Chapter - 4. A new route for preparation of sodiumsilicate-based. via ambient pressure drying

Transcription

1 A new route for preparation of sodiumsilicate-based hydrophobic silica aerogels via ambient pressure drying

2 A new route for preparation 59 A new route for preparation of sodium silicate based hydrophobic silica aerogels via ambient pressure drying 4.1 Introduction Traditionally, silica aerogels are obtained by removing the liquid from a wet gel by supercritical drying without any shrinkage that are composed of highly cross-linked network of silica particles [1]. This method of drying of the gel is expensive, risky to operate, very tedious as well as time consuming. However, in 1968, Prof. S. J. Teichner at University Claud, Bernard in Lyon, France developed a method for producing the silica aerogels within a day using (albeit costly) silicon alkoxide precursors [2]. But, for commercial production, though, there is a need to produce the silica aerogels using lowcost precursor such as sodium silicate as well as for drying the wet gels at ambient pressure. Pure silica aerogels are hydrophilic and became wet with humid atmosphere and get deteriorated with time due to adsorption of water molecule from the humid surroundings because they posses polar OH groups on their surface that can take part in hydrogen bonding with H 2 O [3]. Replacement of H from Si-OH groups by hydrolytically stable Si-R groups through oxygen bond prevents the adsorption of water and hence results in hydrophobic aerogels [4]. In continuation of research work on the hydrophobic aerogels, Schwertfeger and co-workers [5] have produced the silica aerogels using water glass precursor by costly ion exchange resin (lengthy and time consuming process) to remove sodium salt following surface modification and an ambient pressure drying method. However, in this chapter the ion-exchange method for the removal of sodium salt is replaced by simply washing the gels with water followed by solvent exchange, surface modification and drying at ambient pressure. 4.2 Experimental procedure Sample preparation Preparation of the hydrophobic silica aerogels by ambient pressure drying using the sodium silicate solution is depicted schematically in fig. 4.1.

3 A new route for preparation 60 (a) Sol Na 2 SiO 3 solution +Tartaric acid Gelation 50 o C (b) Hydrogel 3 h aging at 50 o C (c) Aged gel (d) Salt-free gel 4 times gel washing with water in 24 h Drying at R.T. for 24 h and 50, 200 o C for 1 h each MeOH:TMCS:Hexane 1 : 1 : 1 volume ratio Surface modification Exchange with methanol once in 24 h (g) Hydrophobic Silica Aerogel (f) Surface modified gel (e) Alcogel Fig. 4.1 Schematic preparation of silica aerogels The chemicals used were: sodium silicate solution (Na 2 SiO 3, LOBA, India, Na 2 SiO 3 content 36 wt%, Na 2 O:SiO 2 = 1:3.33) of specific gravity 1.05 diluted from 1.36 specific gravity as a precursor, tartaric acid (C 4 H 6 O 6 ) (Merck Company, Mumbai) as a catalyst and reactant, trimethylchlorosilane (TMCS) (Fluka, Pursis grade, Switzerland) as a surface modifier, methanol (MeOH, CH 3 OH) and hexane (C 6 H 14 ) (Merck, India) as solvents. Double distilled water was used to prepare the sodium silicate and tartaric acid solutions. Silica hydrosols were prepared by adding 3.6 M tartaric acid dropwise to a sodium silicate solution of 1.05 specific gravity while stirring for 5 minutes and kept for gelation at 50 o C in a temperature controlled oven. After gelation, the gels were aged for 3 h at 50 o C to strengthen the gel network. The gels were then washed four times with water over the course of 24 h. Next, methanol was exchanged into the gels and surface modification was carried out by soaking the gels in a mixture of methanol:tmcs:hexane with a volume ratio of 1:1:1, respectively, for 24 h. The position of gels in water, methanol and silylating mixture is shown in fig Notably, gels sank in the water and methanol but floated in the silylating mixture. After decanting the

