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Effect of polymer molecular weight on the absorption of polyacrylic acid at the alumina-water interface
!i~
COLLOII~ Coltoid~a~ds.~a~
ELSEVIER
SURFACES
A
A: Physicocheraicaland EngineeringAspects133(1998)157 163
Effect of polymermolecularweight on the adsorptionof polyacrylic acid at the a l u m i n a - w a t e r interface D . S a n t h i y a ~, G . N a n d i n i ~, S, S u b r a m a n i a n ~, K . A . N a t a r a j a n ~, S . G . M a l g h a n b., a Deparonent t~/'MetallurgX, b~dian brstittJte oJScience. Bungtdore 560 012. b~dia b Ceramics Divishm. Nathmal btstitute ¢~]'Shmdards and Tec]lllohJg); Gaithersburg. MA 20899. USA
Accepted 1 April 1997
Abstract The adsorption and electrokinetic characterislics of alumina suspensions in the presence of polyacrylic acid as a dispersant have been studied. The adsorption isotherms exhibit high-affinity Lanmnuirian behaviour. The adsorption density decreases with increasing in pH, while it increases with increasing molecular weight of the polymer. Electrokinetic studies indicate spc,cillc adsorption at and above the isoelcctric point of the alumina sample. Possible t~cchanisms of interaction bctwccn alumina and polyac~lic acid are discussed. ,~'~t998 Elsevier Science B.V. /~trwords: Adsorption: a-Alumina; Electrokinetic; Electrostatic and chemical interaction: Hydrogen bonding;
P !yacrylic acid ( PAAI
I. lntrnduction
Ceramics processing tcchnology is used 1o produce commercial products which are very diverse in size, shape, detail, complexily, slruclure, composition and cost. ,x-Alumina is widely used in ceramic industries for a wlriety of applications. In the development and production of the more adwmced ceramics, extr,',tordinary control of the materials and processing operations is a requisite to minimise microstructural defects. Recent successes in developing, producing and applying advanced ceramics in high-technology applications have kindled the interest in ceramic processing. Surface and interface phenomena play an important role in ~eramic processing because powder systems have a relatively high surt:ace-area to mass ratio, and adsorption and distribution of additive * Correspondingauthor. ()927-775798,,SI9.011~ 1998ElsevierScienceB.V.All rightsreserved. PII S0927-7757 ( 97)00132-5
pha~es on the surface may alter the microstruclure and processing behavior quite markedly. Additionally, it is imperative that the rheology of the suspension is controlled properly, and this is accomplished by the addition of dispersants. The adsorption of polyelectrolytes has been investigated with respect to several oxides and nitrides [ I-7]. In ceramic powder processing, inorganic surfactants, low molecular-weight fatty acids, non-ionic polyelectrolytes and ionic polymers are used as dispersants [8-13]. it has been reported that the conformation suitable for better dispersion can be manipulated by controlling the mode of adsorption of the polymers [ 14]. The factors responsible for polyacrylic acid adsorption and the overall stability behavior of 'x-Al20s. m-ZrO2 and their binary suspcns':ons have been cited as acid-base phenomena leading to charging of the oxide particulate surfaces and the polymer [15]. A new approach to increase the
158
D. SanthO'a ct aL / ('olloidx Surfitce~" A: Physic¢~chem. Ettg. A.spects 133 ( 10081 157 163
viscosity of colloidal slurries through coagulation in the presence of repulsive hydration-layer forming ions has been demonstrated by Velamakanni and co-workers [16]. The adsorption of polyacrylic acid (PAA) onto a!umina was found to be enhanced in the presence of calcium, and a critical ratio of calcium iov,s to PAA was found to govern the adsorption process [17]. On the basis of the zeta potential of the colloidal a-aluminapolyacrylic acid complexes, the evolution towards stability or instability has been predicted [18]. Studies on the interaction of soluble aluminium ions from alumina with polyacrylic acid have indicated that the ratio of carboxylic acid to dissolved aluminium ions affects the adsorption kinetics and colloid stability [19]. Recently, direct evidence has been obtained by in-situ ATR-FTIR spectroscopy for the interaction between tire carboxylate groups of citric acid and the surface aluminium(tn) atoms of alumina [20]. A stability map has been constructed based on adsorption and viscosity data as a function of pH for the processing of alumina suspensions using sodium salts of poly(methacrylic acid) by Cesarano and co-workers [21 ]. In this investigation, the adsorption and electrokinetics of alumina suspensions in the presence of polyacrylic acid have been studied. The efl;~ct of pH and the molecular weight of the polymer on the adsorption density have been assessed, and the adsorption isotherms have also been determined. Electrophoretic mobility measurements have been carried out as a function of pH. concentration and molecular w i g h t of the polymer.
