10598
Langmuir 2007, 23, 10598-10602
Unsaturated Fatty Acids in Alkane Solution: Adsorption to Steel Surfaces Sarah M. Lundgren,*,†,‡ Karin Persson,† Gregor Mueller,§ Bengt Kronberg,† Jim Clarke,⊥ Mohammed Chtaib,# and Per M. Claesson†,‡ YKI, Institute for Surface Chemistry, Box 5607, SE-114 86, Stockholm, Sweden, Department of Chemistry, Surface Chemistry, Royal Institute of Technology, Drottning Kristinas Va¨g 51, SE-100 44, Stockholm, Sweden, ThyssenKrupp Steel AG, Center of Materials Excellence, Surface Analysis, Kaiser-Wilhelm-Strasse 100, D-47166 Duisburg, Germany, Arizona Chemical B.V., European Technical Center, Transistorstraat 16, NL-1322 CE Almere, The Netherlands, and Laboratories of LUXCONTROL SA, 1 AVenue des Terres Rouges, L-4004-Esch, Alzette, Luxembourg ReceiVed March 29, 2007. In Final Form: July 2, 2007 The adsorption of the unsaturated fatty acids oleic, linoleic, and linolenic acid on steel surfaces has been investigated by means of a quartz crystal microbalance (QCM). Two different solvents were used, n-hexadecane and its highly branched isomer, viz., 2,2,4,4,6,8,8-heptamethylnonane. The area occupied per molecule of oleic acid at 1 wt % corresponds to what is needed for adsorption parallel to the surface. At the same concentration, the adsorbed amount of linoleic acid and linolenic acid indicates that they adsorb in multilayers. The chemisorbed amount estimated from static secondary ion mass spectroscopy (SIMS) measurements was found to be similar for the three unsaturated fatty acids. In the case of linolenic acid, it was found that the presence of water significantly alters the adsorption, most likely because of the precipitation of fatty acid/water aggregates. Furthermore, static SIMS results indicate that the amount of water used here inhibits the chemisorption of linolenic acid.
Introduction
* To whom correspondence should be addressed. E-mail: sarah.lundgren@ surfchem.kth.se. † Institute for Surface Chemistry. ‡ Royal Institute of Technology. § ThyssenKrupp Steel AG. ⊥ Arizona Chemical B.V. # Laboratories of LUXCONTROL SA.
the solubility of the fatty acids, “chain matching” has also been reported to be of importance for fatty acid adsorption. The heat of adsorption has been found to be largest when the fatty acid is adsorbed from a solvent of similar chain length, i.e., hexanoic acid from heptane or hexadecanoic and octadecanoic acid from n-hexadecane.5 This has been interpreted as solvent forming a mixed film with the fatty acids when the chain lengths are matched. Purified crude tall oil, which is a byproduct in papermaking, is a mixture of, among other components, unsaturated fatty acids including oleic, linoleic, and linolenic acid. These mixtures are currently used as renewable and environmentally friendly additives in diesel fuels. A first step in the understanding of the adsorption of mixtures onto surfaces is to fully characterize the interfacial properties of the single unsaturated fatty acids. Only a few studies have been performed using these fatty acids.2,7,8 For example, the surface area is larger for oleic acid at the benzene/ metal interface compared to that for stearic acid, indicating that the double bond prevents close packing at the surface. The fact that the adsorbed amount is lower for oleic acid could also be due to the fact that oleic acid is more soluble in benzene compared to stearic acid.2 In hexane, the adsorbed amount onto ferric oxide was found to be similar for stearic and oleic acid at the plateau region but was significantly smaller for linoleic acid.7 For all investigated fatty acids, it was found that the calculated area per molecule was in fair agreement with those estimated for horizontal rather than perpendicular arrangement of the fatty acid molecules. The effect of solubility of the fatty acids has been discussed in an investigation on the adsorption of oleic, linoleic, and linolenic acid onto porous magnetite powder.8 The higher adsorption from hexane than from carbon tetrachloride is explained by the poorer solvation ability of hexane as estimated from enthalpies of
(1) Greenhill, E. B. Trans. Faraday Soc. 1949, 45, 625-631. (2) Daniel, S. G. Trans. Faraday Soc. 1951, 47, 1345-1359. (3) Smith, H. A.; Allen, K. A. J. Phys. Chem. 1954, 58, 449-452. (4) Smith, H. A.; McGill, R. M. J. Phys. Chem. 1957, 61, 1025-1036. (5) Groszek, A. J. ASLE Trans. 1970, 13, 278-287. (6) Cook, E. L.; Hackerman, N. J. Phys. Colloid Chem. 1951, 55, 549-557.
