Langmuir 2001, 17, 7907-7911
7907
Alkyl Phosphonate/Phosphate Coating on Magnetite Nanoparticles: A Comparison with Fatty Acids Yudhisthira Sahoo,† Hillel Pizem,‡ Tcipi Fried,† Dina Golodnitsky,§ Larisa Burstein,§ Chaim N. Sukenik,‡ and Gil Markovich*,† School of Chemistry and Materials Research Center, Tel Aviv University, Tel Aviv 69978, Israel, and Department of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel Received May 10, 2001. In Final Form: August 15, 2001
Coated magnetite nanoparticles with a 6-8 nm average diameter were prepared. The surfactants used to stabilize the nanoparticles and disperse them in organic solvents were oleic acid (OA), lauric acid , dodecyl phosphonate, hexadecyl phosphonate, and dihexadecyl phosphate. Transmission electron microscopy analyses of the aggregation of the coated particles suggest that carboxylate surfactants provide the particles with better isolation and dispersibility as compared with phosphonate surfactants. However, Fourier transform infrared spectra of the phosphonate and phosphate coated particles suggest that these surfactants cover the surface of the nanoparticles in islands of high packing density. The thermogravimetric and differential scanning calorimetry measurements suggest that there is a quasi-bilayer of these surfactants covering the surface of the nanoparticles, with varying amounts of surfactant in the outer layer and with the second layer weakly bound to the primary layer through hydrophobic interactions between the alkyl chains. The desorption temperatures of the alkyl phosphonates and phosphate are higher than those of the carboxylate coated particles. The enthalpy of binding of the ligands suggests strong P-O-Fe bonding on the surface. Nevertheless, regardless of binding strength, the OA coated particles are better dispersed in organic solvents. Their higher hydrophobicity is likely due to different interactions among the oleyl chains and/or a smaller tendency to form bilayer structures.
Introduction Broad scientific attention, both fundamental and applied, has been devoted to the study of magnetic nanoparticles. These particles serve as ideal systems for the study of phenomena like superparamagnetism,1 magnetic dipolar interactions,2,3 single-electron transfer,4-6 and magnetoresistance.7-10 They have the potential for improved high-density data storage,11-13 ferrofluids,14 and biomedical applications.15 †
School of Chemistry, Tel Aviv University. Department of Chemistry, Bar-Ilan University. § Materials Research Center, Tel Aviv University. ‡
(1) Neel, L. Rev. Mod. Phys. 1953, 25, 293. (2) Luo, W.; Nagel, S. R.; Rosenbaum, T. F.; Rosensweig, R. E. Phys. Rev. Lett. 1991, 67, 2721. (3) Legrand, J.; Petit, C.; Bazin, D.; Pileni, M. P. Appl. Surf. Sci. 2000, 164, 186. (4) Molotkov, S. N.; Nazin, S. S. Phys. Low-Dimens. Struct. 1997, 10, 85. (5) Weisendanger, R. MRS Bull. 1997, 22, 31. (6) Bird, J. P.; Ishibashi, K.; Stopa, M.; Taylor, R. P.; Aoyagi, Y.; Sugano, T. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 49, 11488. (7) Barthelemy, A.; Cros, V.; Duvail, J. L.; Fert, A.; Morel, R.; Parent, F.; Petrof, F.; Steren, L. B. Nanostruct. Mater. 1995, 6, 217. (8) Yue, D. F.; Banerjee, G.; Miller, A. E.; Bandyopadhayay, S. Superlattices Microstruct. 1996, 19, 191. (9) Perez, A.; Melinon, P.; Dupuis, V.; Jensen, P.; Prevel, B.; Tuaillon, J.; Bardotti, L.; Martet, C.; Treilleux, M.; Broyer, M.; Pellarin, M.; Vaille, J. L.; Palpant, B.; Lerme, J. J. Phys. D: Appl. Phys. 1997, 30, 709. (10) Lopezquintela, M. A.; Rivas, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 806. (11) Gibson, G. A.; Smyth, S. F.; Shultz, S.; Kern, D. P. IEEE Trans. Magn. 1991, 27, 5187. (12) Chou, S. Y.; Wei, M. S.; Kraus, P. R.; Fischer, P. B. J. Appl. Phys. 1994, 76, 6673. (13) Kawanishi, Y.; Tamaki, T.; Sakuragi, M.; Segi, T.; Swuzki, Y.; Ichimura, K. Langmuir 1997, 13, 5244. (14) Rosenweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, 1985. (15) Roger, J.; Pons, J. N.; Massart, R.; Halbreich, A.; Bacri, J. C. Euro. Phys. J.: Appl. Phys. 1999, 5, 321.