2. Experimental 2. I. MuteriaL~"
:t-Alumina (A-11) was obtairtcd from the Aluminium Company of America. The specilic surface area of this sample by the multipoint BET method using nitrogen as the adsorbate was found to be 8.76 m-' g Polyacrylic acid (PAA] saruples of weightaveraged n'|olecular weights of 91) 000. 511000, 500(} and 20IX) were procured from Polysciences Inc,,
USA. All polymer dosages are represented in parts per million (ppm). All other reagents used in the experiments were of analytical reagent t A R ) grade. Deionised, double-distilled water of specific conductivity less than 1.5 ~)'cm was used in ull studies. 2.2. M e t h o d s
The adsorption test procedure was standardised as follows: I g of alumina sample was pulped to a volume of 50 ml using 10 3 M KNO~ anti me pH was adjusted to the desired value. The suspension was equilibrated for 24 h by agitation in a Remi orbital shaking incubator at 250 rpm at 27 _+0.4 C. This was followed by the addition of PAA of the desired molecular weight and concentration and preadjusted pH equivalent to that o[" the suspension pH, and ionic strength of 10 3 M KNO 3 such that the total volume was 100 ml, This was stirred for 2 h at 250 rpm and allowed to equilibrate tbr a further period of 22 h without agitution |bllowing tile method used by Gebhardt and Fuerstenau [ I ]. Subsequently, the suspension was centrifuged lit 5000rpm for 30rain and filtered through Whatman 42 lilter paper. The clear supernatant solution was analysed for the residual polymer concentration using Hyamine, based on tile proccddrc of Crummet and Hummel [221. Electrophoretic mobilities were measured with a micro-electrophoresis zeta mclcr model 3.0 m~mufactured by Zeta-Meter Inc.. New York, USA. Ionic strength was maintained at 10 -3 M using KNO 3. Tile PAA was added slowly to solutions containing 0.01% solids under agitated conditions. :rod allowed to equilibrate Ibr 3 h. pH adjustments were made with the help of analytical-grade HNO.~ and KOH. The sample was uhrasonicatcd for 3(Is in a sonicator bath before taking the measurements.
3. Results and discussion 3. I. .,Idvorption tests
Tile adsorption isotherms of polyacrylic acid of molecular weights 2000. 5iX)0, 5[)000 and 90 000
D. S nthi "a et ~ I. ' Colh, kLr 5)lr/ir'es tt." Physic, chem. Eng. tlspecls 133 (1998) 157--163
for alumina suspensions in the pH range 3-3.5 are depicted in Fig. 1. These tests were carried out at a background concentralion of 10 ~ M KNO~. It is readily apparent that all the isotherms show a steep rise in the adsorption density at lower concentrations of polyacrylic acid, and thereafter reach a saturation value. It is also evident that the adsorption density increases with increasing molecular weight of the polymer. For example, the adsorption density at saturation coverage tbr 90 000 M w is about 1.6 times greater than that at 2000 M W, aboul 1.4 limes that lbr 5000 M~ and close to 1.2 times greater than that for 50 000 M,. At pH 3 3.5, the isotherms are of the high affinity type and exhibit Langmuirian behaviour. The observation of high-affinity type adsorption has been reported by several workers [3,15,211.
The effect of pH on the adsorption density of polyacrylic acid of different molecular weights onto alumina is depicted in Fig. 2. it can be observed that the adsorption density decreases with increasing pH in all cases. Here again, as expected, the molecular-weight dependence on the adsorption density is similar to that observed in the ease of the isotherms. The adsorption density is reduced to near-zero values in the alkaline pH range at and above the PZC of the alum:'na sample, which is found to lie at p H 9 . 2 (Fig. 4). The surface charge of the alumina particles as well as the degree of ionisation of the polymer are affected by the variation in the pH of the suspension. The surface charge on oxides is generally considered to be controlled by the MOH_,+, MOH and M O sites at the surface, depending on the pH. The nature of the mineral surface, the presence of solutes and the functional group of the polymer
i ~
1.6
1.4
~-s...%
1.6 I-~
Initial ¢on~ntralion 200 PPM .
1.4 F-
90,00I) M w O 50,000 M w
% 1.2
1,2 I-~
m E~I.0 >, "2
1.01--
.8_
159
,~.