4 A new route for preparation 61 solvents, the silylated gels were then ambiently dried for 24 h followed by heating at 50 o C for 1 h and then 200 o C for 1 h. After cooling of oven up to room temperature, the aerogels were removed from the oven, and the resulting aerogels were used for characterization. (a) (b) (c) Fig. 4.2 Position of gel in (a) water, (b) methanol and (c) silylating mixture Methods of characterization Bulk density of the aerogels was calculated using a known volume of the aerogels and dividing by their mass (measured by microbalance, 10-5 g precision). Volume shrinkage and porosity of aerogels were calculated as explained in our previous paper [6]. The degree of hydrophobicity was quantified by measuring the contact angle (θ) of a water droplet placed on the aerogel surface. It was measured by using a travelling microscope (least count cm) using the formula [7], θ = 2 tan -1 (2h/b) --- (4.1) where h is the height and b is the base width of the water droplet on the aerogel surface. Contact angle was also measured with a contact angle meter (rame-hart instrument, USA). The surface modification of the aerogels was confirmed by Fourier Transform Infrared Spectroscopy (FTIR) studies. Thermal stability of the aerogels was tested by Thermogravimetric Analysis- Differential Thermal Analysis (TGA-DTA) using a 2960 TA Universal Instrument, USA.

5 A new route for preparation Results and discussion Effect of gel washings with water on optical transmission (%) Gel washing with water removes trapped salt in the pores of gel network. The effect of the gel washings with water on the optical transmission (%) of the aerogels was studied by keeping the Tartaric acid: Na 2 SiO 3 molar ratio constant at 1.08 and varying it from 1 to 4 in 24 h Optical transmission (%) Optical transmission (%) Number of gel washings 10 Fig. 4.3 Effect of number of gel washings on the optical transmission (%) and density It was observed that with the increase in number of washings from 1 to 4, the aerogel optical transmission (%) increased from 20 to 50 % while aerogel density decreased from to g/cc (see Fig. 4.3). This is due to the fact that sodium tartarate, which is formed during hydrolysis, becomes trapped in the pores of the gel network causing a decrease in the optical transmission and increase in the density of the aerogels. Since the solubility of sodium tartarate in water is low (29 g/100 ml). Therefore, multiple washings are required to remove the salt from the pores of the gel to enhance the transparency of aerogels. The best method of quantitative

6 A new route for preparation 63 extraction of solute from one solvent to another is to employ the several washings instead of one [8]. The quantity of Na + ions present in the pores of the aerogels was estimated by Atomic Absorption spectroscopy (AA) and found to be 1.23 %, while based on stoichiometry the hydrosol is known to contain 36.5 % Na + ions. Hence, washing the gel with water after aging decreases Na + ions percentage from 36.5 to 1.23 producing the transparent aerogels Effect of hexane (or methanol) percentage in silylating mixture In silylation process, hexane is used as an inert dilution medium and MeOH is used to eliminate remained water from the pores of alcogel. The effect of hexane percentage on physical properties of the silica aerogels was studied by varying it from 0 to 100 % while keeping the Na 2 SiO 3 :H 2 O:Tartaric acid:tmcs molar ratio constant at 1:146.67:0.86:9.46 (Table 4.1). Fig. 4.4 shows the gel position for mixtures containing 0, 50 and 100 % hexane in methanol. In panel (a) the gel did not float for 0 % hexane, while in panels (b) and (c) gel floated completely in the solution with 50 % hexane and partially floated in 100 % hexane respectively. This is because, for complete silylation of the gel, an inert medium (hexane) is one of the requirements. (a) (b) (c) Fig. 4.4 Position of gel in (a) 0%, (b) 50% and (c) 100% hexane (or methanol) As shown in fig. 4.5, it was observed that volume shrinkage (%) and density of the silica aerogels decreased with an increase in hexane concentration to 50 % in silylation mixture, and then increased with a further

