polymer adsorption and its effect on the stability of hydrophobic colloids. ii. the flocculation...

Polymer Adsorption and Its Effect on the Stability of Hydrophobic Colloids

II. The Flocculation Process as Studied with the Silver Iodide-Polyvinyl Alcohol System

G. J. FLEER AND J. LYKLEMA

Laboratory for Physical and Colloid Chemistry of the Agricultural University, De Dreijen 6, Wageningen, Netherlands

Received October 30, 1972; accepted July 13, 1973

In a series of investigations on the interaction between hydrophobic colloids and polymers, a study has been made on the flocculation of silver iodide sol by polyvinyl alcohol.

The most salient finding is that the extent of flocculation depends critically on the way of mixing of sol and polymer. The flocculation is most effective if equal amounts of nearly completely covered AgI particles and uncovered particles are mixed. An explanation is found in terms of bridging of particles by the polymer. The apparent irreversibility of the adsorption of polymer molecules is essential for the dependence of the flocculation on the mixing order.

As a certain critical amount of salt is needed to bring about flocculation, the flocculation process has to be referred to as sensitization. The critical salt concentration decreases strongly with increasing valency of tile counterion, approximately in the ratio 100: 10:1 for KNO~, Ca (NO~)~ and La (NO3)3. The salts reduce the Stern potential and compress the diffuse double layer to such an extent that the attachment of adsorbed loops on other particles is no longer electrostatically inhibited.

The flocculation efficiency increases somewhat with increasing molecular weight and acetate content of the polymer. The better the polymeric flocculant, the less critical the way of mixing.

From preliminary kinetic measurements it followed that the aggregation between polymer covered and uncovered particles is a bimolecular process.

Silver iodide sediments formed upon flocculation with polyvinyI alcohol are loose and open, their volume decreases only little upon standing. On the other hand, silvei iodide sediments obtained by coagulation with salts have a higher volume during the first hours after their formation, but this volume decreases gradually.

The influence of the mixing order on flocculation has probably a wide array of practical applications.

INTRODUCTION

This paper deals wi th the f locculat ion ~ of

hyd rophob ic sols by polymers . T h e work has

1 Throughout this paper we shall follow La Mer and ttealy's (2) distinction between flocculation (aggregation of colloidal particles by polymers or polyelectrolytes) and coagulation (aggregation of colloidal particles by low molecular weight electrolytes). If no salt is needed for flocculation the process is referred to as adsorption flocculation, otherwise as sensitization.

been u n d e r t a k e n b o t h in v iew of i ts prac t ica l

impor t ance (e.g., for soil s t ruc ture , food

technology, wa te r pur i f ica t ion and biology)

and because the processes i nvo lved are still

poor ly unders tood , even if t h e y cons t i tu te one

of the cores of m o d e r n colloid science.

As a qua l i t a t i ve exp lana t ion usual ly a k ind

of br idging is i nvoked (1), for example in the

p ioneer ing work by La M e r and H e a l y (2)

Copyright ~) 1974 by Academic Press, Inc. All rights of reproduction in any form reserved.

1

Journal of Colloid and Interface Science, Vol. 46, No. 1, January 1974

2 FLEER AND LYKLEMA

and that of many others (3-6). Alternative mechanisms have also been proposed, either or not applying to special cases. As a recent example we mention work by Gregory (7) on charge compensation as a factor in the flocculation of latices. Nemeth and Matijevi~ (8) postulated a mechanism based on the surface charge reduction due to polymer adsorption.

Our work is intended as a contribution to the understanding of the mechanisms involved and to provide experimental information on which theories can be based. As a model the system silver iodide-polyvinyl alcohol (PVA) in aqueous solution has been chosen. One of the great advantages of this system is that the electrostatic and dispersion contributions to the particle interaction energy are fairly well known (also in the presence of adsorbed PVA) so that the additional contributions to the interaction energy are solely attributable to the polymer. In part I of this series (9), the adsorption of PVA onto AgI was characterized through the adsorbed amount, the effective adsorbed layer thickness and the polymer segment distribution. In the present article, the experimental results of the flocculation of AgI sols by PVA will be described. Preliminary results have been reported earlier (10); an extensive description has been given elsewhere (11). The most salient result is that the extent of flocculation depends critically on the way of mixing of polymer and sol. Furthermore it has been found that the influence of electrolytes differs quantitatively from that with coagulation.

In subsequent papers (12) we intend to present a more elaborate study of the kinetics of the flocculation process and to give a particle interaction theory based on the information obtained in the preceding papers. A preliminary report has been given by one of us (11).

EXPERIMENTAL METHODS

Materials

The preparation and properties of the silver iodide (9, 11) and the types of polyvinyl

alcohol (9, 11, 13) have been described previously. The AgI particles were negatively charged (pI ~ 5), the average particle radius was about 50 nm. Four samples of PVA were used, designated as 3-98.5, 13-98.5, 60-99 and 13-88, with viscosity averaged molecular weights M 15,000, 56,000, 101,000 and 63,000, respectively. The first three samples contain I-1.5°-/o of acetate groups, the PVA 13-88 12%. The spread of the molecular weights was rather broad.