It is a technological challenge to prepare nanoparticles of customized size and shape. Physical methods such as gas phase deposition and electron beam lithography are elaborate procedures that suffer from the inability to control the size of structures16,17 in the nanometer size range. The wet chemical routes to magnetic nanoparticles are simpler, more tractable, and more efficient, with appreciable control over the size, composition, and sometimes even the shape of the particles.18-20 Very recent examples of such work include Co particles,21,22 Co/Pt core/ shell particles,23 and γ-Fe2O3 particles24 syntheses. In the preparation and storage of nanoparticles in colloidal form, the stability of the colloid is of utmost importance. Most surfactants adhere to surfaces in a substrate specific manner. For example, it is well-established that thiols are strongly adsorbed on gold and silver, carboxylic acids adhere to aluminum oxide surfaces, and nitriles and amines bind to platinum.25 There is very little reported in the literature about surfactants with phosphonate or phosphate headgroups as protecting agents. In one such piece of work, Maoz and Sagiv reported that C18 phosphates form robust self-assembled monolayers on polar surfaces such as glass and ZnSe,26 unlike sur(16) Krauss, R. P.; Chou, S. Y. Appl. Phys. Lett. 1997, 71, 3174. (17) Winzer, M.; Kleiber, M.; Dix, N.; Weisendanger, R. Appl. Phys. A 1996, 63, 617. (18) Meisel, D. Curr. Opin. Colloid Interface Sci. 1997, 2, 188. (19) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Phys. Chem. 1994, 98, 4506. (20) Yin, J. S.; Wang, Z. L. Phys. Rev. Lett. 1997, 79, 2570. (21) Sun, S. H.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (22) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (23) Park, J. I.; Cheon, J. J. Am. Chem. Soc. 2001, 123, 5743. (24) Hyeon, T.; et al. Submitted for publication. (25) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221.
10.1021/la010703+ CCC: $20.00 © 2001 American Chemical Society Published on Web 11/09/2001
7908
Langmuir, Vol. 17, No. 25, 2001
factants with -OH and -COOH terminal functional groups. In a more recent study involving phosphates, Textor et al. have shown that octadecyl phosphate adsorbs onto tantalum oxide surfaces forming a monolayer with direct, coordinative complexation of the phosphate headgroup to Ta(V) cations27 through strong P-O-Ta bonding. By employing a number of complementary surface analysis techniques, they confirmed that the adlayer orients in a tails-up configuration with local hexagonal order. Consequently, alkyl phosphates or phosphonates should be considered for applications where strong binding of organic molecules to surfaces is needed. As for the adsorption of phosphonates or phosphates on nanoparticle surfaces, the literature is extremely sparse. In one such work, Gedanken et al. studied the adsorption of alkanesulfonic and alkanephosphonic acids on the surface of amorphous ferric oxide particles and proposed two possible bonding schemes for the phosphonate ions on Fe3+, i.e., one O or two O atoms of the phophonate group binding onto the surface.28 Our present work is motivated by the possibility of using alkyl phosphonate and phosphate surfactants as efficient binding ligands on metal oxide nanoparticle surfaces in general and magnetite nanoparticle surfaces in particular and as stabilizers in dispersions of the particles in organic solvents. We report the surface derivatization of magnetite (Fe3O4) by oleic acid (OA), lauric acid (LA), dodecylphosphonic acid (DDP), hexadecylphosphonic acid (HDP), and dihexadecyl phosphate (DHDP). While magnetic nanoparticles have been passivated by LA and OA in previous work,21,29,30 we compare herein properties of carboxylic acid coated particles to colloidal nanoparticles coated with phosphonate or phosphate ligands. For ease of reference, the coated nanoparticles are referred to by their surfactant names, i.e., OA-MP, LA-MP, DDP-MP, HDP-MP, and DHDP-MP (MP means magnetite particle). Experimental Section OA, LA, and DHDP were purchased from Aldrich. Hexadecylphosphonic Acid Synthesis. The literature procedure31 was modified as follows. In a round-bottom flask equipped with a magnetic stirring bar and an air condenser were placed 1-bromohexadecane (61.3 g, 0.2 mol) and triethyl phosphite (68 mL, 0.39 mol). The flask was kept at 150 °C overnight. Excess triethyl phosphite and volatile byproducts were removed by vacuum distillation. Concentrated hydrochloric acid (50 mL) was added to the flask, and this mixture was stirred at 100 °C overnight. After the mixture was cooled, a white precipitate appeared. The solid was collected by suction filtration, washed with acetonitrile, and dried under reduced pressure. Recrystallization from ethyl acetate gave 30 g of pure HDP (50% yield). 