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Surface tension of silane treated natural zeolite
J.Y. Leea, S.H. Leeb, S.W. Kima,*
aDepartment of Chemical Engineering, The University of Seoul, 90 Jeonnong-Dong, Dongdaemun-Gu, Seoul, 130-743, South KoreabDepartment of Environmental Engineering, Anyang University, 708-113 Anyang 5-Dong, Manan-Gu, Anyang City, Kyungki-Do 430-714, South Korea
Received 14 May 1999; received in revised form 30 September 1999; accepted 20 October 1999
Abstract
To investigate the surface tension of the natural zeolite to be used in polymer composites, wicking method was employed on the base of
Washburn's equation and that of the zeolite treated with 0.5 wt.% -aminopropyltriethoxysilane ( -APS) was also studied and compared.
The apolar component LWSV of the untreated natural zeolite was 19.22 mJ/m2 and the polar one, AB
SV was 15.27 mJ/m2. The polar component
LWSV of the treated zeolite with 0.5 wt.% -APS was 27.08 mJ/m2 and the polar one AB
SV was 12.23 mJ/m2. The value of LWSV component
increased, while that of ABSV component decreased with -APS treatment and it was due to the increasing effect of alkyl group
(hydrophobic) in -APS coupled on the zeolite surface. # 2000 Elsevier Science S.A. All rights reserved.
Keywords: Natural zeolite; Surface tension; Silane coupling agent; Wicking method
1. Introduction
Many researchers have investigated to characterize the
surface of clay minerals and polymer matrix, and the inter-
face between them in order to use the clay minerals as
inorganic ®llers in polymer composite, paint, cosmetic, etc.,
and they have found that the surface of solid materials are
divided into two categories [1±4]. One is hydrophilic solid
whose surface tension is high and the other is hydrophobic
solid whose surface tension is low [5±7]. To facilitate the
adhesion of two different materials, the apolar surface
should be modi®ed into a high-energy polar surface or
the polar surface should be changed to apolar surface by
a number of modifying methods. So, the information about
solid surface tension is very important for better insight in
the processes occurring at the interfaces, both in pure and
applied science.
To get the surface characteristics of solids, the contact
angle method has been used. When a liquid drop is put on the
solid surface at equilibrium as shown in Fig. 1, the interfacial
tensions of solid/liquid, SL solid/vapor, SV, and liquid
vapor, LV are expressed by the following Young's equation
[8±11].
LVcos� � SLÿ SV (1)
where, � is contact angle. However, the contact angle for a
particle can't be measured by the same method in Fig. 1. To
get the contact angle formed between a liquid and a fine
powder, column wicking method is used at constant tem-
perature by determining the rate of penetration of probe
liquids (usually methylene iodide, water and formamide)
through the packed beds of the powder, using the following
Washburn's equation [8,11].
h2 � tR LVcos�
2�(2)
where, h(cm) is the penetrated distance of the probe liquid in
a selected time t(sec), R(cm) is the effective interstitial pore
radius between the packed particles in column and �(cP) is
the viscosity of the probe liquid.
Eq. (2) has a fundamental dif®culty that there are two
unknowns (R and cos�). So, R should be ascertained inde-
pendently before determining the cos� and this dif®culty can
be easily solved by using the spreading liquids which can
completely wet the solid surface, so that the contact angle
� � 0, that is cos� � 1. Generally, n-alkanes are used as
spreading liquids and Eq. (2) can be changed as follows
[2,8].
R � 2�h2=t
LV
(3)
After ascertaining the value of R, the contact angle for each
probe liquid with the solid powder is calculated via Eq. (2)
and the surface tension can be determined.
Inorganic materials of many species such as silica, alu-
mina, talc, mica, etc. have been investigated to characterize
Materials Chemistry and Physics 63 (2000) 251±255
* Corresponding author. Tel.: �82-2-2210-2447; fax: �82-2210-2310.
E-mail address: [email protected] (S.W. Kim).