•
5,000 M w
1:3
2,000 Mw
0.8}--
K
153M KNO 3
x:} <
0.61--
pH 3.0-3.5 ~, 90.000 M w 0 50,Q00 Mw
0.,~
0.2
•
5,000 M ~
D
2,000 M w
0.41-"
0.21II
,
0
I
,
I
L
I
h
I
,
r
20 40 6o 80 100 Equili brium c o n c e n t r o 6 o n , mg I L
Fig I. Ads~,rptit.l isotherms of polyacrylic aCld of different
lrlol¢ctllarweighlson alumina.
0
2
4
6
pH
8
10
12
Fig. 2. Ell'colof ptl on the adsorption density of polyacrylic acid of dilt~:rcnlmolecularwdghls o, alumina.
160
D. Santhiya c1 aL
Colloid~ Su(faces A: Pllysu'ochem. Eng. A,~pects 133 ! 1998) 157 163
affect the adsorption behaviour. Such a dependence of the pH on the adsorption density has also been observed by other workers [1,2,23,24]. It is well known that the carboxyl group of polyacrylic acid can act as a proton donor or acceptor, and thus adsorption may take place by hydrogen bonding between surface MOH++ sites and the carboxyl groups [25]. In the acidic pH range studied, electrical interactions also enhance the adsorption density as the alumina is positively charged and the polyacrylic acid molecules are ionised. At alkaline pH values the adsorption of polyacrylic acid is prevented due to electrostatic repulsive forces as both polymer and alumina surface are negatively charged. However+ hydrogen bondir: could govern the adsorption process at basic pH. With respect to hydrogen bonding, the decrease in the number of hydrogen donor groups ( O H ~ ) with increasing pH decreases the amount of polymer adsorbed. The changes in the conformation of polyacrylic acid adsorbed on alumina as a function of pH have been elucidated by Tjipangandjara and co-workers [4]+ They have reported that at pH 4 polymer chains are coiled due to the fact that they are only very slightly ionised and do not exhibit intrapolymer electrostatic repulsion, while in the pH range above the PZC, electrostatic repulsions cause the polymer to acquire a stretched conformation [4]. Dissolution tests on alumina as a /+unction of pH have indicated the release of alunfinum ions at acidic pH and aluminate ions at alkaline pH [26]. Additionally+ coprecipitation tests in the simuhaneous presence of polyacrylic acid (PAA) and aluminium ions its a function of pH have confirmed PAA aluminum interaction [26]. Thus. at low pH the positively charged surface sites phty an important role in the adsorption process, being responsible for chemicul, electrostatic and hydrogenbonding tbrecs of interaction. The influence of the molecular weight of the polymer on the adsorption at saturation coverage at pH 3-3.5 for an initial concentration of the polymer of 200 ppm is highlighted in Fig. 3. These results complement the data reported in Fig. I and Fig. 2. It is well known that as the molecular weight of the polymer increases, the number of segments available Ibr adsorption also increases.
Initial concentration 21Xl PPM 10-3 M KNO 3 E
~1.5 "6 1.4
8 ~1.2
1,1
0
20 40 60 80 1(30 Molecular weight (1+,4w) • 103
Fig 3. Adsorptiondensityof pol.xacrylicacid tm alumirmm; a ftlnctionof the nlolectllar~.+eighlof the pol+',mcr.
High molecular-weight polymers generaUy provide better stabilization for a given system than a low molecuhtr-weight polymer of the same composition. In a related sense, the adsorption of a high molecular-weight polymer will be irreversible. Although each polymer segment may be adsorbed reversibly, since many segments of a given chitin will be adsorbed at any given instant the probability of all adsorbed segments of that chain being desorbcd simuhaneously will be extremely small. ]+his is not the case lbr low molecular-weight fractions, in which there are only a few points tbr attachment. When particles with an adsorbed polymer of a low molecular weight approach, hydrodynamic lbrces may lead to the dcsorption of the protecting molecules, decreasing its protective power, while high molecular-weight polymers will not be displaced so easily [27]. The physic,chemical aspects of polymer adsorption Ibr a number of systems have been pointed out by several workers [28 31]. The increase in adsorption density with
D Sonth(va et aL / Colloids Surfilces A: Physicochem. Eng./Ispects 133 (1998) 157-163
increasing molecular weight has similarly been reported for various systems [2,28,30,32]. 3.2. E l e ctro kin etic s t u d i e s
The electrophoretic mobilities of the alumina particles as a function of pH and polymer concentration for different molecular weights of polyacrylic acid are portrayed in Fig. 4, The isoelectric point (IEP) of the alumina sample is tbund to be located at pH 9.2. On the addition of 0.1 ppm of polyacrylic acid of different molecular weights, the IEP of the alumina-polyacrylic acid system is shifted towards acidic pH values in proportion to the molecular weight of the polymer. In other words, 0.1 ppm of 90000 M,,. polymer results in the maximum shift of the IEP, while the 2000 M w polymer causes the minimum shift for
