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Surface characterization and adsorption abilities of cellulose fibers.

1. INTRODUCTION

1.1. Action of Alkaline Pretreatment on Cellulose Fibers and Their Structure

Among the various pre-treatment or purification processes of cellulose textile fibers, only scouring and mercerization employ concentrated alkaline agents. Although these two alkaline processes use the same reagent, they have different objects.

Scouring (NaOH concentration [less than] 10%) is a purifying treatment of cotton a-cellulose, removing impurities such as waxes, pectins, hemi-celluloses, and mineral salts. Native cotton contains a certain amount of natural waxes (0.5-1%), which bestow a soft touch on the fiber but are undesirable for the subsequent dyeing and finishing operations. These compounds are consequently retained and then have to be eliminated to ensure water absorbency, which is necessary for the uniformity of bleaching, dyeing, printing, and chemical finishing treatments (1).

Mercerization (NaOH concentration [greater than] 12%), is not a fiber purification process; it induces changes of the super-molecular structure and morphology of cellulose fibers [ILLUSTRATION FOR FIGURE 1 OMITTED].

The most important effect of mercerization is the modification of the crystallographic cell from cellulose I to cellulose II.

The cellulose II content reaches a maximum value at a sodium hydroxide concentration of 300 g/l; this maximum of conversion to cellulose II is connected to maximum amount of presence of the solvated dipole hydrate NaOH. 7[H.sub.2]O (1). The majority of changes in the fine structure due to mercerization are decreased by the application of increasing tension. It appears that the ratio of conversion of cellulose I to cellulose II is more pronounced as the applied tension is lower.

Because of the changes in the primary structure of the fibers, the desirable changes as mechanical properties, dyeing properties, and luster are induced in cotton yarn and fabric. Mercerization causes a decrease in the volume of crystalline region or an increase of the accessible area; moreover, this treatment modifies the texture of the fibers and principal their accessibility in an aqueous medium.

Consequently, mercerized fibers have greater a moisture absorption capacity, are more reactive to aqueous chemical reagents, and are better accessible to dye molecules. Mercerization increases the dye absorption, the rate of dyeing, and also the visual color yield - the mercerized material is of darker shade than the non-mercerized.

1.2 Correlation Between Changes in Morphology, Accessibility, and Electrokinetic Properties of Processed Fibers

Alkaline modifications and oxidation processes of cellulose fibers are performed in aqueous environment. During these processes the fibrillar structure of cotton is loosened, causing swelling of the polymer as well as an increased accessibility of active groups on the fiber surface. It is assumed that this modification of the surface of the solid phase mainly causes changes of the electrokinetic properties and of the interaction with components of a liquid phase.

The fine structure of the cellulose fiber is already quite well understood (3) contrary, to the reaction abilities of the fiber surfaces. Recently, some work was published introducing new aspects about the reactivity and accessibility of the inter-fibrillar surface area (4). Nevertheless this aspects were rarely (5-7) correlated with changes of the electrokinetic properties that are caused by chemical purification processes. It can be assumed that they are mainly responsible for different kinds of solid - liquid interactions.

In general, only a few data are available from literature discussing the influence of the electrokinetic properties - zeta potential ([Zeta]) on the interaction between fibers and dyes or surfactants. The zeta potential is the term describing the electrokinetic properties at that position of the solid/liquid interface that. is accessible for interactions. The electrokinetic properties of cellulose polymers are especially important since their parameters very often reflect technologically relevant interaction phenomena with the ingredients of the liquid phase as several kinds of ions, specific enzymes, surfactants, dyes.

It was therefore of interest to investigate the relationship between the changed adsorption abilities of alkaline modified fibers monitored by zeta potential measurements and changes in the fibers morphology analyzed by image processing of microscope images of fibers.

2. IMAGE PROCESSING AND ANALYSIS OF FIBER MORPHOLOGY

To understand changes that cotton undergoes during various chemical processes, it is necessary to understand first the morphology of the fiber as it appears under the microscope (8). Cotton fibers have the characteristic shape of a convoluted tube, which at low magnification resembles a twisted ribbon (9). Mature raw cotton has a thick fiber wall with a hollow tube - lumen - in the middle. The general geometric shape for most mature fibers cross section is elliptical to circular.

