Supplementary Information The Surface Chemistry of Hydrophobic Silica Aerogels Wim J. Malfait, Shanyu Zhao, Rene Verel, Subramaniam Iswar, Daniel Rentsch, Resul Fener, Yucheng Zhang, Barbara Milow and Matthias M. Koebel TABLE OF CONTENT SAMPLE SYNTHESIS 2 SAMPLE APPEARANCE 4 NITROGEN SORPTION 4 SILICA SPECIATION 5 MASS OF ELEMENT IN ROTOR 7 PARTICLE DIAMETER AND SURFACE AREA 8 1
SAMPLE SYNTHESIS Abbreviations TEOS: tetraethoxysilane PEDS: polyethoxydisiloxane HMDZ: hexamethyldisilazane HMDSO: hexamethyldisiloxane TMCS: trimethylchlorosilane MTMS: methyltrimethoxysilane DMDMS: dimethyldimethoxysilane CTAC: cetyltrimethylammoniumchloride 1. TEOS based aerogel hydrophobized with HMDZ TEOS: H 2 O: EtOH: HCl: Ammonia (molar ratio) =1: 3.5: 3.9: 7.8 1-4 : 5.7 1-3. Hydrolysis and sol formation. TEOS (22.3 ml) is mixed with 1.8 ml TEOS:H 2 O with molar ratio of 1:1, the mixture is stirred for 3 minutes at room temperature..2 ml 37% HCl is diluted with 22.7 ml ethanol, added to the solution, and the solution kept in a closed vessel and stirred at 6 C for 4 hours. The solution is cooled to room temperature. Gelation..2ml NH 3 in H 2 O is mixed with an additional 4.5 ml H 2 O and added to the solution. After 2 min. stirring, the solution is separated into 1 boxes (5ml per sample) and kept at 55 C for 1 hr. Aging and solvent exchange. The samples are washed at 65 C in ethanol twice within 3 hours. Subsequently washed at 5 C in n-heptane four times within 48 hours. Modification. Hydrophobization is initiated by immersing the gels in 2% HMDZ in heptane for 15 hours at 55 C. Solvent exchange. The samples are washed four times at 55 C in heptane within 3 hours. Drying. Drying performed at ambient pressure at 65, 9, 12, 18 C for 6 hours each. 2. PEDS based aerogel, single step exchange, hydrophobized with HMDSO Gelation and aging. 1.5 ml of PEDS (P75E2 from PCAS) and.2 ml of H 2 O were diluted with 3.5 ml of ethanol..25 ml of 1M NH 4 OH was added to induce gelation. Samples were aged for 96 hours at 55 C Modification. Modification is performed by immersing the alcogels for 24 hours at 65 C in a modification solution consisting of 1 ml of HMDSO and.25 ml of 2M HCl in ethanol. Drying. Drying performed at 15 C for 2 hours. 2
3. Waterglass based aerogel, single step exchange, hydrophobized with TMCS Waterglass : H 2 O (volume ratio)=1:3. 15 ml waterglass is mixed with 45 ml H 2 O, and passed through Amberlyst-15 ion exchange resin. Gelation. 2.5M ammonia is added to change the ph to 5.1 to induce the gelation at 25 C. Gelation and aging for 3 hours. Modification. Modification is performed by immersing the gels at for 69 hours 65 C in EtOH/TMCS/nheptane solution with EtOH:TMCS molar ratio =2:3, V(TMCS):V(Hydrogel)=1:1, heptane is 8 ml. Drying. Drying performed at 65 C for 4 hours and then at 9, 12, 18 C for 2 hours each. 4. Waterglass based aerogel, multiple solvent exchanges, hydrophobized with TMCS Waterglass : H 2 O (volume ratio)=1:3. 