Silica–Surfactant–Polyelectrolyte Film Formation: Evolution in the

Article pubs.acs.org/Langmuir

Silica−Surfactant−Polyelectrolyte Film Formation: Evolution in the Subphase Bin Yang,† Robben Jaber, and Karen J. Edler* Department of Chemistry, University of Bath, Claverton Down, Bath, Avon, U.K. BA2 7AY S Supporting Information *

ABSTRACT: We have previously reported that robust mesostructured films will grow at the surface of alkaline solutions containing cetyltrimethylammonium bromide (CTAB), polyethylenimine (PEI), and silica precursors. Here we have used time-resolved small-angle X-ray scattering to investigate the structural evolution of the micellar solution from which the films form, at several different CTAB−PEI concentrations. Simple models have been employed to quantify the size and shape of the micelles in the solution. There are no mesostructured particles occurring in the CTAB−PEI solution prior to silica addition; however, after the addition of silicate species the hydrolysis and condensation of these species causes the formation of mesophase particles in a very short time, much faster than ordering observed in the film at the interface. The mesophase within the CTAB−PEI−silica particles finally rearranges into a 2D hexagonal ordered structure. With the aid of the previous neutron reflectivity data on films formed at the air/water interface from similar solutions, a formation mechanism for CTAB−PEI−silica films at the air/water interface has been developed. We suggest that although the route of mesostructure evolution of the film is the same as that of the particles in the solution, the liquid crystalline phase at the interface is not directly formed by the particles that developed below the interface.

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INTRODUCTION Self-assembly in surfactant-templated materials has been extensively studied since Mobil scientists reported the formation of highly ordered M41S mesoporous silicates templated by cationic quaternary ammonium surfactants.1 The resulting materials are composites containing an organized surfactant micelle array embedded in an inorganic matrix. Removal of the surfactant generates nanoscale pores which replicate the highly organized micelle phase. A proper understanding of how the inorganic species interact with the surfactant micelles during the self-assembly process will greatly assist synthetic control over the structure and help to improve properties required for potential applications. The earliest proposed mechanisms suggested that silica infiltrated into a surfactant aggregate which already had a liquid crystalline structure; however, this mechanism was quickly discarded because in most syntheses the surfactant solution was initially too dilute for concentrated phases to form. At the same time, a mechanism involving inorganic-coated micelles, which directly aggregated to form the ordered mesophase composite, was suggested.2−4 Since then, this proposed mechanism has been further refined to include the charge interaction between the inorganic species and micelles in alkaline solutions.5 Inorganic species are however now also recognized to play a prominent role in structure formation since the gradual binding of surfactant ions to the polymeric silica species causes initial aggregation to form the neutral mesomorphous silica− surfactant composite.6−8 A phase separation resulting from this binding of surfactant to silica oligomers was first put forward by Chan et al.,9 who observed a liquid−liquid phase separation which formed droplets of concentrated silica © 2012 American Chemical Society

oligomers and surfactant. These droplets rearranged and further silica condensation caused microphase separation within the droplets, producing the organized mesostructure. This inorganic-driven phase-separation mechanism has gained further experimental support, suggesting that generally interacting inorganic and organic species aggregate into larger liquid-like particles, where microphase separation of the inorganic and surfactant under high concentration conditions occurs, resulting in the formation of the final mesostructure.10,11 Spontaneous growth of mesostructured films of surfactant and silica at an interface was first reported from acidic solutions on mica substrates,12 while Schacht et al. showed the possibility of growing silica films at the oil−water interface also from acidic solutions.13 In 1996, the spontaneous growth of free-standing mesostructured silica films at the air−solution interface was separately reported by Yang3 and Aksay.14 The strategy of fabricating films at the air−solution interface, based on the spontaneous buildup of ordered structure, has provided an efficient way to study the formation mechanism of surfactant templated mesoporous materials under acidic conditions. Initial experiments suggested that films formed through the packing of mesostructured particles from the bulk solution at the interface.15 Further X-ray reflectivity experiments showed that the formation mechanism was concentration dependent. At low and high silica-to-surfactant molar ratios, film formation occurred through the slow addition of single silica-coated Received: April 7, 2012 Revised: May 7, 2012 Published: May 9, 2012 8337

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forms.17,36−38 However, there are no previously reported timeresolved studies on the evolution of CTAB micelles in the presence of PEI and silica precursors. The aim of this work was to observe the real time evolution of structure in CTAB−PEI−silica film-forming solutions using synchrotron-based SAXS. We studied PEI with two different molecular weights, and different CTAB and PEI concentrations with a fixed silica precursor concentration. The growth of surfactant micelle−polyelectrolyte complexes in the presence of silica precursor was continuously followed as a function of time. With the aid of neutron reflectivity and grazing incidence diffraction data from films grown at the air/water interface under the same conditions, a formation mechanism of these CTAB−PEI−silica films at the air/water interface has been developed. This study will assist synthetic control over the structure of inorganic films templated by surfactant−polyelectrolyte complexes to direct efforts to improve the properties of inorganic oxide films grown at the air/water interface toward specific applications.

