JOURNAL OF COLLOID AND INTERFACE SCIENCE
ARTICLE NO.
199, 99–104 (1998)
CS975302
Effect of Tannic Acid on the Surface Free Energy
of Polyester Dyed with a Cationic Dye
Alfonso Ontiveros-Ortega,* Manuel Espinosa-Jiménez,* ,1 Emil Chibowski,† and Fernando González-Caballero‡
*Department of Applied Physics, Faculty of Experimental Sciences, University of Jaén, 23071 Jaén, Spain; †Department of Physical Chemistry,
Faculty of Chemistry, Maria Curie-Sklodowska University, 20031 Lublin, Poland; and ‡Department of Applied Physics,
Faculty of Sciences, University of Granada, 18071 Granada, Spain
Received September 24, 1997; accepted November 4, 1997
properties of these materials. Investigations of dye adsorption on the materials and the electrical and thermodynamic
properties of the fiber/solution interface are fundamental for
understanding the dyeing mechanism and the finishing of
textile materials (1, 2). Results of the investigation of surface free energy can be correlated with some important technical properties of the textile (3–6). The polyester fibers
are highly crystalline and markedly hydrophobic and usually
do not contain basic or strongly acidic groups. Therefore,
generally they do not dye easily with anionic or cationic
dyestuffs (3, 4). Hence, the presence of surfactants during
the dyeing process, in some cases, favors the uptake of dyes
by the fibers. However, for polyester fibers little work has
been done (4, 5), and practically no papers have been reported in the literature in which the application of tannic
acid as a dyeing assistant was studied.
The purpose of this paper is to investigate the changes
in surface free energy of polyester fiber, which are due to
adsorption of tannic acid and of a cationic dye on surfaces
with preadsorbed tannic acid, and the electrokinetic potential
of the polyester fabric. Results of this study may be helpful
in improving the dyeing properties of polyester. To determine the changes the thin-layer wicking method was used.
The method was originally applied by Giese et al. (7) and
later modified by Chibowski and Holysz (8, 9). This method
seems especially suitable for investigation of the surface
free energy of textiles in the process of surfactant and dye
adsorption (6). Details of the experimental procedure and
theoretical background can be found elsewhere (7–13). The
approach of van Oss et al. (10–12) to the formulation of
surface and interfacial free energies will be applied in this
paper. According to this approach, the surface free energy
of a phase i is expressed as a sum of apolar Lifshitz–van
der Waals and polar acid–base components:
The effects of tannic acid and Rhodamine B on the zeta potentials and surface free energy components of polyester fabric (Dacron 54, Style 777) were studied. Knowledge of these parameters
may be helpful in improving the dyeing process of the polyester
fabric. It was found that the zeta potential of the fabric in tannic
acid solutions (10 06 –10 02 M) was negative and its absolute value
decreased with increasing concentration of the acid. It was concluded that H / ions from the tannic acid adsorbed on the polyester
surface caused a decrease in the negative zeta potential. The adsorption of tannic acid probably occurs via hydrogen bonding between the hydroxyl phenolic groups of the acid and the carboxyl
end-groups of the polyester, which hydrolyze in the acidic medium.
The free energy components determined show that the untreated
polyester surface is practically monopolar basic with the electron
2
donor component, g 0
s , equal to 56.7 mJ/m . This relatively high
value probably results from the presence of highly polar carboxyl
end-groups. The adsorbed tannic acid causes an increase in the
2
g0
s component to 66.4 mJ/m due to the presence of more polar
groups on the fabric surface. The subsequent dyeing of the surface
with Rhodamine B causes a further increase of this interaction to
2
g0
s Å 70.6 mJ/m . Rhodamine B contains two amine groups and
one carboxyl group which can strongly interact with the polyester
surface as well as with the adsorbed tannic acid molecules. Because
of the cationic character of Rhodamine B molecules, the electrostatic interaction with the negatively charged polyester surface
plays an essential role. The thin-layer method for determining the
surface free energy components of the fabric appeared to be very
useful, especially because the contact angle method cannot be applied here. q 1998 Academic Press
Key Words: polyester fabric; tannic acid; Rhodamine B; zeta
potential; surface free energy.
INTRODUCTION
Because of the importance of polyester fabrics in the textile industry, there is great interest in improving the dyeing
1
gi Å g LW
/ g AB
i
i .
The acid–base component results from interaction of an
To whom correspondence should be addressed.
99
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All rights of reproduction in any form reserved.