7 A new route for preparation 64 increase in hexane concentration up to 100 %. The reason for this is that at 0 % hexane (100 % MeOH) due to absence of inert medium, which reduces the reaction rate of TMCS with pore water, the silylation of the surface does not occur systematically. Also, the low surface tension of hexane helps to reduce the capillary pressure which is associated with drying shrinkage. Hence due to incomplete surface modification more shrinkage occurs in the gels producing the dense aerogels. And at 100 % hexane (0 % MeOH) due to absence of MeOH, which facilitates polar intermediates in silylation, silylation does not occur as effectively and again the density of the aerogels increases. The effects of presence of hexane and MeOH in mixture are dependent on each other. On the other hand, at 50 % hexane and 50 % MeOH, sufficient surface modification occurs resulting in low shrinkage and low density (0.084 g/cc) aerogels. 220 Volume shrinkage (%) Volume shrinkage (%) Hexane percentage 20 Fig. 4.5 Effect of hexane percentage in silylating mixture on density and volume shrinkage (%) of the aerogels

8 A new route for preparation 65 Table 4.1 Porosity, pore volume, contact angle and thermal conductivity of silica aerogels with variation of the sol-gel parameters Sr. No. Variation Porosity (%) Pore volume (cc/g) Contact angle (θ, deg.) Thermal conductivity (W/m.K) A Effect of percentage of hexane (or methanol) (%) B Effect of aging period (hours) C Effect of weight % of silica Influence of Tartaric acid: Na 2 SiO 3 molar ratio (A) The influence of Tartaric acid: Na 2 SiO 3 molar ratio (A) on the physical properties of the silica aerogels was studied by varying it from 0.27 to 1.2 (Table 4.2). The gel aging period and Na 2 SiO 3 :H 2 O:TMCS molar ratio were kept constant at 3 h and 1:146.67:9.46, respectively. During the gel formation the hydrolysis and condensation reactions take place as follows,

9 A new route for preparation 66 Hydrolysis: Tartaric acid OH HO H Na 2 SiO 3 + H 2 O HO Si OH + NaOOC C C COONa ---(4.2) C 4 H 6 O 6 OH H OH Sodium silicate solution Silicic acid Sodium tartarate Condensation: OH OH OH OH HO Si OH + HO Si OH HO Si O Si OH + H 2 O ---(4.3) OH OH OH OH Silicic acid 200 Log [Gelation time (min)] Log [Gelation time (min)] Tartaric acid:na 2 SiO 3 molar ratio Fig. 4.6 Effect of Tartaric acid:na 2 SiO 3 molar ratio on the gelation time and density The amount of catalyst added strongly affects the gelation time and density of the silica aerogels. As shown in fig. 4.6, it was observed that with

10 A new route for preparation 67 an increase in A to 0.51, the gelation time decreased and density increased. This is because, with increase in catalyst concentration, the rate of hydrolysis and condensation reactions increases, and as a result, silica clusters aggregate at a relatively faster rates to form a three dimensional, dense silica network in short time [9]. Further, the gelation time increased with increase in A (>0.51), however, possibly since the silica particles are negatively charged and, therefore, particles crosslinking is slowed down by charge repulsion. At lower A (<0.51) the gelation time may be high because silica particles are positively charged, and hence, repel each other [9]. The density of aerogels decreased with increase in A up to 1.08 due to the presence of excess tartaric acid which enhances the rate of hydrolysis and condensation reactions that lead to cluster formation, in turn resulting in denser aerogels [10]. At A~1.08, this is believed to occur because of complete hydrolysis and condensation of particles, formation of uniform network takes place, which led to low density (0.100 g/cc) aerogels. Table 4.2 Effect of Tartaric acid:na 2 SiO 3 molar ratio and TMCS percentage on physical properties of silica aerogels Sr. No. Variation D Volume shrinkage (%) Porosity (%) Pore volume (cc/g) Contact angle (deg.) Effect of Tartaric acid/na 2 SiO 3 molar ratio Thermal conductivity (W/m.K) E Effect of TMCS percentage (%)