micelles to the interface; however, in a narrow range of concentrations, initial silica−surfactant particle formation was responsible for rapid film growth. As in the phase separation mechanism observed for alkaline syntheses of powders above, large liquid-like particles formed in the subphase. Microphase separation of the inorganic and surfactant then occurred under high concentration conditions, forming the mesostructured particles that added to the surface, resulting in growth of a film with the final 2D hexagonal mesostructure at the solution interface.16,17 Polymerizing silica acts as a neutral polyelectrolyte during the formation of nanostructured silica−surfactant films;18 thus, substituting the silica with a carbon-based polyelectrolyte also results in formation of a film having an ordered liquid crystalline structure at the air/water surface. Freestanding surfactant−polyelectrolyte films of this type have been extensively studied by our group.18,19 Polymer molecular weight, pH, ionic strength, cross-linker, and temperature all play an important role in defining the film structure.20,21 Smallangle neutron scattering results indicate there is no liquid crystalline ordering in the bulk solution under these films, and with the aid of neutron and X-ray reflectometry, an “evaporation-driven steady-state” formation mechanism has been proposed.22 Dehydration of the upper layer of the solution via evaporation prompts aggregation from the bulk solution to the interface, and as the distance from the interface increases the dehydration degree obtained will decrease until the enthalpic benefit becomes too small to promote further aggregation. Both phase separation and mesophase ordering only occur within the film in these polymer−surfactant systems.21 Here we investigate use of these polymer−surfactant film forming systems as an alkaline, rather than acidic, route to robust mesoporous silica films. Using complexes of oppositely charged polyelectrolytes and surfactants as templates in the presence of a silica source results in a series of mesoporous silica powders with different pore structures and morphologies, templated on the entire polymer−surfactant composite (i.e., both species occupy the pores).23 In contrast, we reported spontaneous silica film formation using cetyltrimethylammonium bromide (CTAB)− polyethylenimine (PEI) complexes both as template and to drive formation as a film at the air/water interface. In this case interactions between the cationic surfactant and the largely neutral polymer are responsible for film formation while the surfactant micelles alone template the pores in the polymer− silica matrix. These films could be easily removed from the surface to produce dried freestanding silica−polymer−surfactant films. Most films displayed 2D hexagonal mesostructures and retained their mesostructures even after the removal of template. Notably these films form at high pH, whereas previous work on film formation, including dip and spin coating, as well as interfacial growth has required acidic solutions.24 Thus, investigation of the formation mechanism of this robust CTAB−PEI−silica film at the air/water interface is of interest to develop new routes to mesoporous membranes. Self-assembly processes between inorganic and organic species and the formation of mesostructure have been followed by many techniques, including TEM,10,25 SEM,9,26 EPR,4,27,28 NMR,29,30 small-angle X-ray diffraction,31 and small-angle neutron scattering (SANS).32,33 In addition, in situ smallangle X-ray scattering (SAXS)33−37 has also been used to obtain kinetic details of the formation and phase transformations in these materials even after a film or precipitate

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EXPERIMENTAL SECTION

Branched PEI (MW = 750 000 (LPEI); 2000 Da (SPEI)) as 50 wt % solutions in water, cetyltrimethylammonium bromide (CTAB), and tetramethoxysilane (TMOS) were purchased from Sigma-Aldrich and used without further purification. Ultrapure Milli-Q water (18.2 MΩ cm resistance) or D2O (Sigma-Aldrich) was used as the solvent. Film forming solutions were prepared using the same conditions used to prepare CTAB−PEI−silica films at the air/water interface in our previous report.24 Briefly, CTAB and PEI stock solutions were prepared separately and then mixed to give the desired final concentration. TMOS was then added to this solution under stirring. The CTAB concentration was varied from 0.009 to 0.037 M and the PEI concentration from 15 to 40 g/L. The TMOS concentration was constant at 0.084 mol/L. The solution pH was between ∼9 and 10, depending on the PEI concentration and was not otherwise adjusted. A purpose-built flow system consisting of a reservoir containing the surfactant−polymer−silica solution under continuous stirring, tubing connecting the reservoir to the sample X-ray capillary and a peristaltic pump to circulate the solution was used on the small-angle scattering instrument I22 at Diamond, UK. An X-ray wavelength of 0.0827 nm and a 3.2 m flight tube was used with the RAPID 2D SAXS detector, giving a Q range of 0.7−8 nm−1. During the SAXS measurements, the aqueous CTAB−PEI mixture was first circulated continuously through the 1.5 mm diameter quartz capillary while a pattern was collected for 5 min. The interval of data collection was then decreased to 20 s, and SAXS data collection restarted. Tetramethoxysilane was added to the solution reservoir after 80 s, and this mixed CTAB−PEI−TMOS solution continued to circulate while patterns were collected for 30 frames of 20 s each. At this point the solution had become cloudy and the interval of data collection was increased to 100 s, and data were collected until no further changes were observed in the SAXS patterns. The SAXS data was analyzed using a method of a simultaneous nonlinear least-squares fitting using models in the SANS analysis software developed by the SANS group at the NIST Centre for Neutron Research, within the IGOR PRO platform (WaveMetrics).39 The SANS models were modified with a multiplying scale factor to account for the fact that the X-ray data is not on an absolute scale. Small-angle scattering measures the absolute scattering cross section I(Q) of a sample as a function of the modulus of momentum transfer Q, where Q = (4π sin θ)/λ and 2θ is the scattering angle while λ is the wavelength. I(Q) is proportional to the product of the form factor P(Q) and the interparticle structure factor S(Q) 2 I(Q ) = NV i i (Δρ) P(Q )S(Q ) + B

(1)

where Ni is the number of scattering bodies, Vi is the volume of one scattering body, Δρ is the difference in scattering length density (contrast) between the scattering body and the surrounding medium, 8338

