Hydrophobic aerogels comprising low occupancy of monofunctional units

Abstract

The problem addressed by the invention is that of producing aerogels which have as high and permanent a hydrophobicity as possible and which have a reduced combustibility, that is as low a carbon content as possible, and are simultaneously less rigid and brittle than known systems, i.e. which with reduced combustibility have a high flexibility and high stability at the same time, that is high mechanical load-bearing capacity. Said problem is solved in that the invention provides gels chosen from lyogel or aerogel, which are synthesised from oxide units and [R.sub.xSiO.sub.(4-x)/2] units, wherein the primary particles have a change of concentration in [R.sub.xSiO.sub.(4-x)/2] units from the inside to the outside, wherein x can be the same or different and is 1 or 2, and R can be the same or different and is hydrogen or an organic substituted or unsubstituted radical, and wherein the oxide units contain [SiO.sub.4/2] units, and a method for producing same. The gels provided can be used in cosmetic, medical or for chromatographic applications, and as a catalyst or catalyst support. If the gels are aerogels, same are preferably used for thermal and/or acoustic insulation.

Claims

1. A gel comprising lyogel- or aerogel-containing primary particles comprising oxidic units and [R.sub.xSiO.sub.(4-x)/2] units, wherein the primary particles have an increase in a concentration of [R.sub.xSiO.sub.(4-x)/2] units from inside to outside, indices x can be identical or different and are in each case 1 or 2, radicals R can be identical or different and are each hydrogen or an organic, substituted or unsubstituted radical, interparticular linkages of the primary particles comprise [R.sub.xSiO.sub.(4-x)/2] units, and the oxidic units contain [SiO.sub.4/2] units.

2. The gel as claimed in claim 1, wherein the primary particles are built up in a form of a core-shell model, where a core contains a concentration of [R.sub.xSiO.sub.(4-x)/2] units of less than 20 mol % and a shell contains a concentration of [R.sub.xSiO.sub.(4-x)/2] units of more than 80 mol %.

3. The gel as claimed in claim 1, wherein the radical R is a methyl group.

4. The gel as claimed in claim 1, wherein a degree of coverage with monofunctional units on a surface is less than 1.5 groups per nm.sup.2.

5. The gel as claimed in claim 1, which is an aerogel having a density of less than 0.25 g/cm.sup.3.

6. The gel as claimed in claim 1, which is an aerogel having a carbon content of less than 15% by weight.

7. The gel as claimed in claim 1, which is an aerogel having a density of less than 0.15 g/cm.sup.3, a degree of coverage with monofunctional units of not more than 1 per nm.sup.2, a carbon content of not more than 8% by weight and a BET surface area of greater than 300 m.sup.2/g.

8. The gel as claimed in claim 1, wherein only [SiO.sub.4/2] units are present as oxidic units.

9. A process for producing gels of claim 1, comprising: i) placing a sol containing colloidal particles, where the colloidal particles contain oxidic units, in a reaction vessel, where the oxidic units contain [SiO.sub.4/2] units and the sols are produced by neutralization of strongly basic alkali metal silicates or by hydrolysis of alkoxysilanes, ii) reacting the particles in the sol with [R.sub.xSiO.sub.(4-x)/2] units, where the indices x can be identical or different and are in each case 1 or 2 and the radicals R can be identical or different and are each hydrogen or an organic, substituted or unsubstituted radical, iii) and forming a gel from the sol.

10. The process as claimed in claim 9, wherein solutions of colloidal particles containing [SiO.sub.4/2] units are used as a starting material, where the particles have an average diameter in a range from 1 to 8 nm.

11. The process as claimed in claim 9, wherein the gel is covered with monofunctional units by surface modification.

12. The process as claimed in claim 9, wherein the gel is dried.

13. The gel as claimed in claim 1, which is an aerogel effective for thermal and/or acoustic insulation.

14. The gel as claimed in claim 1, which is effective for cosmetic, medical or chromatographic applications or as catalysts or catalyst supports.

15. The gel as claimed in claim 1, wherein the primary particles are built up in a form of a core-shell model, where a core contains a concentration of [R.sub.xSiO.sub.(4-x)/2] units of less than 20 mol % and a shell contains a concentration of [R.sub.xSiO.sub.(4-x)/2] units of more than 80 mol %.

16. The gel as claimed in claim 15, wherein the radical R is a methyl group.

17. The gel as claimed in claim 16, wherein a degree of coverage with monofunctional units on a surface is less than 1.5 groups per nm.sup.2.

