polarity characteristics -...
TRANSCRIPT
Chapter 3
POLARITY CHARACTERISTICS
Abstract
This chapter deals with the polarity characteristics of banana fiber.
Surface polarity of banana fiber has been characterized by
solvatochromic and electrokinetic methods. The Kamlet–Taft hydrogen-
bond-donating ability (α), the hydrogen-bond-accepting ability (β),
dipolarity/polarizability (π*), Gutman’s acceptor number (AN) and
Reichardt’s ET (30) parameter have been used to quantitatively represent
the polarity of the untreated and chemically treated banana fiber.
Different chemical treatments used to modify the banana fiber, have
decreased the hydrogen-bond-donating ability of the fiber. Acid-base
parameters obtained from solvatochromic measurements were found to
be consistent with polarity parameters obtained from electrokinetic
measurements. The isoelectric point of banana fiber was found to have
increased after the chemical treatment.
A part of the results in this chapter has been published in Biomacromolecules 9, 1802, 2008
104 Chapter 3
3.1 Introduction Quantification of the general properties of solvents and micelle
environments has been studied by physical organic chemists for many
years. The response of solvatochromic indicators on changing the
solvent environments have been used as the phenomenological basis for
several empirical solvent polarity scales. Among such polarity scales, the
Kamlet -Taft system is the most comprehensive with respect to all solvent
types and it is well supported by the theoretical reaction field models for
the solvent influences upon the solvatochromic probes [1-2]. The
simplified linear solvation energy (LSE) relationship in the Kamlet-Taft
system is given by the following equation [3]. In this equation, three
major intrinsic solvent
( ) ( )0XYZ = XYZ s * +d α +baπ δ β+ (3.1)
properties are included: the solvents’s dipolarity-polarizability (π* and its
dδ correction term for the polarizability), hydrogen-bonding acidity (∝)
and hydrogen bonding basicity (β). (XYZ)0 is the solute property of a
reference system (e.g., a non-polar medium or the gas phase). The term
d is a polarizability correction term that is 1.0 for aromatic solvents, 0.5
for polyhalogenated solvents, and 0 for aliphatic solvents.
Empirical solvent polarity scales based on spectroscopic measurements
usually employ changes in the UV/Vis absorption, (solvatochromism) or
the fluorescence spectrum (fluorochromism) of a solvatochromic probe
molecule which serves as an observer at the molecular level (Reichardt,
1994). UV/Vis spectroscopy of solvatochromic probe dye molecules has
been established for investigating the surface polarity of solid surfaces
[4-6]. The property of a probe dye to change its characteristic UV/Vis
absorption maximum in dependence of the polarity of its surroundings,
Polarity Characteristics 105
polarity parameters of the surface such as the acidity α or the basicity
β can be determined by means of empirically established equations [7].
The interaction between the functional groups on the surface and the π
electron system of the solvatochromic dye can be observed as a shift of
the UV/Vis absorption maximum of the probe dye [7]. The interaction of a
surface environment with a solvatochromic dye is a combination of many
effects. Acid-base, ion-dipole, dipole-dipole, dipole-induced dipole, and
dispersion forces contribute to the overall adsorption energy of a probe
with a polymer surface [8, 9]. Multiple intermolecular solute/solvent
interactions can be described by the linear solvation energy relationship
(LSER) of Kamlet and Taft. Certain dyes can be used as probe
molecules to characterize the Lewis acid-base properties as well as the
polarity of solid surfaces. In an extensive experimental study, Spange
and Reuter [10] showed that cis-dicyano-bis (1,10-phenanthroline)-iron
[Fe(phen)2(CN)2] (1) can be used as an indicator to quantify surface
acidity α, Michler’s ketone (4,4’-bis(N,N-dimethyl amino benzophenone)
(2) allows to estimate surface’s dipolarity/polarizability π*, and the
indicator 3-(4-amino-3-methylphenyl)-7-phenyl-2,6 dihydrofurano [2’,3’:4,5]
benzofuran-2,6-dione (ABF) (3) is sensitive for surface basicity β of
modified silica particles.
Spange et al. [11] studied the α, β, π* parameters of native cellulose
batches, carboxymethyl celluloses, cellulose tosylates and other
derivatives with different degree of substitution and found that α depends
on both the amount and strength of the accessible acidic surface groups
on the cellulose surface. Glass fibers with different surface properties
were investigated by inverse gas chromatography and UV/Vis
spectroscopy of solvatochromic probe dye molecules by Dutschk et al. [7].