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Figure 1. Time-resolved SAXS patterns showing the evolution of CTAB−LPEI−silica solutions, for the concentrations indicated on each graph and in Table SI1. The TMOS concentration was 0.084 M, and the pH of all initial LPEI solutions was between 9 and 10. and B is the background. The form factor describes the intensity due to scattering by different parts of the same scattering body, while the structure factor describes interactions between particles. The scattering patterns from solutions of CTAB with high molecular weight LPEI prior to silica addition and just after addition of TMOS are similar. Attempts to fit this data to a core−shell prolate micelle previously used to fit neutron scattering patterns for static PEI/CTAB solutions21 did not give physically realistic results, and a range of possible micelle models from spherical to cylindrical and lamellar were also tried without success, leading to use of a more complex model. Since LPEI alone shows appreciable small-angle X-ray scattering, a model summing Debye40 scattering with a charged prolate core−shell ellipse41,42 model was used to account for contributions from the dissolved polymer and the micelles, respectively. The Debye function, 2(e−x + x − 1)/x2, where x = qRg2 represents the form factor for a polymer chain with Gaussian coil in dilute solution, giving a radius of gyration Rg. The micelles in solution were fitted as charged prolate ellipsoid particles with a uniform shell thickness.41,42 As the silicate species hydrolyze and condense and the silica− surfactant micelles began to aggregate, the combined Debye-prolate ellipse model no longer fitted the data so a combined fractal and prolate ellipse model was used instead. The fractal model describes the scattering from fractal-like aggregates built from spherical units,43 here assumed to be silica-coated micelles. The form factor P(Q) is the scattering from randomly distributed “building block” particles, having radius R0, and volume fraction ϕ: P(q) = ϕVpΔρ2F(qR0)2 where Vp = (4/3)πR03; the interference from building blocks in fractal-like clusters was calculated as

S(q) = 1 +

sin[(D − 1) tan−1(qξ)] (qR 0)D

Interactions between charged micelles were accounted for using the Hayter−Penfold mean spherical approximation (MSA) structure factor.45,46 The MSA allows for inclusion of interparticle interference effects due to screened coulomb repulsion between charged spherical particles. For all fits the dielectric constant of the solution was approximated to be close to that of water (78), the temperature set to 298 K, and the micelle charge (20) was also held. The monovalent salt concentration was calculated according the PEI concentration in solution for LPEI and the expected charge on this polymer at the synthesis pH (around 10)21 (Table SI1), and this was held during fitting. The SLD of the micelle cores was calculated to be 7.45 × 10−6 Å−2, by assuming that only hydrocarbon groups from the surfactant were present. The initial SLD of the solution was calculated from the proportions of PEI and water to be 9.41 × 10−6 Å−2; however, after TMOS hydrolysis, which generates 0.336 M methanol (assuming hydrolysis is complete), this value decreases to 9.02 × 10−6 Å−2. Micelle volume fractions were calculated according to the surfactant concentration, molecular volumes,42 and total solution volume (Table SI1) and were also held during fitting. In total therefore for the SAXS patterns from solutions containing LPEI six variables were fitted when the combined Debye and core−shell ellipse model was used (Rg, major and minor radii, shell thickness and SLD and a scale factor) and 10 for the combined fractal and core shell ellipse model (replacing Rg; fractal dimension, block radius, correlation length and the block SLD must be added), while for SPEI using the simpler charged uniform ellipse model only four variables were fitted (major and minor radii, salt concentration and contrast). Neutron reflectivity profiles were recorded for films grown on solutions at the same PEI, silica and CTAB concentrations as used for the SAXS experiments, on the INTER instrument47 at the ISIS Pulsed Neutron and Muon source, Rutherford Appleton Laboratories, Chilton, England. The incident angle used for the reflectivity experiment was 2.3°, with a range of wavelengths so that data were collected between 0.035 and 0.326 Å−1 on D2O at room temperature for 30 s per pattern. Final interfacial silica film structures were investigated in situ by grazing incidence X-ray diffraction (GIXD) on films grown in open 15 × 4 cm Teflon troughs at 28 °C on beamline ID10B48 at ESRF, using a wavelength of 1.55 Å.

D Γ(D − 1) ⎡ ⎢⎣1 +

1 2 2

q ξ

⎤(D − 1)/2 ⎥⎦

(2)

where ξ is the correlation length and D is the self-similarity fractal dimension. The mean number of blocks per cluster, the aggregation number, can be calculated as G = Γ(D + 1)(ξ/R0)D, and the Guinier radius of the cluster can be estimated as Rg = D(D + 1)ξ2/2. To fit scattering from CTAB and low molecule weight SPEI mixtures and the initial stage after adding TMOS, a simple charged uniform ellipse model was employed.44 This model describes the form factor for an ellipsoid with uniform scattering length density (SLD) since this data does not support use of a more complex model. SPEI is molecularly dissolved and solutions at equivalent concentration in the absence of micelles do not have significant small-angle scattering, so in these solutions any contribution from the polymer scattering was neglected.

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RESULTS SAXS patterns, with a time resolution of 20 s, were collected during precipitate formation in film-forming solutions. SAXS profiles as a function of time for CTAB−LPEI−silica film forming solutions at different CTAB and LPEI concentrations are shown in Figure 1. Immediately after the addition of TMOS 8339

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Figure 2. (a) SAXS patterns for CTAB-LPEI solutions (A−E) before TMOS addition. (b) Time-resolved SAXS patterns for solution E at 0.037 M CTAB, 20 g/L LPEI after TMOS addition for different times. Lines are fits to the data using the models described in the text.

