THE INTERFACE SYMPOSIUM-3
Zeta Potential Measurement Applications in the Paper Industry RUDOLF SCHMUT
Even in very complex systems, the properties of colloids can be assessed by simplified approaches. Electrophoretic mobilities, as measured by the Zeta-Meter, are descriptive, reproducible, and useful in control of the dispersion ver since W. Ostwald’s pioneering book, “Die Welt
E der vernachlassigten Dimensionen” in 1915, the study of colloidal phenomena bas been largely an experimental science. The formulation of exact theories has been hindered by the large number of variables involved and the difficulty or complexity of accurate measurements. Theories, therefore, are largely limited to particular phenomena. The paper industry has several problems which are based at least partially on colloidal phenomena. These include formation, flocculation, retention of fillers and fines, dispersion of additives, treatment of water and effluent and waste gases, foam, sizing, and preventing pitchy deposits. The aqueous solutions are extremely complex, and many interrelated variables affect the nature of these colloids. For technical applications, one attempts to stabilize or destabilize a dispersion by mutual repulsion or attraction of particles through adsorption of charged ions, using as guides flocculation indexes, sedimentation values, zeta potential measurements, or related descriptive test methods. Recently developed instruments, for example, the Zeta-Meter, permit improved measurements of electrophoretic mobilities. We have found that studies of electrophoretic mobilities, with or without conversion of readings to zeta potentials, can be simplified so far as to permit rapid evaluations of additives and their effects in complex aqueous industrial systems. The results are both descriptive and quantitative depending upon the experience and judgment of the investigator. Modern instrumentation permits measurements which are reliable and reproducible in standardized systems. 28
INDUSTRIAL AND ENGINEERING CHEMISTRY
Though absolute measurements may be doubtful, the relative values obtained are sufficient. Some of the results of tests using a Zeta-Meter to control deposits in papermill systems are given in this article, The effects of variables in such systems (dilution, pH, ionic effects, additives, mixing effects, and so forth) are illustrated. Since this article is a review as well as a report of industrial experiences, some duplication of previous information is unavoidable and the industrial rather than the academic approach is used.
*
r
Electrokinetic Colloidal Phenomena
Four groups of related electrokinetic colloidal phenomena have been reported at boundary surfaces in systems containing at least one liquid phase. Electrophoresis. An externally applied potential difference causes suspended particles to migrate to the pole which carries the charge opposite to that of the particles. Electrosmosis. An externally applied potential difference causes a liquid to move through a capillary tube or a membrane in a container. Streaming or j%w potential. An electrical potential results when a liquid is moved against another phase (e.g., liquid flowing through a tube). Migration potential or f d l potential. An electrical potential results when suspended particles are forced to migrate through a dispersion medium. These four phenomena provide most of the experimental data on behavior and effects of charged small particles. Detailed exposition is given in available textbooks on colloid chemistry. Sols and Some of Their Properties
Sols are colloids of a solid dispersed in a liquid and are classified according to the liquid used as medium of the dispersion. The dispersed particles range usually between 1 and 500 mp in sue. If the medium of dispersion shows an attraction to the dispersed solids, the sol is named lyophilic (hydrophilic in case of water). If there is little or no attraction between the medium of dispersion and the dispersed solid the sol is named lyophobic (hydrophobic in case of water). A beam of light passed through a sol becomes visible because of the Tyndall effect (light scattering by the dispersed solids). The Tyndall effect is the underlying principle of the ultramicroscope and the Zeta-Meter ; both use beams of light focused in a sol and observations of the images obtained at angles to the direction of the beam. Particlescan be observed directly, or as flashes
&
of scattered light, or by adsorption on larger carriers if they are much below the wave length of visible light. Particles of a hydrophobic sol are electrically charged and will migrate toward the electrode of opposite charge in an electric field. This migration is called electrophoresis. The charge on the dispersed solid is usually caused by adsorption of ions. The stability of colloidal particles in a lyophobic sol is a function of the zeta potential (mutual repulsion of particles with equal charges). Hydrophilic sols are prepared by adding the solid to water and heating. They are reversible, that is, a solid obtained by evaporation can be made into a sol again by wetting. Hydrophilic sols may be both charged and hydrated. Coagulation and Precipitation
Though the charge on a dispersed solid in a lyophobic sol is usually caused by adsorption of ions, additional factors (polarity, hydration, swelling, macromolecular structure, and so forth) affect lyophilic sols and obscure the picture. The exact source of all charges on cellulose or wool, for example, has not been determined. For industrial applications, however, this is of secondary importance. As stated previously, the problem becomes that of stabilizing or destabilizing a dispersion by mutual repulsion or attraction of particles using adsorption of charged ions, measured by flocculation indices, sedimentation values, conductance, zeta potential, and related descriptive test methods.
