Hydrophobization of Silica Aerogels: Insights from Quantitative Solid

Article pubs.acs.org/JPCC

Hydrophobization of Silica Aerogels: Insights from Quantitative Solid-State NMR Spectroscopy Wim J. Malfait,*,† Rene Verel,‡ and Matthias M. Koebel*,† †

Laboratory for Building Science and Technology, Swiss Federal Laboratories for Materials Science and Technology, Empa, Ü berlandstrasse 129, 8600 Dübendorf, Switzerland ‡ Laboratory for Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 1-5/10, 8093 Zurich, Switzerland ABSTRACT: Silica aerogels have exceptional physical and chemical properties related to their nanoporous structure and high specific surface area. The hydrophobization of the silica surfaces, for example by modification with trimethylsilyl groups (TMS), is of central importance for silica aerogel production by ambient drying methods, particularly on an industrial scale. This study monitored the chemical modification of silica aerogels by quantitative and two-dimensional solid-state NMR spectroscopy. A series of two-step, acid/basecatalyzed silica alcogels were hydrophobized for different times in hexamethyldisiloxane (HMDSO) and subsequently dried at ambient pressure. Two-dimensional 1H−29Si heteronuclear correlation NMR spectroscopy confirms that both ethoxy and TMS groups are chemically bonded to the silica surfaces. For the single-pulse spectra, a procedure to calibrate the absolute 1H, 13 C, and 29Si NMR signal intensities with external references was developed. The quantification procedure is validated by the internal consistency between the 1H, 13C, and 29Si results and the agreement between the measured sample mass and that predicted from the NMR data. The quantitative speciation data on the aerogel samples show that the silica surface is covered by a monolayer of ethoxy, TMS, and silanol groups. Silanol groups are progressively replaced by TMS groups with increasing modification time, and the TMS content has a strong effect on the density of the final aerogel. present and catalytic amounts of an acid catalyst.8 Gelation is then initiated by the addition of a Brønsted or Lewis base such as ammonia,9 ammonium fluoride or a mixture of the two.10 Freshly formed gels are typically aged for several hours, resulting in buildup of additional silicate at the interparticle necks, thus strengthening the particle network structure.11 Unless specific measures are taken, strong capillary forces occur during ambient pressure drying, which, when coupled with condensation reactions of the silanol terminated gel network (2 SiOH ↔ SiOSi + H2O), lead to a progressive collapse and densification of the gel structure. In classical supercritical drying of gels,12 this collapse is strongly suppressed, allowing the direct preparation of nonfunctionalized silica aerogels. For safety reasons at both laboratory and industrial scales, supercritical drying techniques often use CO2 as a solvent after an exchange from alcohol to either liquid13 or supercritical14 CO2 prior to drying. The main challenge in ambient drying of silica aerogel is to prevent irreversible condensation and to maintain the porosity during solvent evaporation.15−18 This is achieved by a hydrophobization

1. INTRODUCTION Silica aerogels are semitransparent, low-density solids composed of a nanoporous network of silica particles. Their openporous nature and large specific surface area (typically between 250 and 800 m2/g) make them ideally suited for a wide range of applications such as thermal insulation,1,2 oil-spill cleanup,3 or support for catalysis applications.4 They are also the best thermal insulators at ambient conditions known to man, with thermal conductivities as low as 12 mW/(m·K). Their exceptional thermal and acoustic insulation properties combined with their excellent fire resistance place silica aerogels among the most promising, high-performance thermal insulation materials for large-volume applications in the building and construction sector. Silica aerogels are commonly prepared by gelation and aging of a colloidal silica sol, optional modification of the resulting aged gel, and subsequent removal of the solvent. Colloid formation can occur either prior to gelation (e.g., the two-step acid− base-catalyzed process5) or during gelation (e.g., the single-step base-catalyzed process6). The single-step acid-catalyzed route7 is far less common nowadays. Two-step gels are prepared from sols obtained through hydrolysis of alkoxide precursors such as tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS) solutions in their parent alcohols with small amounts of water © 2014 American Chemical Society

Received: August 15, 2014 Revised: October 13, 2014 Published: October 13, 2014 25545

dx.doi.org/10.1021/jp5082643 | J. Phys. Chem. C 2014, 118, 25545−25554

The Journal of Physical Chemistry C

Article

Figure 1. Silica xerogel and aerogel granulate. The same mass of sample (0.24 g) is shown for each modification time.

