Adsorption of Fatty Acids on Iron (Hydr)oxides from Aqueous

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Adsorption of Fatty Acids on Iron (Hydr)oxides from Aqueous Solutions Irina V. Chernyshova,* Sathish Ponnurangam, and Ponisseril Somasundaran NSF I/UCRC Center for Particulate & Surfactant Systems (CPaSS), Columbia University, New York, New York 10027, United States

bS Supporting Information ABSTRACT:

The interaction of iron (hydr)oxides with fatty acids is related to many industrial and natural processes. To resolve current controversies about the adsorption configurations of fatty acids and the conditions of the maximum hydrophobicity of the minerals, we perform a detailed study of the adsorption of sodium laurate (dodecanoate) on 150 nm hematite (R-Fe2O3) particles as a model system. The methods used include in situ FTIR spectroscopy, ex situ X-ray photoelectron spectroscopy (XPS), measurements of the adsorption isotherm and contact angle, as well as the density functional theory (DFT) calculations. We found that the laurate adlayer is present as a mixture of inner-sphere monodentate mononuclear (ISMM) and outer-sphere (OS) hydration shared complexes independent of the solution pH. Protonation of the OS complexes does not influence the conformational order of the surfactant tails. One monolayer, which is filled through the growth of domains and is reached at the micellization/precipitation edge of laurate, makes the particles superhydrophobic. These results contradict previous models of the fatty acid adsorption and suggest new interpretation of literature data. Finally, we discovered that the fractions of both the OS laurate and its molecular form increase in D2O, which can be used for interpreting complex spectra. We discuss shortcomings of vibrational spectroscopy in determining the interfacial coordination of carboxylate groups. This work advances the current understanding of the oxide carboxylate interactions and the research toward improving performance of fatty acids as surfactants, dispersants, lubricants, and anticorrosion reagents.

’ INTRODUCTION Long, straight-chain monocarboxylic or fatty acids and their salts (soaps) have long been in the use for laundry and personal care since the ancient Egyptian period. They represent by far the highest volume of all surfactants.1 These reagents are green: They have low toxicity and high biodegradability and can be obtained from renewable resources by hydrolysis of plant or animal fat.2 In modern technologies, fatty acids and their derivatives are used to modify surface and dispersion properties of iron (hydr)oxides in the personal care, biomedical, micro- and opto-electronics, abrasive, paint, and plastics formulations, as well as in mineral processing, corrosion protection, lubrication, and tar-sands extraction operations. The interaction of iron (hydr)oxides with fatty acids is also related to many processes in nature. In particular, being a component of the natural organic matter (NOM), aliphatic carboxylic acids regulate the fate and transport of the soil components, contaminants, and different types of bacteria and pathogens in the environment.3,4 The adsorption of fatty acids on iron (hydr)oxides from aqueous solutions has previously been addressed using both r 2011 American Chemical Society

macroscopic and spectroscopic experimental methods. It was found that oleate (cis-9-octadecanoate) imparts maximum hydrophobicity to hematite (R-Fe2O3) at circumneutral pH, which is close to the surface tension minimum of the surfactant.5 9 These two properties were suggested to relate to each other, both originating from the formation of premicellar associates in the bulk solution. However, this model is unable to uniformly interpret variations of the maximum hydrophobicity (as determined by flotation) of hematite conditioned with C6 C18 homologues of saturated fatty acids.10 Moreover, it conflicts with the chemisorption mechanism of the fatty acid adsorption6 and neglects the important role of the oxide surface chemistry.8 A related problem, which hinders the mechanistical understanding of the adsorption process, is the poor current knowledge of the surface speciation of fatty acids. On the basis of the adsorption envelopes, it was previously postulated that fatty acids Received: May 10, 2011 Revised: June 22, 2011 Published: June 28, 2011 10007

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Langmuir are preferentially physisorbed on hematite at pH < IEP but are chemisorbed at pH > IEP (IEP stands for isoelectric point).10,11 The chemisorption was inferred based on the presumption that physisorption is unable to attract the anionic surfactant to the negatively charged adsorbent at pH > IEP. However, the generality of this common presumption was recently questioned.12 It was demonstrated that the cooperative effects of both interfacial water structure and the formation of the surfactant hemimicelles at the interface can overcome the electrostatic repulsion. The other arguments in favor of the chemisorption, such as the adsorption increase with temperature or the solution ionic strength,7,13 can also break down in the case of surfactants due to the strong contribution of the hydrophobic interactions (water-induced effective attraction) between the hydrocarbon chains. On the other hand, FTIR spectra of oleate adsorbed on iron oxides both at pH < IEP and pH > IEP at concentrations below CMC were interpreted exclusively in terms of chemisorption based on the spectrum similarity in the wide pH range and spectral correlations.14,15 If so, it is unclear why the hydrophobicity maximum is coincident with the highest solution activity of the premicellar associates of fatty acids. In order to resolve the above controversies and to mechanistically understand formation of the fatty monolayer on iron (hydr)oxides, we undertook a comprehensive spectroscopic, macroscopic, and theoretical study of the adsorption of sodium laurate (SL), CH3(CH2)10COONa, which is a commonly used anionic surfactant. As a model ferric (hydr)oxide, we used hematite. Hematite is the most stable ferric oxide polymorph under oxidizing conditions in the bulk phase. It has the corundum structure with ferric cations octahedrally coordinated.16

