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The effect of mixing alkoxides on the Stöber particles size
Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255
The effect of mixing alkoxides on the St¨ober particles size F. Branda ∗ , B. Silvestri, G. Luciani, A. Costantini Dipartimento di Ingegneria dei Materiali e della Produzione, Piazzale Tecchio, 80-80125 Napoli, Italy Received 10 July 2006; received in revised form 7 November 2006; accepted 28 November 2006 Available online 2 December 2006
Abstract Submicrometric particles were obtained, through the St¨ober method, from alcoholic (ethanol) solutions of tetraethoxysilane (TEOS) and 3aminopropyl-triethoxysilane (APTS), at total alcoxide concentration 0.15 M, in the presence of ammonia (0.1 M) and water (8.8 M) and fluorescein sodium (0.40 mM). Nano-sized particles were obtained from solutions 0.15 M of TEOS or APTS. A surprising particle size increase was observed when mixing TEOS and APTS at constant overall alkoxide content (0.15 M). The result was explained, on the basis of the known aggregation model, by taking into account that the substitution of TEOS to APTS and vice-versa changes the ratio of nucleation and aggregation rates and that nuclei of different size are expected to form when the alkoxides are mixed. The mixing appears to change the evolution of particle population favouring the establishment of a size distribution wherein the dominant aggregation event can be between the freshly generated nuclei and large aggregates, which is the faster aggregation event. The closer is the TEOS/APTS ratio to 1 the sooner particles size differentiation would occur and the fewer but greater particles would form, in short times. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; St¨ober method; Tetraethoxysilane (TEOS); 3-Aminopropyl-triethoxysilane (APTS)
1. Introduction As it is known [1,2] colloidal particles with well-defined size and shape and with narrow size distribution can be produced by means of St¨ober method by hydrolysing tetraethoxysilane (TEOS) in water/alcohol/ammonia mixtures. The method opens the possibility to produce organically modified particles by introducing compounds of the type Rn Si(OR )4−n (n = 1–3; R and R = alkyls) into the starting mixture. The Si–C exhibits, in fact, a high hydrolytic as well as thermal stability. Modified silicas are of interest in many fields of chemistry. Organic compounds [3] and metal ions [4] can, so, be selectively absorbed. This is valuable for metal complexes immobilization for use as catalyst centers [5]. In the biochemical field enzymes can be immobilized [6]. Nano-structured composites of improved interface properties can be produced [7,8]. 3-Aminopropyltriethoxysilane (APTS) is one of the more interesting such modifying compounds. Fluorescein sodium (C20 H10 O5 Na2 ) is useful to produce fluorescent particles.
∗
Corresponding author. E-mail address: [email protected] (F. Branda).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.11.048
Particle formation and growth was recently, successfully, interpreted [9–12] on the basis of an aggregation model, which neglects monomer addition to the primary particles (nuclei) formed. In fact solids are generated from a homogeneous liquid in a process referred to as nucleation. Once formed the solid particles increase in size by molecular addition, where soluble species deposit on the solid surface, or by aggregation with other solid particles. In the aggregation model [10] particles are assumed to be colloidally unstable and to grow solely by aggregation. As it is known [11] silica nanoparticles of size lower than 100 nm can be obtained from alcoholic solutions of TEOS (0.15 M) ammonia (0.1 M) and water (8.8 M). In this paper the effect of the equimolar substitution of tetraethoxysilane (TEOS) with 3-aminopropyl-triethoxysilane (APTS) on the final particle size was studied and interpreted on the basis of the aggregation model. 2. Experimental Silica particles were prepared from mixtures of two alkoxides: tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTS) differing for the substitution of an
F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255
253
Fig. 1. SEM micrograph of samples: (a) xAPTS = 0, after 2 h; (b) xAPTS = 1, after 2 h.
