1998 Electrokinetic Properties of Processed Cellulose Fibers

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Colloids and Surfaces

A: Physicochemical and Engineering Aspects 140 (1998) 127–138

Electrokinetic properties of processed cellulose fibersK. Stana-Kleinschek a, V. Ribitsch b

a University Maribor, Department of Textile Chemistry, Maribor, Slovenia

b University of Graz, Department of Physical Chemistry, Graz, Austria

Abstract

Natural cellulose fibers (cotton) comprise several noncellulose compounds (hemicellulose, wax and pectin substances)

and cationic impurities which cause problems during diff erent adsorption processes such as dying, or final fiber

finishing and coating. Therefore the chemical purification (NaOH boiling, enzymatic purification, demineralization,extraction or oxidative bleaching) is the most important step in cellulose textile finishing. Alternative ways to describe

the success of diff erent processes in fiber purification which result in distinct surface charge and hydrophilicity are the

determination of electrokinetic properties and the water uptake of textile fibers. The zeta-potential (f) was determined

by streaming potential measurement as a function of the pH. From the f –pH functions the adsorption potential for

all ionic species Wi (i.e. (W

K+, W

H+, W

Cl−, W

OH−, in the case of potassium chloride solutions), the charge densities sk

and the pK values are calculated according to the Borner and Jacobasch model.

The degradation and removal of hydrophobic noncellulose compounds which cover the primary hydroxyl and

carboxyl groups of the cellulose polymer is clearly shown by an increase of the negative f of the plateau, which is in

good agreement with the electrokinetic parameters of cotton samples determined by the Borner and Jacobasch model.

The electrokinetic parameters determined by the Borner and Jacobasch model can be used to describe the

adsorption/ dissociation ability of textile fibers. The progress of the fiber processing (cleaning) is reflected by the

surface charge as well as the hydrophilicity of the fiber. © 1998 Elsevier Science B.V. All rights reserved.

Keywords: Adsorption potential; cellulose fibers; charge density; chemical purification; electrokinetic properties;

streaming potential

1. Introduction demineralization, oxidation and swelling processes,

various operations such as alkaline treatment,

Cotton cellulose fibers contain about 10% of demineralization, oxidative bleaching and mercer-

diff erent noncellulose compounds such as hemicel- ization are employed.

lulose, pectic substances, proteins, waxes, pigments Although these processes have been used for

and mineral salts which have to be removed by many years, they are only partially understoodseveral pre-treatment-purification processes. and require a better basic understanding in order

Almost all noncellulose compounds, which creates to gain some knowledge about their influence on

fiber adsorption ability, hydophilicity or the acces-problems — poor absorbency, poor rewettabil-

ity — are located in the primary shall of the fiber, sibility of components of the final fiber finishing

as dyes or surfactants.on the fiber surface (see Table 1 and Fig. 1). To

achieve the aim of the cotton pre-treatment, the All chemical purification and oxidation pro-

cesses are performed in an aqueous environment,cleaning of the raw fabrics by means of extraction,

0927-7757/ 98/ $19.00 © 1998 Elsevier Science B.V. All rights reserved.

PII S 0 9 27 - 7 7 57 ( 9 7 ) 0 0 3 0 1- 4



128 K. Stana-Kleinschek, V. Ribitsch / Colloids Surfaces A: Physicochem. Eng. Aspects 140 (1998) 127–138

Table 1

Average cotton fibers chemical composition

Contents of Percentage in Percentage in

the cross-section the primary wall

Cellulose (and hemicelluloses) 88–96 52

Pectins 0.7–1.2 12

Waxes 0.3–1.0 7

Proteins 1.1–1.9 12

Ashes 0.7–1.6 3

Other components 0.5–1.0 14

Total 100

and the main steps are degradation of noncellulose quite well understood [1, 2], contrary to the reac-

compounds and oxidation of natural pigments. tion abilities of the fiber surfaces. Recently some

During these processes the fibrillar structure of work was published introducing new aspects on

cotton is loosened, causing on one side swelling of the reactivity and accessibility of the interfibrilliarthe polymer as well as increased accessibility of the surface area [3,4 ]. Nevertheless, this aspects were

active groups on the fiber surface. It is assumed rarely [5–7] correlated with changes of the electro-that this modification of the surface of the solid kinetic properties which are caused by chemicalphase causes mainly changes in the electrokinetic purification processes. It can be assumed that theyproperties and alters the interaction with compo- are mainly responsible for diff erences of solid– nents of the liquid phase. liquid interactions.