’ MATERIALS AND METHODS Hematite with the mean particle size of 150 nm was acquired from Fisher. Sodium laurate was sourced from Fluka (purum, 99%) and used without further purification. In all the experiments, Nanopure water with conductivity of ∼18 MΩ/cm and total organic carbon content of less than 3 ppm was used. All the glassware was acid-washed and rinsed thoroughly with Nanopure water. The primary particle size and morphology were characterized by transmission electron transmission microscopy (TEM). A Jeol JEM 100CX transmission electron microscope (TEM) operated in the bright field mode at 100 keV. The X-ray diffraction (XRD) analysis was conducted using a Scintag model X2 X-ray powder diffractometer. A Cu KR (λ = 0.154 nm) radiation source operated at 45 kV and 35 mA was used. The scan step size was 0.05 deg. Estimates of the surface area were made with a single-point BET N2 adsorption isotherm (Monosorb, Quantachrome surface area analyzer). The samples were degassed with N2 at 80 °C for 1 h before measurements were taken. Two replicate measurements were performed, which gave results that agreed within less than 5%. Electrokinetic measurements were carried out using a Zeta Sizer instrument, Nano-ZS from Malvern. These measurements were performed within a pH range of 4 10 and at a particle loading of 0.01 by vol %. The background electrolyte concentration was 0.01 M NaNO3 and the pH of each subsample was adjusted using NaOH and/or HNO3. The pH-adjusted hematite suspensions were allowed to equilibrate for 24 h and pH was readjusted to initial values two hours before the actual measurement. Adsorption isotherms were measured at room temperature of 23 ( 1 °C using the depletion method and total organic carbon (TOC) analysis (TOC-5000A, Shimadzu). The laurate concentration was

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estimated using a calibration line calculated from the measured TOC values of a series of 8 laurate concentrations with 3 duplicates between 0.1 to 1 mM in 5 mM NaCl aqueous solutions with final pH 7.10 ( 0.05. The calibration line (R2 value of 0.9965) was obtained just before measuring each of the adsorption isotherms. Hematite at concentration of 7.5 g/L (10 g/L in the duplicate experiments) was suspended in 20 mL of a 0.005 M NaCl aqueous solution with dissolved sodium laurate at naturally established pH of 7.5 9.0. Afterward, pH of the suspensions was adjusted to 7.1 using an HCl solution. The adsorption was allowed to proceed for 4 h under stirring conditions. The suspensions were then centrifuged. The clear supernatants were pipetted out for the total carbon analysis, while the separated particles were used for the contact angle measurements. Opaque high-density polyethylene vials were used in the experiments to prevent photochemical reactions. Contact angle was measured using a CAM 200 optical contact angle meter (KSV Instruments Ltd., Helsinki, Finland). The experiments were conducted at ∼25 °C and 30% ambient humidity, with a temperature oscillation of (0.2 °C and a humidity oscillation of (1%. The initial droplet volume is controlled to be ∼2 μL. For the measurements, a thick suspension of the particles was spread onto a Ti plate and allowed to dry in air. The FTIR spectra were measured in situ using a Perkin-Elmer Spectrum 100 FTIR spectrometer equipped with an MCT detector in the horizontal attenuated total reflection (HATR) geometry. The sample spectra were collected for 200 400 scans at a resolution of 4 cm 1 and represented in the absorbance scale. An HATR accessory consisting of a ZnSe internal reflection element (IRE) (10 internal reflections, angle of incidence 45°) and a cap on IRE to prevent evaporation of water was employed. A particulate film of hematite was deposited onto the ZnSe IRE. This film was air-dried and then rinsed several times with water to remove any detached particles. As a background, the HATR spectrum of the ZnSe IRE covered by the hematite layer and pre-equilibrated with water with pH 7.15 or 10.0 was used. Afterward, the background water was replaced by the surfactant solution and the sample spectra were measured after selected time intervals. All the spectroscopic measurements were conducted in water without adjusting ionic strength. HATR spectra of the dry reference compounds were measured upon applying pressure to the powders spread on the ZnSe IRE. All the FTIR spectra were measured in duplicate and were qualitatively reproduced. We also verified (by measuring the spectrum of the 1.0 mM surfactant solution alone on ZnSe) that the 1 mM bulk solution of SL does not contribute to the reported spectra of the hematite solution interface. In the H2O/D2O exchange experiments, to convert the pH meter readings (pH*) into a pD value, we use the relationship pD = pH* + 0.44. XPS spectra were collected with a Perkin-Elmer PHI 5500 instrument using monochromatic AlKR X-rays (1486.6 eV) with pass energies of 17.6 eV, at takeoff angle of 45°, and at pressures of less than 1 10 9 Torr, calibrated using the Ag 3d peak. Regional XPS scans were collected at 0.1 eV steps. A low-energy electron flood gun was employed for charge neutralization. All samples were prepared by spreading a thin layer of the hematite paste on a UHV metallic holder followed by quick rinsing with water and air-drying. XPS spectra for each sample were measured twice after depositing a fresh layer on the XPS holder. The spectra were reproduced in terms of peak widths and shapes as well surface atomic concentrations, within the experimental error of the method. However, the peak position was reproduced only if binding energies (BE) were corrected for the charging effect by assuming a common BE for the O 1s core-level electrons of lattice oxygen, which was taken as 530.1 eV, in agreement with previous XPS studies of ferric oxides.17 19 The reason is that common referencing to the C 1s signal can be ambiguous.20 Shirley function was used to subtract the background. The C/Fe atomic concentration ratios were evaluated using C 1s and Fe 2p3/2 peaks and PHI atomic sensitivity factors. 10008

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Figure 2. XRD pattern of hematite.

Figure 1. TEM images of hematite NPs.