the same concentration. It is interesting to note that on the addition of I ppm of polymer, the trend with respect to the molecular weight is more or less maintained up to about pH 5. Beyond pH 5, irrespective of the molecular weight of the polymer, at I ppm concentration the electrophoretie mobilities are almost constant up to pH 10. In the pH range of 5 to about 10, at l ppm polymer dosage the alumina particles are quite stable, as attested to by the higher negative mobilities in this pH range. At this polymer dosage, the polymer-coated alumina particles possibly assume the charge characteristic of the polyacrylic acid at the respective pH values. These results are in good agreement with those reported earlier [I,33-35]. Around pH 3 4.5 the polymer is almost uncharged, and addition of 0. I ppm of PAA of different molecular weights leads to good adsorption, resulting in a PAA[M w)
AIz,03( A -11} 10"3M KNO3
4.0
0.~ ppm 1.Oppm 0 2c00 • ,~. 5000 • 0 90,030 • O 90,000 • ~ NO PAA
V.. 3,2
x~...,. ~ ' " ; X ,
\\\\
E
~ 0~.~
:~
0
_
3
\
4
sh,
161
q,',,, _7..
8 \
\
g', ~o
~-1
"4.(3 Fig. 4. Efti:clor pit, mokx:ularweightand com:c,anaionof polyacrylicat:id on the declrophorclicmobilili,.:sof alumina.
16?2
D. Samhoa et al. : Colhnils Sto?lJlce~ .4: Phrshwchcm. Eng. Aspects 133 (19~R) 157 163
decrease in the positive electrophoretic mobilities in proportion to the molecular weight. The less positive value for particles with a higher molecular weight is due to the slipping plane being displaced fimher from the surface. The negative mobilities observed at and above the PZC may be attributed to specific adsorption. In this pH range, it is apparent that non-electrostatic forces are governing the adsorption process.
weight of the polymer, consequent to higher adsorption. The A6"~pvalues calculated are indicative of strong interaction of alumina with polyacrylic acid. Based on the restdts obtained in this study, both hydrogen and electrostatic attraction have been postulated to govern the adsorption process. Recent cxperimems have provided evidence in favour of chemical interaction [I 9.26].
3.3. Adsorption mechanisms 4. Conclusions The specific free energies of adsorption for the alumina-polyacrylic acid systems of different molecular weights are summarized in Table I, following the explicit relationship between the characteristic shift in the isoelectric point (IEP) and the concentration of polymer provided by Pradip [36]: ApHn~n,= 1.0396C, exp(-AG~,~w'RT
( I)
where ApHu~p is the shift in thc isoelectric point at the dispersion conecntration ('o, and - A 6 ~ , reprcscnts the corresponding specific encrgy of interaction between the ceramic powder surface and the dispersant. R and T are the standard gas constant and the temperature in K, respectively. From Table I it is evident that the free energy of adsorption increases with increasing moh.x:ular Table 1 Spccilic fi'cc energies of adsorption lilr die alttmina PAA s}slcnl System
(~, ( p p m l
ApH.~.
Alunlina + PAA 121100 M~}
alo
(}r711
060 Alumina + P A A ( 5f)ll[I M~)
Alunlina + PAA ( 50 I}(lll M~ }
Alumina + PAA 1911 (lllll M~ )
A(~,'~lRTttllilsl 16.41 +0.30
I00
5911 6"~5
Ib 75±0.51 . 16.31 ±0,23
(}.I0
3,30
- ISS7 ! 11.'~11
0.20 O,N) 11115
5,511 1it11 I.~0
18.67 • 0 Sl} - 1773 t 1123 - 211.~111:t ().4q
0 IO 11,20 (LOg
360 5.40 I,:;I)
21,21) ± 0.13 - 211.911~ 11,50 21.611_.11,42
(I. 11) II.2 0
4.2/) 5.40
--
22,0~) :~ 11.IO 21.60 ± 11.43
Based on this investigation, tile Ibllowing conc[usions can be drawn. ( 1 ) Tile adsorption isotherms of r,olyacrylic acid ( PAA ) onto alumina at acidic pH exhibit highaltinity Langmuirian behaviour for all the molecular weights studied. (2) The adsorption density d~x:rel,;cs with increasing pH due to electrostatic forces of repulsion between the alumina surldce and the polymeric groups in the alkaline pH range. (3) The adsorption density increases with increasing molccuhtr weight of the polymer. (4) Electrokinetic studies show that the electrophoretic mobilities of the alumina particles become more negative following polymer adsorption. At and above the PZC. the negative mobilities are attributed to specific adsorption. 15) The shift in the PZC of the ahtmina PAA system increases proportionally to the increase in the molecular weight of the polymer. 16) The specific free energy of adsorption wits found to increase with increasing molecular weight of Ihe polymer, attesting to higher adsorption.