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100
ONTIVEROS-ORTEGA ET AL.
electron donor (Lewis base), g s/ , and an electron acceptor
(Lewis acid) and is expressed as
g AB
Å 2( g i0 g i/ ) 1 / 2 .
i
Usually, one apolar liquid (n-alkane) and two polar liquids
(water and formamide) are used as the probe liquids for the
wicking experiments.
[2]
MATERIALS AND METHODS
The acid–base interactions are not additive as the Lifshitz–
van der Waals interactions are (11–15). The total interfacial
free energy between phases i and j is expressed as
gij Å g LW
/ g LW
0 2( g LW
g LW
) 1 / 2 / 2[( g i0 g i/ ) 1 / 2
i
j
i
j
/ ( g j0 g j/ ) 1 / 2 0 ( g i0 g j/ ) 1 / 2 0 ( g i/ gj ) 1 / 2 .
The polyester fabric used was 100% pure polyester, Dacron 54, Style 777, from TESTFABRICS, kindly supplied
by the Instituto de Investigación Textil y de Cooperación
Industrial, Tarrasa, Barcelona, Spain. The monomeric unit
of polyester used is
[3]
HO©(CH¤©CH¤©OOC©
The technique used for the determination of surface free
energy components is based on Washburn’s equation (16).
The modified equation (8, 9, 13) relates specific changes in
the surface free energy, DG, accompanying the wicking
process:
x2 Å
Rt
DG.
2h
[4]
In the experiment the time t needed by a liquid (having
viscosity h ) to penetrate a given distance x through a porous
layer (a strip of the textile) is measured. According to our
previous work (8, 9, 13), four cases can be distinguished
in which DG has different values, namely:
(i) DG Å DGp Å g1 , for apolar liquid (n-alkane) completely wetting the solid surface on which the duplex fim of
the liquid is present (the surface has been precontacted with
the liquid vapor) ahead of the penetrating front.
(ii) DG Å DGb Å Wa 0 Wc , for the same liquid as above,
but the solid surface is bare (no duplex film). Wa is the work
of liquid adhesion and Wc is the work of liquid cohesion.
(iii) DG Å DGp , for a polar liquid partially wetting the
solid surface (defined contact angle on the flat surface),
which was precontacted with the liquid vapor.
(iv) DG Å DGb , for the same polar liquid, but a bare
solid surface.
A relationship between DG values for (iii) and (iv) holds
(8, 9, 13):
DGb 0 DGp Å Wa 0 Wc .
[5]
From Eqs. [3] and [5] it is seen that to determine three
unknown components of the solid surface free energy,
0
/
g LW
s , g s , g s , three liquids of known components of their
surface tension must be used for the wicking experiments.
This is because
LW 1 / 2
) / 2( g s0 g l/ ) 1 / 2 / 2( g s/ g l0 ) 1 / 2 .
Wa Å 2( g LW
s gl
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©COO)n©CH¤©CH¤©OH
where n Å 104. The molecular weight is about 20,000 g
mol 01 . The samples of the fabric were rinsed repeatedly
with boiling deionized water until constant conductivity was
reached. Finally, they were dried in a desiccator at 313 K.
The tannic acid and cationic dye (Rhodamine B) used in
the experiments were A.R. grade from Merck (Germany)
and were used without further purification.
The tannic acid used, C76H52O46 , is a derivative of glucose
in which five hydroxy groups are substituted for digallic
acid. In this way a large number of phenolic hydroxy groups
in tannic acid are generated (17). Its molecular weight is
1701.23 g mol 01 .
The molecular structure of the cationic dye Rhodamine B
(C.I. 45170) is
(C¤Hfi)¤N
1
O
N(C¤Hfi)¤
Cl2
C
COOH
C¤°H‹⁄N¤O‹Cl: mol. wt. 479.024
The samples of polyester fabric were conditioned with the
solution of tannic acid at a constant temperature (293 K)
for 24 h. They were dried in a desiccator and then they were
conditioned with the solution of Rhodamine B for 24 h. This
time was sufficient to attain equilibrium. The amount of dye
adsorbed on the fabrics was determined from the difference
between initial and final concentrations of dye in the solution
after the equilibrium had been attained. A Hitachi U-2000
spectrophotometer was used for determination of dye concentration in the solution and the wavelength of maximum
absorbance occured at 557 nm.
The pH of the solution was measured with a Radiometer
pH meter (Model PHM 62). The adsorption of tannic acid
and Rhodamine B occurred at the natural pH of the solutions,
2.70 and 3.25, respectively.
