Langmuir 2007, 23, 10063-10068
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Preparation of Nanosize Silica in Reverse Micelles: Ethanol Produced during TEOS Hydrolysis Affects the Microemulsion Structure Francesco Venditti,† Ruggero Angelico,† Gerardo Palazzo,*,‡ Giuseppe Colafemmina,‡ Andrea Ceglie,† and Francesco Lopez*,† Consorzio InteruniVersitario per lo sViluppo dei Sistemi a Grande Interfase (CSGI), c/o Department of Food Technology (DISTAAM), UniVersity of Molise, I-86100 Campobasso, Italy, and Department of Chemistry and CSGI, UniVersity of Bari, I-70126 Bari, Italy ReceiVed June 12, 2007. In Final Form: July 12, 2007 Microemulsions have been widely used as microreactors for the synthesis of nanoparticles and mesoporous materials. The correlation between the microstructure of a microemulsion and the features of the obtained materials is the most intriguing problem. On this point, many investigations have their ground on the structure of the precursor microemulsion, i.e., the system before the reaction takes place. Nevertheless, any reactions usually involve the formation of byproducts (aside from the nanoparticles). Several of these byproducts (e.g., ions, amphiphilic molecules) could modify the microemulsion structure during the course of the reaction. Here we examine the hydrolysis of tetraethoxysilane (TEOS) in the water-in-oil microemulsion hexadecyl-trimethylammonium bromide (CTAB)/pentanol/hexane/water. Conductivity and NMR measurements performed during the course of the reaction, in combination with dynamic light scattering and pulsed field gradient spin-echo NMR investigation performed on the microemulsion upon the addition of ethanol, indicate that a byproduct (ethanol) modifies the microreactor structure. The ethanol produced by the TEOS hydrolysis drives the microemulsion structure from small disconnected reverse micelles toward large connected aggregates until (for high enough ethanol loading) the system phase separates into two coexisting liquid phases (a dense interconnected network and a dilute reverse micellar phase).
1. Introduction Nanosize inorganic particles are of high interest, and their synthesis is currently the subject of intensive investigations.1,2 In this regard, the microheterogeneous nature of microemulsions offers attractive possibilities as demonstrated by a plethora of investigations.3-6 Microemulsions are composed of very small (1-100 nm) polar and/or apolar domains, separated by surfactant barriers (interfaces), providing a suitable environment for the control of the particle nucleation and growth kinetics. In this connection, several attempts have been done to correlate the microstructure of the microemulsion to the features of the nanoparticles obtained. Often the microstructure was assumed to be unaffected by the presence of the parent compounds (reagents) of the nanoparticles. In some cases, usually when one of the reagents is a modified surfactant, the aggregates present in the microemulsion before the reaction is triggered have been characterized.7,8 However, the reactions required for nanoparticle synthesis usually involve the formation of other byproducts, many of which (ions, amphiphilic molecules, and so on) could, in principle, modify the microemulsion structure during the course * Corresponding author. E-mail:
[email protected] (F.L.); palazzo@ chimica.uniba.it (G.P.). † University of Molise. ‡ University of Bari. (1) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (2) Pileni, M. P. Langmuir 1997, 13, 3266. (3) Lopez-Quintela, M. A. Curr. Opin. Colloid Interface Sci. 2003, 8, 137. (4) Arriagada, F. J.; Osseoasare, K. AdV. Chem. Ser. 1994, 234, 113. (5) Xu, X. J.; Gan, L. M. Curr. Opin. Colloid Interface Sci. 2005, 10, 239. (6) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393. (7) Pileni, M. P.; Motte, L.; Petit, C. Chem. Mater. 1992, 4, 338. Lisiecki, I.; Andre´, P.; Filankembo, A.; Petit, C.; Tanori, J.; Gulik-Krzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 9168. Lisiecki, I.; Andre´, P.; Filankembo, A.; Petit, C.; Tanori, J.; Gulik-Krzywicki, T.; Ninham, B. W.; Pileni, M. P. J. Phys. Chem. B 1999, 103, 9176. (8) Curri, M. L.; Palazzo, G.; Colafemmina, G.; Della Monica, M.; Ceglie, A. Prog. Colloid Polym. Sci. 1998, 110, 188.
