602
Langmuir 1998, 14, 602-610
Directing Oleate Stabilized Nanosized Silver Colloids into Organic Phases Wei Wang,† Shlomo Efrima,*,† and Oren Regev‡ Department of Chemistry and Department of Chemical Engineering, Ben Gurion University, Beer Sheva, Israel 84105 Received September 8, 1997. In Final Form: November 21, 1997 Nanosized hydrophobic, oleate stabilized silver organosols in various organic solvents are obtained using a solvent exchange method. The silver particles are initially prepared as a hydrosol in the presence of sodium oleate (surfactant). Then a transfer of the colloid to an organic phase is induced by a low concentration of several agents such as orthophosphoric acid, with a transfer efficiency of 50-70%. The hydrophobic colloid is stable and the particles retain their integrity even after the solvent is evaporated and the dried deposit is resuspended in a variety of other solvents. We present the preparation method in detail and characterize the hydrosol and organosol particles by electron microscopy, electrophoresis, and UV-visible extinction spectroscopy. On the basis of IR spectroscopy we discuss the conformation of the surfactant adsorbed on the silver cores and the changes in it as the particles transfer into the organic environment.
Introduction Nanometer-sized particles of metals and semiconductors have been investigated intensively in recent years because of their size-dependent properties and the possibility of arranging them in microassemblies (and nanoassemblies).1 Intriguing prospects for the development of novel electronic devices, electrooptical applications, and also catalysis have been established.2 A variety of methods can be used for the formation of ultrasmall crystallites, such as molecular beam epitaxy,3 chemical vapor deposition,4 reduction by ionizing radiation,5 thermal decomposition in organic solvents,6 chemical reduction or photoreduction in reverse micelles,7 and chemical reduction with8 or without9 stabilizing polymers. From all this work it can be assessed that colloidal stability, particle size, and its properties depend strongly on the specific method of preparation and the experimental conditions applied. One of the main challenges in the preparation of nanosized colloids is developing the means of directing the particles into specific physicochemical environments, such as organic nonpolar liquids, specific regions within ordered surfactant phases, monolayer assemblies, etc. In this report we focus on the dispatching of metal colloids into bulk organic phases. This should enable the routine formation of dispersions of particles in both polar and † ‡
Department of Chemistry. Department of Chemical Engineering.
(1) (a) Ozin, G. A. Adv. Mater. 1992, 4, 612. (b) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (c) Belloni, J. Curr. Opin. Colloid Interface Sci. 1996, 2, 184. (d) Brus, L. Curr. Opin. Colloid Interface Sci. 1996, 2, 197. (e) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (2) (a) Weller, H. Angew. Chem., Int. Ed. Eng. 1993, 32, 41. (b) Schmidt, G. Chem. Rev. 1992, 92, 1709. (c) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (3) Bahnemann, D. W. Isr. J. Chem. 1993, 33, 115. (4) Satoh, N.; Kimura, K. Bull. Chem. Soc. Jpn. 1989, 62, 1758. (5) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (6) Esumi, K.; Tano, T.; Torigoe, K.; Meguro, K. Chem. Mater. 1990, 2, 564. (7) Pileni, M. P.; Lisiecki, I.; Motte, L.; Petit, C.; Cizeron, J.; Moumen, N.; Lixon, P. Prog. Colloid Polym. Sci. 1993, 93, 1. (8) Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993, 89, 2537. (9) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120.
nonpolar solvents. The particles are formed initially in an aqueous medium, which is especially convenient for such a task (mostly due to its solubilization power for ions and a variety of molecules) and then transferred into the organic environment. The importance of such a solvent exchange method is for the application of metal particles as catalysts for organic reactions in nonpolar solvents10 and in the study of media effects on colloids and on the state of adsorption of molecules on the surface of solid particles in contact with various environments.11 In addition, and perhaps more importantly, this is one necessary step in developing and generalizing methods of directing nanoparticles into prescribed assemblies involving organic and inorganic constituents.12 Silver nanocrystallites, mostly hydrosols, have been widely studied because of the ease of their preparation and stabilization and due to their application as catalysts13 and their role in photographic processes14 and as substrates for surface-enhanced Raman spectroscopy (SERS).15 Colloidal dispersions of silver in nonaqueous liquids are rarer and more difficult to prepare and stabilize. Often they are prepared by physical methods such as the gas flow cold trap method.16 Chemical methods for preparation of colloidal dispersions of silver in nonaqueous liquids were limited to a few nonaqueous media, such as methanol,17 ethanol,17b,18 cyclohexane,18b,19 and haloalkanes.20 (10) (a) Andrews, M. P.; Ozin, G. A. J. Phys. Chem. 1986, 90, 2929. (b) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1989, 131, 186. (11) (a) Efrima, S. Heterog. Chem. Rev. 1994, 1, 339. (b) Mulvancy, P. Langmuir 1996, 12, 788. (12) Wang, W.; Regev, O.; Efrima, S. Directing Silver Nanoparticles into Lyotropic Lamellar Surfactant Systems. Submitted. (13) (a) Sun, T.; Seff, K. Chem. Rev. 1994, 94, 857. (b) Verykios, X. E.; Stein, F. P.; Coughlin, R. W. Catal. Rev.sSci. Eng. 1980, 22, 197. (14) Mostafavi, M.; Marignier, J. L.; Amblard, J.; Belloni, J. Radiat. Phys. Chem. 1989, 34, 605. (15) Matejka, P.; Vlckova, B.; Vohlidal, J.; Pancoska, P.; Baumrunk, V. J. Phys. Chem. 1992, 96, 1361. (16) Kimura, K.; Bandow, S. Bull. Chem. Soc. Jpn. 1983, 56, 3578. (17) (a) Hirai, H.; Nakao, N.; Toshima, N. Chem. Lett. 1978, 545. (b) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1979, A13, 633. (18) (a) Siiman, O.; Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983, 100, 163. (b) Liz-Marzan, L. M.; Lado-Tourino, I. Langmuir 1996, 12, 3585. (19) Hirai, H.; Aizawa, H.; Shiozaki, H. Chem. Lett. 1992, 1527. (20) Zeiri, L.; Efrima, S. J. Phys. Chem. 1992, 96, 5908.