Acknowledgment The authors gratefully acknowledge the financial support for this research programme granted under the Indo US collaborative scheme coordinated by the Department of Science and Technology. New Delhi. India.
D. S(mtlffva ct t:L ; ('rJlloitL~ Su(];u'es A: PIo'sicochem, Eng. A,~p(,cts 133 (1998) 157-163
References [ I ] J , E Gcbhardt, D.W, Fuerstenau. Colloids Surfitces 7 ( 19831 221. [2] A. Foissy, A. El Attar. J.M. Lamart:he, J. Colloid Interface Sci. 96 119831 275. [31J. Drzymahl, D.W. l:uerstenau, in: Y.A Atria (Ed.L Fh)cculation in Biot¢chnoh)gy and Separation Systems. Elsevier, Amsterdam. 1987, pp. 45 60. [4] K.F. Tjipangandjara, Yi Bin Huang, P. Somasundaran. N.J. Turro. Colloids Surhlces 44 ( 1990l 229. [5] E. Ringenbat:h. G. Chau~cleau, E. Peffi:rkorn. J. Colloid [ntt:rlace St:i, 161 { 1993) 223. [6] S.(i Malghan, Colloids Surfilces 62 ~1992) g7 [7] A.R. Bowt:rs. C.P. [-hllnlg, J. Colhfid Intcrl~lce Sci. 1115 ( 19851 197. [81 S, Sum[to, W.E. Rhiuc, FLK. Bowen. J. Am. Ceram. Soc. 74 119911 2189. [91 K. Esumi. T. Nagahama, K Meguro, Colloid Polym. Sci. 269 11991 ) 1274. [111] K. Chou, L Lee. J. AI}I. Ct:ram. Soc. 72 (1989) 1622. [11] A. Bleier. C,G, Weslnloreland. J. Am, ('eraln. Soc. 74 11991 ) 3100, [ 12} A. Bleier. Colloids Surfaces h6 110921 157. [131 A. Bleit:r, P.F. Beet:her. K.B. Alexandt:r. C.G. Westmoreland. J. Am, Ceram Sot:. 75 110921 2649. [ 141 A. Takahashi, Polym. J. 23 i 1991 ) 715. [15]A. Blcit:r. C.G. Westmoreland. in: Y.A. Atlia, BM. Moadgil. S. Chander {Eds.), Intcrlhcial Phcnome~a in Biotechnology and Malerials Prot:essing, Elscvit:r. Amsterdam. 1988, pp. 217 2~6. 1161 BN. Vel.mmkanni. J.C. Chang. I:.l:. Lange. I t S Pearson, Langmuir 6 (lOt/t1) 1323. 1171 L. Dupont. A. Foissy, R. Mt:rcicr. B. Moiler, J, Colloid Interface Sci, 161 t 19931 455. [181 E, Ringcnhach. G. ('hauxclcau. E Pcll~:rkorn. J. Colloid Interlace Sci. 172 ( 19951 203.