During mercerization, the secondary wall swells causing a reduction or total disappearance of the lumen. The primary wall of the fiber is not extended during the mercerization, so that the stretched swollen secondary wall and fiber loses convolution and becomes more circular in cross section [ILLUSTRATION FOR FIGURE 2 OMITTED]. The inner structure becomes more organized during mercerization. So, the uniformity of dimensions and distribution of holes are increased.

The cotton fiber (except cuticle, which contains waxes and pectins) has a fibrillar structure, which can be observed by electron microscopy. Upon swelling in chemical reagents microfibrils increase in diameter and shorten (8).

Immersing cotton fiber into aqueous solutions causes changes of fiber morphology. These changes can be observed by light or better by electron microscopy and these changes can be measured and evaluated with the help of image analysis.

In textile research, image processing and analysis was mainly used to measure the fibers maturity (10-11) and to determine the distribution of wool fiber-diameter (12). Because of large deviations in fiber cross section shapes, the determination of dimensional features of cotton fibers requires a large number of measurements per sample. The manual microscope measurements are very slow and tedious; an automatic image processing solves this problem successfully. Computer processing is the way to obtain numerical information from images, more accurate, less time-consuming, and more reproducible, than other methods (13). Image processing is a tool that converts the analogue information of the image in a numerical form and is usually used for two somewhat different purposes (14): improving the visual appearance of images to a viewer, and preparing images to measure features and structures.

3. DETERMINATION OF THE ELECTROKINETIC PROPERTIES OF THE FIBERS

Interfaces are characterized by chemical and electrochemical potentials different from those of the bulk phases since the molecules at the boundary are subject to interaction forces from both adjacent phases. The net charge at a solid polymer/aqueous solution interface is generally attributed to the dissociation of functional surface groups and/or preferential adsorption of ions from the solution. The resulting surface charge is balanced by counter-ions of the solution. The description of the charge distribution and the corresponding potential course in relation to the distance from the solid surface is the object of the electrochemical double layer model. Figure 3 shows a double layer model according to Gouy-Chapman-Stern-Grahame (GCSG model), for negatively charged surfaces. A distinction is made between two layers of fixed charge carries at the inner - the plane of centers of the anions (IHP), and the outer Helmholtz plane - that of the centers of the cations (OHP) and the diffuse layer of ions. At increasing distance from the solid surface they are exposed to electrostatic attractive forces from the surface and to thermal motion. The shear plane that represents the effective localization of the solid-liquid interface is approximately localized between the fixed and diffuse ion layer (as described by the Stern model). The potential at this position is called the zeta potential, and it can be measured by any relative motion between solid and liquid compounds. The potential change between the Helmholtz layers is linear, depending on the size and hydration of the counter ions. In the diffuse layer an exponential potential drop is observed according to a Boltzmann distribution.

A description of the formation of electrochemical double layers suggested by Borner and Jacobasch (15) enables consideration of the dissociation of the surface groups as well as preferred adsorption of electrolyte ions for potential determining processes. An essential part of this description is the theory of Esin and Markov (16) regarding specific occupation of adsorption sites in the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) by hydronium ions, hydroxide ions, electrolyte anions, and cations (two ion types for each Helmholtz plane have to be defined as preferentially adsorbed). Furthermore, the dissociation of acidic or basic groups is considered to be equivalent to the adsorption of hydronium or hydroxide ions according to Lyklema (17).