18. The gel as claimed in claim 17, which is an aerogel having a density of less than 0.25 g/cm.sup.3.

19. The gel as claimed in claim 18, which is an aerogel having a carbon content of less than 15% by weight.

20. The gel as claimed in claim 19, which is an aerogel having a density of less than 0.15 g/cm.sup.3, a degree of coverage with monofunctional units of not more than 1 per nm.sup.2, a carbon content of not more than 8% by weight and a BET surface area of greater than 300 m.sup.2/g.

21. The gel as claimed in claim 20, wherein only [SiO.sub.4/2] units are present as oxidic units.

22. A process for producing gels of claim 20, comprising: i) placing a sol containing colloidal particles, where the colloidal particles contain oxidic units, in a reaction vessel, where the oxidic units contain [SiO.sub.4/2] units and the sols are produced by neutralization of strongly basic alkali metal silicates or by hydrolysis of alkoxysilanes, ii) reacting the particles in the sol with [R.sub.xSiO.sub.(4-x)/2] units, where the indices x can be identical or different and are in each case 1 or 2 and the radicals R can be identical or different and are each hydrogen or an organic, substituted or unsubstituted radical, iii) and forming a gel from the sol.

23. The process as claimed in claim 22, wherein solutions of colloidal particles containing [SiO.sub.4/2] units are used as a starting material, where the particles have an average diameter in a range from 1 to 8 nm.

24. The process as claimed in claim 23, wherein the gel is covered with monofunctional units by surface modification.

25. The process as claimed in claim 24, wherein the gel is dried.

26. The gel as claimed in claim 21, which is an aerogel effective for thermal and/or acoustic insulation.

27. The gel as claimed in claim 21, which is effective for cosmetic, medical or chromatographic applications or as catalysts or catalyst supports.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The figures illustrate the invention by way of example without restricting it.

(2) FIG. 1: Schematic depiction of the subsequent modification of existing [SiO.sub.4/2] gel networks having interparticular linkages (sintering bridges) composed of [SiO.sub.4/2] units (A=core, light-colored center) with [R.sub.xSiO.sub.(4-x)/2] units (B=shell, black outer region). Since the interparticular linkages (C) have been formed before the modification, these are shown in light color.

(3) FIG. 2: Schematic depiction of a gel network consisting of [R.sub.xSiO.sub.(4-x)/2] units. The entire gel network, i.e. both the core (A), the shell or outer region (B) and the interparticular linkages (C) consist of [R.sub.xSiO.sub.(4-x)/2] units.

(4) FIG. 3: Schematic depiction of the gradient structure of the primary particles with [R.sub.xSiO.sub.(4-x)/2] concentration increasing towards the outside and [R.sub.xSiO.sub.(4-x)/2]-rich interparticular linkages (sintering bridges, C), where the light-colored center reflects oxidic units with the maximum concentration in the core (A) and the coloration which becomes increasingly dark from the inside to the outside reflects the concentration of [R.sub.xSiO.sub.(4-x)/2] units which is low on the inside and increases toward the outside. The sintering bridges (C) have the same composition as the outer shell of the primary particles (B).

(5) FIG. 4: Schematic depiction of the core-shell structure of the individual primary particles with oxidic core (A, light-colored center) and [R.sub.xSiO.sub.(4-x)/2] shell (B, black outer region) and interparticular linkages (C, sintering bridges) of the individual primary particles. Since the sintering bridges have the same composition as the shells, they are shown in black.

EXAMPLES

(6) The examples illustrate the invention in more detail without restricting its scope.

(7) Analytical Methods:

(8) Determination of the Density

(9) The density of the aerogel pieces was determined by means of pycnometry. For this purpose, the aerogel pieces were weighed on an analytical balance (m.sub.1) and to determine the volume the displacement of water was measured in a 25 ml pycnometer (Gay-Lussac glass pycnometer in accordance with DIN ISO 3507 from Blaubrand) at room temperature. For this purpose, the following masses were determined on an analytical balance: m.sub.2: mass of the pycnometer filled with distilled water m.sub.3: mass of the pycnometer filled with the aerogel pieces and distilled water

(10) The volume of the aerogel piece (V.sub.1) corresponds to the volume of the displaced water (V.sub.2). The volume and the density of the aerogel piece were calculated according to the following formulae:
V.sub.1=V.sub.2=.sub.w*(m.sub.2(m.sub.3m.sub.1))
.sub.aerogel=m.sub.1/V.sub.1

(11) where .sub.w is the density of water at room temperature (0.998 g/cm.sup.3).