163
[19] E. Ringenbach, G. Chauveteau. E. PeB'erkorn, Colloids Surfaces A: Physicochem. Eng. Aspects 99 ([995) 161. [20] P.C. Hidbcr, TJ. Grau[e, L.I. Gaucklcr, J. Am. Ceram. Soc. 79 119961 1857. [21 ] J. Cesarano, |.A. Aksay, A. B[eier, J. Am. Ceram. Soc. 71 119881 250. [22] WB. Crummett, RA. Hummel, J. Am. Water Works Assoc. 55 ( 1963} 209. [23] N.V. Sastry. J.-M. Scquaris, M.J. Schwuger, J. Colloid Interfat:e Sci. 171 (19951 224. [24] M R . Bohmer, Y. El Attar Soft, A. Eoissy, J. Colloid Interlhce St:i. 164 (1994[ 126. [25] S,N. Vinogradov. R.H. Linnt:lL Hydrogen Bonding. van Nostrand Reinhold, New York. 1971. [26lS. Subramanian, D. ~mthiya. SB. Shanbhag, K.A. Natarajau, S.G. Malghan, in: S.P. Mehrotra, Rally Shekhar ( Eds, I, Mineral Processing: Recent Advances and Future Trends, Allied Publishers. New Delhi, [995, pp. 4 5 52. [27] D. Myers. Surfaces. Inlt:rfaccs and Colloids Principles and Applicalions. VCH, New York, 1991. [2Sl W.F. Linke, R.B. Booth. Trans. AMIE 217 { 19591 _'~4. [29] M E . McCarty. R.S. Olsen. Mining Eng. II { 1959l 61. [30] S.F. Kuzkin. W.P. Nebera. S N . Zolm. in: N. Arbiter (Ed.). Proce~xlings of the Seventh International Minerals Processing Congress. Gordon :~nd Breach, New York, 1965, pp. 347 357. [311 HW. Walker. S.B. Grant. L Colloid InterFace Sci. 179 ( 19961 552. 1321 J. Blaakmeer, M R . Bohmer. M.A. Cohen Stuark G.J. Fleer. Macromoleculcs 23 119911) 23OI. [33] G.C. Sresty. P. Somasundaran. Int. J. Miner. Process. 6 119801 303. [34] ll.M. Rics, Nature 226 ( 19701 72. [35] R S Pradip. A. Prt:machandran. S.G. Malghan. Bull. Mater. St:i. 17 ( 19941 911 136] R.S. Pradip. Trans. Indian Inst. Met. 41 ( 198g[ 15.
COLLOII~ Coltoid~a~ds.~a~
ELSEVIER
SURFACES
A
A: Physicocheraicaland EngineeringAspects133(1998)157 163
Effect of polymermolecularweight on the adsorptionof polyacrylic acid at the a l u m i n a - w a t e r interface D . S a n t h i y a ~, G . N a n d i n i ~, S, S u b r a m a n i a n ~, K . A . N a t a r a j a n ~, S . G . M a l g h a n b., a Deparonent t~/'MetallurgX, b~dian brstittJte oJScience. Bungtdore 560 012. b~dia b Ceramics Divishm. Nathmal btstitute ¢~]'Shmdards and Tec]lllohJg); Gaithersburg. MA 20899. USA
Accepted 1 April 1997
Abstract The adsorption and electrokinetic characterislics of alumina suspensions in the presence of polyacrylic acid as a dispersant have been studied. The adsorption isotherms exhibit high-affinity Lanmnuirian behaviour. The adsorption density decreases with increasing in pH, while it increases with increasing molecular weight of the polymer. Electrokinetic studies indicate spc,cillc adsorption at and above the isoelcctric point of the alumina sample. Possible t~cchanisms of interaction bctwccn alumina and polyac~lic acid are discussed. ,~'~t998 Elsevier Science B.V. /~trwords: Adsorption: a-Alumina; Electrokinetic; Electrostatic and chemical interaction: Hydrogen bonding;
P !yacrylic acid ( PAAI
I. lntrnduction
Ceramics processing tcchnology is used 1o produce commercial products which are very diverse in size, shape, detail, complexily, slruclure, composition and cost. ,x-Alumina is widely used in ceramic industries for a wlriety of applications. In the development and production of the more adwmced ceramics, extr,',tordinary control of the materials and processing operations is a requisite to minimise microstructural defects. Recent successes in developing, producing and applying advanced ceramics in high-technology applications have kindled the interest in ceramic processing. Surface and interface phenomena play an important role in ~eramic processing because powder systems have a relatively high surt:ace-area to mass ratio, and adsorption and distribution of additive * Correspondingauthor. ()927-775798,,SI9.011~ 1998ElsevierScienceB.V.All rightsreserved. PII S0927-7757 ( 97)00132-5
pha~es on the surface may alter the microstruclure and processing behavior quite markedly. Additionally, it is imperative that the rheology of the suspension is controlled properly, and this is accomplished by the addition of dispersants. The adsorption of polyelectrolytes has been investigated with respect to several oxides and nitrides [ I-7]. In ceramic powder processing, inorganic surfactants, low molecular-weight fatty acids, non-ionic polyelectrolytes and ionic polymers are used as dispersants [8-13]. it has been reported that the conformation suitable for better dispersion can be manipulated by controlling the mode of adsorption of the polymers [ 14]. The factors responsible for polyacrylic acid adsorption and the overall stability behavior of 'x-Al20s. m-ZrO2 and their binary suspcns':ons have been cited as acid-base phenomena leading to charging of the oxide particulate surfaces and the polymer [15]. A new approach to increase the
158
