Langmuir 1999, 15, 1993-2002
1993
Formation and Morphology of Calcium Sulfate Nanoparticles and Nanowires in Water-in-Oil Microemulsions Gareth D. Rees,*,† Richard Evans-Gowing,‡ Stephen J. Hammond,§ and Brian H. Robinson† Schools of Chemical Sciences and of Biological Sciences, University of East Anglia, Norwich NR4 7TJ, U.K., and Shell Additives International Limited, Shell Research and Technology Centre, Thornton, Chester CH1 3SH, U.K. Received August 13, 1998. In Final Form: December 4, 1998 Nanosized insoluble metal sulfate aggregates, predominantly CaSO4, have been prepared in a variety of water-in-oil (w/o) microemulsions stabilized by either nonionic or ionic surfactants. Particles were visualized by transmission electron microscopy (TEM) and the identities of the aggregates confirmed by energy-dispersive X-ray analysis (EDXA). BaSO4 prepared in n-heptane microemulsions stabilized by the sodium salt of Aerosol-OT (AOT) appeared as slightly irregular aggregates, 8-50 nm in diameter. In contrast, BaSO4 in n-heptane microemulsions stabilized by ammonium diethylhexyl phosphate (NH4DEHP) existed in the form of submicron-sized “flocs” comprising nanospheres 5-7 nm in diameter. BaSO4 synthesis in cyclohexane microemulsions stabilized by tetraethylene glycol monododecyl ether (C12E4), produced discrete essentially monodisperse nanospheres 8-10 nm in diameter. A wider variety of morphologies were encountered in the synthesis of CaSO4 which produced nanospheres, ellipsoids, rods, nanohairs, nanowires, and nanobundles. The greatest structural diversity was obtained in systems stabilized by C12E4 where product morphology was sensitive to the mole ratio of water to surfactant (ω0) in the reaction medium, the overall water content, surfactant concentration, reactant concentration, and incubation time. The growth of CaSO4 nanowires was monitored as a function of time and was fastest at high overall reactant concentrations. Nanowires and nanobundles were often observed to span completely the individual sections of the copper grid used in TEM measurements, indicating lateral growth potential on the order of hundreds of microns. In contrast, CaSO4 formed in microemulsion systems stabilized by AOT in dodecane yielded only nanospheres whose size was largely independent of composition and reaction conditions.
1. Introduction Recent years have seen a remarkable growth in the use of surfactant-based “structured media” for micro- and nanocompartmentalization. This approach has already been extensively applied in the field of biotechnology where water-in-oil (w/o) microemulsion media have been used to solubilize enzymes at the molecular level in individual bionanoreactors.1,2 Currently, there is considerable interest in exploiting this technology in the area of materials science, particularly for directed polymer and particle synthesis, and employing surfactant-based systems as constrained reaction environments has proven a popular strategy. The developing protocols often have the twin aims of combining effective reaction control with the prospect of a molecular templating effect.3 Microemulsions, and in particular w/o microemulsions, have received considerable attention as reaction media for the synthesis of nanomaterials. Interest stems from the ease and reproducibility of microemulsion formation * To whom correspondence should be addressed. Present address: SmithKline Beecham, Research & Development, St. George’s Avenue, Weybridge, Surrey KT13 0DE, U.K. Phone: 01932 822179. Fax: 01932 822120. E-mail:
[email protected]. † School of Chemical Sciences, University of East Anglia. ‡ School of Biological Sciences, University of East Anglia. § Shell Additives International Limited. (1) Oldfield, C. Biotechnol. Genet. Eng. Rev. 1994, 12, 255. (2) Rees, G. D.; Carlile, K.; Crooks, G. E.; Jenta, T. R.-J.; Price, L. A.; Robinson, B. H. In Engineering of/with Lipases; Malcata, F. X., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1996; Vol. E317, p 577. (3) Eastoe, J.; Warne, B. Curr. Opin. Colloid Interface Sci. 1996, 1, 800.