Table 1. Results of Fitting CTAB−LPEI Solutions before TMOS Addition to a Summed Debye and Prolate Core−Shell Ellipse Modela PEI:CTAB molar ratio LPEI (g/L) CTAB (M) Debye Rg (Å) major core radius (Å) minor core radius (Å) shell thickness (Å) SLD shell (Å−2) micelle core volume (nm3) aggregation number shell volume (nm3) shell volume occupied by CTAB headgroups (nm3) PEI monomer volume in shell (nm3) water volume in shell (nm3)

A

B

C

D

E

103 40 0.009 152 77 15 7 9.44 × 10−6 69 152 96 16 15 66

50 40 0.0185 152 86 15 11 9.59 × 10−6 80 175 188 18 67 103

25 40 0.037 80 87 19 15 9.64 × 10−6 125 274 341 28 145 168

19 30 0.037 119 96 16 12 9.79 × 10−6 104 228 251 23 140 88

13 20 0.037 134 89 17 13 9.84 × 10−6 108 235 265 24 159 82

Volumes of the micelle core, shell, surfactant headgroup, PEI monomer, and water as well as the aggregation number were calculated from fit results, using values for the molecular volume of the CTAB head and tail from Berr et al.42 For PEI monomers the molecular volume was approximated from the density of ethylamine. Errors in radius of gyration are ±10 Å, in the minor core radius and shell thickness are ±2 Å, in the major core radius are ±5 Å, and in the shell SLD values are ±0.5 × 10−7 Å−2. The errors in the calculated values are ±5 nm3. The errors specified indicate the sensitivity of the fit to each parameter. a

observed SAXS curves. Our previous work on CTAB−LPEI solutions without silica suggested that PEI only weakly interacts with CTAB micelles at concentrations similar to those used here. The dipole−cation interaction, between dipoles on the polymer amines and the charged CTAB quaternary ammonium group,21 caused elongation and narrowing of ellipsoidal CTAB micelles as the LPEI concentration increased but was not a strong effect. Thus, the polymer largely retains its random coil configuration in solution. Therefore, the initial CTAB−LPEI solution patterns, and the patterns taken immediately after TMOS addition were fitted with summed Debye and charged prolate core−shell ellipse models, which describe the CTAB micelles in solution with the hyperbranched polymer. Fitted SAXS patterns of the initial CTAB−LPEI mixtures (Figure 2a, fitting parameters in Table 1 and Table SI1) and the CTAB−LPEI−TMOS film forming solution in the induction period (Figure SI2, fitting parameters in Table SI3) are presented. In the present study, the polymer Rg decreased from 152 to 79 Å as the CTAB concentration increased from 0.0185 to 0.037 M (Table 1A−C), indicating that CTAB molecules pull PEI into a more compact configuration at the highest CTAB concentration. As the molar ratio of PEI monomers to CTAB molecules is then further decreased, the Rg increases

there is little change in the scattering pattern, and this lasts for a few 20 s frames of data taken on the circulating solutions. After this a diffraction peak grows rapidly, and this peak continues to evolve for some time. The evolution of the subphase species will be discussed in two sections, during the induction period before the appearance of the diffraction peak, and in the period after the appearance of liquid crystalline structure in the solutions. Induction Period in CTAB−LPEI−Silica Film Forming Solutions. During the induction period, after addition of TMOS but before any precipitate was observed in the transparent flow tubes, the SAXS patterns do not show any Bragg diffraction peaks but do show significant structural evolution (Figures 1 and 2). The first SAXS profiles of the CTAB−LPEI−TMOS solution, 20 s after TMOS was added, are similar to those of the initial CTAB−LPEI solutions since the silica precursors had only just begun to hydrolyze and interact with the micelles. Attempts to fit this early data to a simple shape (with structure factor) did not give physically realistic results. Given the X-ray SLDs for CTAB and PEI compared to the solvent (CTAB tails: 7.45 × 10−6 Å−2; CTAB head groups: 9.01 × 10−6 Å−2; PEI: 1.08 × 10−5 Å−2; water: 9.45 × 10−6 Å−2), both species contribute significantly to the 8340

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Figure 3. (F−J) Time-resolved SAXS patterns showing the evolution of CTAB−LPEI−silica solutions, for the concentrations indicated on each graph and in Table SI1. The TMOS concentration was 0.084 M. (K) Example SAXS pattern fitted to a charged uniform ellipse model for solution H, 20 s after the addition of TMOS. Fits to the other patterns are shown in the Supporting Information, SI5 and SI6.

again, suggesting that at lower PEI concentrations micelles interacting with the polymer chains cause them to swell and take on an expanded configuration. The fits suggest the prolate ellipsoid micelles have a minor core radius around 16 Å, and the major core length grows from 154 to 192 Å as the relative CTAB concentration increases. The shell thickness is between 7 and 15 Å, giving a total micelle length about twice that reported by O’Driscoll et al. using a uniform ellipsoid model to fit SANS patterns of CTAB−PEI solutions at similar concentrations.21 The micelle shell is composed of a mixture of water, CTAB headgroups, and some PEI, reflected in the fitted SLD values for this region. Assuming the volume of the CTAB headgroup and aggregation number calculated using the CTAB tail volume,42 the shell SLD was used to calculate the relative volumes occupied by water and PEI monomer units in the shell (Table 1). At low CTAB, high PEI concentration (sample A) the shell is thin, with a low SLD indicating a significant amount of water within the shell. Increasing CTAB concentration at high PEI concentrations leads to an increase in aggregation number, SLD and shell volume, suggesting that larger micelles hold more PEI close to the micelle surface (Table 1A−C). The relative amount of PEI monomer units and water in the shells is maintained at a ratio of ∼1.4 for this PEI concentration. When the CTAB concentration is held constant and the amount of PEI reduced (Table 1C−E), the proportion of water in the shell decreases, although the amount of PEI in this region remains roughly the same. This suggests that there is a positive interaction between the CTAB headgroups and PEI, which is proportional to the surface area of the micelle. However, the hydration of the branched PEI within the shell decreases as the overall solution PEI concentration decreases possibly due to lower molecular crowding enabling a single (or few) branched polymer chains to bind more closely to the micelle surface. As shown in Figure 1, immediately after TMOS was added to the LPEI−CTAB solution, most of the SAXS patterns are similar to that of the initial CTAB−LPEI solution since TMOS rapidly undergoes partial hydrolysis at this pH and is dispersed in the solvent. Fitting of patterns taken 20 s after TMOS

