Langmuir 1994,lO, 4451-4458
4451
Isotropic and Birefringent Dispersions of Surface Modified Silica Rods with a Boehmite-Needle Core Albert P. Philipse,*9?Ana-Mariana Nechifor,* and Chellapah Patmamanoharant Van? H o f f Laboratory for Physical and Colloid Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands, and Research Center for Macromolecular Materials and Membranes, Spl. Indepentei 202, Sector 6, 79611 Bucharest, Romania Received April 22, 1994. In Final Form: August 15, 1994@ A method is presented for the preparation of charged and sterically stabilized rodlike silica particles. First, boehmite needles are synthesized by hydrothermal treatment of aqueous aluminum alkoxide solutions. Then controlled silica growth on the needles takes place in an aqueous sodium silicate solution without formation of aggregates or homogeneous silica particles. Finally grafting reactions are performed with (fluorescent)silanes and octadecyl alcohol to obtain stable dispersionsin organic solvents. Special attention is given to the silica-coveredrods grafted with a dense octadecyl brush, which form isotropic (flowbirefringent) dispersionsand translucent (permanent birefringent)incompressiblelow-densitysediments in cyclohexane. It is argued that the dominant interparticle force in these systems is a steep hard-rod repulsion, which also accounts for the novel observation of the turbidity decrease at increasing densities. 1. Introduction
This communication deals with preparation and properties of stable dispersions of charged silica rods in water and ethanol and uncharged alkane-grafted rods in cyclohexane. Such systems are ofinterest to study colloidal phenomena, which specifically depend on the particle anisotropy. Examples are the orientation of elongated colloids in a shear flow (streaming birefringence) and the rich liquid-crystal-like phase behavior of rods with a suficiently high aspect rati0.l The majority of inorganic colloids in nature and technology are nonspherical, but only a small number is suitable for quantitative study of mentioned phenomena and their explanation in terms of underlying interparticle interactions. Such studies require particles of fairly well-defined size, shape, and surface properties, the latter including grafted organic layers to manipulate particle interactions. Particle dimensions should be small enough (in most cases I0.5pm) to avoid excessive settling during experiments, and for light scattering purposes dispersions must have a low turbidity. Buining et recently developed in this context a hydrothermal alkoxide synthesis of fairly well-defined boehmite (AlOOH) needles (L 5 300 nm) in an aqueous solution. They also prepared polysiobutene-grafted uncharged needles in stable organic dispersions4 and demonstrated for the first time the isotropic-nematic phase transition in a system of uncharged rods.5 We made attempts to further diversify the boehmite systems by applying other surface modifications, which are well-established for the case of silica spheres. Examples are silica surface coatings with octadecyl alcohol,6 a l . 2 3 3
t Van’t Hoff Laboratory for Physical and Colloid Chemistry.
*
Research Center for Macromolecular Materials and Membranes. Abstract published inAdvanceACSAbstracts, October 1,1994. (1)Lekkerkerker,H. N. W.; Vroege, G. J.Philos. Trans.R . Soc.London A 1993,344,419. (2)Buining, P. A.; Pathmamanoharan, C.; Bosboom, M.; Jansen, G. B. H.; Lekkerkerker, H. N. W. J.Am.. Ceram. SOC.1990, 73, 2358. (3) Buining, P. A.; Pathmamanoharan, C.; Jansen, G. B. H.; Lekkerkerker, H. N. W. J. Am. Ceram. SOC.1991, 74, 1303. (4)Buining, P. A.; Veldhuizen, Y. S. J.; Pathmamanoharan, C.; Lekkerkerker, H. N. W. Colloids Surf. 1992,64, 47. (5) Buining, P. A.; Lekkerkerker, H. N. W. J.Phys. Chem. 1993,97, 11511. (6)Van Helden,A. K.; Jansen, J. W.; Vrij, A. J.Colloid Interface Sei. 1981, 81, 354.