(7) Wheeler, D. H.; Potente, D.; Wittcoff, H. J. Am. Oil Chem. Soc. 1971, 48, 125-128. (8) Korolev, V. V.; Ramazanova, A. G.; Yashkova, V. I.; Balmasova, O. V.; Blinov, A. V. Colloid J. (Translation of Kolloidnyi Zhurnal) 2004, 66, 700-704.
Fatty acids are widely used as friction modifiers in fuels and lubricating oils. This has initiated research into the adsorption behavior of fatty acids to different metal surfaces since the beginning of the 20th century. Topics that have been investigated are the type of surface,1-4 fatty acid chain length,2,5 chemisorption versus physisorption,6 and chain matching effects.5 For example, the adsorption at oxide-free surfaces has been shown to be low, while it is much higher at metal oxide surfaces.3 The adsorption of fatty acids onto the oxide-free surfaces is small, while mechanically activated surfaces increase the adsorption.4 In addition to the number of chemisorption sites at the surface, the ability to form fatty acid soaps, and thus the surface oxidation state, is important for the adsorption. The focus of most work has been on the adsorption of saturated fatty acids. Both chemisorption and physisorption mechanisms are possible for fatty acids at metal oxide surfaces.6 The number of chemisorption sites on a surface is independent of the type of fatty acid. However, it can vary from surface to surface and result in both less than a monolayer of the adsorbate as well as the formation of a close-packed monolayer. In contrast to chemisorption, the amount of fatty acid that can physisorb at the surface depends on the type of fatty acid and is closely correlated to the solubility of the fatty acid in the solvent.2 In addition to
10.1021/la700909v CCC: $37.00 © 2007 American Chemical Society Published on Web 09/05/2007
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dissolution. However, this is correct only if fatty acid dimer formation is similar in the two solvents. Even though interest in the unsaturated fatty acids has increased, it is not yet fully understood how they adsorb onto surfaces from alkane solutions. In the present investigation, the adsorption of the unsaturated fatty acids oleic, linoleic, and linolenic acid onto steel surfaces is investigated. Steel was chosen because of its relevance in tribological applications. The physisorbed amount was determined with a quartz crystal microbalance (QCM), while the chemisorbed amount was estimated with static secondary ion mass spectroscopy (SIMS). The solvents used were nhexadecane and its highly branched isomer, viz., 2,2,4,4,6,8,8heptamethylnonane. The choice of solvent was directed by the fact that linear alkanes have been reported to organize at solid surfaces.9-13 Such organization may affect the QCM measurements of the linear alkane but would not interfere with the measurements of the branched isomer. Experimental Materials. n-Hexadecane (99%), 2,2,4,4,6,8,8-heptamethylnonane (98%), cis-9-octadecanoic acid (oleic acid, 99.5%), cis,cis-9,12octadecadienoic acid (linoleic acid, 99%), and cis,cis,cis-9,12,15octadecatrienoic acid (linolenic acid, 99%) were all obtained from Sigma-Aldrich. The chemicals were used as received. For some measurements, linolenic acid was saturated with water by exposing the pure linolenic acid, for one week under nitrogen atmosphere, to a saturated lead nitrate solution, providing a relative humidity (RH) of 97-98% at 20-25 °C.14 