SiO₂ aerogels modified by perfluoro acid amides: a precisely controlled hydrophobicity

Abstract

Five series of novel fluorinated aerogels based on acylated (3-aminopropyl)trimethoxysilane (APTMS) (MeO)₃Si(CH₂)₃NHC(O)R_F (R_F = CF₃, C₂F₅, CF₃CF(OCH₃), C₆F₁₃ and C₈F₁₇) were prepared by a sol–gel method followed by supercritical drying in four different solvents, including isopropanol, CO₂, methyl tert-butyl ether (MTBE) and hexafluoroisopropyl alcohol (HFIP). The hydrophobicity of the aerogels was shown to increase smoothly with the increase in the length of the fluorinated substituent – the water drop contact angle θ changed from 0° and reached 143° for isopropanol dried samples, 133° for CO₂ dried samples, 136° for MTBE dried samples, and ∼140° for HFIP dried samples. The results obtained provide an easy method for the preparation of aerogels with precisely controlled hydrophobicity.

---

1. Introduction

Aerogels (AGs) have attracted much attention due to their unique properties, such as ultra-low density, high porosity and high specific surface area. AGs are usually used as very effective thermal and acoustical insulators,¹ sorbents,^2–5 catalyst supports,⁶ optical materials,^1,7 etc. The preparation of aerogels is a multistage process, typically involving gel preparation by a sol–gel technique, aging and washing with a chosen solvent, followed by supercritical drying (SCD). Alcohols, carbon dioxide (CO₂) and ethers are the most conventional types of supercritical fluids for aerogel preparation.^8–11

High hydrophobicity is often of critical importance for aerogels, as poor humidity resistance is considered to be one of the major drawbacks preventing their practical use. One more promising application of highly hydrophobic aerogels is the removal of non-polar organic pollutants (e.g., oil spills) from the water surface.^3,4 Typically, hydrophobic aerogels are prepared by trimethylsilylation of surface M–OH groups of SiO₂- and Al₂O₃-based aerogels,^12,13 or by the use of methyltrialkoxysilanes CH₃Si(OR)₃ as gel-forming or gel-modifying precursors.^14–17 Highly hydrophobic organic or hybrid aerogels can also be obtained by cold CCl₄ plasma treatment.^18–20 The use of this method allowed synthesizing cellulose-based aerogels possessing the water drop contact angle up to 150°. The use of perfluorinated organic substituents is the other possible way to obtain highly hydrophobic materials. Surprisingly, we could find only a few papers concerning the synthesis of such aerogels. In all the cases, polyfluoroalkylsilanes were used as surface modifiers²¹ or as co-precursors for AGs' preparation.^3,22 The resulting aerogels demonstrated excellent hydrophobicity, but starting polyfluoroalkylsilanes are precursors of limited availability synthesized by multistage procedures, so these materials are very expensive.

Recently, we have shown that fluorine-containing aerogels can be prepared from trifluoroacetylated(3-aminopropyl)triethoxysilane (CF₃C(O)NH(CH₂)₃Si(OEt)₃) by a standard sol–gel procedure.¹¹ The obtained aerogels were both transparent and hydrophobic, their water drop contact angle (θ) reached 139° and depended significantly on the type of solvent used for SCD.

A number of non-aerogel type SiO₂-based materials were made hydrophobic via a similar approach, based on the acylation of aminosilanes by long-chain polyfluorocarbonic acids, followed by the hydrolysis of thus obtained fluoroorganosilanes.^23–27 Fluorinated superhydrophobic polysiloxane materials were successfully prepared in this way but no aerogels of this type were previously reported.

Thus, the present work is focused on the elaboration of novel APTMS-based aerogels bearing fluorinated substituents of various lengths. We paid special attention to the effect of the length of fluorinated substituent on the properties of materials obtained. To prepare aerogels, we have used an original protocol,¹¹ comprising the fluoroacylation of aminopropyl silanes, followed by their gelation and supercritical drying. A number of methyl esters of fluorinated acids were used as acylation agents, namely methyl trifluoroacetate (CF₃COOCH₃), methyl pentafluoropropionate (CF₃CF₂COOCH₃), methyl tetrafluoro-2-(methoxy)propionate (CF₃CF(OCH₃)COOCH₃), methyl perfluoroheptanoate (n-C₆F₁₃COOCH₃), and methyl perfluorononanoate (n-C₈F₁₇COOCH₃). SCD of wet gels was performed in different solvents, namely isopropanol, carbon dioxide, methyl-tert-butyl ether, and hexafluoroisopropanol, in order to study the possible influence of the media on the properties of the resulting fluorinated aerogels.

