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Influence of surfactant surface coverage and aging time on physical properties of silica nanoparticles
Colloids and Surfaces A: Physicochem. Eng. Aspects 350 (2009) 33–37
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Influence of surfactant surface coverage and aging time on physical properties of silica nanoparticles Gui-Mei Gao a , Hai-Feng Zou a , Da-Rui Liu b , Li-Na Miao a , Gui-Juan Ji a , Shu-Cai Gan a,∗ a b
College of Chemistry, Jilin University, 6 Ximinzhu Street, Changchun 130026, PR China College of Materials Science and Engineering, Jilin University, 142 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2009 Received in revised form 24 August 2009 Accepted 27 August 2009 Available online 2 September 2009 Keywords: Oil shale ash Polyethylene glycol Aging time Silica nanoparticles
a b s t r a c t The experimental results on the influence of surfactant surface coverage and aging time on physical properties of silica nanoparticles were reported. The spherical silica nanoparticles have been synthesized using polyethylene glycol (PEG) as the surfactant and oil shale ash (OSA) as a new silica source. In order to identify the optimal condition for producing the best quality silica nanoparticles with the good dispersion and uniformity, the effects of surfactant surface coverage and aging time were investigated. It was found that the particle size and distribution of silica nanoparticles depend on the concentration of PEG in dispersion. At relatively low concentration, 0–2 wt.%, the existing PEG is not sufficient to prevent further growth of the initially formed silica nanoparticles, leading to large aggregates of silica particles. When the PEG concentration increases to 3 wt.%, self-assembled PEG layer on the surface stabilizes the initially formed silica nanoparticles and the silica particles with average diameter of 10 nm are uniformly distributed. With further increasing the concentration of PEG, the number of PEG aggregates increases and silica nanoparticles are mainly formed inside the entangled PEG chains, resulting in an observation of clusters of silica nanoparticles. Moreover, it was found that as the aging time increased, the shape of silica nanoparticles becomes regular and the particle size distribution becomes narrow. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The importance and advantages of nanometer-sized particles are shown not only in the scientific field, but also in various industrial applications, e.g. catalysts, pigments, pharmacy, etc. [1]. Of these particles, silica nanoparticles are used to make electronic substrates, thin film substrates, electrical insulators, thermal insulators, humidity sensors, etc. [2–5]. The silica particles play a different role in each of these products. The quality of some of these products is highly dependent on the size and size distribution of the silica particles. At present, nanoscale silica materials are prepared using several methods, including vapor-phase reaction, sol–gel and thermal decomposition technique [6,7]. Among these methods, the sol–gel with low cost of preparation and easily controlling the size and its distribution is wide applied. With respect to silica particle growth, in wet colloid, the excess water molecules interact with the free hydroxyls on the surface of the silica colloid particles through hydrogen bonds. When particles get close, these molecules will draw neighboring particles together to form big particles. Since the initially formed particles were easily agglomerate into large particles, which requires additional surfactant to cover them.
∗ Corresponding author. Tel.: +86 0431 87652736. E-mail address: [email protected] (S.-C. Gan). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.030
Nonionic surfactants which are a group of chemical compounds are useful in most applications where surface activity is required and including wetting, emulsification, solubilization, spreading, detergency and permeability enhancement [8–10]. However, they have many advantages over cationic, anionic and amphoteric surfactants as a result of their intrinsic electrical neutrality, such as low sensitivity to electrolytes, high degree of compatibility with other product ingredients, good chemical stability, and a low toxicity [11–14]. Like other types of nonionic surfactants, PEG is used in most applications where surface activity is required and including wetting, emulsification, solubilization, spreading, detergency and permeability enhancement. The structure of PEG is comprised of nonionizable polar and nonpolar portions, which is easily absorbed at the surface of colloid particles to form hydrophilic membrane. When the surface of the colloid adsorbs this type of polymer, the activities of colloid greatly decrease and the growth rate of the colloids in some certain facet will be confined. Therefore, linear PEG has been widely used in the synthesis of a series of nanoparticles and 1 D materials in solution [15–19]. In our previous work, the synthetic method of silica nanoparticles using oil shale ash was researched [20], whereas the influences of synthesis parameters on the particle size and its distribution, such as PEG concentration and aging time were not investigated. In the present work, the effects of surfactant surface coverage and
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G.-M. Gao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 350 (2009) 33–37
Fig. 1. Flow diagram of the procedure used to produce silica nanoparticles from OSA.