The surfaces used were quartz crystals coated with stainless steel, supplied by Q-Sense, Gothenburg, Sweden. The steel that was sputtered onto the gold-coated quartz crystal was Swedish standard steel number 2343. For the static SIMS measurements, the same steel was used but it was now sputtered onto silicon wafers. Quartz Crystal Microbalance. The adsorption measurements were carried out employing a QCM with dissipation facilities (QCM-D) from Q-Sense, Gothenburg, Sweden. In this technique, a quartz crystal resonates at its fundamental resonance frequency. Any added mass to the crystal will induce a frequency shift, which can be directly related to the added mass according to the Sauerbrey equation:15 ∆m ) -
Fqtq∆f Fqνq∆f C∆f )) nf0 n 2nf 2
(1)
0
where Fq and νq are the specific density and the shear-wave velocity in quartz, respectively, tq is the thickness of the quartz crystal, f0 is the fundamental resonance frequency, and n is the overtone order (Fq ) 2648 kg/m3, νq ) 3340 m/s, tq ) 0.33 mm, f0 ) 5 MHz, and C ) 0.177 mg/m2 in our system). The QCM used in this investigation measures the dissipation factor and the resonance frequency simultaneously.16 The measured change in dissipation is due to changes in coupling between the oscillating sensor and the surrounding medium. It is affected by any energy dissipating process and is thus influenced by the layer viscoelasticity and slip of the adsorbed layer on the surface. The dissipation is defined as D ) Edissipated/(2πEstored), with Edissipated being the energy dissipated during one period of oscillation and Estored being the energy stored in the oscillating system. (9) Findenegg, G. H. J. Colloid Interface Sci. 1971, 35, 249-253. (10) Kern, H. E.; Findenegg, G. H. J. Colloid Interface Sci. 1980, 75, 346356. (11) Balasubramanian, S.; Klein, M. L.; Siepmann, J. I. J. Chem. Phys. 1995, 103, 3184-3195. (12) Balasubramanian, S.; Klein, M. L.; Siepmann, J. I. J. Phys. Chem. 1996, 100, 11960-11963. (13) Christenson, H. K.; Gruen, D. W. R.; Horn, R. G.; Israelachvili, J. N. J. Chem. Phys. 1987, 87, 1834-1841. (14) Rockland, L. B. Anal. Chem. 1960, 32, 1375. (15) Sauerbrey, G. Z. Phys. 1959, 155, 206-222. (16) Rodahl, M.; Kasemo, B. Sens. Actuators, B 1996, 37, 111-116.
The Sauerbrey equation is only valid when the added mass is rigid and firmly attached to the quartz surface. If the adsorbed mass is viscoelastic, corrections may be required.17,18 Furthermore, it has been shown that, for some systems, corrections for the bulk properties, viscosity and density, have to be made.19-21 The measurement procedure is described in detail in a previous paper.21 A description of the corrections is summarized in the Supporting Information. The adsorbed amounts reported in this paper are results corrected for the bulk properties and the viscoelastic properties of the adsorbed mass in contact with the liquid. Surface Analysis. X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition of the steel-coated crystals (Kratos AXIS HS X-ray photoelectron spectrometer from Kratos Analytical, Manchester, U.K.). The analyzed area was approximately 1 mm2, and the analysis depth was 5-10 nm. The samples were analyzed in fixed analyzer transmission mode using a Mg KR X-ray source operated at 180 W (12 kV/15 mA). The sensitivity factors used were 0.25 for C1s, 0.66 for O 1s, 3.00 for Fe, 1.53 for Cr 2p, 0.23 for Si 2s, 2.60 for Mn 2p, and 