0254-0584/00/$ ± see front matter # 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 2 5 4 - 0 5 8 4 ( 9 9 ) 0 0 2 3 1 - X
the surface tension, however, we can ®nd few papers
concerned with natural zeolite. In this study, the surface
tension of a natural zeolite was studied to use as an inorganic
®ller in epoxy composites [12]. It is well-known that zeolite
are crystalline aluminosilicate minerals whose surface
areas are so high due to the channels and pores in which
metal ions are located [13±15]. To improve the wettability
between the epoxy matrix and the inorganic ®ller, the
natural zeolite was treated with silane coupling agent and
the effect of silane treatment on the surface characteristics
was investigated.
2. Experiment
The natural zeolite from Kampo area in Korea was found
to be a clinoptilolite type zeolite whose average particle size
was 9.89 mm. -aminopropyltriethoxysilane (A-1100, -
APS) was supplied by Union Carbide and the formulation
was
H2Nÿ�CH2�3ÿSiÿ�OC2H5�3� ÿaminopropyltriethoxysilane�
The silane treatment process of the natural zeolite was as
follows. 0.5 wt.% silane coupling agent to the natural zeolite
was hydrolyzed in the mixture of ethanol/water � 95/5
(vol%) for 15 min at room temperature, and then the natural
zeolite was treated with the hydrolyzed solutions for 30 min
at 508C and dried for 24 h at 1208C. The dried zeolite was
ground and sieved into 325 mesh under. They were stored at
desiccator to prevent water molecules from being adsorbed.
The sample was analyzed by FT-IR spectroscopy at a
resolution of 2 cmÿ1 from 4000 cmÿ1 to 400 cmÿ1.
Column wicking experiment was carried out in the
following manner. Glass tube with 100 mm long and
9 mm diameter was plugged at the bottom with a small
wad of cotton wool. 3 g of well-dried zeolite powder was
®lled into the tube with the packing height of 7 cm by means
of a hypodermic syringe. The packed zeolite was wicked
with n-hexane to promote further settling and the tube was
vertically placed in an oven at 1108C overnight to remove
the hexane. Wicking was done with n-alkanes (hexane,
heptane, octane and decane), methylene iodide, water and
formamide at 208C, and the penetration rates of the liquids
were measured.
3. Results and discussion
Fig. 2 shows the penetrated distances squared, h2 as a
function of time, t for untreated natural zeolite wicked with
a series of n-alkanes. The linear relationship between h2 and
t in Eq. (2) should go precisely through the origin. However,
all lines do not so in practice, and it would appear in most
cases owing to various hydrodynamic disturbances at the
moment of immersion [8]. So, the best approach is to make a
plot of h2 versus t as shows in Fig. 2.
n-Alkanes are known as spreading liquids whose contact
angles with a solid surface is � � 0, that is cos� � 1. To
determine the value of R, the surface tension components,
LV and the viscosities, � of n-alkanes in Table 1 are joined
to Eq. (3) and the relationships between 2�h2/t and LV are
displayed in Fig. 3. This shows the straight line and the value
of R is obtained from the slope, which is 6.10 � 10ÿ5 cm.
The contact angles between the untreated natural zeolite
and the probe liquids in Eq. (2) are obtained from each slope
of the straight line in Fig. 4 by joining the value of R and the
data of Table 2. The contact angles for methylene iodide,
water and formamide are � � 76.78, 68.88 and 54.28, respec-
tively.
The relationships between 2�h2/t and LV for the liquids
(nonspreading liquids) are compared with those of n-
alkanes in Fig. 3 and they fail to meet with the straight
line of the spreading n-alkanes. So, it is found that only
Fig. 1. Contact angles between liquid and solid surface.
Fig. 2. Plots of h2 versus t obtained by wicking with n-alkanes on the
untreated natural zeolite.
Table 1
Viscosity, LV, LWLV , �LV and ÿLV of n-Alkanes at 208C [8]
n-Alkanes � (cP) LV LWLV AB
LV �LV ÿLV
(mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2)
n-Hexane 0.326 18.4 18.4 0 0 0
n-Heptane 0.409 20.1 20.1 0 0 0
n-Octane 0.542 21.6 21.6 0 0 0
n-Decane 0.907 23.8 23.8 0 0 0
252 J.Y. Lee et al. / Materials Chemistry and Physics 63 (2000) 251±255
spreading liquids must be used to determine the value of R
and it is due that the spreading liquids appear to pre-wet the
solid surfaces over which they are subsequently spreading
[8].