For the preferential adsorption of hydroxide ions (O[H.sup.-]) and mono-valent electrolyte anions ([X.sup.-])in the IHP, and the preferential adsorption of hydronium ions ([H.sup.+]) and mono-valent electrolyte cations ([K.sup.+]) in the OHP, one gets:

[c.sub.i(IHP)] = [c.sub.i(bulk)] exp[(-F[[Psi].sub.IHP] - [[Phi].sub.i])/RT] (1)

[c.sub.i(OHP)] = [c.sub.i(bulk)] exp[(-F[Zeta] - [[Phi].sub.i])/RT] (2)

[[Sigma].sub.IHP] = [e.sub.0]N([[Chi].sub.[Chi]-(IHP)] - [[Chi].sub.OH-(IHP)]) (3)

[[Sigma].sub.OHP] = [e.sub.0]N([[Chi].sub.K+(OHP)] - [[Chi].sub.H+(OHP)]) (4)

[Mathematical Expression Omitted] (5)

[[Sigma].sub.OHP] + [[Sigma].sub.diffuse] = [C.sub.HP]([Zeta] - [[Phi].sub.IHP]) (6)

where [c.sub.i] is the electrolyte concentration, [Psi] electrical potential, [[Sigma].sub.IHP] and [[Sigma].sub.OHP] are electrokinetic charge densities of the inner and outer Helmholtz planes, [Epsilon] dielectric constant of liquid, [[Epsilon].sub.0] permittivity of free space, [C.sub.HP] capacity between Helmholtz planes, [Zeta] zeta potential [[Phi].sub.i] adsorption potential of all ionic species, F Faraday constant, [Mathematical Expression Omitted] molar fraction of the ionic species in the shear plane, [e.sub.0] elementary charge and N number of adsorption sites, R gas constant, and T temperature. The concentration of the ionic species in the shear ([Kappa]) plane can be expressed by the bulk concentration by means of a Boltzmann approach that contains the adsorption potential for ionic species, [[Phi].sub.i]; (see Eqs 1 and 2).

According to Lyklema, adsorption potentials and pK values are correlated by (17):

[Mathematical Expression Omitted] (7)

[Mathematical Expression Omitted] (8)

The resulting system of equations contains two independent variables (pH, c), two dependent variables ([Zeta], [Psi]) and enables the calculation of the following parameters:

the adsorption potential for all ionic species [[Phi].sub.i] (i.e [[Phi].sub.K+], [[Phi].sub.H+], [[Phi].sub.CI-], [[Phi].sub.OH-], in the case of potassium chloride solutions); the charge density [[Sigma].sup.k]; and the pK values (18, 19).

The molar adsorption potentials, [[Phi].sub.i] represents the non-electrostatic adsorption and does not describe the electrostatic influence on the adsorption equilibrium between the charged surface and ionic species i, from the liquid phase. Hence [[Phi].sub.i] is a characteristic value to describe the adsorption equilibrium at the iso-electric point. The adsorption of (for example) a cationic surfactant on an anionic fiber causes a decrease of the negative zeta potential. At a critical concentration of surfactant the zero point of charge is reached (zeta potential = 0). With the addition of more cationic surfactant the [Zeta] and hence the sign of the surface charge of the fibers is changed into a positive one. This surfacrant concentration is called the charge reversal concentration (CRC).

4. MATERIAL AND METHODS

The material used for all experiments was 100% pure cotton fabric produced by Lenzing AG., with following physical and chemical characteristics: humidity content 6.55%; degree of polymerization is app. 4700.

4.1 Preparation Methods

A) Purifying treatments (the cellulose structure re-mains unchanged - cellulose I)

Boiling - Removal of non cellulose compounds - interfibrillar swelling: (20 g/l NaOH; pH = 11.5; t = 90 min; T = 95 [degrees] C)

Oxidative bleaching - Degradation of natural dyes and pigments:

the material was first boiled and afterwards [H.sub.2][O.sub.2] bleached; (9.5 g/l 35% [H.sub.2][O.sub.2]; pH = 11.2; t = 30 min; T = 95 [degrees] C)

B) Treatment causing a modifications of mechanical and adsorption abilities (the cellulose structure is changed - cellulose II)

Mercerization - inter-fibrillar and intra-fibrillar swelling

raw, boiled and boiled + bleached materials were mercerized;(24% NaOH, pH = 13, t = 60 s, T = 15 [degrees] C)

After each treatment the fibers were washed with distilled water until a conductivity less then 3 ms/m was reached, The processed material was air dried.