DFT Computational Details. The hematite surface was modeled using a ferric trioctahedral cluster obtained from the (104) hematite surface containing two corner-shared and one edge-shared ferric sites (Supporting Information Figure S4). The (104) surface is one of the naturally available surfaces of hematite.16 A butanoate ion (C3H7COO ) was used to model laurate to reduce the calculation time. This simplification is justified by the absence of any significant impact of the chain length on the electrostatic or steric structure of the carboxyl headgroup.21 Full geometry optimization of the ISMM and OS complexes of butanoate with the ferric cluster were performed using DMol3 package available in Material Studio (Accelrys Inc., San Diego, CA, USA). The exchange-correlation energy was calculated using the nonlocal Perdew Burke Ernzerhof (PBE) functional.22 The double numerical polarization (DNP) basis set, which is capable of an accurate description H bonding, was employed. A real space cutoff value of 3 4.3 Å was chosen for orbitals. Spin-polarized orbitals were utilized to account for the antiferromagnetic properties of hematite. The core electrons of Fe3+ and O2 ions were replaced by a single effective core potential (ECP) to reduce the computation cost. A continuum solvation model, COSMO, with dielectric constant, εr, of 12 was employed to model the interfacial species.23 A two-point finite difference scheme of analytical forces was employed to calculate vibrational frequencies of the model systems. We neither scale nor correct frequencies for anharmonicity, because we compare only the trends in computed vibrational frequencies with experimental ones and do not intend to match the exact values. Vibrational modes were assigned through a visual inspection of the atomic displacement trajectories created during the frequency calculations.

’ RESULTS Characterization of Hematite. Size and morphology of hematite particles were characterized using TEM (Figure 1). The particles have a mean size of 150 nm and a rounded polyhedral shape. The XRD analysis shows only the hematite X-ray diffraction pattern, with no lines due to other iron-containing impurity phases (Figure 2). XPS detected only ferric iron and oxygen, apart from adventitious carbon. BET surface area was found to be 9.5 m2/g. The isoelectric point (IEP) is 7.10 ( 0.05 (Figure S1 in Supporting Information). Adsorption Isotherm and Contact Angle. Laurate is classified as a weak electrolyte.24 Its state in water is controlled by hydrolysis to lauric acid (pKa = 5.0), precipitation (SL precipitates at 3 mM at pH 7.1), and micellization.24,25 Critical

micellar concentration (CMC) depends on pH, being 1 and 20 mM for SL at pH 7.1 and 10.0, respectively.24 The range from 25 to 42 °C was reported for the Krafft point.24 Moreover, according to refs 5,26,27, fatty acids can form acid soap dimers or low-molecular weight submicellar aggregates, although others have argued against their existence.28 30 The adsorption measurements were conducted in 0.005 M NaCl at initial pH 7.10 ( 0.05. During the process, pH of the hematite suspensions increased suggesting that, at least partially, laurate is chemisorbed. An important feature of the adsorption isotherm (Figure 3) is the presence of only two regimes of adsorption. Surface concentration of the surfactant linearly increases with increasing bulk concentration, reaching a plateau region at ∼1 mM, at the onset of the surfactant micellization in the bulk solution.24 The plateau value, which corresponds to the maximum surface concentration Γmax at one monolayer (ML) coverage, is 4.3 ( 0.1 molecules/nm2. This value is slightly below the value of 4.8 molecules/nm2 typical for the tilted condensed phase of fatty acids at the water air interface31 but much higher than adsorption densities of both 1 mM sodium dodecyl sulfate (SDS)32 (an anionic surfactant that forms exclusively OS complexes) and 1.9 mM sodium acetate33 on goethite NPs at pH 7 (∼0.5 and ∼0.06 molecules/nm2, respectively). Attempts to measure contact angle on the air-dried particles covered by 1 ML of laurate failed: Water droplets bounce off from the particulate layer, suggesting its superhydrophobicity. At a coverage of 0.57 ML, the layer is hydrophobic and is characterized by a contact angle of 123° ( 4°. Finally, TEM analysis of the particles conditioned with 1 mM SL revealed 3 5 nm particles. These nanoparticles are tentatively assigned to ferric laurate produced by the ligand-promoted dissolution of hematite. FTIR Results. The first question we address is how laurate is adsorbed at 1 ML coverage. To find an answer, an in situ FTIR spectrum was acquired after 25 min of conditioning of hematite with a 1 mM SL solution in water at pH 7.15 ( 0.05, without changing the solution during the conditioning. Given the adsorption isotherm (Figure 3) and the fact that the layer thus formed is superhydrophobic, we conclude that the conditioning produces ∼1 ML of the surfactant in the form of a hemimicelle with the hydrophobic tails pointing toward water. The FTIR spectrum of the monolayer (Figure 4) is characterized by the asymmetric νasCO2 and symmetric νsCO2 stretching bands of the carboxylate group at 1540 and 1410 cm 1, respectively. We assign these bands (see below) to an inner-sphere monodentate mononuclear (ISMM) complex with H-bonded second carboxylate oxygen (Scheme 1). Asymmetry of these bands suggests the presence of at least one more pair of bands at 1530 and 1425 cm 1 assigned below to an OS hydration-shared complex. Another important 10009


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Figure 3. (a) Adsorption isotherm of SL on 150 nm hematite at 5 mM NaCl and initial pH 7.10 ( 0.05 and (b) its representation in the initial surfactant concentration scale: (squares) 0.15 g and (circles) 0.2 g particles suspended in 20 mL of solution.

Figure 4. In situ FTIR-HATR spectra in the region of (a) νCO2 and (b) δscisCH2 vibrational modes of (1) hematite reacted for 25 min with a 10 3 M solution of sodium laurate at pH 7.15 ( 0.05 with no added electrolyte. For comparison, shown are spectra of (2) aqueous 10 2 M sodium laurate at pH 10.2 measured with a Ge IRE, (3) solid sodium laurate solid, and (4) solid lauric acid. The baseline in the 1600 1660 cm 1 region of the adsorbed laurate was corrected by adding the spectrum of water (water is removed from the interface upon the adsorption). Spectral regions of the νCdO and δscisCH2 modes of the surfactant are shadowed. Band assignment is taken from ref 39.

spectral feature is the presence of a band at 1707 cm 1 of the νCdO mode of carboxylic group ( COOH). On the basis of the valence saturation argument, lauric acid can exist at the NP water interface only as an OS complex. The band position is below 1720 cm 1 typical for fatty acids at the air water interface34,35 but much higher than the duplet at 1696 and 1686 cm 1 characterizing cyclic dimers of carboxylic groups in solid lauric acid (Figure 4). It follows that the CdO 3 3 3 H bonding of the lauric acid coordinated to the hematite surface has an intermediate strength.

