Laboratory Experiment pubs.acs.org/jchemeduc
Preparation and Characterization of Colloidal Silica Particles under Mild Conditions Frances Neville,*,†,‡ Azrinawati Mohd. Zin,‡ Graeme J. Jameson,† and Erica J. Wanless‡ †
School of Environmental and Life Sciences and ‡School of Engineering, The University of Newcastle, Callaghan, New South Wales 2308, Australia
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S Supporting Information *
ABSTRACT: A microscale laboratory experiment for the preparation and characterization of silica particles at neutral pH and ambient temperature conditions is described. Students first employ experimental fabrication methods to make spherical submicrometer silica particles via the condensation of an alkoxysilane and polyethyleneimine, which act to catalyze the reaction in the presence of phosphate buffer. This is then followed by methods to characterize the particles by size and imaging and allows the students to describe particle growth and improve their skills in explaining results obtained while using different methods. This silica particle synthesis has the advantage over the Stöber method in that no extremes of pH or temperature are needed. In addition, the chemicals used in this experiment are less hazardous than those commonly used to make silica particles. KEYWORDS: Upper-Division Undergraduate, Laboratory Instruction, Physical Chemistry, Hands-On Learning/Manipulatives, Colloids, Materials Science, Microscale Lab, Nanotechnology, Precipitation/Solubility olloidal silica is traditionally synthesized via a “sol−gel” method, originally carried out by Stöber and Fink1 that involves harsh chemical conditions such as 5 M methanol and 14 M ammonia.1,2 Initial acidification of a tetraalkoxysilane solution yields Si(OH)4, which polymerizes into extremely small silica particles or nuclei (Scheme 1) that are typically in
C
Greenblatt.3 Size is controlled by reaction time and the ratios of the different reagents used. It is the aim of this experiment to produce colloidal silica particles with a more rapid method that has milder conditions than has previously been demonstrated.3 This experiment involves the preparation and characterization of colloidal silica particles and involves fundamental studies of the control of particle size. It is carried out on the microscale to reduce waste and enhance safety control as spillage is minimized due to the small volumes used. The experiment is based on a previous research project4 that has been adapted as a laboratory experiment. The experiment has been carried out by undergraduate third-year science or science-teaching majors for the last two years as part of a chemistry course on polymers and colloids. The illustrated method of colloidal silica synthesis (Scheme 1) generally involves the hydrolysis and condensation of the alkoxysilane tetraethyl orthosilicate (TEOS) or tetramethyl orthosilicate (TMOS), which are flammable, harmful, and toxic (TMOS) substances, together with ammonia as the basic catalyst in aqueous ethanolic solutions. Here the less hazardous trimethoxymethylsilane (TMOMS) is used and a mild acid hydrolysis is undertaken at room temperature (Table S1, Supporting Information).4 The polymer polyethyleneimine is used as the basic catalyst as it is less hazardous than ammonia and it also has a similar structure to the polyamines found in nature where silication also occurs, for example, in marine algae.5,6
Scheme 1. General Method of Colloidal Silica Synthesis
the range of 1−5 nm. Whether these nuclei grow depends on the conditions of polymerization. If the pH is reduced below 7 or if salt is added, the nuclei units tend to fuse together in chains. These products are often called “silica gels”. If the pH is kept on the alkaline side of neutral, the nuclei stay separated, and they gradually grow to yield particles of tens to thousands of nanometers in size. A chemical description of the reactions involved in colloidal silica formation is given by Buckley and © 2012 American Chemical Society and Division of Chemical Education, Inc.
Published: April 27, 2012 940
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instrument and then their particle sizes were measured by a fully trained operator in their presence or by the students themselves. This will depend on the availability of the instrument and the time constraints of the laboratory class. In our case, an instrument, which can measure particle size up to 3000 nm, was used (Zetasizer Nano, Malvern Instruments). The particles may also be observed with optical microscopy, but this will only give qualitative results and depends on the size of the particles and the magnification of the microscope (Figure S1, Supporting Information).
The initiation of the condensation reaction depends on the molar ratio of reagents and can be observed a few minutes after adding the hydrolyzed TMOMS. After this transformation, a turbid white dispersion is formed. The size and morphology of the colloidal silica particles formed depends on the reaction time. The size range can be controlled from tens of nanometers up to micrometer-sized particles, and because of the small size, the surface area of the resultant colloidal silica is very high.
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EXPERIMENT
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Chemicals
HAZARDS TMOMS is highly flammable and should not be handled near open flames. PEI is classified as harmful, harmful if swallowed, toxic to aquatic organisms, and dangerous for the environment, and should not be used without appropriate safety measures. The neat PEI should only be handled by the technical staff. The students will be provided with a dilute solution of PEI. Use gloves and eye protection and carry out the whole procedure in the fume hood. The silica and silane waste generated in the experiment should be consolidated separately to the polymer waste and labeled properly for appropriate disposal (see Supporting Information for more detail).