charge, which is surrounded by stationary positive charges which are in turn surrounded by a diffuse layer of negative charges. The zeta potential is the difference between the charge of such a diffuse or movable layer and the bulk of the other phase (suspending liquid). The charges involved are electrokinetic (adsorption of ions) and not electrostatic (excess or absence of electrons). Most natural substances show a negative zeta potential. These charges prevent flocculation and precipitation of colloids by electrokinetic repulsion. Polyelectrolytes will change the system and the zeta potential, permitting precipitation or further dispersion. For precipitation it is necessary to adjust the zeta potential close to zero and to assist floccing with polymers or flocculants. The Zeta-Meter is more applicable to lyophobic than to lyophilic dispersions since their stability is controlled by the zeta potential, they respond to small amounts of electrolytes, the electrokinetic charge is stable, and they respond always to an applied potential. I n technical application ranges, however, this consideration may be neglected if one controls, or at least monitors, electrolyte concentrations, ionic strengths, pH, hydration, conductance, temperature, time, etc. A standardized procedure is fairly easily established for a system. Since water is the essential medium of dispersion in current pulping and papermaking, this paper is limited to common materials and additives in such a system. The readings obtained from the Zeta-Meter are converted to the zeta potential by monographs based on the Helmholtz-Smoluchowski formula, assuming spherical colloidal particles. This assumption is incorrect for pulp and papermill systems because of size and shape of the majority of investigated materials; however, for the sake of simplicity and for fast comparison of various samples it is adequate. Electrophoretic mobilities reduced to standard conditions seem more appropriate. Description of the Zeta-Meter
Figure 2.
The Zeta-Meter, an instrument f o r measuring electro-
p horetic mobility Ions of opposite charge precipitate sols and the effectiveness of precipitation increases with the valence of the precipitating ions (Schulze-Hardy rule). Ions of the same valence and sign differ somewhat in their precipitation and flocculation values depending on their size, expressed as ionic radius. In extreme cases this effect can obscure the Schulze-Hardy rule (Hofmeister series). Lyophilic sols added to lyophobic sols may coat the lyophobic colloidal particles and stabilize them against precipitation by electrolytes (protective colloids). Since hydrophilic sols may be charged and hydrated, both electrical discharge and dehydration may be required for precipitation or coagulation. Application of the Zeta-Meter
The Zeta-Meter measures the electrophoretic mobility (converted to zeta potential) of suspended colloids and suspensoids, by indirect determination of the ionized or electrokinetic charge surrounding the particles (Figure 1). Most colloid particles have a negative 30
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
The instrument has been described in connection with pitch control and water treatment (7, 2). The system consists of an illuminator, a plastic electrophoresis cell and an electrical control unit (Figure 2). It permits quick and convenient measurements of electrophoretic mobilities. Tests on pulp and paper stocks show discernible differences between various samples, as will be discussed below. The instrument has advantages and drawbacks. T h e main advantages lie in its simplicity of operation and its suitability for samples of wide ranges in particle size and concentration. Its drawbacks include the use of a binocular microscope (a long focus monocular appears sufficient), the individual focusing required for each sample (a predetermined focal plane would obviate this), the size of the cell (a shorter cell with less volume would AUTHOR Rudolf Schmut is Senior Research Chemist, Covington ( V a . ) Research Dept, of the West Virginia Pulp @ Paper
Co. T h i s article is part of the symposium on the Chemistry and Physics of Interfaces, sponsored by the Diu. of Industrial and Engineering Chemistry of the ACS, Washington, D. C., June 1964.
TABLE 1.
DILUTION EFFECTS
-
Sample A Dilution
1
EM
C
Bz
I
EM
ZP
ZP
-
2.35 -30.5 2.48 -32.0 2.48 -32.0 2.45 -31.6 2.44 -31.4 2.45 -31.5 2.47 -31.8 2.20 -28.7 2.35 -30.5 7.85 to 7.35
5
1
Et
ZP
EM
I
ZP
27 2.3 2.5
- 35 -29.9 -32.5
2.3
-29.9
2.4
-31.1 -36.3 -35.0 -32.5
0.73 0.86 0.89
-
9.3 8.6 - 8.9
-7.0
-28.5
2.8 2.7 2.5
0.54
2.2
0.46
-5.9
2.4
-31.1
0.81
-10.4
0.39
-5.1
2.7
-35.0
8
E1
4 . 8 tf15.3
4 . 7 to 5 . 4
7 . 7 to 7 . 4
7.7 to7.5
7
6
EM
-32.5
Samples A , B2,and C are from unsized, unbleached furnishes, DI and
4
Di ZP
2.5
as is
1:l 1: 2 1:2.5 1:5 1:7.5 1:lO 1:15 1 :20 1 :25 1:50 1:lOO pH range
1
EM
from sired, bleached furnishes.