D. SanthO'a ct aL / ('olloidx Surfitce~" A: Physic¢~chem. Ettg. A.spects 133 ( 10081 157 163
viscosity of colloidal slurries through coagulation in the presence of repulsive hydration-layer forming ions has been demonstrated by Velamakanni and co-workers [16]. The adsorption of polyacrylic acid (PAA) onto a!umina was found to be enhanced in the presence of calcium, and a critical ratio of calcium iov,s to PAA was found to govern the adsorption process [17]. On the basis of the zeta potential of the colloidal a-aluminapolyacrylic acid complexes, the evolution towards stability or instability has been predicted [18]. Studies on the interaction of soluble aluminium ions from alumina with polyacrylic acid have indicated that the ratio of carboxylic acid to dissolved aluminium ions affects the adsorption kinetics and colloid stability [19]. Recently, direct evidence has been obtained by in-situ ATR-FTIR spectroscopy for the interaction between tire carboxylate groups of citric acid and the surface aluminium(tn) atoms of alumina [20]. A stability map has been constructed based on adsorption and viscosity data as a function of pH for the processing of alumina suspensions using sodium salts of poly(methacrylic acid) by Cesarano and co-workers [21 ]. In this investigation, the adsorption and electrokinetics of alumina suspensions in the presence of polyacrylic acid have been studied. The efl;~ct of pH and the molecular weight of the polymer on the adsorption density have been assessed, and the adsorption isotherms have also been determined. Electrophoretic mobility measurements have been carried out as a function of pH. concentration and molecular w i g h t of the polymer.
2. Experimental 2. I. MuteriaL~"
:t-Alumina (A-11) was obtairtcd from the Aluminium Company of America. The specilic surface area of this sample by the multipoint BET method using nitrogen as the adsorbate was found to be 8.76 m-' g Polyacrylic acid (PAA] saruples of weightaveraged n'|olecular weights of 91) 000. 511000, 500(} and 20IX) were procured from Polysciences Inc,,
USA. All polymer dosages are represented in parts per million (ppm). All other reagents used in the experiments were of analytical reagent t A R ) grade. Deionised, double-distilled water of specific conductivity less than 1.5 ~)'cm was used in ull studies. 2.2. M e t h o d s
The adsorption test procedure was standardised as follows: I g of alumina sample was pulped to a volume of 50 ml using 10 3 M KNO~ anti me pH was adjusted to the desired value. The suspension was equilibrated for 24 h by agitation in a Remi orbital shaking incubator at 250 rpm at 27 _+0.4 C. This was followed by the addition of PAA of the desired molecular weight and concentration and preadjusted pH equivalent to that o[" the suspension pH, and ionic strength of 10 3 M KNO 3 such that the total volume was 100 ml, This was stirred for 2 h at 250 rpm and allowed to equilibrate tbr a further period of 22 h without agitution |bllowing tile method used by Gebhardt and Fuerstenau [ I ]. Subsequently, the suspension was centrifuged lit 5000rpm for 30rain and filtered through Whatman 42 lilter paper. The clear supernatant solution was analysed for the residual polymer concentration using Hyamine, based on tile proccddrc of Crummet and Hummel [221. Electrophoretic mobilities were measured with a micro-electrophoresis zeta mclcr model 3.0 m~mufactured by Zeta-Meter Inc.. New York, USA. Ionic strength was maintained at 10 -3 M using KNO 3. Tile PAA was added slowly to solutions containing 0.01% solids under agitated conditions. :rod allowed to equilibrate Ibr 3 h. pH adjustments were made with the help of analytical-grade HNO.~ and KOH. The sample was uhrasonicatcd for 3(Is in a sonicator bath before taking the measurements.
3. Results and discussion 3. I. .,Idvorption tests
Tile adsorption isotherms of polyacrylic acid of molecular weights 2000. 5iX)0, 5[)000 and 90 000
D. S nthi "a et ~ I. ' Colh, kLr 5)lr/ir'es tt." Physic, chem. Eng. tlspecls 133 (1998) 157--163
for alumina suspensions in the pH range 3-3.5 are depicted in Fig. 1. These tests were carried out at a background concentralion of 10 ~ M KNO~. It is readily apparent that all the isotherms show a steep rise in the adsorption density at lower concentrations of polyacrylic acid, and thereafter reach a saturation value. It is also evident that the adsorption density increases with increasing molecular weight of the polymer. For example, the adsorption density at saturation coverage tbr 90 000 M w is about 1.6 times greater than that at 2000 M W, aboul 1.4 limes that lbr 5000 M~ and close to 1.2 times greater than that for 50 000 M,. At pH 3 3.5, the isotherms are of the high affinity type and exhibit Langmuirian behaviour. The observation of high-affinity type adsorption has been reported by several workers [3,15,211.
