co ill) IDS AND SURE ACES Role of sequential adsorption of polymer/surfactant mixtures and their conformation in dispersion/flocculation of alumina

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1 Reprinted from co ill) IDS AND SURE ACES AN.KX.RW.. A: PHYSICOCHEMICALAND ENGINEERING ASPECTS Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) Role of sequential adsorption of polymer/surfactant mixtures and their conformation in dispersion/flocculation of alumina A. Fan, P. Somasundaran *, N.J. Turro l.aiigjtiiiir Caler for CoI1oids and Int"rfaca. CohlmbiD U/I;vers;ty..500 Wsf 12tkh Street, N- York, NY 10027, USA ELSEVIER

2 COLLOIDS AND SURFACES A: PHYSICOCHEMICAL AND ENGINEERING ASPECTS AN INTERNATIONAL JOURNAL DEVOTED TO THE PRINCIPLES AND APPLICATIONS OF COLLOID AND INTERFACE SCIENCE Editors-in-Chief P. Somasundaran, 911 S.W. Mudd Bldg, School of Engineering and Applied Science, Columbia University, New York, NY 10027, USA H. Mbhwald. Max-Planck-lnstitut fur Kolloid- und Grenzflachenforschung, Instituteil Berlin- Adlershof, Rudower Chaussee 5, Geb 9-9, D12489 Berlin, Germany Co-Editors D.N. Furlong, CSIRO, Molecular Science, Private Bag 10, Sth. Clayton, MDC, Clayton 3169, Australia Dr. D.C. Prieve, Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA. Tel: 1 (412) ; Fax: 1 (412) ; dcprieve@andrew.cmu.edu Honorary Editor Th.F. Tadros, c/o Ms. Sarah Conway, Elsevier Science ltd., The Boulevard, Langford Lane. Kidlington, Oxford, OX5 1GB, UK (Tel: (44) ; Fax: (44) ; . s.conway@elsevier.co.uk Founding Editor E.D. Goddard, 349 Pleasant Lane, Haworth, NJ 07641, USA Editorial B08rd R. 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 Journal Colloids and Surfaces A: Physicochemical and Engineering Aspects is an international journal devoted to the science of the 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 critical review 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 of catalysis; dispersion preparation. characterization and stability; aerosols. foams and emulsions; surface forces; micelles and microemulsions; light scattering and spectroscopy; detergency and wetting; thin films, liquid membranes and bilayers; surfactant science; polymer colloids; rheology of colloidal and disperse systems; electrical phenomena in interfacial and disperse systems. These and related areas are rich and broadly applicable to many industrial, biological and 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 and processing; oil recovery; household products and cosmetics; pharmaceutical preparations; agricultural. soil and food engineering; chemical and mechanical engineering. Audience Surface and Colloid Chemists, Separation Chemists, Powder Technologists, Mineral Processors, Petroleum Engineers. Environmental. Soap, Cosmetic and Textile Scientists. Biological and Bioengineers. Tribologists. i Copvrighl 01. ElSEVIER SCIENCE B.V. All RIGHTS RESERVED "/S1g.00 Publication Schedule and Subscription Information Colloids and Surfaces A: Physicochemical and Engineering Aspects (ISSN ). For 1999 volumes are scheduled for publication. Subscription prices are available upon request from the publisher. Subscriptions are accepted on a prepaid basis only and are entered on a calendar year basis. Issues are sent by surface mail except to the following countries where air delivery via SAL mail is ensured: Argentina. Australia. Brazil. Canada. Hong Kong. India. Israel. Japan. Malaysia. Mexico. New Zealand. Pakistan. PR China, Singapore. South Africa, South Korea. Taiwan, Thailand, U:SA. For all other countries airmail rates are available upon request. Claims for missing issues must be made within SIX months of our publication (mailing) date. #,

