Controlled Formation of Silica Particles from Tetraethyl Orthosilicate in

Two-Phase Synthesis of Monodisperse Silica Nanospheres with Amines or Ammonia Catalyst and Their ... The Journal of Physical Chemistry C 0 (proofing),...
0 downloads 0 Views 695KB Size
Download PDF
Langmuir 1997, 13, 3295-3307

3295

Controlled Formation of Silica Particles from Tetraethyl Orthosilicate in Nonionic Water-in-Oil Microemulsions Chia-Lu Chang and H. Scott Fogler* Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received November 1, 1996. In Final Form: March 27, 1997X

The formation of silica particles by the ammonia-catalyzed hydrolysis of tetraethyl orthosilicate (TEOS) in nonionic water-in-oil microemulsions was investigated as a function of the molecular structure of the surfactant, the type of oil, and the concentrations of surfactant and water. Three types of nonionic surfactants and two oils, heptane and cyclohexane, were used. Microemulsion solutions were characterized using photon correlation spectroscopy, electrical conductivity measurement, and titration of phase separation points. The hydrolysis of TEOS and the size of synthesized silica particles were measured by FTIR spectroscopy and transmission electron microscopy measurements, respectively. It was found that the size distribution of silica particles correlates well with the size, connectivity, and stability of microemulsion droplets. This correlation indicates that the final size distribution of silica particles is predetermined in the silica particle’s nucleation stage. In this stage, hydrolyzed (monomeric and polymeric) silica reacting species undergo interdroplet dynamic exchanges to contact one another and grow into nuclei. The results of this study showed that the nucleation of silica particles is enhanced (as to result in smaller final particles) by increasing the number of microemulsion droplets that compartmentalize reacting species in solutions or by increasing the strength of surfactant films in sterically hindering the dynamic exchange of reacting species between droplets. The effect of each variable described above on the formation of silica particles in microemulsions is detailed in the paper.

Introduction Water-in-oil (W/O) microemulsions (i.e., reverse micellar solutions) are transparent, isotropic, thermodynamically stable liquid media with a continuous oil domain and an aqueous domain thermodynamically compartmentalized by a surfactant as nanometer-sized liquid entities. These surfactant-covered entities (referred to as W/O microemulsion droplets irrespective to their morphology) offer a unique microenvironment for inorganic precipitation reactions; that is, they act not only as microreactors for hosting the reaction but also as steric barriers for inhibiting the polymerization of reacting species among different droplets during the reaction period. As a result, inorganic particles synthesized in W/O microemulsions are, in general, of a nanometer scale (i.e., less than 100 nm); some examples are metal (Pt, Au), sulfide (CdS), oxide (SiO2, Fe3O4), and carbonate (CaCO3).1-6 In this study, the synthesis of silica particles from the hydrolysis of tetraethyl orthosilicate (TEOS, Si(OC2H5)4) in nonionic W/O microemulsions was investigated. Reactions involved in this silica synthesis include the hydrolysis of TEOS and the condensation (i.e., polymerization) of the hydrolyzed silica species (i.e., monomeric and polymeric silica reacting units) in the presence of a base catalyst, ammonia, i.e., * Author to whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Fender, J. H. Chem. Rev. 1987, 87, 877. (2) Nagy, J. B.; Derouane, E. G.; Gourgue, A.; Lufimpadio, N.; Ravet, I.; Verfaillie, J. P. In Surfactants in Solution; Mittal, K. L., Ed.; Plenum Press: New York, 1989; Vol. 10, p 1. (3) Robinson, B. H.; Khan-Lodhi, A. N.; Towey, T. In Structure and Reactivity in Reverse Micelles: Pileni, M. P., Ed.; Elsevier: New York, 1989; p 198. (4) Ward, A. J. I.; Friberg, S. E. MRS Bull. 1989, 41. (5) Sugimoto, T. Adv. Colloid Interface Sci. 1987, 28, 65. (6) Kon-no, K. In Surface and Colloid Science; Matijevic, E., Ed.; Plenum Press: New York, 1993; Vol. 15.

S0743-7463(96)01062-1 CCC: $14.00

hydrolysis tSi-OR + H2O S tSi-OH + ROH

(1)

alcohol condensation tSi-OR + tSi-OH S tSi-O-Sit + ROH (2) water condensation tSi-OH + tSi-OH S tSi-O-Sit + H2O (3) overall reaction OH-

Si(OR)4 + 2H2O98 SiO2 + 4ROH

(4)

where R is ethyl group -C2H5. Silica particles synthesized in nonionic W/O microemulsions usually have a narrow size distribution with the average value between 25 and 70 nm.6-12 The concentrations of both water and surfactant affect the size of silica particles synthesized in microemulsions. At a constant surfactant concentration, the silica particle size generally decreases with an increase in the water concentration.6,7 However, for a given water concentration, at low surfactant-to-water molar ratios, increasing the surfactant concentration leads to a reduction of silica particle size, whereas at high molar ratios the opposite trend occurs.7,9,11 Both water and surfactant are necessary for the formation of stable silica suspensions in microemulsions.9 Residual water provides a means of hydrogen (7) Chang, C.; Fogler, H. S. AIChE J. 1996, 42, 3153. (8) Yanagi, M.; Asano, Y.; Kandori, K.; Kon-no, K.; Kitahara, A. Proc. 1986 Shikizai Technical Conference; Osaka, Japan, 1986; p 86. (9) Osseo-Asare, K.; Arriagada, F. J. Colloids Surf. 1990, 50, 321. (10) Osseo-Asare, K.; Arriagada, F. J. Colloids Surf. 1992, 69, 105. (11) Arriagada, F. J.; Osseo-Asare, K. In The Collidal Chemistry of Silica; Bergna, H. E., Ed.; Advances in Chemistry Series 234; American Chemical Society: Washington, DC, 1994; p 113. (12) Arriagada, F. J. Synthesis of Submicron Particles in Reverse Micellar Systems: Nanosize Silica via Hydrolysis of Tetraethoxysilane; Ph.D. Thesis, The Pennsylvania State University, 1991.

© 1997 American Chemical Society

3296 Langmuir, Vol. 13, No. 13, 1997

Figure 1. A schematic illustration of the coagulation of hydrolyzed polymeric silica species through the mass exchange among W/O microemulsion droplets. The collision of microemulsion droplets forms a transient dimer with an open water channel for the smaller silica species to pass through and physically contact with the larger one.

bonding between silica particles and surfactants and, hence, facilitates surfactant molecules to adsorb onto silica particles to sterically stabilize particles in the oil continuum.10 Although the concentration of water and surfactant was found to be influential on silica particle formation in microemulsions, previous research has not elucidated on how microemulsion characteristics, such as micellar size and stability, affect the size distribution of synthesized silica particles. These issues need to be clarified in order to further understand the formation of silica particles in microemulsions and to establish a scientific basis for synthesizing oil-dispersible silica nanoparticles with controlled sizes. Due to the complexity of particle formation in microemulsions, and to place our results in perspective, a brief description of the general phenomena on particle formation in W/O microemulsions follows. Formation of colloidal particles in W/O microemulsions occurs not only through the polymerization of monomeric reactants into polymeric reacting species but also through the subsequent polymerization of polymeric species into even larger species. This polymerization process usually occurs through the dynamic fusion and fission of W/O microemulsion droplets which host reacting species and undergo incessant Brownian motions. As shown schematically in Figure 1, the Brownian collision of two droplets results in a transient dimer with an open water channel formed across the surfactant films.13 Subsequently, reacting species in the droplets can pass through the channel to contact one another and undergo polymerization.4,14,15 In general, the formation of channels for (13) Bommarius, A. S.; Holzwarth, J. F.; Wang, D. I. C.; Hatton, T. A. J. Phys. Chem. 1990, 94, 7232. (14) Modes, S.; Lianos, P. J. Phys. Chem. 1989, 93, 5854. (15) Towey, T. F.; Khan-Lodhi, A.; Robinson, B. H. J. Chem. Soc., Faraday Trans. 1990, 86 (22), 3757.

