Zeta Potential Measurements of Irregular Shape Solid Materials - ACS

Chapter 22

Zeta Potential Measurements of Irregular Shape Solid Materials 1

2

D. Fairhurst and V. Ribitsch

Downloaded by UNIV OF ARIZONA on May 24, 2013 | http://pubs.acs.org Publication Date: September 24, 1991 | doi: 10.1021/bk-1991-0472.ch022

1

Brookhaven Instruments Corporation, 750 Blue Point Road, Holtsville, NY 11742 Institute for Physical Chemistry, University of Graz, Austria

2

An instrument is described to measure the electrokinetic potential of solid bodies whose size and/or shape precludes determination by either conventional electrophoresis, or electroacoustic, techniques. Based on the established principles of streaming potential/current, and using specially designed measuring cells, the zeta potential offibresand films is calculated from two simple methods. Examples of measurements are presented to demonstrate the straightforward use of the instrument for the investigation of industrially important applications. The electrokinetic surface properties of materials are important to many industrial processes (1), and also biological and medical applications (2-4). These properties are generated by the electrochemical double layer ( E D L ) which exists at the phase boundary between a solid and a solution containing ionic moieties in which the solid is dispersed or suspended. The net charge at the surface of a material in contact with a polar medium is governed by three processes viz ionization/dissociation of surface chemical groups, adsorption of ionic species and dissolution of ions from the material into solution. The distribution of the electrical charges at the interface is different from that in the bulk phase. The simplest model is that of Stern (5). The charges at the surface are compensated by ions of opposite charge (counter-ions) in solution forming two different layers. Hence the term electrochemical double layer. The layer closest to the surface, (the Stern layer), is considered immobile whilst the outer layer, (the diffuse layer), allows diffusion of ions through thermal motion.(Figure 1). The potential decreases linearly from \ / , (the surface, or wall, potential), to y , (the Stern potential), and then decays exponentially to zero in the diffuse 0097-6156/91/0472-0337$06.00/0 © 1991 American Chemical Society

In Particle Size Distribution II; Provder, T.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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P A R T I C L E S I Z E D I S T R I B U T I O N II


layer. Unfortunately, the fundamental interfacial property y£ , (and hence charge d e n s i t y , ^ ) , is experimentally inaccessible (6). What can be derived is the electrokinetic, or zeta, potential (ZP). This quantity is defined as the potential at the surface of shear - so called because any relative movement of the surface with respect to the solution will cause some of the counter-ions to be sheared off resulting in only partial compensation of the surface charge. The location of the surface of shear is never precisely known. In reality it is a region of rapidly changing viscosity rather than a mathematical plane. Thus the Z P is not well defined. Nevertheless the Z P has become a usefiil parameter to monitor electrokinetic behavior, especially changes in such behavior (1). In theoretical calculations the Z P is frequently taken to be identical with the Stern potential.

Methods to determine the Zeta Potential. Any relative motion between the rigid and mobile parts of the E D L will result in one of four related electrokinetic effects (Figure 2). Measurements made using techniques based upon each effect should, in principle, result in the same calculated value for the Z P . Very often a variety of factors conspire to produce results that are not compatible. These include assumptions in the model for the EDL> inadequacies in the theory, experimental limitations in the design and construction of instrumentation and differences in sample preparation for each method (7). Since the electrochemical properties at the solid- liquid interface are critically dependent upon the extent of that interface and the, concomitant, interfacial chemistry then it is crucially important that the correct measurement technique be used. Instruments, based on the different electrokinetic effects, have practical working limits.The most common procedure used is (micro)electrophoresis. This requires that the material under investigation be in the form of a stable dispersion of microscopically visible particles. Only extremely dilute sols (typically less than 0.001%) can be studied and calculation of the Z P assumes a spherical particle. Most industrial dispersions are concentrates; dilution is, often, not an innocuous process. Also, the particles under investigation must not be so large, nor so dense, that they sediment from the field of view before the measurement can be completed. The new electroacoustic technique (8), which is an extension of the sedimentation potential method, will work with dispersion concentrations in the range l w / w % to 50w/w%. However, the technique is in its infancy; various technical/practical complications arise as the particle concentration increases making calibration difficult. Furthermore, calculation of Z P values from the raw (vibration)potential data has not been definitively established. Figure 3 is an example of the limitations of each electrokinetic technique using the constraints of particle density and solids volume fraction. Note, there is no completely satisfactory method for systems containing material of density less than approximately 2 g/cc and at concentrations between about 0.1% to about



22.