3 ELSEVIER Colloids and Surfaces A: Physicochemical and Engineering Aspects 146 (1999) COLLOIDS AND SURFAC A Role of sequential adsorption of polymer/surfactant mixtures and their conformation in dispersion/flocculation of alumina A. Fan, P. Somasundaran., N.J. Turro Langmuir 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 studied with particular attention to the confonnation of the polymer. It was found that the sequence of addition is of critical importance in detennining the dispersion/flocculation of this system. When SOS was added first. both P AA conformation and suspension stability varied with SOS concentration. Whereas when PAA was added first, the subsequent 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 completely masked 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 then reaches equilibrium. Interestingly, a small amount of pre-adsorbed PAA was found to facilitate the adsorption of SOS. e 1999 Elsevier Science B.V. All rights reserved. x..,words: Sequential adsorption; POlymer/surfactaDt mixtures; Dispersion/floa:ulatioo; Alumina I nttoductjon Since both polymers and surfactants adsorb on solids in certain operations, it is useful to determine the effect each has on the adsorption properties of the other. Most studies in the past have focused on systems where there are interactions between the polymer and the surfactant with very. Corresponding aulhor. Tel.: ; fax: ; e-rnaij: ps24@ooiumbia.edu. little work on co-adsorption of non-interacting polymers and surfactants. In one of the rare examples of the latter, Gebhardt and Fuerstenau [I), while confirming the absence of any interaction between polyacrylic amide (PAAm) and sodium dodecyl sulfonate (SDSO3) in solution, demonstrated that pre-adsorbed P AAm on hematite had no effect on either the adsorption of SDSO3 (except at high concentrations of the latter) or on its electrophoretic mobility. On the other hand, Hollander [2) determined that each ingredient in a S7{99/S - see front matter Elsevitr Science B.V. All rights reserved. PO: (98)

4 398 A. Fan et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1999) PAAmjdodecyl benzyl sulfonate (DDBS) system had a depressing effect on the adsorption of the other on kaolinite. Such an effect was found also by Moudgil and Somasundaran [3] for the adsorption of the SDSO3jPAAm pair on hematite. Some attempt has been made to explain the adsorption in the above studies based on data for some bulk properties. There is however no direct, systematic measurement of the microscopic properties involved in such interactions. The aim of this work is to correlate the bulk properties with microscopic properties for illustrating mechanisms behind the interfacial processes in the systems. The model system chosen for this study is that of polyacrylic acid (PAA) and sodium dodecyl sulfate (SDS) in an aqueous suspension of alumina. Both PAA and SDS are anionic in nature and they will therefore adsorb on positively charged alumina mainly by electrostatic attraction. The adsorption behaviors of both PAA and SDS as well as the effects of their adsorption on alumina suspension stability have been well studied, however no data are found on their role in determining alumina stability for the case where P AA and SDS co-exist. a power setting of 30 Watts (Lab-Line UJtratip Labsonic System, Lab-Line Instruments), the desired amount of polymer or surfactant was then added to the suspension with the prope)]er rotating at 300 rev. min - 1. After the stirring had been stopped for 5 min, 16 m1 of the supernatant from the top was transferred to a turbidimeter (HF Scientific) Fluorescence measurements Fluorescence spectra were recorded on a Photon International PTI-LS 100 Spectrophotometer. For slurry samples, the fluorescence experiments were conducted in a 2 rom flat quartz cell. The relative intensities of pyrene emission peak at 373 nm (monomer) and 475 nm (excimer) obtained with excitation at 335 om were recorded. The ratio of intensities of excimer to monomer peak, Ie/1m, was calculated and termed the coiling index, which reflects the extent of coiling of the polymer 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 additions of sodium dodecyl sulfate (SDS) and polyacrylic Alumina powder (Praxair Surface Technologies) possesses an average size of 0.3 J.1In and a acid (PAA) are given in Fig. I and it can be seen that the stability of alumina suspensions BET surface area of api- ;T1}ately 15 m2jg. Polyacrylic acid of molecular,. '. 90 k was ob- are very much dependent on the addition sequence. When SDS is added to the suspension tained from Polysciences. Pyrene JClVi:led P AA of before PAA(SDSjPAA mode), the turbidity the same molecular weight was supplied by the National Chemical Laboratory of India. Ionic changes markedly as a function of the added SDS concentration; whereas when P AA is added first strength was kept constant in all the experiments (PAA/SDS mode), there is very little dependence at 0.03 M (NaCI). Triply distilled water (TDW) on the SDS additions. When the three curves was used throughout the experiments. corresponding to SDS/PAA mode, PAA/SDS mode and SDS alone are compared, it can be seen 2.2. Flocculation tests that the SDS/PAA curve lies between those for PAA/SDS mode and for SDS alone, the suspension For each experiment, a 0.5 g sample of alumina was transferred into a 40 ml solution of 0.03 M NaCI in a 50 ml beaker fitted with a propeller with four baffle plates. After 30 s of sonication at stability being similar to that for adding SDS alone at high SDS concentrations and approaching that for P AA/SDS mode at low SDS concentrations [6-8].