4.2. Analytical Methods

Determination of the Morphological Changes of Fibers

Light microscopy

Sample preparation method:

The cross sections were prepared by the classical stainless steel plate method. Microscopic images of fiber-cross-sections were obtained by reflection light microscopy. All measurements were performed with a ZEISS microscope Axiotech 25 HD (+ pol), at magnitude 500X (200 cross-section measurements per sample). The following features of fiber-cross-sections were determined: lumen area, fiber diameter (feretmax) and formfactor (factor of roundness). A schematic presentation of the measured features, is given in Fig. 4.

Electron microscopy

Fiber surface images were obtained by Electron microscope. On these images width of wrinkles located on the outer face of the primary wall of fiber was measured.

Image processing and analysis

Measuring equipment:

CCD SONY video camera model DXC-151AP connected to the light microscope, frame grabber (gray and [much greater than] true color [much less than] signal; 8 bit resolution/channel (RGB 8:8:8); image memory 3MB to digitize the image and a host computer with Kontron KS 300 software for image processing.

Determination of the Zeta Potential

The streaming potential method was used, as it has been shown to be the most appropriate electrokinetic technique to study electrokinetic properties i.e. the zeta potential ([Zeta]) of fibers systems (5-7).

The [Zeta] was calculated from streaming potential (Us) data by the Smoluchowski equation,

[Mathematical Expression Omitted] (9)

where [Zeta] is the zeta potential, [U.sub.s] is the streaming potential, [Delta]p is the hydrodynamic pressure difference across the plug, [Eta] is the liquid viscosity, e is the liquid permittivity, [[Epsilon].sub.o] is the permittivity of free space, L is the length of the plug, Q is the cross-sectional area of the plug and R is the electrical resistance across the plug. For most practical systems Eq 9 is perfectly adequate.

The term (L/Q), consists of two parameters, neither of which can be easily measured, Various approaches have been suggested to address the problem. Two different, but simple, methods are those of Fairbrother and Mastin (20) and Chang and Robertson (21). In the Fairbrother and Mastin (FM) approach the term (L/Q) is replaces by ([R.sub.s] [[Chi].sub.s]), where [R.sub.s] is the electrical resistance of the plug when the measurement cell is filled with an electrolyte whose specific conductance, [[Chi].sub.s], is accurately known (22). Thus, Eq 9 becomes:

[Mathematical Expression Omitted] (10)

For most practical systems Eq 10 is perfectly adequate. If the surface conductance has to be taken into account a 0,1 n KCl is used (22) instead of the standard electrolyte.

The measurements of [R.sub.s] and R are the usually made sequentially. The cell is first filled with the 0,1 n electrolyte and [R.sub.s] measured. The cell is then flushed and refilled with the experimental liquid (0,001 n) and R is measured. Because a complete exchange of electrolyte may not be possible without changing the packing density of the plug, this extended FM approach is limited to streaming potential measurements through single channels (membranes), or across surfaces with well defined, fixed, open geometry (films).

Measurements were always performed with fabrics in the fiber cell using 0,001 n KCl as electrolyte solution. The pH of the electrolyte solution was varied always in the identical way. It was first adjusted to pH 10 using 0,1 n NaOH and afterwards decreased stepwise with 0,1 n HCl. The here mentioned zeta potential values are always these obtained at the constant part of the zeta potential - pH function in the alkaline region at pH = 9.

[Zeta] was investigated as a function of pH. From the [Zeta] -pH functions electokinetic parameters are calculated (see Table 1) according to the Borner and Jacobasch model, as described before. The [Zeta] was also investigated as a function of surfactant concentration. The surfactant (N-cetylpyridiniumchlorid -N-CPC) concentration in the electrolyte solution was increased step wise until the fiber was oppositely charged.

Measuring equipment: Electrokinetic Analyzer EKA, A. Paar KG, fiber and fiat plate cell.

