Measurements and Correlation of Octyltriethoxysilane Solubility in

Sep 23, 2009 - Measurements and Correlation of Octyltriethoxysilane Solubility in Supercritical CO2 and Assembly of Functional Silane Monolayers on th...
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Ind. Eng. Chem. Res. 2009, 48, 9952–9960

Measurements and Correlation of Octyltriethoxysilane Solubility in Supercritical CO2 and Assembly of Functional Silane Monolayers on the Surface of Nanometric Particles Carlos A. Garcı´a-Gonza´lez,*,† Julio Fraile,† Ana Lo´pez-Periago,† Javier Saurina,‡ and Concepcio´n Domingo*,† Instituto de Ciencia de Materiales de Barcelona, ICMAB-CSIC, Campus de la UAB s/n, E-08193 Bellaterra, Spain, and Department of Analytical Chemistry, UniVersity of Barcelona, Diagonal 647, E-08028 Barcelona, Spain

Trialkoxysilanes are widely used as primers to functionalize inorganic nanoparticles and to facilitate their interaction with organic phases. In this work, the silanization of nanometric fillers (titania and maghemite) was successfully carried out using supercritical carbon dioxide as the carrier solvent for octyltrialkoxysilane. First, the solubitility behavior of octyltriethoxysilane in compressed CO2 was evaluated at different pressures and temperatures. The measured solubility data were correlated using the Chrastil equation. Next, nanometric powders of either titania or maghemite were silanized at different pressures, temperatures, and reaction times. The prepared samples were characterized by electron microscopy, N2 adsorption-desorption, and laser scattering. Finally, the operating conditions that led to optimal material performance as a function of the intended application for the coated titanium dioxide powder (UV filter in cosmetics and filler in plastics) were evaluated by means of conveniently defined objective functions. 1. Introduction The high-quality dispersion of inorganic fillers in organic phases is essential in the production of homogeneous hybrid composite materials used as paints, plastics, adhesives, restorative biomaterials, electronic packages, and so on.1,2 To overcome the problem of the incompatibility of organic or polymeric phases with high-surface-area nanosized mineral fillers, the surface of the nanoparticles must first be altered from hydrophilic to organophilic. Many organic systems have been investigated for surface modification, with the formation of selfassembled monolayers of alkylsilanes being one of the most studied to date.3-6 Moreover, surface coating by silanization has also been applied to surface protection against corrosive environments,7 water repellency,8-10 protection of labile biomolecules,11 metal recovery,12 and design of chromatographic stationary phases.13 The self-assembly of monolayers of a wide variety of silane molecules on flat, polished, and particulate metal oxide surfaces has been extensively studied using either vapor- or liquid-phase reactions14-23 or supercritical methods.16,24-28 The use of anhydrous vapor-phase silane deposition routes is limited to volatile and thermally stable organosilanes. Instead, silane deposition from aqueous/alcohol liquid solutions often leads to the formation of low-quality self-assembled monolayers because of the difficulty in controlling the amount of water in the solvent mixture.26,29 On the other hand, the use of clean and low-emission technologies is attracting great interest because of tightening environmental restrictions and might be of increasing economic importance. Supercritical carbon dioxide (scCO2) represents an environmentally friendly alternative to traditional solvents, because it can avoid the production of large amounts of * To whom correspondence should be addressed. E-mail: cgarcia@ icmab.es (C.A.G.-G.), [email protected] (C.D.). † Instituto de Ciencia de Materiales de Barcelona, ICMAB-CSIC. ‡ University of Barcelona.