The fine structure of the cellulose fiber is already In general only a few data are available from

the literature discussing the influence of the electro-

kinetic properties on the interaction between fibers

and dyes or surfactants. In some articles the inter-

action of anionic dyes (direct dyes) with cotton

fibers is discussed with the viewpoint of the fibers’

zeta-potential [8, 9].The zeta-potential (f) is the term describing the

electrokinetic properties at that position of the

solid–liquid interface which is accessible for inter-

actions. The contribution of electrokinetic proper-

ties of cellulose polymers are especially important

since these parameters very often reflect technolog-

ically relevant interaction phenomena with the

ingredients of the liquid phase as several kinds of

ions, acid complex forming agents, specfic

enzymes, surfactants and dyes. It was therefore of

interest to investigate if the zeta-potential is anappropriate tool to describe the surface modifica-

tions obtained by purification, which is responsible

for the adsorption of components of the liquid

phase — the surface charge of cellulose fibers as a

function of diff erent purification process steps —

and to correlate this data with the followingFig. 1. Supermolecular structure and morphology of cotton

fibers. phenomena:



129K. Stana-Kleinschek, V. Ribitsch / Colloids Surfaces A: Physicochem. Eng. Aspects 140 (1998) 127–138

$ wetting (swelling) of the fibers; The shear plane is approximately localized between

the fixed and diff use ion layer (as by the Stern$ the final state of the fiber surface:$ the degree of purification; model) and corresponds to the zeta-potential. The

potential change between the Helmholtz layers is$ the shade of the color.

linear, depending on the size and hydration of the

counter ions. In the diff use layer an exponential

potential drop is observed in accordance with the2. Determination of the electrokinetic properties of the fiber-forming polymers Boltzmann distribution.

A description of the formation of electrochemi-

cal double layers suggested by Borner et al. [10]Interfaces are characterized by chemical and

electrochemical potential values diff erent from enables consideration of the dissociation of the

surface groups as well as preferred adsorption of those of bulk phases since the molecules in the

boundary are subject to interaction forces from electrolyte ions for potential determining pro-

cesses. An essential part of this description is theboth adjacent phases. The net charge at solid

polymer–aqueous solution interfaces is generally theory of Esin and Markov [11 ] regarding specific

occupation of adsorption sites in the innerattributed to the dissociation of functional surface

groups and/ or preferential adsorption of ions from Helmholtz plane (IHP) and outer Helmholtz plane

(OHP) by hydronium ions, hydroxide ions, electro-the solution. The resulting surface charge is bal-anced by counterions of the solution. The descrip- lyte anions and cations (two ion types for each

Helmholtz plane must be defined as preferentiallytion of the charge distribution and the

corresponding potential course in relation to the adsorbed). Furthermore, the dissociation of acidic

or basic groups is considered to be equivalent todistance from the solid surface is the object of the

electrochemical double layer model. Fig. 2 shows the adsorption of hydronium or hydroxide ions

according to Lyklema and Sidorova [12].a double layer model according to the

Gouy–Chapman–Stern–Grahame model (GCSG For the preferential adsorption of hydroxide

ions (OH−) and monovalent electrolyte anionsmodel), for negative surface charge. A distinction

is made between two layers of fixed charge carries ( X−) in the IHP, and the preferential adsorption

of hydronium ions (H+) and monovalent electro-at the inner — the plane of centers of the anions

(IHP), and the outer Helmholtz plane — the lyte cations (K+) in the OHP, one obtains

centers of the cations (OHP) and a diff use layerci(IHP)

=ci(bulk)

exp[(−F YIHP−W

i)/ RT ], (1)

of ions at increasing distance from the solid surface

where they are subject to electrostatic attractive C i(OHP)

=C i(bulk)

exp[(−F f−Wi)/ RT ], (2)

forces from the surface as well as thermal motion.sIHP=e

0N [x

X−(IHP)−x

OH+(IHP)], (3)

sOHP=e

0N [x

K+(OHP)−x

H+(OHP)], (4)

sdiffuse

={2e0eRTc

bulk∑ n0

i [exp(−F f/ RT )]−1}1/2 ,

(5)

sOHP+s

diffuse=C

HP(f−W

IHP), (6)

where c i is the electrolyte concentration, Y electri-cal potential, s

IHP and s

OHP are electrokinetic

charge densities of inner and outer Helmholtz

planes, e dielectric constant of liquid, e0

permittiv-

ity of free space, C HP

capacity between Helmholtz

planes, f zeta-potential, Wi adsorption potential of

all ionic species, F Faraday constant, xki

molarFig. 2. Presentation of the electrochemical double layer accord-

ing to the GCSG model [ 14]. fraction of the ionic species in the shear plane, e0




elementary charge, N number of adsorption sites, $ degree of polymerization, approximately 4700.

R gas constant, and T temperature [ 13].