ethoxy group with the non-hydrolysable 3-aminopropyl one. They were prepared by dissolving TEOS and APTS in ethanol in weight ratios corresponding to mixtures of TEOS and APTS at APTS molar fractions, xAPTS : 0, .05, .10, .30, .50, .70, .90, .95, 1. The total alcoxide amount was, in all cases, taken constant at 0.15 M in ethanol. Ammonia and, at last, water were added in concentration 0.1 and 8.8 M, respectively. Fluorescein sodium (C20 H10 O5 Na2 ) in a constant very low concentration (0.40 mM) was also present to have fluorescent particles. Scanning electron microscopy (SEM) was performed through a Leica Stereoscan 440 microscope Oxford Instruments. All sampling for SEM observation was done by dipping thin glassy slides into the suspension. 3. Results Fig. 2. SEM micrograph of sample xAPTS = 0.10, after 2 h.
When one type of alkoxide was used nanoparticles of diameter less than 100 nm were obtained as shown by the micrograph of Fig. 1a and b, relative, respectively, to the sample xAPTS : 0, 1. A similar result is reported in the literature for the xAPTS = 0 sample [11]. The consequence is that the colloidal suspensions appeared, after 2 h, transparent. When substituting a little part of TEOS or APTS with the other alkoxide (.05 ≤ xAPTS ≤ .95) a milky suspension was obtained in short times. In Fig. 2 the micrograph of xAPTS = 0.10 is reported. In Fig. 3 the particles formed after 15 min during
preparation of xAPTS = 0.50 are shown. Fig. 3b is a particular of Fig. 3a. The micrographs of Figs. 2 and 3 confirm that particles of size comparable to the light wavelength were rapidly formed. The times at which transparency began to be lost, ti , and a completely milky aspect was obtained, tf , were recorded and plotted in Fig. 4. Fig. 4 suggests that the rate of formation of submicro-sized particles changes with xAPTS . The curves are not symmetric: the effect of the substitution of TEOS to APTS is more dramatic than the reverse substitution.
Fig. 3. SEM micrograph of sample xAPTS = 0.50, after 15 min (a); a particular (b).
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F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255
event is between the freshly generated nucleus and a large aggregate. In fact the aggregation rate for large/large pairs was shown [11] to be slower than for small/small or (better) small/large pairs. The nanometric size of the particles obtained both when xAPTS = 0 and when xAPTS = 1, involves that, to justify the dramatic size increase recorded when the two alkoxides are mixed (.05 ≤ xAPTS ≤ .95), we must look for something just linked to the mixing. When mixing alkoxides we can speculate that:
Fig. 4. Times at opalescence appearing, ti (), and time to have milky aspect, tf (), vs. xAPTS (mole of APTS/total alcoxide moles).
4. Discussion The outline of particle formation in the aggregation model is reported in Fig. 5. The alcoxide hydrolysis and polycondensation reactions reported in Fig. 5 are well known [13]. The result is the formation of solid particles from a homogeneous liquid in a process referred to as nucleation. When changing the alkoxide or the nature of the alcoholic medium, differences in the nuclei sizes were observed; they were attributed [12,14] to differences in thermodynamic interactions between the solvent and the hydrolyzed intermediates. Once formed, the solid particles may increase in size by molecular addition, where soluble species deposit on the solid surface, or by aggregation with other solid particles. In the aggregation model, particles are assumed [10,11] to be colloidally unstable and to grow solely by aggregation. It was shown [11] that the final size depends on the balance of nucleation and aggregation rates and that systems giving the largest particles are characterized, after a short transient, by a size distribution wherein the dominant aggregation