2% of methyl ethyl ketone). Gelation was induced by the addition of 1 mL of NH4OH (5.5 M in water) and occurred within 7−10 min after being placed in an oven at 55 °C. The alcogels were then covered with ethanol and aged for 24 h at 55 °C. One of the alcogels was dried without modification at ambient pressure for 2 h at 150 °C, resulting in a dense, glassy xerogel. The other alcogels were hydrophobized at 65 °C in a modification solution containing 150 mL of hexamethyldisiloxane (HMDSO), 5.6 mL of ethanol, and 0.66 mL of concentrated HCl (37 wt % in water). After variable modification times, the gels were dried at ambient pressure for 2 h at 150 °C. The sample prepared without modification (Xero 11) collapsed to a silicate glass (Figure 1); the samples dried after surface modification (Aero A, B, and C) retained a significant fraction of their porosity and displayed the typical blue hue associated with the strong Rayleigh scattering of silica aerogels (Figure 1). It should be noted that the unmodified xerogel (Xero 11) was not stable over time (water adsorption, release of ethanol similar to the behavior observed in supercritical alcohol dried aerogels). Because 57 days elapsed between the NMR measurements with 7 mm rotors and those with 4 and 2.5 mm rotors, Xero 11 was dried for a second time prior to the second set of NMR analysis, resulting in the observed discrepancies between the two data sets (see below). The surface-modified aerogels (Aero A, B, and C) were stable over time. 2.2. Envelope Density and Specific Surface Area. The envelope density was derived from the envelope volume, which includes both particles and open and closed pores. The envelope volume was determined by the powder displacement method (GeoPyc 1360, Micromeritics) (Table 1). The specific surface area and pore size distributions were determined by nitrogen sorption (ChemiSorb ASAP2020C, Micromeritics). The specific surface area was derived from the sorption curves by Brunauer−Emmet−Teller (BET) analysis.53 The total pore volume has been derived in three ways: (i) from the envelope and skeleton density, (ii) as a single-point adsorption volume, and (iii) by Barrett−Joyner−Halenda (BJH) analysis.54 The gas sorption results ii and iii systematically underestimate the pore volume (by up to 30%), indicative for the presence of macropores. The average pore size distribution can be determined either from the pore volume and surface area or from the BJH analysis. The former method assumes spherical pore geometries; the latter may suffer from sample deformation during the sorption experiments due to capillary forces.55

treatment of the inner surfaces prior to drying, that is, by replacing polar silanol groups SiOH, with nonpolar hydrophobic groups such as SiR, with RCH 3 , CH2CH3, or OSi(CH3)3. With the exception of structurally engineered gels,19−21 surface modification is the only way to obtain silica aerogel materials by ambient drying methods. In silica gel chemistry, solution NMR has been used extensively to study the hydrolysis and condensation reactions during sol formation.22−25 In addition, solid-state NMR has been applied to dry silica gels,26−39 pyrogenic silica,40−43 silica nanoparticles,44 and, to a much lesser extent, silica aerogels.45−51 Recently, dynamic nuclear polarization has also been applied to silica materials to increase the sensitivity.52 These NMR studies demonstrated that the surface modification, including those on silica aerogels, can be successfully monitored by solid-state NMR, but to the best or our knowledge, fully quantitative solid-state NMR data have not been collected for silica aerogel materials prepared by ambient drying. In this study, we use multinuclear, multidimensional, 1H−29Si solid-state NMR to provide direct evidence for the existence of chemical bonds between the siloxane of the silica particles and ethoxy and trimethylsilyl (TMS) groups in HMDSO modified silica aerogels. In addition, we use 1H, 13C, 29Si solid-state NMR to quantitatively determine the efficiency of the hydrophobization process for silica aerogels prepared by ambient drying. Our procedure relies on comprehensive measurements of the spin−lattice relaxation times (T1), acquiring NMR spectra after full relaxation of the magnetization (pulse delay >5T1), and calibration of the absolute signal intensities using external standards. The quantitative NMR data show that the amount of residual silanol decreases with increasing modification time. We also observe that the envelope density of the ambient dried aerogels/xerogels correlates with the amount of residual silanol groups.

2. EXPERIMENTAL SECTION 2.1. Aerogel Preparation. A series of silica aerogels/ xerogels was prepared with different degrees of hydrophobization (Figure 1). All parameters were kept constant, apart from the modification time, which was varied between 0 (no modification) and 26.2 h. Sols were prepared by diluting 24 mL of ethanolic silica sol concentrate, made by acidcatalyzed hydrolysis of TEOS in a water/alcohol mixture at room temperature with a molar TEOS/water/HCl ratio of 1:1.6:0.003, with 56 mL of absolute ethanol (denatured with 25546



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Table 1. Physical Properties of Aerogel Samples sample modification time (h) density ρ (g/cm3) SBET (m2/h) pore volume (cm3/g) from density (V = 1/ρ − 1/ρ0)a single point (at P/P0 = 0.97) BJH adsorption average pore diameter (nm) from 4V/SBET from BJH adsorption a

Xero 11 0 0.869 ± 0.013 741

Aero A 2 0.399 ± 0.007 916

Aero B 6.4 0.306 ± 0.004 908

Aero C 26.2 0.275 ± 0.005 890

0.7 0.5 0.7

2.1 1.6 1.7

2.8 2.0 2.3

3.2 2.2 2.5

3.8 5.5

9.0 6.1

12.4 8.1

14.3 8.9

ρ0 = skeleton density, approximated by the density SiO2 glass (2.20 g/cm3).