2. Materials and methods

2.1 Materials

Methyl trifluoroacetate (MTFA, Acros, 99%), methyl pentafluoropropionate (P&M-Invest, 99%), methyl tetrafluoro-2-(methoxy)propionate (P&M-Invest, 97%), methyl perfluoro-heptanoate (P&M-Invest, 99%), methyl perfluorononanoate (P&M-Invest, 99%), tetramethylorthosilicate (TMOS, Acros, 99%), (3-aminopropyl)trimethoxysilane (APTMS, Acros, 95%), isopropanol (IPA, Acros, 99.5+%), hexafluoroisopropyl alcohol (HFIP, Aldrich 99%), methyl tert-butyl ether (MTBE, Acros 99%), HF (Acros, 40% aqueous solution) were used without further purification.

2.2 Preparation of monomers

To synthesize (3-trifluoroacetylaminopropyl)trimethoxysilane (monomer M1, CF₃CONH(CH₂)₃Si(OMe)₃), 2 mL of methanol and 2.98 g (0.0167 mol) of APTMS were placed into a glass flask equipped with a reflux condenser. Then, 2.55 g (0.02 mol) of methyl trifluoroacetate were added dropwise under stirring and the mixture was heated at 50 °C for two hours. The product obtained was evacuated (25 °C, 6.7 kPa) to remove all volatile products.

The M2 (CF₃CF₂CONH(CH₂)₃Si(OMe)₃), M3 (CF₃CF(OCH₃)–CONH(CH₂)₃Si(OMe)₃), M4 (n-C₆F₁₃CONH(CH₂)₃Si(OMe)₃), M5 (n-C₈F₁₇CONH(CH₂)₃Si(OMe)₃) monomers were synthesized by the same technique.

¹H, ¹³C, ¹⁹F NMR spectral data, as well as mass-spectral (m/z) data for M1–M5 monomers, are presented in S1.†

2.3 Preparation of lyogels

To synthesize trifluoroacetate-modified silica lyogels (L1), 3.23 g (0.0212 mol) of TMOS and 0.65 g (0.00236 mol) of M1 were cooled and mixed with 4.26 g (0.071 mol) of isopropanol in a plastic beaker. 1.7 g (0.094 mol) of de-ionized water and 0.12 g of 40% HF solution were mixed and cooled in another plastic beaker. The second solution was added to the first solution in one portion, stirred for several seconds, poured into cylindrical polypropylene containers, sealed and allowed to age for 24 h. The resulting lyogels were soaked in isopropanol, MTBE or HFIP for 24 h to exchange the pore liquid for the solvent chosen. This procedure was repeated five times. The gels formed were placed into an autoclave for supercritical drying.

The L2–L5 lyogels were synthesized by the same technique from the corresponding monomers.

2.4 Supercritical drying

Supercritical drying was performed as follows: a lyogel sample in a glass tube containing 14–16 mL of an appropriate solvent was placed into a stainless steel autoclave (V ∼ 40 mL). The autoclave was sealed and heated to a temperature exceeding the critical temperature of the solvent. The heating rate was approximately 100 °C h⁻¹. For isopropanol, hexafluoroisopropyl alcohol, and methyl tert-butyl ether, the drying temperatures were 250–260 °C (the measured pressure in the autoclave at the beginning of the drying procedure reached 6.0–7.0 MPa), 200–210 °C (4.5–5.5 MPa) and 240–250 °C (6.0–7.0 MPa), respectively. After reaching the desired temperature, the valve was opened; the pressure was evenly decreased to the atmospheric pressure over two hours. The hot autoclave was then evacuated for 30 min, cooled to room temperature and opened.

The supercritical drying in CO₂ was carried out in an installation composed of a high pressure CO₂ pump (SSI Supercritical 24, USA), a 50 mL steel reactor, and a Waters BPR (USA) back pressure regulator. The isopropanol lyogel samples were washed with liquid CO₂ for 2 h at 20 °C at a pressure of 15 MPa, then the temperature in the reactor was elevated to 50 °C, and the sample was washed with supercritical CO₂ (15 MPa) for 2–2.5 h. Next, the pressure in the heated autoclave was gradually decreased to atmospheric; the autoclave was cooled to ambient temperature and opened.