(1) Owing to different thermodynamic interactions, a different nuclei size can be expected [12,14] in the hydrolysis and poly-condensation of TEOS and APTS. In the case of mixed alkoxides the nuclei size should depend on the ratio of TEOS and APTS hydrolized units they are made of. Therefore, when xAPTS = 0 or 1, nuclei of different size are, also, expected to form: the nuclei size distribution may be expected to be the widest the closer is xAPTS to 0.5. (2) Owing to differences in the hydrolysis and polycondensation rates of TEOS and APTS the substitution of TEOS to APTS and viceversa is expected to change the ratio of nucleation and aggregation rates. By this way the experimental results can be explained. The result of effect 1 may be to change the evolution of particle population: the particles size differentiation would occur sooner. This would speed up the establishment of a size distribution wherein the dominant event would be the aggregation between the freshly generated nuclei and large aggregates, which is recognised to be the faster aggregation event [11]. The expected result is just that the closer is xAPTS to 0.5 the fewer but greater particles are formed, in short times. Nevertheless the effect of the substitution of TEOS to APTS and vice-versa on the ratio of nucleation and aggregation rates are expected to be opposite. Therefore effect (2) could explain the asymmetry of the curves of Fig. 4. 5. Conclusions Nano-sized particles were obtained from solutions 0.15 M of TEOS or APTS in ethanol in the presence of ammonia (0.1 M) and water (8.8 M) and Fluorescein sodium (0.40 mM). A surprising particle size increase was observed when mixing TEOS and APTS at constant overall alkoxide content (0.15 M). The result was explained on the basis of the aggregation model. The substitution of TEOS to APTS and vice-versa changes the ratio of nucleation and aggregation rates. More than all, however, the mixing appears to change the evolution of particle population. The closer is the TEOS/APTS ratio to 1 the sooner particles size differentiation would occur and the fewer but greater particles would form, in short times. References
Fig. 5. Flow sheet of the process.
[1] W. St¨ober, A. Fink, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69.
F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255 [2] G.H. Bogush, M.A. Tracy, C.F. Zukosky, Preparation of monodisperse silica particles: control of size and mass fraction, J. Non-Cryst. Solids 104 (1988) 95–106. [3] B. Porsch, J. Kratka, Chromatographic stability of silica-based aminopropyl-bonded stationary phases, J. Chromatogr. 543 (1991) 1–7. [4] A.M. Klonkowski, C.W. Schlaepfer, Cu(II) complexes in organically modified silicate gels, J. Non-Cryst. Sol. 129 (1991) 101–108. [5] D.E. Leyden, in: D.E. Leyden, W.T. Collins (Eds.), Silylated Surfaces, Gordon & Breach, London, 1980. [6] K.M.R. Kallury, W.E. Lee, M. Thompson, Enhanced stability of urease immobilised onto phospholipid covalently bound to silica, tungsten and fluoropolymer surfaces, Anal. Chem. 65 (1993) 2459–2467. [7] E.P. Pluddeman, Silane Coupling Agents, Plenum Press, New York, 1991. [8] S. Kang, S. Hong, C.R. Choe, M. Park, S. Rim, J. Kim, Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process, Polymer 42 (2001) 879– 887.
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[9] G.H. Bogush, C.F. Zukosky, Studies of the kinetics of the precipitation of uniform silica particles through the hydrolysis and condensation of silicon alkoxides, J. Colloid Interface Sci. 142 (1991) 18. [10] G.H. Bogush, C.F. Zukosky, Uniform silica particle precipitation: an aggregation growth model, J. Colloid Interface Sci. 142 (1991) 19–34. [11] K. Lee, A.N. Sathyagal, A.V. McCormick, A closer look at an aggregation model of the St¨ober process, Colloids Surf. A: Physicochem. Eng. Aspect. 144 (1998) 115–125. [12] D.L. Green, S. Jayasundara, Y.F. Lam, M.T. Harris, Chemical reaction kinetics leading to first St¨ober silica nanoparticles-NMR and SAXS investigation, J. Non-Cryst. Solids 315 (2003) 166–179. [13] C.J. Brinker, in: C.J. Brinker, G.W. Sherer (Eds.), Sol–Gel Science: the Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, 1990. [14] S. Sadasivan, A.K. Dubey, Y. Li, D.H. Rasmussen, Alcoholic solvent effect on silica synthesis—NMR and DLS investigation, J. Sol–Gel Sci. Technol. 12 (1998) 5–14.
