Real-Time Monitoring of in Situ Polyethyleneimine-Silica Particle

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Real-Time Monitoring of in Situ Polyethyleneimine-Silica Particle Formation Frances Neville*,† and Ahmad Seyfaee‡ †

School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia School of Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia

‡

S Supporting Information *

ABSTRACT: Silica particles are traditionally made via the hydrolysis and condensation of tetraalkoxysilanes with the use of methanol and ammonia as a basic catalyst. More recently, bioinspired polyamines have been used in place of ammonia. Particle formation via the use of tetraalkoxysilanes typically occurs extremely quickly with cloudy precipitates forming immediately, making it practically impossible to characterize the reaction in real time. Our study uses trimethoxymethylsilane (TMOMS) and the polyamine polyethyleneimine (PEI) to form PEI-silica particles via a reaction that takes place over several minutes, allowing us to study the reaction in real time. The acidic hydrolysis of TMOMS and basic polymerization condensation of TMOMS via PEI to form solid PEI-silica particles were observed in situ over time using attenuated total reflectanceFourier transform infrared (ATR-FTIR) spectroscopy and dynamic light scattering (DLS) for the first time. The ATR-FTIR data suggest that dimer formation occurs during acidic hydrolysis followed by PEI-catalyzed condensation to form silsesquioxane structures. The results for the particles formed in situ were then compared with those for particle samples that had been washed to remove excess reactants. The ATR-FTIR results were corroborated via scanning electron microscopy and DLS, which suggest that the growth of PEI-silica particles occurs by aggregation of smaller particles to larger ones, because the data show the presence of small particles and much larger particles at the same time throughout the whole particle growth process.

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INTRODUCTION Silica particles are important in a wide range of applications, including catalysis, coatings, separation materials, enzyme immobilization, and sensors.1−4 The current global demand for silica is forecast to grow by more than 6% per year.5 To control silica particle growth, it is extremely useful to know how the particles form. If the formation of the particles can be controlled because of the knowledge of the chemistry that occurs during synthesis, it would be possible to obtain an absolute control of particle size and morphology with little batch-to-batch variation. It is also advantageous if the silica particles can be fabricated using methods with neutral conditions and reagents more environmentally friendly than those commonly used in methods that often involve harsh reaction conditions.6−9 The sol−gel process, originally conducted by Stöber and Fink,10 is used to make silica gels and solid particles, and the difference between the formation of the gel and solid particles is due to the pH and the presence of salts in the reaction mixture.8,11,12 Silica gels are produced from silica particles suspended in a liquid (a sol) that interconnect and form a continuous network of particles with the liquid between them. Once the liquid is removed, the result is a silica gel.6,12 Silica gels are highly porous, nanostructured materials and are made © 2013 American Chemical Society

of silicon alkoxides such as tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), and trimethoxymethylsilane (TMOMS).11 Silica gels are used in a number of applications, including nuclear waste storage and thermal insulation in solar energy systems, as well as for cleaning up oil spills.13 Therefore, they have a number of uses in important areas of energy and environmental science. TMOMS has been used for porous gel production and to conduct surface modifications of silica particles, but it is not usually used as the main alkoxysilane for solid particle synthesis.9 This is most likely due to the structure of TMOMS, which has only three alkoxy groups and a methyl group around each silicon atom rather than being tetraalkoxylated like TMOS and TEOS. Most of the studies of the use of alkoxysilanes for solid particle production have been based on TEOS. ,14−20 This is because it gives a reaction that is slow enough to observe over tens of minutes, rather than those of other alkoxysilanes such as TMOS where particle precipitation is virtually instantaneous (precipitation is complete within seconds). However, the Received: August 7, 2013 Revised: October 31, 2013 Published: November 2, 2013 14681