Flocculation Sludies. The ESect of the Order of Mixing

Unlike many low molecular weight substances, polymers tend to adsorb irreversibly. The consequence of this is that the way of attachment between polymer and sol particle (as well as the extent of flocculation it induces) depends critically on the mode of mixing sol and polymer.

In the earlier stages of this study we found that if a given amount of pol)aner was added to a given volume of sol only partial and irreproducible flocculation occurred, if it occurred at all. This irreproducibility was supposed to originate from inhomogeneous mixing, by which the AgI particles became not uniformly covered by PVA. In order to improve the reproducibility of the flocculation, we subsequently modified the adding procedure as follows. From a burette a volume of water or dilute salt solution was added carefully on top of the sol, avoiding mixing with the denser sol. In turn, on top of this the polymer solution was brought using a pipette (see Fig. la). After mixing by hand the extent of floceulation was observed; it was now found to be reproducible but virtually zero. Apparently a certain degree of mixing inhomogeneity is necessary for effective flocculation. This consideration led us to adopt a two-portion mixing procedure, in which part of the sol (viral), salt and polymer solution were carefully brought in the cylinder, in this order. These components were mixed by hand and after a certain contact time h (usually 15 rain) the second

Journal of Colloid and Interface Science, Vol. 46, No. 1, J anua ry 1974

I, 'LOCCULATION BY POLYMERS

@ ®

L - . J

2 cm 3 PVA sotution

i!i:i!ii 5 crn 3 salt solution

3 cm 3 Agl soL

L - __1

\ /

v2crn 3 A g I so!.

2 cm 3 PVA solution

5 cm 3 sal t solut ion

~.'..:.; v~crn3Agl sol

,1 / --.., , / / "-....

FIG. 1. The way of mixing of PVA and AgI sol. (a) One-portion method, giving no measurable flocculation. (b) Two-portion method, resulting in efficient flocculation. The second portion of sol (v2) was added h minutes after mixing of the polymer with the first portion of sol (v~). v, -[- v2 = 3 ml.

portion of sol (v2 ml) was added (Fig. lb). (This contact time is necessary to ensure adsorption of PVA on the particles of the first portion.) Then the cylinders were stoppered and rotated end-over-end during a given flocculation time t2. This procedure led to very reproducible flocculation.

In our experiments the volume of sol used (vl + re) was always 3 ml. The final volume in the cylinders was 10 mI. The variables used are the (final) concentrations of sol (C~ol), salt (c~ait) and polymer (c~), the mixing fraction, i.e., the fraction of sol in the first (covered) portion E~bl = vl/(vl + v2)-] and the amount of PVA added per mmole of AgI in the first portion (pl in mg PVA/mmole AgI).

The extent of flocculation was judged by measuring, after centrifugation under mild conditions (2 min at 900 rpm), the absorbance Arel of the supernatant (relative to the absorbance of the original sol) in a Unicam SP 690 spectrophotometer with 1, 2, 5 or l0 mm cells. The wavelength used in these measurements was 430 nm. At this wavelength AgI has an absorption peak. The absorbance is then roughly proportional to the amount of material left in the supernatant after flocculation and not sensitive to the particle size. A low relative

absorbance corresponds to effective flocculation and vice versa. Measuring at other wavelengths is not recommended since, because of scatter- ing, any remaining clusters in the supernatant would contribute strongly to the absorbance. These clusters are especially present when the system is on the verge of flocculation. As a consequence humps in Arel could then be encountered, not reflecting stability regions, see Fig. 2.' Perhaps the "pseudo-stability" regions reported by Levi and Stepanova (14) must also be looked upon as such an artifact.

Sediment Volumes

Sediment volumes were measured as a function of settling time in wide-necked tubes tapering to a narrow cylindrical end to improve sensitivity. The length of the narrow part of the tubes was about 8 cm and its volume 2 cm 3, the total volume of the tubes was 10 cm a.

RESULTS AND DISCUSSION

A dsorplion Isotherms

In Fig. 3 adsorption isotherms for several types of PVA on AgI sol are given. This figure is taken from the previous paper (9) and the


4 FLEER AND LYKLEMA

A~el

%5

0.5

4

O

o ~3o nm .650 nrn

v,

3 Cm J

FIo. 2. The flocculafion of AgI sol as measured by the relative absorbance. In the region where partial flocculation occurs, typical humps are found at a wavelength of 650 nm. PVA 3-98.5; pt = 2.5 mg PVA/mmole AgI; c~oi = 3 mmoles/iiter; c~1, = 10 mmoles/liter of KNO3; h = 15 mln; h = 1 hr.

adsorpt ion results have been extensively discussed there. The adsorpt ion isotherms are reproduced here to facil i tate comparison with the flocculation results.

