Reprinted from

co ill) IDSANDSURE ACESAN ~ .KX.RW..

A: PHYSICOCHEMICALANDENGINEERING ASPECTS

Colloids and SurfacesA: Physicochemical and Engineering Aspects 146 (1999) 397-403

Role of sequential adsorption of polymer/surfactantmixtures and their conformation in dispersion/flocculation

of alumina

A. Fan, P. Somasundaran *, N.J. Turrol.aIIgJtIIIir Caler for CoI1oids and Int"rfaca. CohlmbiD U/I;vers;ty. .500 W~sf 12tkh Street, N- York, NY 10027, USA

~

ELSEVIER

COLLOIDS AND SURFACESA: PHYSICOCHEMICAL AND ENGINEERING ASPECTS

AN INTERNATIONAL JOURNAL DEVOTED TO THE PRINCIPLES AND APPLICATIONS OFCOLLOID AND INTERFACE SCIENCE

Editors-in-ChiefP. Somasundaran, 911 S.W. Mudd Bldg, School of Engineering and Applied Science, ColumbiaUniversity, New York, NY 10027, USAH. Mbhwald. Max-Planck-lnstitut fur Kolloid- und Grenzflachenforschung, Instituteil Berlin-Adlershof, Rudower Chaussee 5, Geb 9-9, D12489 Berlin, Germany

Co-EditorsD.N. Furlong, CSIRO, Molecular Science, Private Bag 10, Sth. Clayton, MDC, Clayton 3169,AustraliaDr. D.C. Prieve, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh,PA 15213, USA. Tel: 1 (412) 268 2247; Fax: 1 (412) 268 7139; E-mail: dcprieve@andrew.cmu.edu

Honorary EditorTh.F. Tadros, c/o Ms. Sarah Conway, Elsevier Science ltd., The Boulevard, Langford Lane.Kidlington, Oxford, OX5 1GB, UK (Tel: (44) 1865 843583; Fax: (44) 1865 843348; E-mail.s.conway@elsevier.co.uk

~i

Founding EditorE.D. Goddard, 349 Pleasant Lane, Haworth, NJ 07641, USAEditorial B08rdR. Aveyard (Hull, UK) R.A. Mackay (Potsdam, NY, USA)J.C. Berg (Seattle. WA. USA) C.A. Miller (Houston. TX, USA)V. Degiorgio (Pavia, Italy) K. Papadopoulos (New Orleans. LA, USA)S.S. Dukhin (New York, NY, USA) B.A. Pethica (Parsippany, NJ. USA)G.H. Findenegg (Berlin, Germany) D.C. Prieve (Pittsburgh. PA. USA)T.W. Healy (Parkville. Australia) A.I. Rusanov. (St Petersburg. Russia)K. Higashitani (Kyoto. Japan) P .J. Scales (Parkville, Australia)E.W. Kaler (Newark, DE, USA) J. Sjoblom (Bergen. Norway)T. Kunitake (Fukuoka. Japan) C. Solans (Barcelona. Spain)K. Kurihara (Sendai, Japan) K. Stebe (Baltimore. MD, USA)D. Langevin (Pessac, France) B.V. Toshev (Sofia, Bulgaria)R. Y. Lochhead (Hattiesburg. MS. USA) F .M. Winnik (Hamilton, Ont., Canada)

Scope of the JournalColloids and Surfaces A: Physicochemical and Engineering Aspects is an international journal devoted to the science ofthe fundamentals, engineering fundamentals. and applications of colloidal and interfacial phenomena and processes.The journal aims at publishing research papers of high quality and lasting value. In addition, the journal contains criticalreview papers by acclaimed experts. brief notes, letters. book reviews. and announcements.Basic areas of interest include the followin9: theory and experiments on fluid interfaces; adsorption; surface aspects ofcatalysis; dispersion preparation. characterization and stability; aerosols. foams and emulsions; surface forces; micellesand microemulsions; light scattering and spectroscopy; detergency and wetting; thin films, liquid membranes andbilayers; surfactant science; polymer colloids; rheology of colloidal and disperse systems; electrical phenomena ininterfacial and disperse systems. These and related areas are rich and broadly applicable to many industrial, biologicaland agricultural systems.Of interest are applications of colloidal and interfacial phenomena in the following areas: separation processes;materials processing; biological systems (see also companion publication Colloids and Surfaces B: Bioinferfaces);environmental and aquatic systems; minerals extraction and metallurgy; paper and pulp production; coal cleaning andprocessing; oil recovery; household products and cosmetics; pharmaceutical preparations; agricultural. soil and foodengineering; chemical and mechanical engineering.