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removing aliquots that were observed in situ via ATR-FTIR spectroscopy or DLS at different time points. For the overall reaction volume not to decrease significantly, the same reaction was conducted in several tubes. When the volume had decreased by 5%, a new tube was sampled. The reaction proceeded as a condensation polymerization reaction with the maximal amount of cloudy precipitate of particles being visible after approximately 20 min. For the samples made with different TMOMS concentrations, 50 or 25 μL of 1 M hydrolyzed TMOMS was added with 50 or 75 μL of water for 0.05 and 0.025 M TMOMS, respectively, in place of 100 μL of 1 M hydrolyzed TMOMS for the 0.1 M condition. Washed PEI-Silica Particle Samples. Particles were prepared as described in In Situ Formation of PEI-Silica Particles, but the reactions were left to react in individual tubes without removing aliquots. At the same time points at which the reaction mixture was analyzed for the in situ samples, the particle sample (precipitate) was separated from any remaining reactants by centrifugation at 16500g for 1 min to sediment the particles. The particles were then washed twice by resuspension in water followed by sonication and then centrifugation at 16500g for 1 min to sediment the particles, before finally being resuspended in water. The washed particle suspensions from each time point were then analyzed using ATR-FTIR, SEM, and DLS. ATR-FTIR Spectroscopy. A Spectrum Two (Perkin-Elmer) instrument was used for ATR-FTIR spectroscopy measurements. Before the samples were analyzed, an air background was taken. A further water background was taken as water is the solvent and the main component of the reaction mixture. The acid-hydrolyzed TMOMS, PEI-silica reaction, and washed PEI-silica particle suspension were analyzed directly using the viscous liquid mode. A 10 μL aliquot was used for each measurement, and the data produced were the average of four runs for each sample. The background signal of water was subtracted from the data using Spectrum version 10.03.08.0135 to facilitate comparison. The peak area was obtained using Spectrum with the baseline being defined by the minima of the particular peak being studied. In addition, the wavenumber values used as minima were kept the same for the analysis of a particular peak to allow comparison of all the data over time. Dynamic Light Scattering. Particle sizes were determined using a Malvern NanoZS ZetaSizer instrument. The intensity average (hydrodynamic diameter) size distributions were obtained. The approximate particle concentration was 0.5 g/L for the washed particle samples, and a sample volume of 1 mL was used for the in situ measurements. The mean hydrodynamic value was calculated using the mean of six measurements where the result of the measurement was the mean value over a period of 1 min. Scanning Electron Microscopy. The PEI-silica particles were analyzed directly from the particle suspension using SEM. A drop of a particle suspension was dried onto an aluminum stub before being sputter-coated with gold. A Philips XL30 SEM instrument was used for the characterization. The SEM images were analyzed for particle size using the freeware UTHSCSA Image Tool.26 In general, 100−150 different particles were analyzed per image. The standard deviation of the particle diameters was obtained.

literature about the growth of particles using TMOMS is scarce.8,9 A study of the fabrication of solid particles made with TMOMS and the polyamine polyethyleneimine (PEI) has recently been reported. This method8,9 used no added methanol or ammonia and PEI as the basic catalyst. The use of PEI has an additional advantage in that PEI-functionalized silica particles can be used for the capture of carbon dioxide from the air21 as well as for separation applications,22 and PEIfunctionalized particles are also used for gene delivery.23 This suggests that the PEI-silica particles produced via our one-pot method have several future useful applications because the TMOMS particles are PEI-modified without a further step being required.8,9 Neville et al.9 studied the effect of using different anions during the basic condensation of TMOMS to form solid particles. They proposed that the PEI aggregates with anions to form nucleation points from which silica particles can grow. In addition, because TMOMS is slightly hydrophobic compared to TMOS, the reaction formed spherical particles, unlike TMOS particles formed in the same way.6,24 However, no information about the molecular structure during in situ hydrolysis of the TMOMS or growth of the particles was presented. In contrast to previous studies, this work focuses on the growth mechanism of “PEI-silica” particles9 that are made using PEI as the basic catalyst instead of ammonia. Hereafter, the particles formed with TMOMS and PEI will be termed PEIsilica particles. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy was used to observe the changes in molecular vibrations during the acidic hydrolysis of TMOMS and during basic particle formation in situ in real time. Dynamic light scattering (DLS) was also used to measure particle growth in situ with time. The washed particles formed at different time points were also characterized using ATRFTIR and DLS. Scanning electron microscopy (SEM) was used to confirm our hypothesis that the changes in polymerization condensation of TMOMS correspond to the increase in the size of the particles as well as that there are different sizes of particles present during the particle growth process.

