Advancrd Powd" T«/tliol.. Vol. 3. No.2. pp. 119-127 (1992)~ VSP and Society of Powd~r TechnololY. Japan 1992

Paper

Effects of the conformation of poly acrylic acid on thedispersion-flocculation of alumina and kaolinite fines

K. F. TJIPANGANDJARA and P. SOMASUNDARAN.Langmuir Center for Colloids and Interfaces. Henry Krumb School of ."fines. Columbia c,-niversit.v.New York City, .VY /0017. USA

Received for APT 1~ December 1990; accepted S August 1991

Abstract-Conformational characteristics of polyacrylic acid (PAA) adsorbed on alumina were studiedalong with stability of its suspension in order to delineate the mechanism by which polymer conformationaffects the stability of fines. Fluorescence spectroscopy, with pyrene as the probe, was used to monitorthe conformation of the polymer at the solid-liquid interface. The stability of these fines with PAA incoiled, stretched and coiled-transformed-to-stretched conformations was investigated. With PAA incoiled and stretched forms, the suspensions were slightly flocculated. Under the coiled-to-stretchedconditions, achieved through manipulation or polymer conformation, flocculation was drasticallyenhanced at lower polymer concentrations, while a stable dispersion was observed at higher polymerconcentrations. This difference in dispersion-flocculation behavior is explained on th~ basis of aconcentration-dependent conformational transition. Enhanced flocculation of kaolinite was alsoachieved by manipulation of polymer conformation.

1. INTRODUCTION

Colloidal stability is a key interfacial parameter that controls the efficiency of manyindustrial processes: printing, detergency, cosmetics, microeleCtronics, high perfor-mance ceramics, mineral processing, effluent treatment and food processing [1-5].In most of these processes, dispersion or flocculation of panicles is determined bymacromolecular adsorption both in terms of the amount and the configuration ofthe adsorbed species [4-13]. Polymers can exist in different conformations depend-ing on the physico-chemical conditions of the system (e.g. pH, ionic strength of thesolution, type of solids and solubility). Stability of suspensions will therefore beinfluenced by these faCtors. Due to the non-existence of reliable in situ techniques todetermine the conformation and orientation of adsorbed polymeric species, verylittle work has been done in the past to elucidate the mechanism by which polymerconformation controls suspension stability and to identify means for controllingpolymer conformation to achieve flocculation or dispersion. As a result, colloidalstability has been interpreted mostly on the basis of polymer adsorption density andthe surface charge propenies of the panicles.

Recently, we developed a multi-pronged approach involving simultaneousmeasurements of flocculation-dispersion responses, elecrrokineticbehavior of pani-cles and configuration of adsorbed polymeric species using fluorescence techniqueand pyrene-labelled reagents [14,15]. The rationale behind the use of this technique

. To whom correspondence should be addressed.

120 K. F. Tjipangandjara and P. Somasundaran

is the observation that, in bulk solutions, the extent of excimer formation (i.e. asso-ciation of a ground state pyrene with an excited state pyrene) depends strongly onthe polymer conformation [16, 17}. When a polymer is in a coiled conformation,there is a higher probability of intramolecular excimer formation between pyrenegroups. Similarly, when a pol}mer is in a stretched conformation, there is a lowprobability of such excimer formation. The difference between the two confonna-tions is discernible in the intensity of their fluorescence spectra at certainwavelengths. Here, the ratio of the intensity of the excimer (Ie) to the monomerpeaks (1m) is referred to as the 'coiling index' of the polymer. In the absence ofsignificant intermolecular interactions, a high lei 1m value is obtained for a coiledconformation, while a low ratio is associated with a stretched conformation.

The aim of this investigation was to study the role played by the conformation ofpolyacrylic acid (P .~) in determining the stability of fines, in particular alumina, andto develop methods for controlling the stability of colloids through manipulation ofpolymer conformation. Configurational behavior of the adsorbed polymer was deter-mined along with flocculation and zeta potential properties of the same samples.Use of the same samples for all measurements is considered important, since correla-tion of results from different tests can be misleading due to experimental artifacts.The effects of polymer concentration and conformation (coiled, stretched andcoiled-co-stretched) on the stability response for alumina suspensions were studied.On the basis of information obtained from an alumina/P AA system, the approachof conformation manipulation \vas also applied to a kaolinite/P AA system.