0
9
1 2 3 4 5 STORAGE TIME, DAYS AT ROOM TEMPERATURE
6
PH
Figure 5. Effect of storage time on zeta potential. Little change was observed
Figure 3. Eject of ,bH on zeta potential of pa,ber machine furnishes. Curves B and D, adjusted with HCl and N a O H ; Curve C, sodium carbonate; Curve F, sodium aluminate; Curve G, alum
-30
1:l
2:1
5:1
1O:l
151
30: 1
50: 1
DILUTION WITH DEMINERALIZED WATER
Figure 4. Effect of dilution with demineralized water on zeta potential. (see Table I )
Sam@e H consisted of sediment diluted with supernatant liquid
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be adequate), the electrode arrangement (permanently embedded electrodes with plugs seem preferable to the crocodile clamps), the bulk of the system, its current limitation to aqueous dispersions, and finally its sensitivity to vibrations. Ideally, one would desire two different instruments-a small compact preset one for routine work, and a more versatile one suitable for such uses as low conductance high voltage applications in nonaqueous systems.
Deposits in paper mill systems and additives for their control have been discussed in other papers (7, 3). A list of materials commonly used for deposit control appears on page 33. The relationship between these and the application of electrophoretic mobility is self-explanatory. Electrophoretic mobilities permit 'one, in many cases, to distinguish between chemical and colloidal precipitates. The tables and figures shown below give a comparison of paper machine process waters (white water) under various test conditions. The Zeta-Met6r Co. has published data and bulletins stressing the necessity of careful sample preparation in dilution, proportioning, mixing, ionic strength, viscosity control, and so forth. The test results reported here indicate that sample preparation and handling are much less critical, a t least in the tested system, than reported in these bulletins. Their data on paper mill systems are very limited, except for water treatment and effluent! control. I t was therefore decided to investigate.bleached and un-
bleached papermill systems and to study various test conditions. Application data are, however, not iucluded, and applications are only discussed in general terms. Figure 3 shows the effect of pH variations on paper machine furnishes; the samples were diluted 1 O : l with demineralized water and the p H was adjusted with HCI, NaOH, alum, soda ash, and sodium aluminate. In the absence of multivalent ions the changes in electrophoretic mobilities are slight. Electrophoretic moper volt-sec. bilities (EM) are expressed in lo-' are expressed in millivolts. Specific Zeta potentials (ZP) conductances are not included in the graphs or tables. They follow straight lines if plotted against dilutions with demineralized water (specific conductance 1 to 3 micrornho/cm.). Figure 4 and Table I show the effect of dilution with demineralized water on the zeta potential; there are no definite trends. One sample ( H ) consisted of sediment diluted with supernatant liquid; it was not significantly different. Figure 5 shows the effect of storage time on the zeta potential. Little change, if any, was observed during a storage period of five days at room temperature. The relationship between zeta potential of a sample and the time required for a sedimentation level of 50% in a 1000ml. graduate was checked for several points. As expected, the closer the sample is to the isoelectric point, the shorter is the sedimentation time. Figures 6 and 7 show comparisons of zeta potential distribution curves (as staple diagrams) of white water samples and mixtures of white water samples. Mixtures
Fieura 6. Zeta potential distribution cwucs before and aftar blmdine
Firwe 7. Zcto potenlid dirtribution curues before and
5
Experimental Work
Y
d
25 3 I D
v1
n
RMZ"2"!2"="-I 1 I / I I
I
I I I I ZETA POTENTIAL, MILLIVOLTS
illll ZETA POTENTIAL, MILLIVOLTS 32
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
show only one peak and are flatter than the original samples because of an equilization of charges. The mixtures, due to different solid contents of the original samples, do not show a numerical average of the distributions of the components ; this effect probably overshadows differences in ionic strengths. Table I1 shows the effects of an anionic oil-ester base defoamer. On an unbleached unsized furnish, the change in the system points to an electrokinetic effect which may lead to precipitation with counter-ions present and may deactivate the defoamer, particularly at excessive defoamer applications. The mechanism of foaming is complicated and pulp and paper defoamers are designed in many cases for specific applications. The emulsifier may be anionic, nonionic, or cationic; the same applies to the active ingredients. Electrokinetic incomptabilities can lead to defoamer breakdown, deposits, defoamer inefficiences, interference with the resin-size-alum system, and so forth. Table 111shows a comparison of two commercial polyphosphate-chelate complexes which are used as anionic dispersants. Both behave in a similar manner-they exhibit an optimum point of addition beyond which the effect flattens out or is reversed. Various other anionics were evaluated in a similar manner, particularly naphthalene sulfonates, permitting cost-efficiency comparisons in presence of various counter-ions. TABLE I I .
