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On the Formation Mechanism of Pluronic-Templated Mesostructured Silica Andreas Sundblom,† Cristiano L. P. Oliveira,‡ Jan Skov Pedersen,*,‡ and Anders E. C. Palmqvist*,† Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, SE-412 96 Go¨teborg, Sweden, and Department of Chemistry and iNANO Interdisciplinary Nanoscience Center, UniVersity of Aarhus, DK-8000 Aarhus, Denmark ReceiVed: January 5, 2010
The synthesis of mesostructured silica formed at mildly acidic conditions using the nonionic surfactant Pluronic P123 has been studied with the aim of showing the importance of the degree of condensation of the silica source. The experimental investigation employed was in situ small-angle X-ray scattering (SAXS) in combination with an advanced model. The results are further supported by dynamic light scattering (DLS) and transmission electron microscope (TEM) analyses. Tetraethyl orthosilicate (TEOS) has been used as silica source, using a two step procedure involving a prehydrolysis of the TEOS, so that the degree of condensation of the silica at the start of the reaction could be varied. The results obtained demonstrated that, by controlling the degree of condensation of the silica at the start of the synthesis, it is possible to control the formation rate and the degree of order of the final material. The findings are summarized in a formation mechanism comprising three steps: Entropy driven surfactant-silica attraction that results in cylindrical silica containing micelles, particle formation induced by micelle-micelle association caused by the surfactantsilica interaction, and finally rearrangement of the internal particle structure into a hexagonal arrangement that also affects the morphology of the entire particle, creating elongated particles. Introduction Syntheses of mesoporous silica using the nonionic block copolymer Pluronics have been studied for more than ten years and considerable progress in understanding the mechanisms involved has been made. The increased use of in situ techniques such as electron paramagnetic resonance (EPR),1 nuclear magnetic resonance (NMR),2 and scattering techniques3–5 in recent years has been central in this improvement. However, the attraction between the silica and Pluronics is still not fully understood. It is generally accepted that the main attraction is between the hydrophilic part of the surfactant, the poly(ethylene oxide) (PEO) blocks, and the silica. It is well-known that PEO6 and surfactants containing PEO7 adsorb on silica surfaces. The fact that the silanol groups have been shown to be essential for the adsorption has led to the conclusion that hydrogen bonding between the ether oxygen in the PEO and the silanol groups is responsible for the adsorption.6 The attraction between silica and the nonionic surfactants is thus most often attributed to hydrogen bonding.8 It has, however, been argued that hydrogen bonding alone is not sufficient as the syntheses are performed in water, which also forms hydrogen bonds with the silica and the surfactant. It has therefore been suggested that an additional hydrophobic attraction must be operational.2 Further information on the PEO-silica interaction has been presented in studies of the sol-gel process. There it was shown that the addition of PEO of varying molecular weights affects the reaction kinetics. From these results it has been concluded that the interaction between PEO and silica arises only when the silica oligomers have reached a certain size.9–11 * To whom correspondence should be addressed. (J.S.P.) Phone: +45 8942 3858. Fax: +45 8619 6199. E-mail:
[email protected]. (A.E.C.P.) Phone: +46 31 772 29 61. Fax: +46 31 16 00 62. E-mail:
[email protected]. † Chalmers University of Technology. ‡ University of Aarhus.
SBA-158,12 is one of the most well-known and studied mesoporous materials synthesized using nonionic surfactants. Syntheses of SBA-15 are most often performed under very acidic conditions, pH 15) was used, the fits were good and the influence from changes of ∆Fout/∆Fin was small. A fixed value of 20 was thus used for all full model fits for the later times. Another important parameter that can be extracted from the model is the disorder parameter, σa, shown in Figure 5B. It describes how disordered the material is and it is a kind of static Debye-Waller factor.
