POLARITY CHARACTERISTICS -...

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


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]


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.


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].


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


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.


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


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)


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


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


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.

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