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Effects of seeds concentration on the formation of colloidal silica
Materials Science and Engineering B 123 (2005) 238–241
Effects of seeds concentration on the formation of colloidal silica Ming-Shyong Tsai ∗ , Chien-Hsin Yang, Po-Yuan Huang The Department of Chemical and Material Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan, ROC Received 3 February 2005; received in revised form 8 July 2005; accepted 9 August 2005
Abstract The effects of seed concentration on the mechanism of colloidal silica formation via sodium silicate were studied in this article. When the seed concentration is less than 0.62 wt%, the mechanism of particle formation is dominated by the homogenous nucleation of the active silicate acid. Increasing the seed concentration increases the importance of heterogeneous nucleation via surface growth on seeds. In the seed concentration range of 0.62–1.36 wt%, the formation mechanism is homogenous nucleation parallel to surface growth. Two kinds of particles, one has a small particle size and the other has a large particle size, were observed by transmission electron microscope pictures. A maximum mean particle size of the final product is observed at 0.74 wt% seed concentration. When the seed concentration is in the range of 1.36–2.48 wt%, the mechanism of particle formation is surface growth. The final particles are uniform in diameter. The particles are aggregated and gelled when the seed concentration is greater than 2.48 wt%. The particle size of the final colloidal silica was estimated by the assumption that the additional silica of added as active silicic acid grows on the seeds. The results are compared with the particle size measurement from dynamic light scattering (DLS). © 2005 Elsevier B.V. All rights reserved. Keywords: Colloidal silica; Sodium silica; Particle formation
1. Introduction Recently, colloidal silica has been applied in many modern industries, e.g. as binder for inorganic paint, stiffener for hard coating reagent and especially as abrasive particles for chemical mechanical polishing slurries [1,2]. St¨ober and Rang [3] developed the famous process of synthesizing mono-disperse silica from tetraethylorthosilicate (TEOS), ethanol, ammonium hydroxide and water as raw materials. Coenen and De Kruif [4] used the St¨ober process modified it by introducing commercial colloidal silica (Dupont, Ludox) as seeds to synthesize the colloidal silica particles via a surface growth process. The particle size of the resulting silica colloids was estimated according to the assumption of surface growth and mass balance in the formation process. Chen et al. [5] indicated some of the conditions of new particle formation. But the details of the results were not shown in their report. In this article, the effects of seed concentration in the constant volume process were studied. The particle sizes of colloidal silica were estimated by the assumption of a surface growth mechanism. The additional amount silica of active silicic acid is precipitated on the surface of seeds ∗
Corresponding author. Tel.: +886 6 2533131×3725; fax: +886 6 2425741. E-mail address: [email protected] (M.-S. Tsai).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.08.084
to increase their diameter. The calculated results were compared with the particle size data from dynamic light scattering measurement. The mechanisms of colloidal silica formation in different ranges of the seed concentrations were discussed in this article. 2. Experimental procedure 2.1. Formation of colloidal silica The process to prepare active silicic acid from sodium silicate via ion exchange process was described in our previous report [6]. The surface growth process was carried out by a titration process. The titration rate was equal to the evaporation rate of the heated solution; therefore, the volume of the heated solution was almost kept constant and the solid contend of the heated solution is increased. As titrating solution an aqueous solution contained 3.24 wt% active silicic acid was used. The heated solution contained the seeds and 1.25 g KOH in 260 ml DI water was heated at 100 ◦ C during titration. After titrating with 250 ml active silicic acid, 1.25 g KOH should be added to the heated solution to prevent gel. The total weight of KOH added to the heated solution is 2.5 g. The final products were characterized by dynamic light scattering (DLS, Malvern Zetasizer 3000HSA) to determine the
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mean particle size and transmission electron microscope (TEM) observation. 2.2. Particle size calculation The calculating equations of particle size are the following: The assumed formation mechanism is a surface growth process. Therefore, the number of particle of the initial solution ns is equal to the number of particle of the final solution nf . ns = nf
(1)
If the particles are uniform in size (i.e. mono-disperse), the number of particle is equal to the total weight of silica divided by the weight of a single particle. Ws Ws + Wa 3 = 3 = ns = nf ρs 43 π D2s ρf 43 π D2f
(2)