An interesting observation is the absence of lauric acid at low surface coverage of the surfactant at both pH 7.15 and 10.0 (Figure 5). Lauric acid appears at pH 10.0 at coverage higher than 0.25 ML, which is reached after ∼1 h of the conditioning with 1 mM SL. Given that laurate is present in the 1 mM solution at pH 10 exclusively as an anion,24,25 its protonation on the hematite surface can be attributed to the adsorption-induced decrease in the local interfacial pH. This conclusion is supported by our DFT results (vide infra) and the well-known8,9,36 fact that 10010


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Scheme 1. Cluster Models of Different Laurate Adsorption Complexesa

a

Blue octahedra

Fe3+; red spheres

oxygen; gray spheres

carbon; green spheres

hydrogen.

Figure 5. In situ FTIR-HATR spectra of 150 nm hematite reacted successively with SL at (a) pH 7.15 and SL concentration of (black) 0.1 mM, (blue) 0.5 mM, and (green) 1.0 mM and (b) pH 10.0 and SL concentration of 1 mM. Time of the interaction is shown in the graphs. It was measured from the moment of the introduction of a SL solution into the FTIR-ATR cell until the beginning of the spectrum scanning (which takes 5 min). The background spectrum was that of the deposited particles pre-equilibrated with water of pH 7.15 for 1 h. Spectra are shifted along the Y axis for clarity.

metal oxides are negatively charged upon the alkanoate adsorption. A similar effect is observed for Langmuir monolayers of fatty acids.35,37 It is instructive to note that, in contrast to lauric acid, oleic acid appears on magnetite15 and hematite14 only at the surfactant concentrations above the precipitation limit.38 The difference can be attributed to a much lower surface density of 1.8 molecule/nm2 of the oleate monolayers (due to the adsorption conformation with carboxylate and double bond near or at the surface).9 A looser packing results in a higher interfacial pH, which impedes protonation. Finally, two different surface species rather than the same species in two different surface arrangements are responsible for the 1545/1410 and 1530/1425-cm 1 pairs in the FTIR spectra. As seen from Figure 6, these two pairs are decoupled from each other: The very first species that arises at pH 10.0 is characterized by the 1545/1410 cm 1 pair. At the same time, both pairs appear in the spectrum of ∼0.1 ML formed at pH 7.15. Taking into account that the hematite surface is negatively charged at pH 10 and almost neutral pH 7.15, it can be concluded that the 1545/1410 cm 1 pair belongs to a more strongly adsorbed species. Assignment of the Adsorbed Species. Drawbacks of the ΔνCO2 Correlation Approach. Traditionally, the splitting

between the νasCO2 and νsCO2 modes is used for specifying the adsorption configuration of the carboxylate groups.40 42 According to this approach, the bands at ∼1530 and 1425 cm 1, with the splitting ΔνCO2 of 105 cm 1, which is much less than 135 cm 1 observed for laurate in the solution (Figure 4), should be assigned to a chelating complex (Scheme 1). The closeness of positions of the other pair at 1545 and 1410 cm 1 to the corresponding bands of the aqueous sodium laurate would imply that the former originates from either an OS (hydration-shared or hydration-separated) or ISBB complex. However, this correlation is not general. Apart from isomerization observed for Zn(II) complexes,43 the correlation can be invalid due to vibrational couplings of the carboxylate modes. Specifically, the νasCO2 mode of the carboxylate group can couple with the δH2O mode of coadsorbed water.33 The νsCO2 mode usually interacts with the δscisCH2 mode of the methylene group in the R position relative to carboxylate group.44 Both νasCO2 and νsCO2 modes can be affected by the corresponding modes of coadsorbed carboxylates.45 In addition, the ΔνCO2 splitting depends on the hydration state of the carboxylate group46,47 and the size and polarizing power of the coordinating cation,34,42 as well as by rotation of the carboxylic group relative to the aliphatic chain skeleton.48 10011


Langmuir Another possible source of error is the assumption that the carboxylate group in the bulk solution is “free”. However, at high concentrations (>10 2 M) required for measuring the reference infrared spectra of the solution species, both the contact ion pair49 52 and solvent shared ion pair51,53 can be formed with a sodium countercation. Moreover, fatty acids in the bulk solution can form acid soap dimers or low-molecular-weight submicellar aggregates.5,25,26 Hence, spectra of hydrated laurate cannot be used as a standard for the band assignment unless its structure is clarified. Finally, the ΔνCO2 correlation approach also neglects the ISMM complex in which the carboxylate oxygen that is not coordinated to a cation is H bonded to a proximately physisorbed water molecule (Scheme 1). Such a complex was found for the carbonate adsorption on hematite NPs.23 Below we show that, in conflict with the general correlation, the bands at ∼1530 and 1425 cm 1 are more consistent with an OS hydration-shared carboxylate complex, while the bands at ∼1545 and 1410 cm 1 most probably are due to the H-bonding stabilized ISMM complex.

Figure 6. Comparison of in situ FTIR-HATR spectra of 150 nm hematite reacted with SL for 5 min at (1) pH 7.15 and SL concentration of 0.1 mM and (2) pH 10.0 and SL concentration of 1 mM. Spectra are normalized by intensity of the νsCO2 band.