functional silanes,’ and fluorescent dyes.6 Such coatings greatly enlarge experimental possibilities, as is demonstrated for silica in refs 9 and 10. However, little progress was made with respect to surface-modified boehmite needles, because the needles easily aggregate during grafting reactions, in (non)polar organic solvents. For example, the grafting with octadecyl alcohol to produce “hard rods”, analogous to the preparation of hard c16coated silica spheres in cyclohexane,6 yields flocculated systems. Therefore we developed a method to grow silica on boehmite needles in aqueous suspension, followed by surface modifications which yield stable dispersions of rodlike particles in organic solvents. (A similar method was used to prepare silica colloids with a magnetic core.11) The silica is the anchor for covalent bonds with organic surface groups, and its deposition also allows control of the particle thickness (which is hardly possible in the boehmite-core synthesis). Moreover, the silica layer also screens destabilizing van der Waals attractions because of the small difference in polarizability (refractive index) between silica and solvents of interest such as cyclohexane. The silica deposition takes place in a supersaturated sodium silicate solution and is based on work of Iler,12J3 dealing extensively with the role of pH and sodium content in the heterogeneous nucleation and growth of silica in water. An alternative for this deposition is the use of boehmite needles as heterogeneous nuclei for silica growth in ethanoywater using the well-known Stober ~ynthesis.’~ Elsewhere we have shown that this inevitably leads to bridging flocculation of the positively charged needles by the slowly formed negative silicate species.15 In aqueous silicate solutions, the boehmite needles are covered at much higher rate with silica. This reduces (but certainly not eliminates) possibilities for needle aggregation. Af-
@
(7) Philipse, A. P.; Vrij, A. J. Colloid Interface Sei. 1989,23, 55. ( 8 )Van Blaaderen, A.; Vrij, A. Langmuir 1992,8, 2907. (9)Philipse, A. P.; Vrij, A. J. Chem. Phys. 1988,88, 6459. (10)Van Blaaderen, A. Adv. Mater. 1993,5, 52. (11)Philipse, A. P.; Van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994,10, 92. (12)Iler, R. K. The chemistry of silica; Wiley: New York, 1979. (13)Iler, R. K. US Patent, 2,885,366, 1959. (14)StiSber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sei. 1968,26,
62. (15)Philipse, A. Colloid and Surfaces A: Physicochemical and Engineering Aspects 1993, 80, 203.
0 1994 American Chemical Society
Philipse et al.
4452 Langmuir, Vol. 10, No. 12, 1994 terward, the silica-coated needles can be used to seed the Stober synthesis, as described briefly in section 2. The content of this paper is as follows. In the Experimental Section we first describe the boehmite core synthesis and examine conditions which influence the particle aspect ratio. Then silica growth on the cores is described, paying attention to details which are essential to avoid boehmite aggregation and separate homegenous silica nucleation. Surface modifications of the “silicarods” are reported for silanes, fluorescent groups, and octadecyl alcohol. The synthesis results are evaluated in section 3. The discussion of modified silica rods focuses on properties of C18-graRed particles in cyclohexane, such as their van der Waals attractions and surface densities of CIS chains. A first application of the Cls-silica rods is their sedimentation into systems which have intriguing properties. They can be more transparent than dilute dispersions, exhibit permanent birefringence, and are already incompressible at low densities. The origin of these phenomena is qualitatively discussed. 2. Experimental Section 2.1. Materials and Methods. All glassware was cleaned with aqueous 8%HF solution, hot tap water, and double-distilledwater. Freshly double-distilledwater was used in all experiments unless stated otherwise. Dowex 50 Wx4 acid ion exchange resin was regenerated by subsequent rinsing with hot water, 3 N HC1, and cold water. In all experiments only freshly regenerated resin was used which had not been previously exposed to a silicate solution. Otherwise silica nuclei may be present in the resin. Universal indicator pH 0-14 (Merck) was used to check the pH. The following reagents were used as received: aluminum tri-sec-butoxide (ASB, Fluka AG), aluminum triisopropoxide (AIP, Janssen Chimica), tetraethoxysilane (TES, Merck), (3-aminopropy1)triethoxysilane (APS,Janssen Chimica), (3-methacryloxypropy1)trimethoxysilane (TPM, Merck), fluoresceine isothiocyanate (FITC, isomer I Sigma), ammonia (25%, Merck), 1-propanol(Baker), absolute ethanol (Nedalco),octadecyl alcohol (Merck) and sodium silicate solution (NazO(SiO&-S; ABCR-Karlsruhe; 28 wt % SiOz). Particle mass densities for octadecyl-coated rods were calculated from mass densities of concentrated dispersions and the solvent cyclohexane. Elemental analysis was performed by Mikroanalytisches Labor Pascher (Remagen, Germany). Streaming birefringence was checked qualitatively by shaking illuminated dispersions between crossed polarizers. 2.2. Boehmite-CoreSynthesis. Aqueous dispersions of crystalline boehmite needles were prepared according to Buining‘s m e t h ~ d .For ~ boehmite particles BU2, with an average length of 182nm, the procedure was as follows. To a stirred mixture of 2902 mL of water and 21.8 mL of HC1 (37%), 59.8 mL of alkoxide ASB was added, after which a white precipitate of Al(OH)3was formed. Then 45.95 g ofAIP powder was added which dissolved within a few hours. The mixture was stirred at room temperature in a closed vessel for a week. The now clear solution was autoclaved, as described e l s e ~ h e r e ,for ~ ?22 ~ h at 150 “C. The resulting boehmite dispersion was dialyzed in cellophane tubes for 2 weeks against demineralized water to remove alcohols and salts. The dialyzed dispersion, coded BU2, had the appearance of a (permanent birefringent) gel. This is due to the strong double layer repulsion between the particles in the saltfree dispersion. After dilution, boehmite dispersions showed streaming birefringence. This manifests the colloidal stability of the boehmite needles: they do not aggregate and are easily oriented by a flow field. (When
Table 1. Average Particle Dimensions from TEMa polydispolydislength persity width persity system Llnm &% D ad% LID boehmite Coresb 48 10.1 29 -9.9 BU1 -100 32 8.5 24 -21.4 BU2 -182.1 38 7.7 26 37.5 BT2 288.8 silica rodsC 57 15.9 20 4.6 BUlSC 73.6 30 12.9 29 16.7 BU2SC 215.4 25 10.2 22 28.6 BT2SC 292 a Particle counts in image analysis: 30 for the length of BU1 and BU2, 500 counts for all other dimensions. Boehmite needles in water. Octadecyl-grafted silica rods in cyclohexane. CIS coating does not contribute to D.