aging time on physical properties of silica nanoparticles were studied. 2. Materials and methods 2.1. Properties of raw material Oil shale ash (OSA) is a by-product of oil shale processing. Previous experimental results indicated that OSA is a high-silica ash with a great quantity of SiO2 (50%). In addition, this material is heat treated because it is produced at high temperatures and this implies that OSA is activated [21–23]. So, an alternative manner is the conversion of this ash into a high-grade silica product, which is considered an environmental friendly product. There is no available information about preparation of silica nanoparticles from OSA. The OSA used in this experiment was taken from oil retorting factory of Jilin province. The chemical compositions of the OSA were reported in our previous publication [20]. 2.2. Reagents All chemicals were analytical grade reagents supplied by Beijing Chemical Reagent Research Institute. They were used as-received without any further purification. These include sodium hydroxide (99%), sulfuric acid (98%), polyethylene glycol (10000). Redistilled water was used throughout all the experiments. 2.3. Experimental procedure 2.3.1. Preparation of sodium silicate solution The silica was extracted from OSA using our previously reported method [20]. Fig. 1 shows the flow diagram of the procedure used to produce silica nanoparticles from OSA. Briefly, the calcined OSA (100 g) was stirred with 30 wt.% sulfuric acid solution (500 g) at 100 ◦ C for 2 h. Then the slurry was filtered and washed with double distilled water until the filtrate was free from acid. The pretreated OSA was mixed with 30 wt.% sodium hydroxide solution (400 g) to dissolve the silica, and produce a sodium silicate solution. The sodium silicate formed was filtered to remove undissolved particles and washed with boiling distilled water. The filtrate and washing were heated to reduce the total mass to 550 g for making about 8.0 ± 0.5 wt.% sodium silicate solution.
2.3.2. Synthesis of silica nanoparticles Firstly, a calculated amount of polyethylene glycol (PEG) was slowly added to OSA derived sodium silicate solution (30 g) that was being sonicated. After PEG completely dissolving, 0.5 mol/L sulfuric acid solution was added gradually into the solution in order to initiate the hydrolysis-condensation reaction. The change in pH value was monitored by using a pH meter. A white gel started to form when the pH value decreased to less than 11.0. The gel was adjusted until the pH value equaled 4. The resultant gel mixture was aged at 50 ◦ C for a certain period. Subsequently, the silica gel aged was filtered and washed with distilled water to wash out the remaining surfactant and soluble materials. The filtration cake was distilled with n-butanol, and then calcined at 550 ◦ C for 2 h in atmospheric condition to remove the surfactant. The silica nanoparticles were obtained finally. The resulting silica nanoparticles were characterized by transmission electron microscopy (TEM, JEM-2000EX microscope). Silica nanoparticles were dispersed ultrasonically in ethanol, and a drop of suspension was deposited on a carbon coated copper grid. Dynamic light scattering measurements were carried out with Malvern HPPS laser particle size analyzer (Malvern, UK).
3. Results and discussion 3.1. Effect of surfactant on the particle size and its distribution of silica nanoparticles To optimize the formation condition of silica nanoparticles, the effect of the PEG concentration was investigated. The experiments were performed varying its concentration from 0 to 4 wt.% of total mass of sodium silicate solution. The TEM micrographs of silica nanoparticles obtained at different PEG concentrations were exhibited in Fig. 2a–e. Fig. 2a shows the TEM image of the silica nanoparticles formed without PEG. It can be clearly seen that the formed silica particles were serious aggregate and the size ranged from a few nanometers to several hundred nanometers. The reason of this phenomenon produced can be explained by the following fact. Usually, in wet silica colloid, the excess water molecules interact with the free hydroxyls on the surface of the silica colloid particles through hydrogen bonds. When particles get close, these molecules will draw neighboring particles together to form big particles. These bridging water molecules can be removed when the silica colloid begins to be dried and the hydrogen bonds between hydroxyls on the surface of two neighboring particles will draw them closer. Further drying process will cause the formation of strong chemical bonds between neighboring particles, and then the hard agglomerate appears (Fig. 3a). Since the initially formed silica nanoparticles were easily agglomerate into large particles, which requires additional surfactant to cover them. Fig. 2b and c shows the TEM images of silica nanoparticles with PEG concentration of 1% and 2% in weight. Obviously, the addition of PEG can effectively prevent the aggregation of silica nanoparticles and change the particle size. This is due to the fact that the PEG molecules can absorb on the surface of newly formed silica nanoparticles through hydrogen bonds to form space steric effect, which prevents further growth of the particles. However, aggregations of silica nanoparticles were clearly observed. This result indicates that the low concentration of PEG molecules cannot fully cover the newly created surface of silica nanoparticles, or particle–particle collisions occur faster than PEG molecules can adsorb to cover the exposed hydrophobic patches, leading to further aggregation of silica particles with increasing reaction time. Fig. 2d exhibits TEM image of silica nanoparticles with PEG concentration of 3% in weight. Obviously, the smallest spherical silica
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Fig. 2. TEM micrographs of silica nanoparticles obtained at the same aging time (8 h) with different PEG concentrations: (a) 0 wt.%; (b) 1 wt.%; (c) 2 wt.%; (d) 3 wt.%; (e) 4 wt.%.
particle size with narrow and uniform distribution can be observed, and the average size of particles is about 10 nm. This result indicates that PEG molecules can fully cover the newly created surface of silica nanoparticles, preventing to further aggregation of silica particles during the reaction and drying process. The TEM image of silica nanoparticles with relatively high PEG concentration (4 wt.%) is shown in Fig. 2e. The shape of particles is irregular and the particle size distribution is wide, ranged from 5 to 30 nm. Moreover, slight clusters of silica nanoparticles were observed. It is attributed to individual silica nanoparticles with self-assembled PEG layers and PEG aggregates with formed silica nanoparticles inside the entangled chains, respectively.