I t m a y be added tha t both our adsorpt ion and flocculation experiments have been per- formed at re la t ively low polymer concentra-

tions. I t is very unlikely tha t under this condition the polymer does adsorb in the same coiled shape i t had in solution, as has extensively been discussed in par t I. The peculiar mixing order effects, discussed in more detail below, are characterist ic for the low polymer concentrat ion region.

The Way of Mixing and the Bridging Model

The most surprising resul t of the s tab i l i ty experiments is tha t the flocculation efficiency depends very strongly on the way of mixing. This is clearly borne out by the fact that , using the sharp boundary method of Fig. l a (i.e., excluding premature contact between sol and polymer) no flocculation at all was found if a given por t ion of sol was added to a given volume of polymer solution or vice versa (qh= 1 or $ 1 = 0). This is a very typical result, obta ined a t any concentrat ion of sol, salt and polymer.

The effect of the way of mixing is shown in Fig. 4 where the relat ive absorbance is p lo t ted as a function of the volume of the first sol por t ion for various polymer concentrat ions and four types of polymer. In these experiments the contact t ime (h = 15 rain), the flocculation t ime (& = 1 hr), the sol concentra-

Adsorption

• rig L 1 rngPVA/mmole AgI , ~,..__..____.-.~-.~ 13-88

1.50 ~ i ...>~....-----¢~* * '-"* , ' ~ ~ . ~ o _ . . _ . . . _ ~ _ - - - - ~ 6 O - 99

1,25 * / * / o " ~ °

| I / ~ ~ ~ " _._____o_----- -°------- 3-98.5 I / J o-.-------~-°.T.~--~ ~° o

100 l l ~ o / ~ . ~ - ~ - - -

o.s0 il

o.25 I Residual concentration

510 I - - I r I I 0 0 I00 150 200 250 300 ppm

Fro. 3. The adsorption of polyvinyl alcohol on silver iodide sol. C~ot = 50 mmoles/liter; time of adsorption 1 hr.


FLOCCULATION BY POLYMERS 5

Arl |

1.0~ . . . . - - 7 -0 , f

'?i]-\ / t/ t,~ o Cp=2.0 ppm

• cp:/.:75pp m

c.=5.25pp rn

~ x __...~ x'~ v 1

I

Atel

1.0 ~, o "° '°--° 'o-o-o-- x-x--x-,o ~

\ \ \ . i as \ \ ~ I a / o %=2.0 ppm

I/ x cp=3.75 pprn ~ " " ~ " ~ i ~ " = cp:5.25ppm

• • ) v I l f ~ x - - x - - x - - x - - t * x I

1 2 3 crn 3

.Arlt

,0= o o ?_Oo_

1 2 3 cm ~

At* 1

& , \ i /®,V,l,= I i \ c . - \k )L / / . c . °.° -

1 2 3 cm 3

(~) PVA 13-98.5

o cp=2.0 p p m

x cp =3.7 5 pp m

= cp=525 p p m

Fro. 4. Effect of the way of mixing on the flocculation efficiency at constant polymer concentration. c=oi = 3 mmoles/liter; c=~it = 10 mmoles KNO3/liter; h = 15 mln; t~ = 1 hr.

tion (Csol = 3 mmoles/l i ter) and the salt concentration (c==lt = 10 mmoles/l i ter) were fixed. Ftocculation occurs only at intermediate values of vl. I t should be noted tha t for any curve of Fig. 4 the total amounts of poljyTner and sol are constant over the entire range of vl. The fact tha t nevertheless the flocculation depends so strongly on vl [-or on ~bl = vl/ (vl -t- @ 7 points clearly to the importance of the mixing method.

For sake of closer inspection let us look at Fig. 4b. The flocculation is most efficient at c~ = 3.75 p p m and less so at higher and lower polymer concentrations. Furthermore, the minima in the curves of Fig. 4b occur under conditions where the amount of polymer added per mmole of sol of the first port ion (pl, which is proportional to cp/v~) is approximately constant and equal to pt = 2.5 mg/mmole . Apparent ly the coverage of the first port ion at this value of px corresponds to a polymer configuration on the particles of the first portion which is most conducive to flocculation. Comparison with the adsorption isotherm

(Fig. 3) shows that an added amount of 2.5 mg PVA/mmote AgI leads to an adsorption which is only slightly below the pseudo-plateau, while at the same time a very small amount of polymer remains in solution (high-affinity adsorption). Hence the pol}nner layer on the first portion of sol is relatively thick, whereas the coverage on the particles oI the second portion is small because too little polymer is left in solution to produce significant adsorption on this second portion. As the adsorption of whole polymer molecules is irreversible, a t least on a t ime scale of several hours, this situation of inhomogeneous coverage of the sol particles persists long enough for flocculation to occur. The flocculation can then be explained easily by the bridging model: long loops of polymers protrude from the particles of the first portion of sol, and the outermost segments of these loops can a t tach onto the (nearly) naked particles of the second sol portion.