2. EXPERIMEl':T AL

2./. iWateria/s

Linde Alumina (L'nion Carbide) ofO.3.um size was used in this study. The iso~lectricpoint of the sample was pH 8.3. Another mineral used in this investigation \Vashomoionic Georgia kaolinite obtained from the clay repository of the University of~Iissouri, and prepared using a standardized technique ".hich involves repeatedwashing with distilled water and treatment with ~aCI solutions to remove exchange-able ions, such as Ca2..., and freeze-drying of the final product [8,18]. Particle sizedistribution of this sample was determined, using Microtrac Particle Size Analyzer,to be 8011!0 under 5.um and 10011(0 under 22.um. The isoelectric point of the kaolinitesample was pH ~.S. A pyrene-labelled PAA sample of molecular weight 88000 wasused in this investigation [16,17], along with an unlabelled polyacrylic acid ofmolecular weight 90000 purchased from Polysciences, Inc. To minimize inter-molecular excimer formation at higher polymer concentrations, a mixture of pyrene-labelled PAA with pyrene-free PAA was used. Concentration of the pyrene-labelledpolymer was maintained at 20 p.p.m. while that of the unlabelled polymer wasvaried. All polymer solutions were prepared in 0.03 kmol/m3 NaCI solutions.Fisher-certified ~aOH and HCl were used for pH adjustment, while constant ionicstrength was maintained by the use of reagent grade NaCl.

2.2. Equipment

Emission spectra were obtained using a SPEX FLUOROLOG fluorescence spectro-photometer. The pH was measured with an Orion Research Digitallonalyzer 501

E//«IS 0/ PAA on alumina and kaolinite fines 1:1

To determine the electrokinetic properties of the suspension. a Zeta-Meter was used.Residual polymer concentration of supernatants was determined using a DohrmannDC 90 Total Organic Carbon (TOC) analyzer.

2.3. PrO('edures

A 10 g mineral sample was equilibrated with 194 ml of 0.03 krnol/mJ NaCI solutionfor 45 min by stirring with a magnetic bar. After pH adjustment, the suspension \\'asfurther equilibrated for 45 min. The magnetic bar was removed, a baffle with four0.63 ~m wide plates was inserted and the suspension was stirred for 3 min using a2.5 ~m diameter propeller, with three blades at 45° inclination, at 600 r .p.m. Usinga Sage syring pump, 6 ml of polymer solution was added to the suspension drop wiseat a rate of 6 ml/min. The polymer-containing suspension was further stirred for5 min before flocculation measurements were made. For the system in which pHadjustment was made after the polymer was added, an additional 5 min of stirringwas done. Aliquots for emission spectra and zeta potential determinations were col-lected from the same sample.

2.4. Flocculation tests

After allowing the suspension to settle for 45 s, the upper-half of the suspension wasremoved by suction and the percent solids settled was estimated by measuring thesolid content of the lower half. It is to be noted that flocculation tests have been per-formed for both the system containing pyrene-labelled P AA and that containingunlabelled P AA. The results showed no measurable difference between the floccula-tion responses obtained using them, suggesting that the pyrene labelling had noeffect on the flocculation behavior itself.

2.5. .-tdsorption tests

Supernatant solutions were removed from these samples, centrifuged and analyzedfor residual concentration using the TOC analyzer. Under all conditions. P AA \\'ascompletely adsorbed and no residual concentration could be detected in thesupernatant.

3. RESt:LTS A~D DISCt:SSION

Figure I shows the excimer-to-monomer ratio (lei 1m) of the adsorbed P AA. at thealumina-liquid interface as a function of PAA concentration at pH 4.~ and 10.5. Itcan be seen from Fig. I that lei 1m, and hence the degree of coiling, is higher atpH 4.~ than at pH 10.5. This can be attributed to the low degree of ionization of thepolymer at pH ~.4 (pKa of PAA = 4.5; refs 10 and 19) which will result indiminished intramolecular electrostatic repulsion. A significant reduction isobserved in the zeta potential of alumina due to adsorbed P AA (Fig. 2). Eventhough the reduction in charge led to agglomeration and some settling of fines,

. For the two systems studied. under all the e~perimental.;onditions there was complete adsorption of the

polymer so that the fluorescence signal was entirely from the adsorbed polymer.

122 K. F. Tjipangandjara and P. Somasundaran

0 10 z ::I) «150 53 108) ~Pa..Y~YLIC FICiCI CCIIC.(pp1t)

100

FilaR 1. Excimer-to-monomer ratio, 1';1.. of PM at the alumina-liquid interface as a funCtion ofP AA concentration at pH ~.~ and 10.S (IS - 0.03 kmovmJ ~aCI. Sit a 10 g/200 ml).