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XPS: Chemisorption of Laurate. To begin with, we performed XPS analysis of the reacted hematite. The main spin orbit Fe 2p3/2 and Fe 2p1/2 photoemission peaks of this oxide originate from ligand-to-metal valence charge transfer accompanying Fe 2p core hole creation with the 2p53d6L final state of the Fe(III) cation (L denotes an O 2p ligand hole).54 As seen from Figure 7a, the Fe 2p3/2 peak of initial hematite exhibits a characteristic shoulder (marked by an arrow). This shoulder originates from the multiplet splitting in the final state, which strongly depends on the ligand field around the ferric cation.54,55 The splitting is still distinguishable for the Fe 2p3/2 peak of the hematite covered by ∼0.2 ML of laurate from a 0.22 mM SL solution but is practically absent in the photoemission of the particles covered by a complete monolayer from a 1.0 mM SL solution. Such a shape change can be explained by a weakening of the ligand field acting on the surface ferric cations,54 which in our case takes place due to substitution of surface hydroxyls by weaker carboxylate ligands. The decrease in the surface concentration of the OH groups is evidenced by the regular decrease of the corresponding peak at 531.5 eV in the O 1s spectra (Supporting Information Figure S2). An additional support of the laurate chemisorption follows from analysis of the outer-valence band region, which spans from 0 to 10 eV (Figure 7b). This region displays three maxima at ∼3.5 (I), 5.7 (II), and 8.0 (III) eV assigned to the d5L final-state multiplet produced from the d6L ground state by ligand-to-3d charge transfer screening of 3d holes.56 58 The presence of the multiplet indicates the strong Fe 3d O 2p hybridization.58 Upon surfactant adsorption, an extra shoulder appears at binding energy below ∼2 eV, which is suggestive of the appearance of the band gap states. In addition, intensity of peak I decreases and its position shifts toward the Fermi level, which can be attributed to a change of the Fe O hybridization and electron enrichment of the hematite (n-type semiconductor) upon laurate chemisorption, respectively. Hence, laurate is chemisorbed on hematite, which is consistent with the pH increase observed during the adsorption isotherm measurements. It is worth noting that no trace of the Na 1s peak at 1071 eV is observed in the XPS survey spectra (Supporting Information Figure S3), which is confirmed by the corresponding regional scans. This fact allows us to discard the formation of the Na+bridging OS complexes of laurate.

Figure 7. XPS (a) Fe 2p3/2 core-level and (b) valence-band spectra of hematite conditioned in solutions of sodium laurate of different concentrations. The C/Fe atomic ratio evaluated using the C 1s and Fe 2p3/2 peak areas is 0.6 for the laurate-free hematite, while 1.1 and 5.0 for hematite conditioned in 0.22 and 1.0 mM solutions of sodium laurate at pH 7, respectively. Charging is accounted by shifting the O 1s and 2s peaks of the hematite lattice to 530 and 22 eV in (a) and (b), respectively. The C 1s, O 1s, and Fe 2p peaks are shown in the Supporting Information. 10012


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Figure 8. Solvent H/D isotope effect on in situ FTIR spectra of sodium laurate adsorbed on (a) 9 nm hematite from a 1 mM solution at pH/pD 10.0, which is mostly presented by an H-bonded ISMM complex, and (b) 150 nm interacted with 0.5 mM sodium laurate at pH/pD = 7.1: (blue) in H2O and (red) in D2O. The spectra are normalized by peak intensity of the νsCO2 component of ISMM laurate at 1410 cm 1. The background spectra were the spectra of the NPs pre-equilibrated with H2O/D2O at the corresponding pH/pD. Time of interaction is 30 min. Asterisks show artifacts due to uncompensated subtraction of the H2O and D2O absorption.

FTIR: ISMM Configuration of Chemisorbed Laurate. Given the XPS results, the more strongly adsorbed species that is characterized by the 1545/1410 cm 1 pair is assigned to chemisorbed laurate. To refine its structure, we studied the solvent H/D isotope effect on its vibrational spectrum. On the basis of earlier works,23,33,59 this effect was expected to be pronounced for the OS and ISMM complexes (due to hydration of their head groups) but negligible for ISBB and chelating complexes where the carboxylate group is directly bound to ferric cations. Spectra were measured on 9 nm hematite at pH 10.0, because for 150 nm hematite we failed to find the conditions under which the chemisorbed laurate dominates in heavy water too (see below for more detail). The detailed TEM, XPS, and XRD characterization of the 9 nm NPs was reported earlier.17 As shown in Figure 8a, the νasCO2 mode at ∼1540 cm 1 shifts upward and increases in relative intensity in heavy water as compared to water. Such a spectral picture is typical for hydrated monodentate complexes of carboxylates (e.g., monodentate ferric oxalate dissolved in water33) and dissolved sodium acetate.42 It is caused by vibrational decoupling of the νasCO2 mode from the δH2O mode of water. Therefore, the chemisorbed species has ISMM rather than chelating or ISBB configuration. This conclusion agrees with our DFT data (vide infra). FTIR: OS Complex. The weakly adsorbed interfacial species characterized by the 1530/1425 cm 1 pair develops at higher surface coverage at pH 10 (Figure 5b). This species is preferentially stripped off the surface by washing with ethanol (Figure 9) or water at pH 8.5 (not shown), confirming that it is more weakly bound to the surface as compared to the ISMM counterpart. Taking into account that the trinuclear ferric laurate complex is characterized60 by main carboxylate bands at 1536 and 1443 cm 1 and the observation of precipitates in the batch adsorption experiments (vide supra), the 1530/1425 cm 1 pair could be assigned to a ferric laurate precipitate. However, we discard this possibility and assign this pair to an OS complex based on the following reasoning. First, removal of the surfactant solution and drying of the hematite particles covered by ∼1 ML of laurate results in an increase in the surface population of this species, while washing with ethanol removes it (Figure 9). The

Figure 9. Effects of drying and washing on FTIR-HATR spectra of hematite particles (1) reacted with SL at pH 7.15 and SL concentration of 1 mM for 55 min, (2) followed by removal of the solution, rinsing with water, and drying in air, and (3) washing in a flow of ethanol for 5 min and drying and in air. Spectra are shifted along the Y axis for convenience.