S0743-7463(97)01017-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/03/1998
Directing Ag Colloids into Organic Phases
Another approach21 relied on the extraction of metallic ions (Au, Pt, Ni, and Pt-Pd) from an aqueous solution to organic solvents induced by such surfactants as trioctylphosphine oxide and distearyldimethylammonium chloride. The ions were then reduced in the organic phase. A direct extraction of silver colloids from water into several nonhydroxylic organic solvents of low polarity occurred as byproducts of the preparation of silver metal liquidlike film (MELLF).20 It was also demonstrated that silver and gold colloids stabilized by oleate molecules can be extracted into various organic solvents by adding relatively large concentrations of salts (in the hundreds of mmoles per liter range).19,22 Doubly charged Mg2+ ions and triply charged La3+ ions required lower concentrations (2-8 mmol/L). The mechanism for this phase transfer has not yet been resolved, though tentative suggestions were advanced.19,22a Pioneering work using capping of gold colloids by thiols and amines made them easily transferable between different liquids and stable over a large variety of experimental conditions.23,24 Along similar lines we report below on a study of the silver oleate colloidal system and the transfer of the particles from the hydrosol to form an organosol, in several organic liquids. The transition is induced by low concentrations of orthophosphoric acid, or its monosodium salt, or perchlorate acid (and its salt). We find that the colloid is capped by the oleate moieties, similar to the thiol capped23 and amine capped24 gold colloids. On the basis of IR spectroscopy we discuss the molecular arrangement of the oleate on the silver surface and the changes in it that enable the phase transfer. Experimental Section 1. Materials. AgNO3 (99+%), NaBH4 (99%), NaH2PO4‚H2O (98+%), cyclohexane (99.5%), dodecane (99+%), and 9-octadecenoic acid (oleic acid, technical 90%) were purchased from Aldrich. H3PO4 (85%), HClO4 (GR, 70%), 1-chlorobutane (synthesis grade), and CC14 (spectroscopy grade) were Merck products. Isooctane (HPLC) was from Fluka, hexane (Analytical) was from Bio Lab, and absolute ethanol (spectroscopy grade) was from BDH, respectively. NaOH (analytical) and chloroform (analytical) were provided by Frutarom. All of the chemicals were used as received. Some experiments were repeated with high-purity (99%) oleic acid and gave the same results. Sodium oleate and sodium perchlorate are obtained by stoichiometrically reacting oleic acid or perchlorate acid with sodium hydroxide in aqueous solutions. Silver oleate is obtained by reaction of silver nitrate with oleic acid in an aqueous solution. Water is of ∼18 MΩ cm resistivity, obtained from a Barnsted E-pure water purifier. 2. Measurements. (a) Absorption spectra are taken with a 8452A HP diode array spectrophotometer with a resolution of 2 nm and usually a 0.5 s exposure time. (21) (a) Meguro, K.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 341. (b) Meguro, K.; Tano, T.; Torigoe, K.; Naka, H.; Esumi, K. Colloids Surf. 1988/9, 34, 381. (c) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torigoe, K.; Meguro, K. Langmuir 1991, 7, 457. (d) Duteil, A.; Schmid, G.; Meyer-Zaika, W. J. Chem. Soc., Chem. Commun. 1995, 31. (22) (a) Hirai, H.; Aizawa, H.; Shiozaki, H. J. Colloid Interface Sci. 1993, 161, 471. (b) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (c) Andres, R. P.; Averback, R. S.; Brown, W. L.; Brus, L. E.; Goddard, W. A., III; Kaldor, A.; Louie, S. G.; Moscovits, M.; Peercy, P. S.; Riley, S. J.; Siegel, R. W.; Spaepew, F.; Wang, Y. J. Mater. Res. 1989, 4, 704. (23) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Leff, D. V.; Ohara, P. C.; Heath, J. R.; Gelbart, W. M. J. Phys. Chem. 1995, 99, 7036. (c) Ohara, P. C..; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (d) Terrill, R. H.; Postwiathwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Favlo, M.; Johnson, C. S., Jr.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (24) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723.
Langmuir, Vol. 14, No. 3, 1998 603 (b) Direct imaging of the particles is obtained by a JEOL 1200 EXII electron microscope under an acceleration voltage of 100 kV with a nominal underfocus of about 4 µm. A drop of the silver sol is placed on a holey carbon film supported by a 300 mesh copper grid (TED PELLA-LTD), and the solvent is allowed to evaporate. The size distributions of the silver particles are obtained by digitizing the printed micrographs and analyzing 500 particles using the public domain NIH Image 1.60 software developed at the U. S. National Institute of Health. (c) Infrared spectra are recorded at 4 cm-1 resolution with a Nicolet Impact 410 FTIR spectrometer. The silver sol is repeatedly spread on the KRS-5 crystal and dried forming a colloidal-particle cast film. A Barnes attenuated-total-reflection (ATR) apparatus is used at a 45° angle of incidence. For ATR spectra and transmission spectra respectively, 600 and 64 multiple scans are accumulated. No smoothing is applied to the spectra. (d) Electrophoretic mobilities of hydrosols are measured by moving boundary-type experiments with platinum electrodes at 18 ( 1 °C. 3. Preparation of Sols. Three different types of silver sols are prepared according to the following procedures: (a) Native Ag hydrosols. A hydrosol without any additional stabilizer was prepared according to literature.25 Typically, one part of ice cold 1 × 10-3 M AgNO3 and another equal volume of 4 × 10-3 M NaBH4 are mixed dropwise, with stirring, forming a clear yellow sol almost immediately. The sol is continuously stirred while it is allowed to warm to room temperature. The sol is then heated to about 60-80 °C to decompose any excess NaBH4 and allowed to cool back to room temperature. (b) Ag hydrosols Stabilized by Surfactant. A typical preparation procedure calls for adding 25 mL of 1 × 10-3 M AgNO3 into 25 mL of 4 × 10-3 M NaBH4 containing 2.5 × 10-4 M sodium oleate (surfactant) with vigorous stirring at an ice cold temperature. A brown-yellowish colloidal solution stabilized by sodium oleate is obtained. (c) Ag Organosols. Typically one adds with vigorous stirring 0.2 mL of 0.1 M H3PO4 (inducer) to a mixture of 25 mL of the silver hydrosol stabilized by sodium oleate and 25 mL of the organic solvent such as cyclohexane or dodecane. A phase transfer is quickly induced, and the aqueous phase becomes colorless. The final pH value of the aqueous phase is controlled in the range of ∼4.0-5.0. Alternatively, one can add NaH2PO4 as an inducer instead of the orthophosphoric acid. A 0.2 g amount of NaH2PO4 solid is used for 20 mL of the hydrosol and the organic solvent. After 20 min of vigorous stirring, we allow the mixture to separate out into two liquid layers: the organic phase is a colored colloidal dispersion, while the aqueous solution loses its color. Other agents that can be used are HClO4 or NaClO4 at concentrations similar to that of the orthophosphoric acid or its salt. They induce a similar transfer of the colloid to the organic phase.
