Charge-Related Measurements – a Reappraisal. Part 1. Streaming

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Martin A. Hubbe and Junhua Chen Charge-Related Measurements – A North Carolina State University, Reappraisal. Dept. of Wood and Paper Science Part 1. Streaming Current

In a 1995 article in this journal Jaycock(1) (a) various factors can interfere with the A 1995 article in this maga- expressed concerns about the accuracy and analyses, and zine raised concerns about interpretation of two types of charge-related (b) one should not stray too far outside of the use and interpretation of measurements that have become increasingly the ranges of conditions under which the two kinds of measurements common in papermaking applications, methods can be expected to give reliable that are being carried out in namely, the streaming current (SC) and the answers. paper mills to evaluate the SP (c) In addition to these two reasons, there electrical charges at sur- streaming potential ( ) methods. faces in fibre slurries. In the intervening years there does not are concerns about the interpretation of SC This article relates to the appear to have been any attempt either to data obtained in paper mills. streaming current method, support or to refute those cautionary state- To start, it is worth quoting Jaycock’s which is widely used for ments. Rather, there has been increasing use statements about the SC method in the Con- (1) endpoint detection when in the paper industry of the test methods that clusions section of the article: (2-10) testing the charge demand of Jaycock cautioned us about. The present “The piston type streaming current detec- whitewater or filtrate sam- article deals with SC measurements. A tor has no established theoretical basis! It is ples from fibre stock. companion article will deal with a second assumed that the measured potential is related Although there is still type of measurement discussed in the same to the zeta potential, and that the adsorbed (11) some truth in the statement article. layers responsible for the generation of this that the “streaming current It seems that there are two ways in which potential are characteristic of the papermak- detector has no established one could view the set of circumstances ing system as a whole, being in equilibrium theoretical basis,” subse- described in the first paragraph. with it. There are doubts about the validity of quent work has helped to i) On the one hand, maybe the increased both of these assumptions.” define ranges of experimen- use of SC devices – even without considering In addition, Jaycock advocated use of tal conditions within which other factors – is sufficient proof that the microelectrophoresis(10,21) as the best way to the test gives reliable results. method must be providing practical value to obtain zeta potential data for a papermaking Also, some specific sources the users. Otherwise, the thinking goes, cost furnish. of interference have become conscious managers of paper companies and To address Jaycock’s concerns, it is first better understood. chemical supply companies would not con- necessary to describe the main equipment and tinue to invest in that technology. a little bit about the procedure of analysis ii) Another way to view the situation is to used in a typical papermaking application. assume that Jaycock’s worst fears were cor- rect, and that much of the data obtained by SC The streaming current (SC) method measurements in paper mills over the inter- Figure 1 (next page) is a schematic diagram vening years have been either inaccurate or of a typical SC device. It is worth bearing in misleading. mind that different brands of equipment can Fortunately, due to some ongoing research be quite different in appearance, despite their during the period between 1995 and now, we sharing the same basic operating principles. are now in a better position to answer some of Suppliers of SC equipment, in different forms, the concerns raised by in the 1995 article.(1) include Rank Brothers, Mütek, Chemtrac, and Some of that continued work was conducted Milton-Roy companies. by us North Carolina State University(12-14) and As noted by others,(1,22-24) the main wetted some was done by others.(15-20) parts of the SC device consist of a plastic pis- The purpose of this review article is to re- ton that moves in a sinusoidal manner back examine the main concerns raised by Jaycock and forth within a dead-ended plastic cylin- and to give qualified support for continued der, usually at a frequency of about 4 Hz. efforts to implement SC measurements, both Because the gap between the piston and cylin- in the laboratory and online in paper machine der is narrow (often less than 1 mm) systems. The reason that the support needs to compared to the diameter of the piston (often be “qualified” is that about 12 mm), the motion of the piston gives