I t is impor tan t to realize tha t in the explanation given for the efficiency of the mixing


6 FLEER AND LYKLEMA

method the irreversibility and the high- affinity character of the polymer adsorption play a crucial role. Thus, in a system where these both factors are less pronounced the extent of flocculation may be expected to be less sensitive to the way of mixing.

A pictorial representation of the bridging and the way of mixing is presented in Fig. 5. Also from this picture it is clear that with the one-portion method (Fig. 5a) no flocculation occurs because the adsorbed layer is too thin and because no free surface is available. If anything, the stability with respect to that of the original sol is enhanced due to configura- tional and/or osmotic repulsion contributions. In the two-portion method (Fig. 5b) long loops are present on the particles of the first portion. They adsorb onto the newly added bare particles, thus giving rise to effective flocculation. Note again that the total amounts of polymer and sol are the same in both cases, only the order of addition is different. With reversibly adsorbing substances such a distinct flocculation behavior difference due to different modes of mixing would be out of question.

The occurrence of a flocculation optimum as a function of vl can now be explained on the basis of the distribution of the polymer over the first portion and the solution. I t has already been observed that for pl = 2.5 mg/mmole

(~) AgI+PVA

5 @

(~) (Agl +PVA)+ AgI

FIG 5. Schematic representation of two ways of mixing. (a) One-portion method; no flocculation. (b) Two-portion method; effective flocculation. The total amoun t of polymer and silver iodide is the same in both cases.

flocculation is optimal because the extension of the loops on the first portion is (nearly) maximal and the polymer concentration in solution is small. Both lowering and raising of vl make flocculation less effective, lowering because too much polymer remains in solution (this would adsorb on the second portion) and raising because the loops are not sufficiently extended.

I t is concluded that three features play a key role: the adsorption should be irreversible (on the time scale of the experiment), it should have a high-affinity character and it should lead to extended layers. These three features are typical for polymer adsorption.

Returning to Fig. 4b, it may be seen that it still depends on the polymer concentration how effective the flocculation in the optimum is. At cp = 3.75 ppm this process is most efficacious. The minimum at this polymer concentration occurs at vl = v2 or ¢1 = 0.5, hence in this case the number of covered particles equals that of uncovered ones. If the flocculation is caused by Brownian encounters between covered and uncovered particles this is quite conceivable.

The results for other types of PVA are essentially the same as those for PVA 13-98.5, as shown in Fig. 4. The flocculation is somewhat more effective for higher molecular weights: at equal polymer concentrations the relative absorbance is lower, while the range in cp where flocculation occurs is broader. Also a lower degree of hydrolysis of the polymer favors the flocculation (compare Fig. 4d and b). The optimal value for pl is nearly the same for all types of polymer, although there is a slight tendency for the optimal pl to increase with molecular weight and to decrease with the degree of hydrolysis, all of this in agreement with the shift in the adsorption levels (Fig. 3). Recalling that our PVA samples had a broad spread in molecular weight it can be expected that these trends would show up still more clearly if fractionated samples would have been used.

From the above discussion it follows that optimal flocculation occurs if: (a) the pol3~ner

Journal of Colloid and Interface Science, Vol. 46. No. 1, January 1974


coverage on the covered sol particles is nearly Ar~, maximal, corresponding with a value of 1.0 pl = 2.5 mg PVA/mmole AgI and (b) the number of covered particles equals that of uncovered ones (qh--0.5) . The validity of this conclusion is also demonstrated in Figs. 6

0.5 and 7. In the experiments shown in Fig. 6, the mixing fraction ¢~ was varied at the optimal value of p~, whereas Fig. 7 gives the results of varying polymer dosage p~ at optimal q~. From both graphs it is clear that the best flocculation is obtained if ¢~ and p~ have their optimal value simultaneously. I t is again found that higher molecular weights and less hydro- lyzed polymers are more effective.

The curves of Fig. 6 are nearly symmetrical with respect to ¢~ = 0.5 as should be expected if the number of covered particles and that of bare ones play an equivalent role. Figure 7 is not symmetrical. Flocculation starts for all types of polymer at a polymer dosage of 2 mg/mmole, indicating that long loops, capable of bridging, start to develop at this dosage. At this very dosage the adsorption isotherms leave the ordinate axis (Fig. 3). Upwards in Fig. 3, the number of available surface sites becomes less abundant and around 2mg/mmole not all the molecules can attach themselves to the surface any more. This transition apparently occurs at about the same dosage where long loops are formed, hence this dosage of 2 mg/mmole plays a critical role both in Figs. 3 and 7. To the right in Fig. 7 the flocculation is inhibited at a polymer dosage A~,L which increases with molecular weight and acetate content. This inhibition occurs when 1.0 the second portion of sol becomes covered to a certain degree. I t follows from the adsorption isotherms that the polymer dosage needed to obtain this degree increases with M and 0s acetate content. Hence the shapes of the curves in Fig. 7 completely reflect the adsorption behavior.