(12) When filling the pycnometer with the aerogel piece and the water, care was taken to ensure that no air bubbles were included. Owing to the high hydrophobicity of the aerogel samples, penetration of water into the pores of the samples is ruled out. As a control, the weight of the aerogel pieces was confirmed by renewed weighing after the measurement.

(13) Determination of the BET Surface Area

(14) The specific surface area of the aerogels was determined by the BET method in accordance with DIN 9277/66131 and 9277/66132).

(15) Determination of the Carbon Content

(16) The carbon content (C content) of the samples was determined on a Leco CS 230 analyzer. The analysis was carried out by high-frequency combustion of the sample in a stream of oxygen. Detection was effected by means of nondispersive infrared detectors.

(17) Determination of the pH

(18) The pH was determined using a pH meter from Mettler Toledo, Seven Multi; electrode: In Lab Science.

(19) Calculation of the Degree of Coverage with TMS Groups

(20) The coverage with the TMS groups after surface modification was calculated by means of a method analogous to EP 0 948 395 B1 by means of the following formula:
Degree of coverage=([C.sub.with TMS][C.sub.without TMS])/BET)*K; unit: [nm.sup.2]

(21) K=6.022*1023/100*12*3*1018=167.28; unit: [g.sup.1]

(22) [C.sub.with TMS]: C content after surface modification in % by weight

(23) [C.sub.without TMS]: C content before surface modification in % by weight

(24) [BET]: BET surface area; unit: [m.sup.2/g]
C[% by weight]=C.sub.with TMS[% by weight]C.sub.without TMS[% by weight]

EXAMPLES

(25) Sources:

(26) Water glass (Sigma-Aldrich: SiO.sub.2 content: 26.5% by weight, Na.sub.2O content: 10.6% by weight)

(27) Potassium methylsiliconate (SILRES BS 16 from Wacker Chemie

(28) AG: aqueous solution containing 34% by weight of active compound and 20% by weight of K.sub.2O)

(29) SiO.sub.2 nanosol (Bindzil 17/750 from Akzo Nobel: SiO.sub.2 content: 15% by weight, average particle diameter according to manufacturer: 4 nm, pH 10.5)

(30) Hexamethyldisiloxane (AK 0.65 from Wacker Chemie AG)

(31) Trimethylchlorosilane (SILAN M3 from Wacker Chemie AG)

(32) Methyltrimethoxysilane (Sigma-Aldrich, Grade: 98%)

(33) Cetyltrimethylammonium bromide (Sigma-Aldrich)

(34) All further laboratory chemicals were, unless indicated separately, procured from Sigma-Aldrich.

Example 1: Production of an Aerogel from Water Glass and Potassium Methylsiliconate (by Sequential Addition of Potassium Methylsiliconate)

(35) In a glass beaker, 55.5 g of water and 55.5 g of water glass were mixed and cooled to 10 C. in an ice bath. In a second glass beaker, 55.5 g of water and 55.5 g of potassium methylsiliconate were mixed and cooled to 10 C. in an ice bath. 200 g of hydrochloric acid (7.5% by weight) were placed in a screw-cap bottle, cooled to below 10 C. in an ice bath and stirred at 500 rpm by means of a magnetic stirrer.

(36) The cooled water glass solution was added slowly via a dropping funnel to the hydrochloric acid solution while stirring. During the introduction, care was taken to ensure that the dripping rate was so slow that the temperature did not rise above 10 C. After the addition, the reaction mixture was stirred further at room temperature for two hours and cooled back to below 10 C. in an ice bath before addition of the second component. The potassium methylsiliconate solution which had likewise been cooled to below 10 C. in an ice bath was subsequently added slowly via a dropping funnel while stirring, with it being ensured during the introduction that the temperature does not rise above 10 C. The stirring was subsequently stopped and the sol was warmed to room temperature, as a result of which gel formation to form the lyogel took place.

(37) For aging, the lyogel obtained was incubated for 3 hours at 60 C. in a closed vessel in a drying oven. The gel was then pressed through a sieve having a mesh opening of 5 mm in order to obtained pieces smaller than 5 mm.

(38) To remove the salts, the gel pieces were incubated five times for 24 hours in each case in weakly alkaline water having a temperature of 60 C. (300 ml of water per 100 g of gel). Weakly alkaline means that the water was set to a pH of 8.5 using NaOH. The water was separated off by decantation after each 24 hours and subsequently replaced by fresh, weakly alkaline water.