15 ml waterglass is mixed with 45 ml H 2 O, and passed through Amberlyst-15 exchange resin. Gelation. The ph is adjusted with 2.5M ammonia to induce the gelation. Gelation and aging for 3 hrs. Solvent exchanges. The samples are washed in 1% ethanol 4 times at 65 C within 48 hours. Subsequently washed in n-heptane 3 times at 5 o C within 3 hours. Modification. Modification is performed by immersing the gels for 69 hours at 65 C in EtOH/TMCS/nheptane solution with EtOH:TMCS molar ratio =2:3, V(TMCS):V(Hydrogel)=1:1, heptane is 8 ml. Drying. Drying performed at 65, 9, 12, 18 C for 2 hours each. 5. MTMS based open-porous foam MTMS: MeOH:H 2 O (molar ratio)=1:35:8 Hydrolysis. 5ml MTMS is diluted with 49.6 ml MeOH and the hydrolysis is carried out by adding drop wise a partial amount of water (2.5 ml) in the form of.1m oxalic acid, 24 hours hydrolysis at room temperature. Gelation. The remaining half of water (2.5 ml), in the form of 1M NH 4 OH, is added to the hydrolyzed sol. The sol is transferred to 1 boxes and kept in a close vessel at 3 C (13.5 hours of gelation/aging). Drying. Drying performed in 65 C for 24 hours and then 1, 15 and 2 C for 1 hour. 6. MTMS+DMDMS based open-porous foam MTMS:DMDMS:C 2 H 4 O 2 :H 2 O:urea:CTAC=1:,67:.4:39,65:3,97:,15 Hydrolysis MTMS. Urea is dissolved in distilled water; acetic acid is added to reach ph 3.5. Addition of MTMS and hydrolysis. Addition of surfactant CTAC. Total duration 15 minutes. Hydrolysis DMDMS. Addition of DMDMS to MTMS sol. Reaction time 45 minutes. Gelation. Sol is placed in closed vessel at 8 C, gelation occurs around 2-4 hours. Aging for 24 hours. Solvent exchange. Washing in water (2x) and then in ethanol (2x). Drying. From ethanol at 8 C in a drying cabinet. 3
SAMPLE APPEARANCE Figure S1. Photograph of six archetypal hydrophobic silica aerogels. Samples 1-4 display the typical blue hues associated with silica aerogel. Samples 5-6 are white due to the larger pore sizes that are comparable to the wavelength of visible light. NITROGEN SORPTION Quantity Adsorbed (cm³/g STP) 3 25 2 15 1 5 3 25 2 a) TEOS/HMDZ (1).5 1 d) WG/SE/TMCS (4) 3 25 2 15 1 5 3 25 2 b) PEDS/HMDSO (2).5 1 e) MTMS (5) 3 25 2 15 1 5 3 25 2 c) WG/TMCS (3).5 1 f) MTMS+DMDMS (6) 15 15 15 1 1 1 5 5 5.5 1.5 1.5 1 P/P Figure S2. Nitrogen sorption curves for five archetypal hydrophobic silica aerogel materials. Samples 1-4 display typical sorption curves for silica aerogel materials. Sample 5 is predominantly macroporous without a significant fraction of mesopores. Sample 6 (MTMS+DMDMS) displayed no measurable nitrogen sorption, indicative of its very low surface area and large pore sizes. 4
SILICA SPECIATION The Q n speciation was determined from unconstrained Gaussian fits to the quantitative NMR spectra (Fig. S3, Table S1). The T n and D n speciation was determined by integration of the quantitative 29 Si NMR spectra (Table S2). Q 4 4) WG/SE/TMCS Q 3 Intensity (a.u.) 3) WG/TMCS 2) PEDS/HMDSO 1) TEOS/HMDZ -85-9 -95-1 -15-11 -115 29 Si chemical shift (ppm) -12-125 Figure S3. Gaussian fits through the Q n region of the 29 Si MAS NMR spectra. Dots mark the experimental data, lines denote the fitted components, fitting envelope and residual to the fit. 5