The concentration of the ionic species in the 3.1. Preparation methodsshear (k ) plane can be expressed by the bulk

concentration by means of a Boltzmann approach 3.1.1. Removal of noncellulose compounds using which contains the adsorption potential for ionic textile purification processes

species, Wi [see Eqs. (1) and (2 )].According to Lyklema and Sidorova, adsorption $ Alkaline treatment (20 g l−1 NaOH; pH=11.5;

potentials and pK values are connected by [12] t=90 min; T =95°C ).$ Demineralization (acid treatment), using

exp C−W

H+

RT D=10−pKb · c[H

2O]

K W

( l mol−1), (7) sequestering agents (i.e. polyphosphonic acids)

in acid environment — (2 ml l−1 sequestering

agent; pH=2.7; t=30 min; T =98°C )exp C

WOH−

RT D=10−pKa · c[ H

2O]

K W

( l mol−1)(8) $ Enzymatic treatment — purification by use of

diff erent enzymes in the enviromnent of 0.05 M

citrate buff er at pH=4.5; T =30°C; t=4 h. TheThe resulting system of equations contains two

applied amounts of specific enzymes were:independent variables (pH, c ), two dependent vari-

Supernatant Schizophyllum commune (S.c.):ables (f, Y ) and six parameters:xylanase activity; 3770 XU ml−1; 500 XU g−1$ the adsorption potential for all ionic species W

icotton=133 ml g−1 cotton;(i.e. W

K+, W

H+, W

Cl−, W

OH−, in the case of

Pectinase-polygalacturonase:potassium chloride solutions);

pectinase activity 90 PU ml−1; 1 PU g−1$ the charge density sk;

cotton=11 ml g−1 cotton;$ the integral capacity between Helmholtz planes

Lipase solution:C Hp

[14].

lipase activity 88 000 LU ml−1; 1:1000 dilution;The molar adsorption potentials, Wi, represent

1 LU g−1 cotton=12 ml of the diluted enzymethe nonelectrostatic adsorption and do not describe

sample/ g cotton.the electrostatic influence on the adsorption equi-$ Oxidative bleaching — in diff erent ways purifiedlibrium between the charged surface and ionic

materials were afterwards bleached with H2O2species i , from the liquid phase. Hence Wi is a (9.5 g per 135% H2O

2; pH=11.2; t=30 min;characteristic value to describe the adsorption

T =95°C ).equilibrium at the isoelectric point.

It is possible to describe the pki dependence on

pH and cbulk

, with respect to the electrostatic term 3.1.2. Removal of individual noncelluloseof the molar free adsorption energy, D

adsG hi [13]: compounds applying specific methods

Dads

G hi=W

i+z

i · F · f(pH, c

bulk). (9)

$ Petrol–ether extraction (Soxhlet extraction;

minimum six circuits per hour);$ HCl treatment (0.1 M HCl, 20 h at 20°C ).3. Material and methodsAfter each treatment the fibers were washed in

distilled water until a conductivity value less thenThe material used for all experiments was 100% 3 ms m−1 was reached.pure cotton fabric produced by Lenzig AG, with

the following physical and chemical characteristics:$ humidity content (%), 6.55 3.2. Analytical methods$ metal content in ppm/ 0.5 g fibers (ICP-MS

inductively coupled plasma-mass spectroscopy): 3.2.1. Determination of the zeta-potential

The streaming potential method was used as it$ Mg=249; Ca=1830; Fe=39.4; Cu=1.73;

Mn=1.26; Co=0.022; Ni=0.85; Zn=20.2; has been shown to be the most appropriate electro-




kinetic technique to study the zeta-potential of source D 65; each sample was measured on the

same place three times.fiber systems [15,16].

The streaming potential of every sample was

measured in 0.001 N KCl, the surface conductivity

was not taken into account. The f was calculated 4. Results and discussion

according to the Smoluchowski equation. The f

values are always obtained at the plateau part of In order to obtain information about the purifi-cation influence on the electrokinetic behavior of the pH function in the alkaline region at pH=9.

The f value was investigated as a function of textile surfaces, the cotton samples were in the first

step diff erently purified (textile specific or bypH. From the f –pH functions electokinetic param-

eters are calculated according to the Borner and specific methods) and the f –pH function deter-

mined (Fig. 3).Jacobasch model:

the adsorption potential for all ionic species Wi

The natural cellulose fibers are negatively

charged (fplateau

=−11 mV ) due to the presence of (i.e. WK+

, WH+

, WCl−

, WOH−

, in the case of

potassium chloride solutions); carboxyl and hydroxyl-groups. In the case of raw

(untreated) material these groups are covered bythe charge densities sk;

the pK values. noncellulose compounds present in the primary

wall of the natural fiber.In order to show the good agreement between The textile purification processes are applied in

order to degrade, remove or complex these com-both methods the pK values are determined

according to Eqs. (7 ) and (8 ), but also according pounds, which is clearly represented by the results

obtained. The increase of the negative f can beto Eq. (10) [17]:

explained by the improved accessibility of anionic

groups (see Fig. 3).pK=pH

1/2fp+0.4343 ·

F · fp

2RT , (10)

The classical NaOH treatment degrades and

removes practically all noncellulose compoundswhere pH

1/2fpis the corresponding pH value where except waxes which remain to about 50% on the

f=1/ 2f of the plateau, and fp

is f of the plateau. fiber. The NaOH boiling causes swelling of theAn Electrokinetic analyzer EKA, A.Paar KG, fiber surface layers, increasing the size of the active

and flat plate cell was used.