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rise to a more rapid movement of aqueous These “extra” ionic charges will exist very fluid back and forth within the annulus. near to the surface in a region that is often Also, there are two metal electrodes built called the “double layer.” The average distri- into the cylinder wall at different axial posi- bution of counter-ion charges in the double tions. The probes are connected to an layer depends on electronic system capable of rectifying and (a) attraction to the surface smoothing any electrical signal that results (b) random diffusion of the ions in all direc- from the reciprocating motion. tions, and Except in some unusual cases,(25) the origin (c) screening of the electrostatic effects by of most of the electrical signal that is salt ions.(26-28) observed with SC devices arises due to the electrically charged nature of the plastic sur- The movement of aqueous solution past the faces, with the understanding that the word charged surface causes net movement of the “surface” includes anything that becomes counter-ions, especially in the “tail” of the adsorbed onto the plastic itself. counter-ion distribution lying farthest from Many SC devices are made with the charged surface. poly-tetrafluoroethylene (PTFE), a contamina- The most common use of SC devices in the tion-resistant substance which, if absolutely paper mill has been to determine the cationic pure, would have no surface charge of its demand or anionic demand of samples of own. Practical use of the SC method, espe- process water, usually in the absence of cially in the case of paper machine fibres.(5-8,12,17) In other words, the device is used applications, requires that the plastic becomes as the means of detecting the endpoint of a charged due to the adsorption of polyelec- titration. trolytes and colloidal materials from the A solution of charged polymer is gradually sample. added to a stirred aliquot of process water, The user makes an implicit assumption and a “zero” reading on the SC device is taken that any excess of charged materials existing as evidence that a sufficient amount of poly- in the sample will determine the sign of the mer solution has been added to the mixture to electrical current signal that is generated. just neutralize any excess of electrical charge. Setting aside, for the moment, the question This excess is understood to exist at the about whether the water-loving, charged surfaces of particles, very finely divided materials present in paper mill water samples materials (i.e. “colloidal matter”), dissolved actually adsorb to a significant extent onto the polyelectrolytes, and various surface-active low-energy plastic surfaces, the reciprocating molecules in the mixture, such as salts of flow of aqueous solution past a charged sur- resin acid molecules from the wood.(29) face gives rise to an electrical signal due to (15-20) the presence of counter-ions. Theoretical limitations to the SC method For example, if the net charge of the PTFE, Before considering practical uses of SC with its covering of colloidal materials, is tests, let’s consider what Jaycock meant by negative, then there needs to be an equal and saying that the SC method has “no theoretical opposite excess of positively charged ions in basis”.(1) To help justify these strong words, it the solution phase adjacent to the surface. is worth noting that typical raw output of an SC device does not have a simple, quantitative relationship to zeta potential. The inventor of the SC method used only a rudimentary theoretical approach to rationalize the observed output signals in terms of a zeta potential at the plastic surfaces of the piston and cylinder in the SC device.(22) Though there has been important progress more recently in quantifying the SC signal in ideal cases,(16-20) the calculation depends criti- cally on the annular distance between the piston and cylinder. That distance is subject to change due to wear, a “ribbed” pattern of some SC pistons, and possible changes in alignment of the piston within the device. Further support of the words “no theoretical basis” involves the fact that one is measuring an electronic quantity related to a coated PTFE surface, despite the fact that PTFE has a completely different surface character Figure 1 Schematic diagram of wetted parts of a common type of streaming compared to that of the fibres, fine particles, current detector or colloidal materials in a sample of process