M

~ 4.4

00

a:JI

~ «J. :))~

~ ~~Z IQ~wI-0 'Q.

l-10

a: !I- IW -~N !\

-X~\; ,.~I. "-

: I -~~~ja;!.--- L ~

.~- .

:J 10 20 :D 40 50 !t 70 B) ~ 100POLYACRYLIC ~C:J CONC'<Fcm)

FIIUR 2. Zeta potential of alumina suspension as a function of PAA concentration at pH ~... and 10.S(IS = 0.03 kmol/mJ ~aCI. 51 L = 108/200 ml).

adsorbed P AA in the coiled conformation was apparently not able to bridge theseagglomerated fines and affect the percent solids settled (Fig. 3). From this study itis clear that P AA in coiled conformation does not have any significant effect on theflocculation and sedimentation of alumina under the tested conditions.

At pH 10.S, the values of lei 1m are lower than that obtained at pH 4.4 whichindicates that the polymer is adsorbed in a relatively stretched conformation (Fig. 1).This can be attributed to the higher charge density of the polymer (due to ionizationof carboxylic acid groups) at pH 10.S, compared with that at pH 4.4, and this willcause enhanced intramolecular repulsion. In contrast to the behavior at pH 4.4, atpH 10.5 the conformation of polyacrylic acid depends on the concentration of the

Effects of PAA on alumina and kaolinite fines 123

a p-I 4.4. p-I IO.~

m.

....,'" -- -.--~'

I "---~,~

@,oJ

polymer. At lower polymer concentrations. the adsorbed chains appear to be morestretched (lei 1m = 0.25). As the concentration of the polymer is increased, thepolymer assumes a less stretched conformation (I"; 1m = 0.35), apparently due to thecrowding of polymer chains on the panicles. Stability tests, as presented in Fig. 3.however. did not show similar concentration-dependent behavior. The amount ofsolids settled passes through a maximum at 20 p.p.m. Simultaneous examination oflei 1m (Fig. 1) shows that at this concentration (20 p.p.m.) the lei 1m increasessuggesting that the more stretched polymer conformation causes enhanced floccula-tion in this case. This inference is also supponed by the fact that the amount ofsolids settled at pH 10.5 when the polymer is stretched is higher than that at pH 4.4.

A relatively low value of lei 1m at pH 10.5 ~'ould only indicate that the polymer isin a stretched conformation. It does not provide any information as to whether ornot the stretched polymer is lying flat on the surface or dangling into the liquid. Theflocculation results, however, suggest that the stretched polymer is likely to be dan-gling into liquid since enhanced flocculation by bridging, compared to when thepolymer is coiled, is possible under this condition. The proposed conformation isalso consistent with the fact that both the polymer and the surface are negativelycharged and it is unlikely that the polymer would lie flat on the surface under thiscondition, as such a conformation would require a large number of attachmentpoints between the polymer and the surface. However, adsorption can still occur (asalso determined experimentally) since even at pH 10.5 there will be sufficientnumber of unionized carboxylic acid groups (-COOH) which can attach to thesurface through hydrogen bonding.

The results discussed above clearly show that the conformation of P AA has amajor effect on the stability of alumina suspensions. The percent settled solid washigher with the stretched polymer chains which effectively bridged the fines toproduce large floccs. In an attempt to control the stability of alumina with P .-\.-\, ascheme involving pH changes after polymer adsorption was investigated. In thiscase, PAA was first adsorbed on alumina at pH 4 and then the pH was raised to 10

~SI'lHa ,~70v:a W c

IfN~

BJ10 20 :I) 40 BJ eo 70 SI ~ laI

PQYFCRYLIC FC!D CGC.(~)

Fiaure 3. Percent solid Settled of alumina suspension as a funCtion of PAA at pH 4.4 and 10.5(IS: 0.03 kmoVmJ NaC!. Sit = 10 g/200 ml).

124 K. F. Tjipangandjara and P. Somasundaran

a :0 20 :D ~ 50 &) 70 8) aI IIJJ

PQY~YLIC A::J CC)IC.CFfJIn)

Filure 4. Percent solid settled of alumina suspension as a funCtion of P AA concentration at pH 4.4 antpH 10.5. and under Shiit~ pH Conditions (from pH ~ to 10) (IS = 0.3 kmol/mJ, SIt - 10 Ic1OO mil

(shifted pH condition), As shown in Fig. 4, a marked increase in flocculation isobtained at lower polymer concentrations when the pH was raised. The system wasbetter tlocculated under shifted pH conditions than either at pH 4.4 or at 10. Athigher concentrations, the alumina suspensions were highly dispersed.

