SYNTHESIS AND APPLICATIONS OF SUPERHYDROPHOBIC
SILICA AEROGELS
Venkateswara Rao
Air Glass Laboratory, Department of Physics, Shivaji University, Kolhapur,
Maharashtra, India
Abstract. Synthesis of non-wetting (hydrophobic) solid surfaces is an active area of
research in recent years because it forms the basis for multidisciplinary applications
such as frictionless flow of liquids through nano- and micro-channels and pipes,
waterproof and corrosion resistance coatings and drug delivery systems.
In this lecture, the experimental results on the synthesis, physico-chemical
properties and applications of superhydrophobic silica aerogels, are presented. The
aerogels have been produced using methyltrimethoxysilane (MTMS) precursor by
two step sol-gel process. The contact angle has been found to be as high as 175°. The
elastic properties of these aerogels have been studied and the Young's modulus (Y)
has been found to decrease from 14.11 x 104 to 3.43 x 104 N/m2 with a decrease in the
density of the aerogels from 100 to 40 kg/m3, respectively. The aerogels are thermally
stable up to a temperature of 753 K and above which they become hydrophilic. The
criticality of the water droplet size on the superhydrophobic surface has been found to
be 2.7 mm. The velocity of the water droplet on such a superhydrophobic surface has
been observed to be 1.44 m/s for 55° inclination, which is close to the free fall
velocity (~1.5 m/s). Further, the as produced aerogels have been used for organic
liquid (i.e. alkanes, aromatic compounds, alcohols and oils) absorption and desorption
studies. The superhydrophobic aerogels showed a very high uptake capacity and high
rate of uptake of the organic liquids. Among the alkanes, the mass of octane absorbed
was a maximum of 15.62 gm per unit mass (1 gm) of the aerogel samples. While,
among the alcohols, the mass of butanol absorbed was maximum (~19 gm). The
desorption of solvents and oils was studied by maintaining the as absorbed aerogel
samples at various temperatures. The vapour pressure is very high (732.7 mm of Hg at
30oC) for pentane which led to faster evaporation. Hence the rate of desorption of
pentane is more. In the case of alkanes, after the desorption, the aerogels regained
their original shape and size. The best quality elastic superhydrophobic aerogels in
terms of contact angle (175o), density (37 kg/m3), shrinkage (6%), porosity (98%) and
thermal conductivity (0.057 W/mK) have been obtained for the molar ratio of MTMS:
MeOH: acidic water: basic water:: 1: 35: 3.97 : 3.97, respectively. The hydrophobicity
has been confirmed by Fourier Transform Infrared (FTIR) spectroscopy and the
contact angle measurements. The microstructure of the aerogels has been studied by
transmission electron microscopy (TEM). The Young's modulus of the aerogels has
been determined by an uniaxial compression test measurements.
1. INTRODUCTION
Silica aerogels are sol-gel-derived materials consisting of interconnected nano
particle building blocks, which form an open and highly porous three-dimensional
silica network. Typical silica aerogels have high surface area (~ 1000 m2/g), high
optical transmission (~ 93%), low density (40 kg/m3) and low thermal conductivity
5-2
(0.02 W/mK) [1-4]. These features have led the aerogels to various applications in
science and industry such as Cerenkov radiation detectors [5], inertial confinement
fusion (ICF) targets [6], lightweight thermal and acoustic insulation [7], catalytic
supports [8], microfilters [9], supercapacitors for electric cars [10] and controlled
release of drugs [11].
Despite of all these fascinating properties, the aerogels have major drawbacks
that they are fragile, brittle and moisture sensitive, which limit their applications in
various fields. Furthermore, considering the adverse impact to ecosystems and the
environmental pollution by the accidental and deliberate release of oil and other
organic liquids during transportation and storage, experiments were conducted to
synthesize
flexible
and
superhydrophobic
silica
aerogels
using
methyltrimethoxysilane (MTMS) as a precursor and to test their usability as efficient
and effective absorbent of oil and other organic liquids. The as produced silica
aerogels were found to be highly flexible and superhydrophobic with excellent
absorption properties of oils and other organic liquids. The aerogels showed a very
high uptake capacity, high rate of uptake and above all. They could be recycled to get
back the organic liquids and the aerogels could be reused as absorbents. It was
observed that aerogels absorbed the organic liquids by more than 20 times and oils by
nearly 14 times of their own mass.