FAIRHURST & RIBITSCH

Zeta Potential Measurements

Figure 1. Stern Model of the Electrochemical Double Layer.

Figure 2. Relationship between the four types of electrokinetic phenomena.


339

340


10%. Other constraints, such as refractive index and particle size, can also be used to construct alternative "technique limitation' diagrams. The choice of method should be predicated primarily by the particular application that needs to investigated. Too often, the system under investigation is adapted to fit the available instrumentation; care must be taken that the results of any measurement are not instrument dependent. This can be illustrated by the example of paper wet end chemistry. The Z P of the furnish must be measured and optimized in order to maximize process parameters such as drainage, water removal and retention (9-11). The "furnish" is a fibrous wood pulp slurry from which paper is made. Wood fibres are thinner than human hair but can vary in length from tens of micrometres to tens of millimetres. The furnish can also contain various additives to provide desired physical and processing properties. These additives include fillers to provide opacity, sizing to give water resistance and dyes for colour. It is clear from Figure 3, however, that (micro)electrophoresis is only suitable for use with a small, selected, portion of the fines. This fraction will have different interfacial properties to the bulk furnish. Since the entire furnish cannot assessed results obtained using (micro)electrophoresis will be limited in their usefulness. Further, wood fibres are hollow and stiff so they are beaten ("refining") which causes them to roughen, fray and collapse. This gives the mixture better bonding quality, allowing the fibres to adhere to each other. The fibres are beaten to varying degrees of "freeness"; the freeness affects how quickly water can be removed from the pulp. The beating essentially changes the viscoelastic behaviour of the pulp. It also has the effect of exposing more surface functional groups. The major chemical constituents of wood fibre are cellulose and lignin. Thus, the surface groups are anionic (negatively charged).


1

It is common practice to add so-called "drainage aids" to neutralize this surface charge and create a structure which will have maximum drainage while maintaining the desired physical properties. These drainage aids are, in effect, cationic (positively charged) surfactants, polyelectrolytes and polymers. Thus, an important parameter which needs to be monitored is the cationic demand of the pulp. This can readily be made by measuring the Z P of the pulp as a function of added cationic chemical. Comprehension of the relation between cationic demand and the level of furnish freeness is fundamental to understanding wet end chemistry. A meaningful correlation clearly can not be obtained using (micro)electrophoresis. Measurement of the Z P of materials of irregular size and shape, such as textile fibers, hair, polymer films, paper pulp, metal foil and coarse dispersions of mining material, can be made most satisfactorily using the streaming potential method. A n intensive review of the theoretical background, and methodical details, of Z P measurements on fibers has been made by Jacobasch(12).


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Principles Measurement of Streaming Potential: Basic Theory. When an electrolyte solution is forced, by external pressure, through a porous plug of material, or across a channel formed by two plates, or down a capillary, a streaming potential develops between the ends (Figure 4). The development of this potential which arises from the motion of ions in the diffuse layer is, in fact, a complex process transfer of charge and mass occurring simultaneously by a number of mechanisms. The potential difference developed for a given applied pressure difference is measured by electrodes placed at either end of the plug or capillary. The fundamental equations relating mechanical and electrical forces were first derived by Helmholtz and Smoluchowski; an excellent review of the topic has been made by Hunter (1). The Z P is calculated from: t = (Es/Ap).(n / €€«).(L/A).(1/R)

(1)