Ws and Wa are the masses of silica introduced as seeds and the additional silica of active silicate acid introduced during titration, respectively. One can assumed that the density of the newly formed silica ρf and the density of seed ρs are both 2.2 g/cm3 [5]. Then Eq. (2) can be simplified. Df Ws + Wa 1/3 = (3) Ds Ws and ns =
Ws 3 ρs 43 π D2s
(4)
If Ds , Ws and Wa are known, then Df and nf can be calculated. If the surface growth dominates the formation process, then the calculated particle size is closes to the real particle size from the experiments which can be measured by DLS. On the other hand, if the calculated particle size is quite different from the real particle size, then other formation mechanisms might be involved. Other possible mechanism may be homogenous nucleation or aggregation. If the number of particles calculated by the data of DLS measurement is smaller than the original particle number of seeds, the possible mechanism is aggregation. However, if the number of particles calculated by the data of DLS measurement is larger than the initial number of seeds, it may be due to the formation of new particles under these conditions. 3. Results and discussion The effects of the seed concentration on the mean particle size of colloidal silica are shown in Fig. 1. A maximum mean particle size was found at 0.74 wt% of seed concentration. The particle size of colloidal silica increases with increasing seed concentration in the range of very low seed concentration. In the range above 0.62 wt%, it decreases with increasing seed concentration. Fig. 2 shows the morphologies of the colloidal silica of different seed concentrations. The observations of TEM are consistent with the measurements of DLS. The products formed in the low
Fig. 1. The effect of the seed concentration on the mean particle size of colloidal silica.
seed concentration range are mono-disperse and small particles as shown in Fig. 2(a and b). The particle size is increased with increasing the seed concentration in this range. When the seed concentration exceeds 0.62 wt%, two kinds of particles were found as shown in Fig. 2(c–e). The larger particles might be formed by the surface growth process and the smaller particles might be formed by the homogenous nucleation process. The particles became more uniform at seed concentration of 2.48 wt% as shown in Fig. 2(f). Increasing the seed concentration, the smaller particle size is increased and the number of larger particles is reduced as shown in Fig. 2(a–f). In general speaking [7], surface growth is faster than homogenous nucleation at the low supersaturation. However, homogenous nucleation occurs at the lower seed concentration (i.e. low supersaturation) in this case. According to the experimental results, it can be concluded that there exist an unknown barrier for surface growth in the very low seed concentration range. The reason is not well understood yet. When the seed concentration become is >0.62 wt%, surface growth and homogenous nucleation occur at the same time. Two kinds of particles are observed in these cases. At a seed concentration of 2.48 wt%, the particles become more uniform. The surface growth seems to dominate the colloidal silica formation process under these conditions. Fig. 3 shows the calculated results of number of particles versus seed concentration. The large deviation between the final number of particle and the initial number of particles (the number of seeds) at low seed concentration shows that the process is not dominated by the mechanism of surface growth. The calculated number of particles is slightly larger than the number of seed in the seed concentration range between 0.62 wt% and 1.86 wt%. It is concluded that the homogenous nucleation and surface growth of heterogeneous nucleation occurs together in this range. At a seed concentration of 2.48 wt%, the calculated value of number of particle is very close to the initial number of seeds. The sur-
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Fig. 2. TEM micrographs of colloidal silica formed at the seed concentration of (a) 0.12 wt%, (b) 0.50 wt%, (c) 0.62 wt%, (d) 0.74 wt%, (e) 1.24 wt% and (f) 2.48 wt%.
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face growth plays a very import role at this seed concentration. The colloidal silica slurry becomes a gel at seed concentration above 2.48 wt%. According to the results of calculation of DLS measurements and TEM observations, the mechanisms of colloidal silica formation can be divided by three intervals as shown in Fig. 4. In the very low seed concentration range, the colloidal silica is formed by the process of homogenous nucleation. If the seed concentration is in the range of 0.62–1.86 wt%, the homogenous nucleation and surface growth occurs. The surface growth becomes more important at seed concentration of 2.48%. However, if the seed concentration exceeds 2.48 wt%, the colloidal silica became unstable and a gel was formed in the formation process. 4. Conclusion
Fig. 3. The effect of seed concentration on the particle number of colloidal silica: (a) the particle number calculated from DLS measurement and (b) particle number of seeds.