hazardous organic or aqueous waste. For these reasons, in conjunction with its moderate critical temperature and pressure (304 K and 7.3 MPa, respectively), scCO2 has become the basis for many innovative applications.30 For the specific application of nanoparticle surface modification or coating, scCO2 has the advantage of its near-zero surface tension, allowing the easy penetration of the fluid into the internal volume of the mesoporous aggregates often formed by nanoparticles. The main objective of this work was to explore the feasibility and limits of using an scCO2 method for the silane coating of different inorganic nanoparticles (titania and maghemite). Nanometer-sized titania (TiO2) is one of the most common fillers incorporated into rubbers and thermoplastics and is a pigment added to paper and cosmetic.31 Coated nanometric maghemite (γ-Fe2O3) is used in applications as diverse as color laser printer technology32 and magnetically guided drug targeting.33 Quantifying the solubility of solutes in compressed CO2 as a function of pressure and temperature is fundamental for any industrial application. Hence, solubility data were first obtained for the system under study: octyltriethoxysilane-scCO2. Next, we performed a detailed study of silanecoated nanoparticles obtained by the scCO2 method using electron microscopy, N2 adsorption-desorption, and laser scattering. The influence of the operating conditions (pressure, temperature, reaction time, and depressurization rate) on the surface structure, hydrophobicity, and dispersibility of the silane-coated nanoparticles was evaluated. Characterization of the system in terms of infrared and Raman spectroscopies and thermogravimetric analysis has been reported elsewhere.28,34 Data compilation of the full study allowed for the selection of the optimal operating conditions for the silanization of TiO2 nanoparticles according to two potential applications: as a UV filter in the cosmetics industry and as an inorganic filler in the plastics industry.

10.1021/ie900775z CCC: $40.75  2009 American Chemical Society Published on Web 09/23/2009

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Figure 1. Schematic setup of the high-pressure equipment in which cooled (EX1) CO2 (T1) is compressed by a syringe pump (TharDesign SP240, P1). V1-V6 are valves used to regulate the flow of CO2, straight lines correspond to stainless steel tubing, and lines with three-slashed groups represent electrical connections. For control instruments, the first letter denotes the measured property (T, temperature; P, pressure; L, level; and F, flow rate), whereas the second and following letters denote the instrument function(s) (I, indicator; C, controller; and G, sight glass). The main parts of the solubility and reaction apparatus are (a) a variable-volume cell (Re1, from ∼5 to 15 mL) and (b) an autoclave (TharDesign, Re2 with a total volume of ∼115 mL). The variablevolume cell is used for solubility measurements. The body has a piston that is moved inside the cell using a Ruska manual pump (P2). The effective cell volume is a function of the piston position and is indicated by a precalibrated level indicator (LI-101). The system is heated using embedded heaters (H1) and stirred with a magnet (S1). A CCD camera provides the capability to view inside the cell through sapphire windows (LG-102). The autoclave is used for silane deposition. The reactor is heated using resistances (H2) and stirred with a magnetic device (S2). During depressurization, the flow rate can be controlled by means of valve V4 and measured with a digital mass flowmeter (Bronkhorst Hi-Tec, FI-101).

2. Materials and Methods 2.1. Materials. Bare TiO2 nanoparticles (TiO2 P25) of ∼20nm diameter and (C8)Si(OMe)3 silanized TiO2 (TiO2 T805, sample labeled TiC8-D) were supplied by Degussa. Nanometric γ-Fe2O3 powder (∼5-nm diameter) was synthesized in the CSIC laboratories following a reported procedure.33 Octyltriethoxysilane (C8 from Fluka) and CO2 (Carburos Meta´licos S.A.) were used as a solute and as a solvent, respectively. 2.2. Equipment and Procedures. 2.2.1. Solubility Measurements. The solubility of octyltriethoxysilane in compressed CO2 was determined using a variable-volume cell with sapphire windows (Figure 1a, Phase Equilibrium Analyzer from Thar Technologies, Inc.). For solubility measurements, compressed CO2 (∼6 MPa) was introduced from a cylinder into the view cell, previously brought to the minimum volume, which contained a weighed amount of silane. The cell was then isolated and heated to the desired temperature (318 or 348 K). The pressure inside the solubility cell was increased by pumping more CO2 until formation of a single phase (with no discernible liquid silane-supercritical CO2 interphase) was clearly observed in a video output. Phase transitions were visually determined by observing the phase condition in the cell with stirring. The pressure in the cell was then increased by ∼1 MPa, and the system was allowed to reach equilibrium for 10 min. An internal stirrer helped equilibrium conditions between the liquid silane and the scCO2 to be reached rapidly. Under the working interval of pressures and temperatures, increasing the equilibrium time to 30 min did not significantly affect the values of the measured solubility data. The amount of CO2 charged to the system was calculated from the density of the compressed CO2 at the settled pressure and temperature35,36 after subtraction of the volume of liquid solute added into the vessel from the inner volume of the cell (10.25 mL at the minimum volume, previously calibrated using water as the fluid). Next, the mixture of scCO2 and the organic liquid was expanded by gradually increasing the volume of the cell (by moving the vessel plunger down) until drops of liquid silane appeared, conditions at which both the pressure and the temperature were recorded. The compression-decom-