Results and Discussion 1. Native Hydrosols. In order to ensure that all of the Ag+ ions are reduced, we applied different RBAg ) [BH4-]/[Ag+] ratios at a fixed [Ag+]. The degree of the reaction is evaluated by integrating the silver plasmon peak at about 400 nm in the UV-visible spectroscopy. We find that the area of the band grows with the borohydride concentration ratio until RBAg ) ∼1, beyond which it does not change anymore. As borohydride reacts directly with water to form hydrogen, a process that is catalyzed by metallic ions,26 it is important to use fresh solutions. Typically we worked with ratios RBAg of 2-4, having thus a large excess of the reductant in all our preparations. If the initial Ag+ concentration is lower than 2 × 10-4 M, the hydrosol is bright yellow. The color deepens with increasing Ag+ concentration. At [Ag+] > 1 × 10-3 M, it (25) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (26) Charle, K. P.; Schulze, W. Ber. Bunsen-Ges. Phys. Chem. 1984, 88, 350.
604 Langmuir, Vol. 14, No. 3, 1998
Wang et al.
Figure 1. Transmission electron micrograph of dried silver particles from an unprotected hydrosol: 5 × 10-4 M silver atoms in water after aging for 3 months at room temperature. Insert: Histogram of the size distribution. 〈D〉 ) 19.0 nm; σ ) 6.9 nm. (〈D〉 is the average diameter, and σ is its standard deviation.)
Figure 3. Transmission electron micrograph of dried silver particles from (a, top) fresh and (b, bottom) 3 month old oleate capped hydrosol: 1 × 10-3 M silver atoms stabilized with sodium oleate in water. Inserts: Histogram of the size distribution. (a) 〈D〉 ) 5.0 nm; σ ) 1.9 nm. (b) 〈D〉 ) 5.5 nm; σ ) 1.7 nm.
Figure 2. UV-visible adsorption spectra of an unprotected hydrosol: 5 × 10-4 M silver atoms. Maintained at room temperature for (a) a half-hour, (b) 2 h, (c) 2 days, and (d) 3 months.
is almost impossible to obtain a stable colloidal dispersion without an additional stabilizer, due to immediate particle aggregation. In comparison, a colloidal dispersion prepared from 5 × 10-4 M Ag+ is stable for at least 3 months at room temperature. Figure 1 shows a transmission electron micrograph (TEM) of the dried particles from a 3 month old colloid. The average diameter of the silver particles is 19.0 nm ((6.9 nm). Possibly coalescence of particles during sample preparation and irradiation is responsible for some of the large particles of the size distribution. Figure 2 shows the UV-visible absorption spectra of a hydrosol containing 5 × 10-4 M Ag atoms taken at various times after its preparation. A very fresh hydrosol manifests a sharp and symmetric absorption peak at 390 ( 2 nm with a full width at half-maximum (fwhm) of 57 nm (Figure 2a). This well-defined plasmon peak is indicative of relatively large, monodisperse, spherical silver particles.26-28 After 2 h at room temperature there (27) Creighton, J. A.; Eaton, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.
is only a beginning of the appearance of a tail on the red side of the band (Figure 2b). In 2 days the absorption band broadens considerably and becomes noticeably asymmetric (Figure 2c). This most probably indicates coalescence.25,29 No further changes are observed even after 3 months (Figure 2d). This spectrum corresponds to the TEM in Figure 1 and suggests there is indeed some limited aggregation of the particles into clusters in suspension. 2. Hydrosols Stabilized with Sodium Oleate. The TEM micrograph of freshly prepared colloidal silver dispersed in water with sodium oleate as a stabilizer is shown in Figure 3a. The average diameter of the particles is 5.0 nm ((1.9 nm) and has a relatively narrow size distribution as shown in the insert. A TEM micrograph of 3 month old oleate capped silver hydrosol is shown in Figure 3b. The average diameter of the particles in the hydrosol is 5.5 nm ((1.7 nm), quite similar to that of the fresh colloid. UV-visible spectra support this finding. The sodium oleate stabilized particles are considerably smaller than those of the native colloid, more monodisperse and less aggregated, even though the silver concentration is much higher (10-3 M in the case shown here). The decrease in particle diameter due to the addition of (28) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (29) (a) Kerker, M. J. Coloid. Interface Sci. 1985, 105, 297. (b) Yonezawa, Y.; Kijima, M.; Sato, T. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 1828. (c) Vogler, A.; Quett, C.; Kunkley, H. Ber. Bunsen-Ges. Phys. Chem. 1988, 92, 1486.
Directing Ag Colloids into Organic Phases
Langmuir, Vol. 14, No. 3, 1998 605
Table 1. Absorption Peaks and Widths at Half-Height of the UV-Visible Spectra of Hydrosols (1 × 10-3 M Ag Atoms with Different Amounts of Stabilizer at Various Aging Times) absorption maximum (fwhm), nm concn of sodium oleate, M
1h
2h
12 h
7 days
1 month
3 months
1.0 × 10-2 7.5 × 10-3 5.0 × 10-3 2.5 × 10-3 1.0 × 10-3 7.5 × 10-4 5.0 × 10-4 2.5 × 10-4 1.0 × 10-4 7.5 × 10-5 5.0 × 10-5 2.5 × 10-5
414 (88) 414 (92) 413 (89) 410 (85) 406 (79) 406 (76) 406 (82) 406 (75) 410 (82) 406 (91) 406 (117) 420 (151)
416 (88) 416 (88) 414 (84) 410 (84) 408 (83) 408 (80) 408 (79) 408 (76) 408 (83) 410 (97) 404 (115) 420 (145)
424 (95) 428 (92) 430 (92) 424 (89) 414 (87) 412 (82) 408 (86) 408 (79) 410 (84) 410 (89) 410 (118) 420 (153)a
432 (97) 432 (96) 432 (92) 426 (89) 412 (83) 412 (82) 408 (84) 408 (79) 410 (83) 410 (87) 412 (109)a 422 (145)a
434 (100) 434 (95) 432 (95) 424 (87) 410 (87) 410 (80) 410 (83) 408 (76) 410 (84) 410 (86) 412 (116)a 422 (148)a
436 (105) 434 (97) 422 (97) 414 (77) 408 (77) 408 (77) 408 (82) 408 (79) 408 (80)a 410 (88)a 414 (111)a 428 (166)a
a Measured values for the supernatant of the hydrosolsssome particles aggregate and precipitate out of solution. Typical uncertainties are (2 nm in the peak positions and (6 nm in the fwhm. Above 12 h or using low oleate concetrations the uncertainties are (4 nm in the peak positions and (12 nm in the fwhm, respectively.