They found that both methods gave useful information on surface
106 Chapter 3
characteristics. Pothan et al. [12] reported the determination of polarity
parameters of chemically modified cellulose fibers by solvatochromic
technique and reported that different silanes, NaOH, and long alkyl groups
used to modify the cellulose fiber surface have changed the hydrogen
bond donating ability of the fiber. Kamlet-Taft’s α, β and π* values of
carboxymethyl celluloses (CMCs) and cellulose tosylates (CTs) with
different degrees of substitution were reported by Fischer et al. [13]. These
authors observed that α values of CMCs and CTs significantly decrease
with increasing degree of substitution due to the decrease of the number of
cellulosic groups (Cell-OH). The π* values of the CMCs show no linear
dependence on the degree of substitution. Spange and Reuter [10] further
reported the application of Fe(phen)2(CN)2 and Michler’s ketone as
suitable surface polarity indicators to analyze the α and π* term of
modified silica particles and also on the application of the Kamlet-Taft
parameters α , β and π* as a reference system to parameterize the surface
polarity. Several other studies on solvatochromism are reported in
literature [14-16].
Surface properties of solids can be investigated by elecrokinetic
methods. Electrokinetic phenomena can be observed by contacting a
solid surface with a polar liquid medium, because of the existence of an
electrical double layer at the solid-liquid interface [17]. The strength of the
interaction is determined by the extent of dispersion and acid-base
interactions. The magnitudes of such interactions depend on the
chemical constitution of the adhering members and adsorption layers. A
better knowledge about the surface properties of lignocellulosic fibers is
needed to use them as effective reinforcements in polymer matrices. The
zeta (ζ)-potential provides information about the formation of the
electrochemical double layer, which forms at the interface between a
Polarity Characteristics 107
solid and an electrolyte solution due to the adsorption of ions and the
dissociation of functional groups [18].
Zeta potential measurements are commonly carried out to investigate the
surface properties and the possible interactions on cellulose fibers [19].
These measurements can be used to characterise the changes produced
after the surface modification of lignocellulosic fiber. Stana-Kleinschek
and Ribitsch [20] discussed the electrokinetic properties of processed
cellulose fibers. They studied the changes in surface properties by chemical
purification of cellulose and the effect on acid-base properties. The
characterisation of solid surfaces by electrokinetic phenomena was reported
by Jacobash [21]. The influence of fiber surface modification on the
thermal and electro kinetic properties of coir and sisal fibers have been
investigated in detail by Bismarck et al. [22]. Pothan et al. [23] studied
the influence of chemical treatments on the electrokinetic properties of
cellulose fiber. The results revealed that chemical treatments with alkali,
acetic anhydride, triazine coupling agent and various silanes reduced the
acidity of the already polar cellulose fiber. Bellmann et al. [24] investigated
the electrokinetic properties of natural fibers. They concluded that the
electrokinetic effect of streaming potential is suitable to analyze the
swelling characteristics of any kind of fiber and the isoelectric point of
fibers treated with various silanes correlates well with the acceptor
number which may be derived from solvatochromic experiments.
Bismarck et al. [25] used zeta potential measurements to confirm
changes at the surface of oxygen plasma treated carbon fibers. The
surface polarity of grafted carbon fibers was determined by contact angle
measurements and confirmed by zeta potential measurements.
108 Chapter 3
0 2 4 6 8 10 12 14 16
-20
-15
-10
-5
0
5
10
15
20
25
pHIEPpHIEPpHIEP
dissociable basic molecule groups amphoteric behaviour dissociable acidic molecule groups
zeta
pot
entia
l (m
V)
pH-value
0 2 4 6 8 10 12 14 16
-20
-15
-10
-5
0
5
10
pHIEP
non-polar surface
zeta
pot
entia
l (m
V)
pH-value
Fig.3.1 Schematic picture of ζ = f (pH) for Bronsted acidic, alkaline and non-polar surfaces
[Ref.: R. W. A. Taft. J. Phys. Chem. 83, 412, 1979]
Polarity Characteristics 109
From the ζ = f (pH) curve, (Fig.3.1), it can be concluded whether
adsorption or dissociation process is predominant. When the formation of
the electrical double layer is caused by the dissociation of the acidic
functional surface groups, a plateau area in the alkaline range for the pH
dependence of the ζ- potential is obtained. If there is a change in the sign
of the ζ- potential in the acidic range, this is, first, caused by repressing the
dissociation of the acidic surface groups, and second, by the adsorption of
the potential determining ions. An analogous trend in the ζ- potential is
present if there are any alkaline surface groups. The isoelectric point, IEP,
where ζ= zero, is also a measure of the acidity or basicity of a solid
surface, if the dissociation of surface groups is the predominant
mechanism for the formation of the electrical double layer. The solid
surface displays acidic character, if the value of IEP is low. If IEP is
situated in the alkaline range, the solid surface contains basic surface
groups. Shifts of the IEP in the ζ = f (pH) due to solid surface modifications
indicate the concentration of dissociable surface groups [26].