Solid state ¹⁹F NMR spectral data for A2 CO₂, A3 CO₂ and A5 CO₂ aerogel samples are presented in S2.†

2.5 Characterization of aerogels

Elemental analysis was performed on a Carlo Erba 1106 CHN analyzer (C, H). The content of Si and F was determined spectrophotometrically (Agilent Cary 100 spectrophotometer).

The specific surface area of aerogels was determined by low-temperature nitrogen adsorption measurements with an ATX-06 analyzer, by an 8-point BET method. Experimental values were plotted against P/P_o according to the BET equation; the correlation coefficient, r, of the linear regression was not less than 0.9975.

The bulk densities of the samples were calculated by their mass to volume ratio.

Porosity of aerogels was calculated in accordance with:²⁰

P (%) = 1 − ρ_b/ρ_s

where ρ_b and ρ_s represent the bulk density of aerogel and skeletal density of amorphous quartz (ρ = 2.2 g cm⁻³) respectively. This equation assumes the skeletal density of the aerogels is close enough to pure quartz glass.

Mass spectra were obtained on a Finnigan MAT INCOS™ 50 mass spectrometer at 70 eV EI.

High resolution ¹H, ¹³C and ¹⁹F NMR spectra were obtained in CDCl₃ on a Bruker Avance III 500 spectrometer at the Larmor precession frequencies of 500, 126 and 470 MHz, respectively, relative to TMS and CFCl₃.

Solid state ¹⁹F NMR experiments were performed on a Bruker Avance III 400 spectrometer, with CFCl₃ as the external reference. Larmor precession frequency was 376.5 MHz. ¹⁹F NMR spectra were recorded under MAS conditions using a 90° pulse of 9 μs duration, 5 s recycle delay and 64 scans for each spectrum. Sample spinning rate was set to 20 kHz.

Microstructure of the samples was studied using a Carl Zeiss NVision 40 high resolution scanning electron microscope (SEM) at 7 kV acceleration voltage (magnifications up to 100 000×). In order to avoid any changes in the samples' appearance at high magnifications we did not coat their surface with a conductive material before measurements.

Infrared Fourier spectroscopy (IR) was performed on a Bruker IFS-113V spectrometer in a 4000–350 cm⁻¹ region (KBr pellets, 0.25–0.5% mass sample content).

Transmittance spectra of CO₂ dried samples (A1–A4) having plane parallel surfaces (4 mm thick) were obtained using an Ocean Optics USB 4000 UV-vis spectrometer equipped with a DH 2000 deuterium-halogen light source.

Sessile water drop contact angles were measured on a Levenhuk DTX 30 Digital Microscope. The volume of water of the drops was fixed at ∼30 μL. Images obtained were analyzed using standard Levenhuk software.

3. Results and discussion

Fluorinated aerogels containing pre-constructed APTMS-based monomers were prepared by a two-step co-gelation procedure¹¹ (Scheme 1).

Scheme 1

To prepare the aerogel sample a mixture of 10 mol% of a monomer and 90 mol% of tetramethyl orthosilicate was hydrolyzed to form a lyogel, which was then supercritically dried. All the lyogels were supercritically dried in four solvents of different chemical nature – polar isopropanol, non-polar aprotic CO₂ and methyl-tert-butyl ether, and highly acidic hexafluoroisopropanol. HFIP was chosen as an SCD solvent, as we have demonstrated earlier that such a solvent gives an additional opportunity to modify the surface of SiO₂-based aerogels and to obtain hydrophobic aerogels bearing fluoroalkyl groups on the surface.^10,28

Hereafter, the trifluoroacetate-modified aerogel series is denoted as A1, the pentafluoropropionate series – as A2, the tetrafluoro-2-(methoxy)propionate series – as A3, the perfluoroheptanoate series – as A4, and the perfluorononanoate series – as A5. Each sample name additionally contains the abbreviation of the solvent used. For example, trifluoroacetate-modified aerogel supercritically dried in isopropanol is denoted as A1 IPA, etc.

IR spectra of the A1–A5 aerogels series contain characteristic amide 1 and amide 2 bands (1717 and 1558 cm⁻¹, 1714 and 1558 cm⁻¹, 1700 and 1558 cm⁻¹, 1711 and 1558 cm⁻¹, 1714 and 1558 cm⁻¹, respectively). These data unambiguously prove that aerogel materials do indeed contain covalently bonded fluorinated acid amide moieties.