1H NMR (300 MHz, CDCl3 + d6-DMSO, δ): 1.6 (m, 4H), 1.26 (m, 26H), 0.98 (m, 3H). 13C NMR (75 MHz, CDCl3 + d6-DMSO, δ): 31.5 (C14), 30.4 (d, J ) 18 Hz, C3), 29.3-28.8 (10 carbons), 26.9 (d, J ) 141 Hz, C1), 22.44 (d, J ) 4.5 Hz, C2), 22.3 (C15), 13.8 (C16). Dodecylphosphonic Acid Synthesis. DDP was made using the same procedure but gave a 40% yield. 1H NMR (300 MHz, CDCl3, δ): 1.6 (m, 4H), 1.26 (m, 18H), 0.98 (m, 3H). 13C NMR (75 MHz, CDCl3, δ): 31.9 (C10), 30.4 (d, J ) 18 Hz, C3), 29.6-29.0 (26) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (27) Textor, M.; Ruiz, L.; Hofer, R.; Rossi, A.; Feldman, K.; Hahner, G.; Spencer, N. D. Langmuir 2000, 16, 3257. (28) Yee, C.; Kataby, G.; Ulman, A.; Prozorov, T.; White, H.; King, A.; Rafailovich, M.; Sokolov, J.; Gedanken, A. Langmuir 1999, 15, 7111. (29) Fried, T.; Shemer, G.; Markovich, G. Adv. Mater. 2001, 13, 1158. (30) Reimers, G. W.; Khalafalla, S. E. U.S. Patent Specification 1 439 031, 1975. (31) Vallant, T.; Brunner, H.; Mayer, U.; Hoffmann, H. Langmuir 1998, 14, 5826.
Sahoo et al. (6 carbons), 25.2 (d, J ) 141 Hz, C1), 22.7 (C11), 22.5 (d, J ) 4.5 Hz, C2), 14.2 (C12). The MPs were prepared by reacting a mixture of Fe2+ and Fe3+ (1:2 ratio) in aqueous solution with 32% ammonia solution at 80 °C under an argon atmosphere, followed by coating their surfaces with the individual surfactants. In a typical experiment, 0.20 g (1 mmol) of FeCl2 and 0.54 g (2 mmol) of FeCl3 are dissolved in 10 mL of distilled and deoxygenated water. The resulting solution is vigorously stirred and heated to 80 °C under an argon atmosphere. Subsequently, 5 mL of ammonia is injected into the flask and heating with stirring is continued for another 20 min to allow the growth of nanoparticles. After the solution is cooled to room temperature, the resulting MPs are subjected to magnetic decantation and washed with a few portions of distilled water. The pH of the suspension is brought to neutral by adding dilute HCl, and the particles are further washed with pure water. At this stage, the surfactants are added to the precipitate in the following manner: DDP: An ethanolic solution of this surfactant (0.125 g in 10 mL) was added with sonication, and the resulting suspension was stirred for 1 h. HDP and DHDP: The precipitate was washed once in ethanol, and about 20 mL of 1% solutions of either of the surfactants in chloroform were added. The suspensions were stirred (as above) for 2 h each. For OA and LA, the sequence of neutralization and surfactant addition was reversed; that is, surfactant addition preceded neutralization. The suspensions were separated by centrifuging at 4000 rpm for 10 min, and the resulting precipitate was dispersed in chloroform. Size selection to narrow the polydispersity was done by adding progressive volumes of ethanol to an aliquot of the chloroform dispersion, followed by centrifugation. The final dispersion of the nanoparticles had a relatively narrow size distribution and was stable over a period of several weeks for all surfactants. Transmission Electron Microscopy (TEM). The chloroform dispersion was drop-cast onto a 300 mesh carbon coated copper grid, and TEM pictures were taken on a JEOL JEM1200ex microscope at an accelerating voltage of 80 kV. Fourier Transform Infrared (FTIR). FTIR spectra were recorded in the transmission mode on a Bruker Vector 2200 spectrometer. The neat solid surfactants were ground with KBr and compressed into a pellet at 20000 psi, and their spectra were recorded as reference spectra to be compared with those of the magnetite colloids. For the coated particles, a few drops