3.2.2. Numerical determination of degree of

whiteness

The numerical determination of whiteness was

used for the calculation of the degree of whiteness

(W ) according to the CIE equation as a measure

for the degree of purification:

W CIE=Y +800(xn−x)+1700( y

n− y), (11)

where Y is the tristimules value of the sample, x, ythe x, y chromaticity coordinates of the sample

and xn, y

n the chromaticity coordinates of the

perfect diff user, all for the CIE 1931 standard

colorimetric observer. An ACS Spectro Seiizor

SS-II was used; software ACS-Croma-Calk

( USA). Datacolor; measuring geometry d/ 8; meas- Fig. 3. f diff erent cleaned — purified cotton fibers as a function

of pH; electrolyte solution 0.001 N KCl.uring region 400–700 nm; halogen light, CIE light




surface, but the amount of dissociable groups that the waxy substances influence electokinetic

properties [18 ].should not be changed. The swelling itself causesEnzymatic purification of cellulose fibers area reduction of the f, because of the shift of the

potential alternatives to the classical textile pro-shear plane into the liquid phase.cesses because of their low environmental pollu-Petrol ether extraction is used to remove thetion. Such a method of purification was used towaxes entirely. This treatment causes the highest

remove the individual noncellulose componentsf values (fplateau=−17.4 mV ). It can be concludedusing specific enzymes as pectinase and lipase infrom these results that the waxes are the onethe first step and to obtain the information aboutcomponent mainly influencing the surface chargethe influence of this components on the f (Fig. 5).of cellulose fibers.The degradation of waxes by lipase results in theA very similar result (f

plateau=−16.5 mV) is

similar fplateau

value of −16.3 mV as the petrolobtained by the so-called acid demineralizationether extraction.treatment using complex forming agents (i.e. poly-

The application of an enzyme cocktail contain-phosphonic acid)-demineralization process. Theing a mixture of the enzymes (pectinase, lipase,cations on the surface of the fibers are complexedS.c. supernatant) leads to the highest negative f,and dissolved by this treatment.indicating the total removal of the impurities.The nontextile typical HCl treatment which

The f –pH functions represented by enzyme-should remove the noncellulose compounds creates cleaned materials are very similar to that obtaineda completely diff erent situation. The f –pH functionby the classical textile or specific treatment of

is changed and the fplateau

values (−9.5 mV) arecellulose fibers — the f values of the plateau region

even lower than that of the raw materialare a function of the degree of removal of noncellu-

(fplateau

=−11 mV). This observation can belose compounds.

explained by the formation of hydrocellulose inThe results of step-wise extraction and enzy-

acid medium (pH=2.5). During the primary stepmatic purification show that only the total purifi-

of the hydrolyzation H3O+ cations and/ or alde-

cation of cotton leads to a simultaneous increasehyde groups are present and the dissociation equi-

of the fplateau

values and shifting of the IEP tolibrium is shifted in such a way that the number

lower pH values (see Figs. 4 and 5, and Tables 2of negative surface groups is decreased causing a

and 3).

reduction of the negative f ( Fig. 3). The oxidation–H2O

2 bleaching process causes a

From Fig. 3 it can be seen that pH– f functionhydrolyzation of the material which is observed

is not changed qualitatively by the treatment: the by the decreased fplateau

. This phenomenon is inf values of the plateau region are a function of agreement with the theory. The f

plateau of NaOH

the degree of removal of noncellulose compounds purified and subsequently H2O

2 bleached fibers

and can therefore be used to describe the progress remains practically the same as that of NaOHof these process steps. The isoelectric points are treated material (f

plateau=−15.0 mV ). This phe-

shifted towards lower pH values during the clean- nomenon shows that by NaOH boiling the fibersing process, due to the increased accessibility of become hydrophilic during the purification stepdissociable surface groups (IEP shifted from and not afterwards during the bleaching steppH 2–3 to a pH value of approximately 1.8, see (Fig. 6). The oxidative process decrease the

Table 2). fplateau of acid demineralized samples; the fibersThe mentioned phenomenon is even more clearly become more hydrophilic and the swelling shifts

demonstrated by step-wise extraction processes the shear plane into the liquid, causing a reduction(Fig. 4). Improving accessibility caused by wax in f. The reduction of f

plateau of demineralized and

degradation, can be detected by the fplateau

which then H2O

2 bleached fibers is evident (Fig. 6).