The effect of mixing alkoxides on the St¨ober particles size F. Branda ∗ , B. Silvestri, G. Luciani, A. Costantini Dipartimento di Ingegneria dei Materiali e della Produzione, Piazzale Tecchio, 80-80125 Napoli, Italy Received 10 July 2006; received in revised form 7 November 2006; accepted 28 November 2006 Available online 2 December 2006
Abstract Submicrometric particles were obtained, through the St¨ober method, from alcoholic (ethanol) solutions of tetraethoxysilane (TEOS) and 3aminopropyl-triethoxysilane (APTS), at total alcoxide concentration 0.15 M, in the presence of ammonia (0.1 M) and water (8.8 M) and fluorescein sodium (0.40 mM). Nano-sized particles were obtained from solutions 0.15 M of TEOS or APTS. A surprising particle size increase was observed when mixing TEOS and APTS at constant overall alkoxide content (0.15 M). The result was explained, on the basis of the known aggregation model, by taking into account that the substitution of TEOS to APTS and vice-versa changes the ratio of nucleation and aggregation rates and that nuclei of different size are expected to form when the alkoxides are mixed. The mixing appears to change the evolution of particle population favouring the establishment of a size distribution wherein the dominant aggregation event can be between the freshly generated nuclei and large aggregates, which is the faster aggregation event. The closer is the TEOS/APTS ratio to 1 the sooner particles size differentiation would occur and the fewer but greater particles would form, in short times. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; St¨ober method; Tetraethoxysilane (TEOS); 3-Aminopropyl-triethoxysilane (APTS)
1. Introduction As it is known [1,2] colloidal particles with well-defined size and shape and with narrow size distribution can be produced by means of St¨ober method by hydrolysing tetraethoxysilane (TEOS) in water/alcohol/ammonia mixtures. The method opens the possibility to produce organically modified particles by introducing compounds of the type Rn Si(OR )4−n (n = 1–3; R and R = alkyls) into the starting mixture. The Si–C exhibits, in fact, a high hydrolytic as well as thermal stability. Modified silicas are of interest in many fields of chemistry. Organic compounds [3] and metal ions [4] can, so, be selectively absorbed. This is valuable for metal complexes immobilization for use as catalyst centers [5]. In the biochemical field enzymes can be immobilized [6]. Nano-structured composites of improved interface properties can be produced [7,8]. 3-Aminopropyltriethoxysilane (APTS) is one of the more interesting such modifying compounds. Fluorescein sodium (C20 H10 O5 Na2 ) is useful to produce fluorescent particles.
∗
Corresponding author. E-mail address: [email protected] (F. Branda).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.11.048
Particle formation and growth was recently, successfully, interpreted [9–12] on the basis of an aggregation model, which neglects monomer addition to the primary particles (nuclei) formed. In fact solids are generated from a homogeneous liquid in a process referred to as nucleation. Once formed the solid particles increase in size by molecular addition, where soluble species deposit on the solid surface, or by aggregation with other solid particles. In the aggregation model [10] particles are assumed to be colloidally unstable and to grow solely by aggregation. As it is known [11] silica nanoparticles of size lower than 100 nm can be obtained from alcoholic solutions of TEOS (0.15 M) ammonia (0.1 M) and water (8.8 M). In this paper the effect of the equimolar substitution of tetraethoxysilane (TEOS) with 3-aminopropyl-triethoxysilane (APTS) on the final particle size was studied and interpreted on the basis of the aggregation model. 2. Experimental Silica particles were prepared from mixtures of two alkoxides: tetraethoxysilane (TEOS) and 3-aminopropyltriethoxysilane (APTS) differing for the substitution of an
F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255
253
Fig. 1. SEM micrograph of samples: (a) xAPTS = 0, after 2 h; (b) xAPTS = 1, after 2 h.
ethoxy group with the non-hydrolysable 3-aminopropyl one. They were prepared by dissolving TEOS and APTS in ethanol in weight ratios corresponding to mixtures of TEOS and APTS at APTS molar fractions, xAPTS : 0, .05, .10, .30, .50, .70, .90, .95, 1. The total alcoxide amount was, in all cases, taken constant at 0.15 M in ethanol. Ammonia and, at last, water were added in concentration 0.1 and 8.8 M, respectively. Fluorescein sodium (C20 H10 O5 Na2 ) in a constant very low concentration (0.40 mM) was also present to have fluorescent particles. Scanning electron microscopy (SEM) was performed through a Leica Stereoscan 440 microscope Oxford Instruments. All sampling for SEM observation was done by dipping thin glassy slides into the suspension. 3. Results Fig. 2. SEM micrograph of sample xAPTS = 0.10, after 2 h.