and the diversity of organized assemblies that form under different compositional and operational conditions.4 Miniaturization of materials has potential benefits not only in terms of efficiency but also because the physicochemical properties of these materials are often markedly altered by virtue of scale. Most notable are quantum size effects observed with semiconductor nanoparticles5 and enhanced electric and magnetic phenomena associated with metal nanoparticles and nanoclusters.6 The microemulsion approach seeks to exploit the ability to compartmentalize reactants at the nanometer level in discrete water domains, the structure of which (spheres, rods, disks, or bicontinuous) is largely determined by the specific surfactant employed. The exchange dynamics control the rate at which reactants come together, and this will influence the relative rates of nucleation and particle growth.3 The dynamics of droplet collision and fusion can be controlled through the use of temperature, choice of surfactant, and the presence of additives, which may enhance or inhibit the fusion process.7-9 Other compositional changes such as surfactant concentration and the mole ratio of water to surfactant (ω0) also exert an (4) Luisi, P. L.; Scartazzini, R.; Pileni, M. P.; Robinson, B. H. Biochim. Biophys. Acta 1988, 947, 209. (5) Towey, T. F.; Khan-Lodhi, A. N.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86, 3757. (6) Lopez-Quintela, M. A.; Rivas, J. Curr. Opin. Colloid Interface Sci. 1996, 1, 806. (7) Fletcher, P. D. I.; Howe, A. M.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1 1987, 83, 985. (8) Fletcher, P. D. I.; Horsup, D. I. J. Chem. Soc., Faraday Trans. 1992, 88, 855. (9) Jain, T. K.; Cassin, G.; Badiali, J. P.; Pileni, M. P. Langmuir 1996, 12, 2408.
10.1021/la981026v CCC: $18.00 © 1999 American Chemical Society Published on Web 02/17/1999
1994 Langmuir, Vol. 15, No. 6, 1999
influence. The time scale of formation of aggregates is generally much slower in w/o microemulsions than it is in the case of surfactant-free systems. This is first because the association/fusion/dissociation mechanism characterizing solute exchange in droplet-containing microemulsions is associated with a large energetic barrier which can inhibit ion-ion encounters. In addition, however, the surfactant almost certainly has a role in the steric stabilization of nucleated and growing particles, although this latter process is much less well understood. As indicated earlier, the constrained reaction environment provided by these and other surfactant assemblies sometimes serves as a molecular-scale template limiting growth.3,6,10 Thus, there are many examples in the literature where the size and shape of the synthesized materials correlate with the size and shape of the organized assemblies from which they originate. This templating effect has been observed in microemulsions which are bicontinuous or contain lamellar phases11-13 and in microemulsions comprising rodlike or cylindrical micellar structures.12,14 However, this effect is most often apparent when nanoparticles such as semiconductors, metals, or polymeric latexes are synthesized in the water pools of spherical w/o microemulsion droplets. In this case, as ω0 and consequently the radius of the water pool increases, there is generally a systematic change in the radius of the synthesized nanoparticle.15-19 Changes in particle size can also be achieved through modifying absolute and relative reactant concentrations,15,17 and this would be expected to be of particular significance in the synthesis of coprecipitates and composites. The templating effect is by no means guaranteed, however, and there are also a number of examples in the literature where there is no clear correlation between the morphology of the synthesized nanoparticle and the molecular dimensions of the microemulsion assemblies.20,21 It has also been reported that the apparent structure of some systems induces a templating effect that is expressed at a dimensional level several orders of magnitude larger than that of the microemulsion.22 Although our understanding is incomplete, our knowledge of the processes and mechanisms involved in the synthesis of nanomaterials in these dynamic ordered systems has improved markedly in the last 2 years both theoretically23,24 and from an experimental standpoint.3,25 In addition to the conventional surfactants employed in the studies described above, block copolymers can also be used to prepare w/o microemulsions. These polymer(10) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (11) Tanori, J.; Pileni, M. P. Adv. Mater. 1995, 7, 862. (12) Pileni, M. P.; Tanori, J.; Filankembo, A. Colloids Surf., A 1997, 123, 561. (13) Tohver, V.; Braun, P. V.; Pralle, M. U.; Stupp, S. I. Chem. Mater. 1997, 9, 1495. (14) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (15) Pellegri, N.; Trbojevich, R.; De Sanctis, O.; Kadono, K. J. SolGel Sci. Technol. 1997, 8, 1023. (16) Moran, P. D.; Bartlett, J. R.; Woolfrey, J. L.; Bowmaker, G. A.; Cooney, R. P. J. Sol-Gel Sci. Technol. 1997, 8, 65. (17) Petit, C.; Pileni, M. P. J. Magn. Magn. Mater. 1997, 166, 82. (18) Arcoleo, V.; Liveri, V. T. Chem. Phys. Lett. 1996, 258, 223. (19) Qi, L.; Ma, J.; Cheng, H.; Zhao, Z. Colloids Surf., A 1996, 108, 117. (20) Eastoe, J.; Stebbing, S.; Dalton, J.; Heenan, R. K. Colloids Surf., A 1996, 119, 123. (21) Kandori, K.; Kon-No, K.; Kitahara, A. J. Colloid Interface Sci. 1987, 115, 579. (22) Tracy, S. L.; Torkelson, J. M.; Jennings, H. M. Mater. Sci. Eng. C 1996, 4, 149. (23) Manjunath, S.; Gandhi, K. S.; Kumar, R.; Ramkrishna, D. Chem. Eng. Sci. 1994, 49, 1451. (24) Tojo, C.; Blanco, M. C.; Rivadulla, F.; Lopez-Quintela, M. A. Langmuir 1997, 13, 1970. (25) Pileni, M. P. Langmuir 1997, 13, 3266.