surfactant (from ∼19 to ∼5 nm) is tentatively attributed to the adsorption of the surfactant on the surface of the silver particles during the course of the reaction. It is also clear that sodium oleate yields much more stable colloids than those produced without it. This, too, is evident in UV-visible spectra, which hardly change in time, contrary to what is observed with the native hydrosol (Figure 2). We tried various preparation procedures, especially with regard to the stage at which the oleate is added, using [Ag+] ) 1 × 10-3 M, [Ag+]/[stabilizer] ) 4, and RBAg ) 4. This specific silver/stabilizer ratio was chosen as it gave the most stable colloid, with the narrowest and most symmetric extinction plasmon band (see as follows and in Table 1). We utilize the UV-visible spectra as characterization criteria for the nature of the colloids that form. (i) Oleate is added after the formation of the colloid. Adding the stabilizer after the completion of the reaction of Ag+ with BH4- , the extinction peak slightly shifts from 390 ( 2 to around 396 ( 2 nm, maybe due to a different mode of the interaction of silver with the stabilizer molecules29a or due to aggregation. As noted above, at this high silver ion concentration, the colloids tend to aggregate and precipitate even before the oleate is added. The aggregation is irreversible with respect to the addition of the surfactant. (ii) Oleate is added during the reduction of the silver. In this case the absorption peak of the colloid appears in the range 396-412 nm, strongly dependent on the time after the commencement of the reduction and the precise manner of adding the stabilizer. The reproducibility of the preparation is rather poor. Furthermore, the spectra usually exhibit a considerable tail toward the red, characteristic of the formation of silver aggregates. This behavior is actually expected, being intermediate between the procedure outlined in i above and those where the oleate is added prior to the reduction of the silver (protocols iii and iv below). (iii) Oleate is added with the silver ions. Here the stabilizer is present in the silver ion parent solution (which turns white-turbid due to the formation of silver oleate), prior to the reduction reaction. Obviously the oleate associates with the silver ions and forms an insoluble product. The absorption peak of the subsequent colloid usually is broad and centers around 430 nm. This is expected as the silver reactant is bound in the solid silver oleate particles suspended in the solution. As the colloid ages, the extinction maximum gradually shifts to shorter wavenumbers, accompanied by a growth of the wing at longer wavelengths.
(iv) Oleate is added with the reductant. When the stabilizer is included in the NaBH4 solution in advance, the subsequent colloid gives a well-defined and reproducible band at 408 ( 2 nm. This procedure enables the oleate to affect the colloid nucleation and growth, as well as its stabilization, but avoids the formation of a precipitate as in protocol iii. We adopt this procedure in all subsequent colloid preparations. It gives a stable suspension even at silver ion concentrations higher than 1 × 10-2 M, while 1 × 10-3 M was too high without the stabilizer. The shift of the spectrum maximum by ∼18 nm to longer wavelengths, compared to that observed in the absence of stabilizer (from 390 nm, Figure 2a, to 408 nm), indicates that the oleate molecules are indeed absorbed on the silver particle surface, probably forming an ionic layer over the particle surface. Even more commendable, drying the particles does not bring about irreversible aggregation, in contrast to the case without the surfactant. One can harvest the dried colloid and easily redisperse it in water, yielding a stable colloid. The position and shape of the silver plasmon peak are very sensitive to the amount of the stabilizer and the aging time, as summarized in Table 1, for an Ag atom concentration of 1 × 10-3 M. With no surfactant, the particles separate out almost immediately. Addition of sodium oleate considerably increases the stability of the hydrosol. Above a “critical surfactant concentration” the particles are stable over long periods, with practically no changes observed even after about 1 month. At a stabilizer concentration of 2.5 × 10-4 M the suspensions are stable for at least 3 months (we have not checked for much longer durations, and the spectrum shows practically no change from its appearance merely 2 h after the colloid was produced). We denote this concentration as the “critical concentration”. Below this oleate concentration, at 1.0 × 10-4 M and lower, the colloid does not keep for long, and a definite extinction tail to the red is observed, progressively increasing as the concentration decreases. It seems that there just is not enough stabilizer to protect all the colloidal particles; they aggregate and tend to precipitate. At a concentration of 1 × 10-3 M silver and 2.5 × 10-4 M of oleate, with particles of a 5 nm diameter (as found above), assuming that all oleate molecules are adsorbed, one has on the surface of the silver particles about 0.08 nm2 per oleate molecule. This is more than 3-fold too small, compared with the minimum head-group area per oleate molecule of 0.27 nm2.30 As we will show later, they are not adsorbed through the carboxylic head-group, (30) Jang, W. H.; Miller, J. D. Langmuir 1993, 9, 3159.
606 Langmuir, Vol. 14, No. 3, 1998
Wang et al.