11 A new route for preparation Effect of gel aging period Aging a gel before drying helps to strengthen the network and thereby reduces the risk of the fracture [11]. The effect of gel aging period on the volume shrinkage (%) and density of aerogels was studied with variation from 0 to 4 h (Table 4.1) by keeping Tartaric acid:na 2 SiO 3 molar ratio constant at At lower and higher gel aging periods (<3h<) the volume shrinkage (%) and density increased while at 3 h aging period volume shrinkage and density decreased as shown in fig This is because during the gel aging, a number of chemical and physical changes take place, such as condensation of surface OH groups, syneresis, coarsening and segregation, all of which strongly affects the properties of the aerogels [12]. The lower volume shrinkage (%) and bulk density of the aged gels indicates that they were coarse, means the dissolution and reprecipitation driven by differences in solubility between surfaces with different radii of curvature occurs. This causes growth of necks between particles, so the capillary pressure was lower and the aerogels were probably stiffer and stronger. 155 Volume shrinkage (%) Volume shrinkage (%) Gel aging period (hours) 40 Fig. 4.7 Effect of gel aging period on density and volume shrinkage

12 A new route for preparation Influence of weight % of silica (B) The influence of the weight % of silica (B) in the hydrosol on the physical properties of the silica aerogels was studied by varying it from 1.5 to 8 wt % (Table 4.1). The aerogels were aged for 3 h keeping Tartaric acid:na 2 SiO 3 molar ratio constant at From fig. 4.8, it can be seen that as value of B was increased to 5, gelation time decreased. As B was further increased (to a value of > 5), the gelation time remained constant. The volume shrinkage (%) of aerogels decreased and then increased with increase in B value from 5 to 8 wt %. This is likely since, at lower B value, lower silica content in the hydrosol slows the rate of hydrolysis and condensation reactions, resulting in longer gelation time and a weaker silica network. Shrinkage during the drying process in turn increases due to weak silica network. At higher B values, the rates of hydrolysis and condensation increase and cluster formation takes place, leading to shorter gelation time and higher silica content per unit volume (i.e., a denser aerogel) Volume shrinkage (%) Log [Gelation time (min)] 3.3 Volume shrinkage (%) Log [Gelation time (min)] Weight % of silica 0.3 Fig. 4.8 Effect of weight % of silica on gelation and volume shrinkage

13 A new route for preparation 70 During the drying process, evaporation of a liquid from the gel creates a capillary tension (P) in the liquid. This tension is balanced by the compressive stresses on the solid network, causing shrinkage of the dried gel. The stresses during the drying depend on the interfacial energies (surface tension of pore liquid), the bulk modulus of the network and the pressure gradient in the liquid. According to Darcy s law, the liquid flow (J) through gel is given by J = (D/η L ) P, ---(4.4) where D is the permeability of the gel, P is the pressure gradient, and η L is the viscosity of the liquid. During liquid evaporation, the pressure (P) in the liquid phase of the gel is related to the volumetric strain rate of the gel (έ) by (D/η L ) 2 P = - έ ---(4.5) The resulting stress in the solid phase of a gel plate of thickness L is given by [13]. σ x C N (Lη L V E /3D), ---(4.6) where C N (1-2N)(1-N), N is Poisson s ratio, and is the liquid evaporation rate. Equation (4.6) indicates that the stress is proportional to the thickness of the gel plate and the liquid evaporation rate. At the same time, if the permeability is high, then the stress is small. Hence at B~4, due to high permeability and low stress, the shrinkage of aerogel decreased resulting in low density silica aerogels.. V E As shown in fig. 4.9, at both lower and higher B value (<4<) the thermal conductivity of the aerogel is more because of greater shrinkage and thus higher density of the aerogel. At B~4, higher pertinent hydrolysis and condensation reactions result in less shrinkage and thus low density (0.084 g/cc) and low thermal conductivity (0.09 W/m.K) of the aerogel

14 A new route for preparation Thermal conductivity (W/m.K) Thermal conductivity (W/m.K) Weight % of silica 0.07 Fig. 4.9 Effect of weight % of silica on density and thermal conductivity Effect of TMCS percentage on silylation Drying of wet gels without surface modification causes the shrinkage of the gel due to continuous condensation of end OH groups leading to dense aerogels. This is because of capillary pressure exerted by pore fluid evaporation causes irreversible shrinkage in the aerogels. Capillary collapse in wet gel can be prevented by replacing hydrophilic OH groups on surface of gel backbone with non-reactive Si CH 3 species by means of surface modification with silane coupling agents such as TMCS. The capillary pressure generated during drying is given by Laplace equation [14]. P = -2 γ LV cos θ r p --- (4.7) where γ LV is the liquid-vapor surface tension, θ is the contact angle of the liquid with a pore wall and r p is the pore radius. The negative sign is due to