Where ^"is the zeta potential, E s is the streaming potential, A p is the hydrodynamic pressure difference across the plug, n is the liquid viscosity, 6 is the liquid permittivity, € is the permittivity of free space, L is the length of the plug, A is the cross- sectional area of the plug and R is the electrical resistance across the plug. The inadequacies of the fundamental equation have recently been reviewed (13). For most practical systems Equation 1 is perfectly adequate. Practical problems, inherent in making the measurement, such as asymmetry potential, plug porosity and surface conductance are well documented (1,7,14,15). The various terms in Equation 1 are known or must be measured. The first term, ( E s / A p ) , is determined by direct measurement of the potential generated for a given, constant, driving pressure. The precision and repeatability of detection can be significantly increased if the potential is monitored as a function of a continuously increasing pressure difference (16). The value of the quotient ( E s / A p ) is determined from the slope. For a given temperature the values of n, 6 , and € o are constant for the liquid used (usually water). The plug resistance, R, can be measured directly using an A C bridge at three different frequencies (500,1000 and 2000 Hz) (17). However, if both the streaming potential, E s and the streaming current, Is, are measured simultaneously then the D C resistance,R, can be determined indirectly from the ratio of the two quotients,(Es/ A p ) / ( I s / Ap) (18). The final term, ( L / A ) , 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 (19) and Chang and Robertson (20). In the Fairbrother and Mastin ( F M ) approach the term ( L / A ) is replaced by (Rs. K s), where R s is the electrical resistance of the plug when the measurement cell is filled with an electrolyte Q


342


d

LoiKdj-d^/dJ

2

11.00

1 Only technique for fibers or flat sheets

2.00 1.10

-1 j Electrophoresis

-2

1.01

0.0001

0.001

0.01


Volume

0.1

Fraction

Figure 3. Operational Envelope for Electrokinetic Techniques under constraints of Density and Volume Fraction.

Electric

Double

Layer

at Rest Counterions

~ Liquid

Caused

to Move Relative

0

Accumulation

of Ions

to

-

Surface v ^ ©

1

_

_

®

Surface

Movement streaming ^

_

©

^

_

_

©

Charge

of ions produces current, I

a

-

Downstream Accumulation causes formation a potential difference, E

®

Backflow

Current

Through

e

©

Liquid E causes backflow through, liquid

current,

® © e © J±L©_J§A steady

state is reached

the measured gives

direct

potential information

the surface-solution

quickly

difference

where I (the

s

- 1 = 0 b

streaming

about the electroststic

and potential)

charge

at

interface.

Figure 4. Schematic representation of Streaming potential at the Solid-Solution Interlace.


I *

of

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343

whose specific conductance, Ks, is accurately known (21). A 0.1M KC1 solution is used to avoid the problem of surface conductance (22). Thus,


(Es/ Ap).(n/e€o).(Rs. K s / R )

(2)

The measurements of R s and R are usually made sequentially. The cell is first filled with the standard electrolyte and R s measured. The cell is then flushed and refilled with the experimental liquid and R measured. Because a complete exchange of electrolyte may not be possible without changing the packing density of the plug, the F M approach is limited to streaming potential measurements through single channels (membranes), or across surfaces with well defined, fixed, open geometry (films). For plugs of granular material and bundles of fibers, where the packing density is not well defined and cannot always be duplicated, the approach of Chang and Robertson (CR) can be adopted. Building on geometric considerations of Goring and Mason (23) they developed an empirical relationship between porosity and packing density. Using their approach Equation 1 can be written as : 10ge(Es/ Ap).(Lm/R) = lOge^ -

Bd

(3)

Where Lm is the distance between the electrodes, B is a constant which depends upon the material under investigation and d is the (fibre) concentration i n the cell i n g/cm - or "packing density". The Z P is obtained from the intercept of a plot of streaming potential versus packing density (Equation 3). 3

Methods and Materials Apparatus. The measurements presented here were all performed using an instrument developed originally at the Technical University Graz, Austria. The design and operation addresses all the requirements discussed above. A schematic diagram and photograph are shown in Figure 5. The instrument comprises an analyzer, data control system and measuring cells. The analyzer consists of a mechanical drive unit to produce, and measure, the driving pressure which causes the electrolyte solution to flow from a reservoir into and through a measuring cell and out back into the reservoir, or to waste. The flow direction can be reversed. Operation is manual or under computer control. The streaming potential and the streaming current are both measured. Temperature, p H and conductivity sensors are also included. Plug resistance is measured by a choice of either A C or D C methods. The Z P is calculated from the streaming potential/streaming current data using either the C R or F M methods. A commercial version of the instrument is now available as the BROOKHAVEN-Paar BI-EKA.