The bridging concept thus satisfactorily explains the experimental results. Moreover, it can be shown ( l l ) that two possible alternative explanations, viz, flocculation in the secondary minimum and heterocoagulation

mg PVA p,=2.5 mmol.e Agl

• o

02 04 0,6 0B 10 ~p,

FIG. 6. The flocculation as a function of the mixing fraction 4~1 at the optimal polymer dosage pl = 2.5 mg PVA/mmole Agl. C~ol = 3 mmoles/liter; c~it = l0 mmoles KNO3/llter; h = 15 rain; t2 = 1 hr.

between a covered and an uncovered particle (having slightly different Stern potentials) are very unlikely.

An entirely different mechanism has been forwarded by Nemeth and Matijevi~ (8). These workers studied the AgBr-gelatin system, i.e., they worked with a po13 electrolyte rather than with an uncharged polymer. However, in their paper they suggested that their picture would have a more general validity and hence it deserves attention. The idea is as follows. (a) Due to polymer adsorption the surface charge on the sol is reduced just as in the case of low molecular weight adsorbates (15). (b) Due to this reduction of charge the sol becomes less stable to electro-

=0.5 o ,- °--l-o-~

I 2 • o ° I P

mg PVA/mmote AgZ

FIG. 7. T h e f locculat ion as a func t ion of the p o l y m e r dosage a t the o p t i m a l mix ing f rac t ion ¢1 = 0.5. C~oi = 3 m m o l e s / l i t e r ; c,~l, = 10 mmoles K N O j / l i t e r ;

h = 1 5 m i n ; t z = l h r .

Journal of Colloid and Interface Science, Vol. 46, No. 1, J anua ry 197g

8 FLEER AND LYKLEMA

lytes. (c) Aggregation is really coagulation by salts and not flocculation by polymer bridging.

The first of these three points is correct. In fact, we may recall that in part I we actually produced a figure showing the decrease of surface charge upon polymer adsorption. However, the statements (b) and (c) are incorrect. There is no simple univocal relation between surface charge and stability. Vincent, Bijsterbosch and Lyklema (16) have demonstrated that under certain conditions a lowering of the surface charge due to adsorption of organic molecules can even lead to an increase of stability towards electrolyte. This is related to the fact that it is not so much the surface charge itself that counts as well as how the countercharge is distributed over Gouy and Stern layer. The conclusion that upon polymer adsorption sols would generally become more susceptible to electrolytes is therefore unten- able. It may be added that, if our process would have been a coagulation by salts, no mixing order effect would have been observed, in contradistinction to the experimental facts. Finally, of course, in the AgBr-gelatin system the charge of the polyelectrolyte attributes also to the electrostatic interaction, but Nemeth and Matijevi~ did not assess this contribution. Actually, they used a negative polyelectrolyte with negative particles. Hence it may be expected that the effective surface charge (including the charge of the polymer sheath) increases upon adsorption, leading to enhanced stability. By virtue of all these arguments we conclude that the mechanism of Nemeth and Matijevi~ is not supported by experiment.

E ~ect of Electrolytes

In connection with the above discussion and, more generally, in order to further the insight in the interaction process it is desirable to find out whether low molecular weight electrolytes are also required and, if so, by which laws their influence is governed. Since we have now established the optimum flocculation conditions (i.e, 4~1 = 0.5 and pl = 2.5 rag/ mmole) it has become possible to study this

influence quantitatively. We recall that hitherto all experiments have been done in 10 mmoles/liter KNO.~. This concentration, as well as the nature of the salt will now be varied.

In Fig. 8, the results are given for KNO3, Ca(NO3)2 and La(NO3)3. The most salient feature is that the extent of flocculation increases from nearly zero to practical complete flocculation over a relatively narrow range of cs,lt. Apparently a certain critical concentration of salt is needed to ensure flocculation. There- fore the flocculation of AgI by PVA has to be considered as sensitizalion and not as adsorption flocculation. For sake of simplicity we define the critical salt concentration (csc, denoted as c½) as that concentration at which half of the material is flocculated (Arol = 0.5). (Thus defined, c½ depends on the flocculation time t2 and on the sol concentration, see later.)

The csc depends very strongly on the valency z oI the counterion (see Fig. 8) and follows approximately a 10 -" law. The following qualitative interpretation for this effect can be offered. For effective bridging it is necessary that the particles are capable to approach each other closely enough. The distance at which bridging is possible will be of the order of magnitude of the thickness of the polymer layer [ ~ 1 0 nm (9, 11)-]. As long as the double layer repulsion is so strong that this particle separation is not attained, the system remains stable, also in the presence of adsorbed layers. The apparent role of electrolytes is to suppress the double layer part outside the polymer layer. From this picture it follows immediately that the concentration needed to obtain sensitization is much less than that to bring about coagulation, because in the latter case the double layer repulsion has to vanish also at much smaller interparticle separations.