5 A. Fan et a/. / Colloids and Swfaces A: Physicochem. Eng. Aspects /46 (1999) In order to elucidate the molecular mechanisms behind the above flocculation/dispersion behavior, polymer conformation was measured under different addition sequences using pyrene labeled PAA(py-PAA) as the fluorescence probe. It can be seen from Fig. 2 that polymer becomes more stretched when it adsorbs on alumina surface from the solution. Furthermore, when SDS is added first, the PAA coiling index is found to increase continuously with SDS dosage and this corresponds to a similar decrease in system turbidity as shown in Fig. 1. In contrast, for the case where P AA is added first, SDS has no effect on either the system turbidity or the PAA conformation at all SDS dosages. On the basis of the above measurements of suspension turbidity and polymer coiling index, it is proposed that when SDS is added first, the pre-adsorbed surfactant causes 'blocking' of the adsorption of the P AA added later, resulting in a smaller fraction of the polymer trains and thus a higher coiling index. However, when P AA is added first, SDS does not 'block' adsorption of P AA which could be either due to masking of the polymer chains or due to prevention of SDS Fig. 2. Polyacrylic acid (PAA) coiling index at alumina/water interface under the SDS/PAA and PAA/SDS addition modes as well as in solution in the presence of SDS. adsorption. Since a measure of the adsorption density of SDS in the presence of pre-adsorbed P AA (Fig. 3) shows SDS adsorption not to be affected by PAA, it can be concluded that when PAA is added first, the subsequent adsorption of SDS does not affect P AA conformation. Because i O.OxIO' S.Ox I 0-4 SOS concentration. I.OxIO.) M x1O Fig. I. Turbidity of alumina suspension for sodium dodecyl sulfate (SDS)fpolyacrylic acid (PAA) addition mode, PAAf SDS addition mode, and addition of SDS only ([PAA] = 10 ppm, (NaCl] = 0.03 M) Fig. 3. Adsorption isotherm of sodium dodecyl sulfate (SDS) on alumina in the presence of pre-adsorbed polyacrylic acid (PAA).

6 400 A. Fan el ai.1 Colloids and Surfaces A: Physicochem. Eng. Aspects 146 (1.999) PM $OS \ 1- concentration and deviate significantly from each other at P AA concentration below 0.5 ppm, i.e. SDS starts to have an effect on the system when the PAA concentration is less than 0.5 ppm. When the P AA concentration is very low, there are not enough chains available to mask SDS; on the other hand, at very high P AA concentrations, a significant fraction of adsorption sites on alumina are pre-occupied by the polymer and thus SDS adsorption density is reduced (Fig. 6). When P AA concentration is higher than 350 ppm, there is no adsorption of SDS Stirring time (8) (b) Fig. 4. Schematic illustration for (a) SDS firstfpaa second addition mode and (b) PAA firstjsds second addition mode. adsorbed SDS is very likely masked by the dangling polymer chains, the suspension stability is not affected also. A schematic representation of the adsorbed states for both addition modes are given in Fig. 4. Since P AA adsorption is irreversible while SDS adsorption is readily reversible, given enough stirring time during the flocculation process, addition sequence should not be expected to make any difference, i.e. both the P AA coiling index and the suspension turbidity for SDSfPAA addition sequence should approach those for PAA/SDS sequence. It can be seen from Fig. 7(a) that it takes about 30 min of stirring for the system to reach equilibrium with the SDSfPAA sequence. During this process, the SDS adsorption density is found 3.2. Effect of amount of pre-adsorbed polyacrylic acid (P AA) Being masked by the P AA chains, SDS species have essentially no effect on the behavior of the whole system. When PAA concentration is decreased, at some point it cannot be expected to be able to mask SDS completely and SDS will then begin to playa role. Fig. 5 shows the turbidity of alumina suspension for PAA alone and PAA/SDS addition mode with SDS concentration kept constant. It can be seen from this figure that the optimum flocculation concentration of P AA when used alone is around 0.5 ppm. This is also the minimum concentration required to mask SDS molecules completely. The two curves in Fig. 5 overlap when the P AA concentration is above this Fig. 5. Turbidity of alumina suspension as a function of polyacrylic acid (PAA) concentration in the absence and presence of sodium dodecyl sulfate (SDS) (PAA/SDS mode).