15 A new route for preparation 72 the negative radius of curvature of the meniscus at the liquid-vapor interface. TMCS minimizes the shrinkage of the gel through the reduction in surface tension of the solvent and contact angle between the solvent and surface of silica network [15]. Hence the hydrophobic aerogels are obtained by replacing the Hs from end capped silanol groups with non-polar hydrolytically stable Si-(CH 3 ) 3 groups [16] using TMCS as follows, Surface modification Si OH Cl Si (CH 3 ) 3 O + Si OH Cl Si (CH 3 ) 3 Silica surface Trimethylchlorosilane Si O Si (CH 3 ) 3 O + 2HCl ---(4.8) Si O Si (CH 3 ) 3 Modified silica surface Percentage of TMCS in silylating mixture was found to be a dominating parameter that affects the silylation and hence physical properties of silica aerogels (Table 4.2). The effect of TMCS percentage on silylation was studied by varying concentration of TMCS from 20 to 40 % while keeping Na 2 SiO 3 :H 2 O:Tartaric acid molar ratio constant at 1:146.67:0.86. Fig shows the decrease in the density and % of optical transmission of the aerogels with increase in TMCS percentage. This is likely due to the fact that at lower percentages of TMCS (< 33 %), incomplete silylation occurs and unsilylated OH groups can undergo condensation in turn causing more shrinkage and thus denser aerogels. Furthermore, because of smaller particle and pore sizes caused by increased shrinkage, the % of optical transmission of these aerogels was higher. At higher percentages of TMCS (> %), complete modification of silanol groups to non-polar, hydrolytically stable Si(CH 3 ) 3 groups occur and causes repulsion between end capped Si(CH 3 ) 3 groups. Because of this, spring back of the gels solid network occurs, facilitating an increase in the aerogel volume with big pores and thus low-density and semitransparent aerogels. At higher percentage of TMCS (> 33 %), the excess TMCS deposited in the pores causing opacity of the aerogels. So, for further studies 33 % TMCS was used. The hydrophobicity of aerogels increased with TMCS percentage, which is quantified by contact angle measurement as shown in fig

16 A new route for preparation Optical transmission (%) Optical transmission (%) TMCS percentage Fig Effect of TMCS percentage on density and optical transmission 15 (a) (b) 133 o 146 o Fig Water droplets on the aerogel surfaces for (a) 20 %, (b) 33 % TMCS The sphericity of a water drop on a solid surface is characterized by the contact angle (θ). Greater is the hydrophobicity of the solid surface, higher is the contact angle and larger would be the sphericity of the water drop. Under equilibrium conditions, the relation between the solid-vapour (γ SV ), solid-liquid (γ SL ) and liquid-vapour (γ LV ) interactions at the intersection of the three phases, is given by the Young s equation [17]:

17 A new route for preparation 74 γ SV = γ SL + γ LV cos θ --- (4.9) For a hydrophobic surface, θ > 90 o and therefore, from the above equation it follows that solid-liquid (γ SL ) interaction is greater than solidvapour (γ SV ) interaction. Fig (a & b) show the water droplets placed on the hydrophobic silica aerogel surfaces modified with 20 % and 33 % TMCS with contact angle (θ) is 133 o and 146 o, respectively. It is observed that the contact angle decreases with decrease in TMCS percentage Effect of silylation period Silylation period plays a significant role in the surface modification of gels. The effect of the silylation period on the physical properties of silica aerogels was studied by varying the silylation period from 6 to 24 h by keeping Na 2 SiO 3 :H 2 O:Tartaric acid:tmcs molar ratio constant at 1:146.67:0.86: Contact angle (degree) Contact angle (degree) silylation period (hours) 137 Fig Effect of Silylation period on density and contact angle Fig shows with increase of the silylation period, density of the aerogels decreased and hydrophobicity increased. This is believed to be