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TANNIC ACID ON DYED POLYESTER
TABLE 1
Surface Tension and Its Components of the Liquid Probes Applied for Thin-Layer Wicking and/or Contact Angle Measurements
Liquid
gl
(mJ/m2)
gLW
(mJ/m2)
g/
(mJ/m2)
g0
(mJ/m2)
n-Nonane
Diiodomethane
Water
Formamide
22.85
50.8
72.8
58
22.85
50.8
21.8
39.0
0.0
0.0
25.5
2.28
0.0
0.0
25.5
39.6
The thin-layer wicking experiments were conducted essentially in the same way as described elsewhere (8, 9, 13,
14, 18, 19). Here strips of the fabric, 25 cm long and 2.5
cm wide, were first equilibrated in the tannic acid solutions
at different concentrations for 24 h at 293 K, then dried in a
desiccator, and used for the wicking experiments. In another
series the tannic-acid-treated strips were then equilibrated
with Rhodamine B, also for 24 h at 293 K, then dried at
313 K and kept in a desiccator before wicking. For the
wicking experiments the following liquids probes were used:
n-nonane and formamide, anal. grade (from Merck), and
water, doubly distilled and deionized (Mili-Q Plus system).
The surface tension components of the liquids are listed in
Table 1. They were used without further purification.
The wicking experiments were conducted in a closed
chamber made of plexiglas. The fabric strip to be tested was
placed on a glass plate which was put into the chamber. The
strip was slightly strained to obtain uniform ‘‘porosity’’ for
the penetrating liquids. As mentioned in the Introduction, to
determine the surface free energy components the wicking
experiments must be conducted on bare strips and strips
precontacted with the wicking liquid vapors (8, 9, 13). The
equilibration of the strips with the liquid vapors was conducted in clossed vessels on the bottom of which the liquids
were poured. The strips were left in the vessels overnight
and after that they were used for wicking with the proper
liquid. For each system three wicking experiments were conducted and the average value was used for the calculations.
The reproducibility of the penetration rates was very good.
To determine the surface free energy components of tannic
acid, its powder was compressed at 150 kg/cm2 and contact
angles of diiodomethane, water, and formamide were measured on the pellets. The contact angle technique was also
used to determine the surface free energy components of
Rhodamine B. In this case a 10% aqueous solution of Rhodamine B was poured onto a glass plate and dried at room
temperature for 24 h and then in a vacuum desiccator over
a dehydrating agent. The contact angles were measured with
a Ramé–Hart 100-00 (USA) goniometer. Contact angles of
each liquid were measured for several droplets and an average value was taken for the calculations.
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For zeta potential measurements of the polyester fabric in
tannic acid solutions a piece of the fabric was equilibrated
overnight in a particular solution of tannic acid, shaken by
hand, dipped into liquid nitrogen for a while, and then
ground in a coffee mill. Then the powder was poured with
tannic acid solution at the same concentration as during
equilibration (10 02 –10 06 M), and after sedimentation of
the bigger particles (during 15 min) the supernatant was
used for zeta potential measurements. The zeta potential
was determined by microelectrophoresis using a ZetaPlus
(Brookhaven Instr. Co.), which uses a dynamic light scattering technique.
RESULTS AND DISCUSSION
Figure 1 presents the changes of the zeta potential of
polyester fabric as a function of the equilibrium concentration of tannic acid. The changes for fabric treated with tannic
acid at varying (only in tannic acid solutions) and constant
(tannic acid and 10 03 M NaCl) ionic strengths, except for
last two concentrations, 10 03 and 10 02 M, are shown. The
zeta potentials are negative. It can be seen (Fig. 1) that at
varying ionic strengths the absolute value of zeta potential
decreases with increasing concentrations of tannic acid. This
seems to be in accordance with the classical Gouy–Chapman
theory of the electrical double layer. Here, two effects determine the zeta potential value, namely, the adsorption of H /
ions onto the negatively charged surface of the polyester and
the compression of the electrical double layer which thickens
with increasing concentration of tannic acid. It was found
(20) that H / ions are potential-determining ones for the
polyester surface. In the presence of NaCl (Fig. 1, practically
constant ionic strength) the negative zeta potential is relatively high and practically constant in the range of 10 06 and
10 03 M tannic acid. This fact can be attributed to a possible
FIG. 1. Changes of zeta potential of the polyester fabric/tannic acid as
a function of tannic acid concentration and in the presence of 10 03 M NaCl.
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102
ONTIVEROS-ORTEGA ET AL.
FIG. 2. Penetration times of n-nonane, water, and formamide into strips
of untreated polyester (bare strips).