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COLLOIDSANDSURFAC~ A

ELSEVIERColloids and Surfaces

A: Physicochemical and Engineering Aspects 146 (1999) 397--403

Role of sequential adsorption of polymer/surfactantmixtures and their conformation in dispersion/flocculation

of alumina

A. Fan, P. Somasundaran., N.J. TurroLangmuir Center for Colloid\' and Interfaces, Columbia University, 500 West I2Oth Street, New York.. NY 10027, USA

Rcccived 21 August 1998; aa:epted )0 October )998

Abstract

The role of sequential adsorption of a non-interacting pair of polymer (polyacrylic acidPAA) and surfactant(sodium dodecyl sulfate-SOS) on alumina particles and in determining the stability of their suspensions was studiedwith particular attention to the confonnation of the polymer. It was found that the sequence of addition is of criticalimportance in detennining the dispersion/flocculation of this system. When SOS was added first. both P AAconformation and suspension stability varied with SOS concentration. Whereas when PAA was added first, thesubsequent adsorption of SOS molecules had no effect on either the polymer conformation or the suspension stability.This is attributed to masking of SOS species by the larger polymer chains. SOS can not however be completelymasked by PAA chains once the PAA concentration is decreased below the optimal concentration fOT flocculation.When the stirring duration was increased, the mode of addition showed no effect suggestjng that the system thenreaches equilibrium. Interestingly, a small amount of pre-adsorbed PAA was found to facilitate the adsorption ofSOS. e 1999 Elsevier Science B.V. All rights reserved.

x..,words: Sequential adsorption; POlymer/surfactaDt mixtures; Dispersion/floa:ulatioo; Alumina

little work on co-adsorption of non-interactingpolymers and surfactants. In one of the rare ex-amples of the latter, Gebhardt and Fuerstenau [I),while confirming the absence of any interactionbetween polyacrylic amide (PAAm) and sodiumdodecyl sulfonate (SDSO3) in solution, demon-strated that pre-adsorbed P AAm on hematite hadno effect on either the adsorption of SDSO3(except at high concentrations of the latter) or onits electrophoretic mobility. On the other hand,Hollander [2) determined that each ingredient in a

I ntToductjon

Since both polymers and surfactants adsorb onsolids in certain operations, it is useful to deter-mine the effect each has on the adsorption proper-ties of the other. Most studies in the past havefocused on systems where there are interactionsbetween the polymer and the surfactant with very

. Corresponding aulhor. Tel.: + 1-212-8542926; fax: + 1212-8548362; e-rnaiJ: ps24@ooIumbia.edu.

0927-77S7{99/S - see front matter 0 1999 Elsevitr Science B.V. All rights reserved.

PO: 80927-7757(98)00865-6

398 A. Fan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 397-403

PAAmjdodecyl benzyl sulfonate (DDBS) systemhad a depressing effect on the adsorption of theother on kaolinite. Such an effect was found alsoby Moudgil and Somasundaran [3] for the ad-sorption of the SDSO3jPAAm pair on hematite.Some attempt has been made to explain the ad-sorption in the above studies based on data forsome bulk properties. There is however no direct,systematic measurement of the microscopic prop-erties involved in such interactions. The aim ofthis work is to correlate the bulk properties withmicroscopic properties for illustrating mechanismsbehind the interfacial processes in the systems.

The model system chosen for this study is thatof polyacrylic acid (PAA) and sodium dodecylsulfate (SDS) in an aqueous suspension of alu-mina. Both PAA and SDS are anionic in natureand they will therefore adsorb on positivelycharged alumina mainly by electrostatic attrac-tion. The adsorption behaviors of both PAA andSDS as well as the effects of their adsorption onalumina suspension stability have been well stud-ied, however no data are found on their role indetermining alumina stability for the case whereP AA and SDS co-exist.

a power setting of 30 Watts (Lab-Line UJtratipLabsonic System, Lab-Line Instruments), the de-sired amount of polymer or surfactant was thenadded to the suspension with the prope)]er rotat-ing at 300 rev. min - 1. After the stirring hadbeen stopped for 5 min, 16 m1 of the supernatantfrom the top was transferred to a turbidimeter(HF Scientific).