ammonia is added, boehmite dispersions flocculate and are no longer birefringent.) The shorter boehmite needles, coded BU1, with an average length of 100 nm, were prepared in the same manner, starting from 2850 mL of water, 9.7 mL of HC1 (37%), and 156 mL of ASB. Autoclaving Conditions. When the alkoxide solution before autoclaving was investigated with transmission electromicroscopy (TEM), no discrete particles could be observed. However, when the solution was stored at room temperature for several months, its turbidity increased and electronmicroscopy revealed the presence of platelike hexagonal particles. We surmized that these platelets, very likely gibbsite, are the precursors for boehmite formation. Therefore the influence of variations in autoclaving time and temperature on particle morphology was investigated, also to demonstrate that autoclaving conditions for the boehmite synthesis (22 h, 150 “C) are indeed optimal for synthesizing high-aspect ratio particles (see Figure 1). Influence of TESAddition on Aspect Ratio. The average boehmite aspect ratio can be varied by changing reagent concentrations, as illustrated by the synthesis of BU1 and BU2, but remains below IJD 30. We attempted to increase IJD by adding additives such as citrates and amino acids (which might influence the particle morphology) and also studied the influence of removing dust and precipitates (which may act as heterogeneous nuclei) from the alkoxide solution by sedimentation or filtration. None of these attempts was successful. We found, however, that the presence of tetraethoxysilane (TES) increases the aspect ratio of the particles. For example, 0.5 mL ofTES was added to 160 mL ofthe alkoxide solution which had been stirred for a week for the BU2 synthesis. After 1.5 h of stirring the solution was autoclaved and dialyzed in the same manner as for the BU2 boehmite. The final particles coded BT2 were clearly longer and thinner than the BU2 rods (see Table 1and Figure 2). This experiment with the TES addition was repeated several times. In all cases this addition increased the aspect ratio. The average particle length, however, was less reproducible and seemed to depend on the age of the alkoxide solution. This point was not investigated further. 2.3. Silica Deposition in Water. Boehmite needles were covered with a silica layer of a few nanometers without loss of colloidal stability using a reproducible procedure which is only described in detail here for boehmite BU2. A 3% sodium silicate solution with pH 12 was prepared by dilution of stock solution with water. To lower the sodium content, ion exchange resin was added to the stirred solution in small portions until pH 11, after which the resin was removed by filtration. (It is essential that
-
Silica Rods the pH does not drop below pH 11otherwise silica may nucleate.) In a three-neck flask 400 mL of dialyzed BU2 dispersion (c = 4 g/L) was diluted with 1200 mL of water. The dispersion was stirred vigorously with a glass stirring rod while it was placed in an ultrasonic bath (Bransonic 8200) which was kept at a temperature of about 20 "C by flowing tap water. After 5 min of sonification, 3%silicate solution was pumped for 15min into the dispersion under the meniscus at a rate of 7 mUmin using a peristaltic pump. The pH was kept in the range 9-10.5 by addition of small portions ofresin. In this pH range silica deposition on the boehmite is rapid enough to prevent particle flocculation,while separate silica particle formation under these conditions usually does not occur. M e r 15 min, ultrasonification was stopped and the addition was continued at a rate of 3.5 mumin until a total of about 700 mL of silicate solution had been added, keeping the pH in the same range as indicated above. (The rate is reduced to suppress homogeneous silica nucleation.) The dispersion (pH 10.5) showed streaming birefringence and there were no signs of aggregation. After 1 week of dialysis in cellophane tubes against flowing demineralized water, TEM pictures were taken: no separate silica particles could be detected, only rods with a thin silica layer. The final dispersion, coded BUBS, had a weight concentration of c = 3.2 g/L. (Some ammonia can be added to dialyzed rods to charge up their silica surface to promote long-term stability.) Influence of Rod Length. When the silica deposition procedure was applied to longer boehmite needles occasionally flocs were formed. Flocculation could be avoided by sufficiently diluting the starting boehmite solution. The observed trend was that longer particles require lower concentrations, for reasons explained in section 3.2. 