addition to the same models gave similar results to those discussed above (Figure SI2 and Table SI3). At most concentrations studied, a diffraction peak appears in the SAXS patterns about 1 min after TMOS addition, as the silicate precursor has begun to condense and to interact with the surfactant micelles and polymer, driving particle formation in solution. At this stage, fitting the data to a simple model becomes less appropriate since it is difficult to describe the interparticle interactions and the correct structure factor for disordered close-packed charged particles is complicated. However, for a CTAB concentration of 0.037 M with a low LPEI concentration of 20 g/L (Figure 2b), the whole micelle evolution process becomes slower. SAXS patterns of the induction period (especially 100 s after TMOS addition) could no longer be fitted with the summed Debye and prolate ellipse model. Thus, considering that in this time period silica and micelle/polymer species are rapidly aggregating, we used the same charged prolate ellipse model for the micelles but combined instead with a fractal structure representing the aggregating silica network. Fitting parameters for the two curves 80 and 100 s after TMOS addition are shown in Table SI4. The results suggest that there is little change in the short axis of the micelles, but they shrink along the long axis during the process of TMOS hydrolysis and initial condensation. The shell thickness increases from 12 to18 Å but then rapidly shrinks to 12 Å, while the SLD of the shell increases from 9.84 × 10−6 to 1.24 × 10−5 Å−2 as the silica condenses. Since the proportions of surfactant headgroups, PEI, and solvent in the shell are not expected to be changing greatly during this period, the increasing shell SLD is determined by the penetration and condensation of silica species within the shell and the methanol released by the TMOS hydrolysis. A similar micelle−silica condensation phenomena has been observed by Boissiere49 and Linton.50 The results of fitting the aggregating silica to a fractal model suggests the aggregate is formed from components with a radius similar in size to the cross section of a surfactant−silica micelle in both the 80 and 100 s patterns. However, the correlation distance is more than twice the surfactant−silica micelle diameter in 80 s pattern, while it is much smaller in the 8341

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Figure 4. Peak area (A) and full width at half-maximum (B) of the first diffraction peak for time-resolved SAXS patterns from CTAB−SPEI−TMOS mixtures. TMOS concentration is 0.084 M. Errors are very small and within the symbols.

peak, so the accumulation of silica around the micelles could not be modeled for these systems. Formation and Evolution of Mesostructured Particles in the CTAB−PEI−Silica Solutions. Time-resolved SAXS has also been used to observe the evolution of the mesostructure after particle formation. Following addition of TMOS, its partial hydrolysis and condensation, the solution quickly becomes opaque and a white solid precipitate was observed after about 1 min. As soon as the precipitate formed, the SAXS patterns show a diffraction peak. The whole process is fast, similar to formation of CTAB−silica precipitates in alkaline preparation where films do not form,51 and much shorter than the time required for precipitation in acidic CTAB−silica solutions where films form without PEI.17 The time required for particle formation had no dependence on the molecular weight or concentration of PEI used. However, the process of mesostructural ordering within the particle differs at different PEI concentrations (Figures 1 and 3). For example, at a constant CTAB concentration of 0.037 M, and very low PEI (e.g., 20 g/L for LPEI Figure 1E and 15 g/L for SPEI Figure 3J), a relatively wide diffraction peak was observed 80 s after TMOS addition in both cases, and this peak does not change with time, indicating a lack of long range mesostructural order in these particles. For increased LPEI concentrations, this initial diffraction peak continuously becomes sharper and other peaks grow in, so that finally the SAXS patterns contain three intense reflections, characteristic of a 2D hexagonal mesostructure. However, at increased SPEI concentrations, the SAXS patterns change abruptly with time from a broad bump to three diffraction peaks. This suggests that the micelles self-assemble very rapidly and directly into a highly ordered 2D hexagonal structure. For SAXS patterns containing diffraction peaks, the peak center, peak area, and full width at half-maximum of the firstorder diffraction peak were measured using a peak fitting routine in IGOR Pro. Examples are given in Figure 4 and Figure SI7. Notably, in all samples, there is a distinct jump in the peak area and peak width between 2 and 4 min after the addition of TMOS. This change is likely to be due to a rearrangement within the liquid crystalline particles although the mesophase transition cannot be specified due to the lack of more than one distinct diffraction peak before this point. During the phase transition the initial micelle ordering is reduced, causing an increase in full width at half-maximum of the first-order peak until the transition is complete, at which point the peak narrows again as the long-range micelle order is restored. The width of the observed peaks is not limited by the instrument resolution so the observed lack of further change in