Figure 2. Quantitative solid-state MAS NMR spectra for silica aerogels prepared with different modification times (indicated at right). All spectra are normalized to the same weight and number of scans: (a) 1H spectra collected with 2.5 mm rotors and a MAS rate of 24 kHz. (b) 13C NMR spectra; (c) 29Si NMR spectra. The peaks related to trimethylsilyl groups (TMS), methylene (−CH2−CH3), and methyl (−CH2−CH3) units originating from ethoxy groups and siloxane (Q3 and Q4) are labeled in the spectra. With increasing modification time, the amount of TMS groups increases and the amount of ethoxy decreases. For the xerogel prepared without modification (Xero11), a set of narrow lines, corresponding to liquid ethanol, is also present (Figure 1b).

line-broadening, and additional 1H NMR spectra were collected in 4 and 2.5 mm zirconia rotors to improve spectral resolution by spinning at 8 and 12 kHz ± 2 Hz as well as at 20 and 24 kHz ± 2 Hz, respectively. For each sample, nucleus, and spinning rate, the T1 relaxation time was determined by saturation recovery and single-pulse spectra subsequently collected with a pulse delay of ≥5T1. Fully relaxed spectra were also collected for adamantane (1H and 13C), octakis(trimethylsiloxy)silsesquioxane (Q8M8) (1H, 13C, 29Si), and/or glycine (1H) to calibrate signal intensities. All samples and standards were measured on completely filled rotors to ensure that potential inhomogeneities in sensitivity due to gradients in the radio frequency (RF) field strength cancel out during the

2.3. NMR Spectroscopy. All NMR spectra were collected with magic angle spinning (MAS) on a Bruker spectrometer equipped with a wide-bore 9.4 T magnet, corresponding to Larmor frequencies of 400.2 MHz for 1H, 100.6 MHz for 13C, and 79.5 MHz for 29Si. All spectra were processed with matNMR.56 Quantitative 1H, 13C, and 29Si MAS NMR spectra were collected with respective spectral widths and acquisition times of 200 kHz and 10 ms, 22 kHz and 93 ms, and 24 kHz and 43 ms. 1H, 13C, and 29Si single-pulse NMR spectra were collected in 7 mm zirconia rotors to maximize signal-to-noise ratio, but limiting the MAS frequency to 4 kHz ± 2 Hz. The 1H NMR spectra collected at this MAS rate suffered from extensive 25547



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Figure 4. 1H−29Si cross-polarization NMR spectra (thick lines) of silica aerogels prepared with different modification times. Compared to the quantitative 29Si NMR spectra (thin lines), the intensity of the TMS, Q3, and Q2 bands is enhanced relative to the intensity of the Q4 band, indicating that the Si in TMS, Q3, and Q2 is closer to hydrogen than the Si in Q4.

a uniform excitation over our spectral ranges: to within 0.03, 0.04, and 0.05% for 1H in 7, 4, and 2.5 mm rotors, respectively; to within 0.5% for 13C; and to within 2% for 29Si, well within the accuracy of our quantification procedure. This prediction was confirmed experimentally for 29Si, which has the softest pulse, by collecting spectra acquired with different transmitter frequencies. For 1H, the signal acquired from an empty rotor was subtracted to remove background contributions from the rotor, caps, and probe. In addition to the quantitative NMR spectra, one-dimensional 1H−13C and 1H−29Si cross-polarization spectra and twodimensional 1H−29Si heteronuclear correlation (HETCOR) spectra have been collected to assist in the peak assignment. Both types of spectra rely on the through-space dipolar coupling and give information about the spatial proximity between the investigated nuclei. The 1H−13C and 1H−29Si crosspolarization spectra were collected with 7 mm rotors, a spinning rate of 4 kHz ± 2 Hz, a contact time of 3000 and 2000 μs, respectively, and a pulse delay of 1.25T1 of 1H.