It has been well-known that the surface tension of a solid,
SV consists of Lifshitz-van der Waals component, LWSV for
apolar and the Lewis acid-base component, ABSV for polar.
The Lewis acid component, �SV acts as an electron acceptor
site and the Lewis base component, ÿSV is electron donor
site [9,11]. To get the values of these components, the values
of the probe liquids in Table 2 were applied to Eq. (4) [9].
Methylene iodide was used as a probe liquid to determine
the LWSV component of the zeolite surface tension, and water
and formamide were for �SV and ÿSV components.
LV�1� cos�� � 2
� �������������������� LW
SV � LWLV
q�
������������������ �SV � ÿLV
q�
������������������ ÿSV � �LV
q �(4)
Using the contact angle � � 76.78, LWLV , �LV and ÿLV for
methylene iodide in Table 2, LWSV � 19.22 mJ/m2 was cal-
culated from Eq. (4). To determine �SV and ÿSV values, two
equations from Eq. (4) for water and formamide were
rewritten and the solutions of the equations were
�SV � 4.28 mJ/m2 and ÿSV � 13.63 mJ/m2. The polar com-
ponent, ABSV is 15.27 mJ/m2 obtained from Eq. (5) and the
total surface tension is calculated from Eq. (6) and it is
34.49 mJ/m2.
ABSV � 2
������� �SV
q ������� ÿSV
p(5)
SV � LWSV � AB
SV (6)
The apolar component is a little larger than the polar and this
means that the characterization of the untreated natural
zeolite surface is some hydrophobic. It is interesting that
this result is different from the general surface character-
istics of zeolites whose surface is hydrophilic due to the
hydroxyl groups located on the surface of the zeolite in the
form of Al-OH, Si-OH, etc. This result may be attributable
to the decomposition of the zeolite structure by grinding.
Work of adhesion between solid and liquid was proposed
by Dupre as follows [16].
WSL � SV � LVÿ SL (7)
Combining Eqs. (1) and (7), we can get the well-known
Young±Dupre equation [16] and the work of adhesion
between the untreated natural zeolite and the probe liquids
is calculated.
WSL � LV�1� cos�� (8)
Those values of WSL for water and formamide (polar
liquids) are 99.13 mJ/m2 and 91.93 mJ/m2 and that for
methylene iodide (apolar liquid) is 62.49 mJ/m2.
Fig. 5 shows the penetrated distances squared, h2 as a
function of time, t obtained by wicking of a series of n-
alkanes on 0.5 wt.% -APS treated natural zeolite. The
slopes for all n-alkanes are wholly decreased compared
with those of untreated natural zeolite and the trend of
slope is similar. The values of Table 1 are joined to Eq. (3) to
determine the value of R and the relationships between 2�h2/
t and LV are displayed in Fig. 6. The value of R calculated
from the slope is 3.72 � 10ÿ5 cm.
Fig. 3. 2�h2/t versus LV for wicking with n-alkanes and probe liquids on
the untreated natural zeolite.
Fig. 4. Plots of h2 versus t obtained by wicking with probe liquids on the
untreated natural zeolite.
Table 2
Viscosity, LV, LWLV , �LV and ÿLV of probe liquids at 208C [8]
Probe liquids � (cP) LV LWLV AB
LV �LV ÿLV
(mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2)
Methylene iodide 2.80 50.8 50.8 0 0 0
Water 1.00 72.8 21.8 51.0 25.5 25.5
Formamide 4.55 58.0 39.0 19.0 2.28 39.6
J.Y. Lee et al. / Materials Chemistry and Physics 63 (2000) 251±255 253
To get the contact angles between the 0.5 wt.% -APS
treated natural zeolite and the probe liquids by using Eq. (2),
each slope of the straight line in Fig. 7 are joined to R value
and the data in Table 2. The contact angles for methylene
iodide, water and formamide are � � 62.68, 68.48 and 64.28,respectively.