by every subsequent two cycles of extraction An decrease of the fplateau

of S.c. purified andincreases, but not by the shift of the IEP values. oxidatively, bleached samples can be seen. By

pectinase and lipase purification the fplateau

of theIn any case this phenomenon is a further proof




Table 2

Electrokinetic parameters of diff erent purified cotton samples defined by Borner and Jacobasch model

Sample WK+

(kJ mol−1) WCl−

(kJ mol−1) WH+

(kJ mol−1) WOH−

(kJ mol−1) sk (mC cm−2) *pKA

*pKB

**pK IEP

Raw −17.32 −32.03 −30.91 −70.38 0.075 4.00 10.27 4.25 2.4

Purification

NaOH −20.44 −48.60 −27.01 −70.54 0.117 3.26 10.96 3.15 2.5

Deminera. −22.47 −48.47 −26.46 −70.86 0.152 3.20 11.06 2.63 1.9

Extraction −26.62 −42.70 −26.72 −74.60 0.324 2.54 11.01 2.49 1.8

HCl −31.12 −46.50 −30.83 −69.76 0.636 3.40 10.29 2.80 2.3

*pK defined according to Eqs. (7) and (8); **pK defined according to Eq. (10).

chemical bleaching remains practically the same

as after purification (Fig. 6).

It can be concluded that the fiber hydrophiliza-

tion process of diff erent purified and afterwards

bleached fibers can indirectly be monitored by the

f –pH function. It is interesting to compare the

results of f –pH function with W . The NaOHsample which was boiled and subsequently

bleached shows the highest degree of whiteness (W

CIE=86.1). Samples purified by the demineraliza-

tion process, which become more hydrophilic even

in the oxidative bleaching step, show much smaller

degrees of whiteness (W CIE=68.1). The white-

ness of all enzymatically purified samples is

extremely high: The samples which have similar fFig. 4. f of step-wise extracted raw cotton as a function of pH,. values after purification and bleaching gain theelectrolyte solution 0.001 N KCl.

highest W (W CIE for lipase or pectinase purified

and bleached sample is approximately 82).It can be concluded that the necessary process

of purification and swelling must be performed in

the purification stage (as obtained by NaOH and

enzymatic purification) and not during the oxida-

tion stage (in the case of demineralized material).

From the f –pH functions the electrokinetic

parameters were calculated according to the

Borner and Jacobasch model and are presented in

Tables 2–4. In Fig. 7 one example of the measured

f –pH– c values of enzymatically purified material

and the fitted function according to the Bornerand Jacobasch model is presented, indicating a

good correlation between both calculated and mea-

sured f –pH– c values.

In Table 2 the electrokinetic parameters of

diff erently purified cotton samples defined byFig. 5. f of enzymatic purified cotton fibers as a function of Borner and Jacobasch model are represented [18].pH,. electrolyte solution 0.001 N KCl.

A vertical comparison of adsorption potentials




Table 3

Electrokinetic parameters of enzymatic purified cotton samples defined by the Borner and Jacobasch model

Sample WK+

(kJ mol−1) WCl−

WH+

WOH−

sk *pKA

*pKB

**pK IEP

(kJ mol−1) ( kJ mol−1) ( kJ mol−1) (mC cm−2)

Raw −17.32 −32.03 −30.91 −70.38 0.075 4.00 10.27 4.25 2.4

Enzymatic purification

S.c. Mathematical 3.64 3.0

calculation was

not possible

Pectinase −8.34 −23.94 −28.88 −72.69 0.098 2.89 10.63 3.85 3.1

Lipase −5.19 −40.83 −24.42 −71,83 0.126 3.03 11.42 3.90 2.0

S.c.+pectinase+lipase −8.93 −43.69 −26.29 −72.96 0.086 2.83 11.09 3.68 2.0


of ionic species indicates diff erences in face is increased. This is shown by the adsorption

potentials, which are higher for all purified materi-WK+

, WCl−

and WOH−

between raw and purified

materials. The diff erences in WH+

are very small. als. The highest diff erences are observed with the

WCl− (for example, WCl− of raw material=The highest WH+ values (adsorption free energy of hydrogen ions on the fiber surfaces) have raw, −32.32 kJ mol−1; WCl−

of NaOH boiled

material=−48.60 kJ mol−1). This means that byHCl and NaOH treated materials, and this is

shown by the f –pH functions. The f plateau of purification steps the adsorption ability for anions

(Cl− in the case of KCl ) is improved, which couldthese samples is the smallest: they exhibit a more

basic character. also be observed from the f –pH functions in Fig. 3.

