A new route for preparation of sodium- silicate-based hydrophobic …shodhganga.inflibnet.ac.in/bitstream/10603/4032/11/11... · 2015-12-04 · A new route for preparation of sodium

Chapter - 4

A new route for preparation of sodium-

silicate-based hydrophobic silica aerogels

via ambient pressure drying

A new route for preparation … Chapter - 4 59

Chapter - 4

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 low-

cost 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 H2O [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 4.2.1 Sample preparation

Preparation of the hydrophobic silica aerogels by ambient pressure

drying using the sodium silicate solution is depicted schematically in fig. 4.1.


The chemicals used were: sodium silicate solution (Na2SiO3, LOBA,

India, Na2SiO3 content 36 wt%, Na2O:SiO2 = 1:3.33) of specific gravity 1.05

diluted from 1.36 specific gravity as a precursor, tartaric acid (C4H6O6)

(Merck Company, Mumbai) as a catalyst and reactant, trimethylchlorosilane

(TMCS) (Fluka, Pursis grade, Switzerland) as a surface modifier, methanol

(MeOH, CH3OH) and hexane (C6H14) (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 oC in a temperature controlled oven. After

gelation, the gels were aged for 3 h at 50 oC 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. 4.2. Notably, gels sank in the

water and methanol but floated in the silylating mixture. After decanting the

Fig. 4.1 Schematic preparation of silica aerogels

Na2SiO3 solution +Tartaric acid

(g) Hydrophobic Silica Aerogel

(d) Salt-free gel

(a) Sol (b) Hydrogel

3 h aging at 50

oC

(c) Aged gel

4 times gel washing with water in 24 h

Exchange with methanol once in 24 h

(e) Alcogel

Gelation

Surface modifi-cation

(f) Surface modified gel

50oC

MeOH:TMCS:Hexane 1 : 1 : 1 volume ratio

Drying at R.T. for 24 h and 50, 200

oC

for 1 h each


solvents, the silylated gels were then ambiently dried for 24 h followed by

heating at 50 oC for 1 h and then 200 oC 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.

4.2.2 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 0.001 cm) using the formula [7],

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.

--- (4.1) θ = 2 tan-1 (2h/b)

Fig. 4.2 Position of gel in (a) water, (b) methanol and (c) silylating mixture

(c) (b) (a)


4.3 Results and discussion

4.3.1 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:

Na2SiO3 molar ratio constant at 1.08 and varying it from 1 to 4 in 24 h.

82

84

86

88

90

92

0 1 2 3 4 5

Number of gel washings

Den

sit

y (

g/c

c)

10

20

30

40

50

60

Op

tical

tran

sm

issio

n (

%)

Density (g/cc)

Optical transmission (%)

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 0.091 to 0.084 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

Fig. 4.3 Effect of number of gel washings on the optical transmission (%) and density


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.

4.3.2 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 Na2SiO3:H2O: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.

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

Fig. 4.4 Position of gel in (a) 0%, (b) 50% and (c) 100% hexane (or methanol)

(c) (b)(a)


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.

70

100

130

160

190

220

0 25 50 75 100Hexane percentage

Den

sit

y (

g/c

c)

20

30

40

50

60

70

Vo

lum

e s

hri

nkag

e (

%)

Density (g/cc)

Volume shrinkage (%)

Fig. 4.5 Effect of hexane percentage in silylating mixture on density and volume shrinkage (%) of the aerogels


4.3.3 Influence of Tartaric acid: Na2SiO3 molar ratio (A)

The influence of Tartaric acid: Na2SiO3 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 Na2SiO3:H2O: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,

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) (%)

1 0 89.8 4.7 132 0.124

2 25 94.8 9.8 144 0.098

3 50 95.5 11.4 146 0.090

4 75 94.8 9.9 145 0.098

5 100 94.5 9 142 0.102

B Effect of aging period (hours)

1 0 92.4 6.4 134 0.118

2 1 92.8 6.8 136 0.113

3 2 93.1 7.2 137 0.112

4 3 94.7 9.5 143 0.099

5 4 92.3 6.3 135 0.117

C Effect of weight % of silica

1 1.5 89.5 4.5 130 0.125

2 2.3 89.8 4.7 132 0.124

3 3 94.7 9.5 143 0.099

4 4 95.5 11.4 146 0.090

5 5 95.7 11.8 146 0.090

6 6 91.7 5.8 134 0.120

7 8 89.4 4.5 130 0.126

Table 4.1 Porosity, pore volume, contact angle and thermal conductivity of silica aerogels with variation of the sol-gel parameters


80

110

140

170

200

0.18 0.43 0.68 0.93 1.18

Den

sit

y (

g/c

c)

0.2

1

1.8

2.6

3.4

Lo

g [

Gela

tio

n t

ime

(m

in)]

Density (g/cc)Log [Gelation time (min)]

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

Hydrolysis: OH

OH

Si HO OH Na2SiO3 + H2O

Tartaric acid

C4H6O6

Sodium silicate solution Silicic acid Sodium tartarate

+ C C NaOOC COONa

HO

OH H

H

---(4.2)

Condensation:

---(4.3)

Silicic acid

+

OH

Si

OH

HO OH

OH

Si

OH

HO OH + H2O Si Si

OH

OH

O OH

OH

HO

OH

Fig. 4.6 Effect of Tartaric acid:Na2SiO3 molar ratio on the gelation time and density

Tartaric acid:Na2SiO3 molar ratio


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.