pression cycle took about 1 h. The cycle was repeated three times for each recorded solubility value. The accuracy of the measured solubility was estimated to be (2%. 2.2.2. Silanization Process. Supercritical silanization was performed using the setup depicted in Figure 1b running in batch mode. In a typical experiment, the reactor was first charged with ∼0.5 g of nanoparticulate powder enclosed in a cylindrical cartridge made of 0.45-µm-pore filter paper that was suspended on the upper part of the autoclave. Liquid silane (∼0.8 mL) was added at the bottom of the reactor. For all of the tested experimental conditions, the added amount of silane was in a large excess (ca. 20-fold) with respect to the theoretical amount needed for the formation of a close-packed monolayer entirely covering the accessible nanoparticle surface. The theoretical calculation was performed using a value of 20 Å2 for the crosssectional area of the silane alkyl chain grafted on the surface and a value of 54 m2 g-1 as the BET (Brunauer-EmmettTeller) surface area measured for bare TiO2.15 Compressed CO2 was then added, and the vessel was heated at the chosen temperature (T). The pressure was increased to the chosen value (P) using a syringe pump. The system was stirred at 300 rpm throughout the running time (t) of the experiment. At the end of each experiment, the stirring was stopped, and samples were washed with a continuous flow of scCO2 for 30 min in order to remove the possible excess of deposited organosilane. The system was depressurized by means of valve V4 at a constant CO2 mass flow rate of 1.2 g min-1 and a constant temperature of either 318 or 348 K. 2.3. Characterization. The quantification of the amount of deposited silane was performed using thermogravimetric analysis (TGA, Perkin-Elmer 7). Measurements were carried out under Ar atmosphere from room temperature to 850 K at a heating rate of 5 K min-1. To confirm the presence of the coupling agent in the treated samples, attenuated total reflectance (ATR) IR spectra were recorded using a Perkin-Elmer Spectrum One spectrometer equipped with a Universal ATR sampling accessory. Micrographs of the samples were recorded in both a scanning electron microscope (SEM, JEOL JSM 6300) and a

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transmission electron microscope (TEM, JEOL JEM-1210). The SEM instrument was equipped with a LINK-ISIS-200 energy dispersive spectrometer (EDS, Oxford Instruments). Textural characteristics of raw and silanized samples were studied by low-temperature N2 adsorption-desorption analysis (ASAP 2000 Micromeritics). Prior to measurements, samples were dried under reduced pressure (50-nm) volume. The rise in macropore volume was related to the foaming of the coated nanostructure and the disaggregation effect promoted by CO2 expansion during system depressurization. To elucidate the validity of the supposition that the variations in mesopore volume arose mainly from silane deposition onto pore walls, whereas the modifications of macropore volume originated principally during the expansion step, a sample of bare TiO2 was processed under experimental conditions similar to those used for sample TiC8-215, but without addition of silane to the reaction medium (sample TiO2-215). For this sample, the measured volume of mesopores was similar to that of bare TiO2,

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Figure 7. Particle size distribution on a volume basis of raw and silanized nanoparticles after dispersion in petroleum: (a) coated TiO2 (15 min), (b) coated TiO2 (50 min), and (c) coated γ-Fe2O3. Symbols in a and b: (, bare TiO2; *, TiC8-1t; ∆, TiC8-2t; O, TiC8-3t; and 0, TiC8-4t.

Figure 6. System depressurization pathways at 348 and 318 K: (a) CO2 density evolution as a function of pressure and (b) rate of pressure drop with time at the constant mass flow rate of 1.2 g min-1.