Purified materials show higher fplateau

values. TheBy the extraction process the surface cuticula is

degraded and the fiber surface groups are set free exception is NaOH boiled material, where waxy

substances partially remain on the material, whichwhich is reflected by the calculated values of the

WOH−

(dissociation potential of solid surface could be detected by the high pK value (pK=3.15). The material is swelling (f

plateau is com-groups) which are increased for extracted samples

(WOH−

raw=−70.38 kJ mol−1; WOH−

of extracted pressed-reduced), but WCl−

shows the highest

value.material=−74.60 kJ mol−1). Thus the fiber sur-faces display a more acidic character. According to calculated W

K+, the cation (K+ in

the case of KCl) adsorption ability is improvedBy purification all adsorption–dissociation inter-

action on the fiber- solid/ electrolyte–liquid inter- by purification processes. All calculated adsorption

Table 4

Electrokinetic parameters of standard purified and afterwards chemically bleached cotton samples defined by Borner and

Jacobasch model

Sample WK+

WCl−

WH+

WOH−

sk *pKA

*pKB

**pK IEP

(kJ mol−1) ( kJ mol−1) ( kJ mol−1) ( kJ mol−1) (mC cm−2)

Raw −17.32 −32.03 −30.91 −70.38 0.075 4.00 10.27 4.25 2.4

Purification and

chemical bleaching

NaOH+H20

2 −9.30 −34.70 −21.01 −77.81 0.113 1.97 12.02 2.86 2.0

Deminer.+H2O

2 −20.86 −33.54 −23.29 −73.88 0.102 1.96 11.62 2.11 1.5

S.c.+H2O

2 −8.78 −16.31 −27.51 −73.85 0.078 2.67 10.88 3.50 2.9

Lipase+H2O

2 −9.51 −42.07 −25.92 −72.88 0.103 2.85 11.16 3.00 2.2

S.c+pectinase+lipase+H2O

2 −11.88 −41.27 −24.61 −73.76 0.086 2.69 11.39 2.68 1.2





degradation of waxy substances or of the forma-

tion of new dissociable groups.

The pK values (Table 2) obtained by two inde-

pendent methods demonstrate the mentioned phe-

nomenon — namely by the purification process

the materials become more acid characters owing

the higher accessibility of characteristic cellulosecarboxyl groups (pK values are smaller for purified

materials, i.e. the pK of the raw material=4.25,

IEP=2.4; pK for extracted material=2.49; IEP=1.8). The exception is NaOH boiled material with

its specific properties.

A horizontal comparison of the calculated

adsorption potentials of cotton materials showsFig. 6. f of diff erent cleaned acid afterwards H

2O

2 bleached that the adsorption energies of anionic surface

cotton fibers as a function of pH; electrolyte solution 0.001 N groups WOH−

(COOH and OH cellulose groups)KCl.

are the highest, as well as the adsorption energies

of ions (WCl−) adsorbed from the liquid phase.This correlation between diff erent adsorptionpotentials of purified materials are higher thanpotentials (dissociation/ adsorption) can bethat of raw cotton, the highest by HCl treatedobserved for raw and purified material; the cottonmaterial (W

K+ of raw=−17.32 kJ mol−1; W

K+ of

fibers display anionic character (Table 2). Inthe HCI purified material=−31.12 kJ mol−1).Table 3 the electrokinetic parameters of enzymati-This phenomenon can be explained by the forma-cally purified cotton samples defined by the Bornertion of new aldehyde groups, which cause betterand Jacobasch model are represented.adsorption of the cations present in the liquid

S.c. enzymatic purified samples could not bephase. This increased adsorption of cations mayapproximated, because the plateau in the basicshield the repulsive forces of anionic groups againstregion was not reached. A vertical comparison of adsorption of anions on the surfaces.

adsorption potentials of ionic species between rawThe charge densities sk

are increased by purifi-end enzymatically treated samples shows diff er-cation (Table 2). The highest sk shows extracted

and HCl-washed material, because of the total ences in the adsorption ability for ions from the

Fig. 7. The fit according to the Borner Jacobasch model of measured f –pH– c values of enzymatic purified material.




liquid phase (WK+

, WCl−

), but there are practically

no diff erences in dissociated surface groups. In all

enzymatically purified samples WK+

is drastically

decreased (WK+

of raw material=−17.32 kJ mol−1;

WK+

of lipase treated sample=−5.19 kJ mol−1),

although the fplateau

is increased by enzymatic

purification (Fig. 4). The pK values calculated bythe two methods are in the same region as that of