When one type of alkoxide was used nanoparticles of diameter less than 100 nm were obtained as shown by the micrograph of Fig. 1a and b, relative, respectively, to the sample xAPTS : 0, 1. A similar result is reported in the literature for the xAPTS = 0 sample [11]. The consequence is that the colloidal suspensions appeared, after 2 h, transparent. When substituting a little part of TEOS or APTS with the other alkoxide (.05 ≤ xAPTS ≤ .95) a milky suspension was obtained in short times. In Fig. 2 the micrograph of xAPTS = 0.10 is reported. In Fig. 3 the particles formed after 15 min during
preparation of xAPTS = 0.50 are shown. Fig. 3b is a particular of Fig. 3a. The micrographs of Figs. 2 and 3 confirm that particles of size comparable to the light wavelength were rapidly formed. The times at which transparency began to be lost, ti , and a completely milky aspect was obtained, tf , were recorded and plotted in Fig. 4. Fig. 4 suggests that the rate of formation of submicro-sized particles changes with xAPTS . The curves are not symmetric: the effect of the substitution of TEOS to APTS is more dramatic than the reverse substitution.
Fig. 3. SEM micrograph of sample xAPTS = 0.50, after 15 min (a); a particular (b).
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F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255
event is between the freshly generated nucleus and a large aggregate. In fact the aggregation rate for large/large pairs was shown [11] to be slower than for small/small or (better) small/large pairs. The nanometric size of the particles obtained both when xAPTS = 0 and when xAPTS = 1, involves that, to justify the dramatic size increase recorded when the two alkoxides are mixed (.05 ≤ xAPTS ≤ .95), we must look for something just linked to the mixing. When mixing alkoxides we can speculate that:
Fig. 4. Times at opalescence appearing, ti (), and time to have milky aspect, tf (), vs. xAPTS (mole of APTS/total alcoxide moles).
4. Discussion The outline of particle formation in the aggregation model is reported in Fig. 5. The alcoxide hydrolysis and polycondensation reactions reported in Fig. 5 are well known [13]. The result is the formation of solid particles from a homogeneous liquid in a process referred to as nucleation. When changing the alkoxide or the nature of the alcoholic medium, differences in the nuclei sizes were observed; they were attributed [12,14] to differences in thermodynamic interactions between the solvent and the hydrolyzed intermediates. Once formed, the solid particles may increase in size by molecular addition, where soluble species deposit on the solid surface, or by aggregation with other solid particles. In the aggregation model, particles are assumed [10,11] to be colloidally unstable and to grow solely by aggregation. It was shown [11] that the final size depends on the balance of nucleation and aggregation rates and that systems giving the largest particles are characterized, after a short transient, by a size distribution wherein the dominant aggregation
(1) Owing to different thermodynamic interactions, a different nuclei size can be expected [12,14] in the hydrolysis and poly-condensation of TEOS and APTS. In the case of mixed alkoxides the nuclei size should depend on the ratio of TEOS and APTS hydrolized units they are made of. Therefore, when xAPTS = 0 or 1, nuclei of different size are, also, expected to form: the nuclei size distribution may be expected to be the widest the closer is xAPTS to 0.5. (2) Owing to differences in the hydrolysis and polycondensation rates of TEOS and APTS the substitution of TEOS to APTS and viceversa is expected to change the ratio of nucleation and aggregation rates. By this way the experimental results can be