Chang and Fogler

larger polymeric species to pass through requires either an unfavorably larger deformation of the surfactant film from its equilibrium or the simultaneous dissociation of more surfactant molecules from droplet surfaces. Therefore, the steric hindrance of surfactant films to the exchange of reacting species between droplets through transient dimer formation usually increases with increasing the species size.2,16 After reaching a critical size, polymeric species become nuclei which are stably confined in the individual droplets and do not undergo interdroplet dynamic exchange because it is not possible to form sufficiently large interdroplet channels. These nuclei then adsorb small monomeric or oligomeric species to their surfaces and grow individually into particles. From the above description, it is clear that the size of colloidal particles formed in W/O microemulsions is directly influenced by two factors. One is the number of microemulsion droplets that host reacting species. In general, the greater the number of droplets there are, the greater will be the physical compartmentalization of reacting species in solutions to form more nuclei. The other factor is the steric hindrance of surfactant films to the interdroplet dynamic exchange of reacting species. For surfactant films with a lower deformability or a stronger attachment to droplets, interdroplet open water channels are less likely to occur for reacting species to pass through. With fewer interdroplet dynamic exchanges, reacting species tend to retain and grow in their respective droplets, leading to the formation of more nuclei and smaller final particles (for a given amount of reactants). Previous fluorescence-quenching studies showed that the hindrance to the interdroplet exchange of small molecules such as fluorescence probes and quenchers was weak in nonionic W/O microemulsions; as a result, the interdroplet exchange rate approaches upper limit of the the Brownian interdroplet collision rate.17 Therefore, it is expected that small hydrolyzed silica species (with sizes as probes) can be easily exchanged between droplets through transient dimer formation in nonionic microemulsions. This result also implies that the critical size for the reacting species to be constrained in individual droplets to become nuclei should be larger than those of fluorescence probes. The size of particles formed in microemulsions is also affected by the rate of converting reactants into active monomeric reacting species before polymerization takes place. For example, the TEOS molecules in this study have to be first hydrolyzed into silica species with reactive silonal groups in order to be condensed into silica particles. In general, for a given reactant concentration, particle nucleation can be enhanced by increasing the rate of conversion of reactants to reactive species. At highreactant conversion rates, many reactive species form at once and grow simultaneously up to the critical size necessary to form nuclei. On the contrary, at low conversion rates, only a small fraction of reactants which are hydrolyzed into reacting species at early reaction times can grow into nuclei; then, these nuclei adsorb all of the subsequently hydrolyzed small reacting species through interdroplet exchanges and grow into final particles. Our previous studies, as well as other research, have found that the growth of silica particles from the surface condensation of small reacting silica species is rate-limited by the extremely slow TEOS hydrolysis with a specific rate constant in the range of 10-6-10-5 s-1. 7,12 Due to this slow TEOS hydrolysis and fast interdroplet matter (16) Monnoyer, Ph.; Fonseca, A.; Nagy, J. B. Colloids Surf., A. 1995, 100, 233. (17) Clark, S. P.; Fletcher, D. I.; Ye, X. Langmuir 1990, 6, 1301.

Controlled Formation of Silica Particles from TEOS

Langmuir, Vol. 13, No. 13, 1997 3297

Table 1. Properties of Poly(oxyethylene) Alkylphenyl Ether Surfactants (n-H(CH2)nPh(OC2H4)mOH) Used in This Study brand name (IGEPAL)

abbreviation

av no. of carbons in aliphatic tail (n)

av no. of oxyethylene groups (m)

av mol wt

water content (%)a

CO430 CO520 RC520

NP4 NP5 DP6

9 9 12

4 5 6

396 440 526

0.04 0.4 0.1

a

Measured by Karl-Fischer titration.

exchange mentioned above, silica particles nucleate in the early reaction period and subsequently grow into final sizes as large as 50 nm in diameter through the continual collection of reacting species. It was also found that the growth of silica particles followed a size-independent rate such that the shape of particle size distributions was preserved during particle growth. This size-independent growth kinetics implies that the final particle size distribution reflects the process of silica nucleation. That is, the number of nuclei (i.e., particles) can be estimated from the average size of final silica particles, and the broadness of the nucleation period can be estimated from the normalized standard deviation of particles.12 In this study, we conducted a systematic investigation of the effect of the molecular structure of surfactants, the type of oil, and the concentrations of surfactant and water on the formation of silica particles in nonionic W/O microemulsions. The influence of these variables on microemulsion properties, including the droplet size, connectivity, and stability, was characterized using photon correlation spectroscopy, electrical conductivity measurements, and phase separation point tests. The rate of TEOS hydrolysis and the size distribution of silica particles synthesized in microemulsions were measured using Fourier transform infrared spectroscopy and transmission electron microscopy, respectively. The measured size and polydispersity of silica particles were then explained in terms of the effect of microemulsion properties on the condensation of hydrolyzed silica reacting species in microemulsions. Experimental Section Sample Preparation. Three nonionic surfactants, Igepal CO430 (NP4), CO520 (NP5), and RC520 (DP6), were kindly supplied by Rhone-Poulenc Inc. and used as received for the preparation of W/O microemulsion solutions. These surfactants are polydisperse mixtures of poly(oxyethylene) alkylphenyl ether molecules with the general chemical structure n-H(CH2)nPh(OC2H4)mOH (where for NP4, n ) 4, m ) 9; for NP5, n ) 5, m ) 9; for DP6, n ) 6, m ) 12). Their properties and the abbreviations are tabulated in Table 1. Tetraethyl orthosilicate (TEOS) (Si(OC2H5)4) with 99.999% purity (Aldrich), reagent grade heptane, and an aqueous ammonia (NH4OH) solution consisting of 71 wt % water and 29 wt % ammonia were used without further purification. Aqueous ammonia acts as both the reactant (H2O) and the catalyst (NH3) for the hydrolysis of TEOS. Microemulsion samples were prepared by first mixing heptane, surfactant, aqueous ammonia, and purified water (deionized at 18 MΩ) together. Then, the resulting mixtures were shaken until becoming optically transparent. The hydrolysis of TEOS and the formation of silica particles occurred immediately after the addition of TEOS to microemulsions. The initial concentration of TEOS in all samples of this study was fixed at 0.018 M. Prepared samples were then kept still at 22 °C for the subsequent measurements. Photon Correlation Spectroscopic Measurement. Photon correlation spectroscopy (PCS) was used to estimate the apparent hydrodynamic diameter of microemulsion droplet entities in the sample solution. For colloidal entities randomly moving in liquid media, their second-order autocorrelation function (g2) measured by PCS is given as

g2 - 1 ) exp(-2q2D τ)