Figure 3. Qn speciation of silica aerogels: quantitative 29Si NMR spectra and the corresponding fits with Gaussian bands. Dots denote the experimental data; lines denote the fit envelope, fitted components, and fit residual. All spectra are normalized to the same maximum intensity. The modification times are indicated at right. The fraction of Q4 increases with increasing modification time.

intensity calibration. For the experiments with the 7 mm rotors, the π/2 pulse lengths for 1H, 13C, and 29Si were 3.4, 5.5, and 11 μs, respectively, corresponding to respective RF field strengths of 74, 45, and 23 kHz. For the 4 and 2.5 mm rotors, the 1H π/2 pulse lengths were 3.9 and 4.9 μs, respectively, corresponding to RF field strengths of 64 and 51 kHz, respectively. These RF field strengths are predicted to produce Table 2. Qn Species Abundance and NMR Parameters sample:

Xero 11

Aero A

Aero B

modification time (h):

0

2

6.4

26.2

Q4

fraction (%) position (ppm) fwhma (ppm)

46.1 ± 0.6 −109.5 ± 0.0 5.8 ± 0.1

59.9 ± 1.2 −109.7 ± 0.1 5.5 ± 0.1

66.1 ± 1.2 −109.5 ± 0.1 5.7 ± 0.1

70.3 ± 1.1 −109.3 ± 0.1 6.2 ± 0.1

Q3

fraction (%) position (ppm) fwhm (ppm)

44.4 ± 0.7 −102.5 ± 0.0 5.1 ± 0.1

40.1 ± 1.1 −103.1 ± 0.1 4.9 ± 0.1

33.9 ± 1.0 −102.9 ± 0.1 4.6 ± 0.1

29.7 ± 0.9 −102.9 ± 0.1 4.2 ± 0.1

Q2

fraction (%) position (ppm) fwhm (ppm)

9.5 ± 1.6 −95.7 ± 0.1 6.0b ± 1.0 0.40 ± 0.01

0.34 ± 0.01

0.30 ± 0.01

NBO/Tc

0.63 ± 0.03

Aero C

fwhm, full width at half-maximum. bValue fixed during the curve fitting procedure. cNBO/T, nonbridging oxygen atoms per tetrahedral cation, [Q3] + 2[Q2]/([Q4] + [Q3] + [Q2]).

a

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Figure 5. 1H−29Si HETCOR NMR spectrum of Aero A (e). The total projections (d, f) and partial projections for certain spectral ranges are also plotted (a−c, g, h). See section 3.1.2 for a detailed discussion of the figure.

HETCOR data (section 3.1.2). The relatively broad nature of the peaks is indicative of the limited mobility of these groups. The 29Si NMR spectra contain a band near 12 ppm associated with Si in the TMS groups and bands associated with siloxane groups: Q4 (−109 ppm), Q3 (−103 ppm), and a weak Q2 shoulder (−96 ppm), where Q4 is a silica tetrahedron coordinated by four bridging oxygen atoms (BOs) that link it to four other silica tetrahedra; Q3 is coordinated by three BOs and one nonbridging oxygen (NBO); and Q2 is coordinated by two BOs and two NBOs. The proportion of Q4, Q3, and Q2 was derived from a Gaussian fit to the NMR spectra (Figure 3; Table 2). Note that the spectra for the xerogel prepared without modification with HMDSO (Xero 11) nevertheless contains minor peaks associated with TMS. This TMS signature most likely originates from gas-phase deposition and incorporation of HMDSO inside the unmodified silica gel during drying, as this sample was dried in the same furnace together with all other, previously hydrophobized “Aero” samples, which were exchanged in modification solution prior to drying. 3.1.2. 1H−29Si Multinuclear NMR. It is generally assumed that the surface groups (ethoxy groups, trimethylsilyl groups)

The HETCOR spectra were collected with 4 mm rotors with a spinning rate of 10 kHz ± 2 Hz: the rotor-size and spinning rate are a compromise between sensitivity (larger samples are better) and spectral resolution in the 1H dimension (faster spinning is better). Homonuclear dipolar decoupling using continuous phase modulation (DUMBO) was applied to improve the spectral resolution in the 1H dimension.57

3. RESULTS AND DISCUSSION 3.1. NMR Spectroscopy of Silica Aerogels. 3.1.1. SinglePulse Spectra. The quantitative, single-pulse NMR spectra are plotted in Figure 2. 1H and 13C spectra indicate the presence of TMS groups with a 1H peak at 0.3 ppm and a 13C peak at 0.7 ppm and ethoxy groups with 1H peaks at 1.4 and 4.1 ppm and 13C peaks at 17 and 59 ppm for CH3 and CH2 units, respectively. The peak assignments are based on the similarity in peak positions when compared to liquid ethanol and HMDSO58 and consistent with previous assignments for modified silica.34,45 The peak assignment is confirmed by the consistency of the quantification of the 1H, 13C, and 29Si data (see below) and by the 1H−29Si cross-polarization and 25549