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water from a paper mill. After all, it takes a Streaming current – the range of reliability brave individual to place their trust in that the For purposes of process control, it is often PTFE (i.e. Teflon® ) surface becomes uni- more important that a measurement be repro- formly and consistently coated by the ducible, regardless of whether the results can colloidal materials in the sample. be described as quantitatively accurate. In the Ordinarily, if your goal is to evaluate zeta case of charge measurements, as long as the potential, then you ought to use some method measured signal has a strong and reliable rela- other than SC. However, it is worth noting a tionship to the added amount of a charged few careful laboratory studies that show chemical, the signal can have value for con- exceptions to this rule. By using relatively trolling the flow of that chemical. pure samples and repeated cleaning of the The usefulness of such a control strategy probe surfaces it is sometimes possible to can be judged later, based on such factors as achieve a high correlation between SC output production efficiency, product quality mea- vs. zeta potential values derived from micro- sures, and opportunities to reduce chemical electrophoresis.(15) costs. The important question then becomes, But don’t try this in the paper mill! There “within what range of process conditions are is just too much variability in electrical con- SC titration results reproducible?” ductivity, hardness, surfactant content, and To answer these concerns, starting in 1999 other attributes of paper mill process water we set out to find out how SC titrations that are likely to throw off any correlation responded to increasing levels of salt. Figure 2 between SC measurements and zeta potential shows some typical results, titrating a solution measurements. of a highly dilute polyvinylsulphate, potassium In practice, some issues related to zeta salt (PVSK) with poly-diallyldimethylammo- potential often can be ignored, depending on nium chloride (poly-DADMAC).(12) how the SC equipment is used. The cited arti- Before describing the effect of salt, it is (1) cle admits that SC measurements are mainly helpful first to consider what happens in the used for determining the endpoint of charge absence of salt, as represented by the lowest titrations – not for zeta potential. For instance, curve in the plot. As shown, the initial signal, (30) an article by Kaunonen and Springer is cited, before the addition of cationic titrant, was saying that “it is possible to use the SCD as a negative. The sign of charge is consistent with detector for cationic demand measurement.” (a) a low, but finite level of negatively However, even those measurements charged sites on the plastic surface, due to involve an assumption that a SC value of zero impurities, and corresponds to a condition of neutral zeta (b) some adsorption of the PVSK. The fact potential. Some important early work with the that negatively charged PVSK adsorbs onto SC device, involving samples from papermak- PVSK is readily apparent from other experi- ing fibre systems, showed cases where such ments showing an increase in the negative an assumption can be quite inaccurate.(31) signal when the PVSK is first introduced. Thus, there has been a need for follow-up The initial, horizontal portion of the titra- tests to determine the range of validity of the tion curve in Figure 2 usually is attributed to SC method, even when it comes to charge consumption of titrant molecules by an excess titrations. of anionic polymer in solution. Since high-mass polyelectrolytes often exhibit “high affinity” adsorption behavior,(32) the fact that some polyelectrolyte complexes (PECs) are forming in the bulk phase does not necessarily imply a change in the amount of adsorbed anionic polymer. In other words, a reduction in the effective concentration of dissolved polymers in the bulk phase does not necessarily cause polymers at the plastic surface to come off. The situation changes as the titration pro- gresses far enough so that most of the polymer in the solution phase has been neu- tralized. Thereafter, the SC signal decreases more rapidly to zero. The decrease can be attributed to either (a) cationic titrant molecules adsorbing directly on the surface (b) cationic titrant molecules complexing Figure 2 Effect of salt on results of titrating a solution of polyvinylsulphate with pre-adsorbed PVSK molecules, and potassium salt (PVSK) with poly-diallyldimethylammonium chloride (c) formation of PECs in the bulk phase, fol- (poly-DADMAC). lowed by their adsorption.

