13801
2008, 112, 13801–13804 Published on Web 08/15/2008
A Potential Novel Rapid Screening NMR Approach to Boundary Film Formation at Solid Interfaces in Contact with Ionic Liquids Maria Forsyth,*,† Thomas F. Kemp,‡ Patrick C. Howlett,† Jiazeng Sun,† and Mark E. Smith‡ Department of Materials Engineering and ARC Centre for Electromaterials Science, Monash UniVersity, Wellington Road, Clayton, Victoria 3800, Australia, and Department of Physics, UniVersity of Warwick, CoVentry, CV4 7AL U.K. ReceiVed: July 10, 2008; ReVised Manuscript ReceiVed: July 23, 2008
The boundary films generated on a series of inorganic compounds, typical of native films on metal and ceramic surfaces, when exposed to various ionic liquids (ILs) based on the trihexyl(tetradecyl)phosphonium cation have been characterized using multinuclear solid-state NMR. The NMR results indicate that SiO2 and Mg(OH)2 interact strongly with the anion and cation of each IL through a mechanism of adsorption of the anion and subsequent close proximity of the cation in a surface double layer (as observed through 1H-29Si cross polarization experiments). In contrast, Al2O3, MgO, ZnO, and ZrO2 appear less active, strongly suggesting the necessity of hydroxylated surface groups in order to enhance the generation of these interfacial films. Using solid-state NMR to characterize such interfaces not only has the potential to elucidate mechanisms of wear resistance and corrosion protection via ILs, but is also likely to allow their rapid screening for such durability applications. Introduction The action of ionic liquids (ILs) in improving the durability performance of metallic and ceramic substrates in tribology1-3 and corrosive situations4-6 has recently been reported. Indeed, some ILs are already being marketed as lubricants in specific situations, although recent reports also suggest that certain IL/ metal combinations may actually lead to corrosion.7 Understanding the reactivity of ILs with solid surfaces, and characterization of the boundary layer that forms would enable better determination of a mechanism of IL action and subsequent optimization of the IL-surface combination for improved durability. Molecular modeling of IL-ceramic surface interactions has shown that the nature of the boundary layer depends on the substrate;8 for example, in the case of Al2O3 and a PF6--based IL, theoretical modeling suggested that the anion bonds to the surface via the fluorine and that the cation is attracted to this layer forming an electrical double layer. In contrast, while the PF6- anion interacted with a silicon nitride surface in a similar manner to Al2O3, the cation was less involved in the boundary layer with weaker anion-cation interactions present. One would expect this to have a significant impact on the tribology and/or corrosion properties of the surface. Other researchers have found that ILs based on PF6- and BF4- anions lead to significant improvements in wear performance for silicon and aluminum. The model proposed, on the basis of some limited scanning electron microscopy (SEM)/ energy dispersive X-ray spectroscopy (EDXS) and X-ray photoelectron spectroscopy (XPS) information, that, as also * Author for correspondence. E-mail:
[email protected]. Tel.: +61399054939. Fax: +61399054940. † Monash University. ‡ University of Warwick.
10.1021/jp806096w CCC: $40.75
Figure 1. Chemical structures of the cation and anion species in the phosphonium ILs used in this work.
suggested by the modeling work, an anion-surface interaction occurs followed by the further interaction of the IL cation species with the adsorbed anion. It is proposed that the solid substrates will be covered by a native oxide film and that the IL interacts with this film.8 In the case of corrosion-resistant IL films that have been reported to form on magnesium alloy surfaces,4-6 it is not clear whether the IL interacts with the metal substrate or the native film, which is ever-present on any metal substrate exposed to atmospheric conditions. It is possible in this case that the IL is chemically changed on the metal surface as a result of electrochemical reduction processes with subsequent deposition of a protective film based on these reduced species.9 Alternatively, a simple adsorption or chemical interaction may occur with the native oxide or hydroxide film, leading to passivation of the metal surface through a chemical reinforcement of this native surface. Understanding the dominant processes would assist in chemical design of the ILs used in either tribology or corrosion protection. Here multinuclear solid-state NMR techniques (including 19F, 31P, and 29Si) are used to examine the strength of interaction 2008 American Chemical Society
13802 J. Phys. Chem. C, Vol. 112, No. 36, 2008
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Figure 2. 19F NMR spectra for (a) P66614NTf2 and (b) P66614PF6 ILs adsorbed onto inorganic surfaces. Typically 300 scans were collected, and spectra were recorded recycle delays between 1 and 3 s.