0.45 for Ni 3p (supplied by Kratos). An atomic force microscope (AFM) (Multimode SPM, Nanoscope IIIA; Digital Instruments, USA) was used to measure the surface roughness of the steel-coated crystals. Images were captured using commercial silicon cantilevers (type: NCH-W) from Nanosensors (Neuchatel, Switzerland). Before measurement, the tip was rinsed with ethanol and cleaned with a Harrich plasma cleaner, (model PDC-32G) for 30 s on low effect (6.8 W) to remove any organic impurities. The resonance frequency of the cantilever supporting the tip was measured to be 276 kHz. The images were recorded in tapping mode with a tip speed of 1 Hz and 512 × 512 points resolution. The measurements were performed in air at ambient temperature. Static SIMS was used to investigate the degree of chemisorption of the fatty acids on the steel surface. The SIMS spectra were obtained in the static mode using a ToF-SIMS5 instrument manufactured by ION-TOF GmbH (Mu¨nster, Germany). The primary ion was Bi3 and the primary acceleration voltage was 25 kV. The target current was 0.27 pA (average), and the scanned area was 500 × 500 µm2 surface. The mass range was 1-500 m/z. Both negative and positive SIMS spectra were recorded. The fatty acids were adsorbed onto steel surfaces at ambient temperature from n-hexadecane prior to analysis. After adsorption, the surfaces were rinsed with ethanol and then dried in a warm stream of air. Bulk Analysis. The 1 wt % water-saturated linolenic acid solution had an opaque appearance. The transmittance through solutions of linolenic acid was measured with a TurbiScan Lab Expert (Formulaction, France). In this instrument, light (an electroluminescent diode in the near-infrared, λ) 880 nm) is shone through the sample while the light transmitted through and backscattered by the sample is recorded. Transmitted and backscattered light flux values are reported as percentages relative to internal standards (silicone oil and a suspension of monodisperse spheres, respectively). The temperature was 25 °C.
Results and Discussion Characterization of the Surface. The steel-coated quartz crystal used for the QCM measurements and the steel-coated silicon wafers used for static SIMS measurements were prepared by sputtering a 500 Å thick steel layer onto the surfaces in argon atmosphere. The surface roughness of the steel-coated quartz crystal was measured with an AFM and was found to be 1.0 nm (Ra) and 1.3 nm (rms) at an area of 1 µm2. Since fatty acids adsorb to different extents onto different metals (both chemisorption and physisorption), the surface composition is of (17) Johannsmann, D.; Mathauer, K.; Wegner, G.; Knoll, W. Phys. ReV. B 1992, 46, 7808-7815. (18) Du, B.; Johannsmann, D. Langmuir 2004, 20, 2809-2812. (19) Kanazawa, K. K.; Gordon, J. G., II Anal. Chim. Acta 1985, 175, 99-105. (20) Kanazawa, K. K.; Gordon, J. G., II Anal. Chem. 1985, 57, 1770-1771. (21) Lundgren, S. M.; Persson, K.; Kronberg, B.; Claesson, P. M. Tribol. Lett. 2006, 22, 15-20.
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Figure 2. The adsorbed amount of oleic acid (O), linoleic acid (]) and linolenic acid (0) in n-hexadecane. Table 1. The Atomic Concentration (%) of the Elements at the Surface as Determined by XPS QCM surface
Figure 1. The adsorbed amount for oleic acid (a), linoleic acid (b), and linolenic acid (c) in n-hexadecane (b) and 2,2,4,4,6,8,8heptamethylnonane (O).