The effects of the seed concentration on the mechanism of colloidal silica formation were studied. The formation mechanism is dominated by homogenous nucleation in the low seed concentration range. The particle size of the calculated result is quite different from the measurement data of DLS. Increasing the seed concentration might lead to an increase of the important of surface growth. A maximum mean particle size of the final colloidal silica is found at a seed concentration of 0.74 wt% at which the particles have two kinds of particle sizes. At the seed concentrations in the range of 0.62–1.86 wt%, homogenous nucleation and surface growth occur. The mechanism of particle formation is dominated by surface growth at 2.48 wt% of seed concentration. Under these conditions more uniform particles are found. References [1] [2] [3] [4] [5]
C.F. Lin, et al., Thin solid films 347 (1999) 248–252. M.S. Tsai, Mater. Sci. Eng. B 104 (2003) 63–67 (SCI). W. St¨ober, A. Rang, J. Colloidal Interface Sci. 26 (1968) 62–69. S. Coenen, C.G. De Kruif, J. Colloidal Interface Sci. 124 (1988) 104–110. S.L. Chen, P. Dong, G.H. Yang, J.J. Yang, J. Colloidal Interface Sci. 180 (1966) 237–241. [6] M.S. Tsai, Mater. Sci. Eng. B 106 (2004) 52–55. [7] T.R. Ring, Fundamental of Ceramic Powder Processing and Synthesis, Academic Press Inc., 1996, pp. 183–192.
Fig. 4. The possible mechanism of colloidal silica in different seed concentration which can be divided by four ranges: (a) homogenous nucleation, (b) surface growth and homogenous nucleation, (c) surface growth and (d) gel.
Effects of seeds concentration on the formation of colloidal silica Ming-Shyong Tsai ∗ , Chien-Hsin Yang, Po-Yuan Huang The Department of Chemical and Material Engineering, Southern Taiwan University of Technology, Tainan 710, Taiwan, ROC Received 3 February 2005; received in revised form 8 July 2005; accepted 9 August 2005
Abstract The effects of seed concentration on the mechanism of colloidal silica formation via sodium silicate were studied in this article. When the seed concentration is less than 0.62 wt%, the mechanism of particle formation is dominated by the homogenous nucleation of the active silicate acid. Increasing the seed concentration increases the importance of heterogeneous nucleation via surface growth on seeds. In the seed concentration range of 0.62–1.36 wt%, the formation mechanism is homogenous nucleation parallel to surface growth. Two kinds of particles, one has a small particle size and the other has a large particle size, were observed by transmission electron microscope pictures. A maximum mean particle size of the final product is observed at 0.74 wt% seed concentration. When the seed concentration is in the range of 1.36–2.48 wt%, the mechanism of particle formation is surface growth. The final particles are uniform in diameter. The particles are aggregated and gelled when the seed concentration is greater than 2.48 wt%. The particle size of the final colloidal silica was estimated by the assumption that the additional silica of added as active silicic acid grows on the seeds. The results are compared with the particle size measurement from dynamic light scattering (DLS). © 2005 Elsevier B.V. All rights reserved. Keywords: Colloidal silica; Sodium silica; Particle formation
1. Introduction Recently, colloidal silica has been applied in many modern industries, e.g. as binder for inorganic paint, stiffener for hard coating reagent and especially as abrasive particles for chemical mechanical polishing slurries [1,2]. St¨ober and Rang [3] developed the famous process of synthesizing mono-disperse silica from tetraethylorthosilicate (TEOS), ethanol, ammonium hydroxide and water as raw materials. Coenen and De Kruif [4] used the St¨ober process modified it by introducing commercial colloidal silica (Dupont, Ludox) as seeds to synthesize the colloidal silica particles via a surface growth process. The particle size of the resulting silica colloids was estimated according to the assumption of surface growth and mass balance in the formation process. Chen et al. [5] indicated some of the conditions of new particle formation. But the details of the results were not shown in their report. In this article, the effects of seed concentration in the constant volume process were studied. The particle sizes of colloidal silica were estimated by the assumption of a surface growth mechanism. The additional amount silica of active silicic acid is precipitated on the surface of seeds ∗
Corresponding author. Tel.: +886 6 2533131×3725; fax: +886 6 2425741. E-mail address: [email protected] (M.-S. Tsai).