Pluronic-Templated Mesostructured Silica The results show that the disorder is at a constant low level for S2-5 for all the collected spectra. This indicates that S2-5 is well ordered from the first spectrum and that the changes of material with time are small. The other four syntheses start at a higher value and show an improved order with time. The time for the first value also differs between the syntheses. This is because the disorder parameter is only included in the full model and therefore the time for the first value for each sample is the same as the times of the appearance of the first Bragg-like peak in Figure 4. It can further be observed that the disorder parameter changes at different rates for the different syntheses and that it levels out at slightly different values. The final value decreases with increasing time between hydrolysis and mixing indicating that a better ordered material is obtained in S2-5 compared to S2-1, which is supported by the spectra shown in Figure 4 showing more narrow peaks for S2-5. The results for the parameters a, Rin and Rout obtained from the model are shown in Figure 5, panels C and D. The obtained results for these parameters correspond well with the results presented in our previous publication.22 Synthesis S3. The third synthesis was performed in order to support the findings presented from our SAXS measurements. The samples were analyzed using TEM and in situ DLS and the yield and composition at different synthesis times were also followed. Dynamic Light Scattering. The results from the in situ DLS study are shown in Figure 6. A measurement of a reference system containing Pluronic P123 at the same conditions as used in the synthesis but in absence of silica, gave a z-average diameter of 19 nm. This correlates well with earlier publications29 and comparing this value to the z-average values presented in Figure 6 shows that as the silica is added the hydrodynamic diameter of the micellar aggregates increases. Since the silica associates with the PEO within the corona the increase cannot be explained by the addition of a silica layer around the micelle. On the other hand, the micellar aggregates cannot grow as a sphere without the addition of a layer since the size of a spherical micelle is limited by the length of the surfactant. The increase in size is thus interpreted as due to that the silica changes the conditions in the corona, which alters the shape of the micelle. The increase in z-average diameter therefore indicates a change of micellar structure into a cylinder. Pluronic P123 is known to change its aggregate structure with increasing temperature and it has been shown that it attains a cylindrical shape in a 1% aqueous solution at 60 °C.29 Also a previous DLS study of a synthesis of SBA-15 claims that the shape of the first micellar aggregates are cylindrical.30 The growth of the micellar aggregates continues with time indicating a further increase of the aggregation number and the formation of longer cylinders. This is most likely caused by an increasing amount of silica present in the corona of the micelles. The two last correlation functions indicate that the sample contains larger particles. This shows that the silica/Pluronic micellar aggregates have started to form larger particles. These two correlation functions were difficult to analyze with both cumulant analysis and CONTIN. Several different settings were tested but the obtained residuals indicated that no reliable result could be obtained. This is probably due to the high polydispersity caused by the formation of the larger particles. The system is most likely ergodic since a nonergodic system would result in a lowering of the starting value for the correlation function, which is not observed here. The first visual observation of turbidity was made at about 150 min. The turbidity increased
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Figure 6. Results obtained from in situ DLS measurements of synthesis S3. Panel A shows the correlation functions obtained at different synthesis times during the synthesis. Panel B shows the values for z-average diameter obtained from the cumulant analysis of the first eight measurements.
rapidly and after 180 min the sample was clearly turbid and further DLS measurements were not possible. Yield and Composition Analysis. The development of the yield and the composition of the formed particles in the S3 synthesis was studied in more detail following the appearance of the larger particles. Small samples of the synthesis solution were collected at different times during the synthesis and analyzed. The formed particles were separated from the mother liquid using ultracentrifugation and the material obtained was used to calculate the yield and composition of the formed material at different times during the synthesis. The results from these analyses are shown in Figure 7. The first sample was collected 210 min after the start of the synthesis. The yield of silica increases fast in the beginning of the synthesis and the rate then decreases. After 24 h it is around 60% and after 15 days around 95%. Also the composition of the product changes with time. It follows the same trend as the yield with a faster change in the beginning. This can be explained as follows; considering the formation of the first particles these are formed from Pluronic micelles when enough silica is added to induce an attractive micelle-micelle interaction, as discussed further in the discussion of the formation mechanism. The surfactant/silica ratio in these particles will therefore be high. After their formation more silica and Pluronic from the solution may be added. However, it can be realized, from these results and the composition of the reaction solution,
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Figure 9. TEM micrographs of samples from synthesis S3 at different times during the synthesis. (A and B) 140 min, (C and D) 210 min, (E) 450 min, and (F) 24 h. Figure 7. Development of the product yield (9) and composition (0) of the dried product at different times during synthesis S3.
Figure 8. SAXS measurements of powder samples obtained from synthesis S3 at different times during the synthesis. The samples shown are representative for all samples studied.