importance. The surface composition of the created surfaces is summarized in Table 1. In the XPS measurement, carbon and oxygen is also observed. Some carbon, typically less than 15%, is always present because of impurities adsorbed from the air after cleaning. The oxygen detected at the surface is bound to the metal as oxide or hydroxide, and the oxide layer is formed upon contact with the lab atmosphere after the sputtering process. It is observed that no manganese was transferred from the original steel to the QCM surface, while small amounts reached the silicon wafer. There is also a much lager amount of silicon on the silicon wafer. Underneath the steel layer of the QCM surfaces, there is a gold layer. As no gold from the underlying layer is observed, the steel layer should be without cracks. Since the coatings on the QCM and the silicon wafers were prepared in the same way, it was assumed that the coating on the silicon wafer is homogeneous and hence, the large amount of detected silicon does not originate from the underlying silica wafer. The absolute value of chemisorbed fatty acids is affected by the surface
element
all elements
iron manganese chromium nickel molybdenum silicon carbon oxygen
25.9 2.2 1.3 0.2 2.8 13.0 56.9
elements excl. carbon and oxygen 77.3 7.7 4.6 0.7 9.8
coated silicon wafer all elements 16.0 0.6 1.5 1.1 0.1 7.4 8.4 64.8
elements excl. carbon and oxygen 59.8 2.3 5.7 4.2 0.4 27.6
composition, particularly the presence of Si, to which it cannot chemisorb. However, as we do not compare adsorbed amounts between the two different surfaces, this is inconsequential. Effect of Solvent. It is well-known that linear alkanes, such as n-hexadecane, may organize at surfaces.9-13 This would have implications for the QCM measurements. Any alignment of n-hexadecane should lead to a baseline that does not correspond to zero adsorption but is equivalent to some n-hexadecane attached to the surface. This would result in an underestimation of the adsorbed amount of fatty acid since the adsorption of the fatty acid would be able to replace adsorbed n-hexadecane with only minor changes in the detected mass by the QCM technique. To evaluate this, the QCM response received when switching between the two solvents was measured. The frequency shift due to solvent exchange was in the range 101.5-102.5 Hz. This frequency shift is close to what is expected because of the differences in viscosity and density between n-hexadecane and 2,2,4,4,6,8,8heptamethylnonane (103 Hz), as calculated with the KanazawaGordon equation.19,20 However, this equation is only correct to within 10%, and some alignment of n-hexadecane molecules at the surface cannot for this reason be ruled out. Nevertheless, alignment of linear alkanes has been detected on molecularly smooth surfaces, such as mica, while the QCM surfaces have a roughness on the order of 1 nm, rendering alignment of the alkane solvent parallel to the surface improbable. Fatty Acid Adsorption to Steel. The adsorbed amount of the unsaturated fatty acids oleic, linoleic, and linolenic acid, from the two alkane solvents n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane onto steel, are shown in Figures 1 and 2. The experimental results have been corrected with regard to changes in the bulk properties (density and viscosity) and the viscoelastic properties of the adsorbed layer. The results demonstrate that, after the corrections have been made, there is only a slight difference between the adsorption from the two solvents. As
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Table 2. The Adsorbed Amount and the Calculated Area Per Molecule for the Different Fatty Acids at 1 wt % in n-Hexadecane as well as the Surface Area at the Air/Water Interface at pH 2 (to Avoid Ionization) area per molecule (Å)
stearic acid oleic acid linoleic acid linolenic acid a
adsorbed amount (mg/m2)
QCM results
0.4 1.0 1.1
120 50 45
air/water interfacea,22
collapse area air/water22
perpendicular arrangement according to model7
20 (S) 41 (L.E.) 48 (L.E.)
18 27 31
20 25 25
parallel arrangement according to model7 110 130 143
S = solid phase; L.E. = liquid expanded phase.
observed in Figure 2, the adsorbed amount reaches a plateau value for oleic acid, while linoleic acid and linolenic acid do not reach a plateau within the investigated concentration range. Conclusions concerning the configuration of the fatty acids at the surface differ significantly between different investigations.2,7 The area per molecule in the solid (S) and liquid-expanded (L.E.) phase and the collapse area of fully protonated fatty acids (at pH 2) as determined from a surface pressure-area isotherm at the air/water interface increases as the unsaturation of the fatty acids increases (Table 2).22 The results were explained by increased steric hindrance due to additional double bonds. The reported packing at the air/water interface is the maximum achievable packing for these fatty acids, as it is not influenced by the presence of specific adsorption sites. The arrangement of the fatty acids can be estimated from the surface area according to molecular models (Table 2).7 Comparing the result from the present investigation with molecular models and results from the air/liquid