2.4. Further Silica Growth in Ethanol. Silica rods were used as seed particles in the Stijber synthesis in the following manner. A mixture of 450 mL of ethanol, 16.8 mL of ammonia, and 10 mL of dialyzed rod dispersion (mixed in this order) was kept in a water bath at 23 "C. Then 0.5 mL of TES at the same temperature was added and the mixture was stirred for about 24 h. Then another 0.5 mL of TES was added followed again by stirring for 24 h. This procedure can be repeated at least another 3 times without secondarysilica nucleation. This Stober synthesis using boehmite-silica seeds was reproduced for a number of other silica rod dispersions. 2.5. Surface Modification Reactions. Coating with Octadecyl Alcohol. The reaction of silica-surface silanol groups with octadecyl alcohol takes place in a melt of the alcohol as in the coating of silica spheres.6 The major problem is to avoid particle aggregation, when the silica rods are gradually transferred from water to an organic medium. The following procedure ultimately yields a stable dispersion of Cls-grafted silica rods. A solution of 64 g of octadecyl alcohol in 1L of ethanol was heated in a 3-L flask. A portion of 10-20 mL of dialyzed BU2S dispersion was diluted with 980 mL of ethanol and added to the flask, followed by 100 mL of propanol to speed up the azeotropic removal of water. (The dilution suppresses, or sufficiently slows down, any flocculation.) After about 1 L of solvent was distilled, another diluted portion of BU2S dispersion was added, again with 100 mL of propanol. This procedure was repeated until 2 L of dialyzed BU2S dispersion, containing 6.4 g of particles, was transferred to the reaction vessel. Distillation was continued until a few hundred milliliters of dispersion remained, which was placed in a 500mL vessel in an oil bath of 120 "C t o distill all the lower alcohols under a moderate nitrogen flow. Finally the oil temperature was raised to 190-200 "C and the magneti-
Langmuir, Vol. 10, No. 12, 1994 4453 cally stirred octadecyl-melt with the silica rods in a 100mL vessel was kept 6 h at this temperature under a weak nitrogen flow. Then part ofthe free octadecyl alcohol was removed by vacuum distillation (oil temperature, 190 "C; cooling water temperature, 80 "C). After cooling, the now solid reaction product was dissolved in warm cyclohexane in sedimentation tubes and purified further from unreacted octadecyl alcohol by three sedimentation-redispersion steps using a Beckman preparative ultracentrifuge at 25 000 rpm for 6 h. The first step was performed at 35 "C, the other steps at 25 "C. The resulting particles were dispersed in cyclohexane to a suspension which displayed streaming birefringence when illuminated between crossed polarizers. (This clearly confirms the stability of the surface-modified silica rods.) Sediments were prepared in cyclohexane with a Beckman table centrifuge at 3000 rpm for several days after which the sediments reached a constant height. Properties ofthese sediments are discussed in section 3.3. Coating with TPM.A stable dispersion of TPM-coated silica rods in pure ethanol was obtained as follows. About 30 mL of dialyzed silica rod dispersion was mixed with 0.15 mL of ammonia (which promotes TPM hydrolysis) and added to a mixture of 180 mL of ethanol, 55 mL of propanol, and 0.6 mL ofTPM. Solvent was distilled until the refractive index of the distillate was equal to that of ethanol. Free TF'M was removed by sedimentationredispersion steps as described above. The final dispersion in ethanol was clearly streaming birefringent. Coating with a Fluorescent Layer. In a closed vessel 0.0849gofFITC,0.938gofAPS, and6mLabsoluteethanol were mixed and stirred under nitrogen in the dark for 24 h. (See for further details ref 8.) To 250 mL of silica rod dispersion in ethanol, prepared by the Stober synthesis in section 2.4,0.2 mL ofTES and 0.1 mL of the dye solution were added. The dispersion was stirred for 24 h and then centrifugated at 3000 rpm in a table centrifuge for 48 h. The green-yellow fluorescent sediment was redispersed in ethanol with ultrasonification. The dispersion sedimented slowly on a time scale of weeks, indicating a (weak) net attraction between the particles. Addition of some TPM resulted in a nonsedimenting dispersion.