Sr. No.

Variation

Volume shrinkage

(%)

Porosity (%)

Pore volume (cc/g)

Contact angle (deg.)


(W/m.K)

D Effect of Tartaric acid/Na2SiO3 molar ratio

1 0.27 70 94 8.6 140 0.105

2 0.51 79 89 4.6 132 0.124

3 0.72 59 93 7.5 137 0.110

4 0.90 50 94.5 9.2 142 0.102

5 1.08 45 94.7 9.5 143 0.099

6 1.20 72 89.5 4.5 130 0.125

E Effect of TMCS percentage (%)

1 20 59 92 6.1 133 0.118

2 26.67 41 94.5 9.2 140 0.100

3 33.33 24 95.6 11.4 146 0.090

4 40 9 95.9 12.3 146 0.089

Table 4.2 Effect of Tartaric acid:Na2SiO3 molar ratio and TMCS percentage on physical properties of silica aerogels


4.3.4 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:Na2SiO3 molar ratio

constant at 1.08. 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. 4.7. 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.

95

110

125

140

155

0 1 2 3 4Gel aging period (hours)

De

nsit

y (

g/c

c)

40

46

52

58

64

Vo

lum

e s

hri

nkag

e (

%)

Density (g/cc)Volume shrinkage (%)

Fig. 4.7 Effect of gel aging period on density and volume shrinkage


4.3.5 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:Na2SiO3 molar ratio constant at 1.08. 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).

10

25

40

55

70

85

100

1 2.5 4 5.5 7 8.5Weight % of silica

Vo

lum

e s

hri

nkag

e (

%)

0.3

0.9

1.5

2.1

2.7

3.3

Lo

g [

Gela

tio

n t

ime (

min

)]

Volume shrinkage (%)

Log [Gelation time (min)]

Fig. 4.8 Effect of weight % of silica on gelation and volume shrinkage


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

where D is the permeability of the gel, ∇P is the pressure gradient, and ηL 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

The resulting stress in the solid phase of a gel plate of thickness L is given

by [13].

where CN ≡ (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.

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

J = (D/ηL)∇P, ---(4.4)

---(4.6) σx ≈ CN(LηLVE/3D), .

.

VE

---(4.5) (D/ηL)∇2P = - έ


50

90

130

170

210

250

1 2.5 4 5.5 7 8.5Weight % of silica

De

ns

ity

(g

/cc

)

0.07

0.085

0.1

0.115

0.13

Th

erm

al c

on

du

cti

vit

y (

W/m

.K)

Density (g/cc)


(W/m.K)

4.3.6 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–CH3 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].

where γLV is the liquid-vapor surface tension, θ is the contact angle of the

liquid with a pore wall and rp is the pore radius. The negative sign is due to

--- (4.7) rp

γLV cos θ P = -2

Fig. 4.9 Effect of weight % of silica on density and thermal conductivity


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-(CH3)3 groups [16] using TMCS as follows,

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 Na2SiO3:H2O:Tartaric acid molar ratio constant at

1:146.67:0.86. Fig. 4.10 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 (> 26.67 %), complete modification of silanol groups

to non-polar, hydrolytically stable –Si(CH3)3 groups occur and causes

repulsion between end capped – Si(CH3)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. 4.11.

Surface modification

+

Silica surface Trimethylchlorosilane

OH Si

OH Si

O

Si (CH3)3 Cl

Si (CH3)3 Cl

Modified silica surface

(CH3)3

(CH3)3 Si

Si O Si

O Si

O + 2HCl ---(4.8)


70

95

120

145

170

18 22 26 30 34 38 42TMCS percentage

Den

sit

y (

g/c

c)

15

30

45

60

75

Op

tical

tra

nsm

issio

n (

%)

Density (g/cc)

Optical transmission (%)

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

(γSV), solid-liquid (γSL) and liquid-vapour (γLV) interactions at the intersection

of the three phases, is given by the Young’s equation [17]:

133 o 146

o

Fig. 4.11 Water droplets on the aerogel surfaces for (a) 20 %, (b) 33 % TMCS

(a) (b)

Fig. 4.10 Effect of TMCS percentage on density and optical transmission


For a hydrophobic surface, θ > 90o and therefore, from the above

equation it follows that solid-liquid (γSL) interaction is greater than solid-

vapour (γSV) interaction. Fig. 4.11 (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.

4.3.7 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 Na2SiO3:H2O:Tartaric acid:TMCS molar ratio constant at

1:146.67:0.86:9.46.