Table S1. Q n species abundance and NMR parameters Sample TEOS/HMDZ PEDS/HMDSO WG/HMDZ WG/SE/HMDZ 1 2 3 4 Q 4 Fraction (%) 89.4 ±.9 87.3 ± 1.4 83.4 ± 1.1 76.2 ±.7 Position (ppm) -11.4 ±.3-11.1 ±.3-11.4 ±.3-11.6 ±.3 FWHM (ppm) 3.5 ±.3 3.4 ±.3 3.3 ±.3 3. ±.3 Q 3 Fraction (%) 1.6 ±.7 12.7 ± 1.1 16.6 ± 1. 23.8 ±.9 Position (ppm) -1.7 ±.3-11. ±.3-11. ±.3-11.9 ±.3 FWHM (ppm) 2.5 ±.3 2.7 ±.3 2.8 ±.3 3.5 ±.3 BO/T 3.89 ±.1 3.87 ±.1 3.83 ±.1 3.76 ±.1 NBO/T.11 ±.1.13 ±.1.17 ±.1.24 ±.1 FWHM: full width at half max. BO/Si: bridging oxygen atoms per Si 4-[Q 3 ]/([Q 4 ]+[Q 3 ]) NBO/Si: non-bridging oxygen atoms per Si, [Q 3 ]/([Q 4 ]+[Q 3 ]) Table S2. T n and D n species abundance; 29 Si NMR parameters Sample MTMS MTMS+DMDMS 5 6 T 3 Fraction (%) 86.4 ± 5.5 52.4 ± 3.9 Position (ppm) -65.4 ±.3-66.3 ±.3 FWHM (ppm) 5.6 ±.3 3.4 ±.3 T 2 Fraction (%) 13.6 ± 1.8 5.5 ± 1.5 Position (ppm) -56.7 ±.3-56.1 ±.3 FWHM (ppm) 4.6 ±.3 2.4 ±.3 D 2 Fraction (%) 38. ± 3.2 Position (ppm) -2.9 ±.3 FWHM (ppm) 1.5 a ±.3 D 1 Fraction (%) 4.1 ± 1.5 Position (ppm) -11.7 ±.3 FWHM (ppm) 1.9 ±.3 BO/Si 2.86 ±.17 2.48 ±.14 FWHM: full width at half maximum BO/Si: 3[T 3 ]+2[T 2 ]+2[D 2 ]+[D 1 ]/([T 3 ]+[T 2 ]+[D 2 ]+[D 1 ]) a Band is split into two partially resolved peaks 6
MASS OF ELEMENT IN ROTOR The total mass of each element in the NMR rotor was determined from the NMR spectra (Table S3). Table S3. Quantitative NMR data: mass of element in rotor sample H a C Si O b Total c Balance 1) TEOS HMDZ 2.2 ±.1 5.8 ±.5 26.1 ± 1.5 24.4 ± 1.5 6.6 ± 3. 61.6 ±.1 2) PEDS HMDSO.8 ±.1 2.6 ±.2 11.1 ±.6 1.5 ±.7 25.9 ± 1.3 26.4 ±.1 3) WG/TMCS 1.8 ±.1 5.1 ±.4 2.8 ± 1.2 19.4 ± 1.3 48.9 ± 2.5 57.6 ±.1 4) WG/SE/TMCS 2.8 ±.2 12.4 ±.9 4.7 ± 2.3 38.3 ± 2.4 97.6 ± 4.7 99.7 ±.1 5) MTMS 5.9 ±.3 24.4 ± 1.7 61.6 ± 3.6 55. ± 3.5 152.4 ± 7.3 142.7 ±.1 6) MTMS+DMDMS 1.9 ±.6 42. ± 2.4 8.6 ± 4.4 61.4 ± 3.7 22.2 ± 8.4 22.7 ±.1 a from 2.5 mm MAS NMR data, recalculated to same total weight as in 7mm MAS rotor b O content calculated from 29 Si NMR data: [O] = 2[Q 4 ] + 2.5[Q 3 ] +.5[TMS] (in moles) c Uncertainties on H, C and Si are uncorrelated, uncertainties on Si and O are correlated Table S4. Quantitative NMR data: comparison to elemental analysis (E.A.). H (wt%) C (wt%) sample NMR E.A. NMR E.A. TEOS HMDZ 3.6 ±.2 2.9 ±.5 9.4 ±.8 11.5 ± 1. PEDS HMDSO 2.9 ±.4 2.9 ±.5 1. ±.8 11.7 ± 1. WG/TMCS 3.2 ±.2 2.9 ±.5 8.8 ±.7 11.1 ± 1. WG/SE/TMCS 2.8 ±.2 3.6 ±.5 12.5 ±.9 14.8 ± 1. MTMS 4.1 ±.2 4.8 ±.5 17.1 ± 1.2 18.5 ± 1. MTMS+DMDMS 5.4 ±.3 6.3 ±.5 2.7 ± 1.2 23.8 ± 1. Table S5. Calibration Standard Formula Mass in rotor (mg) Mass H/C/Si in rotor (mg) I 1 H NMR Adamantane C 1 H 16 8.8 1. 1949 Si(TMS) 4 Si 5 C 12 H 36 5.3.6 116 Q8M8 Si 16 O 2 C 24 H 72 6.6.4 77 13 C NMR Adamantane-2 C 1 H 16 28.1 11.1 129 Adamantane-1 C 1 H 17 28.1 73.4 857 Si(TMS) 4 Si 5 C 12 H 36 16.8 72.2 862 Q8M8 Si 16 O 2 C 24 H 72 249.9 63.7 86 29 Si NMR Si(TMS) 4 (TMS groups) Si 5 C 12 H 36 16.8 56.3 233 Si(TMS) 4 (Central Si) Si 5 C 12 H 36 16.8 14.1 53 Q8M8 Si 16 O 2 C 24 H 72 249.9 49.7 217 Si(TMS)4: Tetrakis(trimethoxy)silane Q8M8: octakis(trimethylsiloxy)silsesquioxane I: Integrated intensity per scan 7
PARTICLE DIAMETER AND SURFACE AREA 1 9 Particle diameter NMR (nm) 8 7 6 5 4 3 2 1 2 4 6 8 1 Particle diameter TEM (nm) Fig. S4. Predicted versus measured particle diameter. Uncertainties of the NMR depend on the degree to which the model assumptions are fulfilled and hard to evaluate (see main text). 1 9 Surface area NMR (m 2 /g) 8 7 6 5 4 3 2 1 2 4 6 8 1 Surface area BET (m 2 /g) Fig. S5. Predicted versus measured surface area. 8