3. Results and Discussions 3.1. Boehmite Cores. Hydrothermaltreatment ofthe acid aluminum alkoxide solutions at 150 "C for 22 h, according to B ~ i n i n gleads , ~ to stable dispersions of needlelike boehmite cores. The autoclave temperature, which is not discussed by Buining? greatly influences the particle morphology, as is illustrated in Figure 1. Details of the morphology also depend on the type of alkoxides in the starting solution, but the following trend is observed. At lower temperatures hexagonal platelets are present, which probably consist of gibbsite, just as the particles which were found in alkoxide solutions aging at room temperature. Apparentlythese platelets dissolve and recrystallize at higher temperatures to the more stable boehmite phase. (In Figure 1 an example can be seen of partly dissolved hexagons coexisting with boehmite needles.) At temperatures above 150-160 "C,the final boehmite particles are elongated plates, whereas a treatment around 150 "C produces particles with the highest aspect ratio. So the reaction conditions reported by Buining3 are essential to obtain the appropriate cores to form silica rods. Addition of tetraethoxysilane to an alkoxide solution before the hydrothermal treatment increases the aspect ratio of the particles, as is illustrated in Figure 2. X-ray diffraction demonstrates that the particles still consist of boehmite and no evidence for a crystalline aluminum silicate is found. However, element analysis of dried BT2
4454 Langmuir, Vol. 10, No. 12, 1994
Philipse et al.
Figure 1. Illustration of the influence of autoclave temperature on the hydrothermal treatment of aluminum sec-butoxidesolutions with the same composition as the solution used for the synthesis at 150 "C of boehmite particles BU1. TEM micrographs were taken after 22 h of autoclaving. Gibbsite hexagons dissolve and recrystallize to boehmite. The bars represent 0.1 pm.
/;I
/2
d
I
I
:-
Figure 2. Boehmite needles BU2 with an average length of
Figure 3. Electron micrographs of silica-coveredBU1 boehmite needles, prepared as described in section 2.3 (left) and silica
about 182 nm (left). Longer and thinner particles are formed (right) when some tetraethoxysilane is added to the BU2 synthesis mixtures before hydrothermal treatment, as described in section 2.2. The bars represent 0.1 pm.
rods prepared using a n aqueous silicate solution which had aged some time at neutral pH, promoting formation of separate silicate particles (right). The bars represent 0.1 pm.
particles from a dialyzed dispersion reveals the presence of silicium (see Table 2). So amorphous silica is present which possibly facilitates the (onset of) chaining of boehmite subunits, leading to longer particles. 3.2. Silica Deposition in Water (andEthanol). The deposition of silica onto the boehmite needles in water as described in section 2.3 is not accompanied by homogeneous silica nucleation: electron micrographs only show silica-covered rods and no separate silica particles (see Figure 3). This point is important, because the dispersions are difficult to purify from these silica contaminants. For example, sedimentation-redispersion cycles hardly separate silica particles and rods. Homogeneous silica nucleation is suppressed here by keeping the pH high enough (see section 2.3),but also the boehmite concentration is relevant. A large boehmite surface area per dispersion volume enhances the probability for silicate species to attach rapidly to a boehmite core and counteracts the build-up of a silicate concentration which would stimulate homogeneous nucleation. (This insight is due to Iler12J3.) However, we observed that when the boehmite concentration is too high, one runs
into another problem, because the dispersion easily aggregates or gelates upon addition of silicate solution. The reason is that in these dispersions of high aspect ratio needles there are many rod-rod contacts per second. Silica precipitation at contact areas simply makes these contacts permanent. So the boehmite concentration must be low enough for silica to precipitate on more or less freely diffusing needles, which requires lower concentrations for longer particles. Once the needles are covered with silica, they are highly negatively charged and will repel1 each other. Thus the boehmite concentration in section 2.3 (found by trial and error) is a compromise between two oppositing tendencies, with the trend that longer rods require lower concentrations. The presence of silica on the particles is clear from the stability behavior, which is the same as for "ordinary" aqueous silica dispersions. For example, the silica rod dispersions are stable upon addition of ammonia, in contrast to the starting boehmite suspensions. From histograms of particle dimensions (Figure 4, Table 1)it follows that the thickness of the silica layer is about 6 nm for boehmite cores BU1 and 4.5 nm for BU2. This layer
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HU2
I3LJ1 sc
I
0
5
10
15
20
25
BT2SC
RU2SC
0
5
10
15 20
25
0
5
10
15
20
25
Diameter (nm) Figure 4. Histograms of particle diameters for boehmite cores BU1, BU2, BT2 and the corresponding thicker silica rods (suffbc sc), obtained from electron micrographs (500 counts for each histogram).