Table 2. Electrophoretic Mobilities of Silver Particles in Hydrosols and Charge Densities on the Particles
[Ag], M
[oleate]/ [Ag+]
[BH4-]/ [Ag+]
mobility, uE, (µ/s)/ (v/cm)
surface charge density, σc × 102, C/m2
1 × 10-3 1 × 10-3 1 × 10-3 1 × 10-2
0 1/4 1/4 1/4
4 2 4 4
2.5 ( 0.4 3.5 ( 0.9 4.3 ( 0.2 4.1 ( 0.4
1.6 ( 0.3 2.2 ( 0.6 2.7 ( 0.1 2.6 ( 0.2
equation:
uE ) Q/(6πηr) e-/particle 7.8 ( 1.3 10.8 ( 2.7 13.2 ( 0.6 12.7 ( 1.2
suggesting that much (∼ >70%) of the oleate does not adsorbe on the particles but is in solution as monomers (the critical micellar concentration, cmc, of sodium oleate is about 1.5 × 10-3 M 31 ). At concentrations above the critical concentration the colloids demonstrate a long-range stability, though the spectra still change in time within the 3 months period. The width is larger than that of the critical concentration during most of that period, and the bands are somewhat asymmetric with a wing arising at lower wavelength. It seems that overcrowding of the surfactant interferes with the colloid production. Indeed, we observe that some surfactant flocs precipitate out within a few days. IR spectroscopy (see below) indicates that these flocs are composed mainly of silver oleate. Excess oleate protects some of the silver from the reduction, or more probably, encourages oxidation by dissolved oxygen of some of the silver in the colloid back into the ionic form. Note that with 1 × 10-3 M Ag atoms the silver/stabilizer molar ratio at the optimum concentration is 4, which is the reason for our common use of this ratio. We find the same ratio, 4, for silver ion concentrations of 5 × 10-4 and 5 × 10-3 M, and it remains approximately the same over most of the range of silver concentrations we investigated (1 × 10-51 × 10-2 M). At the high concentration limit the ratio [Ag+]/[oleate] increases to ∼6, maybe because the particles become bigger. This approximately constant ratio suggests that the size of the silver particles is nearly the same for preparations with constant silver/surfactant ratio. A constant size gives a constant ratio between the surface area (which is utilized by the stabilizer) and the volume (which is a measure of the amount of silver). In agreement with these observations we find that a simple indication of an appropriate concentration is the appearance of just a few bubbles on the surface of the hydrosol suspension when air is softly blown into it. If no bubbles appear, there is a lack of surfactant; if many persistent bubbles are observed, there is too much. The electrophoretic mobilities of the particles, uE, are given in Table 2 and agree with related results reported in the literature.32 The particles possess a negative charge, regardless of the presence or absence of oleate. Frens and Overbeek32a concluded that a negatively charged silver sol consists of positive cores covered with superequivalent specific adsorption of the citrate ions present in their case. A similar situation was suggested for silver sols protected by poly(acrylic acid).32c The sols in the present studies include oleate and BO3- anions which could similarly adsorb on the particles and control their charge. The charge density on the surface of the particles, σc, is evaluated (Table 2) from the Hu¨ckel approximate (31) (a) Flockhart, B. D.; Graham, H. J. Colloid Sci. 1953, 8, 105. (b) Sakharova, M. G.; Shutova, A. I. Lakokras. Mater. Ikh Primen. 1964, 23. (32) (a) Frens, G.; Overbeek, J. Th. G. Kolloid Z. Z. Polym. 1969, 233, 922. (b) Heard, S. M.; Grieser, F.; Barraclough, C. G. J. Colloid Interface Sci. 1983, 93, 545. (c) Ung, T.; Giersig, M.; Dunstan, D.; Mulvaney, P. Langmuir 1997, 13, 1773.
where Q is the net charge on the surface, η is the viscosity of the medium, r is the radius of the particle, and uE is the electrophoretic mobility. We find ∼10 elementary charges per particle. This value is at odds with the results of Ung et al.32c They find by cyclic voltammeter and a potential step technique that ∼1600 electrons are transferred to each colloidal particle. This implies that the charge on the particles is on that same order of magnitude, requiring an exceptionally high surface concentration of charged acrylic groups. Note that 5 nm silver particles have altogether only ∼4000 atoms, some 500 of them at the surface. 3. Organsols Obtained by Solvent Exchange. In the presence of phosphoric acid, or an appropriate amount of its salt, the oleate-capped silver particles can be transferred from the aqueous solution to various organic solvents. Similar results are obtained with perchloric acid or its salt. Here we give details for the phosphate induced transfer. Table 3 gives the transfer yields to several liquids, estimated by comparing the integrated plasmon band in the UV-visible spectra of the aqueous phase just before the solvent exchange and that of the organosol after it. Typical values range from 50 to 70%. The remainder of the colloid concentrates at the liquid/liquid interface, forming an interfacial colloid or an interfacial powder deposit. The aqueous phase clears up entirely. Judging from the UV-visible spectra of the organosols compared to that of the parent hydrosols (Table 3), the phosphate induced phase transfer does not affect the state of the colloid in any significant manner. There are small shifts of the spectra to the red in most of the organic liquids we investigated and hardly any change in the widths of the band. After the sample had aged for 3 months, the spectra still remain without change, indicating high stability. One can harvest the colloid as a dry powder by evaporating the organic solvent, and it still retains its integrity. It can be redispersed in a variety of organic liquids, giving stable colloids with extinction spectra very similar to the original one. Figure 4a shows a UV-visible spectrum of a hydrosol (capped with oleate), then after a phosphate induced transfer of the colloid to cyclohexane (Figure 4b), then the same colloid after it is precipitated from cyclohexane by evaporation and redispersed back in cyclohexane (Figure 4c), and finally after a cycle of evaporation and redispersing in chloroform (Figure 4d). The colloid is exceptionally stable and maintains the integrity of the individual particles, as is demonstrated by the spectra. It is also stable to centrifugation and ultrasonic treatment. Similar redispersibility was demonstrated recently for gold colloids with thiol capping23a or that of amines24 or phosphanes.33 Put together, these results show that this new approach is a promising and viable way to produce and handle a variety of colloids, paving new avenues for interfacial and material science and technology. These systems are in line with the theoretical concepts of “thermodynamically” stabilized colloids34 and related notions of nanocrystal control.35 Another variation on the same theme is a recent application of colloid suspensions (33) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (34) Gelbart, W. M.; Ben-Shaul, A. J. Phys. Chem. 1996, 100, 13169. (35) Whetten, R. L.; Gelbart, W. M. J. Phys. Chem. 1994, 98, 3544.
Directing Ag Colloids into Organic Phases
Langmuir, Vol. 14, No. 3, 1998 607
Table 3. UV-Visible Absorption Peak Position and Width and the Solvent Exchange Transfer Ratios from Hydrosols Containing 1 × 10-3 M Ag Atoms
solvent
absorption maximum, nm (fwhm, nm) (4 nm ((8 nm)
transfer ratio, % (10%
water cyclohexane hexane dodecane isooctane chloroform 1-chlorobutane carbon tetrachloride
406 (71) 412 (70) 410 (70) 410 (67) 410 (66) 408 (63) 412 (68) 412 (70)
70 72 62 57 64 51 55
Figure 5. Transmission electron micrograph of dried silver particles from a colloidal dispersion in cyclohexane containing 1 × 10-3 M silver atoms obtained using H3PO4 as the inducer of solvent exchange after aging for 3 month at room temperature. Insert: Histogram of the size distribution. 〈D〉 ) 4.4 nm, σ ) 1.0 nm.