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Surprisingly, there does not seem to have adsorbs in the form of a polyelectrolyte com- been any reported research as to which of plex, together with the oxidized starch.(39-40) these possible mechanisms tends to be most Another difference, when comparing Fig- important. Partial support for mechanism (b), ures 2 and 3, is that the lower-charge polymer complexation of a cationic titrant with pre- sample was even more susceptible to salt. adsorbed anionic polymer, comes from Figure 3 shows that a clear endpoint was not studies of PE multilayer adsorption.(33-35) Either achieved in the case of oxidized starch if the of the first two mechanisms (a or b) involving salt content was 0.1% or higher, even if twice titrant molecules arriving individually at the the stoichiometrically expected amount of surface can account for a reversal in the sign titrant was added.(12) of charge, based on past work.(32-33,36-37) In fact, it is reasonable to expect that more that both Explaining the effects of salt on SC results mechanisms occur simultaneously. The effect of salt on the initial SC output, The addition of salt affects the SC signal in before addition of titrant, can be explained in three ways, all of which can be observed in terms of two effects. The first effect involves Figure 2, comparing the positions of the plot- the thickness of the ionic double-layer at the ted lines. First, as shown by others,(12,15,22,38) plastic surface. increased salt concentration depresses the ini- To explain this effect, it is worth noting tial signal. Second, salt causes a shift in the that shear flow across a flat surface under pre- endpoint towards greater amounts of titrant. dominantly laminar conditions within an SC And third, at the highest level of salts consid- device is expected to produce an approxi- ered, the device may be unable to detect any mately linear gradient of velocity with respect change in signal due to the addition of titrant. to distance from the surface.(22) That means Before attempting to explain any of these that the flow velocity decreases as you effects, it is worth considering the results of move towards the surface. Meanwhile, the some similar experiments carried out with a reciprocal of the Debye-Hückel parameter, different type of sample. Figure 3 comes from representing an effective distance of counter- an experiment in which the sample was a ions from the surface, decreases in proportion dilute solution of oxidized starch. In contrast to the square-root of the ionic strength of the to PVSK, for which every unit of the polymer aqueous solution.(41) has a negative charge, the oxidized starch In other words, the average distance of sample had a charge content of only about counter-ions from the surface decreases with 2%, based on repeat units of the polymer. increasing salt. Since the flows within an SC The most striking difference, when com- device are fixed by the dimensions of the paring these results to the case shown in device, the stroke length, and the frequency of Figure 2, is the different shape of the curves. piston motion, it follows that increasing salt The shape suggests that added titrant mole- ought to decrease the average velocity at cules are able to reach the probe surfaces even which the counter-ions move relative to the at the very beginning of the titration. Evi- plastic surface. dently, at least some of the poly-DADMAC The second effect can be described as molecules are able to adsorb onto the plastic short-circuiting of the SC measurement sys- surfaces of the device right from the start of tem. This effect is related to the way in which the titration. Possibly the cationic polymer conventional SC devices detect the electrical signal.(22-23) At low levels of ionic strength, the current, which is induced by the ionic motions at the plastic surface, is allowed to complete a circuit between the two electrical probes, see Figure 1, by means of an external circuit. By designing the external circuit with a suitably low resistance, it is possible, in principle, to detect essentially all of the current that is pro- duced by the motion of the piston. However, as the electrical conductivity of the aqueous solution becomes higher and higher, with the addition of salt, a second route for current flow becomes increasingly important. Since the external device cannot detect the part of the current that flows through the liquid phase of the sample, the output signal is decreased. The cause of the shift in titration end- Figure 3 Effect of salt on results of SC titration of oxidised starch solution by poly- points, as illustrated by the results in Figures DADMAC 2 and 3, was not known until recently.(13) An