(5)

where τ is the correlation time and q is the magnitude of the scattering vector, as shown below where θ is the scattering angle

q)

4πn θ sin λ 2

()

(6)

of incident light, λ is the wavelength of light in vacuum, and n is the refractive index of solution. D is the diffusion coefficient of colloidal entities in solution, which relates to the apparent hydrodynamic diameter, Dh, of colloidal entities by the following Stokes-Einstein relation: where

D ) kT/3πµDh

(7)

k is the Boltzmann constant, T is the absolute temperature of solution, and µ is the viscosity of solution. The light-scattering apparatus used in this study contained a photon multiplier tube connected with a 136-channel digital correlator (Brookhaven Instruments BI2030) and a 2-W argon ion laser (Lexel Model 95) which produced a light with a wavelength of 514.5 nm. All experiments were performed at 295 K. Electrical Conductivity Measurements. The measurement of electrical conductivity is a simple means of studying the connectivity of aqueous components in nonionic W/O microemulsions. For these solutions, electrical conductivity sums up the transient electrical currents associated with rapid (∼10-8 s) transport of aqueous ionic species among microemulsion droplets. Although electrical conductivity cannot describe the microscopic dynamics of water transport in microemulsions, it represents a relative scale of the connectivity of water in different microemulsions. In this experiment, the aqueous component of microemulsions was made of 1 M ammonium chloride (NH4Cl) in order to make the solution electrically conductive. Aqueous ammonia was not used because the electrical conductivity of solutions with this weak base was small compared to the resolution of our conductivity measurements (10-3 µS/cm). Titration of Phase Separation Point of Microemulsions. The phase separation point (i.e., the cloud point) of microemulsions was titrated with toluene, a good solvent for the poly(oxyethylene) head group of surfactants. Toluene was increasingly added to single-phase W/O microemulsions initially containing heptane oil and fixed amounts of water and surfactant until phase separation occurred. The heptane/toluene composition of resulting solutions corresponds to the minimum polarity of oil needed to dissociate surfactant molecules away from microemulsion droplets to cause the destabilization of droplets. Therefore, this heptane/toluene composition represents a qualitative scale for the strength of the surfactant to associate with droplets. The higher the toluene-to-heptane ratio at the phase separation point, the stronger the attachment of surfactant to droplets. Fourier Transform Infrared Spectroscopic Measurement. Fourier transform infrared (FTIR) spectroscopy was used to study the kinetics of TEOS hydrolysis in microemulsions. The concentration of TEOS during the reaction was measured by the absorbance of the Si-O-C stretching band of TEOS located at 970 and 795 cm-1. The transmission FTIR spectra of microemulsions were obtained by a single-beam FTIR spectrophotometer (Mattson CYGNUS 100) equipped with a wide-band mercury-cadmium-telluride detector. A triangular apodization was employed for the Fourier transformation of interferograms. A sample holder with a pair of zinc selenide (ZnSe) windows and 0.2 mm thick Teflon spacer (Harrick Scientific) was used to accommodate the solution. Transmission Electron Microscopic Analysis. The size and morphology of silica particles synthesized in the W/O microemulsion solution were measured using a transmission

3298 Langmuir, Vol. 13, No. 13, 1997

Chang and Fogler

electron microscope (Jeol 2000 FX). The sample for transmission electron microscopy (TEM) analysis was made by placing a small amount of microemulsion solutions on a TEM carbon-coated copper grid specimen (Structure Probe, 3520C). Because oven drying was not effective in evaporating the surfactant residue from the copper grid, dichloromethane (CH2Cl2) was used to wash it off. TEM micrographs of silica particles were converted to digitized images from which the diameters, D, of about 200 silica particles were measured. From the silica particle size distribution, the average diameter, 〈D〉, and the standard deviation, σ, defined below

σ ) [〈D2〉 - 〈D〉2]1/2

(8)

were calculated.

Results and Discussion Characteristics of Microemulsion Solutions. In this study, the synthesis of silica particles through the ammonia-catalyzed hydrolysis of TEOS was carried out in single-phase W/O microemulsions using different types of surfactant and solution compositions. The solution compositions for the ternary surfactant/water (aqueous ammonia)/heptane system used to form single-phase W/O microemulsions are detailed in the Appendix. In this section, the size, connectivity, and stability of microemulsion droplets are described. 1. Sizes of Microemulsion Droplets. The size of W/O microemulsion droplets was characterized in terms of the hydrodynamic diameter, Dh, using photon correlation spectroscopy (PCS). Figures 2, parts a-c, illustrate the effect of surfactant concentrations on the hydrodynamic diameter of droplets in microemulsions containing heptane and different surfactants, NP4, NP5, and DP6 (detailed in Table 1). For each type of surfactant, two types of aqueous media of the same weight were used; one is pure water at 0.25 M and the other is aqueous ammonia at 0.178 M H2O and 0.077 M NH3. It is clear that for all surfactants at low concentrations, the hydrodynamic diameter of droplets containing aqueous ammonia is smaller than that with pure water. This trend can be due to ammonia (and ammonium hydroxide, NH4+OH-) which possibly weakens the hydrogen-bonding of water to the surfactant’s poly(oxyethylene) head group.18,19 Hence, the hydration swelling of head groups is decreased in the presence of ammonia, which leads to the decrease in the surfactant’s head-to-tail area ratio and the formation of smaller droplets. Besides, the association between surfactants may also be weaker with ammonia as to cause smaller droplets to occur in microemulsions. Figure 2, parts a-c, also shows that for all types of surfactant the hydrodynamic diameter of droplets decreases significantly from approximately 30 nm to about 10 nm as the concentration of surfactant increases and moves away from phase boundary. Near the phase boundary, the concentration of water is large compared to that of the nonionic surfactant. This large water-tosurfactant molar ratio not only causes the hydration and swelling of head groups of surfactant but also results in the formation of large aqueous cores in the droplets due to the large water (volume)-to-surfactant (surface) ratio. As the surfactant concentration increases from the phase boundary value, the decline of both the hydration of the surfactant’s head groups and the size of aqueous cores causes the droplet size to decrease significantly. When the surfactant concentrations are 0.05 M above the phase boundary value, there is sufficient surfactant (18) Kahweit, M.; Strey, R.; Busse, G. J. Phys. Chem. 1990, 94, 3881. (19) Kabalnov, A.; Olsson, U.; Wennerstrom, H. J. Phys. Chem. 1995, 99, 6220.