NaOH-treated material. The pK of raw material

is for example 4.25, and for lipase-treated sample

3.90. An explanation for such behavior could be,

that the WCl−

are increased primarily by lipase and

enzyme cocktail treated samples (by a simulta-

neous strong decrease in WK+

), resulting in anFig. 8. W

K+, W

Cl−, W

OH− and W CIE as a function of purifica-improved adsorption ability for anions of the

tion and chemical bleaching of cotton fibers.liquid phase. By all enzymatic purification the pK

values are decreased, but not as much as compared

to other chemical purification processes — enzy- ment is a better adsorption ability of anions of theliquid phase. At the same time this sample showsmatic purified samples show other electrokinetic

properties as other samples. Probably the enzy- the highest WOH−

(dissociation potential of the

solid surface groups). By enzymatic treatmentmatic treatment causes similar changes in the

morphology or structure of cellulose fibers as WK+

is reduced already in the purification step,

demonstrating an improved adsorption ability forNaOH boiling, and this may be the reason of the

improved adsorption process. This could be the anions of the liquid phase. In the bleaching step

the potential remains practically the same.proof for the influence of cellulose structure on

the adsorption ability of cotton fibers. At demineralized and afterwards chemically

bleached samples a strong increase of WK+

isThe sk of enzymatic treated samples show very

similar values as NaOH-treated material. By puri- observed, indicating a preferred adsorption of

cations.fication the number of adsorption places is not

increased by the same factor. The diff erences in the WCl−

between purified andbleached samples are slightly decreased by a simul-Horizontal comparison between calculated elec-

trokinetic properties shows that the highest diff er- taneous decrease or increase in the WK+

value. By

diff erent bleaching processes the materials obtainences are in the WK+

, WCl−

values. Hence an anion-

preferred adsorption process can be expected. either an anion-preferred adsorption character or

a cation-preferred adsorption character. SamplesIn Table 4 the electrokinetic parameters of

diff erent purified and subsequently chemically with strong adsorption ability of anions of the

liquid phase (NaOH boiled or lipase purified andbleached cotton samples are represented. The

electrokinetic character [adsorption potentials afterwards bleached samples) with simultaneous

increased WOH−

show the highest degree of white-WK+

, WCl−

, WOH−

of the liquid–solid system (fiber-

solution)] and W CIE as a function of diff erent ness after the chemical bleaching step and after

the fluorescent whitening process (Fig. 8). Thepurification and chemical bleaching is presentedin Fig. 8. f

plateau in the f –pH function of this samples remains

practically the same as after the purification stepThe oxidation-bleaching changes the electro-

kinetic properties of cellulose fibers. The WK+

is or is slightly increased probably due to the pre-

ferred liquid phase anions adsorption, and/ or sur-drastically decreased in the case of NaOH-treated

and oxidized material (WK+

of raw material= face OH− groups dissociation. It can be concluded,

that for an optimal interaction between fibers−17.32 kJ mol−1; WK+

of the mentioned

sample=−9.30 kJ mol−1). The result of the treat- (solid phase) and anions of the liquid phase (dyes)




a preferred adsorption of anions has to be of anionic species. At all enzymatic purifiedsamples W

K+ is drastically decreased, although theobtained.

The pK values of all diff erent bleached cotton fplateau

value is increased. An explanation for suchbehavior could be that the W

Cl− is increased pri-samples are shifted to higher acidity, due to the

oxidation of the primary OH cellulose groups into marily by lipase and enzyme cocktail treatedsamples (with a simultaneous strong decrease inaldehyde, carboxyl and ketone groups.

The sk

is not drastically increased by bleaching WK+), and this results in an improved adsorptionability for anions of the liquid phase.processes, although the material becomes more

hydrophilic, which can be explained by diff erences The electrokinetic parameters of diff erent puri-fied cotton samples defined by the Borner andin the fiber morphology obtained during the oxida-

tion and not in formation of new surface groups. Jacobasch model are in a good agreement withf –pH functions and are an excellent tool for char-

acterizing the adsorption/ dissociation ability of textile fibers.5. Conclusions

5.2. Electrokinetic properties of purified and 5.1. Electrokinetic properties of purified cottonafterwards bleached cotton material material

The fiber hydrophilization process can be moni-The fplateau

in the f –pH function increases duetored indirectly by the f –pH function for diff erentto increased accessibility of dissociable carboxylpurified and afterwards bleached fibers. This isgroups by removal of wax and cationic compo-observed by the reduction of f

plateau. The necessarynents during the cleaning processes (extraction and

steps of purification and swelling must be carrieddemineralization). The waxy substances are theout during the purification stage (as in the case of components mainly influencing the surface chargeNaOH purified+bleached material, where f

plateauof cellulose fibers (the results of step-wise

remains practically the same after bleaching asextraction).after the purification step) and not during theNaOH, pectinase and S.c. enzymatic purificationoxidation-bleaching stage (as in the case of processes cause an increase of the active surface,demineralized+bleached material, where a reduc-but the amount of dissociable groups is not

tion of f after bleaching step is observed). This ischanged. The swelling itself causes a reduction of also shown by enzymatically purified and subse-the f, because of the shift of the shear plane intoquently bleached samples.the liquid phase.