In order to evaluate the valency influence quantitatively more information is needed. In the absence of adsorbed polymers, electro- lyres exert their influence in two ways: (a) by reducing the potential of the diffuse double layer ~bd and (b) by compressing the diffuse

Journal of Colloid and Interface Science, Vol. 46, No. 1, J a n u a r y 1974


Arlt

1.0'

La(N03) 3

0.5

0.01 0,02 0.05 0.1

Ca(N03) 2 KNO 3 z~ PVA 3-98.5 o PVA 13-9&5

o . , . '~&~ . * eVA 13-88

' 2\5o \

0.2 0.5 1 2 5 10 20 mmotes/ t

Fla. 8. Dependence of the flocculafion efficiency on the salt concentration, p1 = 2.5 mg PVA/mmole AgI; ¢~ = 0.5; C~ol = 3 mmoles/liter; h = 15 min ; h = 1 hr.

double layer. Since both effects work the same way and since both are stronger with higher ~, the stability is generally very sensitive to z. This (qualitative!) statement is also known as the Schulze-Hardy rule. What kind of dependence csc(z) is observed, depends on the nature of the system. If ~d is very high, the DLVO theory predicts the csc to be proportional to z -6. However, very high Cd values are seldom encountered with ordinary hydrocol- loids under conditions of coagulation. More frequently low ~d values occur, in which case the csc is proportional to ~e4z -~. In practice this usually leads to a stronger dependence on valency than a z -2 proportionality because ~d itself decreases with increasing z and cs~l~.

Due to the presence of adsorbed polymers the situation is modified in two respects: (a) Except the two interaction free energies occurring in the DLVO theory there are now also attractive and repulsive contributions stemming from the adsorbed polymer. As a consequence, the above-mentioned relationship between the csc and z no longer applies. (b) In order to evaluate the electrostatic contribution to the total interaction, ~d must again be known as a function of z and c~lt, this time in the presence of adsorbed polymer. With our system double layer measurements are under- way to obtain such information (17).

Provisional results of a semiquantitative analysis incorporating these two modifications agree very well with the observed 10 -~ depen-

dence (11). We hope to publish this work after bringing about some refinements (12).

I t may be noted that Nemeth and Matijevi~ found also a strong valency effect but, as shown above, this may not be invoked as an argument in favor of a coagulation mechanism due to surface charge reduction as long as the dependency 6d (z) and the free energy contributions due to the polymer are not established.

The csc is slightly lower in the presence of PVA of higher molecular weight and higher acetate content (Fig. 8). This suggests a somewhat thicker polymer layer in these cases, in agreement with inference from adsorption and protection measurements (9, 11).

The results of a few experiments with other salts are collected in Table I. From the first two columns of Table I it follows that the same lyotropic order is found as for the coagulation of uncovered AgI (18, 19). However, the differences between K + and Rb + amount to about 7~o, compared to 36~o for the coagulation of Ag[ sols. As only a fraction of the AgI surface is void of polymer segments and as measurable specificity shows up only as far as ions are very close to the surface, this reduced specificity is to be expected. The effect of the co-ion is small, both in flocculation and coagulation. The dependence of the csc on pH in the presence of La(NO3)~ is caused by the hydrolysis of the lanthanum ion (20). The resulting complexes adsorb specifically, herewith lowering the Stern potential which


10 FLEER AND LYKLEMA

TABLE 1

CRITICAL SALT CONCENTRATIONS C½ (mmoles/liter) OF DIFFERENT ELECTROLYTES FOR THE

]~LOCCIILATION OF Agl sol BY

PVA 13-98.5"

Nitrates Potassium salts La (NO3) 8

LiNO3 4.6 KNO3 4.5 pH 5 0.041 KNO3 4.5 ½K2504 3.9 plt 7 0.03 RbNO3 4.3 ½K~Fe(CN)~ 3.6 pil l0 0.1

a p~ = 2.5 mg PVA/mmole AgI; ff~ = 0.5; h = 15 min; h = 1 hr; csol = 3 mmoles/liter.

in turn decreases the csc. At high pH lanthanum hydroxide precipitates which leads to an increase of the csc.

The effect of the polymer dosage pl on the csc for KNO3 is given in Fig. 9. With growing pl the csc increases again because the second portion of sol becomes covered: the sensitization by salt is gradually changed into protection against salt. For p l ~ 0, c½ approaches the coagulation value for potassium nitrate (--~130 mmoles/liter). Also from Fig. 9 the flocculation efficiency of PVA 13-88 appears to be higher than that of PVA 13-98.5; the small shift to the right reflects the higher adsorption level for the former polymer.

Kinetics of Flocculalion

As a sequel to the mixing procedure we have carried out a number of experiments regarding the kinetics of the flocculation process. I t is our intention to deal with the results on a forthcoming occasion, but some main results can already be reported.