2.3. Fluorescence measurements

Fluorescence spectra were recorded on a Pho-ton International PTI-LS 100 Spectrophotometer.For slurry samples, the fluorescence experimentswere conducted in a 2 rom flat quartz cell. Therelative intensities of pyrene emission peak at 373nm (monomer) and 475 nm (excimer) obtainedwith excitation at 335 om were recorded. Theratio of intensities of excimer to monomer peak,Ie/1m, was calculated and termed the coiling in-dex, which reflects the extent of coiling of thepolymer chains [4,5].

3. Results and discus..,ion

2. Experimental 3.1. Effect of addition mode

2.1. Materials Results obtained for different sequences of ad-ditions of sodium dodecyl sulfate (SDS) and poly-acrylic acid (PAA) are given in Fig. I and it canbe seen that the stability of alumina suspensionsare very much dependent on the addition se-quence. When SDS is added to the suspensionbefore PAA(SDSjPAA mode), the turbiditychanges markedly as a function of the added SDSconcentration; whereas when P AA is added first(PAA/SDS mode), there is very little dependenceon the SDS additions. When the three curvescorresponding to SDS/PAA mode, PAA/SDSmode and SDS alone are compared, it can be seenthat the SDS/PAA curve lies between those forPAA/SDS mode and for SDS alone, the suspen-sion stability being similar to that for adding SDSalone at high SDS concentrations and approach-ing that for P AA/SDS mode at low SDS concen-trations [6-8].

Alumina powder (Praxair Surface Technolo-gies) possesses an average size of 0.3 J.1In and aBET surface area of apI- ;T1}ately 15 m2jg. Poly-acrylic acid of molecular ,. '. 90 k was ob-

tained from Polysciences. Pyrene JClVi:led P AA ofthe same molecular weight was supplied by theNational Chemical Laboratory of India. Ionicstrength was kept constant in all the experimentsat 0.03 M (NaCI). Triply distilled water (TDW)was used throughout the experiments.

2.2. Flocculation tests

For each experiment, a 0.5 g sample of aluminawas transferred into a 40 ml solution of 0.03 MNaCI in a 50 ml beaker fitted with a propellerwith four baffle plates. After 30 s of sonication at

A. Fan et a/. / Colloids and Swfaces A: Physicochem. Eng. Aspects /46 (1999) 397-403 399

In order to elucidate the molecular mechanismsbehind the above flocculation/dispersion behav-ior, polymer conformation was measured underdifferent addition sequences using pyrene labeledPAA(py-PAA) as the fluorescence probe. It canbe seen from Fig. 2 that polymer becomes morestretched when it adsorbs on alumina surfacefrom the solution. Furthermore, when SDS isadded first, the PAA coiling index is found toincrease continuously with SDS dosage and thiscorresponds to a similar decrease in system tur-bidity as shown in Fig. 1. In contrast, for the casewhere P AA is added first, SDS has no effect oneither the system turbidity or the PAA conforma-tion at all SDS dosages.

On the basis of the above measurements ofsuspension turbidity and polymer coiling index, itis proposed that when SDS is added first, thepre-adsorbed surfactant causes 'blocking' of theadsorption of the P AA added later, resulting in asmaller fraction of the polymer trains and thus ahigher coiling index. However, when P AA isadded first, SDS does not 'block' adsorption ofP AA which could be either due to masking of thepolymer chains or due to prevention of SDS

Fig. 2. Polyacrylic acid (PAA) coiling index at alumina/waterinterface under the SDS/PAA and PAA/SDS addition modesas well as in solution in the presence of SDS.

adsorption. Since a measure of the adsorptiondensity of SDS in the presence of pre-adsorbedP AA (Fig. 3) shows SDS adsorption not to beaffected by PAA, it can be concluded that whenPAA is added first, the subsequent adsorption ofSDS does not affect P AA conformation. Because

~i~

S.Ox I 0-4 I.OxIO.)SOS concentration. M

-3

1.5x1OO.OxIO'

Fig. I. Turbidity of alumina suspension for sodium dodecylsulfate (SDS)fpolyacrylic acid (PAA) addition mode, PAAfSDS addition mode, and addition of SDS only ([PAA] = 10ppm, (NaCl] = 0.03 M)

Fig. 3. Adsorption isotherm of sodium dodecyl sulfate (SDS)on alumina in the presence of pre-adsorbed polyacrylic acid(PAA).