5. RESULTS AND DISCUSSION

Figure 5 represents the photomicrographs of the fiber surface. The raw fibers have very corrugated structure. This structure is formed after shrinkage of the fiber when it stops to grow and dries out. During this process the primary wall (waxes on the surface) wrinkles as much as the cellulose in the fiber shrinks. After mercerization under tension these wrinkles are still present but they are less deep and very extended. Their width increases from 168.8 nm of the raw fiber to 244.7 nm at the mercerized one.

The surface of boiled fibers is practically without wrinkles. During purification waxes are removed and the network of fibrils is free and spreads slightly (increasing of fiber diameter; Table 1, [ILLUSTRATION FOR FIGURE 6 OMITTED]). The wrinkles on the surface decrease because of discussed phenomenon. In the picture of boiled fiber surface the network of fibrils of the primary wall is visible, this is an evidence of good purification. Wrinkles arise repeatedly after mercerization. They are even 10 nm narrower than at the raw fiber (due to partially removed layer of waxes). The fibrillar structure on the surface is still detectable.

Bleaching as an oxidative treatment mostly affects the surfaces of the fibers, they have an etched look, and the wrinkles disappear practically. After mercerization the surfaces of these fibers becomes much more smooth, but the etched structures remain.

The results of measurement of formative changes of cross sections are represented on Fig. 6. The lumen area and the shape of fiber cross sections practically does not change after boiling and bleaching but fiber diameters increases. During these two treatments the fiber cuticle (waxy layer on the fiber surface) is partially removed and network of fibrils of fiber structure can slightly spread (ribbon-like Fiber lightly widens); Table 1., Fig. 6 - Chart 3.

It can be seen that drastic changes in fiber cross section shapes occur after mercerization. By this [TABULAR DATA FOR TABLE 1 OMITTED] treatment of raw or boiled fibers the lumen area is reduced for about 45%, bleached and mercerized fiber show even greater reduction of the lumen area. As mentioned earlier mercerization causes intra-crystalline swelling and modification of the crystallographic cell from cellulose I to cellulose II. This is the reason for permanent swelling and changes of fiber appearance. Fiber primary wall swells less than secondary wall. So, the secondary wall can swell only in lumen direction, which causes a reduction of the lumen area.

For that very reason fiber cross sections become more spherical after mercerization (second graph on [ILLUSTRATION FOR FIGURE 6 OMITTED]). The highest degree of cross-section roundness is shown by fibers that have been boiled or bleached before mercerization. This indicates that purified surfaces are very well prepared for the next processing step - mercerization. By mercerization the fiber diameter is drastically reduced ([approximately equal to] 12%) because of changing of fiber appearance from ribbon like to cylindrical form while the primary wall remains unchanged.

The natural cellulose fibers are negatively charged ([[Zeta].sub.plateau] = - 14 mV) due to the presence of carboxyl and hydroxyl - groups. In the case of raw (untreated) material these groups are covered by non-cellulose compounds present in the primary wall of the natural fiber [ILLUSTRATION FOR FIGURE 7 OMITTED]. The NaOH boiling degrades and removes practically all non-cellulose compounds except waxes which remain to about 50% on the fiber. The NaOH boiling causes inter-fibrillar swelling of the surface layers and so the size of the active surface is increased, but the amount of dissociable groups should not be changed. The swelling itself causes a reduction of the [Zeta], because of the shift of the shear plane into the liquid phase. The oxidation - [H.sub.2][O.sub.2] bleaching process causes formation of new surface groups (-CO, -CHO and -COOH) and a second step of hydrolyzation of material which is observed by the decreased [[Zeta].sub.plateau] ([ILLUSTRATION FOR FIGURE 7 OMITTED], Table 1).