According to electrokinetic parameters obtainedThe cleaning process shifts the IEP to higherby the Borner and Jacobasch model for optimalacidity showing the dominance of dissociationinteraction between the fibers (solid phase) andprocesses in the f –pH function. The exceptions areions of the liquid phase, the fibers have to bethe NaOH boiled and some enzymatically purifiedmodified in a way that preferred adsorption of samples, which is also shown by the pK values.anions is enabled.They are decreased by purification processes, indi-

cating higher acidity of the purified material, due The pK values of all diff erently bleached cottonsamples are shifted towards stronger acidity dueto higher accessibilty of the characteristic carboxyl

cellulose groups. to the oxidation of primary OH cellulose groups

into aldehyde, carboxyl and ketone groups.By purification all adsorption/ dissociation inter-actions on the fiber– liquid interface are increased, The sk is not drastically increased by the bleach-

ing processes, although the material becomes morethis can be concluded from the adsorption poten-tials defined by the Borner and Jacobasch model, hydrophilic. This can be explained by diff erences

in the fiber morphology obtained during thewhich are higher for all purified materials. Anexception is the NaOH boiled material, which bleaching step and not by formation of new surface

groups. Samples with a high energy diff erenceswells (the fplateau

is reduced), but shows thehighest value of W

Cl−, indicating good adsorption between W

K+ and W

Cl− show an increased ability




[3] H. Krassig, Cellulose, Structure, Accessibility andto adsorb ions of the liquid phase. This is alsoReactivity, Gordon and Breach Science Publishers, Y-Parc,shown by all classical and enzymatic purified and1992, pp. 17–258.

subsequently bleached samples. A comparison of [4] H. Krassig, Das Papier 38 (1984) 12.

the parameters obtained by the Borner and [5] V. Ribitsch, K. Stana-Kleinschek, S. Jeler, Colloid Polym.Jacobasch model from enzymatically purified and Sci. 247 (1995) 388–394.

afterwards bleached samples shows that the elec- [6 ] K. Stana, C. Pohar, V. Ribitsch, Colloid Polym. Sci. 273

(1995) 1174–1178.trokinetic properties are not changed during the [7] K. Stana-Kleinschek, V. Ribitsch, Prog. Colloid Polym.bleaching step. They remain the same as after theSci. 101 ( 1996) 157–165.enzymatic purification step. This is a strong evi-

[8] F. Gonzales-Caballero, M. Espinosa-Jamenez, J. Colloiddence, that a correct purification is required in

Interface Sci. 123 (1988) 2.order to obtain a good interaction between the [9] F. Gonzales-Femandez, F. Gonzales-Caballero, M.solid and liquid phase and to improve the adsorp- Espinosa-Jamenez, Colloids Surf. 14 (1985 ) 143.

[10] M. Borner, H.J. Jacobasch, F. Simon, N.V. Churaev, I.P.tion abilities of cellulose fibers.Segeeva, V.D. Sobolev, Colloids Surf. A: Physicochem.

Engng Aspects 85 (1 ) (1994) 9–17.

[11] O.A. Esin, B.F. Markov, Zh. Fiz. Khim. 13 (1939) 318.Acknowledgment

[12] J. Lyklema, H.P. Sidorova, Kolloidni J. 38 (1976) 4.

[13] H.J. Jacobasch, K. Grundke, S. Schneider, F. Simon,The authors are very grateful to the Austrian J. Adhesion 48 (1995) 57.

Federal Ministry of Science and Transport and the [14 ] C. Werner, H.J. Jacobasch, J. Biomater. Sci. Polym. Edn

7 ( 1995) 1.Steirmarkische Landesregierung for financial[15] V. Ribitsch, Ch. Jorde, J. Schurz, H.J. Jacobasch, Prog.support.

Colloid Polym. Sci. 77 (1988) 49–54.

[16] V. Ribitsch, M. Jacobasch, M. Borner, Advances in

Measurement and Control of Colloidal Processes,References Butterworth-Heinemann, Oxford, 1991, pp. 354–365.

[17] H.J. Jacobasch, I. Grosse, Textiltechnik 37 (1987)

266–2679.[1] K. Hess, H. Mahl, Kolloid-Z. 155 (1957) 1.[18] K. Stana-Kleinschek, Dissertation, Karl Franzens[2] J.W.S. Hearle, R.R. Peters, Fibre structure, The Textile

Institute, Butterworths, London, 1963, pp. 227, 409. University, Graz, 1996.

1998 Electrokinetic Properties of Processed Cellulose Fibers

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