whereas the volume of macropores increased significantly (Figure 5c). The macropore volume was also largely influenced by the rate of CO2 expansion occurring during depressurization.49 This behavior was tested by performing an extra experiment in which a sample was expanded from the initial conditions of 318 K and 22.5 MPa at 2.8 g min-1 [sample TiC8215(fast) in Figure 5c], and the values of pore volume were compared with those of a similar system expanded at 1.2 g min-1 (sample TiC8-215 in Figure 5b). Upon increasing the depressurization rate from 1.2 to 2.8 g min-1, the macropore volume increased, whereas the mesopore volume did not change significantly. On the other hand, the formation of macropores due to CO2 expansion and particle disaggregation was less noticeable in samples prepared at 348 K than in samples prepared at 318 K (Figure 5a and b, respectively). To further assess the influence of temperature in macropore opening during system depressurization, the CO2 density drop pathways followed as a function of pressure reduction36 (Figure 6a) and the linked depressurization rates (Figure 6b) were calculated for each working temperature. For experiments performed at 348 K and either 22.5 or 10 MPa, both the variation of density with pressure and the depressurization rate were approximately linear. In contrast, three different expansion regions could be distinguished for a system with initial experimental conditions of 318 K and 22.5 MPa. In region I (22.5-12 MPa), the decrease of density with pressure generated a plateaulike section, and the depressurization rate was very high. Region II (12-7 MPa) was characterized by a significant drop of density with pressure, which led to a low depressurization rate. Finally, in region III (7 MPa to atmospheric), variations in density with pressure were again gradual, with an accelerated depressurization rate. For samples prepared at 10 MPa and 318 K, the depressurization pathway included only regions II and III. The quick and sudden alteration of the depressurization rate between regions I-II and II-III for experiments performed at 22.5 MPa and regions II-III at 10 MPa (Figure 6a,b) is thought to be responsible for the larger rise in macropore volume observed for samples prepared at 318 K (Figure 5b) compared to those obtained at 348 K (Figure 5a). An additional effect that should be taken into account for experiments conducted at 318 K is the possible formation of a liquid phase during depressurization. It is well-known that entrainers and other impurities can significantly alter the critical

point of a supercritical fluid, generally by increasing the critical pressure and/or temperature.50 In fact, during the depressurization of the silane-CO2 mixture at 318 K, the phenomenon of critical opalescence was observed through the sapphire windows of the reaction vessel in the pressure interval of 11-9 MPa. Therefore, the CO2 fluid phase likely crossed the liquid region during the expansion, which led to pore architecture modification. To provide evidence for this assumption, a sample of bare TiO2 was processed under conditions similar to those applied to sample TiO2-215 (originally heated at 318 K), but with the sample expanded at a constant temperature of 300 K [sample TiO2-215(liq)]. As a consequence, the formation of a CO2 liquid phase at 300 K had a significant increase in macropore volume (Figure 5c). At 348 K, scCO2 was eliminated from the system without the formation of a liquid-vapor interphase, avoiding the modification of the macropore structure due to capillary forces. 3.4. Hydrophobic Functionalization. The surface treatment of the nanometric powder with the chosen silane was expected to confer water repellency to the samples. The hydrophobic character was first assessed from the intensity of the N2 adsorption on the raw and treated powders by comparing the calculated values of the C constant in the BET equation.20 The C constant is related to the adsorbate/adsorbent interaction energy. Bare TiO2 has a relatively high value of the C constant (∼50), which is consistent with high-energy interactions between N2 and the OH groups on the surface of a hydrated metal oxide.48,51 Silanization decreased the values of the C constant by a factor of ∼4 or 5. This can be explained as weakening of the N2 interactions with the hydrophobic silanized surfaces.20,48 The dispersive behavior of bare and coated nanoparticles in hydrophobic petroleum was studied by measuring the particle size distribution. Treated TiO2/C8 samples exhibited a unimodal distribution with the maximum centered at around 1.5-2.5 µm, whereas bare TiO2 had a maximum centered at around 4 µm (Figure 7a,b). However, some of the samples obtained at 50min reaction time had a shoulder displaced to the region of high particle size. As a result, the values for the 95th percentile [dp(0.95)], the particle diameter at which 95% of the particles are smaller than or equal to this value, were similar to that of bare TiO2 (Table 1). Dispersibility of silanized γ-Fe2O3 particles had the maximum centered at around 2.0 µm (Figure 8c), whereas bare γ-Fe2O3 could not be dispersed at all in petroleum. 3.5. Selection of Operating Conditions. Once the effectiveness of the supercritical silanization process was established for the TiO2-C8 system, the next step consisted of the evaluation of the operating conditions that led to optimal material performance as a function of the intended application. Charac-

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C(∆m) )

∆m(TiC8-350) ∆m

(6)

Finally, coating stability was related with the degree of crosslinking of the silane layer, estimated by the ratio ∆m2/∆m1. Using this approach, the maximum value of cross-linking was detected for sample TiC8-450 (∆m2/∆m1 ) 1.28), which was taken as the reference material (eq 7). D(∆m2 /∆m1) ) Figure 8. Examples of radial graphics that compare pairs of silanized TiO2 samples in terms of parameters A(as), B[dp(0.95)], C(∆m), and D(∆m2/∆m1): (a) TiC8-215 and TiC8-450 and (b) TiC8-350 and commercial TiC8-D.