of the colloidal solution were mixed with KBr and compressed into pellets whose spectra were recorded. X-ray Photoelectron Spectroscopy (XPS). XPS was performed on representative samples of DHDP-MP and DDP-MP. A drop of the colloidal solution was dried on a clean copper foil, and the XPS spectra were taken of the film of the sample. The instrument was a 5600 Multi-Technique System (PHI). The samples were irradiated with an Al KR monochromated source (1486.6 eV), and the ejected electrons were analyzed by a spherical capacitor analyzer using a slit aperture of 0.8 mm. High-resolution measurements were performed at a pass energy of 11.75 eV with a 0.025 eV/step interval. Thermal Study. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) studies were carried out using a TA Instruments module, SDT 2960. TGA and DTA analyses were recorded for powder samples (about 10 mg) introduced into a sample compartment dried and flushed with ultrapure argon. The powder was obtained by precipitating the particles from solution with acetone and collecting the precipitate on a 0.2 µm membrane by vacuum filtration. Analyses were done at a rate of 5 deg/min up to 800 °C. Differential scanning calorimetry (DSC) measurements were performed using a TA Instruments module and a system controller, 2920, at a ramp rate of 5 deg/min in an argon atmosphere.
Results TEM. TEM micrographs of two representative coated particle samples taken after the narrowing of particle size distributions are shown in Figure 1. Particle diameters were analyzed by Scion Image software, and size distri-
Phosphonate-Coated Magnetite Nanoparticles
Langmuir, Vol. 17, No. 25, 2001 7909
Figure 2. Comparison of FTIR spectra of neat DDP surfactant and DDP-MP pressed in a KBr pellet, focusing on the C-H streching frequencies. Figure 1. TEM images of size selected HDP-MP (a) and OAMP (b) samples together with the corresponding size distribution hystograms (panels c and d, respectively).
bution histograms were plotted. Statistical analyses of 160 particles of OA-MP and 200 particles of HDP-MP gave a mean diameter of 6.4 ( 1.5 and 5.9 ( 1.9 nm, respectively. It can be noted that OA-MPs are wellseparated, indicating efficient surfactant coating. However, all phosphonate and phosphate particle samples show particle coalescence in their TEM micrographs. This is probably because of imperfect surfactant coatings or different intersurfactant interactions. Additional evidence for the different behavior of phosphonate-MP as compared with carboxylate-MP (mainly OA-MP) is the lower dispersibility of the phosphonate-MPs in apolar solvents such as hexane or heptane. FTIR. In a detailed FTIR study of alkanethiolate coated gold nanoparticles, Hostetler et al. and Sandhyarani et al. have made peak assignments to all stretching, bending, scissoring, wagging, and rocking modes of the alkanethiolate moiety.32,33 They consider the symmetric (d+) and antisymmetric (d-) stretching of methylene groups of crystalline polyethylene at 2850 and 2920 cm-1 as sensitive indicators of the crystallinity of the alkyl chain. In the liquid phase, they blue shift to 2856 and 2928 cm-1 due to an increase in the number of gauche defects. These authors have studied the adsorption of alkanethiolates of various chain lengths (C3-C24) on nanogold surfaces and observed that chain ordering and a crystalline microenvironment are manifest in all ligands with chain lengths of C6 or longer. Gedanken and co-workers28 have studied alkanesulfonic and alkanephosphonic acids on amorphous iron oxide nanoparticles. They report relatively crystalline alkyl chains despite some peak broadening in the IR indicating packing disorder. When the DDP system is used as an example, the FTIR spectra (transmission mode) presented in Figure 2 show that there are small shifts in the positions of the main peaks between the neat surfactants and the coated nanoparticles, along with a narrowing of the surfactant peaks in the coated nanoparticles. Overall, the spectrum of the ligand chains on the surface of the nanoparticles (32) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604. (33) Sandhyarani, N.; Antony, M. P.; Selvam, G. P.; Pradeep, T. J. Chem. Phys. 2000, 113, 9794.