interface provides insight into the packing at the metal surface. The area per molecule of oleic acid at the steel surface in the plateau region is larger than that at the air/liquid interface and similar to that needed for parallel arrangement to the surface. This indicates that oleic acid adsorbs parallel to the surface. Linoleic acid at 1 wt % has a calculated surface area similar to the surface area at the air/water interface. At this concentration, the fatty acid could adsorb in a close-packed monolayer. However, as the adsorption has not reached a plateau, linoleic acid may also form multilayers at higher concentrations since it is not physically possible to add any more fatty acids to the first layer. For linolenic acid, the surface area at the steel surface is smaller than expected at the air/liquid interface. This, together with the fact that linolenic acid does not reach an adsorption plateau within the investigated concentration range, suggests that also this fatty acid forms multilayers. The dissipation has been measured, and the change in dissipation is very low after corrections for the bulk effects. In fact, they are close to the resolution of the instrument. We note that, if the adsorbed layer is not rigid or firmly attached to the surface, then the dissipation will increase. This is clearly not observed, and it is hence concluded that the adsorbed layer can be regarded as firmly attached and rigid. Furthermore, we note that the corrected changes in dissipation result in small negative values. The effect is too small to allow us to draw any firm conclusions, but one may speculate that the adsorption results in a decreased surface roughness that lowers the energy dissipation somewhat. Chemisorption of Fatty Acids. Fatty acids are known to chemisorb onto metal surfaces, forming the corresponding fatty acid salt.6,23,24 In the QCM measurements, the system was rinsed with the solvent after fatty acid adsorption. However, possibly (22) Tomoaia-Contisel, M.; Zsako, J.; Mocanu, A.; Lupea, M.; Chifu, E. J. Colloid Interface Sci. 1987, 117, 464-476.
because of poor stirring in the QCM instrument, the adsorbed amount after rinsing was not repeatable. Static SIMS was used to determine the extent of chemisorption of the fatty acids adsorbed to steel-coated silicon wafers from a 0.02 wt % solution in n-hexadecane. The surfaces used in the SIMS were rinsed with ethanol to remove any physisorbed fatty acid. Fragmentation of the fatty acids was observed in the negative spectra. At higher mass numbers, peaks are observed on the rinsed surfaces that could be assigned to fragments of iron carboxylates. The peaks are separated with m/z of 14 (a CH2 group), and the m/z suggests that the form of the metal carboxylate fragments is CnH2nO2FeO. The amount of organic material at the surface can be estimated by using the ratio of C3H5+/Fe+ peaks (height).24 After the physisorbed fatty acids are rinsed off, the ratio is also a measure of the chemisorbed amount. For the three fatty acids used in this investigation, the C3H5+/Fe+ ratio is similar (0.64, 0.59, and 0.64 for oleic acid, linoleic acid, and linolenic acid, respectively). This suggests that the fatty acids are bound to specific adsorption sites at the surface and that the chemisorbed amount is not affected by the hydrocarbon part of the fatty acid molecule. The Influence of Water. Linolenic acid is known to be sensitive to water uptake from air. In order to investigate the influence of water on the adsorption, linolenic acid was allowed to take up water at RH ) 98 under nitrogen atmosphere for 1 week. The adsorption results from the QCM are shown in Figure 3, where it is observed that the moist linolenic acid causes increased adsorption at the steel surface. Not shown in the graph is the adsorbed amount at 1 wt %, which was found to be 67 mg/m2! Note that an adsorption plateau was not reached after 30 minutes at this concentration. At low concentrations of moist linolenic acid, the dissipation behavior was similar to that of the untreated sample (∆D ≈ 0). However, at 1 wt %, the change in dissipation was remarkably higher (5 × 106). This suggests that the adsorbed mass is less rigid than that for the other systems, or that the adsorbed mass slips at the surface. It is well-known that oxidation occurs for unsaturated fatty acids, such as linolenic acid, by attack from oxygen in air, by water, and by catalyzed metals. Even though the exposure to oxygen was minimized, it cannot be ruled out that, during exposure to air, the linolenic acid may also have changed properties as a result of oxidation. Gas chromatography measurements on the moist linolenic acid showed that 35 ( 4% of the linolenic acid molecules had transformed into linoleic acid isomers. It was observed with the TurbiScan that the level of transmitted light for neat n-hexadecane and untreated linolenic acid at 1 wt % in n-hexadecane were the same (Table 3), indicating that aggregates are not formed. However, the transmittance of moist linolenic acid at 0.1 wt % in n-hexadecane was slightly reduced, (23) Beentjes, P. C. J.; Van Den Brand, J.; De Wit, J. H. W. J. Adhes. Sci. Technol. 2006, 20, 1-18. (24) Murase, A.; Ohmori, T. Surf. Interface Anal. 2001, 31, 191-199.