2. Experimental Procedures
2.1 Sample preparation
The synthesis of the superhydrophobic silica aerogels involves two major
steps: (1) the preparation of alcogels by a two step acid-base catalyzed sol gel process
and, (2) the supercritical drying of the wet gels to remove the solvent. The chemical
reactions responsible for the formation of three dimensional gel network structure, are
as follows.
Hydrolysis:
OCH3
H3C
OH
C2H2O4
OCH3 + 3H2O
Si
MeOH
H3C
OCH3
Si
OH + 3CH3OH
OH
MTMS
Silanol
Condensation:
OH
OH
2
H3C
OH
NH4OH
H3C
Si
OH
OH
+ HO
Si
O
H3C
Si
Si
O
Si
OH
OH
Si – O – Si Network
5-3
CH3
+ 4 H2O
O
O
CH3
OH
Si
OH
CH3
Initially, methyltrimethoxysilane (MTMS) was diluted in methanol (MeOH)
solvent and was partially hydrolyzed with water under acidic conditions with oxalic
acid (0.001M). In the second step, after one day, the condensation of these hydrolyzed
species was carried out in the presence of a base catalyst, NH4OH (10M), to get the
alcogels. The alcogels were aged for two days in methanol and then they were
supercritically dried in an autoclave at a temperature of 2650C and a pressure of 10
MPa to obtain the aerogels (Figure 1).
Solvent molecules
Silica particles
Sol
Gel
Aerogel
Fig. 1. Schematic diagram of the sol-gel process.
2.2. Methods of characterization
The as prepared aerogels were characterized by the bulk density, porosity,
volume shrinkage, thermal conductivity and contact angle measurements. The
microstructure of the aerogels was studied using Transmission Electron Microscope.
The Young's modulus (Y) of the aerogels was determined by an uniaxial
compression test as shown in the Figure 2. In this test, various loads (e.g. 0.01 kg,
0.02 kg, 0.03 kg etc.) were applied on the cylindrical aerogel sample and the
corresponding change in length (∆ L) was measured using a travelling microscope.
Finally, the Young's modulus of the aerogel samples was calculated by using the
formula:
Young's modulus (Y) = mgL / r2(∆ L) = (Lg/r2) / slope
(1)
where, L is the original length of the aerogel before deformation and r is the radius of
the aerogel. The absorption and desorption studies of the as prepared
superhydrophobic aerogel sample was done by putting it in an organic solvent until it
was completely wetted by the liquid. Then it was removed and maintained at various
temperatures in an oven The rate of desorption was studied by weighing the aerogel
sample in a micro balance before absorption, immediately after absorption and at
various time intervals until all the liquid got evaporated from the aerogel sample.
5-4
A
B'
B
C
D
Fig. 2. Schematic diagram of experimental set-up for the Young's modulus
measurements of the silica aerogels, A: Vertical axis; B: Platform for the application
of the stress; C: Silica aerogel cylinder; D: Flat bottom surface; l: Change in length
after the application of the stress.