1. The flocculation process is bimolecular. The rate of flocculation is proportional to ¢ 1 ( 1 - ¢ 1 ) .

2. Increasing the sol concentration enhances the flocculation efficiency considerably (at given c~,it and given flocculation time &) and strongly decreases the csc (at given t2). By adjusting the flocculation time such that the same total number of collisions takes place (making use of the bimolecular nature of the flocculation) it was found that the csc becomes

Journal of Colloid and Interface Science, Vol. 46, No. 1, January

independent of the sol concentration, as expected.

The Sediment Volume After Flocculation and Coagulation

Generally it is found that flocs formed after flocculation are looser and less compact than coagulated flocs (2, 3). An open structure is, for example, an important feature for soil structure improvement. Some experiments were carried out to investigate this for silver iodide flocculated by PVA. Results are given in Fig. 10, from which it follows that our system does not follow the general experience.

Sediment volumes of a sol coagulated by simple electrolytes [-KNQ, Ca(NO3)2, La(NOa)a-] were found to decrease gradually with time. This decrease becomes exponential after some hours. Contrary to this, the sediment volume of a sol flocculated by PVA is already approximately constant after some hours; its value is less than the sediment volume of a coagulated sol, at least in the initial stage of the coagulation process. Typical

I c~

1( mmotes/I.

8

6

4

2

pvA13911i '3-88

I] \ / / ..'..o//

p, i i i l

1 2 3 4 mg/mmo[e

FIG. 9. D e p e n d e n c e of t he c r i t i ca l s a l t c o n c e n t r a -

t ion on t h e p o l y m e r dosage . ¢1 = 0 .5 ; Csol = 3

m m o l e s / l i t e r ; ti = 1,5 r a in ; & = 1 h r .

1974

FLOCCULATION BY POLYMERS ] l

Sediment volume • KN% 20 mmolesll

Flocculation • Ca(N03) 2 I remote IL

• La(N03) 3 0115 mmoles/l

0,8 - c rn3~ o KN03 150 mmoles/l

Coagulation z~ Ca(N03) ~ 5 mmoles / l ~o~--.....u.....~..~.~..~~.~ D La(N%} 3 0.5 mmoles/l

0.6 O ~ o E~ ~._.~

"~-" . . . . . ._~_~, _~--... I oA _- _, _ _ _, . . . . Z " ~ _ - ~ _

0.L 0 ~ • ....e . . . . . --" 0 _ - - e - , , , , .... --~-----"~'~"=~o--- o -- --- -- o

time

0.2 t I 2tO I 0 J r t 2 5 1~0 5 100 200 500 hours

Fro. 10. The sediment volume of 0.30 mmole AgI after coagulation (open symbols) and flocculation with PVA 13-98.5 (filled symbols) in the presence of different salts. For the flocculation the two-portion method of mixing was applied (pI = 2.5 mg PVA/mmole AgI; $1 = 0.5; h = 15 rain).

differences exist between sediment volumes in the presence of univalent cations and those with multivalent electrolytes. These differences occur both with coagulation and with flocculation. For the former case an explanation has been forwarded by Troelstra (21) in terms of an easier rolling of the particles over one another in the presence of univalent cations. In a flocculated system this mechanism seems less appropriate because no direct surface-to- surface contacts exist. However, pending a more penetrating treatment of interaction curves an alternative explanation is not easily found.

Neither is the denser sediment for the coagulation simply understood. One might expect the reverse on account of the necessarily higher separation between two neighboring particle surfaces due to the polymer layer between them. However, it should be borne in mind that the sediment volume of a coagulated as well as that of a flocculated sol is very loose anyway, the volume fraction of solid being not more than about 3%. Therefore, it is unlikely that the minimum interparticle distance between aggregated particles would be the sole quantity determining the sediment volume. Because of its theoretical and practical importance, further study of sediment volumes obtained by flocculation under controlled conditions would seem desirable.

Application to O/her Systems

The efficacy of the way of mixing has been explained on the basis of the irreversibility of

the PVA adsorption, at least on the time scale

of some hours. As this type of irreversibility is

generally found for polymer adsorption, the

applicability of a similar mixing method to

other systems may be expected. Especially interesting questions arise if

two different suspensions are used. In a few preliminary experiments we flocculated a mixture of AgI and activated silica by PVA. In both suspensions PVA adsorption is irreversible with respect to dilution with solvent. The adsorption onto SiO2 was found to depend strongly on pH, the adsorbed amount at pH 3 being about three times higher than that at pH 8. I t is reversible with respect to pH. For example, if after equilibration at pH 3 the pH is brought to 8, 9,5% of the difference in adsorption levels between the two pH values desorbs within 1 hr. Flocculation of a mixture of AgI and silica by PVA could be achieved at pH 3 by mixing a volume of nearly completely covered AgI sol (Pl = 2.5 mg/mmole) with a volume of SiO2, whereas at pH 8 no flocculation at all took place. I t was found that a mixture which was flocculated at pH 3 could be redispersed almost completely by changing the pH to 8. From this it follows