344


Analog Out

uC

Temperature and pH

Conductivity

Voltage and Current

Pressure Pump

Pressure Sensor

Sample Under Investigation

Figure 5. Photograph and Schematic Diagram of the B R O O K H A V E N - Paar Electrokinetic Analyser.


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Measuring Cells. The choice of cell design depends upon the nature of the sample under investigation. 1. A Cylindrical Cell (Figure 6a) based on earlier designs of Goring and Mason (23) and Joy et al (24), with modifications suggested by Schaumberger and Schurz (18). It has two perforated A g / A g C l electrodes which are able to slide inside a glass body. A known weight of material in the form of fibers or granules is placed between the electrodes. The electrode spacing is measured precisely by means of micrometer screw drives. Either the F M or the C R method of Z P evaluation can be selected. In the latter method, the distance between the electrodes is decreased in a series of sequential steps to squeeze the plug and so change the packing density. 2. A Rectangular Cell (Figure 6b) made of P M M A based on an original idea by Andrade (25,26) with improvements adopted by Jacobasch (27). The necessary hydrodynamic requirements (28) were met to ensure reproducible determination of streaming potential. A channel of well defined dimensions is created by use of P T F E spacers. The sample is usually in the form of plates, film or foil and is cut to size (120mm x 50mm) to fit inside the P T F E formers. Irregular shaped, or formed, material can also be accommodated by mounting in a suitable sealing material such as beeswax in recesses in an optional, modified, version of the cell; each recess has dimensions 100mm x 20mm x 5mm. A g / A g C l electrodes are mounted at each end of the channel. Only the F M method can be used to calculate the Z P . It must be stressed that the streaming potential is valid only if certain hydrodynamic conditions are met. In any cell design the fluid flow must be steady, incompressible, laminar, and established (28). Ensuring such conditions exist is not trivial and, yet, is a basic assumption in the calculation of Z P from streaming potential measurements. Recently, Bowen (29) has extended the theory and provided correction factors to the classical Helmholtz-Smoluchowski equation under conditions where the capillary or channel dimensions are large relative to the double layer thickness.

Procedure. Ideally, before any measurement the material under investigation should be soaked overnight in the initial, or base, electrolyte with agitation to remove air bubbles adhered on the surface. This is more important for hydrophobic surfaces such as polymer/plastic sheets and bundles of polymer fibres. When the cell is assembled with the sample in place it must first be flushed in both directions to remove entrapped air. In the B R O O K H A V E N Paar E K A rinsing and flushing can be done manually or automatically (under computer control). A l l parts of each cell must also be cleaned on a regular basis not only for good laboratory practice but also to achieve the best reproducibility in measurements. For the glass (cylindrical) cell soaking in a suitable laboratory detergent



346


fiber or powder

adjustable piston

M sample plate 1

sealing foil

lower part

Figure 6. Measurement Cells for the Electrokinetic Analyser. aTFor the investigation of fibers and granulates, b) For the investigation of films and plates.


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followed by a thorough rinsing in distilled water is usually sufficient. The inner surfaces of the P M M A (rectangular) cell can be cleaned using 5% sodium carbonate followed by a detergent wash (after Joy et al). The A g / A g C l electrodes must also be regenerated from time to time. A convenient preparation of such electrodes has been described by Ives and Janz (30).


Materials. A l l materials were used as supplied. No special precautions or cleaning procedure was adopted other than the initial soaking prior to measurement. A l l reagents were Analar grade.