Zeta potential beha..'ior of alumina particles in P AA solution alone cannot explainthe observed flocculation-dispersion behavior under shifted pH condition. It mustbe noted that no residual polymer could be detected in the supernatant solutionsunder these conditions even when the pH was raised, confirming the absence ofdesorption.

0.1... "~' "':";!;'Ic'i:~~~~;" !;!c;,..;;!"i!~)~~:':i

o.~.

E 0.5:-I

~ I0."

21) :J)4JS>eo 70 KI ~POL YACRYL I C RC I 0 CONC. ( ppm )

tOO

Figure 5. Excim~r.to-monom~r ratio. L.! L.. of P AA at th~ alumina-liquid int~rfac~ as a func-tion of PAA conc~ntration at pH 10.5 and und~r shift~d pH conditions (from pH ~ to 10)(IS = 0.03 kmol/mJ NaCI. SIt = 10 i/200 ml).

I~£ff«t.r of PAA on alumina and kaolin it' fin,s

Changes in the conformation of adsorbed P A.-\ due to pH increase are illustratedin Fig. S. Adsorbing PAA first in coiled form at pH4 (high 1.llm)' and then raisingthe pH to 10, causes the confoimation of polyacrylic acid to change from coiled tostretched form (lower lei/,.). However, the adsorbed polyacrylic acid is morestretched at low concentrations than at higher concentrations. The marked increasein flocculation at low concentrations (see Fig. 4) is interpreted on the basis of thechanges in the polymer conformation from coiled to stretched upon pH increase.When P AA adsorbs initiaUy on alumina suspension at pH 4, fines are aggregatedinto small floccs (microfloccs); with increase in pH to the intermediate pH range(pH 5-7), the adsorbed polymer chains are extended, due to ionization, possiblycausing aggregation of the microfloccs into macrofloccs. Any further increase in pHabove 7 results in funher stretching of the polymer causing aggregation of macro-floccs into superfloccs and, thus, enhancing flocculation and sedimentation [IS]. Athigher concentrations, when the pH of the aluminaiP AA system is raised from 4 to10, adsorbed PAA is apparently unable to stretch to the same extent as at lower con-centrations due to the crowding of these chains at the solid-liquid interface. Becauseof repulsion among the adsorbed polymer chains, the suspension is dispersed underthese conditions.

The effects of pH shifting on the stability of the suspension can also be seenin Fig. 6, in which the percent settled is plotted as a funCtion of PAA concentrationat pH ~.3, pH 10.4 and under shifted pH conditions (i.e. from pH ~ to 10).Under rtXed pH conditions (pH 4.3 and 10.4), the percent settled solids shows ama.ximum at S p.p.m. PAA and the effects of the conformation of the polymer onthe flocculation behavior are noticeable at these pH values. Kaolinite suspensionis better flocculated at pH 10.4 when P AA is in the stretched conformation.However, adsorption of P AA on kaolinite at pH 4 followed by raising of the pH to10 results in a drastic increase in flocculation at lower polymer concentrations.While 20 p.p.m. PAA addition did not enhance percent settled solids (only SSa-D)