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EXPERIMENTAL SECTION

Materials. Hydrochloric acid, PEI (molecular mass of 25 kDa), sodium dihydrogen phosphate, disodium hydrogen phosphate, and the alkoxysilane precursor trimethoxymethylsilane (TMOMS) were supplied by Sigma-Aldrich. Unless otherwise noted, all reagent-grade chemicals were used as received, and deionized water (18.2 MΩ) was used in the preparation of all aqueous solutions. The PEI-silica particles were produced in the presence of a mixture of NaH2PO4 and Na2HPO4, which hereafter is termed phosphate buffer (PB).9,25 Hydrolysis of TMOMS. TMOMS (1 M) was hydrolyzed for 15 min with 1 mM hydrochloric acid. This was achieved by adding 142.7 μL of TMOMS to 0.5 mL of 2 mM hydrochloric acid and 357.3 μL of deionized water in a 1.5 mL microcentrifuge tube. The contents of the tube were mixed using a vortex mixer, and the reaction was conducted at room temperature (22 °C). The progression of the reaction was observed in situ over a 30 min time period using ATR-FTIR by removing 10 μL aliquots at allotted time points (ATR-FTIR Spectroscopy). In Situ Formation of PEI-Silica Particles. Polyethyleneimine (100 μL, 25 g/L, pH 10.0) was added to 700 μL of water and 100 μL of sodium phosphate buffer (290 mM, pH 7.4) in a 1.5 mL microcentrifuge tube. This was followed by adding 100 μL of 1 M TMOMS that had been hydrolyzed with hydrochloric acid (final concentration of 0.1 M), at which point the reaction mixture was mixed by vortex mixing. The reaction mixture was sampled by

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RESULTS AND DISCUSSION Here we present the first ATR-FTIR data for how trimethoxymethylsilane hydrolyzes and condenses into solid PEI-silica9 particles in situ over time. In addition, we used DLS to characterize in situ particle formation and the washed particles to support the ATR-FTIR data as well as SEM to visualize the particles. Hydrolysis of TMOMS. The first part of the synthesis of the PEI-silica was to hydrolyze the TMOMS to substitute the methoxy groups with hydroxyl moieties that may then polymerize. This hydrolysis has been previously conducted for other syntheses using PEI as the basic catalyst to form the solid particles.8,9 However, the characterization of the acidic hydrolysis of TMOMS has never been conducted using ATR14682

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FTIR (to the best of our knowledge). It is important to note that although this process is termed hydrolysis condensation occurs simultaneously. This will be described further in this section. The hydrolysis of TMOMS occurs as follows:10,11,25−27 CH3‐Si(OCH3)3 + 3H 2O → CH3‐Si(OH)3 + 3CH3OH (1)

Under acidic conditions, this reaction proceeds slowly with the condensation of TMOMS as11,13,27−29

The hydrolysis and condensation of alkyltrialkoxysilane may form disiloxanes (or oligosiloxanes) as shown in eq 2. This suggests that the main groups that will be observed in the ATRFTIR spectra will be the reduction in the number of Si−OH groups and an increase in the number of Si−O−Si linkages during condensation. Because the alkyl group joined to the Si central atom (a methyl group in the case of TMOMS) cannot polymerize, the Si−CH3 bond will also be visible in the ATRFTIR spectra. Figure 1 shows the ATR-FTIR spectra at different time points during the acidic hydrolysis. The results are from one continuous reaction. The data shown are for the two main peaks in the fingerprint region that change over time. The 854 cm−1 peak may be assigned to the Si−O stretch of Si−OH,30 as shown on the figure, or perhaps to the Si−CH3 rock.31 The 920 cm−1 peak is also assigned to Si−OH (antisymmetric stretching vibration).31,32 Both of these peaks decrease in intensity over time, so it seems likely that the main contribution of the 854 cm−1 peak is due to the intensity of the signal of the Si−OH bond that decreases as polymerization condensation occurs and the Si−OH groups convert to Si−O−Si siloxane bonds, rather than the Si−CH3 rock vibration. The condensation of the Si−OH groups to Si−O−Si groups is demonstrated in Figure 2, which shows the change in the 1077 cm−1 peak that is assigned to the Si−O−Si antisymmetric stretching vibration.28,31,32 The intensity of the signal increases until the 30 min point, after which the intensity of the signal stayed the same (data shown to only 30 min for the sake of clarity). To properly assess the decrease and increase in the intensity of the peaks corresponding to Si−OH and Si−O−Si groups, respectively, the area of the peak may be used. Because TMOMS contains three methoxy moieties and one methyl moiety, the methoxy groups can be hydrolyzed and then undergo condensation polymerization. However, the methyl groups (Si−CH3) do not polymerize; therefore, their number remains constant. Therefore, the 1274 cm−1 peak of the Si-CH3 vibration (Figure S1 of the Supporting Information)32 may be used as an internal standard reference peak to which to compare the other peaks.33 Figure 3 shows the ratios of the peak areas of the data shown in Figures 1 and 2 compared to the 1274 cm−1 Si−CH3 peak area (Figure S1 of the Supporting Information). From this figure, it is clear that acidic condensation reaches completion by ∼20 min because the area ratio of the 1077 cm−1 peak reaches a plateau. In addition, the hydrolysis is completed by this point, which can be assumed by the disappearance of the Si−OH peak at 854 cm−1 as well as by visual observation.