that a relatively large amount, ca. 25%, of the added Pluronic is left in the solution also after 15 days. This represents a concentration of Pluronic at which it will probably not associate or aggregate but will remain in solution. The silica, on the other hand, has a comparably low solubility in water and almost all of the added silica will with time condense and form aggregates. This will then affect the composition of the particles with an increasing amount of silica with time. Small Angle X-ray Scattering. The surfactant/silica ratio is known to be very important for obtaining an ordered material and a change in this ratio with time might affect the order of the material. The powder samples from the S3 synthesis collected at different synthesis times were therefore analyzed using SAXS and the results are shown in Figure 8. The results show that already the first obtained material is well ordered. The Bragg-like peaks become narrower with increasing synthesis time and more peaks appear in the later data sets, reflecting an improved order with increasing synthesis time. The fact that the first sample is well ordered may appear
to somewhat contradict earlier in situ observations. However, it is important to remember that since the sample has been centrifuged, it only represents ca. 10% of the silica in the solution and it is therefore concentrated in ordered material. The material may also change during and after the separation. Transmission Electron Microscope. The S3 synthesis was also followed using TEM. The TEM analyses were carried out on samples collected at different times during the synthesis and samples were prepared by diluting some from the reaction solution with water. A drop of the obtained diluted solution was immediately added to a TEM grid, which was then dried in air. The results obtained are shown in Figure 9. The TEM micrographs show the development of the material with time. Figure 9 A shows that there are some indications of order already after 140 min. Since there are no larger aggregates present in the reaction solution according to the DLS results, these structures are probably an artifact of the TEM sample preparation procedure. The structure with elongated cylinders shown in Figure 9B is more representative of the sample at this stage. This structure also resembles the structure presented in an earlier study using cryo-TEM for studying the early stages of an SBA-15 synthesis31 and also supports the suggested structure from the DLS results. TEM images C and D show the structures soon after the formation of the first particles. These are spherical rather small assemblies of several cylinders. Larger ordered particles dominate later in the synthesis, here shown after 450 min in image E. In image F the very well-ordered structure obtained after 24 h is shown. Formation Mechanism. In order to summarize the results presented and relate the different findings to each other a formation mechanism is proposed. How the reaction rate of the proposed mechanism is affected by the degree of condensation of the starting material is also discussed. The mechanism comprises the three following steps. Step 1: Aggregation of Pluronic and Silica. The most detailed results obtained from the very early part of the syntheses are from the in situ SAXS and DLS measurements. Some information could also be gained from the TEM results from this stage. As mentioned in the introduction there is an ongoing debate about what actually causes the attraction between the silica and the Pluronic. The most common explanation is that hydrogen bonds are formed.6,8 It has, however, been argued, by Flodstro¨m et al, that since the syntheses use water as a solvent hydrogen bonds cannot by themselves be a sufficient explanation
Pluronic-Templated Mesostructured Silica for the attraction. The authors claim that there must be an additional contribution from a hydrophobic attraction.2 Based on the results presented in this paper we argue that the main attraction between the Pluronic and silica is due to the entropic part of hydrogen bonding. Since both the PEO and the silica are hydrated with water these bonds has to be broken in order to form bonds between PEO and silica. The breaking of hydrogen bonds to water results in a release of water molecules from a bonded stage into a free stage in the solution and thus an increase of the entropy of the system. A counteracting decrease in entropy accompanies the formation of bonds between the PEO and silica species. If the silica species is an oligomer with possibilities of multiple hydrogen bond formation the entropic gain will increase because more than one water molecule is replaced with the silica oligomer. By this reasoning the importance of the degree of polymerization experimentally observed here can be understood. The interaction can be viewed as a general oligomer-polymer interaction, where the silica is the oligomer and the PEO is the polymer. These types of interactions are expected to be very much affected by the number of active sites or chain length of the oligomer. A critical chain length, where the interaction strongly increases is also often observed.32 Publications studying silica sol-gel systems containing PEO supports this explanation and suggests that the attraction does not dominate in the very early stages of the condensation concluding that the silica oligomer must reach a certain size before there is a net attractive interaction.9–11 Step 2: Formation of Particles. After the formation of discrete silica containing micellar-like aggregates, these first grow in size and then associate, forming larger particles. This process is very similar to clouding of surfactants containing PEO, which occurs with increasing temperature. As the temperature of a micellar solution of these types of surfactants is increased the PEO headgroup becomes less polar. The amount of water in the palisade layer is then decreased. This reduces the headgroup area leading to an increase of the aggregation number of the micelle and thus a growth of the micelles. Favorable overlap between the headgroup regions of micelles generates an attractive micelle-micelle interaction that triggers the phase separation.33 Here the same process can be observed but instead of increasing the temperature the amount of silica aggregated with the PEO is increased. The addition of the silica to the PEO changes the environment in the corona which leads to an increase of the aggregation number. At some point the attractive forces dominate and phase separation occurs. The results presented show that the time between the start of the synthesis and the phase separation is very dependent on the degree of polymerization of the silica solution used. It has previously been shown that the addition of silica into the PEO layer of Pluronic micelles causes a depletion of water from the corona1 indicating a similar behavior of the surfactant as when the temperature is increased. Step 3: Ordering of Formed Particles. After the assembly of the silica containing micelles into particles the silica and surfactant must rearrange to form an ordered structure. This step in the formation is best studied with the in situ SAXS and the TEM results. The TEM results indicate a gradual change where the first particles formed are small and relatively disordered and the final larger particles are well ordered. The in situ SAXS measurements also support this gradual change. However, it is important to remember that the obtained spectra represent an average of the samples studied and that several processes therefore may overlap. Both the formation of new particles from single Pluronic/silica micellar aggregates and the ordering within
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Figure 10. Illustration of the formation mechanism. The addition of the silica to the Pluronic solution induces an elongation of the micelles. As the degree of silica condensation increases the interaction with the Pluronic also increases. The larger amount of silica in the PEO layer induces further growth of the Pluronic/silica micellar aggregates. A phase separation then follows where some of the cylindrical micelles assemble into particles. The final step is the ordering of these particles into a hexagonal structure. The particles start as spherical and disordered. They then grow in size and the ordered domains start to form within the particles. With time the entire particle is one ordered domain. This also affects the shape of the entire particle which evolves into an elongated structure.