100 s pattern. The overall SLD of the fractal object increases, suggesting that the micelles begin to pack closer together into particles and the particles become more condensed as silica polymerization continues. The number density of building blocks was calculated from the model to give the mean number of blocks per cluster and Guinier radius of the cluster.43 For the 80 s pattern, the Guinier radius is 125 Å and the aggregation number is 16, while for the 100 s pattern, the Guinier radius is 100 Å and the aggregation number is 19. Using the equations given in the Experimental Section, an error of roughly 2.5 can be estimated for the aggregation number (which arises largely from the error in cluster radius). Overall, therefore these results describe a trend of micelles aggregating within 80 s while the cluster continues to get smaller but more dense with time as the silica polymerization proceeds. Induction Period for CTAB−SPEI−TMOS Solutions. SPEI has a lower molecular weight than LPEI and is less branched, thus disperses in solution, causing no visible contribution to the SAXS patterns. It was therefore ignored in the fitting except for its effect on screening the charge on the micelles. SAXS patterns from the CTAB−SPEI solutions were fitted to a charged uniform ellipse model as described above, e.g., Figure 3K (further fitting is presented in Figure SI5 and Table SI6). The micelle volume fraction was calculated from the CTAB concentration (Table SI1) and held during fitting and, as for LPEI solutions, the sample background, micelle charge, temperature, and dielectric constant were also held. The results (Table SI6) suggest these micelles are almost spherical in shape with their major and minor radii both around 12 Å. These radii are smaller than those reported for CTAB micelles by Berr et al.42 Considering the scattering length densities, the major change is between the tail region of the micelle, and the headgroup/water/PEI at the micelle surface, since the mixture of headgroup, PEI and water in the shell region will have a combined SLD which is close to that of the SPEI/water solution surrounding the micelles. Thus, the ellipse measured in this fit is largely the area occupied by the surfactant tails. The small dimensions indicate that PEI chains and water have partly penetrated around the surfactant headgroups, reducing the region solely filled by the surfactant hydrocarbon chain. When TMOS was introduced to the CTAB/SPEI solution, the SAXS patterns again do not change immediately, probably, as for LPEI above, because silica has not started to accumulate in the shell within 1 min after addition. After this point evolution of the scattering was too rapid to observe intermediate patterns prior to the appearance of the diffraction 8342

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Figure 5. Peak center position of the first diffraction peak in time-resolved SAXS patterns for CTAB−PEI−TMOS mixtures.

peak width suggests that some factor is limiting domain growth, possibly the overall particle size. The peak center position of the first diffraction peak as a function of time has also been plotted (Figure 5). At the beginning, just after TMOS addition, the peak center positions vary significantly as the micellar solution forms particles and undergoes rearrangement of the mesophase. Most samples show a rapid increase in the first-order peak position (indicating a shrinking d-spacing) followed by a decrease (reswelling), ending about 5 min after addition of TMOS. During this period other peaks appear at higher Q values, and the ratio of the peak intensities for these reflections is changing. As the reaction proceeds, the peak positions stabilize, generally at a higher Q (smaller d-spacing) than the initial Q value observed for that sample. For example, for a solution with a CTAB concentration of 0.037 M with SPEI at 40 g/L (Figure 5C), the Q value of the first peak position increases from 0.145 to 0.147 Å−1, corresponding to decrease of the d spacing from 43.3 to 42.7 Å. This overall contraction of the d-spacing can be linked to the continuing hydrolysis and condensation reactions taking place within the silicate wall and is normal for MCM-41 materials.52 Ten minutes after the addition of TMOS, most of the patterns show highly ordered 2D hexagonal mesostructures, and these mesophase particles are stable under the conditions in the subphase solution. For lower CTAB concentration, different trends are observed and a greater amount of PEI is needed to maintain mesostructural ordering. For example, at 0.009 M CTAB and 40 g/L SPEI (Figure 3F) the particles are less well ordered than those at the same SPEI concentration but higher CTAB content. Peaks in SAXS patterns for solutions with lower CTAB concentration occur at smaller d-spacings (Figure 5B,D). In the final stable particles, at 40 g/L LPEI as CTAB concentration decreased from 0.037 to 0.0185 or 0.009 M, the d-spacings decrease by 0.51 and 0.76 Å, respectively (Figure 5B). This reflects the results from fitting micelles in PEI solution (Table 1) since at the lowest CTAB concentration the volume of LPEI

in the shell region is also lowest, which would enable the micelles to pack closer together after addition of silica, resulting in the observed smaller d-spacings.

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DISCUSSION Previously, we have reported that highly ordered silica films can be templated by CTAB−PEI complexes at the air/water interface. Here, the subphase solution of this film forming system has been studied by time-resolved SAXS. Overall, the data indicate three stages for the evolution of micelles in the bulk solution: an induction period, formation of particles with a disordered close-packed micellar structure, followed by micelle rearrangement into an ordered phase. Prior to TMOS addition, no particles or mesophase ordering were observed in the CTAB−PEI solutions, which is similar to previous reports on formation of CTAB−PEI only films from similar solutions.21 At the very beginning of mixing these micelles are prolate ellipsoids in LPEI film forming solutions and almost spherical in SPEI film forming solutions. During the induction period, just after TMOS addition but before formation of particles, CTAB micelles remain dispersed in the aqueous polymer solution. Nearly 1 min after the silicate precursor was added, we observe formation of particles with a certain degree of micelle close-packing evident from the peak which appears in the time-resolved SAXS data. The solution has thus entered the second stage marked by formation of particles composed of disordered micelles in a more concentrated silica/ PEI/solvent matrix. The particle formation time in the current solution is much faster than that observed for acidic CTAB− silica film forming solutions (around 800 min)17 but is close to nucleation times reported in alkaline CTAB templated silica synthesis which do not form films.31 Thus, the silicate hydrolysis and condensation process is not greatly affected by the presence of PEI, despite the significant effect of this polymer on the macroscopic morphology of the final material. In our experiment, PEI takes the place of other basic species (NaOH, NH3) added in previous work on alkaline systems. 8343