Only a few studies are reported in literature where the natural fibers are
characterized by polarity measurements. This chapter is intended to
describe the characterization of chemically treated BF with three
carefully chosen probe dye indicators, dicyano-bis (1,10-phenanthrolin)
iron(II) 1, Michler’s ketone 2, and aminobenzodifuranone 3. The
calculation of the polarity parameters with different multiple correlation
equations are described. Electrokinetic measurements are carried out
to measure the surface polarity and to determine the acid-base
properties of the BF.
110 Chapter 3
3.2 Results and Discussion
3.2.1 Solvatochromic studies Fig. 3.2 shows the UV/Vis spectra of BF and BF treated with 10%
alkali when probed with dye 1. The spectra of alkali treated BF show a
definite change in the absorption peak compared with the BF, showing
a reduction in the number of acidic sites. Treatment of BF with alkali
gives a hypsochromic shift, as can be seen in Fig. 3.2. The changes in
the absorption peak can be due to the changes in the surface
properties of the BF due to the interaction with alkali. The effect of alkali on
a cellulose fiber is a swelling reaction, during which the natural
crystalline structure of the cellulose relaxes. The alkaline solution
influences not only the cellulosic components inside the plant fiber, but
300 350 400 450 500 550 600 650 700 750 800
1. untreated2. alkali treated
2
1
Abso
rptio
n (a
u)
Wave length(nm)
Fig.3.2 UV/Vis absorption spectra of dye 1 adsorbed onto untreated and alkali treated banana fiber.
Polarity Characteristics 111
also the non-cellulosic components (hemicellulose, lignin and pectin)
which are situated between the cellulose regions that exist inside and
between the elementary fiber. Fig.3.3 shows the UV/Vis spectra of BF
when probed with dyes 1, 2 and 3. Clear absorption peaks are not visible
for dye 3 when adsorbed onto BF probably due to the inability of the dye
to attach with the basic groups present in the fiber.
300 350 400 450 500 550 600 650 700 750 800
3
21
1. iron dye2. michler's ketone3. furan dye
Abso
rptio
n (a
u)
Wave length (nm)
Fig.3.3 UV/Vis absorption spectra of dyes 1, 2, and 3 adsorbed onto
banana fiber
Table 3.1 shows the UV/Vis absorption maxima for the three dyes used
on differently treated fibers. The νmax value for dye 3 is found to be the
lowest when compared to dye 1 and 2. Iron dye can interact with more
acidic groups. The dye becomes protonated when adsorbed on acidic
surfaces. It can interact through the cyano group on HBD. The indicators 2
and 3 can interact in many ways with surfaces of the BF. The β term
112 Chapter 3
using 3 as the probe for BF is not determinable, because 3 also interacts,
via the carbonyl oxygen, with acidic sites of the cellulose -OH group.
Table 3.1 UV/Vis absorption maxima for the three dyes used on untreated and chemically treated banana fiber.
Samples νmax 1 (10-3 cm-1)
νmax 2 (10-3cm-1)
νmax 3 (10-3 cm-1)
Untreated banana fiber 19.7 26.9 18.5
10% NaOH treated fiber 18.6 25.3 16.0
Benzoylated fiber 19.3 26.2 17.6
Stearic acid treated fiber 19.0 25.6 15.1
Aminopropyl silane treated fiber 18.8 24.3 19.0
Triethoxy octyl silane treated fiber 19.0 25.3 16.5
Vinyl trimethoxy silane treated fiber 19.4 25.6 16.2
KMnO4 treated fiber 19.1 24.5 18.0
Fig.3.4 shows the UV/Vis absorption bands of dye 1 adsorbed on
chemically treated BF. The absorption maxima of the dye depend on the
type of the chemical treatment given to the fiber. Absorption peak is
found maximum for untreated BF while it is lowest for stearic acid treated
fiber. Absorption peaks are clearly visible for alkali treated fibers with all
the three dyes [Fig.3. 5].