of the reaction. To the best of our knowledge, detailed investigations on this aspect are still lacking. In a first attempt to fill such a gap, we have focused our study on microemulsions made of hexadecyl-trimethylammonium bromide (CTAB), water, 1-pentanol, and n-hexane that form reverse micelles in a wide range of compositions. Such a system has been previously characterized in detail9,10 and used as a microreactor for the synthesis of gold11 and CdS nanoparticles.8,12-16 In addition, akin microemulsions have been used in the synthesis of YVO4,17 BaMoO4,18 CaPO4,19 and AgBr20 nanoparticles. The reaction investigated refers to the formation of colloidal silica starting from the parent compound tetraethylorthosilicate (TEOS). This multistep reaction21 involves the hydrolysis of TEOS to form silicic acid and 4 mol of ethanol/mol of TEOS. The interactions between the hydrolyzed species result in condensation and polymerization reactions that end with the generation of silica particles and ethanol as schematized below: (9) Giustini, M.; Palazzo, G.; Colafemmina, G.; Della Monica, M.; Giomini, M.; Ceglie, A. J. Phys. Chem. 1996, 100, 3190. Colafemmina, G.; Palazzo, G.; Balestrieri, E.; Giomini, M.; Giustini, M.; Ceglie, A. Prog. Colloid Polym. Sci. 1997, 105, 281. (10) Palazzo, G.; Lopez, F.; Giustini, M.; Colafemmina, G.; Ceglie, A. J. Phys. Chem. B 2003, 107, 1924. (11) Chen, F. X.; Xu, G. Q.; Hor, T. S. A. Mater. Lett. 2003, 57, 3282. (12) Curri, M. L.; Agostiano, A.; Manna, L.; Della Monica, M.; Catalano, M.; Chiavarone, L.; Spagnolo, V.; Lugara, M. J. Phys. Chem. B 2000, 104, 8391. (13) Curri, M. L.; Leo, G.; Alvisi, M.; Agostiano, A.; Della Monica, M.; Vasanelli, L. J. Phys. Chem. B 2001, 243, 165. (14) Mang, P.; Gao, L. Langmuir 2003, 19, 208. (15) Curri, M. L.; Leo, G.; Alvisi, M.; Agostiano, A.; Della Monica, M.; Vasanelli, L. J. Colloid Interface Sci. 2001, 243, 165. (16) Zhang, P.; Gao, L. J. Colloid Interface Sci. 2004, 272, 99. (17) Sun, L. D.; Zhang, Y. X.; Zhang, J.; Yan, C. H.; Liao, C. S.; Lu, Y. Q. Solid State Commun. 2002, 124, 35. (18) Li, Z. H.; Du, J. M.; Zhang, J. L.; Mu, T. C.; Gao, Y. N.; Han, B. X.; Chen, J.; Chen, J. W. Mater. Lett. 2005, 59, 64. (19) Wang, Y. J.; Lai; C.; Wei, K.; Tang, S. Q. Mater. Lett. 2005, 59, 1098. (20) Husein, M. A.; Rodil, E.; Vera, J. H. Langmuir 2006, 22, 2264. (21) Van Blaadern, A.; Van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481.