UV vis analysis of the ethanol extract did not detect absorption in the visual spectral range, which would be observed if the soluble species included ferric cations. Hence, the ferric laurate precipitates are not responsible for the band pair. A further indirect support of this assignment comes from the decrease of the νCdO band upon sample drying, suggesting that the weakly adsorbed species is formed by deprotonation of the OS complex of lauric acid. This effect can be attributed to an increase in the interfacial pH, a decrease in the effective dielectric function, and a change in the interfacial electric field, which causes charge redistribution at the interface. The spectrum assignment is corroborated by the spectral changes in D2O. Specifically, relative intensity of the 1530/ 1425 cm 1 pair increases as compared to the ISMM pair in D2O (Figure 8b). Given the higher capacity of D2O to solvate polar species compared to H2O,61 we suggest that the equilibrium shift toward the OS laurate originates from their higher hydration as compared to the ISMM complex. To our best knowledge, this 10013


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Table 1. Calculated Vibrational Frequencies of IS and OS Complexes of Butanoate frequencies (cm 1) configuration of adsorbed

charge on ferric

butanoate

cluster

νsCO2

νasCO2

ISMM

0

1383

1520

ISMM OS hydration-shared

1 0

1391 1391

1521 1499

OS hydration-shared

1

1419 1382

1480

1405

Figure 10. Geometry-optimized DFT models of a butanoate ion attached as ISMM and OS hydration-shared complexes to a ferric trioctahedral cluster with charge q = 0 which represents the (104) hematite surface. Numbers are the C O and CR C and H bond lengths in Å. Dashed lines show the surface coordinating H bonds.

is the first report of the solvent H/D isotope effect on the adsorption equilibrium between OS and IS complexes. As seen from Figure 8b, heavy water also promotes protonation of the OS laurate, which is explained by the well-known tendency of D2O to weaken acids (increase their pKa).62 Particularly noteworthy is a decrease in the νasCO2/νsCO2 intensity ratio in heavy water. Such an intensity decrease was reported for strong H-bonded carboxylate groups with proton transfer and attributed to weakening of the C—O 3 3 3 H—O bonds by deuteration.63 The participation of the carboxylate head-groups in very strong H bonds is suggested by the fact that the adsorbed monolayer is disordered upon drying and H/D exchange (vide infra). Thus, the effects of the surfactant surface coverage, pH, H/D exchange, drying of the NPs, and washing them with different solvents indicate that the ∼1530/1425 cm 1 and 1545/ 1410 cm 1 pairs of bands are due to OS and ISMM complexes, respectively. DFT Modeling of Carboxylate Binding to Hematite Surface. DFT calculations were performed to verify the foregoing interpretation of the vibrational spectra as well as to determine the structure of the OS complex. In our earlier study of the carbonate adsorption, we suggested a methodology based on analysis of stability of different surface complexes and comparison of the theoretical and experimental frequencies of these configurations with those of the dissolved species.23 However, the uncertainty in the coordination of dissolved laurate (vide supra) hinders the direct application of this methodology. Instead, we compare the stability of the OS hydration-separated and hydration-shared complexes (Scheme 1) as well as the relationship between calculated frequencies of these and ISMM complexes. As a model, a butanoate ion coordinated to a ferric trioctahedral cluster extracted from the (104) hematite surface was used (Supporting Information Figure S4). The geometry optimization of the OS hydration-separated, OS hydration-shared, and ISMM complexes were conducted at the ferric cluster charges q = 0 and q = +1. As opposed to the OS hydration-shared complex, which is stable (Figure 10), the OS hydration-separated counterpart was found to be unstable at both the cluster charges, moving away

from the cluster. This finding can be attributed to the expectable fact that two hydrating water layers better screen the electrostatic attraction of the carboxylate group by the positively charged surface as compared to one layer. Moreover, similar to what is observed experimentally for the weaker bound laurate, the νsCO2 and νasCO2 modes of the hydration-shared complexes relaxed at q = 0 and +1 are higher and lower than the νsCO2 and νasCO2 modes of the ISMM complexes (Table 1), respectively. It follows that the OS complex more likely adopts the hydration shared configuration. Additionally, we observe that at a charge of the ferric cluster q > +2 the OS hydration-shared complex of the alkanoate is protonated by the water molecule that coordinates the complex to a ferric cation. At the same time, protonation by the coadsorbed hydronium cation takes place at a charge as low as q = 1. Therefore, the interfacial protonation of the carboxylate group is more feasible by the coadsorbed hydronium cation rather than by the hematite surface. This result agrees with the relationship between the pKa values of the involved functionalities. Namely, pKa = 5.0 of laurate is lower than IEP of hematite of 7.1 and hence pKa of the adsorbed water (see also ref 64), but higher than pKa = 1.74 of hydronium. The protonation by the coadsobed proton is further supported by the absence of protonated laurate in the layer adsorbed from a 0.1 mM solution at IEP of hematite (Figure 5a). Thus, our DFT results are consistent with coadsorption of the OS hydration-shared and ISMM complexes. In addition, they show that protonation of the hydration-shared OS complexes is more likely caused by a coadsorbed hydronium ion. FTIR: Surfactant Tail Packing. Information about packing of the aliphatic chains of the adsorbed surfactant can be extracted from analysis of the stretching (νCH2), scissoring bending (δscisCH2), and wagging (ωCH2) vibrations of the methylene groups.45,65 At both pH 7.15 and pH 10.0, the position and width of the symmetric νsCH2 mode at 2851 cm 1 are independent of the surface coverage, as well as the presence of the molecular form in the adsorbed layer (Figure 11a). The exception is a blue shift and broadening of this band in the beginning of the surfactant hematite interaction at pH 10.0, suggestive45 of a higher packing disorder of the chains. The invariance of the νsCH2 mode at increasing surface coverage testifies to the domain-like pattern of the surface filling by the surfactant. Indeed, the hydrophobicity of the monolayer (vide supra) allows us to discard the alternative of the adsorption of micelles as observed using vibrational sum frequency generation spectroscopy for aliphatic quaternary ammonium surfactants on bulk silica.66,67 This conclusion is supported by the absence of the 10014