Polarity Characteristics 113
350 400 450 500 550 600 650 700 750 800
7
6
54 3
21
Abso
rptio
n (a
u)
Wave length (nm)
1. untreated2. amino silane treated3. benzoylated4. vinyl silane treated5. triethoxy silane treated6. alkali treated7. Stearic acid treated
Fig.3.4 UV/Vis absorption spectra of dye 1, adsorbed onto chemically treated banana fiber
Table 3.2 and 3.3 contains the Kamlet–Taft polarity parameters
calculated for alkali treated, various silane treated, benzoylated and
stearic acid treated BF. The Kamlet–Taft HBD ability α is lowest for alkali
treated fiber. The decrease in the α value can be attributed to the
changes that occurred to the fiber surface as a result of the alkali
treatment. The process of alkali absorption destroys the hydrogen bonds
within the cellulose, opening up the structure. Hemicellulose, which
consists principally of xylan, polyuronide and hexosan is very sensitive to
the action of alkali. When the hemicellulose is removed, the inter fibrillar
region is likely to be less dense and less rigid, making the fibrils more
capable of rearranging themselves.
114 Chapter 3
300 350 400 450 500 550 600 650 700 750 800
3
2
1
Abso
rptio
n (a
u)
Wave length (nm)
1. Iron dye2. Michlers ketone3. Furan dye
Fig. 3.5 UV/Vis absorption spectra of dyes1, 2 and 3 adsorbed onto
alkali treated banana fiber
Table 3.2 Values of the Kamlet-Taft polarity parameters for untreated and chemically treated banana fiber.
Samples α π* β
Untreated banana fiber 1.6 0.94 0.5
10% NaOH treated fiber 1.3 0.76 1.0
Benzoylated fiber 1.5 0.89 0.5
Stearic acid treated fiber 1.4 0.64 1.2
Amino propyl silane treated fiber 1.39 1.1 1.5
Triethoxy octyl silane treated fiber 1.37 0.74 1.0
Vinyl trimethoxy silane treated fiber 1.43 0.67 0.8
KMnO4 treated fiber 1.41 0.74 0.9
Polarity Characteristics 115
Table 3.3 AN and ET (30) parameters of untreated and chemically treated banana fiber
Samples AN ET (30)
Untreated banana fiber 61 64
10% NaOH treated fiber 51 58
Benzoylated fiber 59.5 63
Stearic acid treated fiber 54.6 59.1
Aminopropyl silane treated fiber 55 56.7
Triethoxy octyl silane treated fiber 54 55
Vinyl trimethoxy silane treated fiber 57 59
KMnO4 treated fiber 56 58
Dissolution of waxy substances exposes the –OH and the –COOH
groups on the fiber surface leading to increased polarity and decreased
acidity of the fiber surface. Thus the concentration and strength of the
acidic sites on the cellulose contribute to α. Because of the concentration
of the most acidic sites is very low, it is not possible to decide at which
surface site the probe is located. The π* value of the alkali treated fiber is
low due to the decreased polarity of the environment. The AN values
calculated for these fibers, which are indicative of the electron-accepting
ability or acidity, also shows the same trend. The ET (30) parameter
reflects the polarity of the environment. This is also found to be lower for
alkali treated fiber.