Results and Discussion Four examples are presented. These demonstrate the versatility, and utility, of the streaming potential technique for the investigation of the electrokinetic properties of surfaces of materials which are directly used in important industrial processes. Example 1. The quality of offset printing is affected by the surface chemistry of the printing plates (31). The bright metal surface is dimmed as corrosion proceeds. Various oxidative species are produced on the surface and these reduce the hydrophobic character. The oxide groups can be ionic and so will dissociate in solution to a degree which depends upon the p H of the solution and, to a lesser extent, on the ionic strength. The magnitude of the Z P is a sensitive measure of the dissociative nature of surface groups ; a Z P of zero indicates an isoelectric point (IEP) and a plateau in the plot of Z P vs p H is indicative of complete dissociation (1,32). Figure 7 shows the p H dependency of the Z P determined for two samples of printing plate which have received different surface treatments. Two pieces of each sample were cut to size to fit into the standard rectangular cell. Surface A is clean with a bright metallic luster. Surface B has been exposed to corrosive elements and appears dim. The oxidized surface exhibits an IEP near pH6 and total dissociation of surface groups can be attained at around pH7. In contrast, dissociation is not complete for the clean surface until around pHIO and there is no evidence of an IEP. Both the wetting and adhesive properties of solids depend on the dissociation constant (pK-value) of the surface chemical composition (33-35); the pK-value of such surface groups can be estimated from Z P measurements (36). Thus, the effects of chemical and physical treatment of surfaces can readily be monitored and analysed using streaming potential measurements. The flow through nature of the streaming potential method also makes it ideal for the study of long term corrosion or oxidation processes using different solution formulations - one example would be printing ink fountain solution.

American Chemical Society Library 16th St, II;N.W. In Particle1155 Size Distribution Provder, T.; ACS Symposium Series;WttMRCtM, American Chemical Washington, DC, 1991. 0JLSociety: 20038


348


Example 2. The lifetime of paper machine felts is highly dependent upon the degree of contamination which occurs during paper manufacture (37). The "paper felt" is a felt blanket which conveys the newly formed paper through press rollers (and steam heated driers) where excess water is squeezed out of the paper and into the felt. The tendency of the felt to become polluted is controlled by the felt surface charge (38). The least contamination is found for very low, but negative, values of the Z P . Usually cationic synthetic resins are added to felts during their manufacture to achieve this low Z P condition. Table 1 lists results of Z P measurements made using samples of polyester fiber felt treated with different levels of resin. Pieces of felt were cut to fit the recesses of the modified rectangular cell and the Z P was determined using distilled water as the flow fluid. The results show that increasing amounts of resin lead to a decrease in the primary negative Z P . Overdosing with the resin produces a positive Z P - an equally undesirable situation. Table 1. Polyester Paper Machine Felt Samples Resin Treatment Untreated 0.1% 0.5% 1.0%

Zeta Potential (mV) -28 -17 -2 +10

Thus, streaming potential measurements can be used to optimize felt manufacturing and also to improve the filtration process in paper production. In addition it is possible to study variations in surface charge behaviour caused by contamination which occurs during paper manufacture as well as cleaning procedures for the felts. Example 3. In the textile industry fibers are coated with agents that reduce the tendency for surface oxidation which will affect processibility of the fiber (39). Figure 8 shows the p H dependency of two samples of viscose fibers. Sample A is coated and Sample B is untreated. No difference can be observed. Thus the nature of any dissociable groups is unaffected by the coating treatment. Cationic quaternary ammonium salts are used as anti-static agents for fibers (40-42). Figure 9 shows the result of titrating the same two viscose fiber samples with cetylpyridinium bromide (CPyBr) solution. There is a dramatic difference in the number of titratable groups between the two fiber surfaces. The untreated fiber required almost three times the concentration of CPyBr (12mg) to obtain an I E P compared with the protected fiber (4mg). The maximum surface coverage was also significantly different.


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349


+10

0.01M

KCI

IEP

0


-io| -20

j4...A..B(dJm)

A

(0

=

-30

I

-40I

N

-50

A(clean)

JL.

4 5

6 7

8

9

10

11

PH

Figure 7. Variation of the Zeta Potential with p H for two pieces of Offset Printing Plate.

20

0.001 M KCI

10

°A

IEP

0 -> -10 E

o

o-

o

N-20

3

5

4

6

7

8

PH xx

Coated

oo Uncoated

Figure 8. Comparison of the p H dependency of the Zeta Potential for two samples of Viscose fibers.