10.1021/la701739w CCC: $37.00 © 2007 American Chemical Society Published on Web 08/30/2007
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NH3
Si(OC2H5)4 + 4H2O 98 Si(OH)4 + 4C2H5OH NH3
Si(OH)4 98 SiO2V + 2H2O
(1)
The choice of this reaction is due (aside from the importance of colloidal silica in many applications)22 to the formation, as a byproduct, of ethanol. Such a short-chain alcohol is known to influence the properties of surfactant aggregates,23 and it has been proposed that this could play a role in influencing the size and shape of colloidal silica synthesized in microemulsions24 and emulsions.25 In order to have an alkaline aqueous milieu, ammonia solution (10 wt %) was used instead of pure water. We have found that adding 10 wt % ammonia aqueous solution to reach a water/ CTAB mole ratio of 30 results in a reverse micellar phase, which is stable upon further addition of ammonia up to a water/CTAB ratio of 45, where a phase separation occurs. According to scheme 1, the aqueous ammonia furnishes both the reagent and the catalyst for the formation of colloidal silica. 2. Materials and Methods All the chemicals were purchased from Sigma-Aldrich. The microemulsion was prepared by weighing in a volumetric flask the amounts of surfactant, alcohol, and 10 wt % ammonia solution in water needed to obtain the mole ratios of pentanol/CTAB ) 5 and water/CTAB ) 30. Hexane was added to obtain a final CTAB concentration of 100 mM. Ammonium hydroxide acts as a catalyst in the hydrolysis of TEOS, triggering the subsequent polymerization reaction. Self-diffusion coefficient measurements were carried out by the Fourier transform-NMR pulsed field gradient spin-echo (PGSE) method26 using a BS-587 A NMR (Tesla) spectrometer operating at 80 MHz for the proton. The magnetic field was locked by an external D2O lock signal. High-resolution 1H NMR spectra were recorded by means of a Varian 400 MHz NMR spectrometer (probe 5 mm for 1H) with a superconductor cryo-magnet, at 11.74 T. Conductivity measurements were performed with a CDM230 conductivity meter (Radiometer Analytical) equipped with a twopole conductivity cell tailored for small volumes (CDC749; cell constant 1.84 cm-1). Dynamic light scattering (DLS) measurements were performed using the commercial instrument Zetasizer-Nano S from Malvern operating with a 4 mW He-Ne laser (633 nm wavelength) and a fixed detector angle of 173° (noninvasive backscattering geometry). The same apparatus was used to evaluate the Rayleigh ratio R from the ratio between the intensities of light scattered from the sample and from pure toluene (Is and Itoluene, respectively) according to R ) Is/Itoluene (nhexane/ntoluene)2R toluene, where ni indicates the refractive index of the substance i, and R toluene is the Rayleigh ratio of toluene. The SEM images were obtained using a Zeiss DSM 940 equipped with a Link System INCA energy-dispersive X-ray analyzer.
3. Results and Discussion 3.1. TEOS-Loaded Microemulsions. The formation of silica nanoparticles was achieved simply by adding, under continuous stirring, the appropriate amount of TEOS to a stable transparent microemulsion placed in a vial within a thermostated bath. After a few minutes, the sample becomes opalescent, and the turbidity (22) Capek, I. AdV. Colloid Interface Sci. 2004, 110, 49. (23) Perez-Casas, S.; Castillo, R.; Costas, M. J. Phys. Chem. B 1997, 101, 7043. (24) Bagwe, R. P.; Yang, C.; Hilliard, L. R.; Tan, W. Langmuir 2004, 20, 8336. (25) Esquena, J.; Solans, C. Colloids Surf., A 2001, 533, 183. (26) Stilbs, P. Prog. NMR Spectrosc. 1987, 19, 1.
Figure 1. Lower panel: Specific conductivity (κ) as a function of reaction time: (9) microemulsion loaded with 0.23 M TEOS (the solid line represents the linear best fit utilized for the extrapolation of τ*); (O) water + hexane biphasic system loaded with TEOS (0.23 M overall concentration). On the left is a photograph of the sample after 5 min; on the right, a photograph of the phase-separated sample after 250 min. Upper panel: SEM microphotograph of silica particles after 250 min.
develops with time until, after several hours, the sample phase separates (see Figure 1 for representative pictures taken at different times). Silica nanoparticles adsorb to the liquid-liquid interface between the two phases and can be recovered by centrifugation and extensive washing with ethanol. As shown in Figure 1, scanning electron microscopy (SEM) analysis reveals two different particle populations, with a main population of approximately 120 nm and a smaller population of much larger aggregates (400 nm). From our point of view, the most interesting feature of this process is the final demixing of an initially stable microemulsion. The time required to achieve such a phase separation depends on the amount of added TEOS, being longer for smaller amounts of this reagent. We interpret this evidence as an indication that some of the reaction products destabilize the microemulsion. Changes in the microemulsion structure during the time course of TEOS hydrolysis and subsequent silica polymerization have been probed through conductivity measurements. For water-in-oil microemulsions, the charge transport depends mainly on the connectivity degree of the polar domains. Reverse micelles dissolved in an apolar continuous phase are characterized by very low conductivity values (a few tenths of microsiemens per centimeter), while large interconnected structures or transient clusters allow ionic conduction along wide (although tortuous) paths. A typical conductivity (κ) versus time curve is shown in Figure 1. The conductivity reading taken a few minutes after the TEOS addition is very low (0.3 µS cm-1) and increases only slightly, always remaining below 1-2 µS cm-1.