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important clue as to the mechanism was the when aluminum ions are present during SC surprising finding that the titration results titrations. depended not only on salt concentration, but Surprisingly, the experimental results also on the direction in which the titration was showed that, under certain conditions of carried out. Regardless of which charged NaOH addition, there was a fixed ratio polymer was used as the sample, and which between the amount of soluble aluminum and was used as the titrant, the amount of titrant the amount of PVSK titrant required to reach needed to reach the endpoint always the SC endpoint.(14,43) In other words, there was increased with increasing concentration of a stoichiometric relationship. salt. The observed stoichiometry was consistent A mechanism to explain such results is with one-to-one interactions between the illustrated in Figure 4, in which the dark lines electrical charges of PVSK and the 7+ represent the polymer used as the “sample” [AlO4Al12(OH)4(H2O)12] ion, which in the and the light-shaded lines represent the scientific literature is sometimes called the (44-45) titrant, which was added gradually as a solu- Al13 ion. By contrast, the SC titration tion to a solution of the sample. The idea is results showed no evidence of interaction of that, at the endpoint of a SC titration, the sam- Al species with PVSK under conditions of pH ple mixture contains charge-stabilized where the Al13 ion is not expected to exist. polyelectrolyte complexes. As shown in the In principle, it may be possible to explain figure, each such complex has an excess of effects of aluminum ions in quantitative titrant molecules on its outer surface. terms, by calculating the amount of different Supporting evidence for this mechanism aluminum species likely to be present at was obtained by microelectrophoresis and different pH values and aluminum concentra- (13) turbidity tests. The results are also consis- tions,(46) However, the practical reality is that tent with some recent theoretical and aluminum chemistry is complicated, espe- (33-35) experimental work by others. It is worth cially after aluminum compounds are added noting that such deviations from 1:1 stoi- to papermaking furnish. chiometry, as well as problems related to In that sense, interference is a good word indistinct titration endpoints, usually can be to describe the practical effect of aluminum minimized by diluting all of the samples compounds on SC measurements in the paper with a fixed ratio, e.g. 10:1, of distilled water mill. For example, results of SC tests carried so that the conductivities of the samples are out in papermaking systems to which alum well below 1000 µS/cm during the titra- has been added are likely to be affected (12-13) tions. greatly by the concentration of aluminum ions, as well as by the pH at which the set is How Al ions affect SC measurements conducted. St. John and Gallagher(42) used the word “interference” to describe the effect of alu- Effects of solid particles minum ions on the results of certain charge Near the beginning of this article it was titrations. The word “interference” suggests noted that double-layer effects at the plastic unpredictability. Experiments were carried surfaces may not be the only significant con- out to find out if the same word is justified tribution to observed SC signals in all cases. In fact, one can expect a second, little-known contribution to observed SC signals if the sample contains solids particles that differ greatly in density from the aqueous solution.(25,47) For example, a strong SC signal is observed if one places a suspension of filler particles in the device.(15,48) Though the resulting “particle charge” signal is sometimes attributed to temporary or lasting attachment of particles to the plastic surfaces of the device,(1,15,17,22-23,49-51) it is not necessary to assume that such attachment takes place. Rather, a contribution to the SC signal also can be explained in terms of an inertial effect. The idea is that the momentum of dense particles, for instance, will cause them to lag behind the reciprocating motion of the sur- rounding fluid. Since each particle is surrounded by a loose atmosphere of counter- Figure 4 Schematic diagram of a polyelectrolyte complex that is stabilised by the ions, the relative motion will produce a net presence of an adsorbed excess of the titrant. current.