(39) To determine the C content before surface modification, a sample of 10 g of the moist lyogel obtained was dried at 180 C. to constant weight in a drying oven and subsequently analyzed as described above.

(40) In parallel, 100 g of the moist gel obtained before surface modification was covered with 200 ml of an ethanol/water mixture (50% by weight of ethanol) and incubated at room temperature in a closed vessel for 16 hours. The gel was subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40). For the surface medication, the gel pieces obtained were admixed with 200 ml of hexamethyldisiloxane and 10.0 g of trimethylchlorosilane in a closed screw-cap bottle, and incubated at 60 C. for 16 hours in a drying oven. The gel pieces were subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40) and dried to constant weight under reduced pressure in a vacuum drying oven (10 mbar, 120 C.), giving an aerogel which was subsequently analyzed by the above-described methods.

(41) Particular analytical data of the 1st example:

(42) Density: 0.10 g/cm.sup.3

(43) BET: 587 m.sup.2/g

(44) C content before surface modification: 9.2% by weight

(45) C content after surface modification: 11.7% by weight

(46) Coverage with TMS groups: 0.71 nm.sup.2

Example 2: Production of an Aerogel from Bindzil 17/750 and Potassium Methylsiliconate

(47) In a closable glass bottle, 56.6 g of SiO.sub.2 nanosol were added while stirring to a solution which was composed of 4.8 g of HCl solution (32% by weight) and 34 g of water and had been cooled to 0 C. 7.6 g of potassium methylsiliconate were subsequently added over a period of 10 minutes while cooling to 0 C., so that a pH of 8 was established. The sample was subsequently warmed to RT, as a result of which gel formation commenced, and this was concluded within 45 minutes. The resulting lyogel, which in this case was a hydrogel, was incubated at 60 C. for 48 hours to effect aging, subsequently broken up in pieces smaller than 5 mm as described in example 1 and the totality of the gel pieces was divided into two parts.

(48) One part (about 50 g) of the gel pieces was, in order to determine the C content before surface modification, washed with water and the solid was dried to constant weight at 120 C. and 10 mbar and subsequently analyzed as described above.

(49) The second part (about 50 g) of the gel pieces was covered with 100 ml of hexamethyldisiloxane. The hydrogel was silylated at 80 C. for 16 hours by addition of 10 g of HCl (32%) and 10 g of ethanol as phase compatibilizer, with the aqueous phase being displaced from the pores. The aqueous phase was separated off and the hydrophobic lyogel obtained was filtered off on a Bchner funnel (Whatman Filter, 125 mm, Grade 40) and dried to constant weight at 120 C. and 10 mbar, giving an aerogel which was subsequently analyzed using the above-described methods.

(50) Particular analytical data of the 2nd example:

(51) Density: 0.11 g/cm.sup.3

(52) BET: 300 m.sup.2/g

(53) C content before surface modification: 3.6% by weight

(54) C content after surface modification: 3.8% by weight

(55) Coverage with TMS groups: 0.11 nm.sup.2

Comparative Example 1: Gel Formation from Pure SiO.SUB.2

(56) (method based on EP 0 948 395 B1)

(57) In a glass beaker, 150 g of water and 150 g of water glass were mixed and cooled to 10 C. in an ice bath. 200 g of hydrochloric acid (7.5% by weight) were placed in a screw-cap bottle, cooled to below 10 C. in an ice bath and stirred at 500 rpm by means of a magnetic stirrer.

(58) The cooled water glass solution was slowly added via a dropping funnel to the hydrochloric acid solution while stirring. During the introduction, care was taken to ensure that the temperature does not rise above 10 C. At a pH of 5.2, the addition was stopped and the reaction mixture was warmed to room temperature, as a result of which gel formation took place. To effect aging, the lyogel obtained was incubated at 60 C. for 3 hours in a closed vessel in a drying oven. The gel was then pressed through a sieve having a mesh opening of 5 mm in order to obtain pieces smaller than 5 mm. To remove the salts, the gel pieces were incubated five times for 24 hours each time in weakly alkaline water having a temperature of 60 C. (300 ml of water per 100 g of gel). The water was for this purpose set to a pH of 8.5 using NaOH. The water was separated off by decantation after each 24 hours and subsequently replaced by fresh, weakly alkaline water.

(59) 10 g of the moist gel obtained were, to determine the C content before surface modification, dried to constant weight at 180 C. in a drying oven and subsequently analyzed as described above.

