iron oxides present on the polished steel sample. Black: magnetite, red: hematite, blue: goethite, magenta: wustite, green: lepidocrocite. 288x201mm (300 x 300 DPI)


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Figure 2. a) XPS depth profile of elements present in a polished S355 steel sample. b) Comparison of results between XPS and EBSD for surface iron oxides present on the polished steel sample. Black: magnetite, red: hematite, blue: goethite, magenta: wustite, green: lepidocrocite. 296x209mm (300 x 300 DPI)


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Figure 3. A typical acid-base titration curve of 100 g L-1 acid-washed almandine garnet suspension in 100 mM sodium nitrate solution. Equivalence points are labelled in red, and equilibrium points are labelled in blue. 288x201mm (300 x 300 DPI)


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Figure 4. a) An EDX element map of the steel surface abrasive blasted with garnet for 10 s at 40 psi. b) Image after subtraction of the iron signal. The non-black area of the image is used to estimate the surface coverage of abrasive. 211x191mm (96 x 96 DPI)


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Figure 4. a) An EDX element map of the steel surface abrasive blasted with garnet for 10 s at 40 psi. b) Image after subtraction of the iron signal. The non-black area of the image is used to estimate the surface coverage of abrasive. 169x154mm (120 x 120 DPI)


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Figure 5. Surface coverage studies of abrasive blasting: a) variable duration at a constant pressure of 40 psi; b) variable pressure at a constant duration of 30 s. 288x201mm (300 x 300 DPI)


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Figure 5. Surface coverage studies of abrasive blasting: a) variable duration at a constant pressure of 40 psi; b) variable pressure at a constant duration of 30 s. 271x207mm (300 x 300 DPI)


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Figure 6. Surface coverage of garnet abrasive material residue on the steel surface for different cleaning methods. 288x201mm (300 x 300 DPI)


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Figure 7. a) Surface X-ray photoelectron spectra comparison between S355 steel after 30 s abrasive blasting with garnet (green) and polished S355 steel (blue). The most intense peaks of various elements are labelled in red. b) Surface elemental composition determined by XPS for polished S355 steel (blue) and after 30 s of abrasive blasting with garnet (green). All error bars are standard deviations. 296x209mm (300 x 300 DPI)


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Figure 7. a) Surface X-ray photoelectron spectra comparison between S355 steel after 30 s abrasive blasting with garnet (green) and polished S355 steel (blue). The most intense peaks of various elements are labelled in red. b) Surface elemental composition determined by XPS for polished S355 steel (blue) and after 30 s of abrasive blasting with garnet (green). All error bars are standard deviations. 288x201mm (300 x 300 DPI)


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Figure 8. A schematic illustration of the proposed model for the attachment of abrasive material at the steel surface. 126x76mm (150 x 150 DPI)


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Figure 9. Representative EDX elemental mapping of samples sent from North Sea Brent offshore facilities, with a) the steel surface, and b) the underside of the gently peeled paint, both analysed. 20320x100mm (1 x 203 DPI)


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Figure 9. Representative EDX elemental mapping of samples sent from North Sea Brent offshore facilities, with a) the steel surface, and b) the underside of the gently peeled paint, both analysed. 20320x100mm (1 x 203 DPI)


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Figure 10. Adsorption isotherms of a range of organic molecules, conducted in organic solvents: a) 2,4,6tris(dimethylaminomethyl)phenol (DMP-30) in dodecane, b) bisphenol A in toluene, c) 4-mercaptophenol in toluene, d) bis(2-ethylhexyl)phosphate (BEHP) in dodecane and e) palmitic acid in dodecane. Black circles: steel, green triangles: acid-washed garnet. 288x201mm (300 x 300 DPI)


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Abstract 126x81mm (150 x 150 DPI)


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Characterizing Surfaces of Garnet and Steel, and Adsorption of

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