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silica interactions with the micelle are disrupted and micelles cannot rearrange within the particles. When the PEI concentration is too high relative to CTAB concentration, coacervation is favored, but the polymer is less tightly bound to the smaller micelle and the silicate hydrolysis and condensation occur at the same rate as for the lower polymer concentrations. At higher concentration within the particles the polymer and the condensing silica network is more viscous, the polymer occupies significant space between the micelles and greater charge screening of the micelles (by both PEI and silica) occurs, so the micelles cannot rearrange into an ordered phase. The optimum ordered structure therefore could only be observed for particles at an intermediate PEI concentration, where the micelle charge density, polymer binding to the micelle, and silica binding to both are balanced. Comparison of these results with time-resolved measurements on films grown on the surface of equivalent solutions suggests that charge screening is also an important factor driving ordering in the films. Comparing the SAXS data with GIXD patterns and timeresolved neutron reflectivity data (samples are shown in Figures 6 and 7, respectively; for further examples please see ref 24) for CTAB/TMOS/PEI solutions, it is interesting to note that the mesostructure type and the degree of mesostructural ordering of the particles in solution are identical to the final structures formed in the films.24 For example, Figure 6, a GIXD pattern from a film grown with 0.037 M CTAB, 40 g/L SPEI, had sharp peaks at 0.152, 0.263, 0.303, 0.403 Å−1, indexed as the (100) (110) (200) (210) reflections of a well-ordered 2D hexagonal phase. From the SAXS patterns of the subphase solution at the same concentration (Figure 3C), the highly ordered 2D hexagonal mesostructure had already formed about 5 min after the addition of TMOS. However, according to timeresolved neutron reflectivity data (Figure 7) for a film grown at

However, like CTAB itself, linear and branched PEI causes silica formation directly from TMOS in aqueous solution. Linear PEI induces hydrolytic condensation of TMOS, creating silicas with different morphologies.53 Silicate−polymer hybrid fibers have also been synthesized via catalysis by PEI.54 It is accepted that the polyamines catalyze silica formation due to the alternating presence of protonated and nonprotonated amine groups in the polyamine chains, which allows hydrogen bond formation with the oxygen adjacent to silicon in the precursor, and this facilitates −Si−O−Si− bond formation.55 Here, the polyamine groups in PEI therefore work synergistically with the quaternary amines in the CTAB surfactant template to hydrolyze and condense the silica precursors. Since there are no mesophase particles observed in the subphase of CTAB−PEI solutions without added silica, the partially hydrolyzed silicate precursor acts to electrostatically screen charges between micelles, as well as hydrogen bonding with PEI amine groups, under the basic conditions. All these interactions facilitate the formation of silica/PEI particles with the micelles trapped inside. The third stage is the evolution of mesostructure within the disordered particles, which proceeds in a similar manner to that observed for surfactant−silica syntheses without added polymer; however, now the influence of the polymer becomes more apparent. The particle mesostructure observed by SAXS is less ordered for very high or very low PEI concentrations. From the fitting in Table 1, at low overall PEI concentration the polymer appears to be more tightly bound to the micelles (Table 1E), which may hinder interaction with the silica and the ordering of the micelles may be constrained by the bound polymer. At high polymer concentration with low CTAB concentration, the micelles are smaller and fewer in number with less bound polymer (Table 1A), and the greater amount of polymer in the surrounding solution may react preferentially with the silica, forming coated polymer strands which may disrupt micelle ordering. The mesophase particles are believed to form due to coacervation where an aqueous solution separates into two immiscible liquid phases, one being more concentrated while the other is more dilute.9,10 Formation of a coacervate depends on the molecular weight, charge density, and concentration in polyelectrolyte−surfactant solutions.56 Increased concentrations may therefore promote phase separation (coacervation) while a low polymer concentration would limit the phase separation, resulting in smaller, less concentrated particles which thus have a less ordered mesostructure. At the low polymer concentration where the polymer is more tightly bound around the micelles, high molecular weight PEI is sufficiently large to bind several micelles, which may also hinder rearrangement into an ordered material. Formation of the coacervate phase in the surfactant solution is clearly primarily driven by the addition of TMOS (in the presence or absence of PEI). The silica ions are thus responsible for charge screening of the micelles which allows this phase separation to occur. At lower PEI concentrations, the micelles will initially be less shielded from each other, thus having a relatively higher charge density which would be expected to promote ordering in the concentrated phase separated particles as is seen in the CTAB/ PEI films grown without silica.21 However, since a less ordered phase is formed as the PEI concentration decreased but greater PEI binding to the micelle occurs at this concentration, this suggests that if the polymer binds tightly to each micelle, then

Figure 6. (A) GID patterns collected on silica films grown at the air− water interface synthesized with 0.037 M CTAB, 40 g/L SPEI, and 0.084 M TMOS. (B) Line profiles at Qxy = 0.007 Å−1 of the GID pattern in (A).

the same concentration, a cubic intermediate structure was formed initially within 10 min, which finally transformed into the 2D hexagonal mesostructure after about 1 h. If the film growing at the air/water interface is formed because particles aggregate from the bulk solution to this interface, the film should have a 2D hexagonal structure from around 10 min after TMOS addition rather than the observed cubic structure. 8344