3. RESULTS AND DISCUSSION
3.1 Effect of MeOH/MTMS molar ratio (S)
Change in length, l x 10-2 (m)
0.7
S = 35
0.6
0.5
S = 28
0.4
0.3
S = 21
0.2
0.1
0
0
10
20 30 40 50 60 70
Mass applied x 10-3 (kg)
Fig. 3. Plots of change in length against mass applied for the silica aerogels prepared
with various MeOH/MTMS molar ratios
The effect of MeOH/MTMS molar ratio (S) on the elastic and other physical
properties of the superhydrophobic silica aerogels was studied by keeping the molar
ratio of H2O/MTMS constant at 8. The oxalic acid (C2H2O4) and ammonium
hydroxide (NH4OH) catalyst concentrations were kept constant at 0.001M and 10M,
5-5
respectively. The Young's modulii (Y) of the aerogels scaled with the bulk density. It
has been observed that with an increase in S value from 14 to 35, the volume
shrinkage and hence the density of the aerogels decreased from 28 to 7% and from
100 to 40 kg/m3, respectively. Figure 3 shows the graphs of change in length against
the mass applied for the calculation of Y. The Y was found to decrease from 14.11 x
104 to 3.0 x 104 N/m2 resulting in increase in the flexibility of the aerogels. Figure 4
shows the flexible aerogel sample which is bended about 90o (Figure 4a) and the
extent of bending of the sample (Figure 4b). Since the silica chains of the aerogel with
S = 35 are quite separated from each other and large empty spaces are available in the
network, it can undergo deformation when the stress is applied. However, if the S
value is decreased, i. e. for S = 14, the degree of polymerization increased and
extensive cross-linking in three dimension resulted in the rigid structure.
a
b
Fig. 4. a The flexible aerogel sample which is bended about 90o. b The extent of
bending of the flexible aerogel sample.
3.2 Hydrophobicity and thermal stability of the aerogels
The hydrophobicity of the aerogel sample was quantified by measuring the
contact angles () of the water droplet placed on the aerogel surfaces under
investigation shown in Figure 5. It was found that the aerogel sample was
superhydrophobic with a contact angle of 175o for a 2.4 mm water droplet, since
MTMS contains one hydrolytically stable methyl group, which is responsible for the
hydrophobicity in the silica aerogels [12]. The photograph showing the water droplet
placed on the surface of a superhydrophobic silica aerogel was depicted in Figure 6.
Furthermore, the hydrophobicity was confirmed by the Fourier Transform Infrared
(FTIR) spectrum shown in Figure 7 indicates strong peaks at 1270, 840 cm-1 and
2900, 1310 cm-1 corresponding to Si-C and C-H bonds respectively [13,14]. Very
small peaks corresponding to O-H bonding around 3500 and 1650 cm-1 were
observed, clearly indicating the hydrophobic nature of the aerogels.
5-6

h

AEROGEL SAMPLE
WOODEN SUPPORTING STAND
Fig. 5. Schematic of contact angle measurement of the water droplet placed on the
aerogel surfaces under investigation.
Fig. 6. The photograph showing the water droplet placed on the surface of a
superhydrophobic silica aerogel.
5-7
Fig. 7. The FTIR spectra of the MTMS based aerogel sample.
Further, as shown in table 1, larger the R/Si ratio, the better is the hydrophobic
covering yielding higher contact angles. However, in case of the MTMS, the contact
angle is high though it is trifunctional. This is due to the fact that there is very less
chances of formation of gel network using mono or di functional precursor. However,
the trifunctional precursor gives three hydrolysable alkoxy groups to form the three
dimensional gel network and one non hydrolysable alky group which led to the
hydrophobic property of the final product.
Table 1: Structure, silicon oxide content, R/Si ratio of the various functional groups.
UnSi
StructStruct
ure
Units
ure
MonofuMonofuncti DifuncDifunction TrifunTrifunctiona QuadriQuadrifunctio
onal
al
l
nal
R
R
O
O
nctional
tional
ctional
functional
O
O
R
R
O
O
O
O
O
O
R
O
R
R
Si
R SiO0.5
SiO
SilicoSilicon
oxide
Si
O1 SiO1
SiO
(n = 0(n = 0
toContent
3)atio
O
Si
SiO
O 1.5
OSiO2
SiO1.5
0.5
Content
R/Si RR/Si
nRatio
oxide
O
3
2
TMES, TMCS,
HMDZ
DMCS
3
SiO2
2
DMCS
1
0
0 TEOS,
TMOS
1MTMS,
R= NonR= Non-hydrolysable
TMCS, group, X= Hydrolysable group,
to 3)
It was found that for the superhydrophobic PTES
silica aerogels, more Laplace
-hydrolysable group,HMDZ
X= Hydrolysable group,
pressure (PL) is required
to fill the water in the pores of the network according to the
MTMS,
following formula:
PTES
5-8
PL = - 2 lv cos  /
(2)
where lv is the interfacial energy of the liquid /vapor interface,  is the contact angle
of the liquid with the solid surface and r is the radius of the pore. Figure 8 shows the
schematic diagram of Laplace pressure water intrusion. When applied pressure P is
less than that of the Laplace pressure (PL), water can not enter into the pores and when
P> PL , water enters the pores.