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Figure 3. 1H NMR spectra in the region of alcoholic R-CH2 at different times along the reaction time course for a sample loaded with 0.06 M TEOS.
Figure 2. Dependence of the conductivity time-threshold τ* (A) and of the initial reaction rate (B) on the starting TEOS concentration. (A) Values of τ* measured by electrical conductivity as in Figure 1; the solid curve represents the prediction without adjustable parameters of eq 2, for the pseudo-first-order rate constant kapp ) 0.36 s-1 determined from the data of panel B and a threshold ethanol concentration [EtOH]* ) 0.29 M (determined by NMR, see text). (B) Initial rates of both ethanol production and ethylsilicate consumption as a function of TEOS concentration; the best fit linear regression has a slope kapp ) 0.36 s-1.
It is only after a threshold time τ* that the conductivity grows dramatically, increasing linearly with time up to a maximum and finally dropping for longer times. Visual inspection indicates that the drop in conductivity coincides with the macroscopic phase separation. The time at which phase separation takes place and thus the position of the κ-maximum depends somehow on stirring conditions. Since the time course of TEOS hydrolysis probed by NMR is independent from stirring conditions (see below), it is likely that this behavior reflects the influence of stirring on the kinetics of nucleation and growth of the new phase. At variance, the onset of conductivity rise was found to be independent from the stirring conditions. This threshold time τ* was evaluated as the time intercept of the extrapolation of the linear portion of the κ versus time curve (see Figure 1). As shown in Figure 2A, τ* decreases upon increase of the initial TEOS concentration ([TEOS]°). The overall reaction of silica formation does not imply the formation of charged species; furthermore, conductivity remains very low (κ < 1 µS/cm) for reactions carried out in a water/hexane biphasic system (open circles in Figure 1), suggesting that the presence of ionic intermediates is negligible. According to this evidence, the dramatic changes in conductivity found as the reaction proceeds are likely related to some increase in the connectivity of microemulsion structure. In order to unravel effects due to the formation of products (changes in chemical composition) from the structural rearrangement of microemulsion (changes in the connectivity of polar domains), we measured 1H NMR spectra at different times along the reaction time course. The starting microemulsion was thoroughly shaken after the addition of a known amount of TEOS and rapidly transferred into a 5 mm NMR tube (the lock signal was provided by an
internal capillary filled with D2O), and NMR spectra were recorded at given times during the reaction time course. Since the methylene resonances for ethanol and ethylsilicate are well separated in the 1H NMR spectra (3.65 and 3.79 ppm, respectively),27 the evolution of TEOS hydrolysis can be probed quantitatively. As shown in Figure 3, both the disappearance of the ethylsilicate and the formation of ethanol signals are easily quantified as a function of time. For selected compositions, the reaction kinetics was followed discontinuously, taking aliquots from the same vessel where the conductivity measurements were carrying out (under stirring); no differences in the rates of TEOS consumption and ethanol production were found between continuous and discontinuous measurements (data not shown). In all cases, TEOS disappearance and ethanol production roughly followed consistent pseudo-first-order kinetics (see Figure 4B). The initial rate was evaluated as the slope at time ) 0 of the third-order polynomial that fits the experimental data in the first hour (see Figure 4A for a representative example). The dependence on the starting TEOS concentration of the initial rates of both ethanol production and ethylsilicate consumption is roughly linear (Figure 2B) and consistent with a pseudo-firstorder rate constant kapp) 0.36 ( 0.06 s-1. A peculiar finding is that the concentration of ethanol produced after time ) τ* is the same (0.29 M), independently of the initial concentration of TEOS. In other words, the threshold for conductivity raise corresponds to a constant ethanol concentration. The τ* values decrease with the reagent concentration because the rate of reaction is proportional to [TEOS]°. Such a condition was demonstrated experimentally only for [TEOS]°< 0.3 M because, for higher TEOS loading, the reaction is too fast to be probed by means of conventional NMR experiments. However, the validity of this condition can also be tested for higher TEOS concentrations by comparing the results coming from NMR with the dependence of τ* on [TEOS]° coming from conductivity. Assuming a pseudo-first-order kinetic law, one expects the following relation between a given (critical) concentration of ethanol produced [EtOH]* and the time required for its formation τ*:
(
τ* ) -kapp ln 1 -
[EtOH]* [TEOS]°
)
(2)
The prediction of the above equation using kapp) 0.36 ( 0.06 s-1 and [EtOH]* ) 0.29 M coming from NMR measurements is shown in Figure 2A (dashed curve). The agreement between (27) The resonance of the hexane terminal methyl was set at 0.9 ppm.