FIG. 4. Penetration times of n-nonane, water, and formamide into strips
of 10 03 M tannic acid-treated polyester (bare strips).
adsorption of Cl 0 ions and a competition with H / ions adsorption on the fabric surface. At the highest concentration
of NaCl (Fig. 1), the double-layer compression, it seems,
affects the zeta potential. From Fig. 1 it may be concluded
also that tannic acid molecules are adsorbed onto the polyester surface in increasing amounts with increasing concentrations. The same was observed for cellulose fabric [2].
To understand better the dyeing of the polyester with the
cationic dye, the samples treated with tannic acid were subsequently dyed with Rhodamine B. These processes should be
accompanied by changes of the surface free energy of the
polyester. The energy components, apolar Lifshitz–van der
0
Waals, g LW
s , polar electron donor, g s , and electron ac/
ceptor, g s , were determined by means of the thin-layer
wicking method. Figures 2 and 3 present the results of the
liquid probe penetration rates obtained for the untreated
polyester strips. Figure 2 shows the results for bare strips
(nonequilibrated with the liquid vapors) and Fig. 3 shows the
same relationships for the strips precontacted (equilibrated)
with the vapors. In Figs. 4 and 5, similar results are shown
for bare strips of the polyester. First tannic acid at 10 03 M
had been adsorbed (Fig. 4) and then Rhodamine B at 10 03
M (Fig. 5) had been adsorbed on the surface. The results
for the precontacted strips are not presented here, as the
relationships were essentially the same, proving that the
Washburn equation holds in the systems tested. Using these
results and the methodology described above the values of
the free energy components of the tested polyester surface
FIG. 3. Penetration times of n-nonane, water, and formamide into strips
of untreated polyester (precontacted with the vapor strips).
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FIG. 5. Penetration times of n-nonane, water, and formamide into strips
of 10 03 M tannic acid-treated polyester and subsequently dyed with 10 03
M Rhodamine B (bare strips).
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103
TANNIC ACID ON DYED POLYESTER
were calculated. First the R parameter was calculated for the
three different surfaces of the fabric (untreated, tannic-acidtreated, and tannic-acid / Rhodamine-treated surfaces).
These values, together with g LW
s , as calculated from the
wicking of bare strips of the polyester by n-nonane, and
g s0 and g s/ , obtained from formamide and water wicking
results, are listed in Table 2. Moreover, in Table 3 the surface
free energy components of tannic acid and Rhodamine B,
as obtained from contact angle measurements, are given. For
comparison, the values for untreated polyester are also given.
From Table 2, it can be seen that the effective diameter
R decreases for the polyester equilibrated in tannic acid solution, from 45 to 37 1 10 05 cm. However, for the polyester
dyed with Rhodamine, the R parameter increases (37 1
10 05 cm) toward its value for bare surfaces. It is difficult to
conclude why the effective diameter changes upon adsorption of the surfactant or dye, but it is not a topic of this
study. As to the g LW
values (Table 2), they are in the range
s
of values found for other materials (21, 22), as well as that
obtained for Leacril fabric (6). The g LW
component results
s
from apolar, mainly London-dispersion, interactions. Because it increases especially after tannic acid treatment, this
acid causes an increase of hydrophobic interaction. However, simultaneously it also causes an essential increase in
polar, electron donor, g s0 , interaction (Table 2), from 56.7
to 66.4 mJ/m 2 . Moreover, while subsequent dyeing with
Rhodamine B only slightly affects the g LW
interaction (Table
s
2), its effect on the electron donor component is predominant, causing a rise to 70.6 mJ/m 2 . The relatively high value
of the g s0 component for the bare polyester surface is probably due to the presence of carboxyl end-groups (23), which
are electron donors. For tannic acid-treated surfaces, the even
higher value of the component is most probably caused by
phenolic hydroxy groups present in its molecule. The same is
true for Rhodamine dyed surfaces, as the molecules possess
strong electron donor groups, two amine and one carboxyl.