Piston
Fig. 8. Schematic diagram of Laplace pressure water intrusion.
The Laplace water intrusion method indirectly provides an idea about the pore
sizes as well as the hydrophobicity because the Laplace pressure is inversely
proportional to pore size and directly proportional to the contact angle.
3.3 Transport of liquids on superhydrophobic aerogels
Transportation of liquids on a nano scale is crucial in the development of
nanofluid based devices for applications in chemical and biological technologies [15].
Therefore, the velocity of the liquid droplets on an inclined surface coated by the
superhydrophobic silica aerogel powder has been studied using a specially prepared
device interfaced with the personal computer as shown in Figure 9. The work of
adhesion (Wa) values for various aerogel surfaces have been given in the table 2. The
Wa values have been calculated using the Dupre’s equation:
Wa = lv (1+ cos )
(3)
where lv is the surface tension of the liquid (for water, lv = 72.99 N/m) [16]. Thus it
can be observed from the values given in table 2, as the contact angle () increases
(approaches to 180o) Wa becomes very small (Wa=0.54 N/m, for 173o) because of
negligible interaction between solid-liquid phases. For complete non-wetting
(=180o), Wa = 0, that is the drop would be levitated.
5-9
PC's parallel
port
+
5V
330

Water
drop
47K

1M

2K

LE
D
1
13
47K

Light
detector
2
5
0
5
4
6
LM32
4
1
1
1
7
O/
P
1
4
Figure 9.a
1
4
t1
3
1
2
t2

2
Figure 9.b
Fig. 9. a Circuit diagram of the instrument for the measurement of the droplet and
marble velocities on an inclined surface. .b Schematic set up for water droplet
velocity measurements: 1: Light emitting diodes (LED), 2: Photo-conductive
detectors, 3: Inclined plane platform, 4: Water droplet (size: ~2.7 mm (0.2mm)).
The aerogel powder was prepared by crushing the superhydrophobic aerogels.
By rolling a water drop on the aerogel powder, liquid marbles were obtained. This
aerogel powder covered water droplet (i.e. liquid marble) was placed on various
substrates like glass, aerogel, paper, wood etc. It was observed that irrespective of the
nature of the substrate, a small sized marble (~1mm) maintains its sphericity with 
175o. However, a liquid marble placed on water deforms the water surface and
hence the apparent contact angle decreases to 165o as seen from the Figure 10 which
shows a typical liquid marble (2.7 mm (0.2mm)) floating on the water surface.
5-10
Table 2. Static contact angle () and velocity (v) of the water droplet on
superhydrophobic aerogel coated surface for various angles of inclination.
Sample
M1
M2
M3
M4
M5
M6
Contact angle ()
162o
160o
160o
159o
173o
162o
Work of adhesion (Wa)
mN/m
3.57
4.40
4.40
4.84
0.54
3.57
Drop centre deviation ()
cm
0.03
0.033
0.035
0.05
0.024
0.03
Inclination ()
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
5o
28.16
29.14
19.25
40
29.6
40
15o
40
42.16
34
52
37.25
52.4
22o
42
67.75
43
70
60
77
35o
75.7
84.40
64.6
83.2
83.8
97.83
52o
96.75
105
92.6
97.83
97.83
144
Fig. 10. Liquid marble (2.7 mm (0.2mm) size) floating on a water surface.
