1 Introduction

Wetability plays an important role in many natural and technological processes. The wettability of solid surface is governed by both chemical composition and geometrical microstructure on the surface [1]. At the present time, hydrophobic silica aerogels have been proved to be promising material in various fascinating applications like absorption of organic liquids/oils [2] transportation of liquid on nanoscale in chemical and biotechnological applications and adsorption of toxic substances [3]. Schwertfeger et al. [4] reported the use of silica aerogels as potential drug carrier materials in medicine. For using aerogels in such applications, the capability of silica aerogels to tune wide range of surface free energy, which plays very crucial role in terms of their surface wettability properties. As organic liquid have low surface tension values, it is important to tune the surface free energy of silica aerogel surface such that the organic non-polar liquid wet the aerogel surface fully and get easily absorbed and at the same time the surface is nonwettable for polar solvents like water, glycerol, etc. that have high surface tension values. The aerogels get deteriorated with time due to the adsorption of water molecules from the humid surroundings because they posses on their surface, polar OH groups that can take part in hydrogen bonding with H2O [5, 6]. The replacement of H from the Si–OH groups by the hydrolytically stable Si–R groups through the oxygen bond prevents the adsorption of water and hence results in the hydrophobic aerogels [7, 8]. In general, a smooth, homogeneous solid surface can be made hydrophobic by reducing its surface energy using a suitable chemical modification [9].

In our previous work, Shewale et al. [10] and Rao et al. [5] have studied the physical properties of sodium silicate based silica aerogels by varying the molar ratio of silylating reagent and sodium silicate. Mahadik et al. [11] studied the effect of variation in % of silylating agent on apparent surface free energy of TEOS based silica aerogels, estimated from Neumann’s equation of state and Young’s equation. But Young’s equation developed by considering the surface is ideally flat. Therefore, in this paper we have selected Lifshitz–van der Waals/acid–base approach to get nearly accurate apparent surface free energy values of chlorotrimethylsilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ) modified silica aerogels by varying the % of silylating reagent during the silylation process. To attach more methyl groups to the surface of silica aerogels % of silylating reagents were varied from the 5 to 12 %. The silica gels were prepared using hydrolysis and condensation of tetraethoxysilane (TEOS) precursor in methanol solvent using two step acid–base catalyzed sol–gel process. The surface modifications of the gels were carried out using TMCS, DMDCS and HMDZ silylating reagents in the volume ratio from 5 to 12 % of the solvent to modify the surface free energy of the silica aerogels. The obtained silica aerogels were studied by FTIR spectroscopy for the confirmation of surface modifications of aerogels. Thermo-gravimetric and differential thermal analysis carried out to study thermal stability and hydrophobicity temperature limit of the silica aerogels. The static contact angle measurement data is used to estimate the apparent surface free energy of silica aerogel surface. The surface free energy of silica aerogel samples were determined by analyzing the contact angles in terms of the Lifshitz–van der Waals/acid–base approach.

2 Method for surface free energy determination

The hydrophobicity of the silica aerogel depends on the chemical composition of the solid surface and surface tension of liquid used [11]. The contact angle measurement data has been used to calculate approximately the surface free energy values of silica aerogel samples. Surface free energy plays a very important role in various physicochemical processes and technological applications and it also appears to be an important surface-characterizing parameter. In recent years many studies on determination of the surface free energy of solids have been carried out, but such determination is still an open problem, despite many attempts to solve it. Since there is no direct reliable method for measuring this quantity, indirect methods have been used.

In the present study, the surface free energy of silica aerogel was determined using Lifshitz–van der Waals/acid–base approach. For this we used two non-polar liquids and one polar liquid. The limitations of Neumann’s method are overcame by this method which gives polar and apolar components of surface free energy of aerogel sample.

2.1 Lifshitz–van der Waals/acid–base approach

The Lifshitz–van der Waals/acid–base (LWAB) approach to determine solid surface free energy proposed by van Oss et al. [12], presented a very useful method for surface free energy formulation, which in most cases is due to hydrogen bonding between hydrogen and oxygen atoms. According to them, noncovalent surface and interfacial interactions give rise to Lifshitz–van der Waals (LW) apolar component and electron-donor/electron-acceptor (or Lewis acid–base) (AB) polar component. In many systems electrostatic interactions can be neglected. Thus, the total surface free energy of solids, \( \gamma_{S}^{TOT} \), is the sum of the apolar, \( \gamma_{S}^{LW} \), and polar \( \gamma_{S}^{AB} \), components:

$$ \gamma_{S}^{TOT} = \gamma_{S}^{LW} + \gamma_{S}^{AB} $$
(1)

The acid–base component of the surface free energy of a solid is the geometric mean of two parameters:

$$ \gamma_{S}^{AB} = 2(\gamma_{S}^{ - } \gamma_{S}^{ + } )^{1/2} $$
(2)

where, \( \gamma_{S}^{ - } \) is the electron-donor (Lewis base) and \( \gamma_{S}^{ + } \) is the electron-acceptor (Lewis acid) parameter.