is much thicker than in previous work,15 where it could not be detected clearly with transmission electron microscopy. This is probably the reason why, in contrast to the previous work,15 the aqueous silica rod dispersions can be dialyzed to remove solublesilicates without gelation taking place. In case of a too thin silica layer, prolonged dialysis also removes part of the surface silica, leading to bare (positivelycharged)boehmite spots, which may cause heteroflocculation with (negatively charged) silica particles. It should be noted that dialysis is an essential step in the preparation of surface-modified rods and also for further silica growth in ethanol. Soluble(sodium) silicates in an unsufficiently dialyzed aqueous silica dispersion easily induce flocculation when e.g. ethanol is added.15 The seeding of the Stober silica synthesis in ethanol with the present silica rods yields stable, nonaggregated dispersions of particles with a morphology as shown in Figure 5. The thick rods occasionally have protrusions. It is not clear where they come from. Possibly they are due to small boehmite fragments which are hardly noticeable in the starting boehmite dispersion, but which now manifest themselves as nuclei for silica growth. The silica rods in water have a more regular appearance and a higher aspect ratio, which is why octadecyl-coatedrods were prepared starting from aqueous dispersions. 3.3. Silane-ModifiedSilica Rods. The TPM-coated silica rods form stable, charge-stabilized dispersions in ethanol, just as their spherical equivalents.' Adsorption of TPM also improves the stability of the fluorescent silica rods. These rods are still fluorescent after removal of free dye molecules in the dispersion (see Figure 6), which clearly indicates that the dye (linked to the silane APS) is incorporated in the silica shell of the rods, in the same manner as for fluorescent silica spheres.8 APS is not an effective stabilizer for the rods; slow sedimentation indicated (weak) net attraction, which is sufficiently screened by an additional TPM coating. The resulting rods could be used for light scattering and (self) diffusion studies, taking advantage of the fluorescent labeling. 3.4. Octadecyl-CoatedSilica Rods. Single-Particle
Figure 5. An example of silica rods formed by the seeding of the Stober synthesis with silica-coveredboehmite needles. Bar is 1pm.
Properties. The rods, shown in Figure 7, are covered with a dense brush of cl8 chains just as &-grafted silica spheres.6 This follows from the elemental analysis results in Table 2. The rods contain the following compounds
Al00H; SiO,; -OC,,H,,;
H,O;impurities
where H20 also includes any silanol groups. Impurities ( -= 1% (w/w))such as Na will be neglected. From elemental analysis and mass densities (see Table 2) we calculate the number of C18chains and the volume u of bare, ungrafted rods per gram of grafted material. Converting u to a bare rod surface s = 4u/D,with D the average TEM diameter, chain number densities in the range 4-9 nm-2 are found (see Table 3). This is a high surface coverage, but we have to bear in mind that the actual available surface area for grafting is larger than s because of small surface irregularities. Anyhow, the chain densities equal values reported for grafted silica spheres,6 which indicates that rods and spheres are covered with a similar CIS brush.
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4456 Langmuir, Vol. 10, No. 12, 1994
Table 3. Surface-Grafting Densities system
Figure 6. Scattering of blue laser light (1 = 488 nm) by a dispersion of TPM-coated silica rods (left). Rods containing a fluorescent dye produce in addition a greenish-yellow fluorescence (right).
BUlSC
BU2SC
BT2SC
densities dispersions at rest are optically isotropic. A weak, rapidly decaying streaming birefringence is observed upon shaking. Charged silica rods or boehmite rods at these concentrations are either permanently birefringent or exhibit a strong, slowly decaying flow birefringence. The difference is due to the fact that the Cis-coated rods in cyclohexane are uncharged. The interaction range is much smaller than for rods surrounded by electrical double layers. So oriented hard rods decay more rapidly to the isotropic state. Cyclohexane has been chosen as a solvent, because it is a good solvent for octadecyl chains and because its refractive index nearly matches that of the octadecylcoated silica surface layer. This reduces the van der Waals attraction between the rods, leaving a steep hard-rod repulsion (resulting from interprenetation of octadecyl layers) as the dominant interaction force, analogous to octadecyl-coated hard-sphere silica dispersions.6 However, the analogy should be made with care, because the van der Waals attractions strongly depend on the rod orientations. Estimates of the attractions, however, indeed show that they are fairly weak. The attraction energy V is strongest for the parallel orientation of two (cylindrical) rods, for which16
y, = -ALa124H312fi where A is the Hamaker constant and H is the surfaceto-surface distance between rods of diameterD and length L. For crossed rods the energy is
v,
Figure 7. Typical transmission micrograph of randomly deposited octadecyl-coated silica rods BUBSC on a grid dipped into a dispersion in cyclohexane. Bar length is 0.1 pm.
C
boehmite cores BU1 BU2 2.0 BT2 silica rods BUlSC 14.6 BU2SC 25.46 BT2SC 20.0 ,
H
0
Si
Al
Na
mass density” 13/gcm-~ 3.01b 3.01
2.9
55.4
1.11 38.8
3.69 43.5 13.8 5.36 38.6c 15.5 4.21 42.7c 21.4
First consider bare, smooth silica cylinders with the dimensions of an average BU2SC particle (L = 215 nm, D = 12.9 nm). The Hamaker constant for silica in cyclohexane is estimated by Jansen17to beA = 0.15kTat T = 298 K. So the attraction energies are
VL
Table 2. Particle Properties elemental analysis w/w %
22.4 0.81 1.86(1.8) 15.1 1.59 (1.5) 11.7 1.64 (1.6)
a Values between parentheses are calculated from elemental analysis using mass densities of silica (2.2))boehmite (3.01))water (1.0)) and stearyl alcohol (0.81). Literature value.3 e Calculated values.