between some ILs based on trihexyl(tetradecyl)phosphonium cations and inorganic substrates. New insights into the interaction mechanism are provided. Experimental Section Powder samples of SiO2 (Merck), Al2O3 (neutral surface, Aldrich), Mg(OH)2 (Ventron), Si3N4 (UBE), and ZrO2 (Aldrich) were used as received. The ILs (see Figure 1) based on trihexyl(tetradecyl)phosphonium (P66614) and hexafluorophosphate (PF6), and bis(trifluoromethanesulphonyl)amide (NTf2) were supplied by CYTEC, and P66614 diphenyl phosphate (dpp) was prepared from P66614Cl and diphenyl phosphate as reported earlier.10 Samples were prepared by combining the inorganic oxide and less than 1 wt% IL using an agate mortar and pestle and vigorously mixing for 30-60 s. The PF6 IL was heated to 50 °C to achieve a molten state prior to mixing with the inorganic particles, as it had a waxy consistency at room temperature. Solid-state NMR was carried out with a 4 mm Bruker H-X probe under magic angle spinning (MAS) at 10 kHz on 7.05 and 8.45T magnets, using Varian Infinity Plus and Chemagnetics Infinity consoles, respectively. Single-pulse experiments were performed for 19F, 31P, and 29Si, as well as 1H-29Si crosspolarization (CP) for SiO2 and Si3N4 samples using a contact time of 1.2 ms. Spectra were secondarily referenced using polytetrafluoroethylene (PTFE), ammonium dihydrogenphosphate, and kaolonite for 19F, 31P, and 29Si, respectively. Results and Discussion The 19F MAS NMR measurements for the mixtures of P66614NTf2 and P66614PF6 on SiO2, Al2O3, Si3N4, and Mg(OH)2 as well as the pure ILs are shown in Figure 2. The chemical shift of -79.3 ppm for the pure NTf2 anion is unchanged when the IL is in contact with the Al2O3 or Si3N4, although both 19F resonances are significantly broadened compared with the pure IL spun at 100 Hz (full width at half-maximum (fwhm) ) 36 Hz, cf 42 Hz (Al2O3) and 67 Hz (Si3N4) in the powder samples). In contrast, when either IL is in contact with SiO2, the line shape changes significantly with additional features observed. In particular, a broader resonance at more negative chemical shift is only seen with SiO2/NTf2. In the case of P66614PF6, the pure IL has a complex 19F spectrum consisting of a narrow doublet (-70.4, -72.9 ppm)
and a significantly broader pair at approximately -68.5 and -71 ppm. The 31P MAS NMR spectrum of the pure material confirmed that these doublets are from 31P-19F J-coupling of 715 Hz. This NMR behavior is typical of plastic crystal type materials often observed in the solid phases of ILs11 and represents the immobile crystalline component and the more mobile species in the vicinity of defects. It is interesting that the broad resonances are completely absent in the case of SiO2 and Si3N4, whereas they are very prevalent for Al2O3. However, there are still distinct differences between SiO2 and Si3N4 with a time-dependent increase of the intensity of the additional 19F resonance (observed at more negative chemical shifts) in the case of SiO2, whereas the PF6 chemical shift on Si3N4 was identical to the position of the narrow resonance observed in the pure IL. The 31P NMR data also indicate a strong interaction between the dpp anion and the hydroxylated surface of Mg(OH)2 as well as the SiO2 (Figure 3); however, much less significant interactions are present with all the other inorganic materials investigated (including ZnO, ZrO2, Al2O3, MgCO3, and freshly calcined MgO not shown here). The PF6- 31P resonances in all cases showed a relatively narrow heptet due to 31P-19F J-coupling, although significant broadening was observed in the case of the IL adsorbed onto SiO2 (Figure 3). The cation 31P resonance in these ILs showed