Figure 3. TGA results: derivative of the weight loss as function of temperature for various surfactants coating the MPs. Note the difference between the phosphonates and the carboxylates.
is similar to that of the neat ligand. The CH2 stretching peaks of the neat surfactant and the peaks of the MP samples are shifted by e2 cm-1. However, the narrowing observed for the MP-surfactant peaks (e.g., the peak width at half-height for DDP-MP is 12 ( 2 cm-1; for neat DDP, it is 18 ( 2 cm-1) is consistent with surfactant molecules that are immobilized on the particle surfaces. TGA, DTA, and DSC Studies. The TGA and DSC results for all of the samples are shown in Figures 3 and 4, respectively. The DTA curves consist of broad complementary features of the possible transitions involved and hence are not presented. We note that the TGA derivative curves show two distinct transitions for all of the samples between room temperature and 600 °C. The transition temperatures and the corresponding percentage weight losses are summarized in Table 1. The bare MPs do not show any significant weight loss or phase transformation in this temperature range. It should also be noted that the derivative peak positions were reproducible for different preparations of the same material but with varying peak areas. Similarly, DSC studies were carried out for the three samples containing phosphonate or phosphate and multiple endothermic transitions were found. The DSC findings complement the TGA weight losses in terms of the higher temperature transitions. The enthalpies of these transitions have been calculated and incorporated
7910
Langmuir, Vol. 17, No. 25, 2001
Sahoo et al. Table 1 TGA
1st weight loss
DSC 2nd weight loss
1st transition
derivative peak (°C)
weight loss (%)
derivative peak (°C)
weight loss (%)
MP-OA MP-LA MP-DDP
240 274 340
2.5 3.3 6.5
378 348 469
6.6 2.7 7.5
MP-HDP
340
5.2
459
5.7
MP-DHDP
250
3.9
376
3.5
sample
Figure 4. DSC curves for the three types of phosphonate/ phosphate coated particles.