Mercerization changes the supermolecular structure and morphology of fibers without any chemical changes. Due to the intra- and inter-fibrillar swelling the accessibility of fibers is drastically changed; this is observed by a strong decrease of the [[Zeta].sub.plateau] ([[Zeta].sub.plateau] of raw material = -14 mV and of raw mercerized -10; [[Zeta].sub.plateau] boiled = -20 mV and of boiled and afterwards mercerized -15 mV; see Table 1). The [[Zeta].sub.plateau] of the very hydrophilic bleached material remains after the mercerization practically the same. It can be concluded from these results that the accessibility of the dissociated groups can be monitored by pH - [Zeta] functions.

From Fig. 7 it can be seen that the [Zeta] - pH function is not: changed qualitatively by the treatment, the [Zeta] values of the plateau region are a function of the degree of removal of noncellulose compounds and can therefore be used to describe the progress of these process steps. The isoelectric points are shifted towards lower pH values during the cleaning process, due to the increased accessibility of dissociable surface groups (IEP shifted from pH 2-3 to pH approximately 1.8; (Table 1).

The surfactant adsorption process [ILLUSTRATION FOR FIGURE 8 OMITTED] shows a similar picture as the [Zeta]-pH functions. The hydrophilic materials (boiled, bleached and mercerized samples) with better accessibility obtained by fiber swelling and enlargement of the primary inter-fibrillar and/or intra-fibrillar places show a better adsorption of cationic surfactant (CRC of bleached material = 64 mg N-CPC/1 KCl). On the other hand, the concentrations at which the sign of the surface charge of raw and raw and afterwards mercerized material fibers is changed are increased (CRC of raw material = 110 mg N-CPC/1 KCl). The results of the [Zeta]-surfactant concentration functions show that different processing of cellulose fibers offers different fibers adsorption properties for cationic surfactants. Samples with changed structure (enlarged pore structure or/and supermolecular structure) adsorb less cations although their active groups and places are better accessible for the liquid phase.

The physical changes that are reflected in the altered organization of fibrillar structures [ILLUSTRATION FOR FIGURE 5 OMITTED] we in good correlation with changes of the electrokinetic character. All fibers with modified structure show smaller zeta potential values in the [Zeta] - pH functions and decreased critical charge reversal concentrations (CRC) in the surfactant concentration - [Zeta] functions (Table 1).

The adsorption behavior of cations (N-cetylpyridiniumchlorid -N-CPC) or anions (direct diazo red dye C.I. 29160) on alkaline modified samples was monitored also spectrometrically by measuring the fibers adsorption ability [ILLUSTRATION FOR FIGURES 9 AND 10 OMITTED]. The adsorbance (A) - time function (isotherm) obtained by adsorption of cations on negative cellulose fibers shows the same character of fibers as the surfactant concentration - [Zeta] functions. The adsorption process of N - CPC on the raw and raw and afterwards mercerized samples (their CRC = 110 mg N-CPC/l KCl) is finished after 6 min. The material which was not purified (waxy layer on the fiber surface; [ILLUSTRATION FOR FIGURE 5 OMITTED]) before mercerization shows the same adsorption characteristic as the raw sample even if an intra-fibrillar swelling is observed. In comparison with other samples (hydrophilic one) this two materials show a pronounced cation preferred adsorption ability. Boiled, bleached and afterwards mercerized samples show a weak adsorption of cations.

The fibers show different adsorption character if anions are adsorbed (direct diazo red dye C.I. 29160) [ILLUSTRATION FOR FIGURE 10 OMITTED]. This large dye molecule (d = app. 2 nm) can only penetrate into spread fiber structures, (enlarged pore structure, formation of cellulose II) which are obtained by mercerization. This fibers show a drastically improved anion adsorption ability. The mercerized fibers adsorb higher amounts of anionic dye than fibers which were only purified. This can also be observed tinder identical coloring conditions. It can be concluded that the changes in the supermolecular structure have more dominant influence on anionic dyes adsorption process where large molecules are adsorbed than the electrokinetic character of the fibers.