7 .4. F.. et ai.! Colloids and SIIrf«rs.4.: P/ty.ricochem. Eitg..4.rj cts 146 (1999) I.OK10.'.. B a O" '8 f 1.0&10' 3.5. Kinetics of SDS adsorption in the presence and absence of P AA Finally, it was observed that the SDS adsorption is facilitated by the presence of small amounts of PAA (Fig. 9). It is suggested that trace amounts of pre-adsorbed PAA can prevent ftoccujation caused by the SDS at certain SDS concentrations. This in turn can also prevent any retardation of adsorption due to inaccessibility of polymer into interior of ftocs if they form..oxio" 0 Fig. 6. Sodium dodecyl sulfate (SDS) adsorption density on alumina as a function of preadsorbed polyacrylic acid (PAA) ([SDS)initial = M). to remain constant (Fig. 7(b», indicating re-distribution of SDS instead of it being replaced, i.e. PAA relaxes and reoriented while SOS redistribute on alumina surface to comply with the polymer relaxation/reorientation IxIO' Effect of time interval between additions of P AA and SDS!/XX 1 AJthough SDS does comply with PAA relaxation as stirring proceeds, when SDS and PAA are added to alumina suspension at the same time, the system turbidity is more similar to that for SDS added alone (Fig. 8). This suggests that SDS adsorbs more rapidly and reversibly on alumina than the polymer. It can also be seen from Fig. 8 that the flocculation/dispersion behavior of alumina suspension is very sensitive to the addition sequence of the two species. When flocculation time is of a few minutes, the species added first is always playing a dominating role, no matter how smaj) the time interval between the two additions. f 900 MXI,.. soo 400 IxIO" I 10 x) fa} 70 Fig. 7. (a) Changes in turbidity SI8Ii8I 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 under SDS/pAA addition mode ([PAA) = 10 ppm; (SDS)initial M) f 8.. f I

8 402 A. Fan et a/./ Colloids and Surfaces A: Physicochem. Eng. Aspects /46 (1999) Fig. 8. The effect of time interval between sodium dodecyl sulfate (SOS) and polyacrytic acid (PAA) additions on alumina stability. 4. Summary I. These experiments show that the effect of a pair of non-interacting polymer and surfactant I jl I.OxlO' Oxl0-6 on the adsorption of each other and on flocculation of adsorbent can be marked, depending particularly on the addition sequence. 2. When SDS, the surfactant, was added first, the confonnation of the polymer, PAA, and the suspension stability were found to vary with SDS dosages. A desired stability level can thus be obtained by manipulating the SDS concentration. When P AA is added first, the SDS adsorbed subsequently is masked by the polymer and contributes neither to the PAA conformation nor to the suspension stability. For example, for the system studied, SDS adsorption density is not hampered by pre-adsorbed PAA as long as PAA concentration is not very high «50 ppm). Also, when the PAA concentration is less than 0.5 ppm (which was also the optimal flocculation concentration) SDS was no longer masked by P AA; under these conditions, SDS had a measurable effect on the system behavior. 3. Time of reagentizing had a significant effect on both the adsorption and flocculation. If the flocculation time is on the order of a few minutes, the species added first plays a predominant role independent of time interval between the two addition. When flocculation time was long, P AA showed a governing role. P AA adsorption is irreversible while SDS adsorption is readily reversible. Both the P AA coiling index and system turbidity for the SDSjPAA addition sequence approached those for the PAA/SDS addition sequence. In the fonder addition sequence, SDS is considered to redistribute itself to comply with P AA adsorption and its conformation. 4. A small amount of pre-adsorbed PAA can retard flocculation caused by SDS of certain concentrations and thereby accelerate the adsorption of SDS on alumina. I.OKIO' 0 10 IS 20 2S SI8IWI&l8IIe.mm. Fig. 9. The kinetics of sodium dodecyl sulfate (SDS) adsorption with and without preadsorbed PAA ([SDS)initial = M). Acknowledgements The authors thank Environmental Protection Agency (EPA R82330l-Ol-O) for the financial support.

9 A. Fan et af. / Col/oidf and Surfaces A: Physicochem. Eng. Aspects 146 (1999) References [I] J.E. Gebhardt, D.W. Fuerstenau, ACS Symp. Ser. 253 (1984) 291. [2] A. Hollander, M.S. thesis, Columbia University, New York, NY, (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.), Innovations in Materials Processing Using Aqueous, Colloid and Surface Chemistry, The Minerals Metals and Materials 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) [7) K.F. Tjipangandjara, P. Somasundaran, Colloids Surf. A 55 (1991) [8) J.T. Kunjappu, P. Sornasundaran, K. Sivadasan, Colloids Surf. A 97 (1995)