The mass densities of the rods are lower than the values for silica and boehmite, as can be seen in Table 2. This is because of the relative large volume of the low-density alkane layer. When mass densities are calculated from elemental compositions, measured values are indeed reproduced. Isotropic Dispersions in Cyclohexane. After the final sedimentation-redispersion steps in the surface modificationprocedure, dispersions in cyclohexane are obtained with typically a few volume percent of rods. At these
= -ADl12H
-3.4kT/H3I2;
VL x -0.16kTIH
(3)
with H expressed in nm. For rods which are decorated with an octadecyl layer with a thickness of about 1.5 nm, contact attractions at the minimal surface-to-surface distance H x 3 nm are
V,,
-0.7kT;
VL zz -0.05kT
These numbers are rough estimates, but nevertheless it is clear that rods in crossed orientations will not “stick”, whereas the parallel contact attraction between rods of a few hundred nm long may be of order IZT. However, parallel encounters with maximal contact area are rare in an isotropic rod dispersion. The majority of collisions will occur in more or less crossed orientations with in this case little influence of van der Waals attractions. So silica rods in cyclohexane will mimic a hard-rod dispersion. (It should be noted that the rod surface is somewhat irregular, see Figure 7. These irregularities decrease contact areas between rods and thus also van der Waals attractions.) (16)Israelachvili,J.N. Intermolecular and Surface Forces;Academic Press: London, 1985. (17) Jansen, J. W.; de Kruif, C. G.; Vrij, A. J. Colloid Interface Sci. 1986,114,471.
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I
Figure 8. Top: permanent birefringent sediments of silica rods BUlSC (left) and BUBSC (right) in cyclohexane,viewed between crossed polarizer's illuminated from behind. (Isotropicstructures appear black. The light spot outside the tubes is due to imperfection of the polarization filter. The colors in the tube are caused by birefringence.) In the right sediment a magnetic stirrer is present which has produced a flow pattern, which does not decay. Bottom: the BUlSC sediment with a rod volume fraction of 4 = 0.316 is also fairly translucent for unpolarized light (left). The turbidity of a dilute BUlSC dispersion (4 < 0.01) in cyclohexane (right) is higher for thermodynamic reasons explained in the text.
In practice one should nevertheless always be aware of attractions, in particular for Cia-coated rods with a thin silicalayer, which does not completely screen the relatively large attraction between the boehmite cores with a large Hamaker constant. Anisotropic Sediments. Here we discuss the consequences of the centrifugation of silica rods in cyclohexane to nondensifying sediments which have intriguing properties. For example, all sediments are quite translucent and have a lower turbidity than dilute dispersions (see Figure 8). This .clearly is not an optical matching effect as the refractive index difference between particles and solvent is in all cases the same. The low turbidity very
likely has a thermodynamic origin and can qualitatively be explained with recent work of Bolhuis and Lekkerkerkerls on light scattering properties of hard rod dispersions. These authors calculatezero-anglescattering intensities I( O=O), which is for small enough rods roughly proportional to the turbidity of a dispersion. They show that at low volume fractions I( O=O) increases with concentration, whereas a t sufficiently high densities I( O=O) decreases. This decrease is due to the strong damping of concentration (18)Bolhuis, P. G.; Lekkerkerker, H. N. W. Physica A 1993, 196, 375.
Philipse et al.