Figure 2. The hydrodynamic diameter, Dh, of W/O microemulsion droplets as a function of the concentration of surfactant: (a) NP4 surfactant, (b) NP5 surfactant, and (c) DP6 surfactant. Composition of microemulsions: heptane, water at 0.25 M, or aqueous ammonia at 0.178 M H2O and 0.077 M NH3.

to solubilize all aqueous components in the surfactant’s head group layer to form reverse micelles with negligible aqueous pools. The hydrodynamic diameters of NP4, NP5, and DP6 droplets become approximately 7.5, 13.0, and 9.0 nm, respectively. The fact that these droplet sizes are at least 2 times larger than the surfactant molecular length (about 2-3 nm) reflects the micellization of surfactant molecules in solution. In addition, although the length of NP5 surfactant molecules is less than that of DP6, the size of NP5 droplets (13 nm) is noticeably larger than those of NP4 and DP6 (7.5 and 9 nm, respectively). This trend indicates that the NP5 surfactant tends to form large micellar entities as compared to NP4 and DP6 surfactants (see Figure 5). Figure 3 shows the effect of water concentration on the size of microemulsion droplets in heptane solutions containing 100 g of surfactant per liter of solution (100 g/dm3). For all surfactant solutions studied, the hydrodynamic diameter of droplets increases steadily with an increase in the water concentration. In addition, the

Controlled Formation of Silica Particles from TEOS

Figure 3. The hydrodynamic diameter, Dh, of W/O microemulsion droplets as a function of the concentration of water. Composition of microemulsions: heptane and 100 g/dm3 of surfactant.

hydrodynamic diameters of NP5 droplets are always significantly larger than those of NP4 and DP6 over the entire water concentration range. The hydrodynamic diameters for both NP4 and DP6 solutions increase slightly from 6 and 7 nm in the absence of water to 8 and 16 nm at phase boundary (1.45 M H2O for NP4 and 1.95 M H2O for DP6), respectively. These relatively small increases in droplet sizes indicate that the NP4 and DP6 microemulsion droplets are small and well dispersed in the solution. On the other hand, the hydrodynamic diameter for NP5 solutions increases significantly from 13 nm in the absence of water to 35 nm at 0.85 M H2O and then to 110 nm beyond phase boundary (0.97 M H2O). This steep rise in the NP5 droplet size in the vicinity of phase boundary indicates that NP5 surfactant molecules associate into large entities. 2. Water Connectivity in Microemulsion Solutions. Electrical conductivity is a simple means of characterizing the connectivity (or, conversely speaking, compartmentalization) of water in nonionic W/O microemulsions. The electrical conductivity of microemulsions containing 100 g/dm3 of surfactant, heptane, and varying amounts of 1 M NH4Cl aqueous solution is illustrated in Figure 4. For both NP4 and DP6 microemulsions, the electrical conductivity increases steadily with an increase in the concentration of water from a value between 0 and 0.02 µS/cm in the absence of water to a value of 0.16 µS/ cm at 0.4 M H2O. At higher water concentrations, the electrical conductivity of both NP4 and DP6 solutions remains constant at 0.16 µS/cm, indicating that aqueous components are highly compartmentalized in solution.20 On the other hand, the electrical conductivity of NP5 solutions increases essentially linearly with an increase in the water concentration up to a value of 0.4 µS/cm at 0.75 M H2O. A further increase in the water concentration toward its solubility limit results in a steep upturn in electrical conductivity. This upturn in electrical conductivity has been recognized as the so-called electrical percolation phenomena where water percolates throughout the entire W/O microemulsion medium.21 The similar trends between the curves shown in Figures 3 and 4 demonstrate that the microstructure of microemulsions is closely related to the connectivity of water in microemulsions. Figure 5 illustrates a schematic microstructural difference between the NP5 microemul(20) Shah, D. O.; Hamlin, R. M., Jr. Science 1971, 171, 483. (21) Langevin, D. Ann. Rev. Phys. Chem. 1992, 43, 341.

Langmuir, Vol. 13, No. 13, 1997 3299

Figure 4. Electrical conductivity of W/O microemulsions as a function of the concentration of water. Composition of microemulsions: heptane and 100 g/dm3 of surfactant.

sion and the other two microemulsions. For NP4 and DP6 surfactants, the tail group is larger than the head group despite that the latter can be swollen by water. As a result, small and dispersed W/O microemulsion droplets are formed over the entire range of water concentrations, which leads to a finite increase in the electrical conductivity of solutions. The dispersed droplet structure of NP4 W/O microemulsions was also found by Caldararu et al.22 using spin probe and fluorescence probe techniques. On the other hand, for the NP5 surfactant, its tail and head groups are considered to be comparable in size such that the average curvature of NP5 surfactant films becomes virtually near zero. As a consequence, the NP5 droplets, perhaps lamella-like,23 are prone to associating together, forming interdroplet open water channels and becoming large coagulated entities with continuous water pools therein.24,25 Hence, one observes that in Figures 3 and 4, both the NP5 droplet size and the electrical conductivity increase correspondingly, and eventually electrical percolation emerges as it approaches the phase boundary where large NP5 microemulsion entities take place. Previous research has shown the concurrence of electrical percolation and the fast interdroplet matter exchange rate for a number of microemulsions.21,24,25 Therefore, it is expected that the interdroplet matter exchange rate in NP5 solutions is to be higher than those in NP4 and DP6 solutions. 3. Stability of Microemulsions. The phase separation point of W/O microemulsions with varying water concentrations was titrated with toluene. As shown in Figure 6, at 0.4 M H2O, NP4, NP5, and DP6 microemulsions become phase-separated when the volume percentage of toluene exceeds 15, 35, and 50 vol %, respectively. This phase separation occurs as the oil continuum becomes sufficiently polar with increasing toluene fractions to cause surfactant molecules to dissociate from W/O microemulsion droplet surfaces. Because higher toluene fractions are needed to destabilize stronger surfactant-droplet interactions, Figure 6 implies that the strength of surfactant to associate with droplets follows the order: DP6 > NP5 > NP4 (see Figure 5). Because this order is also the same as that of increasing the number of oxyethylene (22) Caldararu, H.; Caragheorgheopol, A.; Vasilescu, M.; Dragutan, I.; Lemmetyinen, H. J. Phys. Chem. 1994, 98, 5320. (23) Ravey, J. C.; Buzier, M.; Picot, C. J. Colloid Interface Sci. 1984, 97 (1), 9. (24) Jada, A.; Lang, J.; Zana, R. J. Phys. Chem. 1989, 93, 10. (25) Jada, A.; Lang J.; Zana, R.; Makhloufi, R.; Hirsch, E.; Candau, S. J. J. Phys. Chem. 1990, 94, 387.

3300 Langmuir, Vol. 13, No. 13, 1997

Chang and Fogler

Figure 5. A schematic illustration of the structural difference among W/O microemulsion droplets which contain NP4, NP5, and DP6 surfactants.