teristics of the silanized TiO2 nanoparticles were assessed for their use in two potential manufacturing industries: the cosmetics industry (CI) as a UV filter and the plastics industry (PI) as an inorganic filler in hybrid composites. Samples used for optimization were those of Table 1. Four key characteristics were evaluated: as, dp(0.95), ∆m, and ∆m2/∆m1. To develop the objective function, a parameter was defined for each key characteristic: A(as), B[dp(0.95)], C(∆m), and D(∆m2/∆m1). A limit or target value from a selected reference material was assigned to each of these key characteristics. For characteristics identified principally with the material, such as and dp(0.95), commercial products were used as references. On the other hand, for characteristics defined by the process, such as ∆m and the ratio ∆m2/∆m1, samples obtained under supercritical conditions were used as reference materials. Regarding the specific surface area, the target was to prepare materials with maximum surface area (eq 4). In general, the deposition of silane molecules on the particle surface is considered to imply a decrease in surface area and open pore volume, attributed to both enlarged particle diameters after coating and potential pore blocking. Hence, the reference material for the BET surface area was the bare TiO2, with a specific surface area of as(TiO2) ) 54 m2 g-1. This value was obtained by the BET equipment software, which, by default, used a N2 cross-sectional area of 16.2 Å2. Moreover, the value of 54 m2 g-1 also match the value of surface area giving by the supplier.52 A(as) )

as as(TiO2)

(4)

The parameter dp(0.95) quantifies the dispersibility of the particles in an organophilic medium. High values of dp(0.95) indicate low dispersibility. Therefore, materials with minimum values of dp(0.95) were the target. In this case, the commercially silanized TiO2-D sample was taken as the reference material [dp(0.95) ) 8.1 µm], because this product is marketed as having an excellent dispersibility in organic matrixes (eq 5).52-54 B[dp(0.95)] )

dp(0.95)(TiC8-D) dp(0.95)

(5)

The grafting density calculated through ∆m in the temperature interval of 525-850 K was related with the total silane consumption during processing. Sample TiC8-350 with the minimum amount of silane consumption [∆m(TiC8-350) ) 3.4 wt %] was chosen as the reference material (eq 6).

∆m2 /∆m1 ∆m2 /∆m1(TiC8-450)

(7)

Different samples had different values of the parameters A-D (Table 2). Examples of radial graphics that compare these four parameters simultaneously for pairs of silanized TiO2 samples are shown in Figure 8. Each trapezoid represents a distinct sample and the vertices of the trapezoid correspond to the parameters A, B, C, and D. Because the four parameters were maximized for optimal value, the target value for each characteristic was expected to be as far as possible from the center of the radial graphic. For instance, Figure 8a indicates that sample TiC8-215 had better characteristics with regard to surface area (A), dispersibility (B), and silane consumption (C) than sample TiC8-450, although the thermal stability (D) of the coating was lower in the former. Moreover, sample TiC8-350 in Figure 8b shows better properties than does the commercial TiC8-D sample. The optimal combination of the contributions of the four different parameters was expected to lead to the desired material. However, it should be taken into account that the importance of these key characteristics depends on each specific application. Hence, to develop the objective function (f), a weight factor (wi) was assigned to each of the four defined parameters (i ) 1-4) in order to rank their importance for each intended industrial application. wi had assigned values between 1 and 5. Equation 8 shows the resulting objective function to be maximized for process optimization f ) w1A(as) + w2B[dp(0.95)] + w3C(∆m) + w4D(∆m2 /∆m1) (8) For the cosmetics industry, values of wi(CI) ) 5, 5, 1, and 2 were assigned for the parameters A(as), B[dp(0.95)], C(∆m), and D(∆m2/∆m1), respectively. In cosmetics, the effectiveness of micrometerized TiO2 as a UV filter mainly depends on having a high surface area [w1(CI) ) 5].52-54 Moreover, consumer acceptance of a sun cream is increased if its whitening effect on the skin caused by TiO2 agglomerates is minimized [w2(CI) ) 5].55 On the other hand, for this application, minimizing silane consumption is not crucial because of the high-added value of the resulting product [w3(CI) ) 1]. Finally, cosmetic products are intended for use in humans. Therefore, they are not frequently exposed to high temperature or to aggressive chemical conditions, so the stability of the coating is not a critical parameter [w4(CI) ) 2]. Results from the application of the objective function with the selected weighting factors for cosmetics [f(CI)] are given in Table 2. The best results were achieved with three of the nine studied options (samples TiC8215, TiC8-350, and TiC8-315). Among these three possibilities, minimum process operating cost could be an additional criterion for the final choice. For the plastics industry (PI), values of wi(PI) ) 3, 5, 4, and 4 were assigned for the parameters A(as), B[dp(0.95)], C(∆m), and D(∆m2/∆m1), respectively. For plastics, homogeneity of the composite would be favored by relatively good physical