By introducing the contact angle � � 62.68, LWLV , �LV and
ÿLV of methylene iodide on Table 2, to Eq. (4), the apolar
component, LWLV �27.08 mJ/m2 was obtained and the polar
components, �SV � 4.12 mJ/m2 and ÿSV � 9.09 mJ/m2
were also obtained from Table 2 and Eq. (4). By substituting
the value of �SV and ÿSV to Eq. (5), ABSV � 12.23 mJ/m2 was
obtained and the total surface tension, SV � 39.31 mJ/m2 is
calculated from Eq. (6). The apolar component is 2.2 times
as large as the polar component and this means that the
characterization of the surface of 0.5 wt.% -APS treated
natural zeolite is hydrophobic. Work of adhesion, WSL for
water and formamide (polar liquids) are 99.60 mJ/m2 and
83.24 mJ/m2 and that for methylene iodide (apolar liquid) is
74.18 mJ/m2.
The surface tension components for the zeolite are put
together on Table 3. The value of LWSV component increased,
while that of ABSV component decreased with -APS treat-
ment. It is due to that the surface of the zeolite is more
affected by alkyl group (hydrophobic) than amine or hydro-
xyl groups (hydrophilic) as shown in Fig. 8.
Fig. 8A shows the FT-IR spectra of the natural zeolite.
The strongest vibrations in the range of 1294±837 cmÿ1 are
for the internal tetrahedron symmetric stretch of Si (or Al)-
O. The Si (or Al)-O bending appears at 474.1 cmÿ1 and the
band of 525.4 cmÿ1 is related to the presence of the double
rings in the framework structures. The band of H2O bending
is observed at 1635 cmÿ1, the band of hydrogen-bonded OH
is shown at 3453.2 cmÿ1 and the sharp band of isolated OH
is shown at 3645.5 cmÿ1. The effect of -APS treatment is
shown in Fig. 8B. The characteristic peaks mentioned above
are similar in two spectra, however, the new bands appeared
at 3187 cmÿ1 and 1697 cmÿ1 which are related to the
presence of amine groups in -APS. Therefore, it was found
that the surface of the zeolite is coupled with -APS, so the
surface tension was affected by alkyl group and amine or
hydroxyl groups explained above.
Fig. 5. Plots of h2 versus t obtained by wicking with n-alkanes on the
natural zeolite treated with 0.5 wt.% -APS.
Fig. 6. 2�h2/t versus LV for wicking with n-alkanes and probe liquids on
natural zeolite treated with 0.5 wt.% -APS.
Fig. 7. Plots of h2 versus t obtained by wicking with probe liquids on
natural zeolite treated with 0.5 wt.% -APS.
Table 3
Surface, tension components for the natural zeolite at 208C
Samples LV LWLV AB
LV �LV ÿLV
(mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2) (mJ/m2)
Untreated zeolite 34.49 19.22 15.27 4.28 13.63
-APS treated zeolite 39.31 27.08 12.23 4.12 9.09
254 J.Y. Lee et al. / Materials Chemistry and Physics 63 (2000) 251±255
4. Conclusions
The effects of -APS treatment on the surface tension
of the natural zeolite used for polymer composites
were studied by wicking method using the Washburn's
equation. The components of surface tension for the
natural zeolite were LWSV � 19.22 mJ/m2, �SV � 4.28 mJ/
m2, ÿSV � 13.63 mJ/m2, ABSV � 15.27 mJ/m2 and SV �
34.49 mJ/m2. Those for the -APS treated zeolite were
LWSV � 27.08 mJ/m2, �SV � 4.12 mJ/m2, ÿSV � 9.09 mJ/
m2, ABSV � 12.23 mJ/m2 and SV � 39.31 mJ/m2. The
value of LWSV component increased, while that of AB
SV
component decreased with -APS treatment. It was due
to that the surface of the zeolite was more affected by alkyl
group (hydrophobic) than amine or hydroxyl groups (hydro-
philic).
Acknowledgements
This work was supported by Jeong Moon Information Co.
Ltd. and Keuk Dong Design & Communication Co. Ltd. in
Korea.
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Fig. 8. FT-IR spectra of natural zeolite untreated (A) and treated (B) with 0.5 wt.% -Aps.
J.Y. Lee et al. / Materials Chemistry and Physics 63 (2000) 251±255 255