Figure 6. The minimum volume percentage of toluene in the heptane/toluene oil phase of W/O microemulsions which needs to induce the phase separation of microemulsions.

groups of surfactants, it indicates that the stability of droplets is primarily controlled by the hydrogen-bonding association of the surfactant’s head groups among themselves and/or with the aqueous cores of droplets. Overall, the characteristics of microemulsion droplets in different surfactant solutions can be summarized as illustrated in Figure 5. For NP4 and DP6 surfactants, droplets are small and well dispersed, but the association of NP4 surfactant molecules with droplet surfaces is weak compared with those of the DP6 surfactant. For the NP5 surfactant, the droplets tend to associate together and form interdroplet open water channels. Hydrolysis of Tetraethyl Orthosilicate. Fourier transform infrared spectroscopy (FTIR) was used to study the rate of TEOS hydrolysis in W/O microemulsions containing heptane, 0.0357 M TEOS, 0.235 M H2O, 0.104 M NH3, and 50 g/dm3 of surfactant. A typical evolution of FTIR spectra of microemulsions is shown in Figure 7, part a. The gradual decrease in the Si-O-C stretching bands of TEOS located at both 795 and 967 cm-1 indicates that TEOS is hydrolyzed during the reaction period. The formation of ethanol is evidenced by the increase in both 882 and 1050 cm-1 bands which correspond to the C-C-O stretching of ethanol. The TEOS concentrations measured

Figure 7. (a) The evolution of FTIR spectrum of a NP4/heptane microemulsion solution. Base line: The FTIR spectrum of solution prior to TEOS hydrolysis. (b) The time evolution of the concentration of TEOS hydrolyzed in W/O microemulsions. Composition of microemulsions: heptane, 0.0357 M TEOS, 0.235 M H2O, 0.104 M NH3, and 50 g/dm3 of surfactant.

from the absorbance of the 967 cm-1 band are shown as a function of the reaction time in Figure 7, part b. For all microemulsions with NP4, NP5, and DP6 surfactants, the logarithm of TEOS concentration decreases linearly with time, indicating the rate of TEOS hydrolysis is first order with respect to TEOS concentration. The specific TEOS hydrolysis rate constants, kh, determined from the slope of linear fitting, were found to be near 0.035 h-1 for all microemulsions.

Controlled Formation of Silica Particles from TEOS

Figure 8. The TEM micrographs and size distributions of silica particles synthesized in W/O microemulsions with different surfactants. (a) NP4 surfactant, (b) NP5 surfactant, and (c) DP6 surfactant. Composition of microemulsions: heptane, 0.174 M H2O, 0.075 M NH3, 50 g/dm3 of surfactant, and 0.018 M TEOS. The residual surfactants can be seen as thin threads between silica particles.

The closeness of the TEOS hydrolysis rates for NP4, NP5, and DP6 surfactants suggests these surfactants have a similar hindrance to the contact between oil-soluble TEOS molecules and the aqueous components inside microemulsion droplets. Because the rates of TEOS hydrolysis are similar for these surfactants, any significant variation of the final silica particle size with the type of surfactant used should be primarily due to the different effectiveness of surfactant in hindering the contact and condensation of hydrolyzed polymeric silica reacting species while they grow into nuclei (and particles). Similarly, our previous study found that TEOS hydrolysis in microemulsions had a reaction order of 0 and 0.5 with respect to the concentrations of water and surfactant, respectively.7 From this weak dependence of the TEOS hydrolysis rate on the concentrations of water and surfactant, it is inferred that the variation of silica particle sizes with water and surfactant concentrations is also due to the change in the compartmentalization of polymeric silica species in microemulsions. Formation of Silica Particles. Transmission electron microscopy was used to measure the size distribution of silica particles in W/O microemulsions 10 days after TEOS hydrolysis. Figures 8, parts a-c, illustrate the TEM micrographs and the measured size distributions of silica particles synthesized in solutions with heptane, 0.174 M H2O, 0.075 M NH3, and 50 g/dm3 of different surfactants. Irrespective of the surfactant used, silica particles are essentially spherical and attach predominantly as a monolayer to the TEM copper grid, indicating that the

Langmuir, Vol. 13, No. 13, 1997 3301

Figure 9. The number average diameter, 〈D〉, and the normalized standard deviation, σ/〈D〉, of silica particles synthesized in W/O microemulsions as a function of surfactant concentration. Composition of microemulsions: heptane, 0.174 M H2O, 0.075 M NH3, and 0.018 M TEOS.

silica particles are well dispersed in the original solution. The measured silica particle size distributions show a single mode at the midpoint (or slightly above). Approximately, 92% of the silica particles span a range of 10-12 nm, and the other 8% are apparently smaller and located in the lower end of the distribution. The average diameter, 〈D〉, and the normalized standard deviation, σ/〈D〉, of silica particles were calculated from the TEM image of particles. As shown in Figures 9 and 12, the diameters of silica particles are in general about 5-7 times larger than the hydrodynamic diameter of microemulsion droplets shown in Figures 2 and 3. From the sizes and volume fractions of silica particles and microemulsion droplets (i.e., water plus surfactant), the number of synthesized particles was found to be 3-4 orders of magnitude less than that of droplets. This result clearly shows that the nucleation of silica particles is carried out primarily by the condensation of hydrolyzed silica species through exchanges between interacting droplets. 1. Effect of Type of Surfactant. The effect of the chemical structure of nonionic poly(oxyethylene) alkylphenyl ether surfactants on the formation of silica particles in microemulsions is illustrated in Figures 9 and 12. One observes that the number average diameters of silica particles synthesized in NP4, NP5, and DP6 solutions differ significantly from one another. The size of synthesized silica particles varies with the type of surfactant by the order: NP5 > NP4 > DP6. In this study, 0.018 M

3302 Langmuir, Vol. 13, No. 13, 1997

Chang and Fogler

Figure 10. A schematic illustration of the effect of surfactant and water concentrations on the formation of silica particles in microemulsions.

TEOS was used in all samples and completely reacted in the end. Because larger particles consume more TEOS, the number of synthesized silica particles follows the order: NP5 < NP4 < DP6. As described in the introduction, the total number and the size distribution of silica particles are influenced significantly by the steric hindrance and the compartmentalization effects that microemulsion droplets have on silica reacting species during the particle nucleation stage. The fact that silica particles synthesized in NP5 solutions are the largest can be attributed to the near zero average curvature of NP5 surfactant films. As illustrated in Figures 5 and 10, NP5 surfactant films tend to associate together to form open channels between droplets which facilitate large silica reacting species to pass through to proceed condensation. On the other hand, in NP4 and DP6 microemulsions, the dynamic exchange of reacting species between the small, well-dispersed droplets is low compared with that in NP5 solution as to result in the formation of smaller particles. Other different trends in silica particle formation between NP5 and the other surfactants also include: (1) In the low surfactant concentration range, a steep reduction of 〈D〉 occurs as NP4 and DP6 concentrations increase while it does not occur with the NP5 surfactant (Figure 9, part a); (2) the average diameter, 〈D〉, decreases with an increase in water concentrations in NP4 and DP6 solutions but remains virtually constant in NP5 solutions until phase separation occurs (at 1.1 M) (Figure 12 part a). Figures 9 and 12 also show that silica particles synthesized in NP4 solutions are larger than those in DP6 solutions although the droplet size and water connectivity are similar for both solutions as shown in Figures 2-4. This trend can be explained by the fact that both hydrophilic and hydrophobic groups of DP6 surfactant are longer than those of NP4 surfactant. The DP6 surfactant with six oxyethylene groups attaches more strongly to microemulsion droplet surfaces than the NP4 surfactant with four oxyethylene groups. The longer