According to elemental analysis data (see Table S1†), A1 CO₂, A2 CO₂ and A5 CO₂ samples have F/Si atomic ratios of 0.33, 0.42 and 1.6 respectively, which is close enough to the theoretical values (0.3, 0.5 and 1.7 respectively). Thus, the procedure used for the preparation of fluorine-substituted aerogels provides nearly quantitative functionalization of the aminogroups in aerogel materials.

The thermal behavior of aerogels modified by fluorinated substituents of various lengths was studied by means of thermal analysis (Fig. 1 and S1†). All the materials obtained contain relatively small amounts of physisorbed solvents, which are withdrawn at temperatures below 150 °C. Thermal analysis data reveal some general features of the thermal behavior of aerogels with various fluorinated substituents. Aerogels modified by short substituents (CF₃, C₂F₅) have relatively low thermal stability – their oxidation in air flow begins at 200 °C and is accompanied by both weight loss in TGA curves and exothermal effect in DTA data. In turn, aerogels modified by longer substituents (CF₃CF(OCH₃), C₆F₁₃, C₈F₁₇) possess higher thermal stability and begin to lose weight only when heated up to ∼280 °C.

Fig. 1 Thermogravimetric curves (a) and differential thermal analysis data (b) for SiO₂ aerogels modified by perfluoro acid amides, supercritically dried in CO₂.

Low-temperature nitrogen adsorption data (see Table 1) indicate that the specific surface area values of aerogel samples decrease with the increase in length of fluorinated substituents. This trend is observed regardless of the nature of the solvent used for SCD. We assume that the key factor governing the texture characteristics of aerogels is the length of fluoroorganic moieties bound to the surface of the aerogel network. The increase in substituent length results in a decrease of pore volume and specific surface area, due to hydrophobic interactions.

Table 1 Specific surface area (m² g⁻¹)/bulk density (g cm⁻³)/porosity (%) of SiO₂ aerogels modified by fluorinated acids amides

Series	SCD solvent
	IPA	CO2	MTBE	HFIP
A1	830 ± 100	880 ± 110/0.29/87	730 ± 90	740 ± 90
A2	810 ± 100	860 ± 100/0.33/85	820 ± 100	780 ± 90
A3	600 ± 70	680 ± 80/0.37/82	460 ± 60	530 ± 60
A4	540 ± 70	520 ± 60/0.63/71	500 ± 60	480 ± 60
A5	560 ± 70	380 ± 40/0.11/95	450 ± 50	290 ± 30

---

The changes in the texture properties of aerogels comprising fluoroorganic substituents can be illustrated by the analysis of full adsorption–desorption isotherms presented in Fig. 2.

Fig. 2 Full adsorption–desorption isotherms (left) and corresponding pore size distributions obtained using a BJH model (right) of SiO₂ aerogels modified by trifluoroacetamide and supercritically dried in isopropanol and CO₂ (A1 IPA and A1 CO₂, respectively), and SiO₂ aerogels modified by perfluorononanamide and supercritically dried in isopropanol and CO₂ (A5 IPA and A5 CO₂, respectively).

Full adsorption–desorption isotherms presented in Fig. 2 are of IV type²⁹ with narrow hysteresis loops, indicating that samples primarily contain relatively small cylindrical both-sides open pores. BJH analysis of the isotherms revealed that the increase in the length of the substituents leads to a pronounced shift in pore size distributions to the smaller pore sizes and to a decrease in the overall pore volume from ∼0.8 cm³ g⁻¹ (A1 series) to ∼0.3 cm³ g⁻¹ (A5 series). Note that in cases of both short and long substituents the size of the pores in aerogels generally does not exceed 10 nm, so these materials mostly contain small-sized mesopores, while microporosity (pores less than 2 nm in size) as estimated using t-plots is almost negligible (less than 0.01 cm³ g⁻¹).³⁰ Analysis of t-plots also evidences that the increase in the length of the substituent results in a notable decrease in the specific outer surface area of mesoporous aggregates from ∼250 m² g⁻¹ for the A1 CO₂ sample to ∼90 m² g⁻¹ for the A5 CO₂ one.

The peculiarities of the structure of fluorinated silica aerogels found by low-temperature nitrogen adsorption are in good accordance with high resolution SEM data (see Fig. 3 and S2†).

Fig. 3 SEM images of SiO₂ aerogels modified by trifluoroacetamide and perfluorononanamide and supercritically dried in CO₂ (A1 CO₂ and A5 CO₂, respectively).