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based microemulsions have modified physicochemical properties but have also been successfully employed in the synthesis of CdS- and PbS-polymer composites,26,27 as well as in the synthesis of gold nanoparticles.28 Oilin-water (o/w) microemulsions have likewise been employed in the synthesis of nanomaterials such as polymers,29,30 CdS,31 lipid nanospheres,32,33 and cobalt ferrite.34 Gold particles have been synthesized in normal micelles based on cetyltrimethylammonium chloride,35 and vesicles have been used in the preparation of magnetic particles and polymers.36,37 The study presented here describes the use of w/o microemulsion systems for the formation of metal sulfate nanoparticles. The synthesis of CaSO4 is of particular interest to us because it is produced in large quantities in commercial oils as a product of the neutralization of sulfuric acid, formed during the combustion of high sulfur content fuels in marine diesel engines. Lubricating oils contain surfactant as stabilizers for various “sol” materials. Surprisingly, we are aware of only two studies relevant to the formation of CaSO4 which involve surfactantstabilized systems. The first describes the synthesis of CaCO3 and CaSO4 in systems stabilized by sodium n-alkylsulfonate (n ) 12-14), sodium n-alkylbenzenesulfonate (n ) 12-18), or calcium n-alkylbenzenesulfonate (n ) 10-14), where n is based on a branched-chain hydrophobe. The reaction media were principally emulsions; however, a small number of microemulsion systems were described.38 The second report describes the synthesis of a polymer-inorganic composite in a didodecyldimethylammonium bromide (DDAB) w/o microemulsion.22 In both cases the materials formed were generally ordered on the micron scale rather than the nanometer scale on which the parent microemulsion was organized. The aim of this work is to further our understanding of these processes by seeking some general features which can control the synthesis of CaSO4 nanoparticles. 2. Materials and Methods Analytical-grade n-heptane, hexane, and cyclohexane were purchased from Fisons. Decane (99+%), dodecane (99+%), tetradecane (99+%), Ca(NO3)2‚4H2O (99%), BaCl2‚2H2O (99%), Na2SO4 (99+%), and 97% diethylhexylphosphoric acid (HDEHP) were supplied by Aldrich. Ammonium diethylhexyl phosphate (NH4DEHP) was synthesized from HDEHP by titration of an ethanolic solution of HDEHP with concentrated ammonium hydroxide. The ammonium salt was recovered after removal of the solvent using a rotary evaporator and vacuum oven. The surfactants sodium Aerosol-OT (AOT) and tetraethylene glycol monododecyl ether (C12E4) were purchased from Sigma and were of the highest available purity. AOT, C12E4, HDEHP, (26) Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178. (27) Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185. (28) Spatz, J. P.; Mossmer, S.; Moller, M. Chem.sEur. J. 1996, 2, 1552. (29) Gan, L. M.; Chew, C. H.; Lee, K. C.; Ng, S. C. Polymer 1994, 35, 2659. (30) Larpent, C.; Bernard, E.; Richard, J.; Vaslin, S. React. Funct. Polym. 1997, 33, 49. (31) Petit, C.; Jain, T. K.; Billoudet, F.; Pileni, M. P. Langmuir 1994, 10, 4446. (32) Boltri, L.; Canal, T.; Esposito, P.; Carli, F. Eur. J. Pharm. Biopharm. 1995, 41, 70. (33) Morel, S.; Ugazio, E.; Cavalli, R.; Gasco, M. R. Int. J. Pharm. 1996, 132, 259. (34) Moumen, N.; Pileni, M. P. Chem. Mater. 1996, 8, 1128. (35) Esumi, K.; Matsuhisa, K.; Torigoe, K. Langmuir 1995, 11, 3285. (36) Yaacob, L. I.; Nunes, A. C.; Bose, A. J. Colloid Interface Sci. 1995, 171, 73. (37) Poulain, N.; Nakache, E.; Pina, A.; Levesque, G. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 729. (38) Eskova, E. V.; Bukanova, E. F.; Tutorskii, I. A. Colloid J. Russ. Acad. Sci. 1993, 55, 366.