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are covalently bonded to the silica surface. The presence of TMS groups in HMDSO-modified aerogels is supported by their hydrophobic nature and by data on hexamethyldisalazane (HMDSZ)-modified silica gels, where a distinct peak for TMS groups was identified in the solid-state NMR spectra.31 However, there is little direct evidence for a chemical bond between the TMS groups and the silica surface. To address this point, we have collected 1H−29Si cross-polarization (Figure 4) and two-dimensional 1H−29Si HETCOR data (Figure 5). Let us be reminded that both the cross-polarization and HETCOR spectra are mediated by the dipolar coupling. As a result, a cross peak in the HETCOR spectrum is direct evidence for spatial proximity but does not necessarily require a chemical bond. In the cross-polarization spectra (Figure 4), the relative intensities of the Q2 and Q3 peaks, compared to that of Q4, are higher after cross-polarization from 1H, indicating that these sites are in close spatial proximity to protons. Furthermore, the band associated with Q2 is resolved as a clear shoulder in the cross-polarization spectrum of Xero 11, but is virtually unresolved in the quantitative single-pulse spectrum. The two-dimensional HETCOR spectrum (Figure 5) allows us to identify which hydrogen atoms are in close proximity to which Si sites. The cross peaks at (−103.1 and 2.7) and (−103.1 and 0.9) ppm correspond to the correlation between Q3 and the hydrogen in the CH2 and CH3 units of the ethoxy groups (Figure 5e), respectively. The correlation between CH2 and Q3 is stronger compared to the correlation for CH3 and Q3, despite the smaller number of protons in the methylene units (Figure 5h), suggesting closer proximity to the silicon atoms. In addition, the hydrogen in CH2 groups correlates more selectively to Q3 (Figure 5a), compared to the hydrogen in CH3, which displays a significant correlation to both Q3 and Q4 (Figure 5b). The stronger, more selective cross peak between Q3 and CH2, compared to Q3 and CH3, provides strong evidence for a chemical bond between Q3 and the CH2 of the ethoxy. The existence of a SiOCH2CH3 bond is consistent with the peak positions of Q3 and Q2, which are shifted less far from the Q4 position (shifted by ca. 7 and 14 ppm, respectively; Table 2) than for Q3 and Q2 attached to silanol groups (typically shifted by ca. 10 and 20 ppm, respectively29,59). A smaller peak shift for Q3/Q2 with SiOC bonds is

Figure 6. Calibration curves for quantitative solid-state MAS NMR: 1 H (large blue symbols, 7 mm rotor; small black symbols, 2.5 mm rotor) (a), 13C (b), and 29Si (c). The 29Si NMR signal for the siloxane groups in Q8M8 was not fully relaxed; therefore, only the signal from the trimethylsilyl groups (TMS) has been considered. All signal intensities are normalized to the same number of scans (one).

Table 3. Quantitative NMR Resultsa: Mass of Element in Rotor (Milligrams)

1

sample:

Xero 11

Aero A

Aero B

Aero C


0

2

6.4

26.2

H C 13 C 13 C 13 C 29 Si 29 Si 29 Si 29 Si 29 Si Ob total (NMR)c total (measured) 13

total −CH2−CH3 (Et) −CH2−CH3 (Et) TMS total TMS Q2 Q3 Q4 total total

6.3 12.5 14.0 1.7 28.3 1.0 7.1 33.5 33.5 75.2 98.1 207.9 210.6

± ± ± ± ± ± ± ± ± ± ± ± ±

0.7 1.3 1.4 0.2 1.9 0.1 0.5 2.5 2.5 3.6 7.3 8.4 0.1

4.2 4.4 4.9 8.2 17.5 5.6

± ± ± ± ± ±

0.5 0.4 0.5 0.8 1.0 0.4

3.2 3.0 3.6 6.6 13.2 4.9

± ± ± ± ± ±

0.3 0.3 0.4 0.7 0.8 0.4

3.4 3.2 3.7 7.6 14.5 5.3

± ± ± ± ± ±

0.4 0.3 0.4 0.8 0.9 0.4

14.6 21.6 41.9 45.5 109.1 110.2

± ± ± ± ± ±

1.1 1.6 2.0 3.4 4.1 0.1

9.5 18.3 32.7 34.4 83.5 81.0

± ± ± ± ± ±

0.7 1.4 1.6 2.6 3.1 0.1

7.6 17.9 30.8 31.2 79.8 77.5

± ± ± ± ± ±

0.6 1.3 1.5 2.3 2.9 0.1

a

Data from measurements with 7 mm rotor−4 kHz MAS rate. bCalculated from Qn species: [O] = 2[Q4] + 2.5[Q3] + 3[Q2] (in moles). cTotal calculated using the 4 kHz data for 1H. 25550



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Figure 7. Validation of the quantification procedure. (a) The mass of sample loaded in the rotor, derived from the NMR data, agrees with the measured mass to within 3%. (b) The 1H NMR data collected with 7 mm rotors (4 kHz MAS rate) are consistent with the data collected with the 2.5 mm rotors (24 kHz MAS). (c) The amount of TMS in the aerogel derived from the 1H, 13C, and 29Si data agrees within mutual uncertainties. (d) The amount of ethoxy groups derived from the 1H and 13C NMR data agrees within mutual uncertainties. The discrepancies for the Xero 11 data (open symbols, panels b and d) is due to the unstable nature of this sample (see section 2.1).