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Essentially the same mechanism has been 8. Denbrok, C., and Peacock, B., “Wet-End Charge used to explain a phenomenon known as sed- Measurement using Streaming Current Online (52) Titrators,” Tappi J. 82 (10): 57-59 (1999). imentation potential. It is fortunate for us 9. van de Ven, T. G. M., “Effect of Fibre Conduc- that this inertial contribution to SC signals tivity on Zeta Potential Measurements of Pulp happens to be in the same direction as the Fibres,” J. Pulp Paper Sci. 25(7):243-245 main signal that is most often described.(25) (1999). 10. Hubbe, M. A., “Selecting and Interpreting Col- loidal Charge Measurements,” in “Proc. Conclusions Scientific & Technical Advances in Wet End 1. Recent work supports continued imple- Chemistry,” Barcelona, Spain, PIRA Intl., mentation of streaming current (SC) tests as a Leatherhead (2000). means of determining endpoints of charge 11. Hubbe, M. A., and Wang, F., “Charge-Related Measurements – A Reappraisal. Part 2. Fibre-Pad demand titrations. Useful and reliable infor- Streaming Potential,” Paper Technol. mation can be obtained, despite the fact that 12. Chen, J., Hubbe, M. A., and Heitmann, J. A., the SC output often is poorly correlated to “Measurement of Colloidal Charge in the Paper more fundamental quantities such as zeta Mill by Streaming Current,” TAPPI 2001 Paper- makers Conf., electronic document, TAPPI Press, potential. Atlanta. 2. High levels of electrical conductivity 13. Chen, J., Heitmann, J. A., and Hubbe, M. A., should be avoided when running routine “Dependency of Polyelectrolyte Complex Stoi- charge evaluations with the SC method. For chiometry on the Order of Addition. 1. Effect of instance, one can adopt a standard practice of Salt Concentration during Streaming Current Titrations with Strong Poly-acid and Poly-base,” diluting certain samples with a ten-to-one Colloids Surf. A 223 (1-3): 215-230 (2003). ratio of high-quality distilled water, and then 14. Chen, J., “Factors Affecting Interactions of Poly- multiplying the resulting cationic demand by electrolytes During Charge Analysis,” Ph.D. a factor of ten.(12) Diss., NC State Univ., 2004. 15. Barron, W., Murray, B. S., Scales, P. J., Healy, T. 3. One can expect results of SC titrations W., Dixon, D. R., and Pascoe, M., “The Stream- to be affected by various “interferences”. ing Current Detector: A Comparison with The key to understanding such effects is to Conventional Electrokinetic Techniques,” Col- recognize that there is a wide range of com- loids Surf. A. 88: 129-139 (1994). plexing ability between sample components 16. Walker, C. A., Kirby, J. T. and Dentel, S. K., “The Streaming Current Detector: A Quantitative and the types of polyelectrolyte titrants used Model,” J. Colloid Interface Sci. 182 (1): 71-81 in SC tests. For instance, the Al3+ cation (1996). interacts only very weakly with PVSK 17. Phipps, J. S., “Some Mechanistic Insights for titrant. On the other hand, an oligomeric Using the Streaming Current Detector to Measure Wet-End Charge,” Tappi J. 82 (8): 157-165 (1999). form of the aluminum ion, as present in 18. El-Gholabzouri, O., Cabrerizo, M. A., Hidalgo- poly-aluminum chloride formulations or Alvarez, R., “Comparative Electrophoretic resulting from hydrolysis of Al3+ ions in Mobility and Streaming Current Study for Zeta- papermaker’s alum, may interact strongly Potential Determination,” Colloids Surf. A 159 (2-3): 449-457 (1999). PVSK SC with giving a sharp endpoint to an 19. Erickson, D., Li, D. Q., “Streaming Potential and titration. Streaming Current Methods for Characterizing 4. Further research is still needed concern- Heterogeneous Solid Surfaces,” J. Colloid Inter- ing the origin and magnitude of SC signals face Sci. 237 (2): 283-289 (2001). under different experimental situations. 