It was found that the formation of silica nanoparticles depends on the concentration of PEG in dispersion. If particle–particle collisions occur faster than PEG molecules can adsorb to cover the exposed hydrophobic patches, then extensive particle aggregation will occur due to hydrophobic attraction between the particles. In the absence of applied shear forces, the kinetics of surfactant adsorption to the freshly exposed hydrophobic patches on the silica nanoparticles surfaces should be mainly diffusion controlled. In this case, the surfactant concentration in the solution will play a significant role since the driving force for the adsorption process decreases with decreasing surfactant concentration [9]. At low excess surfactant concentration, there may be insufficient PEG
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Fig. 3. Schematic depiction of PEG-silica hybrid formation in aqueous solution: (a) silica nanoparticles formed without PEG; (b) silica nanoparticles formed at a low PEG concentration; (c) silica nanoparticles formed at a right PEG concentration; (d) silica nanoparticles formed at high PEG concentration. The gray lines represent PEG molecules and the black dots represent silica nanoparticles.
molecules present to completely cover the silica particle surfaces, or the PEG molecules may not adsorb rapidly enough relative to the rate of particle collisions. Moreover, at low excess surfactant concentration, an optimal surface coverage of the surfaces may not be achieved resulting in less than optimal stabilization of the particle dispersion. Under these circumstances, silica nanoparticles would tend to aggregate (Fig. 3b). With increasing the concentration, the adsorption of PEG onto initially formed silica nanoparticles becomes favorable, leading to more stable self-assembled PEG layers on surfaces of silica nanoparticles. Thus, the further growth of silica nanoparticles can be prevented by self-assembled PEG layers, as shown in Fig. 3c. With further increasing concentration of PEG, the number of secondary PEG aggregates in dispersion increases as described above. Sodium silicate molecules, precursors for the silica nanoparticles, can penetrate into the entangled PEG chains. Therefore, the silica nanoparticles will be formed inside the PEG matrix after hydrolysis and silica clusters can be observed (Fig. 3d). Fig. 4 shows particle size distribution curve of silica. Silica nanoparticles formed in 3 wt.% PEG solution with average diameters of 13.5 nm were observed. Particle sizes reported by dynamic light scattering measurement are slight larger than that shown in TEM image. This result indicates that silica nanoparticles formed are not completely monodisperse. Since they are PEG monomers (rather than micelles) that adsorb to hydrophobic silica surfaces, this may explain why it was not possible to form completely monodisperse silica nanoparticles, even at right concentrations. 3.2. Effect of aging time on the particle size and its distribution of silica nanoparticles In another set of experiments, we further changed the aging time to check the role of different aging times in the particle size distribu-
Fig. 5. TEM micrographs of silica nanoparticles obtained at different aging time: (a) 2 h; (b) 6 h; (c) 8 h.
Fig. 4. Particles size distribution curve of silica formed in 3 wt.% PEG solution.
tion. Fig. 5a–c exhibits the TEM micrographs of silica nanoparticles formed in 3 wt.% PEG solution with different aging times of 2, 6 and 8 h, respectively. From the TEM images, it is clearly seen that the particle size distribution becomes narrow with increasing the aging time. When the aging time reached to 8 h, the silica particles were uniformly dispersed in the solvents. It can be attributed to the fact that the aging time plays an important role in the particle size distribution. The aging is a process of dissolution and reprecipitation driven by differences in solubility. Base on the aging theory, during the aging process of silica gel, the smaller silica particles dissolved
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and the silica particles reprecipitated onto larger particles with the increase of aging time. As the aging time increased to 8 h, the silica gel reached dissolution equilibrium. So the silica particles were uniformly dispersed in the solvents. 4. Conclusions The effects of surfactant surface coverage and aging time on physical properties of silica nanoparticles were investigated. The experimental results show that the formation of silica nanoparticles depends on the PEG concentration in dispersion. At relatively low concentration, 0–2 wt.%, the existing PEG is not sufficient to prevent further growth of the initially formed silica nanoparticles, leading to large aggregates of silica particles. When the concentration of PEG increases to 3 wt.%, self-assembled PEG layer on the surface stabilizes the initially formed silica nanoparticles and silica particles with average diameters of 10 nm are found to be uniformly distributed. With further increasing the concentration of PEG, the number of PEG aggregates increases and silica nanoparticles are mainly formed inside the entangled PEG chains, resulting in an observation of clusters of silica nanoparticles. Moreover, it was found that as the aging time increasing, the silica particles were uniformly dispersed in the solvents. The results of this study are useful for the preparation of valuable and widely applicable nanoscale silica from OSA, also helping to solve disposal and pollution problems. Acknowledgements This work was supported by foundation from the scientific research program no: 20051015 and Development Program of China (863 Program, Grant 2007AA06Z202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2009.08.030. References [1] S.K. Park, K.D. Kim, H.T. Kim, Preparation of silica nanoparticles: determination of the optimal synthesis conditions for small and uniform particles, Colloids Surf. A: Physicochem. Eng. Aspects 197 (2002) 7–17. [2] S. Sadasivan, D.H. Rasmussen, F.P. Chen, R.K. Kannabiran, Preparation and characterization of ultrafine silica, Colloids Surf. A: Physicochem. Eng. Aspects 132 (1998) 45–52. [3] C.V. Suciu, T. Iwatsubo, S. Deki, Investigation of a colloidal dampe, J. Colloid Interface Sci. 259 (2003) 62–71.