This strong basic character of these two compounds can be
seen in Table 3, where g l0 for tannic acid and Rhodamine
B is 58.7 and 51.7 mJ/m 2 , respectively. As mentioned above,
TABLE 2
Effective Pore Radius R and the Surface Free Energy Compo/
nents, Lifshitz-van der Walls gLW
s , Electron Acceptor gs , and
0
Electron Donor gi , of Untreated and Treated Polyester Fabric
with Tannic Acid and Rhodamine B
R 1 105
gsLW
g/
g0
s
s
(cm)
(mJ/m2) (mJ/m2) (mJ/m2)
Polyester fabric
Untreated
Treated with 1003 M tannic acid
Treated with 1003 M tannic acid
and then with 1003 M
Rhodamine B
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37.0
47.5
58.4
0.15
0.10
56.7
66.4
43.0
50.7
0.07
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TABLE 3
Surface Free Energy Components of Untreated Polyester Fabric
(from Thin-Layer Wicking), Tannic Acid and Rhodamine B (from
Contact Angle Measurements)
Material
gsLW
(mJ/m2)
g/
s
(mJ/m2)
g0
s
(mJ/m2)
Untreated polyester
Tannic acid
Rhodamine B
47.5
38.2
38.6
0.15
0.41
1.42
56.7
58.7
51.7
tannic acid possesses strong electron donor phenolic hydroxy
groups and Rhodamine B two amine and one carboxyl
groups. Adsorption of the molecules onto the polyester surface increases the basic character of the surface, and the
adsorption process probably occurs via hydrogen bonding
between carboxyl end groups of the polyester and the hydroxyl groups of tannic acid. The g s0 value for the surface
of untreated polyester (Table 2) is smaller than that after
tannic acid adsorption, 56.7 and 66.4 mJ/m 2 , respectively,
and also higher than that of tannic acid (Table 3), resulting
in the polyester carboxy-end groups becoming stronger electron donor moieties in an acidic environment (pH 2.7 in
these experiments) (23). Also, the adsorbed tannic acid molecules themselves cause increases in the electron donor properties. Dyeing the surface with Rhodamine B introduces
more strong electron donor groups, which appears to further
increase in g l0 parameter (Table 2).
The electron acceptor interaction, which is represented by
the g l/ component, should also be mentioned. As can be
seen from Table 2, the values of this component are negligible for both bare polyester surfaces and tannic acid/Rhodamine B-treated surfaces. Thus, we have a practically monopolar surface of the material. However Rhodamine B shows
remarkable electron acceptor interaction, g l/ Å 1.42 mJ/m 2 ,
which together with high g l0 interaction (70.6 mJ/m 2 )
causes strong adsorption of this dye onto tannic acid-treated
polyester surfaces via H-bonding. Also, attractive electrostatic interaction between the negatively charged polyester
surfaces (Fig. 1) and the positive groups of Rhodamine B
molecules probably plays an essential role here.
SUMMARY AND CONCLUSIONS
In light of the results obtained and the above discussion
it may be stated that:
(i) Zeta potential of the polyester fabric is negative in
tannic acid solutions of 10 06 –10 02 M. Its absolute value
decreases as the concentration increases. This behavior can
be ascribed to both H / ion adsorption and compression of
the double-layer thickness. In the presence of 10 03 M NaCl
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ONTIVEROS-ORTEGA ET AL.
the zeta potential was practically constant and more negative,
except for 10 02 M tannic acid. This indicates that Cl 0 adsorption may also take place on the polyester surface.
( ii ) It may be concluded that the adsorption of tannic
acid on the polyester surface occurs via hydrogen bonding
between carboxy end-groups of the polyester and phenolic
hydroxy groups of tannic acid, as well electrostatic interaction.
(iii) The Lifshitz–van der Waals component of surface
free energy g LW
increases by ca. 10 mJ/m 2 after tannic acid
l
adsorption and decreases again after dyeing with Rhodamine. This could be explained in terms of the orientation of
hydrocarbon groups on the polyester surface, although it is
difficult to speculate about the details of this mechanism.
(iv) The surface of the polyester is practically a monopolar Lewis base with a strong electron donor interaction. It
is believed that the interaction originates from the carboxy
end-groups.
(v) Adsorption of tannic acid and subsequent adsorption
of Rhodamine B enhance the electron donor interaction,
which can also be explained via hydrogen bonding interaction and the presence of highly polar functional (phenolic
hydroxy) groups of tannic acid and acidic hydrolysis of the
polymer carboxy groups in a low pH 2.7.
(vi) The high g s0 Å 70.6 mJ/m 2 obtained for the surface
dyed with Rhodamine, which is higher then Rhodamine itself, may be attributed to the presence of many different
polar groups on the polyester surface, which may cause a
synergetic effect.
(vii) The thin-layer wicking method seems to be very
useful for textile fabric free energy determination, especially
because the contact angle method fails for such materials.
ACKNOWLEDGMENTS
We thank the Dirección General de Investigación CientıB fica y Técnica
(DGICYT), Spain, for providing the funds for Research Project PB940462,
of which this study is a part. Also, financial support for this cooperation
from the Committee for Scientific Researches, Warsaw, Poland (Project
KBN No.z/15/2/97), is very much acknowledged.
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