350



The protected fiber has a 50% larger Z P value than the uncoated fiber and would, therefore, exhibit much better anti-static properties. Thus, streaming potential is an effective tool to study both deviations in the chemical composition of fibres and the activity of the preparation agents used in textile processing. Example 4. In the mining industry rock destruction or machining uses high speed drills with diamond bits. The rate of diamond abrasion is a function of the machining speed. Drilling fluids which are used as coolants are formulated to contain auxiliary additives (43) which critically affects both parameters viz drill abrasion and machine speed. Optimum conditions i.e. minimum diamond abrasion and maximum machining speed are achieved when the zeta potential of the rock surface is zero with respect to the liquid coolant (44,45). To date this "enhanced drilling phenomenon" is not well understood; the economic implications are, however, enormous. Also, no statistically reliable trend exists between rock property tests (such as hardness) and mechanisms proposed to explain this point of zero charge phenomenon (46). For negatively charged rock surfaces cationic surface active agents are used in the coolant formulation. Figure 10 shows the result of an experiment which used two plate-like pieces of granite rock. The Z P was measured as a function of the concentration of dodecylpyridinium chloride (DoPyCl). For the material under investigation only a relatively low concentration (less than 0.01M DoPyBr) was needed to completely reverse the sign of the charge on the rock surface. Thus, streaming potential measurements can be used to directly determine optimum additive concentrations for zero point of charge conditions of mineral surfaces. For geological material, fracture of the surface can expose a completely different surface chemistry. Measurement of fines alone, using (micro) electrophoresis, can give incomplete, and possibly misleading, information. This may be one reason for the varied, often contradictory, Z P data for mineral surfaces. The streaming potential method uses material of macroscopic dimensions and is, thus, more appropriate for real, practical systems.

Conclusion A new instrument, the B R O O K H A V E N - P a a r B I - E K A , is available to measure the Z P of materials of industrial importance whose size, or shape, precludes measurement by (micro)electrophoresis. Reproducibility and comparability of results is ensured through the measuring cell geometries. However, the precision of Z P values from the streaming potential method is typically about 10% and so is less than that obtained using (micro)electro phoresis. Investigation as to the nature, and type, of various processing agents for selected applications - such as optimization of surface chemical composition can readily be made. A major advantage of the technique is the ability to


22. F A I R H U R S T & R I B I T S C H

351


+20 0.001MKCI


+10

N

-10

-20

20

60 80 100 [CePyBr] (mg/g fiber)

40

• • • Coated

120

140

ooo Un coated

Figure 9. Effect of the Adsorption of Cetylpyridinium Bromide on the Zeta Potential of Viscose fiber. +4 +2 0 Q. N

-2 R

10"

4

10" [DoPyCl]

3

10"

Figure 10. The Reversal of Charge of a Granite Surface using a Quaternary Ammonium Salt.


352


investigate adsorption phenomena in situ. The flow through nature of the method makes it ideal for the study of long term corrosion or oxidation processes. The use of material of macroscopic dimensions provides data which is more appropriate to process conditions.