Figure 4. UV-visible spectra of (a) oleate protected silver hydrosol, (b) silver organosol in cyclohexane transferred from hydrosol a, (c) redispersed silver colloid in cyclohexane with dried particles from organosol b, and (d) redispersed silver colloid in chloroform with dried particles from organosol b.
onto liquid surfaces and subsequent formation of Langmuir-Blodget films.36 TEM micrographs of silver cyclohexane organosols obtained by the solvent exchange method using H3PO4 (Figure 5) and NaH2PO4 (Figure 6) show highly monodisperse particles, with average diameters of 4.4 nm ((1.0 nm) and 5.5 nm ((1.2 nm), and a narrow size distribution. This is quite similar to what has been registered for the parent surfactant capped hydrosol. When the solvent exchange process is attempted without sodium oleate (as a stabilizer), the colloidal particles do not transfer to the organic phase but, instead, precipitate out of the water. Thus, the surfactant acts in two (related) ways: it prevents colloidal sedimentation and promotes the transfer of the particles into the organic phase. An interesting feature seen in the TEM pictures is the hexagonal, close-packed, 2D ordered layer of the dried silver particles on the holey carbon copper grid. Large ordered regions are observed, indicating the high monodispersity, as well as long-range interparticle interactions. Figure 7 shows the fast Fourier transform (FFT) of such a region, clearly demonstrating the 6-fold symmetry of the particle arrangement. We also observe areas with two or even three layers, retaining the hexagonal structure. The particles of the second layer sit in the locations (36) (a) Dabbousi, B. O.; Murray, C. B.; Rubner, M. F.; Bawendi, M. G. Chem. Mater. 1994, 6, 216. (b) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. J. Chem. Soc., Faraday Trans. 1994, 90, 673.
Figure 6. Transmission electron micrograph of dried silver particles from a colloid dispersion in cyclohexane containing 1 × 10-3 M silver atoms obtained using NaH2PO4 as the inducer of solvent exchange after aging for 3 months at room temperature. Insert: Histogram of the size distribution. 〈D〉 ) 5.5 nm; σ ) 1.2 nm.
Figure 7. Fast Fourier transform of an ordered region of the TEM picture of hydrophobic silver particles.
of 3-fold symmetry, as expected in bulk hexagonal lattices. In polydisperse systems such long-range ordering is impossible. Under appropriate conditions, one might get radial size segregation34 and nearly hexagonal local ordering.37 Similar ordering on TEM grids was reported with iron oxides,38 where ordered multilayers were also observed, and with cadmium chalcogenides.39 Large 3D and 2D superlattices were formed by self-assembly of cadmuim selenide particles40 and silver sulfides.41 Very
608 Langmuir, Vol. 14, No. 3, 1998
Wang et al.
recently, similar ordered arrangements were obtained for electrophoretically deposited thiol stabilized gold colloids,42 self-assembled thiol stabilized silver colloids,43 and LB colloidal films.44 The preparation of densely packed monolayers has been reported by a few groups,45 but usually the crystallinity was rather limited. Twodimensional crystalization is of much interest, for a variety of reasons, and it very recently has drawn some attention and was demonstrated in various experimental systems.46 As far as we are aware, the two-dimensional ordering of the oleate stabilized silver particles is the first example for two-dimensional crystallinity of non-thiol stabilized nanosized metal colloid particles. The important point to note is that the binding of the oleate to the silver surface is expected to be much weaker than that of thiol to gold. The binding is reversible to some degree, yet the integrity of the particles and their packing is maintained even under harsh conditions. 4. IR Measurements. FTIR serves here to understand the role of the stabilizer, the oleate, in the formation of the colloid. IR frequencies of long-chain alkanes47 and unsaturated carboxylic acids48 have been widely studied, and it is possible to anticipate with some certainty the bands expected for adsorbed oleic acid and oleate. IR spectra of the free and adsorbed oleic acid/oleates as measured by us are shown in Figure 8, and their band characteristics are summarized in Table 4. The results for oleic acid and sodium oleate agree with published values.47,48 For both oleic acid and oleate a series of bands in the region 2850-2970 cm-1 is expected, attributable to the symmetric and asymmetric stretching of the CH2 groups and the terminal CH3 group. In addition a band attributable to the stretching of the olefinic CH is expected around 3004 cm-1. In the 1200-1750 cm-1 region, both oleic acid and oleate are expected to show a strong band in the vicinity of 1465 cm-1 due to CH2 deformation. The oleate anion is expected to show asymmetric and sym-
Figure 8. Infrared spectra of (a) liquid oleic acid, (b) a cast film of sodium oleate in an aqueous solution, (c) silver oleate powder precipitated from an aqueous solution, (d) a cast film of a silver hydrosol stabilized with sodium oleate, (e) a cast film of a silver cyclohexane sol using H3PO4 as an inducer of solvent exchange, (f) a cast film of silver cyclohexane sol using NaH2PO4 as an inducer of solvent exchange, and (g) a floc centrifuged from silver hydrosol stabilized by excess surfactant.
(37) (a) Ohara, P. C.; Leff, D. V.; Heath, J. R.; Gelbart, W. M. Phys. Rev. Lett. 1995, 75, 3466. (b) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (38) (a) Bentzon, M. D.; van Wonterghem, J.; Morup, S.; Tholen, A. Philos. Mag. B 1989, 60, 169. (b) Bentzon, M. D.; Tholen, A. R. Ultramicroscopy 1991, 38, 105. (39) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (40) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science 1995, 270, 1335. (41) (a) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425. (b) Motte, L.; Billoudet, F.; Lacaze, E.; Douin, J.; Pileni, M. P. J. Phys. Chem. B 1997, 101, 138. (42) Giersig, M.; Mulvaney, P. Langmuir 1993, 9, 3408. (43) (a) Harfenist, S. A.; Wang, Z. L.; Alvarez, M. M.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. 1996, 100, 13904. (b) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (44) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 189. (45) (a) Dusemund, B.; Hoffmann, A.; Salzmann, T.; Kreibig, U.; Schmid, G. Z. Phys. D 1991, 20, 305. (b) Meldrum, F. C.; Kotov, N. A.; Fendler, J. H. Langmuir 1994, 10, 2035. (46) (a) Garvey, M. J.; Mitchell, D.; Smith, A. L. Colloid Polym. Sci. 1979, 257, 70. (b) Pusey, P. N.; van Megen, W.; Bartlett, P.; Ackerson, B. J.; Rarity, J. G.; Underwood, S. M. Phys. Rev. Lett. 1989, 63, 2753. (c) Pieranski, P. Contemp. Phys. 1983, 24, 25. (d) He, C.; Donald, A. M. Langmuir 1996, 12, 6250. (e) Onoda, G. Y. Phys. Rev. Lett. 1985, 55, 226. (f) Yeh, S.-R.; Seul, M.; Shraiman, B. I. Nature 1997, 386, 57. (g) Clarke, S. M.; Rennie, A. R.; Ottweill, R. H. Langmuir 1997, 13, 1964. (47) (a) Synder, R. G. J. Chem. Phys. 1967, 47, 1316. (b) Synder, R. G. J. Mol. Spectrosc. 1961, 7, 116. (c) Synder, R. G. J. Chem. Phys. 1979, 71, 3229. (d) Casal, H. L.; Mantsch, H. H.; Cameron, D. G.; Synder, R. G. J. Chem. Phys. 1982, 77, 2825. (48) (a) Kobayashi, M.; Kaneko, F.; Sato, K.; Suzuki, M. J. Phys. Chem. 1986, 90, 6371. (b) Koyama, Y.; Ikeda, K. Chem. Phys. Lipids 1980, 26, 149. (c) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975; Chapters 2, 3, and 10. (d) Colthup, N. B.; Daly, L. H.; Wiberley, S. E. Introduction to Infrared and Raman Spectroscopy, 2nd ed.; Academic Press: New York, 1975; Chapters 7 and 9.