(60) 100 g of the moist gel obtained were, before surface modification, covered with 200 ml of an ethanol/water mixture (50% by weight of ethanol) and incubated for 16 hours at room temperature in a closed vessel. The gel was subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40). To effect the surface modification, the gel pieces obtained were admixed with 200 ml of hexamethyldisiloxane and 10.0 g of trimethylchlorosilane in a closed screw-cap bottle, shaken and incubated at 60 C. for 16 hours in a drying oven. The gel pieces were subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40) and dried to constant weight under reduced pressure in a vacuum drying oven (10 mbar, 120 C.), giving an aerogel which was subsequently analyzed using the methods indicated.

(61) Particular analytical data of the 1st comparative example:

(62) Density: 0.20 g/cm.sup.3

(63) BET: 521 m.sup.2/g

(64) C content before surface modification: <0.1% by weight

(65) C content after surface modification: 8.4% by weight

(66) Coverage with TMS groups: 2.70 nm.sup.2

Comparative Example 2: Gel Formation from Water Glass and Potassium Methylsiliconate (by Cocondensation)

(67) In a glass beaker, 150.0 g of water, 75.0 g of water glass and 75.0 g of potassium methylsiliconate were mixed and cooled to 10 C. in an ice bath.

(68) 200 g of hydrochloric acid (7.5% by weight) were placed in a screw-cap bottle, cooled to below 10 C. in an ice bath and stirred at 500 rpm by means of a magnetic stirrer.

(69) The cooled water glass-potassium methylsiliconate solution was slowly added to the hydrochloric acid solution via a dropping funnel while stirring. During the introduction, care was taken to ensure that the temperature does not rise above 10 C. At a pH of 5.3, the addition was stopped and the reaction mixture was warmed to room temperature, as a result of which gel formation took place. To effect aging, the gel obtained was incubated for 3 hours at 60 C. in a closed vessel in a drying oven. The gel was then pressed through a sieve having a mesh opening of 5 mm in order to obtain pieces smaller than 5 mm. To remove the salts, the gel pieces were incubated five times for 24 hours each time in weakly alkaline water having a temperature of 60 C. (300 ml of water per 100 g of gel). The water was for this purpose set to a pH of 8.5 using NaOH. The water was separated off by decantation after each 24 hours and subsequently replaced by fresh, weakly alkaline water.

(70) 10 g of the moist gel obtained were, to determine the C content before surface modification, dried to constant weight at 180 C. in a drying oven and subsequently analyzed as described above.

(71) 100 g of the moist gel obtained were, before surface modification, covered with 200 ml of an ethanol/water mixture (50% by weight of ethanol) and incubated for 16 hours at room temperature in a closed vessel. The gel was subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40). To effect the surface modification, the gel pieces obtained were admixed with 200 ml of hexamethyldisiloxane and 10.0 g of trimethylchlorosilane in a closed screw-cap bottle, shaken and incubated at 60 C. for 16 hours in a drying oven. The gel pieces were subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40) and dried to constant weight under reduced pressure in a vacuum drying oven (10 mbar, 120 C.), giving an aerogel which was subsequently analyzed using the methods indicated.

(72) Particular analytical data of the 2nd comparative example:

(73) Density: 0.11 g/cm.sup.3

(74) BET: 644 m.sup.2/g

(75) C content before surface modification: 8.7% by weight

(76) C content after surface modification: 14.8% by weight

(77) Coverage with TMS groups: 1.57 nm.sup.2

Comparative Example 3: Gel Formation from [(CH.SUB.3.)SiO.SUB.3/2.]

(78) (based on a method of Shan Yun et. al., RSC Adv., 2014, 4, 4535-4542)

(79) 315 g of water and 3.0 g of cetyltrimethylammonium bromide (Sigma-Aldrich) were placed in a screw-cap bottle, admixed with 81.8 g of methyltrimethoxysilane while stirring (magnetic stirrer, 500 rpm) and stirred at room temperature for 20 minutes. 3.0 ml of an ammonia solution (1.0 M) were subsequently added while stirring, the mixture was stirred for another one minute and the stirrer was removed, whereupon gel formation commences. To effect aging, the gel obtained was incubated for 16 hours at 60 C. in a closed vessel in a drying oven.