Table 4. Quantitative NMR Results: Fractions of Various Species (Weight Percent) sample:

Xero 11

Aero A

Aero B

Aero C


0.00

2.00

6.40

26.20

± 2.02 ± 1.50 ± 1.00

16.23 ± 2.34 16.64 ± 1.66 15.92 ± 1.19

18.33 ± 2.64 19.96 ± 2.00 17.69 ± 1.32

± 1.62 ± 1.08

10.44 ± 1.50 10.66 ± 1.07

10.24 ± 1.48 11.41 ± 1.14

± 3.58

76.77 ± 3.73

73.15 ± 3.58

± 3.72

103.35 ± 3.92

102.26 ± 3.82

TMS TMS TMS

1

H 13 C 29 Si

24 kHz 4 kHz 4 kHz

ethoxya ethoxya

1

24 kHz 4 kHz

siloxaneb

29

totalc

H C

13

Si

4 kHz

Trimethylsilyl Groups 2.18 ± 0.31 14.04 1.68 ± 0.17 15.04 1.26 ± 0.09 13.33 Ethoxy Groups 9.45d ± 1.36 11.21 16.10 ± 1.61 10.84 Siloxane (Si−O−Si) 81.77 ± 3.87 74.15 Total 99.14 ± 3.89 98.33

a

Weight percent of ethoxy groups (−CH2CH3 part only) derived from the 1H or 13C methyl peak intensity. bSi from 29Si NMR, O calculated from Qn species: [O] = 2[Q4] + 2.5[Q3] + 3[Q2] (in moles). cDerived from 29Si for siloxane and TMS, from 13C for ethoxy groups, and from 1H for silanol groups. d24 kHz 1H data collected after storage and renewed drying. Ethoxy/water content changed as a result.

the aerogel. Although it was suggested earlier45 that the replacement of silanol by TMS groups is the reaction that occurs during hydrophobization, the HETCOR data provides the most direct evidence so far to corroborate this mechanism. 3.2. Calibration of NMR Intensities. The integrated solidstate NMR intensity is a linear function of the number of spins and hence also of the mass of the element of interest in the rotor if a number of conditions are fulfilled, most notably the full relaxation of the magnetization, a constant filling factor of

expected on the basis of the correlation between peak shift and electronegativity of the cation associated with the NBO.60 The HETCOR cross peak at (12.5 and 0.3) ppm corresponds to the correlation between the Si and hydrogen atoms in TMS. In addition, a partially resolved correlation appears between the hydrogen in TMS and Q4 (Figure 5c,e,h), consistent with the presence of an interaction between TMS and Q4. In summary, the HETCOR data provide strong evidence that both ethoxy and TMS groups are chemically bonded to the silica surfaces in 25551



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the 1H and 13C data result in consistent estimates for the amount of ethoxy in the sample (Table 4; Figure 7d). These combined observations provide strong evidence for both accuracy and precision of quantitative solid-state NMR in these porous solids, opening up new possibilities for the systematic study of the modification and functionalization of the surfaces of silica aerogels. 3.3. Surface Modification of Silica Aerogels. Because hydrophobized wet gels are generally known to undergo changes during ambient drying (e.g., the loss of silanol groups due to condensation reactions, elimination of ethanol), the solid-state NMR data on the silica aerogels prepared with different modification times do not directly probe the kinetics of the modification/silylation reactions. However, the NMR data provide direct data on the surface modification of the final aerogel product, and these data are directly relevant for aerogel properties such as hydrophobicity, stability in high-humidity conditions, aging, density, or thermal conductivity. The coverage of the silica surface with ethoxy and TMS groups can be directly derived from the quantitative NMR data (Figure 8). For silanol groups, the 1H NMR signal is generally broad and, depending on the strength of the hydrogen bonding, overlaps with the 1H signal of the ethoxy.44 As a result, it is not possible to directly determine the silanol content from the 1H NMR data for our aerogels with high ethoxy contents and low silanol contents. Alternatively, we can assume that all of the NBOs are occurring either as ethoxy or silanol groups, which then allows us to approximate the amount of silanol from the difference between the amount of NBO and ethoxy in the sample (Figure 8). Note that this is an indirect way of detecting silanol groups, and the results are accompanied by a large uncertainty. With increasing modification time, the coverage with TMS groups increases at the cost of silanol groups (Figure 8b,c), consistent with the following reaction: 2≡SiOH + (CH3)3 SiOSi(CH3)3 HCl