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[4] S.T. Bromley, E. Flikkema, A new interatomic potential for nanoscale silica, Chem. Phys. Lett. 378 (2003) 622–626. [5] R.R. Zaky, M.M. Hessien, A.A. El-Midany, Preparation of silica nanoparticles from semi-burned rice straw ash, Powder Technol. 185 (2008) 31–35. [6] G.G. Wu, J. Wang, J. Shen, T.H. Yang, Q.Y. Zhang, B. Zhou, Z.S. Deng, F. Bin, D.P. Zhou, F.S. Zhang, Properties of sol–gel derived scratch-resistant nano-porous silica films by a mixed atmosphere treatment, J. Non-Cryst. Solids 275 (2000) 169–174. [7] M. Tomozawa, D.L. Kim, V. Lou, Preparation of high purity, low water content fused silica glass, J. Non-Cryst. Solids 296 (2001) 102–106. [8] M. Sadoqi, C.A. Lau-Cam, S.H. Wu, Investigation of the micellar properties of the tocopheryl polyethylene glycol succinate surfactants TPGS 400 and TPGS 1000 by steady state fluorometry, J. Colloid Interface Sci. 333 (2009) 585– 589. [9] T. Helgason, T.S. Awad, K. Kristbergsson, D.J. McClements, J. Weiss, Effect of surfactant surface coverage on formation of solid lipid nanoparticles (SLN), J. Colloid Interface Sci. 334 (2009) 75–81. [10] S.Y. Zhang, D.J. Sun, X.Q. Dong, C.F. Li, J. Xu, Aqueous foams stabilized with particles and nonionic surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 324 (2008) 1–8. [11] F. Podczeck, P. Alessi, J.M. Newton, Pellet formulations containing non-ionic 498 surfactants prepared by extrusion/spheronization, Int. J. Pharm. 361 (2008) 33–40. [12] J.J. Pan, H.N. Zhang, M. Pan, Self-assembly of Nafion molecules onto silica nanoparticles formed in situ through sol–gel process, J. Colloid Interface Sci. 326 (2008) 55–60. [13] N. Duerr-Auster, R. Gunde, R. Mader, E.J. Windhab, Binary coalescence of gas bubbles in the presence of a non-ionic surfactant, J. Colloid Interface Sci. 333 (2009) 579–584. [14] Z.W. Wang, G.Z. Li, X.Y. Zhang, R.K. Wang, A.J. Lou, A quantitative structure–property relationship study for the prediction of critical micelle concentration of nonionic surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 197 (2002) 37–45. [15] S. Dai, K.C. Tam, Isothermal titration calorimetric studies on the interaction between sodium dodecyl sulfate and polyethylene glycols of different molecular weights and chain architectures, Colloids Surf. A: Physicochem. Eng. Aspects 289 (2006) 200–206. [16] O.G. Chu, S.G. Chen, B. Che, G. Xue, Aggregation of azo dye Orange I induced by polyethylene glycol in aqueous solution, Colloids Surf. A: Physicochem. Eng. Aspects 301 (2007) 346–351. [17] M. Sertkol, A. Baykal b, H. Kavasa, A.C. Bas-aran, Synthesis and magnetic characterization of Zn0.6 Ni0.4 Fe2 O4 nanoparticles via a polyethylene glycol-assisted hydrothermal route, J. Magn. Magn. Mater. 321 (2009) 157–162. [18] L. Dong, Y. Chu, Y. Zhang, Y. Liu, F. Yang, Surfactant-assistant and facile synthesis of hollow ZnS nanospheres, J. Colloid Interface Sci. 308 (2007) 258–264. [19] X.H. Liu, J. Yang, L. Wang, X.J. Yang, An improvement on sol–gel method for preparing ultrafine and crystallized titania powder, Mater. Sci. Eng. A 289 (2000) 241–245. [20] G.M. Gao, H.F. Zou, S.C. Gan, Z.J. Liu, Preparation and properties of silica nanoparticles from oil shale ash, Powder Technol. 191 (2009) 47–51. [21] X.X. Han, X.M. Jiang, Z.G. Cui, Change of pore structure of oil shale particles during combustion. 2. Pore structure of oil shale ash, Energy Fuel 22 (2008) 972–975. [22] J. Reinik, Synthesis and characterization of calcium–alumino-silicate hydrate from oil shale ash—towards industrial applications, Fuel 87 (2008) 1998– 2003. [23] G.M. Gao, H.F. Zou, D.R. Liu, S.C. Gan, Synthesis of ultrafine silica powders based on oil shale ash by fluidized bed drying of wet-gel slurry, Fuel 88 (2009) 1223–1227.