metric COO- stretch frequencies in the regions 15501620 and 1300-1450 cm-1, respectively; the latter is normally more difficult to locate. For oleic acid a band in the vicinity of 1700-1730 cm-1 is anticipated due to the CdO stretch. Less readily identifiable bands are anticipated around 1400 cm-1 (C-O-H in-plane bend) and 1300 cm-1 (C-OH stretch). Previous studies have shown that the C-OH stretch and C-O-H in-plane bend modes are strongly coupled and the resulting bands are variable in position and intensity.48c,d In all of the spectra we see the ∼720 cm-1 CH2 rocking mode. Reference transmission spectra of pure liquid oleic acid, a cast thin film of sodium oleate from an aqueous solution, and silver oleate powder precipitated from an aqueous solution are given in Figure 8a-c, respectively. The strong CdO stretching band of oleic acid appears at 1700 cm-1. For sodium oleate and silver oleate the CO2 asymmetric bands appear at 1560 and 1517 cm-1, respectively. Figure 8d is an attenuated total reflection (ATR) spectrum of a cast film of silver hydrosol particles stabilized by sodium oleate. Before recording the spectrum, the thin sample film was immersed in water for 2 min in order to remove any soluble salts, such as nitrates and borates. Parts e and f of Figure 8 exhibit the spectra of cast films of hydrophobic silver particles from cyclohexane organosols produced using H3PO4 and NaH2PO4, respectively. Similarly to the spectra of oleic acid and oleates, those of the colloids, cast from water and from cyclohexane, exhibit clear characteristics of the hydrocarbon chain. One can identify the CH2 asymmetric and symmetric stretching bands around 2921 and 2851 cm-1, the CH3 asymmetric
Directing Ag Colloids into Organic Phases
Langmuir, Vol. 14, No. 3, 1998 609
Table 4. Observed Bands for Various Species of Oleic Acid and Their Assignmentsa frequencies, cm-1 organosol oleic acid
Na oleate
Ag oleate
flocb
3004 v 2955 sh 2925 vs 2854 vs 1709 vs
3002 v 2955 sh 2921 s 2851 s
3004 v 2953 sh 2917 vs 2849 vs
3004 v 2953 sh 2920 vs 2851 vs
1560 vs 1463 w 1443 m 1406 vw
1541 sh 1517 vs 1468 m 1419 m 1406 m
1541 sh 1519 m 1468 m 1421 m 1407 m
1464 m 1436 m 1412 m
hydrosol 3013 v 2954 sh 2922 vs 2852 vs 1559 vs 1461 m 1446 w 1398 m,b 1375 m,b
1285 m 937 m,b 724 m
826 m 720 m
823 m 719 m
822 w 719 m
727 w
cyclohexanec
cyclohexaned
assignment47,48
3003 v 2955 sh 2922 vs 2852 vs 1709 m 1547 m 1519 m 1464 m
3005 v 2955sh 2922 vs 2852 vs 1732 w,b 1539 m 1516 m 1464 m
1398 vs
1397 s
1262 m
1261 m
804 m 721 w
805 m 721 w
ν(dC-H) νas(CH3) νas(CH2) νs(CH2) ν(CdO) νas(CO2-) νas(CO2-) δ(CH2), scissor νs(CO2) + δ(COH) + δ(R-CH2) νs(CO2) + δ(COH) + δ(R-CH2) νs(CO2) + δ(COH) + δ(R-CH2) ν(C-O) δ(O-H), out-of-plane ω(dC-H) ? F(CH2), rock
a vs, very strong; s, strong; m, middle; w, weak; vw, very weak; sh, shoulder; b, broad; ν , asymmetric stretching; ν , symmetric stretching; as s δ, deformation; F, in-phase rock; ω, wag. b Flocs separated from a silver hydrosol with excess oleate. c H3PO4 is the inducer for the solvent d exchange. NaH2PO4 is the inducer for the solvent exchange.
and symmetric bands around 2953 and 2872 cm-1, CH2 scissoring bands around 1464 cm-1 , and the CH2 in-phase rocking band around 722 cm-1. Undoubtedly, molecules of surfactant are associated with both the hydrophilic and the hydrophobic silver colloidal particles. Later we show they are actually adsorbed on the particles. Figure 8g shows the spectrum of the flocs collected from the aqueous colloidal solution when excess surfactant is used. A comparison to the spectrum of pure silver oleate (Figure 8c) reveals it to be the main constituent of the floc. The CO2 stretch band of the oleate species on the surface of hydrophilic silver particles appears as a band at 1559 cm-1 (Figure 8d), which is different from that seen for silver oleate and is similar to that of sodium oleate (Figure 8b). This indicates that the carboxylate is rather distant from the surface and faces the dispersant liquid (water). In the organosols, in contrast¸ two bands appear at 15161519 and 1539-1547 cm-1, close to the positions observed for silver oleate (1517, 1541 cm-1). This implies that in the organosols there is a direct interaction between the polar head-group of the oleate molecules and the silver particles. Thus, a clear picture emerges, where the oleate switches direction on the silver particle. In the hydrosol the carboxylate is directed outwardly, away from the solid surface, imparting a hydrophilic nature to the exterior of the dressed particle. In the organosol the carboxylate serves to anchor the oleate to the surface. Consequently the hydrophobic “tail” is directed toward the solvent. Furthermore, in the hydrosol the dC-H stretch band of the oleate appears at 3013 cm-1, considerably blue shifted by ∼9 cm-1 relative to pure oleates or oleic acid. This indicates a significant interaction between the CdC double bond of the adsorbed oleate and the silver surface in the hydrosols. Thus, one can suggest that in the hydrosol the anchoring to the silver surface is made via the double bond. As an additional support for this view note that the surface pressure-molecular area (π-A) isotherms show that the unsaturated CdC bond is more hydrophilic than the saturated C-C bond.30 Note also that in the organosols this dC-H stretch is not shifted compared to the salts or the free acid, indicating that this group is positioned away from the surface, as expected if the attachment is through the carboxylate. In silver oleate, too, the bonding is at the carboxylate, as evidenced from the CO2- and the C-H bands.