significant broadening in the case of SiO2 compared to the pure ILs; however, there were no other observable changes in the chemical shifts. The strong effect of the Mg(OH)2 surface on the dpp anion, and the relative lack of interaction of this anion with the other metal oxides, are consistent with our preliminary observations of film formation on complex magnesium metal alloys containing Zn, Zr, and Al. Both 31P and 19F NMR results indicate that there is a strong interaction between the anion of the IL (i.e., NTf2, PF6, and dpp) and the SiO2 surface, whereas very little interaction is observed in the case of the more inert Al2O3 surface. The presence of a hydroxylated surface clearly enhances the interactions with the IL anions, and this accounts for the strong effect of the SiO2 and, to a lesser extent, the Si3N4 materials. SiO2 is known to have a reactive surface covered with -OH species, while commercial Si3N4 nearly always contains residual surface oxide, which will also contain hydroxy groups.12 In contrast, the other inorganic surfaces such as Al2O3, ZrO2, and ZnO are significantly less reactive, and thus their effect on the NMR of
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J. Phys. Chem. C, Vol. 112, No. 36, 2008 13803
Figure 3. 31P NMR spectra for (a) P66614dpp and (b) P66614PF6 ILs adsorbed onto inorganic surfaces. Typically 300 scans were collected, and spectra were recorded with recycle delays between 1 and 3 s.
Figure 4. 1H-29Si CP MAS spectra of SiO2 and Si3N4 inorganic powders combined with the ILs. Contact times of 1.2 ms were used in the CP experiment with delay times of 1 s. The number of scans varied from 4000 to 50000. Under identical conditions, the CP MAS spectrum for pure SiO2 (not shown) was very weak. A single-pulse 29Si experiment of the pure SiO2 was also performed with delay times of 30s and is also shown.
the IL is less noticeable, indicating that the anions are unlikely to strongly adsorb onto such surfaces. 1H-29Si CP experiments were used to probe more directly the surfaces of SiO2 and Si3N4. In the case of the NTf2 and PF6 anions, any strong CP effect indicates close proximity of the cation, which contains more than 60 protons per cationic species. On the other hand, the dpp itself already contains 10 aromatic protons, and so direct bonding of the anion to the SiO2 surface would be expected to yield strong CP effects. The CP data for SiO2 surfaces is shown in Figure 4. In the case of NTf2 and PF6, the 29Si CP NMR signal developed quite rapidly with a good signal-to-noise ratio (S/N) obtained after just 4000 scans. On the other hand, and somewhat surprisingly, the surface with P66614dpp took significantly longer to collect a spectrum with good S/N under similar CP conditions (>50 000 scans). Pure SiO2 had minimal signal under these conditions. The spectra, when compared with a single pulse 29Si experiment for the pure SiO2 powder, indicate a prevalence of the Q3 and Q2 species13 compared with the dominant Q4 species observed in the one pulse experiment. This suggests that the surface interactions predominantly arise through those sites. The fact that such a strong CP effect is observed is evidence of a close proximity