0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.08.084
to increase their diameter. The calculated results were compared with the particle size data from dynamic light scattering measurement. The mechanisms of colloidal silica formation in different ranges of the seed concentrations were discussed in this article. 2. Experimental procedure 2.1. Formation of colloidal silica The process to prepare active silicic acid from sodium silicate via ion exchange process was described in our previous report [6]. The surface growth process was carried out by a titration process. The titration rate was equal to the evaporation rate of the heated solution; therefore, the volume of the heated solution was almost kept constant and the solid contend of the heated solution is increased. As titrating solution an aqueous solution contained 3.24 wt% active silicic acid was used. The heated solution contained the seeds and 1.25 g KOH in 260 ml DI water was heated at 100 ◦ C during titration. After titrating with 250 ml active silicic acid, 1.25 g KOH should be added to the heated solution to prevent gel. The total weight of KOH added to the heated solution is 2.5 g. The final products were characterized by dynamic light scattering (DLS, Malvern Zetasizer 3000HSA) to determine the
M.-S. Tsai et al. / Materials Science and Engineering B 123 (2005) 238–241
239
mean particle size and transmission electron microscope (TEM) observation. 2.2. Particle size calculation The calculating equations of particle size are the following: The assumed formation mechanism is a surface growth process. Therefore, the number of particle of the initial solution ns is equal to the number of particle of the final solution nf . ns = nf
(1)
If the particles are uniform in size (i.e. mono-disperse), the number of particle is equal to the total weight of silica divided by the weight of a single particle. Ws Ws + Wa 3 = 3 = ns = nf ρs 43 π D2s ρf 43 π D2f
(2)
Ws and Wa are the masses of silica introduced as seeds and the additional silica of active silicate acid introduced during titration, respectively. One can assumed that the density of the newly formed silica ρf and the density of seed ρs are both 2.2 g/cm3 [5]. Then Eq. (2) can be simplified. Df Ws + Wa 1/3 = (3) Ds Ws and ns =
Ws 3 ρs 43 π D2s
(4)
If Ds , Ws and Wa are known, then Df and nf can be calculated. If the surface growth dominates the formation process, then the calculated particle size is closes to the real particle size from the experiments which can be measured by DLS. On the other hand, if the calculated particle size is quite different from the real particle size, then other formation mechanisms might be involved. Other possible mechanism may be homogenous nucleation or aggregation. If the number of particles calculated by the data of DLS measurement is smaller than the original particle number of seeds, the possible mechanism is aggregation. However, if the number of particles calculated by the data of DLS measurement is larger than the initial number of seeds, it may be due to the formation of new particles under these conditions. 3. Results and discussion The effects of the seed concentration on the mean particle size of colloidal silica are shown in Fig. 1. A maximum mean particle size was found at 0.74 wt% of seed concentration. The particle size of colloidal silica increases with increasing seed concentration in the range of very low seed concentration. In the range above 0.62 wt%, it decreases with increasing seed concentration. Fig. 2 shows the morphologies of the colloidal silica of different seed concentrations. The observations of TEM are consistent with the measurements of DLS. The products formed in the low
Fig. 1. The effect of the seed concentration on the mean particle size of colloidal silica.