⎯⎯⎯→ 2≡SiOSi(CH3)3 + H 2O Figure 8. Surface modification of silica aerogels as a function of modification time expressed as (a) wt %, (b) mol/kg, and (c) molecules/nm2. The amounts of ethoxy, TMS, and NBO are determined directly from the NMR data, and the amount of silanol (OH) is determined indirectly (see section 3.3). With increasing modification time, the amounts of ethoxy and silanol groups decrease and the amount of TMS groups increases.

the rotors, consistent tuning and matching of the probe, and constant amplifier and preamplifier settings. Under these conditions, quantitative data on the absolute amount of certain species and elements can be obtained by calibration with standards with known composition (Figure 6). The quantification procedure and intensity calibration is validated by several observations, namely, (i) the mass of the sample loaded in the rotor can be predicted to within 3% from the NMR data (Table 3; Figure 7a); (ii) the 1H data collected with 7 mm rotors are consistent with those collected with 2.5 mm rotors (Figure 7b); (iii) the 1H, 13C, and 29Si data result in consistent estimates for the TMS content of the samples (Table 4; Figure 7c); and (iv)

Figure 9. Silica aerogel density as a function of the surface coverage with TMS groups. 25552



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data, Santhosh Matam for his assistance with the BET analysis, and two anonymous reviewers for their comments.

The replacement of silanol groups by TMS groups has a dramatic effect on the shrinkage during drying and the final density of the aerogel (Figure 9). In total about 4−5 terminal groups (silanol + ethoxy + TMS) are present per nm2 for the silica aerogel samples (Figure 8c). Because of the high surface area of the silica aerogels (Table 1), this coverage of the silica surface with a single layer of mostly ethoxy and TMS corresponds to relatively large mass fractions in the final aerogel: up to 16 wt % ethoxy and 20 wt % TMS (Figure 8a).

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(1) Cuce, E.; Cuce, P. M.; Wood, C. J.; Riffat, S. B. Toward Aerogel Based Thermal Superinsulation in Buildings: a Comprehensive Review. Renewable Sustainable Energy Rev. 2014, 34, 273−299. (2) Koebel, M. M.; Rigacci, A.; Achard, P. Aerogel-Based Thermal Superinsulation: an Overview. J. Sol−Gel Sci. Technol. 2012, 63, 315− 339. (3) Reynolds, J. G.; Coronado, P. R.; Hrubesh, L. W. Hydrophobic Aerogels for Oil-Spill Cleanup − Synthesis and Characterization. J. Non−Cryst. Solids 2001, 292, 127−137. (4) Pajonk, G. M. Aerogel Catalysts. Appl. Catal. 1991, 72, 217−266. (5) Hæreid, S.; Nilsen, E.; Ranum, V.; Einarsrud, M.-A. Thermal and Temporal Aging of Two Step Acid-base Catalyzed Silica gels in Water/ Ethanol Solutions. J. Sol−Gel Sci. Technol. 1997, 8, 153−157. (6) Nicolaon, G. A.; Teichner, S. J. On a New Process of Preparation of Silica Xerogels and Aerogels and their Textural Properties. Bull. Soc. Chim. Fr. 1968, 4. (7) Russo, R. E.; Hunt, A. J. Comparison of Ethyl versus Methyl SolGels for Silica Aerogels using Polar Nephelometry. J. Non−Cryst. Solids 1986, 86, 219−230. (8) Tillotson, T. M.; Hrubesh, L. W. Transparent Ultralow-Density Silica Aerogels Prepared by a 2-step Sol-Gel Process. J. Non−Cryst. Solids 1992, 145, 44−50. (9) Rao, A. V.; Bhagat, S. D. Synthesis and Physical Properties of TEOS-based Silica Aerogels Prepared by Two Step (Acid-Base) SolGel Process. Solid State Sci. 2004, 6, 945−952. (10) Guray, J. L.; Nadargi, D. Y.; Rao, A. V. Effect of Mixed Catalysts System on TEOS-based Silica Aerogels Dried at Ambient Pressure. Appl. Surf. Sci. 2008, 255, 3019−3027. (11) Einarsrud, M.-A.; Nilsen, E.; Rigacci, A.; Pajonk, G. M.; Buathier, S.; Valette, D.; Durant, M.; Chevalier, P.; Nitz, P.; EhrburgerDolle, F. Strengthening of Silica Gels and Aerogels by Washing and Aging Processes. J. Non−Cryst. Solids 2001, 285, 1−7. (12) Kistler, S. S. Coherent Expanded Aerogels. J. Phys. Chem. 1932, 36, 52−64. (13) Tewari, P. H.; Hunt, A. J.; Lofftus, K. D. Ambient-Temperature Supercritical Drying of Transparent Silica Aerogels. Mater. Lett. 1985, 3, 363−367. (14) van Bommel, M. J.; de Haan, A. B. Drying of Silica Aergel with Supercritical Carbon Dioxide. J. Non-Cryst. Solids 1995, 186, 78−82. (15) Shi, F.; Wang, L.; Liu, J. Synthesis and Characterization of Silica Aerogels by a Novel Fast Ambient Pressure Drying Process. Mater. Lett. 2006, 60, 3718−3722. (16) Bhagat, S. D.; Oh, C.-S.; Kim, Y.-H.; Ahn, Y.-S.; Yeo, J.-G. Methyltrimethoxysilane Based Monolithic Silica Aerogels via Ambient Pressure Drying. Microporour Mesoporous Mater. 2007, 100, 350−355. (17) Prakash, S. S.; Brinker, C. J.; Hurd, A. J.; Raom, S. M. Silica Aerogel Fils Prepared at Ambient Pressure by Using Surface Derivatization to Induce Reversible Drying Shrinkage. Nature 1995, 374, 439−443. (18) Rao, A. V.; Nislen, E.; Einarsrud, M.-A. Effect of Precursors, Methylation Agents and Solvents on the Physicochemical Properties of Silica Aerogels Prepared by Atmospheric Drying Method. J. Non− Cryst. Solids 2001, 296, 165−171. (19) Noisser, T.; Reichenauer, G.; Hüsing, N. In Situ Modification of the Silica Backbone Leading to Highly Porous Monolithic Hybrid Organic-Inorganic Materials via Ambient Pressure Drying. ACS Appl. Mater. Interface 2014, 6, 1025−1029. (20) Leventis, N.; Palczer, A.; McCorkle, L.; Zhang, G.; SotiriouLeventis, C. Nanoengineered Silica-polymer Composite Aerogels with no Need for Supercritical Fluid Drying. J. Sol−Gel. Sci. Technol. 2005, 35, 99−105. (21) Sadekar, A. G.; Mahadik, S. S.; Bang, A. N.; Larimore, Z. J.; Wisner, C. A.; Bertino, M. F.; Kalkan, A. K.; Mang, J. T.; SotiriouLeventis, C.; Leventis, N. Green Aerogels and Porous Carbons by Emulsion Gelation of Acrylonitrile. Chem. Mater. 2012, 24, 26−47.