hydrocarbon tail of the DP6 surfactant also makes the surfactant film less deformable than the shorter tail of the NP4 surfactant.26 In summary, both the stronger surfactant attachment to droplets and the less surfactant film deformability impart a higher steric hindrance to the interdroplet exchange of hydrolyzed silica reacting species and, hence, lead to the particles synthesized in DP6 solutions being smaller than those in NP4 solutions. 2. Effect of Surfactant Concentration. Figure 9, parts a and b, illustrates the effect of the concentration of NP4, NP5, and DP6 surfactants on 〈D〉 and σ/〈D〉 of silica particles synthesized in microemulsions containing heptane, 0.174 M H2O, 0.075 M NH3, and 0.0180 M TEOS. The variation of silica particle sizes with the concentration of surfactant appears similar for both NP4 and DP6 microemulsions. For NP4 microemulsions, the 〈D〉 value decreases by 30% from 51 nm at 0.05 M NP4 to 38 nm at 0.08 M NP4 and then increases slightly to 42 nm at 0.25 M NP4. Similarly, for DP6 solutions, the 〈D〉 value decreases from 38 nm at 0.35 M DP6 to 28 nm at 0.08 M DP6 and then increases to 34 nm at 0.19 M DP6. The variation of silica particle size as a function of the surfactant concentration is schematically illustrated in Figure 10 and can be explained as follows. When the concentration of NP4 and DP6 surfactants is within 0.04 M of the phase boundary (i.e., less than about 0.08 M), the size of silica particles increases significantly with a decrease in the surfactant concentration. This trend is very similar to the variation in microemulsion droplet sizes shown in Figure 2. In the vicinity of phase boundary, water becomes excessive to the surfactant as to form aqueous pools within the droplets where silica species are fully hydrated. When the surfactant concentration decreases toward the phase boundary, the number of droplets decline significantly but each droplet accommodates more (26) Ben-Shaul, A.; Gilbert, W. M. In Micelles, Membrances, Microemulsions, and Monolayers; Ben-Shaul, A., Gilbert, W. M., Roux, D., Eds.; Springer-Verlag: New York, 1994.

Controlled Formation of Silica Particles from TEOS

Langmuir, Vol. 13, No. 13, 1997 3303

Figure 11. TEM micrographs of silica particles synthesized in W/O microemulsions with different water concentrations. Composition of microemulsions: heptane, 0.075 M NH3, 100 g/dm3 of surfactant and 0.018 M TEOS.

hydrolyzed silica reacting species. These reacting species are polymerized more easily through direct collisions inside droplets than through the interdroplet matter exchange which requires the energetically unfavorable transient dimer formation. Therefore, fewer nuclei form and then grow up to larger final sizes as microemulsions approach the phase boundary. When the concentration of NP4 and DP6 surfactants exceeds 0.08 M, the size of silica particles becomes slightly increased with an increase in the surfactant concentration (Figure 9, part a). This trend is attributable to the hydration of silica surfaces which has proven to be essential to the colloidal stability of silica particles synthesized in W/O microemulsions (see Introduction). As shown in Figure 10, in this concentration range, surfactant molecules become sufficient so that their head groups associate directly with almost all water molecules to form reverse micellar droplets of constant sizes. Increasing the surfactant concentration partitions more water toward associating with surfactant and correspondingly reduces the fraction of water that attaches directly to silica species. Owing to a smaller degree of surface hydration, silica species adsorb fewer surfactant molecules and become less sterically protected by surfactant films; hence, the species can exchange more easily between droplets and condense into larger particles at higher surfactant concentrations. For both NP4 and DP6 microemulsions, the σ/〈D〉 value of silica particles in Figure 9, part b, also displays a similar trend with respect to surfactant concentration; that is, σ/〈D〉 increases gradually from 4.5% at 0.04 M surfactant to 11.5% at 0.2 M surfactant. The fact that σ/〈D〉 decreases significantly toward phase boundary is mainly due to the

steep increase of 〈D〉 and a fairly constant particle size standard deviation, σ. On the other hand, at surfactant concentrations higher than 0.08 M, Figure 9, parts a and b, shows that the extent of the σ/〈D〉 increment is higher than that of 〈D〉, indicating that σ becomes larger. This broader silica particle size distribution at higher surfactant concentrations also suggests that the steric stabilization of larger silica reacting species is reduced due to the decline in the adsorption of surfactant to the surface of lesshydrated silica species. Both 〈D〉 and σ/〈D〉 of silica particles formed in NP5 microemulsions show somewhat different trends from those of NP4 and DP6 microemulsions. For example, phase separation occurs at a NP5 concentration of 0.09 M during the later stages of silica formation. This phase separation is attributable to the TEOS hydrolysis product, ethanol, which easily destabilizes NP5 microemulsion droplets with a near zero surface curvature. When the NP5 surfactant concentration further increases, the 〈D〉 value increases slightly from 47 nm at 0.09 M NP5 to 49 nm at 0.22 M NP5, and the σ/〈D〉 value decreases slightly from 11.5% at 0.09 M NP5 to 9% at 0.22 M NP5. This variation of both 〈D〉 and σ/〈D〉 with the NP5 surfactant concentration is small compared to those of NP4 and DP6 cases. This trend suggests that the nucleation of silica particles in NP5 solutions is influenced not only by the surface hydration of reacting species but also by the fact that the coagulation of NP5 droplets are sensitive to the water-to-surfactant molar ratio. The influence of NP5 microemulsion structures on silica particle formation is further described in the following section. 3. Effect of Water Concentration. Figure 11 shows the TEM micrographs of silica particles synthesized in

3304 Langmuir, Vol. 13, No. 13, 1997

Figure 12. The number average diameter, 〈D〉, and the normalized standard deviation, σ/〈D〉, of silica particles that are shown in Figure 11 as a function of water concentration.

microemulsions with 0.075 M NH3 and 100 g/dm3. The 〈D〉 and σ/〈D〉 of particles shown in Figure 11 are plotted as a function of water concentration in Figure 12, parts a and b, respectively. For NP4 and DP6 microemulsions, the 〈D〉 value decreases gradually from values of 41 and 34 nm at 0.174 M H2O to values of 24 and 23 nm at 1.1 M H2O, respectively. The σ/〈D〉 values for both NP4 and DP6 solutions remain in the range of 10-15%. The fact that the particle size decreases with an increase in the water concentration suggests that more silica particles are nucleated at higher water concentrations. As shown in Figures 1 and 10, the nucleation of silica particles occurs through the Brownian collision of reacting species in the aqueous domain of microemulsions. At higher water concentrations, reacting species are diluted into lower concentrations in the aqueous domain (with a larger volume) and undergo collisions in a slow rate as to facilitate the nucleation process. Previous theoretical and experimental studies found that under a surface reaction limited growth condition, the particle size, D, of silica particles synthesized in alcohol solutions scaled with the initial concentration of TEOS, C, as D ≈ C1/6. 27-29 In this study, (27) Matsoukas, T.; Gulari, E. J. Colloid Interface Sci. 1988, 124, 252; 1989, 132, 13. (28) Bogush, G. H.; Zukoski, C. F., IV. J. Colloid Interface Sci. 1991, 142, 1, 1992, 19. (29) Van Blaaderen, A.; Van Geest, J.; Vrij, A. J. Colloid Interface Sci. 1992, 154, 481.