By purification all adsorption/dissociation interaction at the fiber - solid/electrolyte - liquid interface are increased, this is shown by the adsorption potentials, which are higher for all purified materials. The highest differences are observed at [[Phi].sub.Cl-] (for example; [[Phi].sub.Cl-] of raw material = -32.1 kJ/mol; [[Phi].sub.Cl-] of NaOH boiled material = 48.6 kJ/mol). This means that purification steps improve the adsorption ability for anions from the liquid phase; this could also be observed by spectrometric data [ILLUSTRATION FOR FIGURE 10 OMITTED], The mercerization causes a further improvement of anion adsorption ability of fibers - the materials show increased [[Phi].sub.Cl-] and [[Phi].sub.OH-], potentials and decreased [[Phi].sub.H+] potentials (boiled+mercerized material; [[Phi].sub.Clz] = -47.8 kJ/mol; [[Phi].sub.H+] = -19.92 kJ/mol). Adsorption potentials calculated by the Jacobasch model are in a good agreement with the results obtained by spectroscopically determined adsorption isotherms.

The pK values (Table 1) show that the materials become more acid character by purification process due to the higher accessibility of characteristic cellulose carboxyl groups (pK values are smaller for purified materials i.e. pK of raw material = 4.3, IEP = 2.4; pK of bleached + mercerized sample = 2.4).

Comparison of the calculated adsorption potentials of cotton materials shows that the adsorption energies of anionic groups [[Phi].sub.OH-] (COOH and OH cellulose groups) are the highest, as well as the adsorption energies of ions ([[Phi].sub.Cl-]) adsorbed from the liquid phase. This correlation between different adsorption potentials (dissociation/adsorption) can be observed for raw and for purified material; the cotton fibers have anionic character (Table 1). The boiled and afterwards mercerized fiber with the highest adsorption ability for the anionic dye [ILLUSTRATION FOR FIGURE 10 OMITTED] has the highest charge density [[Sigma].sup.[Kappa]] ([[Sigma].sup.[Kappa]] = 0.82 [[micro]controller]/[cm.sup.2]), showing a high amount of free adsorption places (obtained by boiling; [ILLUSTRATION FOR FIGURE 5 OMITTED]) and their accessibility is higher (mercerization).

6. CONCLUSIONS

There is a good correlation between the changes in fiber morphology and electrokinetic character of the fibers. The changes in the fiber morphology of surfaces and cross-section can be easily monitored by microscope image analysis. Applying this alternative analytical method we were able to discriminate and define the modification of cross-section shapes and surfaces organization caused by different cotton processing.

The streaming potential measurements clearly show, that among other morphological changes, the differences in surface properties, determine the adsorption behavior of solid materials. The method can be used to describe the interaction mechanism between textile fibers (with all difficulties caused by porous materials) and components of the liquid phase. It may also be an excellent tool in the optimization of different technologically relevant processes.

ACKNOWLEDGMENT

Financial support for this work has been provided by the Austrian Federal Ministry of Science and Transport and Styrian Government, Department of Science and Research.

REFERENCES

1. M. Lewin and S. B. Sello, Chemical Processing of Fibers and Fabrics, Fundamentals and Preparation Part A, Marcel Dekker, Inc., New York and Basel (1983).

2. J. Schulz, Acta Polymerica, 36, 80 (1985).

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8. R. T. O'Connor, The Fine Structure of Cotton, An Atlas of Cotton Microscopy, Marcel Dekker, Inc., New York (1973).

9. R. T. O'Connor, Instrumental Analysis of Cotton Cellalose and Modified Cotton Cellulose, Marcel Dekker, Inc., New York (1972).

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11. B. Xu and B. Pourdeyhimi, Proceedings of Beltwide Cotton Conference, Part 2 (1993).

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13. S, Inoue, Video Microscopy, Plenum Press, New York (1986).

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16. O. A., Esin and B. F. Markov, Zh Fiz. Khim., 13, 318 (1939).

17. J. Lyklema and H. P. Sidorova, Kolloidni J., 38, 4 (1976).

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Author:Stana-Kleinschek, Karin; Strnad, Simona; Ribitsch, Volker
Publication:Polymer Engineering and Science
Date:Aug 1, 1999
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