400 A. Fan el aI.1 Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1.999) 397-403

concentration and deviate significantly from eachother at P AA concentration below 0.5 ppm, i.e.SDS starts to have an effect on the system whenthe PAA concentration is less than 0.5 ppm.When the P AA concentration is very low, thereare not enough chains available to mask SDS; onthe other hand, at very high P AA concentrations,a significant fraction of adsorption sites on alu-mina are pre-occupied by the polymer and thusSDS adsorption density is reduced (Fig. 6). WhenP AA concentration is higher than 350 ppm, thereis no adsorption of SDS.

\$OS

1-PM

3.3. Stirring time

Since P AA adsorption is irreversible while SDSadsorption is readily reversible, given enough stir-ring time during the flocculation process, additionsequence should not be expected to make anydifference, i.e. both the P AA coiling index and thesuspension turbidity for SDSfPAA addition se-quence should approach those for PAA/SDS se-quence. It can be seen from Fig. 7(a) that it takesabout 30 min of stirring for the system to reachequilibrium with the SDSfPAA sequence. Duringthis process, the SDS adsorption density is found

(8) (b)

Fig. 4. Schematic illustration for (a) SDS firstfPAA secondaddition mode and (b) PAA firstjSDS second addition mode.

adsorbed SDS is very likely masked by the dan-gling polymer chains, the suspension stability isnot affected also. A schematic representation ofthe adsorbed states for both addition modes aregiven in Fig. 4.

3.2. Effect of amount of pre-adsorbed polyacrylicacid (P AA)

Being masked by the P AA chains, SDS specieshave essentially no effect on the behavior of thewhole system. When PAA concentration is de-creased, at some point it cannot be expected to beable to mask SDS completely and SDS will thenbegin to playa role. Fig. 5 shows the turbidity ofalumina suspension for PAA alone and PAA/SDSaddition mode with SDS concentration keptconstant.

It can be seen from this figure that the optimumflocculation concentration of P AA when usedalone is around 0.5 ppm. This is also the mini-mum concentration required to mask SDSmolecules completely. The two curves in Fig. 5overlap when the P AA concentration is above this

Fig. 5. Turbidity of alumina suspension as a function ofpolyacrylic acid (PAA) concentration in the absence and pres-ence of sodium dodecyl sulfate (SDS) (PAA/SDS mode).

.4. F.. et aI.! Colloids and SIIrf«rs .4.: P/ty.ricochem. Eitg. .4.rj¥cts 146 (1999) 397-403 401

3.5. Kinetics of SDS adsorption in the presenceand absence of P AA

I.OK10.'

Finally, it was observed that the SDS adsorp-tion is facilitated by the presence of smallamounts of PAA (Fig. 9). It is suggested thattrace amounts of pre-adsorbed PAA can preventftoccuJation caused by the SDS at certain SDSconcentrations. This in turn can also preventany retardation of adsorption due to inaccessi-bility of polymer into interior of ftocs if theyform.

..~

Ba- 1.001.O"

'8

f1.0&10'

.OxIO"

0

Fig. 6. Sodium dodecyl sulfate (SDS) adsorption density onalumina as a function of preadsorbed polyacrylic acid (PAA)([SDS)initial = 0.00125 M).

to remain constant (Fig. 7(b», indicating re-dis-tribution of SDS instead of it being replaced,i.e. PAA relaxes and reoriented while SOS redis-tribute on alumina surface to comply with thepolymer relaxation/reorientation. 1200 IxIO'

11003.4. Effect of time interval between additions ofP AA and SDS 1

f8§..

f

I

!/XX

900AJthough SDS does comply with PAA relax-ation as stirring proceeds, when SDS and PAAare added to alumina suspension at the sametime, the system turbidity is more similar to thatfor SDS added alone (Fig. 8). This suggests thatSDS adsorbs more rapidly and reversibly onalumina than the polymer. It can also be seenfrom Fig. 8 that the flocculation/dispersion be-havior of alumina suspension is very sensitive tothe addition sequence of the two species. Whenflocculation time is of a few minutes, the speciesadded first is always playing a dominating role,no matter how smaJ) the time interval betweenthe two additions.