In the case of solid and liquid phases the interfacial free energy can be expressed as:

$$ \gamma_{SL} = \gamma_{S} + \gamma_{L} - W_{A} $$
(3)

where, WA is the work of liquid adhesion to the solid surface, which becomes:

$$ W_{A} = 2[(\gamma_{S}^{LW} \gamma_{L}^{LW} )^{1/2} + (\gamma_{S}^{ - } \gamma_{L}^{ + } )^{1/2} + (\gamma_{S}^{ + } \gamma_{L}^{ - } )^{1/2} ] $$
(4)

Combining Eqs. (3) and (4) with Young equation:

$$ \gamma_{S} = \gamma_{L} \cos \theta + \gamma_{SL} $$
(5)

where \( \theta \) is measured contact angle. The work of adhesion can be expressed by the relationship:

$$ W_{A} = \gamma_{L} (1 + \cos \theta ) $$
(6)

By comparing Eqs. (4) and (6) we get;

$$ \gamma_{L} (1 + \cos \theta ) = 2[(\gamma_{S}^{LW} \gamma_{L}^{LW} )^{1/2} + (\gamma_{S}^{ - } \gamma_{L}^{ + } )^{1/2} + (\gamma_{S}^{ + } \gamma_{L}^{ - } )^{1/2} ] $$
(7)

Equation (7) is often referred to as the van Oss–Chaudhary–Good equation, which provides a simple means to characterize a solid surface using contact angle measurements with three probe liquids, two of which contain different acid and base parameters of surface tension [12]. For determination of \( \gamma_{S}^{ - } \) and \( \gamma_{S}^{ + } \) (using Eq. (7)) two bipolar liquids are needed. The most important of these liquids is water. The other two high surface tension liquids used in present study are formamide and diiodomethane.

3 Materials and method

3.1 Experimental procedure

Initially, the sol was prepared using tetraethoxysilane (TEOS) diluted in methanol (MeOH) solvent and was partially hydrolyzed with water under acidic conditions with oxalic acid (0.0005 M). The sol was stirred for 1 h and kept for hydrolysis for 12 h. The molar ratio of TEOS:MeOH:oxalic acid:NH4OH:NH4F:TMCS was kept constant at 1:16.5:0.71:0.58:0.60:0.98 respectively. After hydrolysis base catalysts were added in the sol and after gelation, gel kept in oven for 45 min, for strengthening the gel network. After aging of the alcogels, they were cut into small cubic pieces. The methanol was decanted out and hexane was added. The methanol in the gel was exchanged with hexane at 50 °C. The alcogels in hexane was shaken at 120 rpm for 4 h in a shaker (Remi instruments, Mumbai, India). To make the gels hydrophobic, the gels were immersed in a liquid mixture containing sylilating agents, TMCS or DMDCS or HMDZ and hexane for silylation and kept in oven for 16 h at 50 °C. The percentage of syllilating agents was varied from 5 to 12 % of the hexane. The untreated sylilating agent was exchanged with the hexane by keeping again in the shaker for 4 h at 50 °C with 120 rpm speed. Finally, the alcogel pieces along with a little solvent were kept in the bottle. The bottle was covered with aluminum foil with 8–10 small pin holes to allow the evaporation of the solvent. These bottles were kept in the oven at 50 °C for 1 h and at 150 °C for 2 h. The retrieved aerogels were cooled to room temperature and used for determination of surface free energy by measurement of contact angle with three different probe liquids.