There are additional indications in the spectra of the validity of the adsorption model in the hydrosol and the organosols we propose. From the spectrum of the deposited hydrosol (Figure 8d), one can see that the CH2 scissors band at 1461 cm-1 shifts to a lower frequency and the CH2 rocking band at 727 cm-1 shifts to a higher frequency with respect to those of sodium oleate or the free acid. In the silver oleate (and the silver oleate floc) the shifts are in an opposite direction. In the organosols it is hardly shifted at all. This demonstrates the larger interaction of the methylene groups with the surface of the hydrosol. Anchoring through the double bond necessarily brings many of the methylene groups into direct contact or near proximity to the silver surface. On this basis one can understand the shifts observed in the hydrosol and their absence in the organosls. A band that is perhaps assigned to a trans dC-H wag mode,48d which appears at 826 cm-1 in the spectra of sodium and silver oleates, is absent from that of the hydrosol but appears in that of the organosols (though shifted to 805 cm-1). This, again, supports the model we described for the adsorption conformation of the oleate in the hydrosol compared to that in the organosol. As noted above, the spectra of oleate on hydrophobic silver particles in cyclohexane (Figure 8e,f) are different than those observed for hydrophilic particles, as well as those of oleic acid and sodium oleate, but have a close resemblance to that of silver oleate. One major difference is, however, the appearance of a band at 1700 cm-1, typical of the oleic acid CdO stretch, especially when using H3PO4. This suggests that on the hydrophobic silver particles adsorption of oleic acid might take place in addition to that of the oleate. At the prevailing basic pH in the aqueous phase, there are no free acid molecules; however, they may be present in the organic phase. It seems that they have a role in the arrangement of the adsorbed layer on the particle. Tentatively we suggest that hydrogen bonding between the acid and the anions might be important. From the discussion above it appears that the specific molecular structure of oleic acid is essential for the exchange process to occur (at least, under the conditions of our experiments). Specifically, the combination of a double bond in a convenient location along the hydrocarbon chain and the carboxylate group seem to be important. In the hydrosol there is a major adsorption by anchoring to
610 Langmuir, Vol. 14, No. 3, 1998
the surface through the double bond. Also Hirai et al.22a note the importance of the double bond for phase transfer of a gold colloid. This allows the polar carboxylates to face the solution side and imparts a hydrophilic character to the particle. Indeed, when we use other surfactants that do not have a CdC group, such as sodium stearate, sodium dodecyl sulfate, or tetradecyltrimethylammonium bromide, we do not observe transfer of the particles to the organic phase. Instead, a complete particle phase separation, precipitation, occurs. Also, Hirai et al. failed to achieve transfer with stearic acid.22a 5. Role of the Inducer of the Solvent Exchange. The role of both NaH2PO4 and H3PO4 in the transfer of the silver particles from the aqueous to an organic liquid is particularly interesting. Hirai et al.22a used high concentrations of salts, or highly charged ions, and infer hydration effects and/or formation of salts with oleate. Both of these explanations are not valid, at least in our system, as we use much lower concentrations of orthophosphoric acid (∼8 × 10-4 M) or its mono salt (∼7 × 10-2 M). Fendler et al.36b also puzzle over the detailed mechanism. Incidentally, other acids, such as H2SO4, HCl, formic acid, or anisic acid, did not induce solvent exchange. We, too, do not have an explanation for the role of the phase transfer agents we use here, the “phosphates” and the perchlorates. However, it seems that the ability to form hydrogen bonding is a common denominator to all of them. Also the concentrations are sufficiently low to indicate that they most probably adsorb onto the silver particles, as they presumably cannot make any significant changes in the solution itself. One could envision a capping layer composed of the oleate moieties and the transfer agents forming a net of hydrogen bonds. With the phosphoric acid or its anion which produce insoluble silver salts, it is plausible to assume that they coadsorb onto the surface of the particles and perhaps force the carboxylate group of the oleate to turn over toward the surface, in order to form this internet of hydrogen bonds. We cannot, however, use the same argument for the perchlorates, which are equally efficient transfer agents. We carried out a few molecular modeling calculations but could not
Wang et al.
find any clue to the mechanism. The energy differences between various combinations of phosphoric acid and oleate did not show any clear trend. Obviously this unsolved important question, of the role of the transfer agent and the mechanism of its action, merits further investigation. Conclusion Sodium oleate stabilized silver particles in nanometersize range were synthesized in water by reduction of AgNO3 with NaBH4. It was found that the size distribution and stability of the silver particles strongly depend on the amount of sodium oleate. Appropriate surfactant concentrations for the preparation of silver particles were determined. Using orthophosphoric or hyperchlorate acids or their sodium salts, a solvent exchange of particles from water to an organic phase takes place and a subsequent hydrophobic silver organosol is obtained. The exchange ratios of the particles from water to various organic solvents are in the range 50-70%. The organosols have a narrow particle size distribution and are stable for at least 3 months at room temperature. The oleate adsorbs via the double bond in the hydrosol and rearranges to a head-on adsorption anchored by the carboxylate group in the organosol. The colloids capped by oleate are highly stable and can be separated out of the solution (by evaporation) without loosing their particular character. The colloid can be stored in the dried powder form and then redispersed in a variety of organic solvents, preserving their monodispersity and the individuality of the metal particles. Finally, using the solvent exchange method, the colloids can be directed to a variety of other chemical environments, in addition to bulk phases, such as to liquid/liquid interfaces, forming interfacial colloidal film,11 or to various surfactant assemblies, such as lamella phases. The details of the latter will be reported elsewhere.12 Acknowledgment. W.W. thanks Dr. Zeiri and Mr. Burshtain for useful discussions. LA9710177