SEM data indicate that all the aerogels are of uniform microstructure and the size of the pores in all cases does not exceed ∼100 nm. The appearance of A1–A4aerogels dried in CO₂ seems almost identical, while the A5 CO₂ sample comprises a network of relatively large particles. The uniform microstructure is typical for silica-based aerogel materials obtained in supercritical CO₂ and other SC fluids.³¹ The differences in the samples' microstructure are in agreement with the above supposition concerning the influence of the fluoroalkyl substituents' length on aerogels' texture properties. Long substituents bonded to the particle surface at the stage of the lyogel formation increase their hydrophobicity, resulting in particles coalescing in a highly polar water–alcohol reaction mixture and the formation of relatively dense particles due to hydrophobic interactions.

We have shown earlier that the nature of SCD fluid has a strong impact on aerogels' texture properties.^9–11,28 In this work, such an influence is relatively weak (with the exception of the A5 series). Probably, fluorinated hydrophobic chains covering the silica surface reduce the ability of solvent molecules to reach the particle surface, to react with it, and to change the size of the aerogel particles due to the dissolution–precipitation process, thus influencing aerogel texture characteristics.

Hydrophobic properties of the aerogels surface were studied using sessile water drop contact angle measurements. The results are listed in Table 2, showing that the hydrophobicity of all the samples increases smoothly with the increase in the fluorinated substituent length and, hence, with the increase in fluorine content, the max θ value reaches 143° for A5 IPA sample (see Fig. S3†). Silica aerogels modified by short trifluoroacetic substituent bearing three fluorine atoms are rather hydrophilic. Such a relationship between the nature of fluorinated substituent and the hydrophobicity of the corresponding material is probably connected to van der Waals interactions of the surface species with water molecules.³²

Table 2 Contact angle values (θ, °) for SiO₂ aerogels modified by different fluorinated acid amides and supercritically dried in different solvents

Series of aerogels	SCD solvent
	IPA	CO2	MTBE	HFIP
a Aerogels are strongly hydrophilic, the aerogels soak up water drops quickly.
A1	90	106	0^a	0^a
A2	135	100	112	0^a
A3	113	104	115	121
A4	135	135	130	140
A5	143	133	136	137

---

A relatively low contact angle value for A3 IPA sample is probably due to the tendency of 2-methoxy-tetrafluoropropionic acid derivatives to loose CH₃-group under heating in acidic conditions.³³ Such a process could result in the decrease in fluorine content due to evolution of CH₃F providing higher hydrophilicity of the resulting aerogel.

The obtained results indicate that the introduction of only 10 mol% of fluorinated amide substituents allows the overcoming of high hydrophilicity intrinsic to the silica matrix and produces strongly hydrophobic aerogels.

To explain the correlation between the contact angle values and the length of fluorinated substituents, the following simple geometric model may be applied (see Fig. 4).

Fig. 4 The schematic representation of a separate aerogel particle (a) [SiO₄] tetrahedron (b) [O₃Si–CH₂] tetrahedron (c).

Let us assume that each aerogel particle is spherical (with the radius R) and consists of [SiO₄] tetrahedrons.³¹ Let us also consider [SiO₄] tetrahedrons as spheres (with the radius r equal to a Si–O covalent bond length) (Fig. 4a and b). Then, the total number of [SiO₄] groups in the volume of an aerogel particle is

The next assumption is that fluorinated substituents are attached only to the outer surface of aerogel particles (i.e., the location of R_F on the surface of a nanoparticle is strongly favorable compared to its location in the bulk) and some surface [SiO₄] tetrahedrons are replaced with [O₃Si–CH₂] moieties bearing a single fluorinated substituent (Fig. 4c).

Let us denote the ratio of surface tetrahedrons (N_surf) to the total number of tetrahedrons in the volume of an aerogel particle as

where and so and .

Taking into account the experimental Z value of 0.1 (10 mol% of M1–M5 in the reaction mixtures) and Si–O covalent bond length (r = 0.18 nm), the radius of an aerogel particle which bears all and only R_F substituents on the surface is approximately equal to 7 nm. In other words, silica particles with 7 nm radius containing 10 mol% of fluorinated substituents are fully covered with R_F groups. In smaller particles the ratio Z is higher, and consequently some of the surface-located [SiO₄] groups will not be bonded to R_F, and will bear surface –OH groups.