Calcium Sulfate Nanoparticles and Nanowires and NH4DEHP were used without further purification subject to satisfactory in-house quality control checks, most notably determination of the water solubilization capacity in standard systems. Water-in-oil microemulsions were prepared by injecting the appropriate volume of an aqueous reactant solution into a reverse micellar solution of surfactant in oil. The protocol is designed to prepare an MSO4 colloid in situ in the dispersed phase of a w/o microemulsion, where M is Ca2+ or Ba2+. This is achieved by simple manual mixing of equal volumes of separately prepared microemulsions containing one or other of the water-soluble reactants Na2SO4 and Ca(NO3)2 or BaCl2. All salt concentrations given in the text refer to moles per unit volume of the dispersed aqueous phase. All remaining concentrations refer to the overall concentrations in the total microemulsion. A JEOL 2000EX transmission electron microscope (TEM) was used to characterize the morphology and structure of the products at an operating voltage of 100 kV. Energy-dispersive X-ray analysis (EDXA) was performed using an Oxford Microanalytical AN10 instrument. Reaction microemulsions prepared using C12E4 were analyzed by placing a 2 µL drop onto a carbon-coated 200 mesh copper grid. Excess liquid was removed by contacting the side of the grid with an absorbent paper tissue. Reaction microemulsions prepared using AOT or NH4DEHP were initially similarly analyzed. However, significant improvements in resolution were obtained by immersing the copper grid in hexane for 10 s before mounting the sample. Even with this additional treatment, AOT samples sometimes required a further immersion in hexane in order to obtain grids that were sufficiently thinly coated to allow penetration by the electron beam. 2.1. Phase Behavior. The phase behavior of a number of systems stabilized by the surfactants used in this work have already been reported, most notably C12E4 in cyclohexane39 and also AOT in a variety of n-alkanes.7 While these studies provide a useful guide to the phase behavior we might expect, introduction of the salts used in this work will impact on the phase behavior. To establish the utility of different microemulsion media (in terms of stability in the presence of large concentrations of added salt) and identify those offering the most scope for more systematic study, the phase behavior of a variety of microemulsion systems was examined. In practice, this involved determining the extent of the single-phase microemulsion region as a function of temperature at a fixed concentration of surfactant in oil. Particular consideration was given to the solubility of the reactant salt solutions, with phase behavior being examined as a function of salt concentration (0.025-0.20 mol dm-3) and dispersed phase volume fraction. An example of the typical form of these phase maps is shown in Figure 1 for the AOT in dodecane series. In the case of AOT microemulsions in n-heptane, addition of salt causes a decrease in the extent of the single-phase region and shifts the position of the lower temperature phase boundary (LTPB) to higher temperatures. Increasing the salt concentration exacerbates these two effects, and of the two most widely used salts, Ca(NO3)2 and Na2SO4, Ca(NO3)2 has the more pronounced influence. There is therefore only a limited compositional regime which allows microemulsions containing high salt concentrations (0.20 mol dm-3) to be mixed at the same temperature. This regime is effectively determined by the extent to which the single-phase region of the phase maps overlap. In the specific case of AOT/ n-heptane microemulsions, this can only be achieved at low values of ω0 (