For the measurement of the droplet velocity on the superhydrophobic aerogel
inclined plane, a droplet of the size ~ 2.7mm ( 0.2mm) was chosen because it is
comparable to the capillary length -1 [17]:
5-11
-1 = (lv /  g)1/2
(4)
3
For water, the surface tension lv = 72.99 N/m, the density  = 1000 kg/m and the g=
9.8 m/s2. Hence the value of -1 is 2.7 mm. A droplet smaller than -1 generally
remains stuck when placed on a solid surface because of the contact angle hysteresis.
On the other hand, gravity would flatten a droplet of radius r > -1 [18].
The velocity of a freely falling water droplet of 2.8mm (0.2mm) size in the
gravitation field was observed to be ~152 cm/s. The difference in the freely falling
drop (~152 cm/s) and a rolling drop (highest v ~144 cm/s) indicates that a contact
zone, however small, must be forming between the drop and the solid surface, which
opposes the flow of liquid. It has been shown that, even for a small droplet of a liquid
having contact angle 180o with a particular solid, a contact zone of radius 'l' forms
between the solid and the liquid due to gravity [19]. The mass center of the droplet
gets lowered by a quantity '' due its own weight. If 'r' is the radius of the drop, then
the '' and 'l' are related to each other by:
l2

(5)
r
The deviation () values observed for a water droplet of 2.8 mm (0.2 mm)
placed on various aerogels are given in table 3. As the contact angle value increased
from 159 to 173o, the  value decreased from 0.05 to 0.024.
Table 3. The velocity (v) of the liquid marbles on non-adhesive side of the tape (i.e.
uncoated surface) for various angles of inclination.
Sample
M1
M2
M3
M4
M5
M6
Drop centre deviation () cm
0.026
0.028
0.030
0.027
0.024
0.024
Inclination ()
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
v (cm/s)
5o
31.75
30
31
31.75
29.5
46.25
15o
35.25
38
37.3
39
38.4
60.5
22o
43.87
54
49.5
52
52
76.33
35o
57.66
82
70
66
73.75
105
52o
82.75
105
89.6
105
95
123.8
3.4 Uptake capacity of organic liquids by the superhydrophobic aerogels
The uptake capacity of the aerogel sample was quantified in terms of the mass
of the organic liquid absorbed by unit mass (1g) of the aerogel sample. For this study,
various organic liquids were used which are shown in table 4.
5-12
Table 4. Mass of various organic liquids absorbed by unit mass (1g) of the aerogel
sample.
Organic Liquid / oil
Mass of the organic liquid / Moles of the organic
oil absorbed per unit mass liquid absorbed per unit
mass (1g) of the aerogel
(1g) of the aerogel
Pentane
9.83 g
0.1365
Hexane
10.95 g
0.1215
Heptane
13.38 g
0.1338
Octane
13.82 g
0.1210
Benzene
19.92 g
0.2553
Toluene
20.64 g
0.2189
Xylene
20.37 g
0.1921
Methanol
14.03 g
0.4410
Ethanol
14.54 g
0.3231
Propanol
19.15 g
0.3191
Butanol
18.93 g
0.2558
Petrol
13.82 g
-----
Kerosene
16.45 g
-----
Diesel
18.55 g
-----
From the table, it follows that the mass of petrol absorbed was minimum
(13.82g) and diesel was maximum (18.55g) among the oils. The aerogel absorbed
nearly the same amount (~20g) of the solvents benzene, toluene and xylene. Among
the alkanes, the mass of pentane absorbed was minimum (9.83g) while the mass of
octane absorbed was maximum (15.62g) and among the alcohols, the mass of
methanol absorbed was minimum (~14g) and that of butanol was maximum (~19g).