Contents lists available at ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Influence of surfactant surface coverage and aging time on physical properties of silica nanoparticles Gui-Mei Gao a , Hai-Feng Zou a , Da-Rui Liu b , Li-Na Miao a , Gui-Juan Ji a , Shu-Cai Gan a,∗ a b
College of Chemistry, Jilin University, 6 Ximinzhu Street, Changchun 130026, PR China College of Materials Science and Engineering, Jilin University, 142 Renmin Street, Changchun 130025, PR China
a r t i c l e
i n f o
Article history: Received 5 June 2009 Received in revised form 24 August 2009 Accepted 27 August 2009 Available online 2 September 2009 Keywords: Oil shale ash Polyethylene glycol Aging time Silica nanoparticles
a b s t r a c t The experimental results on the influence of surfactant surface coverage and aging time on physical properties of silica nanoparticles were reported. The spherical silica nanoparticles have been synthesized using polyethylene glycol (PEG) as the surfactant and oil shale ash (OSA) as a new silica source. In order to identify the optimal condition for producing the best quality silica nanoparticles with the good dispersion and uniformity, the effects of surfactant surface coverage and aging time were investigated. It was found that the particle size and distribution of silica nanoparticles depend on the concentration of PEG in dispersion. At relatively low concentration, 0–2 wt.%, the existing PEG is not sufficient to prevent further growth of the initially formed silica nanoparticles, leading to large aggregates of silica particles. When the PEG concentration increases to 3 wt.%, self-assembled PEG layer on the surface stabilizes the initially formed silica nanoparticles and the silica particles with average diameter of 10 nm are uniformly distributed. With further increasing the concentration of PEG, the number of PEG aggregates increases and silica nanoparticles are mainly formed inside the entangled PEG chains, resulting in an observation of clusters of silica nanoparticles. Moreover, it was found that as the aging time increased, the shape of silica nanoparticles becomes regular and the particle size distribution becomes narrow. © 2009 Elsevier B.V. All rights reserved.
1. Introduction The importance and advantages of nanometer-sized particles are shown not only in the scientific field, but also in various industrial applications, e.g. catalysts, pigments, pharmacy, etc. [1]. Of these particles, silica nanoparticles are used to make electronic substrates, thin film substrates, electrical insulators, thermal insulators, humidity sensors, etc. [2–5]. The silica particles play a different role in each of these products. The quality of some of these products is highly dependent on the size and size distribution of the silica particles. At present, nanoscale silica materials are prepared using several methods, including vapor-phase reaction, sol–gel and thermal decomposition technique [6,7]. Among these methods, the sol–gel with low cost of preparation and easily controlling the size and its distribution is wide applied. With respect to silica particle growth, in wet colloid, the excess water molecules interact with the free hydroxyls on the surface of the silica colloid particles through hydrogen bonds. When particles get close, these molecules will draw neighboring particles together to form big particles. Since the initially formed particles were easily agglomerate into large particles, which requires additional surfactant to cover them.
∗ Corresponding author. Tel.: +86 0431 87652736. E-mail address: [email protected] (S.-C. Gan). 0927-7757/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2009.08.030
Nonionic surfactants which are a group of chemical compounds are useful in most applications where surface activity is required and including wetting, emulsification, solubilization, spreading, detergency and permeability enhancement [8–10]. However, they have many advantages over cationic, anionic and amphoteric surfactants as a result of their intrinsic electrical neutrality, such as low sensitivity to electrolytes, high degree of compatibility with other product ingredients, good chemical stability, and a low toxicity [11–14]. Like other types of nonionic surfactants, PEG is used in most applications where surface activity is required and including wetting, emulsification, solubilization, spreading, detergency and permeability enhancement. The structure of PEG is comprised of nonionizable polar and nonpolar portions, which is easily absorbed at the surface of colloid particles to form hydrophilic membrane. When the surface of the colloid adsorbs this type of polymer, the activities of colloid greatly decrease and the growth rate of the colloids in some certain facet will be confined. Therefore, linear PEG has been widely used in the synthesis of a series of nanoparticles and 1 D materials in solution [15–19]. In our previous work, the synthetic method of silica nanoparticles using oil shale ash was researched [20], whereas the influences of synthesis parameters on the particle size and its distribution, such as PEG concentration and aging time were not investigated. In the present work, the effects of surfactant surface coverage and
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Fig. 1. Flow diagram of the procedure used to produce silica nanoparticles from OSA.