20. Schweiss, R., Welzel, P. B., Werner, C., et al., “Interfacial Charge of Organic Thin Films Char- acterized by Streaming Potential and Streaming References cited Current Measurements,” Colloids Surf. A 195 (1- 1. Jaycock, M. J., “Assumptions Made in the Mea- 3): 97-102 (2001). surement of Zeta-Potential by Streaming 21. Burns, N. L., “Measurement of Electrokinetic Current/Potential Detectors,” Paper Technol. 36 Phenomena in Surface Chemistry,” in Handbook (3): 35-38 (1995). of Applied Surface and Colloid Chemistry, K. 2. Sanders, N. D., and Schaeffer, J. H., “Comparing Holmberg, ed., John Wiley, New York, 2002. Papermaking Wet-End Charge-Measuring Tech- 22. Gerdes, W. F., “A New Instrument – The niques in Kraft and Groundwood Systems,” Streaming Current Detector,” 12th Natl. ISA Tappi J. 78(11): 142-150 (1995). Analysis Inst. Symp., Houston, TX, May 1966, 3. Padovani, E., and Colasurdo, A. R., “Online 181-198. Real-Time Measurement Reduces Wet-End Vari- 23. Cardwell, P. H., “Adsorption Studies using a ability,” Pulp Paper 69 (4): 57-62 (1995). Streaming Current Detector,” J. Colloid Interface 4. Miyanishi, T., and Shigeru, M., “Optimizing Sci. 22 (5): 430-437 (1966). Flocculation and Drainage for Microparticle Sys- 24. Dentel S. K., and Kingery, K. M., “Theoretical tems by Controlling Zeta Potential,” Tappi J. 80 Principles of Streaming Current Detection,” (1): 262-270 (1997). Water Sci. Tech. 21: 443-453 (1989). 5. Spence, G. G., Underwood, R. T., and Yarnell, J. 25. Müller, R. H., Zetapotential und Partikelladung R., “A Titration Procedure for Determining the in der Laborpraxis, Wissenschaftliche Verlagsge- Level of Anionic Impurities in a Pulp Furnish,” sellschaft mbH, Stuttgart, 1996, see pp. 104-108. PaperAge 113 (7): 30-31 (1997). 26. Verwey, E. J. W., and Overbeek, J. Th. G., The- 6. Stitt, J. B., “Charge Control Helps Tissue Pro- ory of the Stability of Lyophobic Colloids, ducers Achieve Quality, Productivity Benefits,” Elsevier, New York, 1948. Pulp Paper 72 (5): 109-114 (1998). 27. Ohshima, H., “Double-layer Potential Distribu- 7. Bley, L., “Latest Experiences of On-line Charge tion and Surface Charge Density/Surface Measurement: A Process Control Concept,” Pulp Potential Relationship for a Nearly Spherical Paper Can. 99 (5): T165-T169 (1998). Spheroid in an Electrolyte Solution,” Colloids

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Surf. A 169 (1-3): 13-16 (2000). 28. Patra, C. N., and Ghosh, S. K., “Structure of Electric Double Layers: A Self-Consistent Weighted-Density-Functional Approach,” J. Chem. Phys. 117 (19): 8938-8943 (2002). 29. Hunter, R. J., Foundations of Colloid Science, Clarendon Press, Oxford, 1987. 30. Kaunonen, A., and Springer, A. M., “Evaluation of Piston Type Streaming Current Detector for Pulp and Paper Applications,” TAPPI Papermakers Conf. Proc., TAPPI Press, Atlanta, 225-230 (1988). 31. Davison, R. W., and Cates, R. E., “Electrokinetic Effects in Papermaking Systems: Theory and Practice,” Paper Technol. Ind. 16 (4): 107-144 (1975). 32. Fleer, G. J., Cohen-Stuart, M. A., Scheutjens, J. M. H. M., Cosgrove, T., and Vincent, B., eds., Polymers at Interfaces, Chapman and Hall, London, ISBN 0412581604, pp. 343-375 (1993). 33. Hoogeveen, N. G., Cohen Stuart, M. A., Fleer, G. J., and Böh- mer, M. R., “Formation and Stability of Multilayers of Polyelectrolytes,” Langmuir 12 (15): 3675 (1996). 