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micelles have a spherical or ellipsoidal structure in the CTAB/ PEI mixture, and addition of TMOS, which infiltrates into the polymer network, drives formation of particles with a low degree of ordering. The silicate precursor interacts not only with polymer due to hydrogen bond formation with amine groups along the polymer chain but also with the surfactant due to the electrostatic interaction between the negatively charged silica oligomers generated under basic conditions and the positively charged surfactant. Thus, hydrolyzed and condensed silicate precursors as well as the polymer will become a network which incorporates the spheroidal micelles to form CTAB/ PEI/silica particles with a pseudocubic liquid crystalline structure. These particles show some degree of initial micelle ordering which increases as they continuously rearrange into a 2D hexagonal structure which has greatest order at intermediate PEI concentrations. Although it is difficult to observe the rapid phase transformation within the CTAB/PEI/silica particles in the subphase, this same transformation was observed within the silica/CTAB/PEI films at the air/water interface. In the viscous PEI/TMOS networks of the film at the solution interface, less water is present within the surfactant/polymer/silica phase, so the micelle transformation and rearrangement within the film will require a longer time. It is possible that during the film reorganization period, as evaporation continues from the solution surface, the level of the solution decreases and particles from the subphase become embedded in the bottom of the film which may assist in nucleating the final 2D hexagonal phase. Given the results from slow evolution of the films, it is clear that the subphase particles must continue to be fluid for some time after the final 2D hexagonal phase is formed even if no further evolution in the phase is noted once an equilibrium structure has been reached. Indeed, work by others suggests that as alkoxysilane silica precursors undergo relatively slow hydrolysis in alkaline solution, silica hydrolysis in the walls of surfactant templated material continues for some time after the initial formation of the mesophase,31 meaning a fully condensed silica network is only achieved after considerable reaction times. Thus, rapid silica condensation is not responsible for disordered particle structures. If particles merge with the film, further rearrangement could still occur, leading to the ordered single-mesophase films observed.

Figure 7. Neutron reflectivity pattern from a film growing at the air/ water interface using 0.037 M CTAB, 40 g/L SPEI, and 0.084 M TMOS.

Therefore, although the route of mesostructure evolution in the film is the same as that of the particles in solution, the liquid crystalline phase at the interface is not in this case directly formed by particles which develop below the interface. This is the opposite observation to that made previously for acidic silica/surfactant film forming systems where particle (or coated micelle) formation in the subphase occurred just before film formation and the films formed from particles packing at the solution surface.16 Instead, our current results support the work of Åberg et al.,22 who suggest that evaporation from the solution surface promotes the formation of a phase-separated layer at the surface. This layer, similar to a coacervated liquid particle in the subphase, is more concentrated than the subphase solution but continues to dry from the top surface, making it much more viscous than the subphase coacervate particles. Thus, although the same route to mesostructure formation is observed, it is retarded in the films relative to the self-assembly in the subphase. The ordering in these films proceeds much more slowly due to their highly viscous nature, but for the same reagent concentrations the same final mesostructure results. Thus, the ordering in the films is not cut short or trapped by silica polymerization prior to the completion of ordering, and the structure formed must reflect only the electrostatic or iondipole interactions at the relative concentrations of the CTAB, PEI, and TMOS. With all the previous data24 and SAXS data reported in the current study, we attempt to summarize the evolution of the CTAB−PEI−silica film forming solution and propose a film formation mechanism in Scheme 1. Initially, in solution the

Scheme 1. Interfacial CTAB−PEI−Silica Film Formation Mechanism and Subphase Processes

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CONCLUSIONS In the current study, SAXS was used to investigate the evolution of micelles in the subphase solutions containing CTAB, PEI, and silica which form films at the air/water interface. Simple models were employed to quantify the size and shape of the micelles formed in solution. For film forming solutions with LPEI, the micelles are prolate ellipsoids dispersed in the polymeric solution immediately after mixing, while micelles in similar solutions with SPEI are almost spherical. There is no liquid crystalline ordering of the surfactant micelles in the bulk solution prior to silica precursor addition. For all the film forming solutions, addition of silicate species does not initially change the micelle size and structure. Hydrogen bond formation between the amine groups in the polyamine chains and the surfactant head groups, with the oxygen adjacent to silicon in the precursor, facilitates TMOS hydrolysis and condensation, causing formation of phaseseparated particles with a disordered mesostructure. The mesophase within the CTAB−PEI−silica particles finally rearranges into a 2D hexagonal ordered structure. Neutron reflectivity data from films formed at the air/water interface demonstrate that the mesostructure evolution process in the film is similar to the mesostructure evolution in the particles in the subphase of film forming solution. The whole process within the films takes a much longer time than that in the subphase solutions because of evaporation causing both film formation and drying, resulting in an increased viscosity of the CTAB−PEI−silica layer at the surface which slows the processes of mesostructural ordering.

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Dr. Alexei Vorobiev for their assistance with grazing incidence diffraction on ID10B (Trö ika II, experiment CH-2727). Matthew Wasbrough and James Holdaway are thanked for their assistance with making the flow system. We also thank the University of Bath and the ORSAS scheme for funding for B.Y.

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ASSOCIATED CONTENT

S Supporting Information *

Table of calculated values used in fitting, fitted SAXS patterns and a table of fit parameters for CTAB−LPEI−TMOS solutions immediately after TMOS addition, a table of fit parameters for solution E at various times after TMOS addition, fitted SAXS patterns and a table of fit parameters for CTAB−SPEI−TMOS solutions just after TMOS addition, and graphs of peak area and full width at half-maximum for SAXS pattern from all CTAB−PEI−TMOS solutions. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Tel +44(0)1225 384192, Fax +44(0)1225 386231, e-mail k. [email protected]. Present Address †

Laboratoire de Physique des Solides, UMR 8502 Universite Paris Sud, Bat. 510-91405 Orsay cedex, France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the beamline scientists Dr. Nick Terrill and Dr. Claire Pizzey for assistance on the SAXS beamline I22 (experiment SM647) at Diamond, Didcot, Oxfordshire. Dr. John Webster is thanked for assistance with the experiments on INTER (RB910127) at ISIS, Rutherford Appleton Laboratories, Oxfordshire. We also thank the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities and the beamline scientists Dr. Oleg Konovalov and 8346

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