of the cation to the inorganic surface, which confirms the hypothesis that an electrical double layer forms on these surfaces in the presence of certain ILs. Given the shape of the phosphonium cation (i.e., flat with an easily accessible positive charge) one can envisage a close association of the cation to the surface via the Coulombic interactions between it and the surface-adsorbed anion. The less efficient CP (1H-29Si) for the dpp-based IL suggests an increase in distance of the P66614 from the surface. This may reflect a difference in interaction mechanism with the inorganic surface; for example, the diphenylphosphate may interact directly via a chemical bond to the hydroxylated surface which is not really possible for the other two anions unless they themselves undergo oxidation or hydrolysis. The presence of a chemical bond no longer requires close proximity of the cation to maintain electroneutrality, and hence the double layer may not be present or perhaps the dynamics of the IL ions may be significantly different. Fitting of the various Qn sites indicate that ∼63% of the species interacting with this IL are Q3, and only ∼17% are Q4, in contrast to the other two fluorinated anion ILs, where up to 40% of the Q4 sites are interacting closely with a proton, and the Q3 sites make up ∼45-50% of the signal intensity. It has been shown here that 29Si NMR clearly indicates that the interaction of the anion with the surface probably results in the subsequent attraction of the cation, forming a boundary double layer. The SiO2 surface has the strongest interaction with the ILs chosen here, whereas the interaction with the Al2O3 is less obvious. It also appears that a surface that is hydroxylated (as would be the case with fine particulates of SiO2) is more surface active. This boundary layer thus formed with the IL can act as a barrier to aggressive species in the case of a corrosive environment, can be tuned to be more or less hydrophobic through choice of cation, and can successfully lubricate solid surfaces. From these initial observations it is apparent that solid-state NMR can provide insight into IL-surface interactions. Future work will utilize these observations to better understand the evolution of passive films on metals, as well as the effect of cation and anion chemical structure on the interactions with inorganic surfaces. Acknowledgment. EPSRC is thanked for funding a visiting fellowship for M.F. to the U.K. (EP/F022913). EPSRC and the University of Warwick are thanked for partial funding of the NMR facility at Warwick. M.F. thanks the ARC for partial support of this work through the Australian Centre of Excellence for Electromaterials Science and DP0451444. References and Notes (1) Yu, G.; Yan, S.; Zhou, F.; Liu, X.; Liu, W.; Liang, Y. Tribol. Lett. 2007, 25, 197–205.
13804 J. Phys. Chem. C, Vol. 112, No. 36, 2008 (2) Liu, X.; Zhou, F.; Liang, Y.; Liu, W. Wear 2006, 261, 1174–1179. (3) Jimenez, A. E.; Bermudez, M. D.; Iglesias, P.; Carrion, F. J.; Martinez-Nicolas, G. Wear 2006, 260, 766–782. (4) Forsyth, M.; Howlett, P. C.; Tan, S. K.; MacFarlane, D. R.; Birbilis, N. Electrochem. Solid-State Lett. 2006, 9, B52–B55. (5) Birbilis, N.; Howlett, P. C.; MacFarlane, D. R.; Forsyth, M. Surf. Coat. Technol. 2007, 201, 4496–4504. (6) Howlett, P. C.; Zhang, S.; MacFarlane, D. R.; Forsyth, M. Aust. J. Chem. 2007, 60, 43–46. (7) Nooruddin, N. S.; Wahlbeck, P. G.; Carper, W. R. J. Mol. Struct.: THEOCHEM 2007, 822, 1–7. (8) Uerdingen, M.; Treber, C.; Balser, M.; Schmitt, G.; Werner, C. Green Chem. 2005, 7, 321–325.
Letters (9) Howlett, P. C.; Izgorodina, E. I.; Forsyth, M.; MacFarlane, D. R. Z. Phys. Chem.: Int. J. Res. Phys. Chem. Chem. Phys. 2006, 220, 1483– 1498, echem. (10) Sun, J.; Howlett, P. C.; MacFarlane, D. R.; Lin, J.; Forsyth, M. Electrochim. Acta, accepted. (11) MacFarlane, D. R.; Forsyth, M. AdV. Mater. (Weinheim, Ger.) 2001, 13, 957–966. (12) Carduner, K. R.; Carter, R. O.; Millberg, M. E.; Crosbie, G. M. Anal. Chem. 1987, 59, 2794–2797. (13) MacKenzie, K. J. D.; Smith, M. E. Multinuclear Solid State NMR of Inorganic Materials; Pergamon: Oxford, 2002.
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