(80) The gel was then pressed through a sieve having a mesh opening of 5 mm in order to obtain pieces smaller than 5 mm. To remove the cetyltrimethylammonium bromide used and the water in the pores, the gel pieces were firstly incubated three times at 50 C. for 24 hours each time in ethanol heated to 50 C. (300 ml of ethanol per 100 g of gel) in a closed vessel. The ethanol was separated off by decantation after each 24 hours and subsequently replaced by fresh ethanol which had been heated to 50 C. The gel pieces were subsequently incubated three times for 24 hours each time in n-hexane which had been heated to 50 C. (300 ml of n-hexane per 100 g of gel) in a closed vessel. The n-hexane was separated off by decantation after each 24 hours and subsequently replaced by fresh n-hexane which had been heated to 50 C. The gel pieces were subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40).

(81) 10 g of the gel obtained were, in order to determine the C content before surface modification, dried to constant weight under reduced pressure in a vacuum drying oven (10 mbar, 120 C.) and subsequently analyzed as described above.

(82) 50.0 g of the gel pieces obtained were, to effect surface modification, admixed with 250 ml of n-hexane and 5.0 g of trimethylchlorosilane in a closed screw-cap bottle, shaken and incubated at 50 C. for 24 hours in a drying oven. The gel pieces were subsequently separated off by filtration on a Bchner funnel (Whatman Filter, 125 mm, Grade 40), washed twice with 250 ml each time of n-hexane and dried to constant weight under reduced pressure in a vacuum drying oven (10 mbar, 120 C.), giving an aerogel which was subsequently analyzed using the methods indicated.

(83) Particular analytical data of the 3rd comparative example:

(84) Density: 0.25 g/cm.sup.3

(85) BET: 615 m.sup.2/g

(86) C content before surface modification: 18.0% by weight

(87) C content after surface modification: 18.8% by weight

(88) Coverage with TMS groups: 0.22 nm.sup.2

Example 3: Production of an Aerogel from Bindzil 17/750, Methyltriethoxysilane and Dimethyldiethoxysilane

(89) In a closable glass bottle, 120 g of SiO.sub.2 nanosol were added while stirring to a solution which was composed of 3.2 g of HCl solution (32% by weight) and 22.6 g of water and had been cooled to 0 C., with a pH of 2.5 being established. A mixture of 15.2 g of methyltriethoxysilane and 6.3 g of dimethyldiethoxysilane was subsequently added over a period of 10 minutes while cooling to 0 C. and the mixture was stirred for about 30 minutes. A pH of 8 was subsequently set using an ammonia solution (1 M). The sample was subsequently warmed to RT, as a result of which gel formation commenced and this was concluded within 45 minutes. The resulting lyogel was incubated at 60 C. for 48 hours to effect aging, subsequently broken up into pieces smaller than 5 mm as described in example 1 and the totality of the gel pieces was divided into two parts.

(90) One part (about 50 g) of the gel pieces was, to determine the C content before surface modification, washed with water, the solid was dried to constant weight at 120 C. and 10 mbar and subsequently analyzed as described above.

(91) The second part (about 50 g) of the gel pieces was covered with 100 ml of hexamethyldisiloxane. The hydrogel was silylated at 70 C. for 16 hours by addition of 10 g of HCl (32%) and 10 g of ethanol as phase compatibilizer, with the aqueous phase being displaced from the pores. The aqueous phase was separated off and the hydrophobic lyogel obtained was filtered off on a Bchner funnel (Whatman Filter, 125 mm, Grade 40) and dried to constant weight at 120 C. and 10 mbar, giving an aerogel which was subsequently analyzed using the above-described methods.

(92) Particular analytical data of the 3rd example:

(93) Density: 0.16 g/cm.sup.3

(94) BET: 360 m.sup.2/g

(95) C content before surface modification: 7.7% by weight

(96) C content after surface modification: 9.2% by weight

(97) Coverage with TMS groups: 0.69 nm.sup.2

(98) TABLE-US-00001 TABLE 1 Overview of the particular analytical data of all examples Degree of coverage C.sub.without TMS C.sub.with TMS C with TMS Density [% by [% by [% by BET groups Example [g/cm.sup.3] weight] weight] weight] [m.sup.2/g] [nm.sup.2] 1 0.10 9.2 11.7 2.5 587 0.71 2 0.11 3.6 3.8 0.2 300 0.11 3 0.16 7.7 9.2 1.5 360 0.69 Comparison 1 0.20 <0.1 8.4 8.4 521 2.70 Comparison 2 0.11 8.7 14.8 6.1 644 1.57 Comparison 3 0.25 18.0 18.8 0.8 615 0.22