aging time on physical properties of silica nanoparticles were studied. 2. Materials and methods 2.1. Properties of raw material Oil shale ash (OSA) is a by-product of oil shale processing. Previous experimental results indicated that OSA is a high-silica ash with a great quantity of SiO2 (50%). In addition, this material is heat treated because it is produced at high temperatures and this implies that OSA is activated [21–23]. So, an alternative manner is the conversion of this ash into a high-grade silica product, which is considered an environmental friendly product. There is no available information about preparation of silica nanoparticles from OSA. The OSA used in this experiment was taken from oil retorting factory of Jilin province. The chemical compositions of the OSA were reported in our previous publication [20]. 2.2. Reagents All chemicals were analytical grade reagents supplied by Beijing Chemical Reagent Research Institute. They were used as-received without any further purification. These include sodium hydroxide (99%), sulfuric acid (98%), polyethylene glycol (10000). Redistilled water was used throughout all the experiments. 2.3. Experimental procedure 2.3.1. Preparation of sodium silicate solution The silica was extracted from OSA using our previously reported method [20]. Fig. 1 shows the flow diagram of the procedure used to produce silica nanoparticles from OSA. Briefly, the calcined OSA (100 g) was stirred with 30 wt.% sulfuric acid solution (500 g) at 100 ◦ C for 2 h. Then the slurry was filtered and washed with double distilled water until the filtrate was free from acid. The pretreated OSA was mixed with 30 wt.% sodium hydroxide solution (400 g) to dissolve the silica, and produce a sodium silicate solution. The sodium silicate formed was filtered to remove undissolved particles and washed with boiling distilled water. The filtrate and washing were heated to reduce the total mass to 550 g for making about 8.0 ± 0.5 wt.% sodium silicate solution.
2.3.2. Synthesis of silica nanoparticles Firstly, a calculated amount of polyethylene glycol (PEG) was slowly added to OSA derived sodium silicate solution (30 g) that was being sonicated. After PEG completely dissolving, 0.5 mol/L sulfuric acid solution was added gradually into the solution in order to initiate the hydrolysis-condensation reaction. The change in pH value was monitored by using a pH meter. A white gel started to form when the pH value decreased to less than 11.0. The gel was adjusted until the pH value equaled 4. The resultant gel mixture was aged at 50 ◦ C for a certain period. Subsequently, the silica gel aged was filtered and washed with distilled water to wash out the remaining surfactant and soluble materials. The filtration cake was distilled with n-butanol, and then calcined at 550 ◦ C for 2 h in atmospheric condition to remove the surfactant. The silica nanoparticles were obtained finally. The resulting silica nanoparticles were characterized by transmission electron microscopy (TEM, JEM-2000EX microscope). Silica nanoparticles were dispersed ultrasonically in ethanol, and a drop of suspension was deposited on a carbon coated copper grid. Dynamic light scattering measurements were carried out with Malvern HPPS laser particle size analyzer (Malvern, UK).
3. Results and discussion 3.1. Effect of surfactant on the particle size and its distribution of silica nanoparticles To optimize the formation condition of silica nanoparticles, the effect of the PEG concentration was investigated. The experiments were performed varying its concentration from 0 to 4 wt.% of total mass of sodium silicate solution. The TEM micrographs of silica nanoparticles obtained at different PEG concentrations were exhibited in Fig. 2a–e. Fig. 2a shows the TEM image of the silica nanoparticles formed without PEG. It can be clearly seen that the formed silica particles were serious aggregate and the size ranged from a few nanometers to several hundred nanometers. The reason of this phenomenon produced can be explained by the following fact. Usually, in wet silica colloid, the excess water molecules interact with the free hydroxyls on the surface of the silica colloid particles through hydrogen bonds. When particles get close, these molecules will draw neighboring particles together to form big particles. These bridging water molecules can be removed when the silica colloid begins to be dried and the hydrogen bonds between hydroxyls on the surface of two neighboring particles will draw them closer. Further drying process will cause the formation of strong chemical bonds between neighboring particles, and then the hard agglomerate appears (Fig. 3a). Since the initially formed silica nanoparticles were easily agglomerate into large particles, which requires additional surfactant to cover them. Fig. 2b and c shows the TEM images of silica nanoparticles with PEG concentration of 1% and 2% in weight. Obviously, the addition of PEG can effectively prevent the aggregation of silica nanoparticles and change the particle size. This is due to the fact that the PEG molecules can absorb on the surface of newly formed silica nanoparticles through hydrogen bonds to form space steric effect, which prevents further growth of the particles. However, aggregations of silica nanoparticles were clearly observed. This result indicates that the low concentration of PEG molecules cannot fully cover the newly created surface of silica nanoparticles, or particle–particle collisions occur faster than PEG molecules can adsorb to cover the exposed hydrophobic patches, leading to further aggregation of silica particles with increasing reaction time. Fig. 2d exhibits TEM image of silica nanoparticles with PEG concentration of 3% in weight. Obviously, the smallest spherical silica
G.-M. Gao et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 350 (2009) 33–37
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Fig. 2. TEM micrographs of silica nanoparticles obtained at the same aging time (8 h) with different PEG concentrations: (a) 0 wt.%; (b) 1 wt.%; (c) 2 wt.%; (d) 3 wt.%; (e) 4 wt.%.
particle size with narrow and uniform distribution can be observed, and the average size of particles is about 10 nm. This result indicates that PEG molecules can fully cover the newly created surface of silica nanoparticles, preventing to further aggregation of silica particles during the reaction and drying process. The TEM image of silica nanoparticles with relatively high PEG concentration (4 wt.%) is shown in Fig. 2e. The shape of particles is irregular and the particle size distribution is wide, ranged from 5 to 30 nm. Moreover, slight clusters of silica nanoparticles were observed. It is attributed to individual silica nanoparticles with self-assembled PEG layers and PEG aggregates with formed silica nanoparticles inside the entangled chains, respectively.