Chang and Fogler

if the apparent concentration of TEOS in the aqueous domain of microemulsions (i.e., C) is assumed to be inversely proportional to the water concentration, we find that the final size of particles (i.e., D) synthesized in NP4 and DP6 solutions (Figure 12, part a) still varies with C by a larger dependence than that predicted by the surface reaction limited particle growth, especially in the small C range (i.e., at high water concentrations). This larger dependence in size reveals that particle growth in microemulsions may be complicated by the microstructure of solutions. As shown in Figure 10, the hydration of both silica reacting species and the head group of the surfactant becomes greater at higher water concentrations. Hence, the surfactant adsorbs to silica species as a more densely packed film barrier to the interdroplet exchange of silica species, which leads to the reduction of particle sizes with an increase in water concentration by a larger dependence than that in alcohol. For NP5 microemulsions, on the other hand, 〈D〉 remains virtually constant at 48 nm in the range of 0.174-0.73 M H2O. When the water concentration increases to 1.1 M H2O, 〈D〉 decreases significantly to 36 nm. Similarly, the σ/〈D〉 value remains at about 10% between 0.174 and 0.73 M H2O and increases steeply to 21% at 1.1 M H2O. The significant change of both 〈D〉 and σ/〈D〉 at 1.1 M H2O is due to the occurrence of phase separation during silica synthesis which results in a secondary nucleation. At lower water concentrations, the size of silica particles appears to be insensitive to the concentration of water. Since this trend differs clearly from that of NP4 and DP6 cases, it suggests that the formation of silica nuclei in NP5 solutions is not determined by the dilution and hydration of reacting species with water but mainly by the characteristics of NP5 microemulsions as shown in Figure 10. The size of NP5 droplets increases with the water concentration much more significantly than those of NP4 and DP6 droplets (see Figures 3 and 5). At higher water concentrations, NP5 droplets with a near zero surface curvature are prone to coagulating and forming interdroplet open water channels through which reacting species undergo collision and condensation without overcoming surfactant film barriers. It seems that this facilitation of silica condensation (which inhibits nucleation) due to interdroplet water channel formation counteracts the negative effect of water dilution on silica condensation. As a consequence, the nucleation of silica particles does not increase with an increase in the water concentration, leading to a virtually constant size of particles formed in NP5 solutions (see Figure 12, part a). Besides, the size distribution of silica particles also appears weakly dependent on the concentration of water as evidenced by the fairly constant σ/〈D〉 value shown in Figure 12, part b. 4. Effect of Type of Oil. The effect of the type of oil on the size distribution of silica particles was studied using microemulsions containing the NP5 surfactant and three oil solvents, heptane, cyclohexane, and a mixture of 50 vol % heptane and 50 vol % cyclohexane. Figure 13, parts a and b, shows the 〈D〉 and σ/〈D〉 of silica particles as a function of the concentration of the NP5 surfactant. When the concentration of the NP5 surfactant is greater than 0.08 M, the 〈D〉 value maintains approximately at 48, 39, and 31 nm for the oil continuum of heptane, the heptane/ cyclohexane mixture, and cyclohexane, respectively. This particle size difference indicates that the formation of silica particles is influenced significantly by the type of oil used in microemulsions. We have also found that NP5 droplets transformed from the associated entities in heptane to the well-dispersed ones in both cyclohexane and the 50% heptane/50% cyclohexane media. This increase in the

Controlled Formation of Silica Particles from TEOS

Langmuir, Vol. 13, No. 13, 1997 3305

In this study, it is clearly demonstrated that narrowly dispersed silica particles with the average diameter between 20 and 70 nm can be produced in nonionic W/O microemulsions from the hydrolysis of tetraethyl orthosilicate by varying the composition of solution and the type of surfactant and oil. Therefore, the hydrolysis of TEOS in nonionic W/O microemulsions provides simple routes to make model oil-dispersible silica nanoparticles. Our finding that the final size distribution of silica particles is sensitive to the compartmentalization and dynamic exchange of aqueous species over microemulsion droplets suggests it may be feasible in characterizing other (nonionic) microemulsion solutions using the size distribution of silica particles synthesized therein. It should also be pointed out that the mechanism of silica particle formation uncovered in this study appears general and should provide a knowledge basis for studying and controlling the formation of other types of colloidal particles in nonionic W/O microemulsions. Summary

Figure 13. The effect of oil types on the number average diameter, 〈D〉, and the normalized standard deviation, σ/〈D〉, of silica particles synthesized in W/O microemulsions at different concentrations of NP5 surfactant. Composition of microemulsions: 0.174 M H2O, 0.075 M NH3, and 0.018 M TEOS.

compartmentalization of silica reacting species in solutions may account for the decrease in the synthesized silica particle size in the presence of cyclohexane. Figure 13, part a, shows that in both the heptane/ cyclohexane mixture and cyclohexane, 〈D〉 increases significantly up to a value of 65 nm when the concentration of the NP5 surfactant decreases from 0.08 M toward the phase boundary. In addition, as illustrated in Figure 13, part b, the σ/〈D〉 values for these solutions steadily increase with an increase in the NP5 surfactant concentration. These trends are very similar to those of NP4 and DP6 microemulsions shown in Figure 9, parts a and b. This similarity further verifies that cyclohexane-contained NP5 microemulsions contain well-dispersed droplets as compared to those in heptane-based NP4 and DP6 microemulsions. The fact that varying the type of oil affects the structure of microemulsion droplets has been previously reported.30,31 Oil molecules with a shorter length or a higher polarity (e.g., cyclohexane) can swell the hydrophobic layer of surfactant films to a larger extent by orienting themselves more easily along the surfactant tail group or by penetrating more deeply toward the surfactant head group. This stronger swelling deforms surfactant films and, thus, reduces the size of droplets. (30) Leung, R.; Shah, D. J. Colloid Interface Sci. 1987, 120 (2), 330. (31) Mitchell, D. J.; Ninham, A. W. J. Chem. Soc., Faraday Trans. 2 1981, 77, 601.