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Figure 5. Specific conductivity as a function of ethanol content: (b) CTAB/ammonium solution/pentanol/hexane model microemulsion (without TEOS) diluted with ethanol; (0) microemulsion loaded with 0.23 M TEOS during the time course of TEOS hydrolysis (same data of Figure 1; the ethanol concentration at different times has been measured via NMR). Also shown are the maximum ethanol concentration before phase separation and the threshold ethanol concentration of conductivity raise during the TEOS hydrolysis. Figure 4. (A) Kinetics of the ethylsilicate consumption and formation of ethanol for [TEOS]° ) 0.12 M as probed by NMR; the lines are the best-fit third-order polynomials used to evaluate the initial rate. (B) First-order behavior of the TEOS hydrolysis and ethanol formation. The semilog ordinate represents the normalized intensity of the TEOS NMR signal and the difference (IEtOH(∞) - IEtOH(t)) between the intensity of ethanol NMR signal at long time IEtOH(∞) and that during the reaction course IEtOH(t). The line is a singleexponential fit.
the predictions of eq 2 (without any adjustable parameters) is quite good over the entire range of initial TEOS concentrations explored. This confirms the idea of a threshold in conductivity related to the presence of a critical amount of products. 3.2. Ethanol-Loaded Microemulsions. The final products of the reaction are ethanol and nanosized silica. The increase in conductivity can be hardly ascribed to the latter. Actually, the addition of silica nanoparticles to the starting microemulsion does not affect the conductivity of the sample (data not shown). On the contrary, upon loading with ethanol, the microemulsion has a clear effect on its conductivity. As shown in Figure 5 (filled circles), for ethanol concentrations higher than 0.02 M ethanol, conductivity increases noticeably to about 80 µS/cm. For ethanol concentrations higher than 0.35-0.38 M, the system phase separates. For comparison, in the same figure are plotted the conductivity data of Figure 1 using as the abscissa the ethanol concentration evaluated at different times through NMR measurements (open triangles Figure 5). The raise in conductivity and the final phase separation, found upon loading with ethanol, are reminiscent of analogous phenomena observed during the reaction kinetics. Interestingly, the ethanol threshold for conductivity raise observed during the TEOS hydrolysis is close to the maximum ethanol concentration before the microemulsion phase-separates (in the absence of added TEOS). As a whole, the results of Figure 5 suggest that the ethanol formed during the TEOS hydrolysis induces the formation of interconnected aqueous channels, allowing ionic conduction by counterions and cationic surfactant molecules. The microemulsion plus ethanol system represents a suitable model to study the changes in microstructure occurring during the silica synthesis. Such a model system can be studied by means of techniques otherwise unsuitable in the presence of chemical reactions. In particular, direct
Figure 6. Self-diffusion coefficients measured on model microemulsions along the ethanol dilution line. The shaded area represents the two-phase region; data in this region refer to the lower phase.
investigations by means of PGSE-NMR and light scattering are not feasible during the time course of the reaction. Indeed, the duration of a PGSE-NMR run is around 10 min (with our instrumental setup) while light scattering should probe mainly the (large) silica particle (in addition silica tends to adsorb on the wall of the glass cuvette). The self-diffusion coefficients associated with the resonances of hexane (Doil), OH (mainly water; DOH), and CTAB (DCTAB) were measured (via PGSE-NMR)28 on the model microemulsion loaded with different amounts of ethanol. As shown in Figure 6, the diffusion coefficients of the three components are very different from each other; within the single region Doil remains around 3 × 10-9 m2 s-1, a value typical of pure hexane and consistent with the presence of a continuous apolar phase. On the contrary, the DOH starts with an initial value of 2 × 10-10 m2 s-1 for the model microemulsion, and increases only slightly upon ethanol loading. The self-diffusion coefficient of CTAB is lower than DOH and grows with ethanol concentration as well. On the basis of our previous knowledge of microemulsion transport properties,9,10 the data of Figure 6 indicate, in the singlephase region, a microstructure mainly composed by slowly (28) Soderman, O.; Stilbs, P.; Price, W. S. Concept Magn. Reson. A 2004, 23A, 121.