Figure 1. ATR-FTIR spectrum of acid-catalyzed hydrolysis and condensation of TMOMS over time (reaction of 1 M TMOMS with 1 mM HCl): (A) 0, 4, 8, and 10 min, (B) 12, 14, and 16 min, and (C) 20, 25, and 30 min. The arrows are in the same positions on each graph to indicate the reduction in the intensity of the 920 and 854 cm−1 peaks over time.

The results are in agreement with those of Amoriello et al.,33 who used a mixture of colloidal silica and trimethoxymethylsilane to observe organic−inorganic thin films using traditional FTIR with KBr and ZnSe disks to observe acidic hydrolysis and condensation of TMOMS. However, for their observations, they used a mixture of 2-propanol, formamide, TMOMS, and colloidal silica and had to dry the films before FTIR analysis because water cannot be used with ionic salt disks. The 14683

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in contrast to the acidic hydrolysis and condensation step described in Hydrolysis of TMOMS, where there was no turbidity observed. However, the precipitation of solid PEIsilica particles occurred much more slowly for TMOMS than for the tetraalkoxysilane TMOS, which occurred instantaneously when TMOS was introduced to PEI and phosphate buffer.6 However, in either acidic or basic condensation, Si−O− Si, or siloxane, bonds form from silane monomers. A siloxane is a molecule that contains a Si−O−Si bond (Figure S2 of the Supporting Information). The formation of siloxane bonds may occur via a number of routes, and siloxanes often contain functional groups such as halogen atoms or hydrophobic moieties.35 Silsesquioxanes, however, differ from siloxanes in that every silicon atom is linked to three oxygen atoms and every oxygen atom is linked to two silicon atoms (Figure S2 of the Supporting Information).35,36 Furthermore, silsesquioxanes may have a number of structures depending on the extent of polymerization (Figure S2 of the Supporting Information). Alkyltrialkoxysilanes may hydrolyze and then condense (polymerize) under basic conditions to form silsesquioxanes with the general formula (RSiO1.5)n.37 However, incompletely condensed silsesquioxanes still contain Si−OH groups that can form additional Si−O−Si linkages via the removal of water.37 Therefore, during the in situ formation of PEI-silica particles, we would expect to see some of the same vibrations as in the acidic hydrolysis of TMOMS as well as new vibration peaks due to the further polymerization condensation of TMOMS via formation of Si−O−Si bonds. Figure 4 shows the ATR-FTIR spectra for different time points during the in situ condensation of hydrolyzed TMOMS

Figure 2. ATR-FTIR spectrum of acidic condensation of TMOMS over time shown by the increase in the intensity of the Si−O−Si peak at 1077 cm−1 (reaction of 1 M TMOMS with 1 mM HCl).

Figure 3. Ratio of peak areas compared to the 1274 cm−1 peak area (Si−CH3) of simultaneous reduction of hydroxyl groups (shown by the decrease in the intensity of the Si−OH asymmetric peak at 920 cm−1) and condensation of TMOMS (shown by the increase in the intensity of the Si−O−Si peak at 1077 cm−1). The disappearance of the peak at 854 cm−1 suggests these molecular vibrations are more likely due to Si−OH (symmetric) vibrations rather than to the Si− CH3 bend.