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Figure 11. Effect of conditioning on symmetric stretching vibrations of the methylene groups of the hydrocarbon chains of laurate adsorbed on hematite: (a) from 1 mM solution at pH 10.0 and 7.15 at different conditioning time indicated in the graph, (b) effects of drying and H/D exchange on the formal monolayer formed at pH 7.15 in a 1 mM SL solution.

hemimicellization range in the adsorption isotherm (Figure 3). The domain-like pattern of the monolayer growth probably originates from the mixed IS and OS composition of the monolayer, where the ISMM carboxylates are able to screen the electrostatic repulsion between the coadsorbed OS carboxylates, as well as from the strong H bonding between the carboxylate headgroups. The packing order of the aliphatic chains slightly decreases in heavy water and, to a larger extent, upon drying of the particles, as judged from the blue shifts of the νsCH2 modes (Figure 11b). Disordering of adsorbed surfactants upon drying of the substrate has been previously reported and attributed to the stabilizing role of the coadsorbed water.45 In contrast, the effect of the solvent H/D exchange has not been reported before. It is consistent with the critical role of H bonding between the surfactant head groups and coadsorbed water in the chain packing. Further details of the chain packing can be extracted from the scissoring bending δscisCH2 modes. As seen from Figure 4b, the adsorbed laurate is characterized by two δscisCH2 modes at ∼1466 and 1458 cm 1. The latter band is associated with gauche isomers of the chains,68,69 suggesting a certain degree of the chain disorder. The ∼1466 cm 1 band is typical for the trans conformation. The band splitting is absent for the hexagonal perpendicular subcell of the hydrophobic tails. For comparison, splitting into components at 1463 and 1470 cm 1 is observed for solid sodium laurate and lauric acid, which is a signature of either an orthorhombic perpendicular or monoclinic subcell. Hence, the hydrocarbon tails, although well-packed in the adsorbed monolayer, are flexible and free to rotate.

’ DISCUSSION Our study shows that, both at pH < IEP and at pH > IEP, the laurate adlayer on hematite is mainly composed of ISMM and OS hydration-shared complexes (Scheme 1). These complexes are characterized by the 1545/1410 and 1530/1425 cm 1 band pairs in the FTIR spectra, respectively. The difference between our assignment of the adsorbed species and that based on the conventional ΔνCO2 correlation40 42 can be attributed to polarization of the adsorbed carboxylate groups by strong H bonding. Indeed, as previously shown,34 polarization of the

carboxylic group by the coordinating cation can have a significant impact on the spectra. Effects of Alkyl Bond Length and the Iron Oxide Polymorphism on the Adsorption of Alkanoates. DRIFTS spectra of oleate adsorbed on natural hematite exhibit bands at 1405, 1430, 1530, and 1580 cm 1,14 which are close to the bands of laurate on hematite. Hence, it can be concluded that fatty acids are similarly coordinated to hematite, forming OS and ISMM complexes. In contrast, only the 1530/1425 cm 1 pair is observed in the in situ FTIR-HATR spectra of oleate adsorbed on magnetite (Fe3O4) at pH > IEP.15 This pair was assigned to the chelating complex based on the conventional ΔνCO2 correlation. However, according to our results, it should be reassigned to the OS hydration-shared oleate. This configuration is also more consistent with a much lower electron accepting capacity of magnetite as compared to hematite. This effect hinderes the formation of the donor acceptor bonds in which magnetite acts as the electron acceptor. In contrast, short-chain monocarboxylates including acetate are adsorbed on goethite (R-FeOOH) as the hydration-separated and hydration-shared OS complexes (Scheme 1) at circumneutral and acidic pH, respectively.33 Given chemisorption of acetate on ferric (hydr)oxides from gaseous phase,70,71 the difference with laurate in water can be attributed to stronger hydration of acetate. At the same time, on the basis of our DFT results, one can expect protonation of the OS acetate at pH < pKa = 4.76, which however is not observed.33 Moreover, the OS model is at odds with indirect evidence of chemisorption of the short chain carboxylates.72 75 Hence, a further multitechnique study is required to resolve these controversies. Monolayer Filling. By analyzing the position and width of the νsCH2 band in the FTIR spectra, we found that the laurate monolayer grows on hematite through the formation of highly ordered domains, similar to the case of monolayers of fatty acids on the aqueous subphases containing Cd2+ cations76 and on fluorite.77 Hence, the surface filling pattern for laurate on hematite differs from the hemimicellization (entropy-controlled hydrophobic interactions) observed for SDS on hematite32 and oleate on magnetite.15 Interestingly, the laurate packing is unaffected by the protonation of the surfactant headgroup, which 10015