It was found that the formation of silica nanoparticles depends on the concentration of PEG in dispersion. If particle–particle collisions occur faster than PEG molecules can adsorb to cover the exposed hydrophobic patches, then extensive particle aggregation will occur due to hydrophobic attraction between the particles. In the absence of applied shear forces, the kinetics of surfactant adsorption to the freshly exposed hydrophobic patches on the silica nanoparticles surfaces should be mainly diffusion controlled. In this case, the surfactant concentration in the solution will play a significant role since the driving force for the adsorption process decreases with decreasing surfactant concentration [9]. At low excess surfactant concentration, there may be insufficient PEG
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Fig. 3. Schematic depiction of PEG-silica hybrid formation in aqueous solution: (a) silica nanoparticles formed without PEG; (b) silica nanoparticles formed at a low PEG concentration; (c) silica nanoparticles formed at a right PEG concentration; (d) silica nanoparticles formed at high PEG concentration. The gray lines represent PEG molecules and the black dots represent silica nanoparticles.
molecules present to completely cover the silica particle surfaces, or the PEG molecules may not adsorb rapidly enough relative to the rate of particle collisions. Moreover, at low excess surfactant concentration, an optimal surface coverage of the surfaces may not be achieved resulting in less than optimal stabilization of the particle dispersion. Under these circumstances, silica nanoparticles would tend to aggregate (Fig. 3b). With increasing the concentration, the adsorption of PEG onto initially formed silica nanoparticles becomes favorable, leading to more stable self-assembled PEG layers on surfaces of silica nanoparticles. Thus, the further growth of silica nanoparticles can be prevented by self-assembled PEG layers, as shown in Fig. 3c. With further increasing concentration of PEG, the number of secondary PEG aggregates in dispersion increases as described above. Sodium silicate molecules, precursors for the silica nanoparticles, can penetrate into the entangled PEG chains. Therefore, the silica nanoparticles will be formed inside the PEG matrix after hydrolysis and silica clusters can be observed (Fig. 3d). Fig. 4 shows particle size distribution curve of silica. Silica nanoparticles formed in 3 wt.% PEG solution with average diameters of 13.5 nm were observed. Particle sizes reported by dynamic light scattering measurement are slight larger than that shown in TEM image. This result indicates that silica nanoparticles formed are not completely monodisperse. Since they are PEG monomers (rather than micelles) that adsorb to hydrophobic silica surfaces, this may explain why it was not possible to form completely monodisperse silica nanoparticles, even at right concentrations. 3.2. Effect of aging time on the particle size and its distribution of silica nanoparticles In another set of experiments, we further changed the aging time to check the role of different aging times in the particle size distribu-
Fig. 5. TEM micrographs of silica nanoparticles obtained at different aging time: (a) 2 h; (b) 6 h; (c) 8 h.
Fig. 4. Particles size distribution curve of silica formed in 3 wt.% PEG solution.
tion. Fig. 5a–c exhibits the TEM micrographs of silica nanoparticles formed in 3 wt.% PEG solution with different aging times of 2, 6 and 8 h, respectively. From the TEM images, it is clearly seen that the particle size distribution becomes narrow with increasing the aging time. When the aging time reached to 8 h, the silica particles were uniformly dispersed in the solvents. It can be attributed to the fact that the aging time plays an important role in the particle size distribution. The aging is a process of dissolution and reprecipitation driven by differences in solubility. Base on the aging theory, during the aging process of silica gel, the smaller silica particles dissolved
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and the silica particles reprecipitated onto larger particles with the increase of aging time. As the aging time increased to 8 h, the silica gel reached dissolution equilibrium. So the silica particles were uniformly dispersed in the solvents. 4. Conclusions The effects of surfactant surface coverage and aging time on physical properties of silica nanoparticles were investigated. The experimental results show that the formation of silica nanoparticles depends on the PEG concentration in dispersion. At relatively low concentration, 0–2 wt.%, the existing PEG is not sufficient to prevent further growth of the initially formed silica nanoparticles, leading to large aggregates of silica particles. When the concentration of PEG increases to 3 wt.%, self-assembled PEG layer on the surface stabilizes the initially formed silica nanoparticles and silica particles with average diameters of 10 nm are found to be uniformly distributed. With further increasing the concentration of PEG, the number of PEG aggregates increases and silica nanoparticles are mainly formed inside the entangled PEG chains, resulting in an observation of clusters of silica nanoparticles. Moreover, it was found that as the aging time increasing, the silica particles were uniformly dispersed in the solvents. The results of this study are useful for the preparation of valuable and widely applicable nanoscale silica from OSA, also helping to solve disposal and pollution problems. Acknowledgements This work was supported by foundation from the scientific research program no: 20051015 and Development Program of China (863 Program, Grant 2007AA06Z202). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.colsurfa.2009.08.030. References [1] S.K. Park, K.D. Kim, H.T. Kim, Preparation of silica nanoparticles: determination of the optimal synthesis conditions for small and uniform particles, Colloids Surf. A: Physicochem. Eng. Aspects 197 (2002) 7–17. [2] S. Sadasivan, D.H. Rasmussen, F.P. Chen, R.K. Kannabiran, Preparation and characterization of ultrafine silica, Colloids Surf. A: Physicochem. Eng. Aspects 132 (1998) 45–52. [3] C.V. Suciu, T. Iwatsubo, S. Deki, Investigation of a colloidal dampe, J. Colloid Interface Sci. 259 (2003) 62–71.