advantage of our method is that ATR-FTIR spectroscopy can be used with aqueous-based suspensions and solutions, which means that we could analyze the hydrolysis of TMOMS in situ over time without having to make separate measurements with dried samples. The hydrolysis and condensation of TMOMS occur more slowly than those of TMOS because there are onequarter fewer Si−CH3 groups in TMOMS than in TMOS.34 Brinker et al.34 show data for acid and base hydrolysis of a number of alkoxysilanes, although they do not include TMOMS. All the alkoxysilanes hydrolyze more quickly under basic conditions than under acidic conditions. However, even under acidic conditions, all of the alkoxysilanes tested were hydrolyzed within 5 min, except for tetramethoxysilane.34 Our data show that under acidic conditions it takes at least 15 min for the TMOMS to hydrolyze (Figure 3). In Situ Formation of PEI-Silica Particles (ATR-FTIR). After the acidic hydrolysis of TMOMS had been characterized, the in situ polymerization using PEI as the basic catalyst was investigated. The basic conditions allowed the particles to form more quickly than under acidic conditions,11 which was observed visually as the solution just started to become turbid after only 4 min when PEI was used as the basic catalyst. This is

Figure 4. ATR-FTIR spectrum of in situ condensation of phosphate buffer, PEI, and TMOMS over time.

under basic conditions using PEI as the basic catalyst. The hydrolyzed TMOMS has a major peak at 1077 cm−1 that corresponds to Si−O−Si bonds found during the acid-catalyzed hydrolysis step28,29,31,32 (Figure 2). The major difference observed in Figure 4 is the shift over time from one peak at 1077 cm−1 to two distinct Si−O−Si peaks, the other being at 1125 cm−1,28,29,31,32 which indeed is not present during acidic hydrolysis and condensation (Figure 2). The production of the two peaks at 1077 and 1125 cm−1 over time suggests that the PEI-silica is growing from a simple siloxane polymer, such as dimethyldisiloxane,32 to a silsesquioxane [(RSiO1.5)n].32,35 Furthermore, the peak at 1125 cm−1 is indicative of a 14684

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silsesquioxane when n = 8, 10, or 12,32,35 as observed by Abe et al.35 Furthermore, the increase in the intensity of the 1125 cm−1 peak can be correlated with the particle growth via condensation by comparing the peak area with that of the 1274 cm−1 peak for Si−CH3 (Figure 5),33 as conducted for the

The PEI-silica diameters of the particles forming in situ and also of the washed particles were measured using DLS. Figure 6

Figure 6. Intensity average (DLS) and SEM diameters of PEI-silica particles at time points from 0 to 60 min suggesting the three stages of growth as (I) aggregation of hydrolyzed TMOMS and PEI/PB, (II) fast growth via particle aggregation, and (III) formation of smooth stable particles: SEM data (▲), DLS in situ data (□), and DLS data from washed particles (●). Figure 5. Ratio of the 1125 cm−1 peak area to the 1274 cm−1 peak area (Si−CH3) of the in situ condensation of TMOMS (shown by the increase in the number of Si−O−Si bonds) into silica particles. The increase in the intensity of the signal corresponds to the time when the condensation product is visible as a cloudy precipitate.

shows the intensity average diameter of the particles formed in situ and also of the washed particles. The diameters of the particles obtained from the SEM image analysis are also given in the same figure. As expected, the SEM diameters are smaller than those obtained from the DLS measurements.8 The differences in particle sizes are due only to the analysis method used as the SEM images are from dried particles that do not allow for any increase in diameter due to PEI on the surface, which may increase the hydrodynamic diameter measured using DLS.8 In Figure 6, three different regions corresponding to different time intervals can be observed. The data for the in situ particle growth and washed particles (Figure 6) clearly show two main regions of rapid growth up to approximately 20 min (region II) followed by a plateau in particle size (region III). In addition to the fact that the in situ particle diameters from 4 to 60 min are remarkably similar to those of the washed particles, there is one significant difference in the data (Figure 6). This difference comes from the first point of the in situ particle growth data that was obtained immediately after the reaction solution had been mixed. At this point of reaction initiation, the basic PEI catalyst and phosphate buffer, which form polyamine−salt aggregates around 15−20 nm in diameter,9 and the hydrolyzed TMOMS come into contact and quickly polymerize (region I). The corresponding DLS particle size increases to around 350 nm in 4 min. After this (4−20 min), there is another period of significant particle growth. This is followed by a plateau at which the particle diameter distribution does not change significantly over the remainder of the 1 h period (not all the particles have the same diameter). Further discussion of the particle growth mechanism is given in Particle Growth Mechanism. In addition to the ATR-FTIR study in situ over time, the corresponding analysis for the washed particles was also conducted. The main difference between the particles formed in situ (Figure 4) and the washed particles (Figure 7) is the shift in peak position from 1077 cm−1 (Figure 4) to 1039 cm−1 (Figure 7). The position of the 1077 cm−1 peak is the same as that of the hydrolyzed TMOMS peak (Figure 2), suggesting