seed concentration range are mono-disperse and small particles as shown in Fig. 2(a and b). The particle size is increased with increasing the seed concentration in this range. When the seed concentration exceeds 0.62 wt%, two kinds of particles were found as shown in Fig. 2(c–e). The larger particles might be formed by the surface growth process and the smaller particles might be formed by the homogenous nucleation process. The particles became more uniform at seed concentration of 2.48 wt% as shown in Fig. 2(f). Increasing the seed concentration, the smaller particle size is increased and the number of larger particles is reduced as shown in Fig. 2(a–f). In general speaking [7], surface growth is faster than homogenous nucleation at the low supersaturation. However, homogenous nucleation occurs at the lower seed concentration (i.e. low supersaturation) in this case. According to the experimental results, it can be concluded that there exist an unknown barrier for surface growth in the very low seed concentration range. The reason is not well understood yet. When the seed concentration become is >0.62 wt%, surface growth and homogenous nucleation occur at the same time. Two kinds of particles are observed in these cases. At a seed concentration of 2.48 wt%, the particles become more uniform. The surface growth seems to dominate the colloidal silica formation process under these conditions. Fig. 3 shows the calculated results of number of particles versus seed concentration. The large deviation between the final number of particle and the initial number of particles (the number of seeds) at low seed concentration shows that the process is not dominated by the mechanism of surface growth. The calculated number of particles is slightly larger than the number of seed in the seed concentration range between 0.62 wt% and 1.86 wt%. It is concluded that the homogenous nucleation and surface growth of heterogeneous nucleation occurs together in this range. At a seed concentration of 2.48 wt%, the calculated value of number of particle is very close to the initial number of seeds. The sur-
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Fig. 2. TEM micrographs of colloidal silica formed at the seed concentration of (a) 0.12 wt%, (b) 0.50 wt%, (c) 0.62 wt%, (d) 0.74 wt%, (e) 1.24 wt% and (f) 2.48 wt%.
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face growth plays a very import role at this seed concentration. The colloidal silica slurry becomes a gel at seed concentration above 2.48 wt%. According to the results of calculation of DLS measurements and TEM observations, the mechanisms of colloidal silica formation can be divided by three intervals as shown in Fig. 4. In the very low seed concentration range, the colloidal silica is formed by the process of homogenous nucleation. If the seed concentration is in the range of 0.62–1.86 wt%, the homogenous nucleation and surface growth occurs. The surface growth becomes more important at seed concentration of 2.48%. However, if the seed concentration exceeds 2.48 wt%, the colloidal silica became unstable and a gel was formed in the formation process. 4. Conclusion
Fig. 3. The effect of seed concentration on the particle number of colloidal silica: (a) the particle number calculated from DLS measurement and (b) particle number of seeds.
The effects of the seed concentration on the mechanism of colloidal silica formation were studied. The formation mechanism is dominated by homogenous nucleation in the low seed concentration range. The particle size of the calculated result is quite different from the measurement data of DLS. Increasing the seed concentration might lead to an increase of the important of surface growth. A maximum mean particle size of the final colloidal silica is found at a seed concentration of 0.74 wt% at which the particles have two kinds of particle sizes. At the seed concentrations in the range of 0.62–1.86 wt%, homogenous nucleation and surface growth occur. The mechanism of particle formation is dominated by surface growth at 2.48 wt% of seed concentration. Under these conditions more uniform particles are found. References [1] [2] [3] [4] [5]
C.F. Lin, et al., Thin solid films 347 (1999) 248–252. M.S. Tsai, Mater. Sci. Eng. B 104 (2003) 63–67 (SCI). W. St¨ober, A. Rang, J. Colloidal Interface Sci. 26 (1968) 62–69. S. Coenen, C.G. De Kruif, J. Colloidal Interface Sci. 124 (1988) 104–110. S.L. Chen, P. Dong, G.H. Yang, J.J. Yang, J. Colloidal Interface Sci. 180 (1966) 237–241. [6] M.S. Tsai, Mater. Sci. Eng. B 106 (2004) 52–55. [7] T.R. Ring, Fundamental of Ceramic Powder Processing and Synthesis, Academic Press Inc., 1996, pp. 183–192.
Fig. 4. The possible mechanism of colloidal silica in different seed concentration which can be divided by four ranges: (a) homogenous nucleation, (b) surface growth and homogenous nucleation, (c) surface growth and (d) gel.