Jourlzal of Colloid and Interface Science, V o l . 4 6 , N o . 1, J a n u a r y 1 9 7 4

12 FLEER AND LYKLEMA

tha t only A g I - P V A - S i Q links were present in the aggregates and no A g I - P V A - A g I contacts. Moreover, the easy redispersibility indicates tha t the particles in a floc find themselves not in a deep pr imary minimum where the van der Waals force plays the major role, but are separated by a certain distance. These features confirm the ideas given before and constitute another argument against coagulation by salts as the aggregation mechanism. One surprising result emerged. By variation of the relative amounts of silica and AgI it was found that op t imum flocculation occurred when the number of pr imary silica particles was about 100 times tha t of AgI particles. This extremely asymmetr ic behavior has not yet been explained. Possibly extensive mutual aggregation of pr imary silica particles plays a role. Further investigations on well- defined systems of different particle size (e.g., latices) are necessary to clarify this point.

Also in m a n y practical systems applications of our mixing method m a y be envisaged. Polymer flocculation is increasingly used in waste water t rea tment and water purification and in several industrial applications such as flotation and mining engineering (22). In these processes a relatively concentrated polymer is locally added to the system to be treated, so tha t in m a n y cases parts of the system with high pob~-ner content come into contact with regions without polymer. Thus our mixing method is simulated and the ground is prepared for better process control. I t m a y be expected that our mixing procedure is also very relevant in a number of other systems of practical interest.

ACKNOWLEDGMENTS

The authors thank Mrs. W. de Kok-Bats foi her skillful technical assistance. Many students have par- ticipated in this work. We mention especially It. A.

Klapwijk for his invaluable help in developing the mixing method.

REFERENCES

1. RUEItRWE1N, R. A., AND WARD, D. W., Soil Sci. 73, 485 (1952).

2. LA MER, V. K., AND HEALY, T. W., Rev. -Pure Appl. Chem. 13, 112 (1963).

3. LINKE, W. F., AND BoomrI, R. B., Trans. A IME 217, 364 (1960).

4. MlCI~AELS, A. S., Ind. Eng. Chem.4 6, 1485 (1954). 5. SLATER, R. W., AN]) KlmCI~ENER, J. A., Discuss.

Faraday Soc. 42, 267 (1966). 6. SOMMERAUER, A., SUSSMAN, D. L., AND STUA{M, W.,

Kolloid-Z. Z. -Polym. 225, 147 (1968). 7. GREGORY, J., Trans. Faraday Soc. 65, 2260 (1969). 8. NEMETH, R., AND MATIJEVI6, Kolloid-Z. Z. -Polym.

225, 155 (1968). 9. FLEER, G. J., Y,~OOPAL, L. K., AND LYKLE~A, J.,

KolIoid-Z. Z. -Polym. 250, 689 (1972). 10. FLEER, G. J., AND LYKLEMA, J., -Proc. Int. Congr.

Surface Activ., 5th, (Barcelona) 1968; 2, 247 (1969).

11. FLEER, G. J., thesis, Agricultural Univ. Wage- ningen, 1971; Meded. Zandbouwhogeseh. Wage- ningen 72-20 (1971) ; available on request.

12. FLEER, G. J., AND LYKLEMA, J., to be published. 13. LANICVELD, J. M. G., ANB LYKLEMA, J., J. Colloid

Interface Sci. 41, 454 (1972). 14. LEVI, S. M., AND STEPANOVA, T. K., Colloid Y.

USSR 27, 42 (1965). 15. BIJSTERBOSCH, B. H., AND LYKLEMA, J., J. Colloid

Interface Sci. 20, 665 (1965). 16. VINCENm, B., BIJSTERBOSCH, B. I-I., AND LYKLEMA,

J., J. Colloid Interface Sci. 37, 171 (1971). 17. KOOPAL, L. K., to be published. 18. OVERBEEK, J. TH. G., in "Colloid Science" (H. R.

Kruyt, Ed.), Vol. 1, p. 307. Elsevier, Amsterdam, 1952.

19. LYKLEMA, J., ]9roe. Int. Vortragstagung iiber grenz- flgchenaktiven Stoffe, 3rd, (Berlin) 1966; p. 542 (1967).

20. OTTEWILL, R. H., AND SHAW, j . N., J. Colloid Interface Sci. 26, 110 (1968).

21. TROELS~RA, S. A., thesis, State Univ. Utrecht, 1941; KRuYm, H. R., ArTD TROELSTRA, S. A., Kolloidchcm. Beih. 54, 225 (1943).

22. KUZ'I~IN, S. K., AND NEBERA, V. P., "Synthetic Flocculants in Dewatering Processes." Moscow, 1963 (Trans. Nat. Lending Library, Boston G.B. 196~).

Journal of Colloid and Interface Science, Vol. 46, No. 1. January 1974

polymer adsorption and its effect on the stability of hydrophobic colloids. ii. the flocculation...

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