Trimethoxymethylsilane (TMOMS), hydrochloric acid, polyethyleneimine (PEI), and all components of sodium phosphate buffer are available from Sigma-Aldrich. Concentrations, purities, CAS numbers, and links to MSDS are provided in the Supporting Information. Procedure
A full description of the method is given in the Supporting Information. In summary, during the first week, a precipitation reaction was carried out by hydrolyzing 1 M TMOMS with 1 mM HCl. This was freshly prepared for every session. A buffer solution of 290 mM sodium phosphate, pH 7.4, was prepared by technical staff as was a solution of 1 mM PEI. Ultrapure water (18.2 mΩ cm) was used in all solutions. The components were added to eight microcentrifuge tubes in the following order: water, buffer, PEI, then hydrolyzed TMOMS. The final concentrations of the components were 29 mM phosphate buffer, 0.1 mM PEI, and 0.1 M TMOMS. The various components were mixed by vortexing and a cloudy precipitate of particles formed after around 15 min. At time points of 30, 60, 90, and 120 min, two tubes were placed in the centrifuge at 2000g for 4 min. The particles were washed by resuspension in water by sonication and then recentrifuging to sediment the particles. The supernatant was removed and the washing process repeated. The centrifugation and washing stops the reaction. After the second wash, the particles were resuspended in just enough water to cover the sedimented particles. This method produced two microcentrifuge tubes of particles for each of the four time points allowing sufficient sample for characterization. For the fabrication of the particles, access to a microcentrifuge (and appropriate tubes) is required. If a microcentrifuge is not available, a larger benchtop centrifuge may be used and the volumes of reagents scaled up appropriately. After the particles have been made, the samples should be stored and then prepared for characterization. The characterization of the particles requires further methods such as scanning electron microscopy (SEM) and dynamic light scattering (DLS). This will depend on the availability of instruments, but in our case, the electron microscope unit manager prepared the students’ samples for the SEM prior to the second week of the experiment. He then gave the students a tutorial on sample preparation and microscope operation and then imaged the student samples during the second session, generally with the students present. In our experiment, the particles were mounted on to aluminum stubs from an aqueous suspension and allowed to dry before sputter coating with gold. The SEM images can be analyzed for particle size using SEM software or freeware such as UTHSCSA Image Tool7 or Image J.8 Furthermore, the particles were characterized by DLS. Ideally, the students were given a brief tutorial on the sizing
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RESULTS AND DISCUSSION This laboratory experiment was run in groups of two students with one or two groups carrying out the experiment at once on a rotating basis with the other experiments running during the lab course. The experiment was carried out over two, 3-h practical sessions with the particle fabrication in the first session and characterization and interpretation of the results in the second session. The mean results from the DLS and SEM particle sizing results are shown in Table 1. These results are Table 1. Diameters of Silica Particles at Different Reaction Times Obtained with DLS and SEM Image Analysis Reaction Time/min 30 60 90 120
DLS Diameter (Intensity Average)/ nm 540 574 632 669
± ± ± ±
72 90 128 49
DLS Diameter (Number Average)/ nm 465 484 508 561
± ± ± ±
65 84 90 35
SEM Diameter/ nm 436 472 508 531
± ± ± ±
92 72 30 72
from eight pairs of students who carried out the experiment in 2010 and 2011. The DLS results are given in terms of the intensity and number average particle hydrodynamic diameter.9 Example images of the particles produced at different reaction times at a magnification of 25,000× are shown in Figure 1. The images clearly show that the particles increase in size between the first and last time points. The diameters of the particles measured by DLS intensity and number average together with the mean SEM value as function of incubation time are plotted in Figure 2. The data demonstrate a clear increase in particle diameter with increasing reaction time. The DLS number average and SEM values are in close agreement, as expected, whereas the DLS intensity average values are higher due to its weighting toward the larger particles in the polydisperse samples.9 More detail on DLS is given in the Supporting Information. 941
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REFERENCES
(1) Stöber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62−69. (2) Bogush, G. H.; Tracy, M. A.; Zukoski, C. F., IV J. Non-Cryst. Solids 1988, 104, 95−106. (3) Buckley, A. M.; Greenblatt, M. J. Chem. Educ. 1994, 71, 599−602. (4) Neville, F.; Pchelintsev, N. A.; Broderick, M. J.; Gibson, T.; Millner, P. A. Nanotechnology 2009, 20, 055612. (5) Schröder, H. C.; Wang, X.; Tremel, W.; Ushijima, H.; Müller, W. E. G. Nat. Prod. Rep. 2008, 25, 455−474. (6) Losic., D.; Mitchell, J. G.; Voelcker, N. H. Adv. Mater. 2009, 21, 2947−2958. (7) UTHSCSA ImageTool. http://ddsdx.uthscsa.edu/dig/itdesc. html (accessed Apr 2012). (8) ImageJ. http://rsbweb.nih.gov/ij/ (accessed Apr 2012). (9) Dynamic Light Scattering. http://www.malvern.com/common/ downloads/campaign/MRK656-01.pdf (accessed Apr 2012).
Figure 1. SEM images of particles produced by one group at (A) 30 min and (B) 120 min after the reaction was started. The scale bar is 500 nm.
Figure 2. Change in average particle diameter with incubation time showing the mean values and linear trends for eight groups of students. Data were obtained using the Malvern Zetasizer Nano where diameter results are in terms of intensity average (diamond), number average (square), and SEM image analysis using Image Tool7 software (triangle). The error bars show standard deviation.
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CONCLUSIONS This experiment has provided upper-division undergraduate students an opportunity to observe and discuss spherical particle nucleation, growth, and sizing and to comment on the colloidal stability of spherical particle suspensions.
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ASSOCIATED CONTENT
S Supporting Information *
Laboratory procedure given to students, notes to the instructors, notes to technical staff for preparation of the analytes and safety considerations. This material is available via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to David Phelan of the Electron Microscope/X-ray Unit for carrying out the scanning electron microscopy. The University of Newcastle students in the undergraduate CHEM3580 laboratory are acknowledged for their participation and pooled results and Vicki Thompson for her technical assistance with the laboratory. 942
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