acidic hydrolysis step (Figure 3). When this analysis was conducted, it was clear that there is a period of slower growth up to approximately the 8 min point, followed by a rapid growth phase from 8 to 24 min, after which the peak area ratio decreases and then plateaus (Figure 5). These observations made from the ATR-FTIR data are corroborated by visual observations of the periods when turbidity is observable only to the point of maximal turbidity. In addition to the Si−O−Si peaks, the intensities of the weaker peaks at 1254 and 1385 cm−1 (Figure 4) that correspond to the C−H bend of the Si−CH3 moiety29,31,33 of the TMOMS molecule were observed to increase over time. This is most likely due to the methyl groups being present at either the edges or corners of the silsesquioxane (depending on if it is a ladder or cube structure),35,37 which are easier to detect than if the Si−-CH3 groups are randomly distributed within the sample. The polymerization condensation of TMOMS to form particles was also studied in situ using DLS. The results will be discussed in the following section through a comparative study with the washed particle samples. Analysis of Washed PEI-Silica Particles (SEM, DLS, and ATR-FTIR). The washed particles were characterized using SEM. In situ measurements cannot be made using SEM due to fact that the samples need to be dried before being placed in the vacuum chamber of the SEM. After the particles had been washed twice, any gel or small particle nuclei were removed, and thus, only the larger particles may be observed in SEM images of the washed samples. Figure S3 of the Supporting Information shows SEM images of the particles made at different time points between 4 and 60 min (no solid particles were visible prior to the 4 min point). The images clearly show the increase in particle size with time until around 20 min, which corroborates the results shown in Figure 5. 14685

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that there are large aggregate and smooth particles present concurrently, suggesting that the particles grow via aggregation but not all at the same rate. In addition to the in situ measurements using 0.1 M TMOMS presented so far, the concentration of TMOMS was altered to observe the effect this had on the growth of the particles in terms of the initial particle growth rate and also the equilibrium diameter of the particles. The additional TMOMS concentrations studied were 0.05 and 0.025 M. We have defined the initial particle growth rate by measuring the linear increase in particle diameter (based on the intensity average diameter from the DLS measurements) over time, over the time it takes for the particles to first reach their approximate equilibrium diameter (or over the first 30 min of the reaction at a concentration of 0.025 M). We have defined the equilibrium diameter as the average intensity diameter of the plateau region, as shown by region III of Figure 6. Our method quickly produces particles for which the maximal intensity average particle size distribution is relatively constant over a short time period, and thus, for future production and applications, it is useful to determine how quickly the particles grow over a short period of time. It is expected that a decrease in particle growth time will ultimately lead to an increase in the level of production. Figure 9 shows the in situ DLS data for reactions using 0.1, 0.05, and 0.025 M TMOMS. All other reaction conditions were

Figure 7. ATR-FTIR spectrum of washed PEI-silica particles over time.

that this Si−O−Si peak is due to the acidic catalyzed polymerization of the TMOMS35 into small gel particles. Because there is a large amount of gel present in the in situ sample that is removed during particle washing, we propose that the gel masks the signal from the 1039 cm−1 peak in the washed particle samples. In addition, the two distinct peaks that are visible (1039 and 1125 cm−1) (Figure 7) are representative of polysiloxane polymers.32 Furthermore, the shift from 1077 to 1039 cm−1 is indicative of the increase in the level of polymerization from a trisiloxane to a silsesquioxane where n ≥ 8.32,35 Furthermore, the presence of small gel particles shown by the Si−O−Si peak at 1077 cm−1 (Figure 4) helps us to propose the mechanism of formation of these particles that will be presented in Particle Growth Mechanism. Moreover, an example of a SEM image of a particle sample where the small gel particles have not been fully washed away is shown in Figure 8A. Panels B and C of Figure 8 also show large aggregate particles after sample washing had been conducted, confirming