ADDtTlON OF DEFOAMER
Defoamer,
yoof liquid
ZP
0 0.05 0.10 0.15 0.20 0.25
-28
2.17 2.38 3.4 3.7 4.0 4.6
-30.8 -42.0 -47.5 -51.8 -69.5
TABLE I l l .
Addition Level, P.P.M.
EM
0 10 20 30 50 100 150
2.12 2.2 2.32 2.35 2.40 2.38 2.39
Compound I
0 10 20 30 50 100 150
00.88 '63 0.88 0.93 0.96 1.46 1.27
1
Compound I1 EM 1 ZP Sample BI-unbleached, unsited
1
ZP
-27.3 -28.5 -30.0 -30.5 -31 .O -30.8 -30.9
i
-
8.2 -11.5 -11.5 -12.0 -12.5 -19.0 -16.7
1
I
I1
2.2 2.12 2.35 2.32 2.39 2.30 2.28
0.63 0.88 0.88 0.94 0.95 1.39 1.25
1
-27.3 -28.5 -30.5 -30.0 -30.9 -29.8 -29.6
-
8.2 -11.5 -11.5 -12.2 -12.4 -18.1 -16.4
Wet End Additives for Deposit Control
ADSORBENTS are characterized by fine particle size, large surface area, and acceptable papermaking qualities (brightness, capacity, low abrasiveness, low reaction rate with other furnish constituents). They have organophilic surfaces or edges, adsorb organics selectively, and keep them dispersed by their own dispersion or by coating the pitch particles. Most other deposit control agents interfere with the action of the adsorbents. POLYPHOSPHATES are negatively charged polyelectrolytes. They complex or sequester some of the polyvalent metal ions which assist in the formation and precipitation of sticky deposits. Negative charges are imparted to particles after adsorption of the polyphosphate on polyvalent cations at their surface. The negative charge causes the particles to repel one another. The formation of surface layers on interfaces tends to inactivate them and to decrease the tendency of deposition. AGENTS, alone or combined with polySEQUESTERING phosphates, sequester excess alkaline earths and polyvalent metals, which augment the agglomeration and precipitation of deposits. I n particular, they sequester calcium, iron, copper, and magnesium-ions. AND POLYELECTROLYTES in general act SURFACTANTS by changing surface charges (electrokinetic charge measured as electrokinetic mobility). Anionic materials impart negative charges leading to mutual repulsion and dispersion at zeta potentials lower than -20 millivolts. Cationic materials act by increasing the zeta potential close to 0 millivolts, or to the positive side, leading to precipitation and retention of materials on the paper fibers. Cationic surfactants may require the use of nonionic hydrophilic additives or of amphoteric additives to prevent agglomeration before the particles are retained on the fibers. It is claimed that some of the cationics dissolve pitch or disperse pitch; however, it seems that a zeta potential of about +20 millivolts for cationic dispersions would be hard to reach and may interfere with the whole system. SULFATES, AND ACIDS harden pitchy deposits SULFITES, and may fix them on the fiber surfaces. ALKALIS assist in the dispersion of pitch, but may lead to difficulties since calcium and magnesium ions contribute to pitch precipitation at pH 6.0 and above. Alkali treatments are usually combined with a subsequent stabilizing treatment, for instance the formation of protective colloids. PROTECTIVE COLLOIDS AND EMULSIFIERS stabilize existing dispersions and lower the surface tension. I n many cases, they depend on pH for other effects (for instance, retention). The changes in surface tension can lead to foaming and from there back to deposits through the precipitating action of defoamers. CLOSEDSYSTEMS AND HARD WATER tend to increase deposit troubles as do large amounts of dissolved solids and salts; these act by salting-out and destabilizing emulsions or dispersions. REFERENCES
A., Schmut, R., Tuppt 47, No. 1, 210h-213A (1964) (2) Riddick, T. M., Cham. Eng. 6 8 , No. 26, 28: 121-26, 141-46 (1961' (3) Schmut, R., Pnpcr Mdl Nems 86, No. 12, 22-3 (1363).
( 1 ) Jacobsen, N.
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