explained. The result of effect 1 may be to change the evolution of particle population: the particles size differentiation would occur sooner. This would speed up the establishment of a size distribution wherein the dominant event would be the aggregation between the freshly generated nuclei and large aggregates, which is recognised to be the faster aggregation event [11]. The expected result is just that the closer is xAPTS to 0.5 the fewer but greater particles are formed, in short times. Nevertheless the effect of the substitution of TEOS to APTS and vice-versa on the ratio of nucleation and aggregation rates are expected to be opposite. Therefore effect (2) could explain the asymmetry of the curves of Fig. 4. 5. Conclusions Nano-sized particles were obtained from solutions 0.15 M of TEOS or APTS in ethanol in the presence of ammonia (0.1 M) and water (8.8 M) and Fluorescein sodium (0.40 mM). A surprising particle size increase was observed when mixing TEOS and APTS at constant overall alkoxide content (0.15 M). The result was explained on the basis of the aggregation model. The substitution of TEOS to APTS and vice-versa changes the ratio of nucleation and aggregation rates. More than all, however, the mixing appears to change the evolution of particle population. The closer is the TEOS/APTS ratio to 1 the sooner particles size differentiation would occur and the fewer but greater particles would form, in short times. References
Fig. 5. Flow sheet of the process.
[1] W. St¨ober, A. Fink, Controlled growth of monodisperse silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62–69.
F. Branda et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 299 (2007) 252–255 [2] G.H. Bogush, M.A. Tracy, C.F. Zukosky, Preparation of monodisperse silica particles: control of size and mass fraction, J. Non-Cryst. Solids 104 (1988) 95–106. [3] B. Porsch, J. Kratka, Chromatographic stability of silica-based aminopropyl-bonded stationary phases, J. Chromatogr. 543 (1991) 1–7. [4] A.M. Klonkowski, C.W. Schlaepfer, Cu(II) complexes in organically modified silicate gels, J. Non-Cryst. Sol. 129 (1991) 101–108. [5] D.E. Leyden, in: D.E. Leyden, W.T. Collins (Eds.), Silylated Surfaces, Gordon & Breach, London, 1980. [6] K.M.R. Kallury, W.E. Lee, M. Thompson, Enhanced stability of urease immobilised onto phospholipid covalently bound to silica, tungsten and fluoropolymer surfaces, Anal. Chem. 65 (1993) 2459–2467. [7] E.P. Pluddeman, Silane Coupling Agents, Plenum Press, New York, 1991. [8] S. Kang, S. Hong, C.R. Choe, M. Park, S. Rim, J. Kim, Preparation and characterization of epoxy composites filled with functionalized nanosilica particles obtained via sol–gel process, Polymer 42 (2001) 879– 887.
255
[9] G.H. Bogush, C.F. Zukosky, Studies of the kinetics of the precipitation of uniform silica particles through the hydrolysis and condensation of silicon alkoxides, J. Colloid Interface Sci. 142 (1991) 18. [10] G.H. Bogush, C.F. Zukosky, Uniform silica particle precipitation: an aggregation growth model, J. Colloid Interface Sci. 142 (1991) 19–34. [11] K. Lee, A.N. Sathyagal, A.V. McCormick, A closer look at an aggregation model of the St¨ober process, Colloids Surf. A: Physicochem. Eng. Aspect. 144 (1998) 115–125. [12] D.L. Green, S. Jayasundara, Y.F. Lam, M.T. Harris, Chemical reaction kinetics leading to first St¨ober silica nanoparticles-NMR and SAXS investigation, J. Non-Cryst. Solids 315 (2003) 166–179. [13] C.J. Brinker, in: C.J. Brinker, G.W. Sherer (Eds.), Sol–Gel Science: the Physics and Chemistry of Sol–Gel Processing, Academic Press, Boston, 1990. [14] S. Sadasivan, A.K. Dubey, Y. Li, D.H. Rasmussen, Alcoholic solvent effect on silica synthesis—NMR and DLS investigation, J. Sol–Gel Sci. Technol. 12 (1998) 5–14.