Considering the results of specific surface area measurements (see Table 1), we can also estimate an average aerogel particle radius from the experimental values, using a well-known relation³⁴

With a ρ taken as a density of quartz glass, 2.2 g cm⁻³ (ref. 35) the R value for A1–A5 aerogels will fall within a 2–4 nm interval, which is significantly lower than the above estimated value of 7 nm. Note that the radius value estimated from specific surface area measurements is in good accordance with the data on particle size in typical SiO₂ aerogels.³¹

The above estimations show that A1–A5 aerogels modified by 10 mol% of fluorinated substituents contain a considerable amount of surface Si–OH groups providing hydrophilic properties to the material. This is the case for short R_F substituents. Longer R_F can shelter neighboring hydrophilic Si–OH groups and thus make the entire material hydrophobic.

The above model implies that the increase in the content of fluorinated substituents can result in a stronger hydrophobization of the aerogel surface. In our previous work,¹¹ we have synthesized trifluoroacetamide-modified SiO₂ aerogels containing 20 mol% of CF₃ moiety. The materials obtained appeared to be a bit more hydrophobic (θ = 99–139° depending on the SCD solvent) than those ones synthesized in this work.

The next consequence from the model is that the increase of Z value in the case of long substituents will prevent gel formation due to the above-mentioned effect of surface Si–OH groups sheltering by bulky fluorinated substituents. In order to verify this assumption we have conducted additional experiments, aiming for the synthesis of lyogels comprising 20 mol% of R_F (R_F = C₂F₅, CF₃CF(OCH₃), C₆F₁₃, and C₈F₁₇). The synthetic procedure used was similar to that described in the Experimental section. In none of the cases did we observe the formation of lyogels – the reaction mixtures remained liquid. These observations undoubtedly corroborate our geometric approach and show that the formation of lyogels requires the presence of naked Si–OH groups, which are practically absent in the case of high content of long fluorinated substituents.

Our data on hydrophobicity of fluorinated silica aerogels correlate well with previously reported contact angle values for polyfluoroalkyl-modified aerogels (139–150°).^3,4,21 Reynolds et al. introduced a 10 mol% of CF₃CH₂CH₂–Si fragment into a SiO₂-based aerogel to obtain a contact angle value of 139°.⁴ Hrubesh et al. have shown that the water drop angle was 150° at the trifluoropropyl substituent content of 30 mol%.³ Roig et al. provided an exhaustive silylation of surface SiOH groups by a long-chain fluorinated silane C₆F₁₃CH₂CH₂Si(CH₃)₂–Cl and reached contact angle values of 150°.²¹ As was mentioned above, the starting fluorosilanes are expensive and of limited availability, so we reached a high hydrophobicity of aerogels in a substantially more convenient and affordable way.

One interesting feature of SiO₂ aerogels modified by perfluoro acid amides is that some of them are highly transparent (Fig. 5 and S4†).

Fig. 5 Appearance of SiO₂-based aerogels modified by perfluoro-acid amides.

High transparency together with hydrophobicity is an advantageous combination of properties, presenting a possibility to use the materials synthesized in industrial applications, e.g., for the manufacturing of window coatings, since polyfluorinated organic compounds are well known for their chemical inertness and the ability to resist harmful environmental factors – solar radiation, chemical pollution of atmospheric air, acidic rains, etc.³⁶ The transparency of aerogels depends on the type of fluorinated substituent and decreases with the increase in substituents' length. A1–A3 aerogels are transparent monoliths, A4 samples are semitransparent yellowish monoliths, and A5 series aerogels are white opaque monoliths. The optical transmittance spectra of 4 mm thick CO₂-dried A1, A2, A3 and A4 samples with plane parallel surfaces are shown in Fig. 6 (the CO₂ dried samples were chosen because they demonstrated the highest transparency and were the least coloured ones).

Fig. 6 Light transmittance spectra of CO₂-dried SiO₂ aerogels modified by trifluoroacetamide (A1), pentafluoropropionamide (A2), tetrafluoro-2-(methoxy)-propionamide (A3), and perfluoroheptanamide (A4).