Literature Cited 1. Hunter,R.J.,Zeta Potential in Colloid Science; Academic Press, New York, 1981. 2. Richmond,D.V. and Fisher,DJ., Advances in Microbial Physiology, 9, 1, 1973. 3. Seaman,G.V.F., in "The Red Blood Cell", Volume 2, Academic Press, New York, 1975. 4. Berreta,D.A. and Pollock,S.R., Journal of Orthopaedic Research, 4, 337, 1986. 5. Kruyt,H.R.(Ed), Colloid Science, Volume 1, Elsevier, London, 1949. 6. Guggenheim,E.A., J.Phys.Chem., 33, 842, 1924. 7. Shaw,D.J., "Electrophoresis", Academic Press, London, 1969. 8. Marlow,B.J., Fairhurst,D. and Pendse,H.P., Langmuir, 4, 611, 1988. 9. McKague,J.F., et at., TAPPI Papermakers Conference, Boston, 1974. 10. Melzer,J., Das Papier, 26, 305, 1972. 11. Anderson,R.G. and Penniman,J.G., Paper Trade Journal, January 14, 1974. 12. Jacobasch,H-J., et al., Coll. and Polym. Sci., 263, 3, 1988. 13. Christoforou,C.C., et al., J.Coll. and Int. Sci., 106, 1, 1985. 14. Somasundaran,P. and Kulkarni,R.D., J.Coll. and Int.Sci., 45, 591, 1973. 15. van den Hoven,T.J.J., Thesis, Wageningen Agricultural University, 1984. 16. Ball,B. and Fuerstenau,D.W., Miner.Sci.Eng., 5, 267, 1973. 17. Ferse,A. and Paul,M., Z.Phys.Chem., 252, 198, 1973. 18. Schausberger,A. and Schurz,J., Ange.Makromol.Chem., 80, 1, 1979. 19. Fairbrother,F. and Mastin,H., J.Chem.Soc., 75, 2318, 1924. 20. Chang,M. and Robertson,A., Can.J.Chem-Eng., 45, 66, 1967. 21 von Stackelberg et al, Kolloid Z.,135, 67, 1954. 22. Bull,H.B. and Gortner,R.A., J.Phys.Chem., 35, 309, 1931. 23. Goring,D. and Mason,S., Can.J.Res., Sect.B., 28, 307, 1950. 24. Joy,A.S., Watson,D. and Botten,R., Res.Techniques in Instr., 1, 6, 1965.



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&

RIBITSCH


25. Van Wagenen,R.A., Andrade,J.D. and Hibbs,J.B.,J.Electrochem. Soc., 123, 1438, 1976. 26. VanWagenen,R.A.and Andrade,J.D., J.Coll.and Int.Sci., 76, 305, 1980. 27. Schurz,J., et al., GITFachz.Lab.,2, 98, 1986. 28. Van Wagenen,R.A, PhD Thesis, University of Utah, 1976. 29. Bowen,B.D., J.Coll. and Int.Sci., 106, 367, 1985. 30. Ives,D. and Janz,G., Reference Electrodes, Academic Press, New York, 1961. 31. Bassemir,R.W., in "Colloids and Surfaces in Reprographic Technology (Eds. M.Hair and M.L.Croucher)", ACS Symposium Series 200, 1982. 32. Ottewill,R,H. and Shaw,J.N., Disc.Faraday Soc., 42, 154, 1966. 33. Bolger,J.C., in "Adhesion Aspects of Polymeric Coatings (Ed. K.Mittal), Plenum Press, New York, 1983. 34. Fowkes,F.M., in "Physicochemical Aspects of Polymer Surfaces (Ed. K.Mittal), Plenum Press, New York, 1983. 35. Jacobasch,H.-J., Angew.Makromol.Chemie, 128, 47, 1984. 36. Schurz,J., et al., Wochenblatt f. Papierfabrik., 525, 1989. 37. Saltman,D.,"Paper Basics", Van Nostrand, New York, 1978. 38. Weigl,J. and Kaestner,M.,Wochenblatt f. Papierfabrik., 16, 559, 1982. 39. Linfield,W., "Fatty Acids and their Industrial Application", Marcel Dekker, New York, 1968. 40. Schwartz,A.M. and Perry,J.W., "Surface Active Agents", Vol.1, Interscience, New York, 1949. 41. Jungerman,E.(Ed), "Cationic Surfactants", Marcel Dekker, New York, 1970. 42. Johnson,D.H., J.Am.Oil Chem.Soc., 55, 438, 1977. 43. Appl,F.C., et al., Ind.Diamond Rev., 41, 312, 1981. 44. Westwood,A.R.C., et al., Colloids and Surfaces, 2, 1, 1981. 45. Engelmann,W.H., et al., J.Electrochem.Soc., 135, 1043, 1988. 46. Tuzinski,P.A., US Dept.of the Interior, Bureau of Mines, private communication. RECEIVED

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Zeta Potential Measurements of Irregular Shape Solid Materials - ACS

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