The mass of the organic liquid absorbed by an aerogel depends upon the
surface tension () of the corresponding liquid. From the Young’s equation [20]:
SV = SL + LV cos  …….... (6)
where SV is the solid-vapour, SL is the solid-liquid and LV is the liquid-vapour
interaction at the intersection of the three phases, it follows that the MTMS based
superhydrophobic (>150o) silica aerogels are low energy surfaces which are not
wettable by water. But the organic liquids wet the surface and also get absorbed by
the aerogels. This is due to the fact that the organic liquids are less polarizable than
the solid aerogel.
Table 4 also gives the moles of solvent absorbed per unit mass of the aerogel.
The oils such as petrol, diesel and kerosene being the extracts from the crude oil, the
molecular weights are not well defined and hence the moles of the oils absorbed is not
5-13
given in the table. It is interesting to note that the graph of desorption time versus the
moles of solvent absorbed per unit mass of the aerogel is linear for all the three types
of solvents- alkanes, aromatics and alcohols as shown in Figure 11.
200
Alkanes
Aromatics
Alcohols
150
Desorption time (min)
100
50
0
0
0.1
0.2
0.3
0.4
0.5
Moles of solvent per unit mass of the aerogel
Fig. 11. Desorption time of solvents as a function of moles of solvent absorbed
per unit mass of the aerogel.
3.5 Rate of desorption of organic liquids by the superhydrophobic aerogels
The rate of desorption was found by measuring the mass of the aerogels at
regular time intervals till the liquid got totally evaporated and the original mass of the
aerogel was restored. Figure 12 shows the photographs showing various stages of
absorption and desorption of hexane from the aerogel. It was observed that the rate of
desorption was very high at the beginning and slowed down with the passage of time.
Further, it was observed that the desorption rate decreased as we go from shorter
chain (pentane) to the longer chain (octane) organic liquids. This is due to the fact that
the process of evaporation takes place in two stages. During the first stage, molecules
are brought from the interior up to the surface, overcoming the surface tension effect
and during the second stage, they vaporize from the surface film depending on the
vapor pressure of the liquid. Therefore, lesser is the surface tension, easier for the
molecules to come on to the surface and hence faster would be the evaporation.
5-14
Figures 13 show the transmission electron micrographs of the aerogels before
absorption and after desorption of methanol.
Fig. 12. The Photographs showing various stages of absorption and desorption of
hexane from the aerogel.
a
b
Fig. 13. Transmission electron micrographs of the aerogels before absorption and
after desorption of methanol.
3.6 Effect of temperature on desorption of organic liquids by the
superhydrophobic aerogels
To study the effect of temperature on the rate of desorption, after absorption,
the samples were put in an oven (Termaks company, Norway) at various temperatures
and the desorption time was recorded. Figure 14 shows the effect of temperature on
the desorption time of petrol. It was observed that with an increase in temperature, the
desorption time decreased significantly. Desorption time for butanol, for example,
reduced from 180 minutes at 30oC to 14 minutes at 100oC. Similarly, for octane, it
reduced from 80 minutes at 30oC to 8 minutes at 100oC.
5-15
700
Desorption time (min.)
600
500
400
300
200
100
0
20
40
60
80
100
120
o
Temperature ( C)
Fig. 14. Desorption time of petrol as a function of temperature.
With an increase in the temperature, the vapor pressure of the organic liquids
increases considerably. For e.g., the vapor pressure of methanol at 30oC is 209.67mm
of Hg, while it is 416.58mm of Hg at 50oC. The surface tension of all the liquids
decreases linearly with the rising temperature, over small temperature ranges, so that
the surface tension, t, at toC is given by [21]:
t = o (1-t)
…….... (7)
where o is the surface tension at 0oC and  is the temperature co-efficient.
As a result, it would be easy for the liquid molecules to come out of the
aerogel pores at higher temperatures due to higher temperature gradient. At the same
time, with an increase in temperature, the vapor pressure increases leading to the early
evaporation of the liquid from the aerogel surface. Therefore, desorption time
decreased remarkably with the increase in temperature.