into Table 1. The temperature of the first weight loss is approximately the boiling or decomposition temperature of the neat surfactant. The enthalpy of the second, higher temperature, weight loss peak is significantly larger than that of the first weight loss for all samples. An estimate of the average number of surfactant molecules on a particle was obtained from the area of the second weight loss peak relative to the final residual sample weight. XPS. The XPS spectra of films of phosphonate and phosphate coated MPs reveal the presence of phosphorus. The relative concentrations of Fe, O, C, and P in the samples were used to estimate the number of surfactant molecules per particle. Because the carbon/phosphorus or organic-oxygen/phosphorus atomic ratios did not give the expected stoichiometry of the ligands, it was assumed that impurities containing carbon and oxygen were present. Consequently, the Fe/P atomic ratio was used to calculate the ligand-to-particle ratio. This ratio was 7.7 for MP-DDP and 13 for MP-DHDP. Discussion The TGA curves strongly suggest that the weight loss from all samples takes place in two distinct steps. While we cannot rigorously exclude the possibility of some weaker bound molecules within the primary surfactant layer, we propose a quasi-two-layer adsorption of these surfactants on the MPs; that is, in addition to a first, strongly bound layer, there are weakly bound surfactant molecules in a second layer farther from the magnetite core. The molecules in the “second layer” are bound by a combination of interchain van der Waals interactions and hydrogen bonding between the headgroups. Bilayers of phospholipids in the form of vesicles or liposomes are wellprecedented. Joniau and De Cuyper studied the adsorption kinetics of phospholipids on LA stabilized Fe3O4 nanoparticles and found evidence for a bilayer architecture.34
peak temp (°C)
2nd transition ∆H (J/g)
no data no data many low-enthalpy transitions many low-enthalpy transitions 253.4 11.8
peak temp (°C) no data no data 466
∆H (J/g)
36.9
469
123.6
391
48.0
Rapuano and Carmona-Ribiero reported a phosphatidylcholine bilayer on the surface of hydrophilic silica,35 and Hatton and co-workers have provided compelling evidence for bilayer formation in fatty acid coated particles.36,37 It is important to note that Gedanken and co-workers28 found that the TGA of phosphonate coated amorphous ferric oxide particles is double-stepped, and they do not attribute this finding to a bilayer structure but rather to two kinds of bonding of the phosphonate group on the Fe surface. Two types of bonding to a surface are also suggested by Spencer and co-workers for phosphate coated Ta2O5.27 We favor the quasi-two-layer interpretation for our work because it is not clear that two types of bonding would have substantially different desorption temperatures and because of the following three considerations: (i) It is certain that the samples always contain some excess of the surfactant that is not strongly bound to the surface of the particles. Unless special efforts are made, the effect of that excess is observed in the preparation of Langmuir-Blodgett films of the particle suspensions.29 It is clear that the surfactant molecules not bound to the surface would have a substantially lower desorption temperature. Occasionally, TGA runs were repeated for two different preparations of the same surfactant coated particles and a large variation in the ratio of the lower temperature peak to the higher temperature peak was found. This correlates well with variations in excess surfactant between different preparations. (ii) The double-desorption step is observed for both carboxylates and phosphonates, though according to ref 28 the carboxylates should have a single binding configuration. This again fits better the excess-surfactant second-layer picture. (iii) The DSC transitions occur at temperatures roughly similar to the TGA peaks but are more prominent in the temperature regime where the second TGA loss occurs. This confirms that the second TGA peak belongs to the chemically bound primary layer. The much smaller enthalpies of transitions corresponding with the lower temperature DSC peaks indicate that the first TGA peak relates to weak binding forces. The numerous small transitions in the DSC curves at lower temperatures may be attributed to the dynamics of chain reorganization as the heating continues. It is interesting to compare the TGA behavior of thiols38 and amines39 (weight loss at 250-260 °C) adsorbed on Au (34) De Cuyper, M.; Joniau, M. Langmuir 1991, 7, 647. (35) Rapuano, R.; Carmona-Ribeiro, A. M. J. Colloid Interface Sci. 2000, 226, 299. (36) Shen, L.; Laibinis, P. E.; Hatton, T. A. Langmuir 1999, 15, 447. (37) Shen, L.; Stachowiak, A.; Fateen, S. K.; Laibinis, P. E.; Hatton, T. A. Langmuir 2001, 17, 288. (38) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (39) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723.