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Table 1. Composition of the Two Phases Formed upon Addition of 3 wt % of Ethanol to the Microemulsiona (Determined via NMR) nhexane/nCTABb nwater/nCTABb npentanol/nCTABb npentanol/nCTAB at the interfaceb,c [pentanol] in the bulk (M)c
lower phase
upper phase
28 ( 3 27 ( 3 3.2 ( 0.5 2.0 ( 0.1 0.33 ( 0.02
150 ( 5 30 ( 4 9.2 ( 0.2 2.0 ( 0.3 0.35 ( 0.02
a Overall composition: mole ratios of pentanol/CTAB ) 5, water/ CTAB ) 30, hexane/CTAB ) 66, ethanol/CTAB ) 5. b ni/nCTAB denotes the mole ratio between component i and CTAB. c Evaluated as described in reference 10.
diffusing reverse micelles of water and CTAB surrounded by a freely diffusing hexane bulk (the finite solubilities of water and ethanol in hexane justify the evidence DOH > DCTAB; see reference 9 for further discussion). For ethanol loading higher than 0.38 M, the system phaseseparates into two microemulsions. The composition of the two phases obtained after demixing was calculated from the areas of the NMR peaks for a system with 3 wt % of ethanol, and it is given in Table 1. In both of the phases, the water/CTAB mole ratio is around 30. The upper phase has a low conductivity (κ < 0.5 µS/cm), and it is likely made of disconnected reverse micelles. The lower phase is rich in water and surfactant and has a high conductivity (κ ∼ 400 µS/cm). The diffusion coefficients of hexane, water, and CTAB measured in the lower phase are mutually closer than analogue values observed in the singlephase system (see Figure 6). This result, combined with the relatively high conductivity found in the lower phase indicates the presence of an extended bicontinuous network. The description of the structural changes induced by ethanol in the single phase region is more delicate. In this region, the conductivity displayed in Figure 5 is between the values typical of reverse micelles and bicontinuous microemulsions. To put this point on a quantitative ground, we recall that, for the water/CTAB/pentanol/hexane microemulsions, the conductivity previously measured for disconnected reverse micelles is around 0.2 µS/cm, while, for bicontinuous structures, κ ranges from 100 (lamellar phases) to 2000 µS/cm (bicontinuous microemulsions at high water loading; water/CTAB ) 80).9,29 As stated above, the diffusional behavior of the main components of the system is consistent with discrete reverse micelles. Since CTAB is barely soluble in the continuous organic phase,9,10 its self-diffusion coefficient allows the determination of an apparent micellar radius through the StokesEinstein equation rapp h ) kBT/6πηDCTAB. Another characteristic diffusional length can be evaluated by means of DLS. The time correlation function of the scattered intensity has been analyzed according to a second-order cumulant expansion30 giving the first cumulant 〈Γ(Q)〉 from which a hydrodynamic correlation length ξh ) Q2kBT/6πη〈Γ(Q)〉 can be evaluated.31 Note that, for monodisperse reverse micelles, ξh) rapp h at infinite dilution. For the present oil continuous systems (where long-range electrostatic interactions are negligible), discrepancies between ξh and rapp h are also expected to be low at finite concentrations for monodisperse reverse micelles (within 40% for volume fractions below 10%).9 Figure 7 shows that, for the ethanol-loaded microemulsion, ξh and rapp h remain mutually close for ethanol (29) Palazzo, G.; Carbone, L.; Colafemmina, G.; Angelico, R.; Ceglie, A.; Giustini, M. Phys. Chem. Chem. Phys. 2004, 6, 1423. (30) Koppel, D. E. J. Chem. Phys. 1972, 57, 4814. (31) Abbreviations: kB is the Boltzmann constant, η is the viscosity of continuous medium, and T is the temperature; the scattering vector Q is defined as Q ) 4πnλ-1 sin(θ/2), where n is the refractive index of the solution, λ ) 633 nm is the wavelength of incident light, and θ ) 173° is the scattering angle.