under fixed pH condition (pH 4.3), under shifted pH conditions almost complete

1m0

.~

~~

~ 11~S)ti.'J) ,°':,0

~~~ \

51

..-~:::::~ '\ \I \'" \. .

I... ""==.~--- .

0 10 I :xl 40 SI 51 ~ a> ~ 100 '10PQYR:RYLIC A:ID CO';c.(oan)

Ficure 6. Percent solid settled of kaolinite as a function of PAA con~entration at pH .4.3 and 10,.4, andunder shifted pH conditions (from pH 4 to 10) (IS - 0.03 kmol/mJ, 5, L - 10 S"200 ml).

3]

a P'i 4.:3. pH 10.40 ~ 4 -->

126 K. F. Tjipan,andjara and P. Somasundaran

flocculation is observed (- 9S8J. solids settled). At higher polymer concentrations,pH shifting does not seem to affect the stability of kaolinite.

CO~CLUSIO~S

It is evident that the conformation of the polymeric species is a major controllingfactor in determining the stability of alumina and kaolinite suspensions. pH shiftingoffers a new means of controlling the conformation of the polymer in the adsorbedstate and thus the stability of suspensions. At low dosages. PAA conformationalchanges from coiled to stretched. due to pH increase. lead to excellent flocculationof alumina and kaolinite suspensions. while dispersion is enhanced at higher

polymer dosages.

Acknowledgements

The authors acknowledge financial support from the International Fine ParticlesResearch Institute and National Science Foundation (CPE-86-I8163). K.F.T. alsothanks the United Nations Commissioner's Office for Namibia for financialsupport.

REFERE~CES

I. D. H. Napper. Polymeric Stabili;ation of Colloidal Dispersions. New York: Academic Press. 1983.:. J. Th. G. Overbftk. Surfaces and Coatings Related to Paper and K'ood. New York: Syracuse

University Press. 1967.). B. A. Stewan. Soil Condition. ~Iadison. NY: Soil Condition Science Society of Americ3. 19-3..$. E. luck et al.. (,'II_nn's EnC'_,"clopediD of/ndwtrial Chemistry, All. G. Wolfgang (Ed.). ~~

York: VCH Publishers. 1988.S. B. \1. ~Ioudail 3nd P. Somasund3ran. On flo.:.:uI3Iion. sedimenl31ion and .:onsolidalion. In: Pro..'

EnJlneerinf Foundation Con!:. ~ Island. Gcoraia. l.'SA. 1985.6. J. Gregory. Scientific &rsis oj Flo,','ulation. K. J. I\es lEd.). Ro.:kville. \ID: Sijlhoil &: :-.Ioordhc:~..

19-.i.-. T. \\". Healy and \'. K. La \Ier. The adsorplion-rl.x..-ulation rea\."tions of a pol~mer with 3n aqueou.

.:olloidaJ dispersion. J. Ph.\'s. Ch..m. 66. 183~-1.i3g. 1%2,S. A. F. Hollander. P. Somasundar3n Olnd C. C. Gr;.1e. Adsorptionjrom Solution, P. H. Tew3ri IEj.I.

~~ York: Plenum Press. 1981. p. I~).9. \\'. F. Linke ~nd R. B. Boolh. Trans. A/.WE 217. 3~-368. 19S8.

10. J. Cesarano. I. A. Aksay and A. Bleier. Stability of aqueous o-AI:O) suspensions \A:i~!tpoly(methacryli~ acid) polyele.."trolyte. J. Am. Ceram. Soc. 71. 1SO-~5. 1988.

II. T. Salo and R. Ruch. Stabili;:ation of Colloidal Dispersions by Pol_\'m~r .-tdsorption. New York:~Iarcel Dekker. 1980.

I:. Th. F. T3dros. The Effect 0.' Pol.,.,ner on Dispo'rsion Properties. Th. F. T3dros lEd,.. ~e" York:A':3demi.: Press. 1981.

I). P. A. ~.illiams er al.. The EJj~.r of Po/.vmer on DispersiUII Properri.-s. Th. F. T;adros (Ed-I. :-"-e\\York: Academi.: Press. 1983,

I~. P. Chandar. P- Somasundar:1n.:-"-. J. Turro and K. C. Waterman. Ex.:imtr t1uorescen\.ooe delt,;min~.lion of solid-liquid inleria.::;ai pyrene-labeled poiY(3.:ryli.: ;al:id) .:oniormations. LanJmuir. J.298-300. 1987.

IS. K. F. Tjipangandjara. p, Somasundaran. Y.-B. Huana and ~. J. Turro. Correlation of AI:O,flo.:culation with ;adsorbed pol~.3':r;.-lic acid I:onfonnation. Colloids Surf. U. 119-2)6. 1990.

16. ~. J, Turro and K. S. Arora. pyrtne as a pholophysic3i probt for intermol«ular imeral:rions 0\waltr-solub~ polymers in dilult wlutions. Pol_\.mer 27.78)-7%. 11}86.

E/1«ts 01 PAA on alumina and kaolinite fines 127

17. N. J. Turro and O. Tusuneo. Polarity at the micelle-water interface under high pressure as estimatedby a pyrene 3-.:arboxaldehyde fluorescence probe. J. Phys. Chem. 86. 159-161. 198~.

18. H. Van Olphen. An Introduction to Clay Colloid Chemistry. New York: Wiley Science. 1977.19. J. E. Gebhardt and D. W. Fuerstenau, Adsorption of polyacrylic acid at oxide/water interfaces.

Col/oidsSurf.1. 221-231,1983.