Fig. 6 shows that the A2 CO₂ sample demonstrates the highest optical transmittance (more than 50%) in the 600–900 nm region. In general, the relatively high transparency of aerogels is rather unusual, and such a property was not observed before for aerogels modified by fluorine-containing organic substituents, except in our previous work.¹¹ To the best of our knowledge, there is only one other paper concerning the optical characteristics of fluorinated aerogels.²¹ In that work, Roig and co-authors have found that initially transparent SiO₂ aerogels become opaque upon treatment with highly fluorinated trialkylchlorosilane C₆F₁₃CH₂CH₂Si(CH₃)₂–Cl, resulting in a formation of the hydrophobic siloxane substituent Si–O–Si(CH₃)₂–CH₂CH₂R_F. Their observation is in agreement with our findings, indicating that A4 and A5 aerogels bearing long fluorinated chains are highly opaque. Previously we have shown that silica aerogel modified by 20 mol% of trifluoroacetamide moieties and synthesized by the similar approach appeared to be transparent.¹¹ F : Si molar ratio in this material was found to be close to the theoretical value of 0.6. The F : Si theoretical value for the most transparent A2 aerogel (10 mol% of perfluoropropionic moieties) is nearly the same (F : Si = 0.5). A1 aerogel (F : Si = 0.3) is also quite transparent. In turn, F : Si molar ratios for poorly transparent A4 and A5 aerogels are much higher (1.3 and 1.7, correspondingly). Thus we can conclude that fluorinated silica aerogels bearing amide group possess high transparency when F : Si ratio is less than ∼1.3. Supposedly, the changes in F : Si molar ratio could affect the mesostructured of aerogels and, consequently, light scattering in these materials. High transparency of highly hydrophobic silica aerogels modified by perfluorinated organic groups is quite an unusual property that needs further detailed study.

4. Conclusions

A facile method of the fine tuning of the hydrophobicity of silica aerogels is proposed, based on acylation of APTMS by polyfluorocarboxylic acids bearing fluorinated substituents of different length (CF₃–, C₂F₅–, CF₃CF(OCH₃)–, C₆F₁₃–, C₈F₁₇–) with a formation of fluorinated amides (CH₃O)₃Si(CH₂)₃–NH–C(O)–R_F. Upon cogelation procedure with Si(OCH₃)₄ and supercritical drying in chosen solvents (CO₂, IPA, MTBE and HFIP), fluorinated aerogels were prepared.

Such an approach allowed the obtaining of aerogel samples with the water contact angle changing from 0 to 143° and the specific surface area from 290 to 880 m² g⁻¹. The influence of the type of SCD solvent on the texture characteristics and hydrophobicity of aerogels was demonstrated. The optical transmittance of bulk aerogels also substantially depended on the length of the fluorinated substituent.

Acknowledgements

Financial support from the Russian Science Foundation (project 14-13-01150, aerogels synthesis, texture properties) and Russian Foundation for Basic Research (project 16-29-10736, hydrophobicity and transparency determinations) is greatly appreciated.