Ind. Eng. Chem. Res., Vol. 48, No. 22, 2009 Table 2. Parameters Used for Maximizing the Objective Function and Values Obtained for the Function When Optimized for Either the Cosmetics [f(CI)] or the Plastics [f(PI)] Industry sample

A(as)

B[dp(0.95)]

C(∆m)

D(∆m2/∆m1)

f(CI)

f(PI)

TiC8-D TiC8-115 TiC8-150 TiC8-215 TiC8-250 TiC8-315 TiC8-350 TiC8-415 TiC8-450

0.91 0.91 0.85 1.05 1.04 0.91 1.02 0.80 0.78

1.00 1.02 0.74 1.21 0.85 1.27 1.31 1.04 1.02

0.94 0.89 0.89 0.89 0.85 0.82 1.00 0.94 0.60

0.95 0.97 0.84 0.87 0.86 0.86 0.94 0.87 1.00

12.4 12.5 10.4 14.0 12.0 13.4 14.5 11.9 11.6

15.4 15.3 13.0 16.3 14.2 15.6 17.3 14.9 13.8

blending at the interface of the polymer and the nanofiller [w1(PI) ) 3]. However, a uniform distribution of the filler in the polymer matrix is extremely important in order to obtain nearisotropic properties in the composite material.53,56 Thus, a great extent of dispersibility of the nanometric particles in the organic medium should be attained [w2(PI) ) 5]. Consumption of silane in the process is considered important for the plastics industry, because raw materials costs along with large production scales make necessary the minimization of this parameter [w3(PI) ) 4]. Thermal and chemical stability of the silane coating are also vital, because multiple potential uses could be intended for the resulting composite [w4(PI) ) 4].17 Values obtained for the objective function for the assigned set of weight parameters for plastic composites [f(PI)] are given in Table 2. According to the requirements of the plastics industry, the optimal operating conditions were those used for sample TiC8-350. 4. Conclusions Surface silanization of TiO2 and γ-Fe2O3 nanoparticles was addressed in this work through silanization aided by scCO2. For the design of the optimized process, it was first necessary to precisely determine the octyltriethoxysilane solubility in scCO2 under working pressures and temperatures (pressure range of 8.0-18 MPa at 318 and 348 K). Experimental data were correlated through the Chrastil equation (AARD values of ∼8%). After supercritical silanization of the nanopowder, the mesoporous character of the agglomerates was maintained, as confirmed by N2 adsorption-desorption analysis and TEM characterization. The deposition of silane molecules on the particle surface resulted in a decrease in mesopore volume, whereas the effect of CO2 volume expansion occurring during system depressurization was to increase the macropore volume. Enhanced dispersibility in oleophilic phases was observed for coated nanoparticles of both TiO2 and γ-Fe2O3 with respect to the bare compounds. Compilation of characterization data allowed for the definition of an objective function to choose the operating conditions that lead to optimal material performance as a function of intended application. Acknowledgment Financial support from the Spanish Projects MAT-200763355 and CTQ2008-05370/PPQ is greatly appreciated. The authors thank Dr. S. Veintemillas-Verdaguer for providing the nanometric maghemite particles. MATGAS 2000 AIE is acknowledged for the provision of its facilities in the supercritical silanization experiments. C.A.G.-G. acknowledges CSIC for its funding support through the I3P program. Supporting Information Available: ATR-IR characterization of silanized inorganic particles. This information is available free of charge via the Internet at http://pubs.acs.org.

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ReceiVed for reView May 13, 2009 ReVised manuscript receiVed September 4, 2009 Accepted September 4, 2009 IE900775Z