Phosphonate-Coated Magnetite Nanoparticles
nanoparticles with the higher temperature peaks of the LA-MP (348 °C) and DDP-MP (469 °C) samples of the present study. Although the carboxylate and phosphonate coated MPs have better thermal stability (probably due to the difficult volatilization of the very polar acids), the dispersed particles readily lose ligands to the organic solvent when the free ligand concentration in the organic solvent is reduced. Repeated precipitation and redissolution of the coated MPs often result in an undispersible precipitate. With the thiolate coated gold particles, ligand volatilization is easier; hence, thermal stability is relatively poor. However, the barrier for the loss of ligands in solution is relatively high, consistent with a strong bond between the thiolate and the gold surface. When the phosphonate/phosphate ligands are compared to the carboxylates, in addition to the strength of adsorption, it is important to compare the density of surfactant molecules on the particles’ surfaces. One qualitative conclusion that could be drawn from both TGA and XPS is that the DHDP coated particles have less molecules adsorbed on their surfaces. This was expected since the DHDP is a double-chain surfactant and therefore each molecule occupies a larger surface area. It was, however, difficult to quantify this difference. There is an order of magnitude discrepancy between the XPS estimates of the number of molecules per particle (on the order of 1000) and the TGA estimates for the different samples (on the order of 100). XPS results are less reliable since they do not provide us with the correct P:O or P:C atomic ratio. It is also possible that the surfactant estimate, based on the Fe:P atomic ratio, is exaggerated due to shielding of particle cores by surfactant molecules, given the high surface selectivity of XPS. As indicated in the TGA results summarized in Table 1, it appears that no substantial differences exist between OA, DDP, and HDP coatings, while LA-MPs seem to have lower surface coverage. Analysis of the ratio between the two TGA peaks for the different surfactant coated MP samples reveals a major difference between the OA-MP sample and the others: The OA-MPs have less surfactant outside the first (more tightly held) layer. This point is important to the discussion of the dispersibility of the particles. The issue of particle dispersibility in solvents is a major criterion for various practical purposes (e.g., stable ferrofluids) and for basic studies such as understanding interparticle magnetic interactions, where it is important to obtain well-isolated particles. Magnetic nanoparticles coated with OA are easily dispersed in a nonpolar solvent like hexane, while the ones coated with LA are not. Hatton and co-workers coated ferrite particles with LA as the primary layer and created a second layer with various (C9-C13) acids.36 The particles were water dispersible as a consequence of the polar groups on the surface of the second layer. In our case, the LA coated particles were well-dispersed in chloroform, only
Langmuir, Vol. 17, No. 25, 2001 7911
partially dispersed in apolar solvents such as hexane, and precipitated in polar solvents such as water, methanol, or ethanol. We believe that the better dispersibility of OAMP vs LA-MP in nonpolar solvents is due to differences in interchain interactions. This may be due to different chain-solvent interactions and/or to a smaller tendency of the OA coated particles to form a second surfactant layer. Chain dependent packing effects have been observed in various types of unsaturated surfactants (e.g., phospholipids) in monolayers and in bilayer structures.40 It is also clear to us that dispersibility of the coated particles varies with the sample preparation procedures. For example, in the LA-MP case, our particles were dispersible in organic solvents due to partial stripping of the second layer while Hatton and co-workers’36 preparation conditions enabled the formation of a full second layer that rendered the particle surfaces hydrophilic. Conclusion It was found that alkyl phosphonates and phosphates could be used for obtaining thermodynamically stable dispersions of magnetic ferrite nanoparticles. The ligands seem to form a quasi-bilayer structure with the primary layer strongly bonded to the surface of the nanoparticles as evident from the temperature and enthalpy of desorption. Despite the strong surface anchoring of these ligands, the phosphonate/phosphate coated particles suffer from lower dispersibility as compared with the OA coated particles. The better dispersibility of OA-MPs in apolar solvents probably arises from their more hydrophobic nature. This is likely due to the different interactions of the unsaturated chains with neighboring chains and solvent molecules. This leads to a smaller number of molecules in the second layer, and as a consequence, less polar headgroups face the hydrocarbon solvent molecules. The good biocompatibility41 of phosphonate and phosphate ligands may advance the utilization of encapsulated magnetic nanoparticles in medical applications such as magnetic resonance imaging and other biophysical purposes. Also, from a more fundamental point of view, this study demonstrates that alkyl phosphonates and phosphates bind efficiently to iron oxide particle surfaces and can serve, in general, as potential alternatives to fatty acids as coating agents for oxide nanoparticles. Acknowledgment. This work was supported by the Israeli Ministry of Science Tashtiot Program and by the Israel Science Foundation Grant 152/99. G.M. is grateful to Prof. J. Sagiv and Prof. S. Efrima for useful discussions. LA010703+ (40) Lefevre, T.; Picquart, M. Chem. Phys. Lipids 1998, 92, 79. (41) Roberts, D.; Zhu, W. L.; Fromenn, C. M.; Rosenzweig, Z. J. Appl. Phys. 2000, 87, 6208.