Langmuir is different from the case of fatty acids at the air water interface2,31,78,79 and aliphatic amines on silicates.80,81 This difference is explained by the presence of the ISMM complexes of laurate on hematite: These complexes screen the electrostatic repulsion between the coadsorbed OS complexes and possibly act as active centers (anchor molecules) for the domain formation. In fact, H bonding between two carboxylate groups can be as strong as 38.6 kcal/mol.82 Along with the hydrophobic interactions between the C12 aliphatic chains of 26 kcal/mol (ref 83), such bonding can overcome the electrostatic repulsion between the carboxylate group and the negatively charged hematite at pH > IEP because this repulsion is partially screened by the mediating water layer. Solvent H/D Isotope Exchange Effect. The solvent H/D isotope exchange can significantly shift the interfacial adsorption equilibrium because heavy water promotes both the formation of the OS complexes and their protonation. This effect can be used for differentiating spectral contributions of the OS and IS adsorption forms. At the same time, it points to the necessity to take extreme care when the H/D exchange is employed for interpretation of FTIR spectra or heavy water is used as a solvent. Although the influence of heavy water on the stabilization/ destabilization of proteins84,85 and surfactant vesicles86 is wellknown, to the best of our knowledge, this effect has not been reported for the surfactant adsorption. Implications of Our Results. Our findings suggest that both the solution activity of the fatty acid and the surface properties of the oxide (surface charge, the adsorption affinity and capacity) control hydrophobicity and hence the flotation and aggregation properties of the oxide. In fact, the hematite surface covered with 1 ML of laurate is superhydrophobic. This condition is attained at the micellization/precipitation edge of laurate, which is 1 mM at pH 7.1. Both the surface coverage and the hydrophobicity decrease as either laurate concentration decreases or pH increases. Furthermore, lauric acid, which is incorporated into the adlayer above a certain critical surface coverage, does not affect the conformation order of the aliphatic chains. Hence, in conflict with the previous model,5 9 concentration of acid soap dimers in the bulk solution does not directly relate to the oxide hydrophobicity. In fact, although for oleate the surface tension minimum coincides with the maximum hydrophobicity of hematite,5 9 this correlation is not observed for C6 C18 fatty acids.10 Macroscopic/thermodynamic and FTIR spectroscopic measurements can provide a wealth of information on the interaction of the carboxylate groups with metal oxides. However, as follows from our results, these measurements alone are unable to elucidate interfacial structures of complex molecules including surfactants, which is the main obstacle for developing mechanistic models. Specifically, as mentioned in the Introduction, the conventional criteria of differentiating between the IS and OS complexes based on the effects of the solution ionic strength or temperature are not applicable to the adsorption of large molecules for which strong association effects can produce the same macroscopic response as the chemisorption. Moreover, the conventional ΔνCO2 FTIR correlation, which is derived from the infrared spectra of bulk complexes, can be invalid (see Results). Hence, to avoid misinterpretation, the vibrational spectra measured under different solution conditions should be complemented by the isotope exchange effects, X-ray photoelectron or absorption spectroscopy analysis, and DFT simulations.

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’ CONCLUSIONS (1) Laurate is adsorbed on hematite mainly as a mixture of ISMM and OS hydration-shared complexes, both at pH > PZC and at pH < PZC. (2) The OS complex is partially protonated by a coadsorbed hydronium cation as either surface coverage increases or solution pH decreases. (3) The protonation does not influence the conformational order of the surfactant tails in the adlayer. (4) Except for the very beginning of the interaction, laurate fills the hematite surface through the growth of hydrophobic domains, in which hydrocarbon tails are hexagonally packed. (5) The laurate monolayer is formed at the edge of the surfactant precipitation/micellization and makes the hematite particle superhydrophobic. The above results suggest that the net solution activity of the fatty acid along with the surface properties of the oxide such as IEP, the adsorption affinity, and capacity rather than the structure of the solution species control the hydrophobicity and hence the flotation and aggregation properties of the oxide. In addition, we demonstrate that (1) the ΔνCO2 correlation is invalid for interpreting vibrational spectra of adsorbed fatty acids and (2) the fraction of the OS complexes and their molecular form increases in D2O, which can be used for interpreting spectra. ’ ASSOCIATED CONTENT

bS

Supporting Information. Zeta potential vs pH plot for 150 nm hematite, survey and regional XPS spectra of adsorbed laurate, the ferric trioctahedral cluster used as a model of the (104) hematite surface. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]

’ ACKNOWLEDGMENT Support from the NSF under grant nos. 0925232 and UCSB KK9157, as well as from the Tata Research Development and Design Center, Pune, India, is acknowledged. The authors thank Dr. X. Fang for the help with the contact angle measurements and Dr. Ananthapadmanabhan for discussion. This work used the shared experimental facilities that are supported primarily by the MRSEC program of NSF # DMR 0213574 and by the New York State Office of Science Technology and Academic Research (NYSTAR). ’ REFERENCES (1) Farn, R. J. Chemistry and Technology of Surfactants; Blackwell Publishing Ltd: Oxford, 2007. (2) Jarek, E.; Jasinski, T.; Barzyk, W.; Warszynski, P. Colloids Surf., A 2010, 354, 188–196. (3) Gu, B. H.; Mehlhorn, T. L.; Liang, L. Y.; McCarthy, J. F. Geochim. Cosmochim. Acta 1996, 60, 1943–1950. (4) Gao, X. D.; Metge, D. W.; Ray, C.; Harvey, R. W.; Chorover, J. Environ. Sci. Technol. 2009, 43, 7423–7429. (5) Somasundaran, P.; Ananthapadmanabhan, K. P.; Ivanov, I. B. J. Colloid Interface Sci. 1984, 99, 128–135. (6) Ananthapadmanabhan, K. P.; Somasundaran, P. Oleate chemistry and hematite flotation. In Interfacial Phenomena in Mineral Processing, 10016


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Adsorption of Fatty Acids on Iron (Hydr)oxides from Aqueous

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