References

1. N. Hüsing and U. Schubert, Angew. Chem., Int. Ed., 1998, 37, 22–45.
2. A. V. Rao, N. D. Hegde and H. Hirashima, J. Colloid Interface Sci., 2007, 305, 124–132.
3. L. W. Hrubesh, P. R. Coronado and J. H. Satcher Jr, J. Non-Cryst. Solids, 2001, 285, 328–332.
4. J. G. Reynolds, P. R. Coronado and L. W. Hrubesh, J. Non-Cryst. Solids, 2001, 292, 127–137.
5. K. Wörmeyer, M. Alnaief and I. Smirnova, Adsorption, 2012, 18, 163–171.
6. W.-C. Li, M. Comotti, A.-H. Lu and F. Schüth, Chem. Commun., 2006, 1772–1774.
7. C. A. Morris, M. L. Anderson, R. M. Stroud, C. I. Merzbacher and D. R. Rolison, Science, 1999, 284, 622–624.
8. K. Tajiri, K. Igarashi and T. Nishio, J. Non-Cryst. Solids, 1995, 186, 83–87.
9. S. A. Lermontov, A. N. Malkova, L. L. Yurkova, E. A. Straumal, N. N. Gubanova, A. Y. Baranchikov and V. K. Ivanov, Mater. Lett., 2014, 116, 116–119.
10. S. Lermontov, A. Malkova, L. Yurkova, E. Straumal, N. Gubanova, A. Baranchikov, M. Smirnov, V. Tarasov, V. Buznik and V. Ivanov, J. Supercrit. Fluids, 2014, 89, 28–32.
11. S. A. Lermontov, N. A. Sipyagina, A. N. Malkova, A. V. Yarkov, A. E. Baranchikov, V. V. Kozik and V. K. Ivanov, RSC Adv., 2014, 4, 52423–52429.
12. H. Yokogawa and M. Yokoyama, J. Non-Cryst. Solids, 1995, 186, 23–29.
13. L. Wu, Y. Huang, Z. Wang, L. Liu and H. Xu, Appl. Surf. Sci., 2010, 256, 5973–5977.
14. A. V. Rao, S. D. Bhagat, H. Hirashima and G. M. Pajonk, J. Colloid Interface Sci., 2006, 300, 279–285.
15. G. Hayase, K. Kanamori and K. Nakanishi, J. Mater. Chem., 2011, 21, 17077–17079.
16. S. Yun, H. Luo and Y. Gao, RSC Adv., 2014, 4, 4535–4542.
17. J. Feng, D. Le, S. T. Nguyen, T. C. Nien, V. D. Jewell and H. M. Duong, Colloids Surf., A, 2016, 506, 298–305.
18. Y. Cheng, L. Lu, W. Zhang, J. Shi and Y. Cao, Carbohydr. Polym., 2012, 88, 1093–1099.
19. J. Shi, L. Lu, W. Guo, J. Zhang and Y. Cao, Carbohydr. Polym., 2013, 98, 282–289.
20. R. Lin, A. Li, T. Zheng, L. Lu and Y. Cao, RSC Adv., 2015, 5, 82027–82033.
21. A. Roig, E. Molins, E. Rodríguez, S. Martínez, M. Moreno-Mañas and A. Vallribera, Chem. Commun., 2004, 2316–2317.
22. T. M. Tillotson, K. G. Foster and J. G. Reynolds, J. Non-Cryst. Solids, 2004, 350, 202–208.
23. E. Y. Ladilina, V. V. Semenov, Y. A. Kurskii, O. V. Kuznetsova, M. A. Lopatin, B. A. Bushuk, S. B. Bushuk and W. E. Douglas, Russ. Chem. Bull., 2005, 54, 1160–1168.
24. E. Y. Ladilina, T. S. Lyubova, V. V. Semenov, Y. A. Kurskii and O. V. Kuznetsova, Russ. Chem. Bull., 2009, 58, 1015–1022.
25. E. Y. Ladilina, T. S. Lyubova, V. V. Semenov, O. V. Kuznetsova, A. I. Kirillov, V. I. Faerman, A. Y. Dolgonosova, M. A. Baten'kin and M. A. Lopatin, Russ. Chem. Bull., 2010, 59, 590–597.
26. S. K. Thanawala and M. K. Chaudhury, Langmuir, 2000, 16, 1256–1260.
27. V. V. Tomina, G. R. Yurchenko, A. K. Matkovsky, Y. L. Zub, A. Kosak and A. Lobnik, J. Fluorine Chem., 2011, 132, 1146–1151.
28. S. A. Lermontov, A. N. Malkova, N. A. Sipyagina, A. E. Baranchikov, D. I. Petukhov and V. K. Ivanov, Russ. J. Inorg. Chem., 2015, 60, 1169–1172.
29. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Mouscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 1985, 57, 603–619.
30. S. Lowell, J. E. Shields, M. A. Thomas and M. Thommes, Characterization of Porous Solids and Powders: Surface Area, Pore Size and Density, Springer, Netherlands, 2004, p. 349.
31. J. L. Gurav, I.-K. Jung, H.-H. Park, E. S. Kang and D. Y. Nadargi, J. Nanomater., 2010, 2010, 1–11.
32. L. Mayrhofer, G. Moras, N. Mulakaluri, S. Rajagopalan, P. A. Stevens and M. Moseler, J. Am. Chem. Soc., 2016, 138, 4018–4028.
33. S. A. Lermontov, L. L. Yurkova and N. V. Kuryleva, J. Fluorine Chem., 2008, 129, 332–334.
34. S. J. Gregg and K. S. W. Sing, Adsorption, surface area and porosity, Academic Press, London, 2nd edn, 1982.
35. B. H. W. S. De Jong, R. G. C. Beerkens and P. A. van Nijnatten, Glass, Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, 2000.
36. Kirk-Othmer encyclopedia of chemical technology, ed. R. E. Kirk and D. F. Othmer, John Wiley & Sons, Inc., New York, 4th edn, 1998, vol. 11.

---

Footnote

† Electronic supplementary information (ESI) available: NMR and mass-spectral data, elemental analysis data, differential thermogravimetric curves, SEM images and appearance of aerogels. See DOI: 10.1039/c6ra15444a

---