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[4] S.T. Bromley, E. Flikkema, A new interatomic potential for nanoscale silica, Chem. Phys. Lett. 378 (2003) 622–626. [5] R.R. Zaky, M.M. Hessien, A.A. El-Midany, Preparation of silica nanoparticles from semi-burned rice straw ash, Powder Technol. 185 (2008) 31–35. [6] G.G. Wu, J. Wang, J. Shen, T.H. Yang, Q.Y. Zhang, B. Zhou, Z.S. Deng, F. Bin, D.P. Zhou, F.S. Zhang, Properties of sol–gel derived scratch-resistant nano-porous silica films by a mixed atmosphere treatment, J. Non-Cryst. Solids 275 (2000) 169–174. [7] M. Tomozawa, D.L. Kim, V. Lou, Preparation of high purity, low water content fused silica glass, J. Non-Cryst. Solids 296 (2001) 102–106. [8] M. Sadoqi, C.A. Lau-Cam, S.H. Wu, Investigation of the micellar properties of the tocopheryl polyethylene glycol succinate surfactants TPGS 400 and TPGS 1000 by steady state fluorometry, J. Colloid Interface Sci. 333 (2009) 585– 589. [9] T. Helgason, T.S. Awad, K. Kristbergsson, D.J. McClements, J. Weiss, Effect of surfactant surface coverage on formation of solid lipid nanoparticles (SLN), J. Colloid Interface Sci. 334 (2009) 75–81. [10] S.Y. Zhang, D.J. Sun, X.Q. Dong, C.F. Li, J. Xu, Aqueous foams stabilized with particles and nonionic surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 324 (2008) 1–8. [11] F. Podczeck, P. Alessi, J.M. Newton, Pellet formulations containing non-ionic 498 surfactants prepared by extrusion/spheronization, Int. J. Pharm. 361 (2008) 33–40. [12] J.J. Pan, H.N. Zhang, M. Pan, Self-assembly of Nafion molecules onto silica nanoparticles formed in situ through sol–gel process, J. Colloid Interface Sci. 326 (2008) 55–60. [13] N. Duerr-Auster, R. Gunde, R. Mader, E.J. Windhab, Binary coalescence of gas bubbles in the presence of a non-ionic surfactant, J. Colloid Interface Sci. 333 (2009) 579–584. [14] Z.W. Wang, G.Z. Li, X.Y. Zhang, R.K. Wang, A.J. Lou, A quantitative structure–property relationship study for the prediction of critical micelle concentration of nonionic surfactants, Colloids Surf. A: Physicochem. Eng. Aspects 197 (2002) 37–45. [15] S. Dai, K.C. Tam, Isothermal titration calorimetric studies on the interaction between sodium dodecyl sulfate and polyethylene glycols of different molecular weights and chain architectures, Colloids Surf. A: Physicochem. Eng. Aspects 289 (2006) 200–206. [16] O.G. Chu, S.G. Chen, B. Che, G. Xue, Aggregation of azo dye Orange I induced by polyethylene glycol in aqueous solution, Colloids Surf. A: Physicochem. Eng. Aspects 301 (2007) 346–351. [17] M. Sertkol, A. Baykal b, H. Kavasa, A.C. Bas-aran, Synthesis and magnetic characterization of Zn0.6 Ni0.4 Fe2 O4 nanoparticles via a polyethylene glycol-assisted hydrothermal route, J. Magn. Magn. Mater. 321 (2009) 157–162. [18] L. Dong, Y. Chu, Y. Zhang, Y. Liu, F. Yang, Surfactant-assistant and facile synthesis of hollow ZnS nanospheres, J. Colloid Interface Sci. 308 (2007) 258–264. [19] X.H. Liu, J. Yang, L. Wang, X.J. Yang, An improvement on sol–gel method for preparing ultrafine and crystallized titania powder, Mater. Sci. Eng. A 289 (2000) 241–245. [20] G.M. Gao, H.F. Zou, S.C. Gan, Z.J. Liu, Preparation and properties of silica nanoparticles from oil shale ash, Powder Technol. 191 (2009) 47–51. [21] X.X. Han, X.M. Jiang, Z.G. Cui, Change of pore structure of oil shale particles during combustion. 2. Pore structure of oil shale ash, Energy Fuel 22 (2008) 972–975. [22] J. Reinik, Synthesis and characterization of calcium–alumino-silicate hydrate from oil shale ash—towards industrial applications, Fuel 87 (2008) 1998– 2003. [23] G.M. Gao, H.F. Zou, D.R. Liu, S.C. Gan, Synthesis of ultrafine silica powders based on oil shale ash by fluidized bed drying of wet-gel slurry, Fuel 88 (2009) 1223–1227.