4458 Langmuir, Vol. 10, No. 12, 1994 Table 4. Sediment Volume Fractions system @I%
LID
BUlSC 31.6 4.6
BU2SC 27 16.7
BT2SC 25 28.6
fluctuations in dense systems of repulsive particles. The intensity versus concentration curve depends markedly on the polydispersity in rod diameter (much less on the length). Reference 18presents calculations for an average aspect ratio LID = 5 and a polydispersity OD = 0.2, which happen to be values for BUlSC rods. The intensity maximum is found at 4 0.1, thus the sediment density 4 = 0.32 for BUlSC is clearly in the region where the intensity (and the turbidity) is suppressed. For larger aspect ratios, not discussed in ref 18, the rods will interact more strongly at a given volume fraction, shifting the intensity (and the turbidity) maximum (probably only slightly) to lower densities. This accounts for the observation that also sediments of the high aspect ratio rods BU2SC and BT2SC are fairly transparent. Bolhuis and LekkerkerkerlBtreat isotropic dispersions, which makes detailed comparison with our silica rod sediments difficult, because they contain permanently anisotropic regions. This follows from the optical birefringence observed between crossed polarizers (see Figure 8). The anisotropy is probably not due to an isotropicnematic phase transition. An isotropic high aspect ratio hard-rod dispersion is expected to become instable near 3.3DIL,l which is below the sediment volume fractions for the longer particles BU2SC and BT2SC (see Table 4). But we observe no coexistence with an isotropic phase. Moreover, the sediments are extremely viscous, especially for the high aspect ratios, requiring laborious stirring to redisperse them. Phase transitions will be very slow in such systems, necessitating very slow densification of dispersions t o avoid kinetical arrest of particles in an immobile microstructure. (This is liable to happen for high LID, see later discussion.) To explain the anisotropy, one could argue that the sedimenting rods are randomly deposited such that their main axes lay in horizontal planes just as in the case of deposition on a microscope grid (Figure 7). These planes indeed can account for the birefringence and also for the observation that the intensity of the birefringence depends on the angle between the sedimentation tubes and polarization of the incident light. However, in this microstructure the relevant length scale for any kind of diffraction is the thickness of a rod layer, thus the rod diameter, which is much too small to produce any diffraction colors. Since the color seen in transmission is often red, one could also argue that it is due to singleparticle Rayleigh scattering, as in the case of a sunset. However, other colors are observed as well (see Figure 8). Moreover, streaming birefringence in less concentrated systems displays similar color effects, just as in dialyzed dispersions in water. These latter effects are certainly due to flow-induced ordering. We speculate that in the sediments similar nematic domains are present, possibly in an isotropic matrix, with a preferred orientation parallel to the centrifugal force, instead of parallel to flow lines in stirred dispersions. Such a microstructure is consistent with the observations. With respect to the origin of this structure we note that any (temporarily)nematic domains in a dispersion will sediment very rapidly compared with single rods, with a minimum of friction if the longest axis of the domain is moving parallel to the sedimentation velocity. These domains can also be formed near the sediment interface where the concentration rapidly increases.
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The idea that sediments mainly grow by the rapid settlement of small domains, somewhere formed in the concentration profile, could be tested in principle as follows. Suppose that in an analytical ultracentrifuge sedimentation velocities are determined as a function of rod volume fraction 4. At low 4 one measures a single (free-particle) velocity. Any domain formation a t large enough 4 will envelope an additional, rapid peak in the refractive index profile. Clearly also scanning electronmicrographs of dried sediments would be useful. Preliminary SEM pictures of dried BUlSC sediments suggest anisotropic structures at least on a level of several micrometers; the available resolution was not suficient to view the required submicrometer details, however. Finally we consider the fact that the sediment densities are fairly low as compared to sphere packings (see Table 4). When rods are elongated a t a given sediment volume fraction, the number of rod-rod contacts increases, leading to a less mobile microstructure. The efficientdensification of the structure by sidewise translations will be frustrated. Instead, rod centers have to be concentrated mainly by lengthwise rod movements, which are very inefficient for that purpose, because in an isotropic system these movements have (by definition) random orientation. So high aspect ratio rods are indeed expected to form an incompressible sediment at low concentrations. We are not aware ofrelevant theory on this point. There are, however, early computer simulations ofVoldlgon the deposition of randomly oriented attractive rods, which irreversibly cohere to each other on initial contact. She indeed finds a decrease in sediment density with increasing particle aspect ratio. The densities for the strongly attracting rods in the simulations are an order of magnitude smaller than for the CIS-coated rods, which once more confirms the absence of significant attractions in our dispersions. 4. Conclusions
A method has been developed for the preparation of charged (fluorescent) silica rods in ethanol and sterically stabilized rods in cyclohexane by the seeded growth of silica on boehmite cores in water, followed by various chemical surface modifications in organic solvents. The final dispersions are stable against aggregation. Rods grafted with octadecylalcohol appear to have a dense alkane layer on their surface, which screens the van der Waals attractions. Dispersions of these particles mimic an isotropic hard-rod system. Translucent sediments illustrate the suppressing of turbidity by interparticle interactions. The permanent birefringence of the sediments is ascribed to the sedimentation of small nematic domains, and the low sediment densities manifest kinetical arrest of rod motions.
Acknowledgment. We thank the following persons for their contributions: Paul Buining and Luis Liz Marzan (synthesis), Pim van Maurik (TEM), J a n den Boesterd (photographs and drawings), Maarten Terlou (IBAS), Professor Henk Lekkerkerker for his stimulating interest and discussions, and Marina Uit de Bulten and Toni Vos for typing the manuscript. The stay of one of the authors (A.M.N.) at the Van’t Hoff Laboratory was made possible by a donation of DSM (Geleen, The Netherlands). (19)Vold, M. J.Phys. Chem. 1969, 63, 1608.