The HBD ability α for different silane treated BF are found to be lower
than that of BF, but greater than that of alkali treated fiber. The three
types of silanes used are aminopropyl triethoxy silane, triethoxy octyl
116 Chapter 3
silane and vinyl trimethoxy silane. BF is treated with 0.5% alkali before
treatment with silane. The advantage of using silane coupling agents are
their inherent natural attraction to both the natural fiber and the resin
matrix. The reaction mechanisms is as follows. First, silane reacts with
water to form silanol and an alcohol. In the presence of moisture, the
silanol reacts with the hydroxyl group attached to the cellulose molecules
of the fiber through an ether linkage with the removal of water. Besides
these reactions, the silanols can condense to give polysiloxanes. Fig. 3.6
shows the reaction of silane with BF. Fig.3.7 shows the UV/Vis spectra of
the three different silanes when probed with dye 3. For the aminosilane
treated BF, the dye is able to give a sharp UV/Vis absorption band. The
absorption spectrum is broad for dye 3 adsorbed onto fibers treated with
the triethoxy octyl silane. The α value for triethoxy octyl silane treated
fiber is lower when compared to vinyl triethoxy silane treated fiber. This
can be attributed to low surface acidity of the functional groups. Moreover
the bulky alkyl groups possibly prevent the dyes from attaching to the
polar surface centers of the fiber. Amino silane treated fiber has acidity
lower than that of vinyl triethoxy silane treated fiber but acidity higher
than that of triethoxy octyl silane treated fiber. The HBD ability is
lowered because of the presence of the polar amino group. But polarity
value π* is found higher for amino silane treated fiber. This can be
explained as due to the differences in the functional groups of the
silanes. The presence of amino group will also significantly improve the
polarizability of the amino silane treated fiber. The β value is found higher
for amino silane treated fiber showing the presence of basic sites on the
treated fiber surface. Spange et al. [27] reported a decrease in the value
of the parameter α through an increased degree of functionalization on
the silica surface.
Polarity Characteristics 117
Fig. 3.6 Schematic representation of interaction of silane with banana fiber; R represents the alkyl group
Fig.3.8 shows the UV/Vis absorption spectra of dyes 1 and 2 adsorbed
onto benzoylated BF. Absorption peaks of the two dyes are clearly
visible. Hydrogen bond donating ability α (Table 3.2) is found higher for
118 Chapter 3
350 400 450 500 550 600 650 700 750 800
1
3
2
1Ab
sorp
tion
(au)
Wave length (nm)
1. aminopropyl silane treated2. triethoxy octyl silane treated3. vinyl trimethoxy silane treated
Fig.3.7 UV/Vis spectra of the three different silanes when probed with dye 3
400 500 600 700 800
2
1
Abs
orpt
ion
(au)
Wave length (nm)
1. Iron dye2. Michlers keton dye
Fig.3.8 UV/Vis absorption spectra of dyes 1 and 2 adsorbed onto benzoylated banana fiber
Polarity Characteristics 119
benzoylated fiber indicating higher acidity when compared to other
treated fibers. This is due to the formation of HCl. The benzene ring
also plays important role in enhancing the acidity. The π* value is also
high. But HBA, which represents basicity, is low. AN (Table 3.3) is
also high indicating high acidity. ET(30) value which depends on α and
π* is also high when compared to other treated fibers. The probable
reaction between BF and benzoyl chloride is represented in Fig.3.9.
+
2Fiber-OH +NaOH Fiber-O Na H O-→ +
Fibre - O-Na+ + Cl -C -
O
Fibre - O-C-
O
+ NaCl
Fig.3.9 The probable reaction between banana fiber and benzoyl chloride
In the case of stearic acid treated fiber, the α value is found to be lower
than that of the untreated fiber showing the reduction in the number of
acidic sites. The polarisability value is also reduced. In stearic acid
treated fiber, the carboxyl group of the stearic acid reacts with hydroxyl
group of the fiber through an esterfication reaction and hence the
treatment reduced the number of hydroxyl groups available for bonding
with water molecules. Another reason for lower acidity is that the bulky
alkyl groups possibly prevent the dyes from attaching to the polar surface
centers of the fiber. The treatment also helped to remove the non-
crystalline constituents of the fiber. AN value shows a decrease when
compared to the untreated fibers due to lower acidic character of the fiber
surface. π* value shows that the overall polarity is high for stearic acid
treated fiber. The β value is also high due to the presence of the alkyl
Fiber Fiber
120 Chapter 3
group. The reaction of stearic acid CH3(CH2)16COOH with BF can be
represented as follows.
( ) ( )3 2 3 216 16CH CH COOH+HO Cellulose CH CH COO Cellulose− → − (3.2)
The indicator 3, could not give sharp absorption peaks in almost all cases
when adsorbed onto the untreated and chemically treated fibers. But dye1
give peaks in almost all cases. Unlike dye 1, dye 2 give sharp UV/Vis
absorption bands in all cases. Fibers treated with the various silanes, are
all characterized by the indicator dye 2. Fig.3.10 shows the UV/Vis
absorption spectra of dye 2, adsorbed onto different silane treated fibers.