Figure 9. In situ DLS and SEM data of particle growth with different TMOMS concentrations over a 30 min growth period. The TMOMS concentrations were 0.1 (◆), 0.05 (■), and 0.025 M (▲). The scale bar is 250 nm.

kept the same as in previous experiments. The increase in the intensity average diameter of the 0.1 M TMOMS particles was again observed, and the data in Figure 9 were independent of the data shown in Figure 6. Nevertheless, the data presented in Figure 9 are extremely similar to those in Figure 6 for the 0.1 M TMOMS condition. The initial particle growth again occurred over the first 8−10 min, after which the average intensity diameter plateaued with relatively little change over time. The equilibrium diameter as derived from the DLS data was approximately 500 nm. The error bars show the standard deviation for three to five repeats of the experiments. We suggest that the error is higher in the case of the 0.1 M TMOMS condition because of the wide particle size distribution, although again the data presented in Figure 9 for the 0.1 M TMOMS condition have errors similar to those in Figure 6 for comparable experiments. Adjacent to the graph

Figure 8. SEM images of (A) TMOMS particles after incomplete washing, showing the simultaneous presence of (1) small gel particles, (2) the formation of particles through aggregation of small particles to larger aggregates before the smoothing of the larger particles, and (3) smooth larger particles, (B) a case of washed PEI-silica large aggregate particles, and (C) an example of washed PEI-silica large aggregate particles and smooth particles that have formed at different rates within the same sample. The scale bar is 500 nm. 14686

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Langmuir 0.17 M TEOS, water (3.8−15 M), and ammonia (0.5−3 M) with KOH or HCl as a catalyst

0.17 M TEOS, 1−1.3 M ammonia, and 1−3.8 M water (in propanol) Bailey and Mecartney20

Bogush and Zukoski15,19 Aggregation Model Small particle nuclei aggregate to form colloidally stable particles. The particles grow via aggregation of the unstable small the point at which the monomer has polymerized sufficiently particle nuclei with the large stable ones. Small unstable and larger stable particles are present throughout the particle to phase separate and form a distinct particle (∼4 nm growth period. diameter), particle nucleus

theoretical models Flory17 and Stockmayer18 Densification Model Particle nuclei form due to collapse of a gel network. The particle nuclei grow by addition of a polymer that collapses to the point at which the monomer has polymerized sufficiently form a larger particle. This model does not apply when hydrolysis is complete before growth commences or when the (microgel cluster) to phase separate and form a distinct water/base concentration is high as the polymers will not be as expanded (so cannot collapse). particle (∼20 nm diameter), “particle nucleus”

Matsoukas and Gulari16

LaMer14

reaction conditions ref nucleation definition model

Table 1. Summaries of Different Particle Growth Models and Nucleation Definitions 14687

Monomer Addition Model All “nuclei” form particles that grow by monomer addition. Small nuclei and larger stable particles are not present Two alkoxysilane molecules condense to form a nucleus concurrently. (dimerization) (∼1 nm diameter).