Figure 7. Light scattering investigation on the ethanol-loaded microemulsion. Solid lines are mere guides for the eyes. (A) Hydrodynamic correlation length ξh (O, determined by DLS) and apparent micellar radius rapp h (1, determined by PGSE-NMR) as a function of ethanol content. (B) Rayleigh ratio R (9, determined from the scattered light intensity) as a function of ethanol content.
concentrations below [EtOH]*; for further ethanol loading, one app observes ξh . rapp h . For polydisperse systems, ξh and rh reflect different averages of the hydrodynamic radius (R) of the particle ensemble; ξh is z-averaged (ξh ) 〈R6〉/〈R5〉, i.e., the experimentally determined average hydrodynamic correlation length is heavily weighted toward the large size particles).32 On the contrary, the measured self-diffusion coefficient DCTAB is averaged, according to the Lindman’s law,33 with respect to the surfactant molecules 2 and therefore to the particle surface, giving rapp h ) 〈R 〉/〈R〉. In other words, the average ξh determined via DLS is more biased toward larger micelles than the average rapp h measured via PGSENMR. Accordingly, Figure 7 indicates that, for ethanol concentrations less than [EtOH]*, the microemulsion is still composed of relatively monodispersed aggregates (ξh ≈ rapp h ). For further ethanol loading, the system evolves toward a coexistence between small reverse micelles and large aggregates (probed mainly by PGSE-NMR and DLS, respectively), somehow preceding the macroscopic coexistence between L2 reverse microemulsion and bicontinuous phases found above [EtOH] ) 0.38 M. The presence of large aggregates is confirmed by the optical appearance of the samples that, at high ethanol concentrations, are slightly bluish. Such an increase in turbidity is also shown in Figure 7B, where the Rayleigh ratio (R) of the scattered light intensity is plotted as a function of the ethanol concentration.
4. Concluding Remarks The insight gained on the ethanol-loaded microemulsion consistently accounts for the conductivity behavior observed during the TEOS hydrolysis and for the final phase separation. The unperturbed microemulsion is made of small monodisperse (32) Pusey, P. N.; van Megen, W. J. Chem. Phys. 1984, 80, 3513. (33) Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1983, 87, 4756. Nilsson, P. G.; Lindman, B. J. Phys. Chem. 1984, 88, 5391.
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reverse micelles. As long as reaction 1 proceeds, the system is loaded with ethanol. The so-produced ethanol induces the formation of some large aggregates and drives the system toward the liquid/liquid coexistence curve. Such a liquid-liquid phase separation is peculiar. For the CTAB/water/pentanol/hexane system, the partition of pentanol between the interfacial film and the continuous bulk was previously investigated in detail.10,27 Starting from the compositions of the two phases listed in Table 1, it is possible to evaluate (as described in the appendix of reference 10) the composition of the interface and of the continuous bulk (last two rows of Table 1). Surprisingly, the two phases are formed by aggregates of identical composition and differ only in the concentration and degree of connectivity. It is already known that, in the present system, pentanol loading results in a Winsor II phase separation because the medium chain alcohol makes the spontaneous curvature more negative.27 Our results indicate that, instead, ethanol increases the spontaneous curvature of the interfacial film, favoring the formation of interconnections among aqueous channels. Theoretical models foretell that, also in the absence of any specific interaction between aggregates, the presence of the junctions induces an effective interaggregate attraction.34 This junction-induced attraction is strong enough to drive phase separation between a dense network
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and a dilute micellar phase, as found in the present system upon increasing the ethanol content. Some points remain to be elucidated. There are differences between the conductivity readings for the model microemulsion and during the reaction (Figure 5). In addition, during the TEOS hydrolysis, the system remains single phase at ethanol concentrations higher than 0.35 M. This behavior is likely due to the consumption of water associated with the formation of ethanol. Indeed, at lower water contents, the conduction of ions in the water channels and the density of branch points are both expected to decrease, and this could explain the lower conductivity. In any case, these aspects are outside the scope of the present report. The main message of this contribution is a caveat to the scientists involved in nanoparticle preparation in microemulsion media: The interpretation of nanoparticle formation (size, morphology, and kinetics) in terms of the structure of the preexisting microemulsion could be (at least in some cases) misleading because some of the products of the synthesis can heavily affect the microemulsion itself. LA701739W (34) Zilman, A. G.; Safran, S. A. Phys. ReV. E 2002, 66, article no. 051107.