The absorption peaks of dye 2 are at a lower wavelength than those due to
dye1. The UV/Vis absorption peaks of dye 2 are due to π- π* transition.
400 500 600 700 800
Abso
rptio
n (a
u)
Wave length (nm)
1.aminopropyl silane treated2.triethoxy octyl silane treated3.vinyl trimethoxy silane treated
Fig.3.10 UV/Vis absorption spectra of dye 2 adsorbed onto different
silane treated banana fiber
As evident from Table 3.1, for samples treated with amino silane and
triethoxy octyl silane, UV/Vis absorption maximum obtained at λ max (2)
Polarity Characteristics 121
are at 390 and 401nm, correlate well with the dipolarity/polarizability of the silanol group environment, as reported by others for silica samples
[28]. Vinyl silane treated fibers also give a UV/Vis absorption maximum at
these wavelengths. Dye 2 can be used to detect silanol groups on
cellulose fiber surfaces also.
Treatment of BF in KMnO4 solution leads to the formation of cellulose
manganate complex [29]. The formation of cellulose manganate complex
is responsible for the low HBD value. Scheme 3.1 represents the reaction
of BF with KMnO4 [29].
Cellulose -H + KMnO4 Cellulose -H - Mn -O-K+
Cellulose -H -O - Mn -O-K+
O
O
Cellulose.
+ H -O - Mn -O-K+
O
O Scheme 3.1 The probable reaction between banana fiber and KMnO4
The UV/Vis absorption maxima obtained for dye 1, 2, and 3 are at λmax =
520 nm, λmax = 370 nm and λmax = 626 nm. For KMnO4 treated fiber HBD
value α is less, π* value is also less but β value is higher. It was found
that permanganate could etch the fiber surface as a result of oxidation
and make it physically rougher so that interfacial properties can be
improved by the induced mechanical interlocking.
3.2.2 Zeta potential measurements A study on the influence of pH on zeta potential provides valuable
information on the nature of the surface of chemically treated BF. The
acidity or basicity of the solid surfaces can be determined qualitatively
from the pH that corresponds to zero of the zeta potential (isoelectric
point, IEP). At this pH, the number of negative charges equals the
122 Chapter 3
number of positive ones [20]. The acidity of the surface is always
characterized by the IEP. In the case of low IEP values, the number of acidic
group dominates. The natural cellulose fibers are negatively charged, due to
the presence of carboxyl and hydroxyl groups [20]. Fig.3. 11 shows the pH
dependence of ζ-potential for the chemically treated BF. From the nature of
the graph, it is clear that BF as well as chemically treated BF is acidic in
nature. The results again show that all the fibers investigated absorb large
quantities of water, as evidenced by the negative ζ plateau values measured
over a wide pH range [18]. For the untreated fiber the low isoelectric point,
where the zeta potential is zero, are caused by the low pKa values of the
functional group present in the fibers such as of carboxyl group present in
cellulose, uronic acids and some proteins present in the fibers. These
functional groups will dissociate at pH values above the pKa value giving rise
to negative zeta potential [18]. In the case of BF, a plateau region is
obtained in the pH range starting from 4.4.
2 3 4 5 6 7 8 9 10 11
-14
-12
-10
-8
-6
-4
-2
0
2
4 untreated benzoyl chloride treated triethoxy octyl silane alkali treated aminopropyl silane
Zeta
pot
entia
l (m
V)
pH-value (in KCl, c=10-3mol/L)
Fig.3. 11 pH dependence of ζ -potential on chemically treated BF
Polarity Characteristics 123
The low ζ plateau value is due to the fact that ion adsorption competes with
water adsorption. i.e, it means that fewer ions can adsorb onto very
hydrophilic surfaces and consequently a smaller ζ-potential is measured.
Another cause for the low ζ plateau value could be the removal of water-
soluble components from the surface/bulk of the fiber. The ζ plateau value is
strongly influenzed by the chemical structure and the concentration of
functional groups and their availability at the fiber/electrolyte interface which
depends on the supra molecular structure of the fibers [18]. The ζ-potential
of BF is found to be -4.4 volt. The surface polarity of BF has been
characterized using Linear Solvation Energy (LSE) relationships. These
data have been represented in Table 3.2 and Table 3.3. The acceptor
number calculated for BF has a value 61 and the ET (30) parameter
calculated for the fiber is 64 kcal/mol. The results are consistent with the ζ-
potential values. The low IEP values and the overall polarity parameter
values agree mutually. The molecular structure of cellulose is responsible
for its supramolecular structure and this, in turn, determines many of its
chemical and physical properties.