The influence of the structure of microemulsions on the formation of silica particles in nonionic W/O microemulsions was investigated. The variables studied include the type and concentration of surfactant, the concentration of water, and the type of oil. Despite the fact that these variables only slightly influence the rate of TEOS hydrolysis, they significantly change the size distribution of silica particles synthesized in microemulsions. This trend indicates that the formation of silica particles is primarily controlled by these variables during the nucleation period where silica nuclei emerge through both intradroplet collisions and interdroplet matter exchanges of hydrolyzed polymeric silica reacting species. In general, silica particles with smaller sizes are synthesized when the parameter values decrease the condensation of silica reacting species by increasing the number of microemulsion droplets that compartmentalize reacting species in solutions or by increasing the steric hindrance of surfactant films that retard the dynamic exchange of reacting species between droplets. The specific effect of each variable is summarized below: Effect of Type of Surfactant. The molecular structure of surfactant affects the size, connectivity and stability of microemulsion droplets. In a heptane medium, NP4 and DP6 droplets are small and well dispersed while NP5 droplets tend to associate and form interdroplet open water channels. Therefore, silica reacting species in NP5 microemulsions are significantly less compartmentalized over droplets than those in NP4 and DP6 microemulsions, leading to the largest silica particles synthesized in NP5 microemulsions. With the longer head and tail groups, the DP6 surfactant seems likely to provide stronger steric film barriers to the interdroplet exchange of silica reacting species than those of the NP4 surfactant. Consequently, more nuclei are formed and become into smaller particles in DP6 solutions than in NP4 solutions. Effect of Surfactant Concentration. In NP4 and DP6 microemulsions with a constant water concentration, silica particles with the smallest size are formed at an intermediate surfactant concentration that is approximately 0.04 M above phase boundary. As the concentration of surfactant is decreased toward phase boundary, microemulsion droplets increase in size but decrease in number; thus, silica reacting species are less compartmentalized and condense into larger particles. On the other hand, when NP4 and DP6 surfactant concentrations increase from the intermediate value, silica particles increase in size due to the reduction in steric stabilization of reacting species with less-hydrated surfaces for the

3306 Langmuir, Vol. 13, No. 13, 1997

adsorption of surfactant. In the vicinity of the phase boundary, NP5 microemulsions with a near zero surfactant film curvature tend toward phase separation during silica particle synthesis. Away from the phase boundary, the size of synthesized silica particles are less influenced by the NP5 surfactant concentration compared with those in NP4 and DP6 cases. Effect of Water Concentration. Water acts as the reaction medium in which silica reacting species undergo collisions and condense into nuclei. In NP4 and DP6 solutions, increasing the water concentration dilutes reacting species within the water pools of droplets. Hence, the rate of condensation between reacting species is reduced as to facilitate nucleation and formation of small particles. In addition, nucleation is further enhanced at higher water concentrations as a consequence that reacting species become more sterically stabilized by the surrounding surfactant films with higher degrees of hydration. On the other hand, for NP5 solutions, the size of silica particles only shows a weak dependence on the concentration of water due to the formation of open water channels among associated microemulsion droplets which significantly reduces the effect of water dilution on the nucleation of silica particles. Effect of Type of Oil. Oil molecules with smaller sizes or higher polarities penetrate more deeply into surfactant films around W/O microemulsion droplets to cause the droplet size reduction and morphological change. Due to this oil-penetration effect, the extent of compartmentalization of hydrolyzed silica reacting species in NP5 microemulsions increases as the oil continuum changes from heptane to cyclohexane; therefore, silica particles synthesized in cyclohexane are smaller than those in heptane. Acknowledgment. The authors express gratitude for the financial support of the Department of Energy and the following industrial sponsors: Aramco, ARCO, Chevron, Conoco, Dowell-Schlumberger, Hurriburton Services, Mobil and Unocal. We also greatly appreciate Professors E. Gulari and F. E. Filisco for the use of FTIR spectrophotometer and LS apparatus. Appendix Phase Diagrams of the Ternary Surfactant/Water (Aqueous Ammonia)/Heptane Systems. In this study, silica particles were synthesized in the single-phase W/O microemulsion solution containing heptane, ammonia, water, and three surfactants, NP4, NP5, and DP6 at temperature 22 °C. Therefore, the compositions of these systems for forming a single-phase W/O microemulsion solution at this temperature were evaluated. Figures 14, parts a, b, and c, illustrates a section of the phase diagrams of the microemulsion systems containing NP4, NP5, and DP6, respectively. For each phase diagram, there are two curves representing the solubilities of water and aqueous ammonia (with 29 wt % NH3), respectively. These curves separate the single-phase region (i.e., L2 phase) from the two-phase region. In the single-phase region, water and aqueous ammonia are completely solubilized in the W/O microemulsion media, while in the two-phase region, the excessive amount of water results in a second water-rich phase. Figure A1, parts a, b, and c, clearly illustrates that the solubility of pure water in each of the surfactant solutions differs from that of aqueous ammonia. The relative value of the solubility of water to that of aqueous ammonia also varies with the type of surfactant. Figure A1, part a, shows that both water and aqueous ammonia become

Chang and Fogler

(a)

(b)

(c)

Figure 14. Phase diagrams of heptane/aqueous media/ surfactant ternary systems: (a) NP4 surfactant, (b) NP5 surfactant, and (c) DP6 surfactant. The aqueous media: water (thick line) and aqueous ammonia with 29 wt % NH3 (thin line). Note that the single-phase W/O microemulsions in this study is usually referred to as L2 phase.

soluble in the NP4/heptane solution when the NP4 concentration exceeds 1.8 wt %. Above this value, the amount of both water and aqueous ammonia solubilized in the solution increase steadily with an increase in the surfactant concentration. In general, the solubility of water is higher than that of aqueous ammonia. Figure A1, part b, illustrates the phase behavior of the NP5/ heptane solution. The solubilities of water and aqueous ammonia become significant only when the NP5 concentration reaches 3 and 3.5 wt %, respectively. The solubility of water is less than that of aqueous ammonia, which is opposite to that for NP4. Figure A1, part c, illustrates the phase behavior of the DP6/heptane solution. Similar to the NP4 case, both water and aqueous ammonia become soluble when the DP6 concentration is higher than 1.8 wt %. However, the solubility of water exceeds that of aqueous ammonia only when the DP6 concentration is higher than 6 wt %. The different solubilities of water and aqueous ammonia in the nonionic microemulsion solutions can be attributed to the fact that the hydration of the poly(oxyethylene) head groups of surfactants by the aqueous ammonia solution is weaker than that by water, as described in Figure 2. However, the occurrence of different relative values in the solubilities of water and aqueous ammonia in different solutions is due to the different spontaneous curvatures associated with different types of surfactant layers. For the NP4 surfactant, the surfactant layer always bends toward water in the presence of water and

Controlled Formation of Silica Particles from TEOS

aqueous ammonia. However, water provides a greater extent of swelling (i.e., hydration) for the surfactant head group and therefore decreases the curvature of the NP4 surfactant layer. With a smaller surface curvature, W/O microemulsion droplets become larger and therefore accommodate more water (than aqueous ammonia). On the other hand, for the NP5 surfactant in heptane media, the surfactant layer bends from toward water to toward oil once the head group of surfactant is swelled (i.e., hydrated) to a certain extent, leading to the occurrence of phase inversion from the W/O microemulsion phase to the O/W microemulsion phase accompanied with a second

Langmuir, Vol. 13, No. 13, 1997 3307

oil-rich phase. Water offers a greater swelling of the head group layer of the NP5 surfactant (than that of aqueous ammonia) and therefore induces phase inversion at the lower concentration, leading to the solubility of water being less than that of aqueous ammonia. For the DP6 surfactant, the surfactant layer seems to have a spontaneous curvature between those of NP4 and NP5. Therefore, at low-surfactant concentrations the solubility of water is lower than that of aqueous ammonia, but at highsurfactant concentrations the opposite trend takes place. LA961062Z