4. CONCLUSIONS We investigated the surface modification of silica aerogels with solid-state NMR spectroscopy. The two-dimensional 1H−29Si HETCOR NMR data demonstrate that for ethanol-based, TMS-modified silica aerogels, both ethoxy and TMS groups are chemically bonded to the silica surface. This technique has great potential to assist in the development of hydrophobized and/or ambient-dried aerogels and organic−inorganic hybrid aerogels. A procedure based on the calibration of the absolute NMR signal intensities with external references was developed and validated to derive quantitative speciation and compositional data from fully relaxed, single-pulse 1H, 13C, and 29Si NMR spectra. The resulting quantitative 1H, 13C, and 29Si NMR data allow us to monitor the surface modification. Ethoxy groups, TMS, and residual silanol groups form a mixed monolayer on the silica surfaces of ambient-dried, hydrophobized, alkoxide-based aerogels. With increasing modification time, silanol groups are progressively replaced by TMS groups, reducing shrinkage during subsequent drying and resulting in lower densities of the final aerogel material. The consistency between the 1H NMR and 13C and 29Si NMR data demonstrates that 1H NMR alone can provide robust, quantitative data on the surfaces of silica aerogels. This observation, combined with the short collection times for 1H NMR spectra, on the order of minutes rather than hours to days for 13C and 29Si, establishes solid-state NMR as a fast, quantitative screening tool for monitoring aerogel surface modification, perhaps the most critical aspect of aerogel production by ambient-pressure drying.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(W.J.M.) Phone: +41 58 765 4983. E-mail: wim.malfait@ empa.ch. *(M.M.K.) Phone: +41 58 765 4780. E-mail: matthias.koebel@ empa.ch. Author Contributions

W.J.M. and M.M.K. designed the study. W.J.M. and M.M.K. prepared the silica aerogels, and W.J.M. and R.V. collected the NMR data. W.J.M. analyzed the data and developed the quantification procedure. The manuscript was written through contributions of all authors: W.J.M. wrote the first draft of the manuscript, which was modified on the basis of comments from M.M.K. and R.V. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Shanyu Zhao for his assistance with the preparation of the aerogel samples, Daniel Rentsch for preliminary NMR 25553



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Hydrophobization of Silica Aerogels: Insights from Quantitative Solid

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