The interaction between cellulose and aqueous alkali hydroxides results
in swelling and uptake of alkali and water. The process of alkali
absorption destroys the hydrogen bonds within the cellulose opening up
the structure. There is a decrease in the supramolecular order in the
chain conformation of cellulose. Usually, the hydroxyl anions are seen to
be responsible for the primary interaction with the cellulosic hydroxyl
groups in the ordered regions of the structure, while the hydrated cations
are responsible for the resulting swelling. The IEP value for chemically
treated fibers is shown in Table 3.4.
124 Chapter 3
Table 3.4 Isoelectric point of chemically treated banana fibers
Samples IEP values
Banana fiber 2.5
10% NaOH treated fiber 3.6
Benzoylated fiber 2.6
Aminopropyl silane treated fiber 3.4
Triethoxy octyl silane treated fiber 3.2
The value in the case of the BF is 2.6, which shows higher acidity. The
IEP value of NaOH treated fiber is 3.6 indicating that the treatment with
alkali decreases the acidity. At lower pH values, the adsorption of
protons, which are potential determining ions, is the predominant
process, and gives rise to the positive value of the ζ-potential. In this
case, the dissociation process of the functional groups causes a negative
value of the zeta potential. Solvatochromic measurements carried out on
alkali treated fiber showed a definite change in the absorption peak
compared to the untreated fiber showing a reduction in the number of
acidic sites (Fig. 3.2). The acceptor number of alkali treated fiber
calculated using the solvatochromic measurements showed a value
lower than that of BF (Table 3.3). The results are consistent with the ζ-
potential measurements.
The IEP of the silane treated BF (Table 3. 4) are found to be higher than
that of BF. The increase in IEP values indicates lower acidity for silane
treated fiber. Silanes used in this work have two functional groups, a
hydrolysable group which can condense with the hydroxyls of the BF and
an organofunctional group capable of interacting with the matrix. The
Polarity Characteristics 125
hydrolysed silane can undergo the condensation and bond formation stage
when influenced by acid or base-catalysed mechanisms. Besides these
reactions, the silanols can condense to give polysiloxanes. The silanol
groups are chemically attached to the fiber through an ether linkage [30].
Fibers subjected to treatment with aminopropyl triethoxy silane and
triethoxy octyl silane after pretreatment with the alkali give IEP values of
3.4 and 3.2 respectively (Table 3.4). The increase in IEP value, indicates
lower acidity. Solvatochromic studies revealed that the HBD for different
silane treated fibers are lower than that of untreated fiber. The chemical
interaction between the aminopropyl triethoxysilane and BF is believed to
take place additionally other than the normal ether linkage of the other
silanes as shown in Fig.3.12.
Fig.3.12 Schematic representation of interaction of aminopropyl triethoxy silane with banana fiber
[Ref.: L.A. Pothan et al. J. Adhesion Sci.Technol.16, 157, 2002]
The IEP shows higher value in the case of aminopropyl silane treated
fiber proving the lower acidity of the fiber (Table 3.4). The lower acidity
126 Chapter 3
and the IEP value 3.4 can be explained as due to the presence of the
NH2 group. Solvatochromic measurements also showed an increase in
the β value when compared to triethoxy octyl silane treated fiber showing
the basicity of the aminopropyl silane treated fiber.
For benzoylated fiber, ζ-potential measurements shows a negative value
and IEP extrapolated to ≈ 2.5. The fiber has higher acidity as seen from
the IEP value. HBD (Table 3.4) is found higher for benzoylated fiber
indicating higher acidity when compared to BF. The π* value is also
high. But HBA which represents basicity β is low. AN (Table 3.3) is also
high indicating high acidity. ET (30) value which depends on α and π* is
also high when compared to other chemical treatments given to the BF.
3.3 Conclusions Banana fiber was characterized by solvatochromic and electrokinetic
method. Empirical polarity parameters were calculated in terms of the
Kamlet-Taft solvent polarity scale. The HBD ability was found lowest for
alkali treated fiber. All the chemical treatments lowered the polarity of BF.