4. CONCLUSIONS
The physicochemical properties of elastic superhydrophobic silica aerogels
have been studied. Also, use of as prepared aerogel in absorption and desorption of
the organic liquids and oils have been studied. The following are the major findings of
the present experimental investigations:
1. Highly flexible and superhydrophobic silica aerogels have been obtained using
MTMS precursor.
2. The aerogels regain its origin shape after releasing the stress.
5-16
3. The powder of the aerogel sample cab\n be used to produce liquid marbles, which
float on the water surface.
4. The aerogel absorbed the organic liquids and oils by nearly 15 times its own mass.
5. The aerogels retained their original structure after the total desorption of the
organic liquids.
6. The rate of desorption increased with an increase in the temperature.
Thus, the elastic superhydrophobic aerogels could be used as efficient
absorbents for the purposes of storage, clean-up, transport and the safe disposal of
organic liquids and oils.
REFERENCES
1. L. W. Hrubesh, Chem.Ind.17 (1990) 824.
2. Bond G. C. and Flamerz S., Appl. Catal., 33 (1987) 219.
3. J. Fricke, A. Emmerting, Structure and Bonding, 77 (1992) 27.
4. C. A. M. Mulder, J. G. Van Lierop, in: J. Fricke (Ed) Aerogels, Springer, Berlin,
1986, p.68.
5. Pestotnik R, Krizan P, Korpar S, Bracko M, Staric M, Stanovnik A, Nuclear
Science Symposium Conference Record, 2001IEEE1 (2001) 372.
6. K. Y. Jang, K. Kim, R. S. Uphadye, J. Vac. Sci. Technol, A 8 (1990) 1732.
7. R. Alfaro, M.I. Martínez and G. Paic, Nuclear Instruments and Methods in Physics
Research Section A: Accelerators, Spectrometers, Detectors and Associated
Equipment, 572 (2007) 437.
8. M. Reim, W. Körner, J. Manara, S. Korder, M. Arduini-Schuster, H.-P. Ebert and J.
Fricke, Solar Energy, 79(2005) 131.
9. A. Venkateswara Rao, Manish M. Kulkarni, Sharad D. Bhagat, J. Colloid Interface
and Sci., 285 (2005) 413.
10. Jun Li, Xianyou, Qinghua, Sergio Gamboa, P.J. Sebastian, J. Power Sources, 158
(2006) 784.
11. Qanil Tang, Yao Xu, Dong Wu, Yuhan San, Jiqing Wang, Jun Xu, Feng Deng, J.
of Controlled Release 114 (2006) 41.
12. A. Venkateswara Rao, M. M. Kulkarni, D. P. Amalnerkar, T. Seth, J. Non-Cryst.
Solids, 330 (2003) 187.
13. N. Hering, K. Schriber, R. Riedel, O. Lichtenberger, J. Woltersodorf, Appl.
Organometal. Chem. 15 (2001) 879.
14. M. Laczka, K.Cholwa-Kowalska, M.Kogut, J.Non-Cryst. Solids 287 (2001)10.
15. J. P. Rolland, R. Van Dam, D. A. Schorzman, S. R. Quake, J. M. DeSimone, J.
Am. Chem. Soc. 126 (8) (2004), 2322.
16. A. Venkateswara Rao, G. M. Pajonk, N. N. Parvathy and E. Elaloui, in:
Y. A. Attia (Ed.), Sol-Gel Processing and Applications, Plenum
Press, New York, 1994, p. 237
17. A. Venkateswara Rao, M. M. Kulkarni, Mater. Res. Bull., 37 (2002) 1667.
18. A. Venkateswara Rao, R. R. Kalesh, D. P. Amalnerkar, T. Seth J. Porous.
Mater. 10 (2003) 23.
19. D. A. Fridrikhsberg, "A Course in Colloid Chemistry", Mir Publishers,
Moscow, 1986, p. 63.
20. Arthur Beiser, The Mainstream of Physics, Addison Wesley Publishing
Company Inc., second printing, 1962, p-124.
21. F.H. Newman and V.H.L. Searle, The General Properties of Matter, Orient
Longmans, Fifth Edition, 1957, p.188.
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