already formed particles may occur at the same time. It is therefore likely that the process of rearranging the silica and the Pluronic within the particles is a faster process than apparent in some of the in situ SAXS measurements. This is supported by the results for S2-5 showing that ordered material can form relatively fast. The SAXS results of the powder samples may seem to contradict the proposed processes of reorganization showing well-ordered material directly after the formation of particles. However, the separation of the particles by centrifugation concentrates the sample in ordered material as compared to the solution in which they form. In addition, it is not clear that the process of rearrangement within the aggregates is stopped as the particles are separated from the reaction solution. To summarize the mechanism the three steps are all schematically described in Figure 10. The proposed mechanism shows many similarities with earlier presented mechanisms for the synthesis of SBA-15.1–5,30,31,34 The initial step of an elongation of the micellar structure is supported in several of the mechanisms using both scattering techniques4,5 and cryo-TEM.31 The results from the cryo-TEM study by Ruthstein et al. also support the formation of larger assemblies from the primary micellar structures and the reorganization and growth of ordered domains with time. These later stages are also described in a similar way by Flodstro¨m et al. where they also emphasize the importance of controlling the rates of the different steps and especially the progression of silica polymerization.2 Reaction Rate. All of the syntheses studied here follow in principle the suggested reaction mechanism. The reaction rates
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are, however, very different and for some syntheses the different steps are difficult to distinguish from each other. Since the main difference between the samples is the time between start of the hydrolysis and the addition of the Pluronic, it can be concluded that the rate of the reaction is strongly affected by the degree of condensation of the silica. It is also shown that this has a strong effect on how ordered the final structure will be. The best example of this is the S1 sample series where S1 and S3 are relatively disordered while S2 is well ordered. Starting with S1-1 this shows that a too low reaction rate in the first step results in a disordered material. Since the concentration of silica is increased in the corona compared to the bulk solution it is reasonable to assume that the rate of the condensation reaction of the silica oligomers is faster in the corona. As the condensation progresses, the degree of polymerization increases and the material solidifies. If the silica that has associated with the corona condenses too much and forms a solid structure before the synthesis reaches the second stage, where the larger particles form, the structure of the material will be disordered. This is because when the single cylinders assemble into larger particles this is a relatively fast process and the first formed particles are not well ordered. The silica and the Pluronic in the particles must be able to rearrange into the most energetically favored structure, which using these compositions is an ordered hexagonal structure. The results presented here indicate that the reorganization is relatively fast but it is still important that the third step occurs. It is therefore essential that the rate of the aggregation between silica and Pluronic and the resulting assembly is fast enough relative to the condensation of the silica present in the corona so that the reorganization can occur before the structure is fixed. For S1-3 the reaction rate is fast enough but the obtained material is still disordered. This demonstrates that if the starting material is too condensed, the organization of the silica into an ordered material will be hindered by the silica structures already formed before the addition of the Pluronic. Conclusions The results presented here focus on the formation mechanism of mesostructured silica using nonionic surfactants and illustrate the importance of the state of the silica source used in the synthesis. The formation mechanism presented can shortly be described as follows: When the silica and Pluronic is mixed there is an entropic attraction between the PEO and the silica. The attraction results in the formation of silica-containing micellar aggregates. As more silica is incorporated the micellar aggregates grow. The incorporation of silica leads to a micelle-micelle attraction, which induces a phase separation, resulting in that the micellar aggregates form particles. A reorganization of silica and Pluronic in the particles is the subsequent and final step in the formation of the hexagonally mesostructured silica. It is also shown that the rate of the reaction can be controlled by the degree of condensation of the silica solution used in the synthesis and that the rate influences the order of the final product. This was illustrated by varying the
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