showing the DLS data are SEM images showing the particles produced with the different TMOMS concentrations at the 30 min time point. The particle in the image (Figure 9) for the 0.1 M TMOMS condition is approximately 450 nm in diameter, which corresponds well with the DLS data (Figure 9), given that the equilibrium diameter was ∼500 nm and the SEM values are usually smaller than those of DLS.8 The data for the 0.05 M TMOMS condition show that the reaction occurs more slowly than for the 0.1 M TMOMS condition, although there is a trend similar to that of the 0.1 M TMOMS concentration condition overall. The initial growth rate for the 0.05 M TMOMS case is approximately 30 nm/min compared to ∼57 nm/min for 0.1 M TMOMS. This is almost twice as slow for 0.05 M TMOMS as for 0.1 M TMOMS and corresponds directly to the change in the concentration of TMOMS. The equilibrium diameter is smaller than that for the 0.1 M TMOMS condition, with the value for the 0.05 M case being approximately 360 nm. The corresponding SEM image (Figure 9) confirms that the equilibrium particle diameter size is smaller for 0.05 M TMOMS than for 0.1 M TMOMS, with a diameter of ∼320 nm. From Figure 9, it is clear that over the 30 min period studied, there is no significant growth of the particles made with 0.025 M TMOMS. The intensity average diameter value is almost constant at ∼15 nm (standard deviation of 3 nm). This value corresponds to the very small particles consisting of PEI−PB polyamine−anion aggregates9 that may be associated with any TMOMS present (see Particle Growth Mechanism) prior to their growth into larger aggregates as was seen with the higher concentrations of TMOMS. These very small particles are visible in the SEM image in Figure 9. Note that the small particles are also visible in the SEM images for the samples made with 0.1 and 0.05 M TMOMS because the images are of the in situ unwashed samples rather than the washed particle samples. If the 0.025 M TMOMS sample was washed, the very small particles would not be able to be separated via the centrifugation step and thus would not be able to be visualized as they would be removed from the supernatant. Particle Growth Mechanism. To date, the majority of growth models of alkoxysilanes have been based on TEOS.14−20 This is because it gives a reaction that is slow enough to observe over tens of minutes, as opposed to that of other alkoxysilanes such as TMOS that produce particle precipitation that is virtually instantaneous (complete within seconds). However, there is very little work on the characterization of TMOMS particle formation via condensation polymerization relating the data to a proposed growth mechanism. There are currently several particle growth mechanisms that have been proposed over the past 80 years. These models differ on how the particle nuclei form and how the particles grow. Because of the number of particle models and the fact that a number of them refer to particle nuclei or nucleation with different definitions, the summaries of the different models and how they define nuclei are given in Table 1 for easy comparison. Further details are given in the text below. The reaction conditions given in the corresponding references are also given in Table 1. The first model that will be discussed here is that of LaMer,14 which proposed that all the particles form from a supersaturated solution during a very rapid nucleation period. The model then suggests that the monodisperse particles then result due to “self-sharpening”15 of particle size, during which the

sulfur in an ethanol/acetone mixture with water 0.087 M TEOS in ethanol or methanol with 1.2−1.6 M ammonia diluted in water (3.2 M)

Article

dx.doi.org/10.1021/la403040u | Langmuir 2013, 29, 14681−14690

Langmuir

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larger particles grow less quickly than the smaller ones. This model considers that particles grow via monomer addition until the equilibrium concentration is achieved and all the particles have the same diameter.15 Matsoukas and Gulari16 proposed a modified LaMer model in which particle nuclei are formed by hydrolysis of two alkoxysilane molecules (TEOS). The particle growth again occurs via addition of the monomer to the nuclei via a diffusion process. This model is based on the assumption that the particle nuclei are colloidally stable and that the rate of TEOS hydrolysis is equal to the rate of particle growth16 so that each nucleus results in a particle. Unlike the monomer addition mechanism of LaMer and Matsoukas, Flory and Stockmayer17,18 referred to nucleation as the point at which the monomer had polymerized sufficiently to become unstable and phase separate to form a small particle. The mechanism of Flory and Stockmayer is quite different from that of Matsoukas in that in the former nucleation forms only when the growing polymer becomes unstable (i.e., at a critical concentration) and collapses forming a nucleus particle. In contrast, in the latter model, nucleation occurs any time two hydrolyzed TEOS molecules condense and nucleation stops when the rate of addition of the monomer to the nuclei is greater than the rate of monomer dimerization.15,19 Bailey and Mecartney20 proposed another model based on the production of particles via the collapse of low-density particle networks to form colloidally stable seed particles required for further growth. They proposed that particle growth occurred by the addition of small low-density particles to the seed particles that then further collapsed to form a larger particle. This mechanism was applicable to their data as the hydrolysis was not conducted prior to the basic condensation of TEOS with ammonia. However, if the hydrolysis is allowed to complete before condensation commences, the formation of dense particles will occur20 as is the case in our particle synthesis. Bogush and Zukoski15,19 tested the mechanism proposed by Matsoukas and Gulari16 and suggested a different growth mechanism that involves particle−particle aggregation. They showed that the concentration of hydrolyzed TEOS is not directly related to the number density and presence of the growing particles.15 In addition, Bogush and Zukoski19 proposed that the final size of the particle is not only due to the reaction rate but also due to the colloidal stability of the small particle nuclei and that nucleation occurs throughout the precipitation reaction as well as aggregation of growing particles. Furthermore, Bogush and Zukoski19 indicated that it is difficult for nuclei to be stable if their diameter is