Of the three silane treated BF, the vinyl silane treated fiber showed
enhanced HBD value. Electrokinetic measurements revealed that the
chemical treatments given to BF increased the IEP value. Benzoylated
fiber showed a lower IEP value when compared to other chemically
treated BF. This implies the higher acidity of benzoylated fiber. The
results obtained were found to be consistent with the polarity values
calculated using solvatochromic measurements.
References 1. R. W. A. Taft. J. Phys. Chem. 83, 412, 1979.
2. J.E. Brady and P.W. Carr. J. Phys. Chem. 86, 3053, 1982.
Polarity Characteristics 127
3. M. J. Kamlet, J-L. M. Abboud, M.H. Abraham and R.W. Taft J.
Org.Chem. 48, 2877, 1983.
4. M.S. Paley, R.A. McGill, S.E. Wallace and J. Harris. Macromolecules
23, 4557, 1990.
5. B.Y. Zaslavsky, L.M. Miheeva, E.A. Masimov, S. F. Djaforow and C.
J. Reichardt. J. Chem. Soc. Faraday Trans. 86, 519, 1990.
6. S. Spange, T. Heinze and D. Klemm. Polym. Bull. 28, 697, 1992.
7. V. Dutschk, S. Prause, and S. Spange. J. Adhesion Sci. Technol.
16, 1749, 2002.
8. H. Zollinger. Color Chemisty, VCH, New York, 142, 1991.
9. C. Reichardt. Chem. Rev. 94, 2319, 1994.
10. S. Spange and A. Reuter. Langmuir 15, 141, 1999.
11. S. Spange, K. Fischer, S. Prause and T. Heinze. Cellulose 10, 201, 2003.
12. L. A. Pothan, Y. Zimmermann, S. Thomas and S. Spange. J.
Polym. Sci.Part B Polymer Physics 28, 2546, 2000.
13. K. Fischer, T. Heinze and S. Spange. Macromole. Chemi. and Phy.
204, 1315, 2003.
14. V. Sasirekha, M. Umadevi and V. Ramakrishnan. Spectrochimi. acta
69, 148, 2008.
15. M.L. Moita, R.A. Teodoro and L.M. Pinheiro. J. Mole. Liquids 136, 15,
2007.
16. U. El-Ayaan, A. A-M. Abdel-Aziz and S. Al-Shihry. European J.
Medicinal Chemi. 42, 1325, 2007.
17. K. L. Mittal. (Ed) Acid-Base Interactions: Relevance to Adhesion
Sci. Technol. Vol 2, VSP, Utrecht, 2000.
128 Chapter 3
18. A. B-y-Jimenez and A. Bismarck. Cellulose 14, 115, 2007.
19. D. Klemm, T. Philipp, U. Heinze and W. Wagenknecht. Compre.
Cellulose Chemi., Wiley-VCH, Vol. 2, 1998.
20. K. S-Kleinschek and V. Ribitsch. Colloids and Surfaces A:
Physicochemi. and Engi. Aspects140, 127, 1998.
21. H. Jacobasch. Prog. Org. Coat. 17, 115, 1989.
22. A. Bismarck, J. Springer, A.K. Mohanthy, G. Hinrichsen and M. A.
Khan. Colloid Polym. Sci. 278, 229, 2000.
23. L. A. Pothan, C. Bellman, L. Kailas and S. Thomas. J. Adhesion Sci.
Technol. 16,157, 2002.
24. C. Bellmann, A. Caspari, V. Albrecht, T. T. Loan Doana, E. M¨ader, T.
Luxbacher and R. Kohl. Physicochem. Eng. Aspects 267, 19, 2005.
25. A. Bismarck, M. E. Kumru and J. J. Springer. J. Colloid Interface Sci.
210, 60, 1999.
26. J. Jacobasch., F. Simon, C. Werner and C. Bellmann. Tech. Mess.
63, 447, 1996.
27. S. Spange, Y. Zimmermann and A. Graeser. Department of Polymer
Chemistry, University of Technology, Chemnitz, Germany, 1999.
Private communication.
28. S. Spange, A. Reuter and D. Lubda. Langmuir 15, 2103, 1999.
29. P. V Joseph, K. Joseph and S. Thomas. Compos. interfaces 9, 171,
2002.
30. K. L. Mittal (Ed.), Silanes and Other Coupling Agents, Vol. 2, VSP,
Utrecht, 2000.