34. Decher, G., “Fuzzy Nanoassemblies: Toward Layered Polymeric Automatic Stock Multicomposites,” Science 227 (5330): 1232 (1997). 35. Bertrand, P., Jonas, A., Laschewsky, A., and Legras, R., “Ultra- thin Polymer Coatings by Complexation of Polyelectrolytes at Chest Cleaning Interfaces: Suitable Materials, Structure and Properties, Macro- mol. Rapid Commun. 21 (7): 319-348 (2000). 36. Schwarz, S., Buschhammer, H.-M., Lunkwitz, K., and with motorised wash units Jacobasch, H.-J., “Polyelectrolyte Adsorption on Charged Sur- faces: Study by Electrokinetic Measurements,” Colloids Surf. A 140: 377-384 (1998). 37. Sukhorukov, G. B., Donath, E., Lichtenfeld, H., Knippel, E., Knippel, M., Budde, A., and Möhwald, H., “Layer-by-Layer Self Assembly of Polyelectrolytes on Colloidal Particles,” Colloids Surf. A 137: 253-266 (1998). 38. Kenaga, D. L., Kindler, W. A., and Meyer, F. J., “Studies of Adsorption of Cationic Polyelectrolytes on Pulp Using Stream- ing Current Detection,” Tappi 50 (7): 381-387 (1967). 39. Philipp, B., Dautzenberg, H., Linow, K.-J., Kötz, J., and Dawyd- off, W., “Polyelectrolyte Complexes – Recent Developments and Open Problems,” Prog. Polym. Sci. 14 (1): 91-172 (1989). 40. Kekkonen, J., Lattu, H., and Stenius, P., “Adsorption Kinetics of Complexes Formed by Oppositely Charged Polyelectrolytes,” J. Colloid Interface Sci. 234 (2): 384-392 (2001). 41. Hiemenz, P. C., Principles of Colloid and Surface Chemistry, Dekker, New York, 1977. 42. St. John, M. R., and Gallagher, T. M., “Evaluation of the Charge 100% surface cleaning by twin sprays State of Papermachine Systems using the Charge Titration rotating in two planes Method,” Proc. 1992 TAPPI Papermakers Conf., Nashville, TN, 479-502. I Safe - no personnel inside tank. 44. Bottéro, J. Y., and Fiessinger, F., “Aluminum Chemistry in Aqueous Solution,” Nordic Pulp Paper Res. J. 4 (2): 81-89 I No need for atmospheric testing. (1989). 45. Exall, K. N., and van Loon, G. W., “Effects of Raw Water Con- I Portable - leave installed or move ditions on Solution-State Aluminum Speciation during Coagulant Dilution,” Water Res. 37: 3341-3350 (2003). from tank to tank. 46. Hayden, P. L., and Rubin, A. J., “Systematic Investigation of the I Hydrolysis and Precipitation of Aluminum(III),” in Aqueous- Consistent, repeatable, complete Environmental Chemistry of Metals, Rubin, A.J., Ed., Ann Arbor coverage. Science. Ann Arbor, MI, 1976 I 47. Marlow, B. J., Fairhurst, D., and Pendse, H. P., “Colloid Vibra- Big savings of water and time. tion Potential and the Electrokinetic Characterization of I Concentrated Colloids,” Langmuir 4 (3): 611-626 (1988). Vessels up to 12.2m diameter. 48. Dentel, S. K., Thomas, A. V., and Kingery, K. M., “Evaluation I of the Streaming Current Detector. 1. Use in Jar Tests,” Water 1-man installation, connection and Res. 23 (4): 413-421 (1989). control. 49. Bley, L., “Measuring the Concentration of Anionic Trash – the PCD,” Paper Technol. 33 (4): 32-37 (1992). 50. Ojala, T., “Charge Measurements of Different Furnishes using Polyelectrolyte Titration with a Streaming Current Detector,” TAPPI Papermakers Conf., 613-626 (1993). 51. Dentel, S. K., “Use of the Streaming Current Detector in Coagu- lation Monitoring and Control,” J. Water SRT-Aqua 44 (2): 70-79 (1995). 52. Hidalgo-Alverez, R., “On the Conversion of Experimental Elec- trokinetic Data into Double-Layer Characteristics in Solid-Liquid Interfaces,” Adv. Colloid Interface Sci. 34: 217-341 (1991). Experts in Spray Technology 23 for Paper Manufacture and Converting