3.2 Reaction mechanism

The hydrolysis and condensation of methanol diluted tetraethoxysilane (TEOS) in the presence of oxalic acid, NH4OH and NH4F is shown in the following chemical reactions:

$$ {\text{Si}}({\text{OC}}_{2} {\text{H}}_{5} )_{4} + 4{\text{H}}_{2} {\text{O}}\mathop{\longrightarrow}\limits^{{{\text{C}}_{2} {\text{H}}_{2}{\text{O}}_{4}}} {\text{Si}}({\text{OH}})_{4} + 4{\text{C}}_{2}{\text{H}}_{5} {\text{OH}} $$
(8)
$$ {\text{Si}}({\text{OH}})_{4} + ({\text{OH}})_{4}{\text{Si}}\mathop{\longrightarrow}\limits^{{\text{NH}}_{4} {\text{OH}}+ {\text{NH}}_{4} {\text{F}} } {\text{(OH)}}_{\text{3}}{\text{Si}}{-}{\text{O}}{-}{\text{Si}}({\text{OH}})_{3} +{\text{H}}_{2} {\text{O}} $$
(9)

The hydrophobicity was achieved by surface chemical modification of alcogels by different percentage of sylilating agents TMCS, DMDCS and HMDZ in hexane, respectively. The foremost chemical reactions are given below [10]:

3.3 Methods of characterization

The organic and inorganic bonds present in an aerogel samples were studied by Fourier transform infrared spectroscopy (FTIR) using a Perkin Elmer (model No. 1760X in 450–4,000 cm−1 range) IR spectrophotometer. Thermal stability of the hydrophobic silica aerogels in terms of retention of hydrophobicity was observed using thermo-gravimetric and differential thermal analysis (TG–DTA, SDT, 2960TA, USA) within the temperature range from room temperature to 1,000 °C. Hydrophobicity of the aerogel samples was tested in terms of contact angle (θ) of the water droplet with the aerogel surface using Rame–Hart contact angle meter, USA. The surface chemical composition of the aerogel samples was analyzed by using X-ray Phtoelectron Spectroscopy (XPS, VG Multilab 2000, Thermo VG Scientific, UK).

4 Results and discussion

4.1 Fourier transform infrared spectroscopy

Figure 1 shows the FTIR spectra of (a) silica aerogel modified using HMDZ, (b) silica aerogel modified using TMCS and (c) silica aerogel modified using DMDCS. In the figure, the absorption peaks around 2,950 and 740 cm−1 are due to C–H bonds and those at 1,260 and 850 cm−1 corresponds to Si–C bonding [13, 14]. The broad absorption band at around 3,450 cm−1 and the sharp peak around 1,600 cm−1 are due to –OH groups. The prominent peak at around 1,100 cm−1 in the spectra is due to the asymmetric bending mode of SiO2. The intensity of the absorption bands due to Si–C at 850 cm−1 and C–H at 2,950 cm−1 are stronger in the case of DMDCS modified aerogels as shown in Fig. 1c. However, in the case of HMDZ modified silica aerogels as shown in Fig. 1a, intensity of the absorption bands corresponding to Si–C and C–H is very less. It clearly confirms that surface modification is more in the case of DMDCS modified aerogels as compare to TMCS and HMDZ modified silica aerogels. The results of FTIR studies have been confirmed by contact angle measurements of the aerogels.

Fig. 1
figure 1

FTIR spectra of the a silica aerogel modified using HMDZ, b silica aerogel modified using TMCS and c silica aerogel modified using DMDCS

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4.2 Contact angle measurement and evaluation of surface free energy

The contact angle measurement was carried out by using sessile drop method for water, formamide and diiodomethane as shown in Table 1. In case of TMCS, DMDCS and HMDZ modified silica aerogel samples the contact angle of water and formamide increased with an increase in the percentage of sylilating agent from 5 to 12 % (Table 1). The increase in contact angle indicate the increase in hydrophobicity of silica aerogels which is due to replacement of polar –OH groups on the surface by non-polar CH3 groups due to sylilation. The diiodomethane is non-polar liquid so the values of contact angle with surface modified silica aerogel samples are less than 90°.

Table 1 Physical properties and surface free energy components of test liquids used [15]
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In wetting, spreading and adhesion processes, surface free energy interactions are of crucial importance. A liquid in contact with solid exhibits a contact angle (θ) which depends upon the surface tensions of the liquid and the solid as well as the interfacial tension between them. The Table 1 shows the known values of the surface tension parameters of the liquids [15] used for the determination of surface free energy of silica aerogel samples modified with different sylilating agents. The contact angles obtained by different test liquids and the surface free energy components calculated on the basis of the Lifshitz–van der Waals/acid–base model of silica aerogel samples are shown in Table 2. The surface free energy components are calculated by using Eq. (7), which constitutes a set of simultaneous linear equations which can be solved mathematically. As can be seen in Table 2, \( \gamma_{S}^{TOT} \)of silica aerogel samples modified with TMCS is varies from 36.96, 45.95, 43.16 and 38.57 mJ/m2 for that of 5, 7, 10 and 12 % TMCS percentage respectively. In case of HMDZ, \( \gamma_{S}^{TOT} \)varies in very small range as shown in the Table 2. For DMDCS, the values of \( \gamma_{S}^{TOT} \) varies from 36.32, 51.42, 46.17, 49.08 mJ/m2 with increasing the percentage of DMDCS from 5, 7, 10 and 12 % respectively shown in Table 2. These values of surface free energy of silica aerogels are approximately matched with the values in our earlier publication [16]. In the present study the suggestion of de Meijer et al. [17] has been applied, i.e. if a negative value was obtained during the calculation of the surface free energy components, the component was taken into account as a negative contribution to the total surface free energy of solid surface. This assumption is necessary to use because of the solution of the linear equation system (derived from Eq. (7)) results in square roots of the surface free energy components are derived. Theoretically, the surface free energy value can only be a positive value, thus its square root can also be only positive. In Ref. [17] the appearance of the negative square roots was explained by the surface roughness of these samples. The application of “negative contributions” results in realistic surface free energy values for the treated aerogel surfaces.