18 A new route for preparation 75 because, for shorter periods of silylation, incomplete surface modification of wet gels occurs leading to dense and less hydrophobic aerogels. For the longer silylation periods the bulk density decreases because of complete surface modification of the gel, increases the hydrophobicity of the aerogel. The effect of the silylation period on the porosity and thermal conductivity of silica aerogels is depicted in fig Increasing silylation period to 24 h led to an increase in porosity and decrease in the thermal conductivity. This may be because, after higher silylation periods, more complete surface modification facilitates better spring back. The thermal conductivity, which depends on the porosity of aerogels, is lower because there is less solid content per unit volume of the aerogel [18]. Spring back implies that the gels densify and then undensify upon drying [19]. Porosity (%) Porosity (%) Thermal conductivity (W/m.K) Thermal conductivity (W/m.K) Silylation period (hours) 0.08 Fig Effect of Silylation period on density and contact angle Fig shows the variation in % of volume change with drying temperature. It has been found that the % of volume change is more up to 100 o C and then decreased above 100 o C and remained constant above 150 o C, clearly indicating the effect of spring back.

19 A new route for preparation % of volume change Temperature (oc) ( o C) Fig Effect of drying temperature on % of volume change Further, the surface modification of the aerogel with the variation of silylation period from 6 to 24 h is confirmed by FTIR spectra of the aerogels as shown in fig % of optical transmission (A.U.) -OH -OH Wave number (cm -1 ) Fig Infrared spectra of aerogels with the variation of the Silylation period, (a) 6 h, (b) 12 h, (c) 18 h, and (d) 24 h.

20 A new route for preparation 77 It was observed that with increase in the silylation period, the intensity of OH bond peaks at 1600 and 3400 cm -1 [20] decreased and the peaks related to C-H at 2960, 1450 cm -1 and Si-C at 840 and 1260 cm -1 increased [21]. Thus, effect of surface modification with silylation period was clearly observed. Thermal stability of the hydrophobic silica aerogels silylated with 33% TMCS for 24 h was tested using TGA-DTA as shown in fig It shows that, the weight loss with an exothermic peak at 435 o C, was considered to correspond to the decomposition of surface -CH 3 groups [22]. The gradual weight loss at higher temperatures could be attributed to the dehydration and condensation of silanols. Thus, it clears that the silica aerogels have high heat-resistance up to around 435 o C. Weight (%) o C TGA DTA Temperature difference ( o C/mg) Temperature ( o C) Fig TGA-DTA of hydrophobic silica aerogel

21 A new route for preparation Conclusions Superhydrophobic, low density and semitransparent silica aerogels were obtained using a sodium silicate precursor by an ambient pressure drying method. Both, the gel washings with water and sol-gel parameters have striking effects on the physical properties of the silica aerogels produced by this technique. It was observed that for more gel washing times, the optical transmission (%) of the aerogel improved. Increasing the silylation period and TMCS percentage reduces the density of the aerogels. Also, the 50 % hexane (or methanol) in the silylating mixture produced the lowest density aerogels. From FTIR spectra of the aerogels, it was observed that the intensity of OH bond at 1600 and 3400 cm -1 decreased and C-H bond at 2960, 1450 cm -1, Si-C bond at 840 and 1260 cm -1 increased with increase in the silylation period. The TGA-DTA showed that the silica aerogels were thermally stable up to 435 o C. The semitransparent aerogels with density ~0.084 g/cc, porosity ~95 %, thermal conductivity ~0.090 W/m.K and hydrophobicity ~146 o were obtained for the molar ratio of Na 2 SiO 3 :H 2 O:Tartaric acid:tmcs at 1:146.67:0.86:9.46 respectively, with 4 times gel washing with water in 24 h, 3 h aging, 24 h silylation period and 50 % hexane (or methanol) in silylating mixture by ambient pressure drying method.

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