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sample. However, for moist linolenic acid, the chemisorbed amount was significantly lower, as estimated from the lower C3H5+/Fe+ ratio. In summary, linolenic acid/water aggregates adsorb/precipitate onto the steel surface. The lower chemisorbed amount is caused by either a water layer adsorbed between the surface and the aggregate or the fact that the carboxylic head group is directed away from the surface into the water droplets.
Conclusions
Figure 3. The adsorbed amount for fresh linolenic acid (b) and moist linolenic acid (O) in n-hexadecane. Note, the concentration given for the moist linolenic acid is the total amount of moist fatty acid, i.e., fatty acid + water. Table 3. Summary of the TurbiScan Results transmission % n-hexadecane linolenic acid 1 wt % linolenic acid wet 0.1 wt % linolenic acid wet 1 wt %
96.9 96.6 94.6 84.1-80.8
while it was much lower for the moist linolenic acid at 1 wt %. The low transmittance suggests that moist linolenic acid at 1 wt % forms aggregates large enough to scatter a significant amount of light. Furthermore, for the 1 wt % solution, sedimentation occurred after storage for one week. Using Stokes law, it was estimated that the size of these droplets was on the order of 1 µm. The large adsorbed amount is therefore suggested to be due to precipitated water/linolenic acid droplets. Hence, the surface acts as a nucleus for precipitation. Note that the QCM technique does not distinguish between different species at the surface, and thus coadsorbed (or precipitated) water may contribute to the measured mass. At a lower concentration (0.1 wt %), the solution has only slightly lower transmittance than the solvent, and this value does not change within 24 h. The size of the scattering droplets is smaller than that for the 1 wt % solution, as no sedimentation is observed. The adsorbed amount at 0.1 wt % is nevertheless much higher for the moist system compared with the untreated solution and is likely affected by the precipitation of fatty acid/water droplets onto the metal surface. In the SIMS measurements, the moist linolenic acid showed peaks at the same mass numbers as the untreated linolenic acid
The adsorbed amounts of oleic acid, linoleic acid, and linolenic acid from n-hexadecane and 2,2,4,4,6,8,8-heptamethylnonane have been measured with QCM. There are only slight differences in the adsorbed amount from the two solvents. Static SIMS shows that the chemisorbed amounts of the three fatty acids are similar, suggesting that the chemisorbed amount is not influenced by the hydrocarbon chain of the fatty acid but is dependent on the interactions between the carboxylic head group and adsorption sites at the surface. The physisorbed amount on the other hand increases with increasing unsaturation of the fatty acids. While oleic acid reaches an adsorption plateau, the other two do not in the investigated concentration range. By comparison with molecular models, and area per molecule from the air/water interface, we conclude that linoleic acid and linolenic acid adsorb in multilayers. Moist linolenic acid adsorbed much more than untreated linolenic acid. It is suggested that linolenic acid/water droplets precipitate onto the surface. In samples of fatty acids, there may be trace amounts of water present, which may promote the formation of iron carboxylates. However, at the water level used in the moist linolenic acid sample, static SIMS showed that the water inhibits the binding of linolenic acid to the surface. Acknowledgment. The VINNOVA Competence Centre “Surfactants Based on Natural Products, SNAP”, supported this project. The authors thank Marie Ernstsson and Anna Hillerstro¨m at the Institute for Surface Chemistry for the XPS measurement and the AFM measurements, respectively. Thanks also to Dr. Martin Raulf at ThyssenKrupp Steel (Duisburg, Germany) for enabling the valuable SIMS measurements. Supporting Information Available: A description of how the adsorbed amount has been corrected is available, including experimental information on how the viscosity and density measurements were performed. Furthermore, information on the gas chromatography measurements is summarized. This information is available free of charge via the Internet at http://pubs.acs.org. LA700909V