60

110

160

210

260

310

3 8 13 18 23 28silylation period (hours)

Den

sit

y (

g/c

c)

137

139

141

143

145

147

Co

nta

ct

an

gle

(d

eg

ree

)

Density (g/cc)Contact angle (degree)

Fig. 4.12 shows with increase of the silylation period, density of the

aerogels decreased and hydrophobicity increased. This is believed to be

Fig. 4.12 Effect of Silylation period on density and contact angle

γSV = γSL + γLV cos θ --- (4.9)


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. 4.13. 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].

83

86

89

92

95

98

4 9 14 19 24Silylation period (hours)

Po

rosit

y (

%)

0.08

0.095

0.11

0.125

0.14

Th

erm

al

co

nd

ucti

vit

y (

W/m

.K)

Porosity (%)Thermal conductivity (W/m.K)

Fig. 4.14 shows the variation in % of volume change with drying

temperature. It has been found that the % of volume change is more up to

100 oC and then decreased above 100 oC and remained constant above 150

oC, clearly indicating the effect of spring back.

Fig. 4.13 Effect of Silylation period on density and contact angle


0

20

40

60

80

100

0 50 100 150 200 250

Temperature (oC)

% o

f v

olu

me c

ha

ng

e

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. 4.15.

Fig. 4.15 Infrared spectra of aerogels with the variation of the Silylation period, (a) 6 h, (b) 12 h, (c) 18 h, and (d) 24 h.

Fig. 4.14 Effect of drying temperature on % of volume change

Temperature (oC)

-OH

Wave number (cm-1)

% o

f o

pti

cal

tran

sm

issio

n (

A.U

.)

-OH


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-1and 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. 4.16. It shows

that, the weight loss with an exothermic peak at 435 oC, was considered to

correspond to the decomposition of surface -CH3 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 oC.

Fig. 4.16 TGA-DTA of hydrophobic silica aerogel

100

Weig

ht

(%)

435oC

Tem

pera

ture

dif

fere

nce (

oC

/mg

)

Temperature (oC)

TGA

DTA

200 300 400 500 600

100

88

90

92

94

96

98

-1

0

1

2

3

0


4.4 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 oC. The semitransparent aerogels with density

~0.084 g/cc, porosity ~95 %, thermal conductivity ~0.090 W/m.K and

hydrophobicity ~146o were obtained for the molar ratio of

Na2SiO3:H2O: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.


References

[1] S. S. Kistler, Nature, 127 (1931) 741

[2] G. A. Nicolaon and S. J. Teichner, Bull. Soc. Chem., France, 5 (1968)

1906

[3] In:F. Schuth, K. S. W. Sing, J. Weitkamp (Eds.), Hand Book of

Porous Solids, Wiley-VCH, Weinheim, vol. 3 (2002) 2014

[4] S. S. Prakash, C. J. Brinker, A. J. Hurd, S. M. Rao, Nature,

347 (1995) 439

[5] F. Schwertfeger, D. Frank, M. Schmidt, J. Non- Cryst Solids,

225 (1998) 24

[6] A. Venkateswara Rao, Sharad D. Bhagat, Solid State Sci., 6 (2004) 945

[7] J. J. Bikerman, Surface Chemistry: Theory and Applications, 2nd edn.

(Academic, New York ), (1958) p.343

[8] A. I. Vogel, Quantitative Inorganic Analysis, (1939) p. 891

[9] P. B. Sarawade, Jong-Kil Kim, Jin-Koo Park and Ho-Kun Kim, Aerosol

and Air Quality Research, Vol. 6, No. 1 (2006) p. 93

[10] C. J. Brinker and G. W. Scherer, “Sol-Gel Science” (Academic

press, Sandiego ), (1990) p. 536

[11] J. Zarzycki, M. Prassas, and J. Plippouha, J. Mater. Sci., 17 (1982) 3371

[12] A. Venkateswara Rao, A. Parvathy Rao, M. M. Kulkarni, J. Non-Cryst

Solids, 350 (2004) 224

[13] G. W. Scherer, J. Non- Cryst Solids, 109 (1989) 171

[14] E. Nilsen, M.-A. Einarsrud and G. W. Scherer, J. of Non-Cryst Solids,

221 (1997) 135

[15] D. M. Smith, G. W. Scherer, J. M. Anderson, J. Non- Cryst Solids,

188 (1995) 191

[16] A. V. Rao, D. Haranath, Microporous Mater., 30 (1999) 267

[17] A. W. Adamson, Physical Chemistry of Surfaces, John Wiley, New York,

1982, p.338.

[18] A. Parvathy Rao, G. M. Pajonk, A. V. Rao, J. Mater. Sci.,

40 (2005) 3481

[19] D. M. Smith, R. Deshpande, C. J. Brinker, Ceramic Transaction,

J. Porous Materials, 31 (1993) 71

[20] N. Hering, K. Schriber, R. Reidel, O. Lichtenberger, J. Woltersodorf,


Appl. Organometal Chem.,15 (2001) 879

[21] Ae. -Y. Jeong, S. -M. Goo, D. -P. Kim, J. Sol-Gel Sci.Technol.,

19 (2000) 483

[22] A. Parvathy Rao, A. Venkateswara Rao, G. M. Pajonk, J. Applied

Surface Sci., 253 (2007) 6032

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