Table 2 Contact angles obtained by different test liquids and the surface free energy components calculated on the basis of the Lifshitz–van der Waals/acid–base model of silica aerogel samples
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4.3 Elemental analysis

The XPS spectrum of DMDCS modified silica aerogel sample is shown in Fig. 2. The strong peak of O1s was observed at 531.7 eV in the spectrum. The carbon peak is observed at 290 eV and demonstrates the replacement of hydroxyl groups with methyl groups in silica aerogels. This was also confirmed from the FTIR study. The peaks observed at Si2p [ 18] and Si2 s with binding energy values 98.7, 148.7 eV are found in the spectrum. The C1s peak at 284.7 eV shows C–H content in the silica aerogel. The surface of silica aerogels is composed of oxygen, silicon, carbon, and hydrogen atoms. For DMDCS modified aerogels, oxygen amount was found to be 45.65 %, carbon amount was found to be 20.85 % and for silica it was found to be 29.12 % from the XPS studies.

Fig. 2
figure 2

XPS spectra of DMDCS modified silica aerogels

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4.4 Thermal stability analysis

Silica aerogel powder has been subjected to TG–DTA studies in order to know at what temperature methyl groups are decomposing. TG– DTA curves were recorded in air at a heating rate of 10 °C/min from room temperature to 1,000 °C. For TMCS modified silica aerogels, the noticeable weight loss was observed at around 452 °C in TGA curve, accompanied with an exothermic peak in the DTA curve as shown in Fig. 3. The sudden weight loss at around 452 °C is due to the decomposition of alkyl (–CH3) groups where as the exothermic peak in DTA curve is due to the oxidation of alkyl groups. In case of DMDCS and HMDZ modified silica aerogels, the decomposition of alkyl groups was observed at 275 and 320 °C respectively as shown in Figs. 4 and 5. This difference in the decomposition temperature of methyl groups may be attributed to the slight changes in the bond length due to difference in the structural formula of the silylating agents. It reveals that the thermal stability of TMCS modified silica aerogels is up to 452 °C and that in case of DMDCS and HMDZ modified silica aerogels is up to 320 and 275 °C. When heated above these temperatures respectively, the aerogels showed hydrophilic nature.

Fig. 3
figure 3

Thermo gravimetric and differential thermal analysis (TG–DTA) of TMCS modified aerogel sample

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Fig. 4
figure 4

Thermo gravimetric and differential thermal analysis (TG–DTA) of DMDCS modified aerogel sample

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Fig. 5
figure 5

Thermo gravimetric and differential thermal analysis (TG–DTA) of HMDZ modified aerogel sample

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5 Conclusions

The wetting behaviour of silica aerogels were studied in detail by determining contact angle and surface free energy. The surface of TEOS based silica aerogels was modified using three different silylating reagents viz. trimethylchlorosilane (TMCS), dimethyldichlorosilane (DMDCS) and hexamethyldisilazane (HMDZ). The silylation process enhances the hydrophobicity of the aerogels due to replacement of hydroxyl groups of silanol with the methyl groups in the retrieved silica aerogels and this is validated by FTIR and XPS studies. The Lifshitz–van der Waals/acid–base approach gives the surface free energy values are 45.95, 51.42 and 45.69 mJ/m2 by modifying their surfaces using 7 % TMCS, DMDCS and HMDZ silylating reagents, respectively and these values of surface free energy are in good agreement with earlier results. Also, from this studies it is concluded that one can change the wettbility properties of aerogels with the variation in precursor chemicals, which leads application of silica aerogels in medical fields as a carrier.