~

fIxIO"MXI

,..~

soo

400I 10 ) 30 40 50 fA} 70

SI8Ii8I Fig. 7. (a) Changes in turbidity and PAA coiling index as a

function of stirring time uDder SDS/pAA addition mode; (b)SOS adsorption dcDsity as a functiou of stirring time underSDS/pAA addition mode ([PAA) = 10 ppm; (SDS)initial-0.00125 M)

x

402 A. Fan et a/./ Colloids and Surfaces A: Physicochem. Eng. Aspects /46 (1999) 397-403

Fig. 8. The effect of time interval between sodium dodecylsulfate (SOS) and polyacrytic acid (PAA) additions on alu-mina stability.

4. Summary

I. These experiments show that the effect of apair of non-interacting polymer and surfactant

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IjlI

on the adsorption of each other and on floccu-lation of adsorbent can be marked, dependingparticularly on the addition sequence.

2. When SDS, the surfactant, was added first,the confonnation of the polymer, PAA, and thesuspension stability were found to vary withSDS dosages. A desired stability level can thusbe obtained by manipulating the SDS concentra-tion. When P AA is added first, the SDS ad-sorbed subsequently is masked by the polymerand contributes neither to the PAA conforma-tion nor to the suspension stability. For exam-ple, for the system studied, SDS adsorptiondensity is not hampered by pre-adsorbed PAAas long as PAA concentration is not very high« 50 ppm). Also, when the PAA concentrationis less than 0.5 ppm (which was also the optimalflocculation concentration) SDS was no longermasked by P AA; under these conditions, SDShad a measurable effect on the system behavior.

3. Time of reagentizing had a significant ef-fect on both the adsorption and flocculation. Ifthe flocculation time is on the order of a fewminutes, the species added first plays a predomi-nant role independent of time interval betweenthe two addition. When flocculation time waslong, P AA showed a governing role. P AA ad-sorption is irreversible while SDS adsorption isreadily reversible. Both the P AA coiling indexand system turbidity for the SDSjPAA additionsequence approached those for the PAA/SDSaddition sequence. In the fonDer addition se-quence, SDS is considered to redistribute itselfto comply with P AA adsorption and its confor-mation.

4. A small amount of pre-adsorbed PAA canretard flocculation caused by SDS of certainconcentrations and thereby accelerate the ad-sorption of SDS on alumina.

I.OKIO'Acknowledgements0 10 IS 20

SI8IWI&l8IIe.mm.

2S 30 ~ 40

The authors thank Environmental ProtectionAgency (EPA R82330l-Ol-O) for the financialsupport.

Fig. 9. The kinetics of sodium dodecyl sulfate (SDS) adsorp-tion with and without preadsorbed PAA ([SDS)initial =0.00125 M).

Oxl0-6

A. Fan et af. / Col/oidf and Surfaces A: Physicochem. Eng. Aspects 146 (1999) 397-403 403

References

[I] J.E. Gebhardt, D.W. Fuerstenau, ACS Symp. Ser. 253(1984) 291.

[2] A. Hollander, M.S. thesis, Columbia University, NewYork, NY, 1979.

(3] Dc Moudgil, P. Somasuodaran, CoUoids Surf. 13 (1985) 87.(4] P. Somasuodaran, J.T. Kunjappu, in: F.M. Doyle, S.

Ragbaven, P. Somasundaran, G. W. Warren (Eds.), Inno-vations in Materials Processing Using Aqueous, Colloid

and Surface Chemistry, The Minerals Metals and Materi-als Society, Las Vegas, 1988, p. 31.

[5] P. Somasundaran, J.T. Kunjappu, Miner. Metall. Process.5 (1988) 68.

[6) K.F. Tjipangandjara, Y.-B. Huang, P. Somasundaran, N.J.Turro, Colloids Surf. 44 (1990) 229-236.

[7) K.F. Tjipangandjara, P. Somasundaran, Colloids Surf. A55 (1991) 245-255.

[8) J.T. Kunjappu, P. Sornasundaran, K. Sivadasan, ColloidsSurf. A 97 (1995) 101-107.