The effect of pH on the adsorption density of polyacrylic acid of different molecular weights onto alumina is depicted in Fig. 2. it can be observed that the adsorption density decreases with increasing pH in all cases. Here again, as expected, the molecular-weight dependence on the adsorption density is similar to that observed in the ease of the isotherms. The adsorption density is reduced to near-zero values in the alkaline pH range at and above the PZC of the alum:'na sample, which is found to lie at p H 9 . 2 (Fig. 4). The surface charge of the alumina particles as well as the degree of ionisation of the polymer are affected by the variation in the pH of the suspension. The surface charge on oxides is generally considered to be controlled by the MOH_,+, MOH and M O sites at the surface, depending on the pH. The nature of the mineral surface, the presence of solutes and the functional group of the polymer
i ~
1.6
1.4
~-s...%
1.6 I-~
Initial ¢on~ntralion 200 PPM .
1.4 F-
90,00I) M w O 50,000 M w
% 1.2
1,2 I-~
m E~I.0 >, "2
1.01--
.8_
159
,~.
•
5,000 M w
1:3
2,000 Mw
0.8}--
K
153M KNO 3
x:} <
0.61--
pH 3.0-3.5 ~, 90.000 M w 0 50,Q00 Mw
0.,~
0.2
•
5,000 M ~
D
2,000 M w
0.41-"
0.21II
,
0
I
,
I
L
I
h
I
,
r
20 40 6o 80 100 Equili brium c o n c e n t r o 6 o n , mg I L
Fig I. Ads~,rptit.l isotherms of polyacrylic aCld of different
lrlol¢ctllarweighlson alumina.
0
2
4
6
pH
8
10
12
Fig. 2. Ell'colof ptl on the adsorption density of polyacrylic acid of dilt~:rcnlmolecularwdghls o, alumina.
160
D. Santhiya c1 aL
Colloid~ Su(faces A: Pllysu'ochem. Eng. A,~pects 133 ! 1998) 157 163
affect the adsorption behaviour. Such a dependence of the pH on the adsorption density has also been observed by other workers [1,2,23,24]. It is well known that the carboxyl group of polyacrylic acid can act as a proton donor or acceptor, and thus adsorption may take place by hydrogen bonding between surface MOH++ sites and the carboxyl groups [25]. In the acidic pH range studied, electrical interactions also enhance the adsorption density as the alumina is positively charged and the polyacrylic acid molecules are ionised. At alkaline pH values the adsorption of polyacrylic acid is prevented due to electrostatic repulsive forces as both polymer and alumina surface are negatively charged. However+ hydrogen bondir: could govern the adsorption process at basic pH. With respect to hydrogen bonding, the decrease in the number of hydrogen donor groups ( O H ~ ) with increasing pH decreases the amount of polymer adsorbed. The changes in the conformation of polyacrylic acid adsorbed on alumina as a function of pH have been elucidated by Tjipangandjara and co-workers [4]+ They have reported that at pH 4 polymer chains are coiled due to the fact that they are only very slightly ionised and do not exhibit intrapolymer electrostatic repulsion, while in the pH range above the PZC, electrostatic repulsions cause the polymer to acquire a stretched conformation [4]. Dissolution tests on alumina as a /+unction of pH have indicated the release of alunfinum ions at acidic pH and aluminate ions at alkaline pH [26]. Additionally+ coprecipitation tests in the simuhaneous presence of polyacrylic acid (PAA) and aluminium ions its a function of pH have confirmed PAA aluminum interaction [26]. Thus. at low pH the positively charged surface sites phty an important role in the adsorption process, being responsible for chemicul, electrostatic and hydrogenbonding tbrecs of interaction. The influence of the molecular weight of the polymer on the adsorption at saturation coverage at pH 3-3.5 for an initial concentration of the polymer of 200 ppm is highlighted in Fig. 3. These results complement the data reported in Fig. I and Fig. 2. It is well known that as the molecular weight of the polymer increases, the number of segments available Ibr adsorption also increases.
Initial concentration 21Xl PPM 10-3 M KNO 3 E
~1.5 "6 1.4
8 ~1.2
1,1
0
20 40 60 80 1(30 Molecular weight (1+,4w) • 103
Fig 3. Adsorptiondensityof pol.xacrylicacid tm alumirmm; a ftlnctionof the nlolectllar~.+eighlof the pol+',mcr.
High molecular-weight polymers generaUy provide better stabilization for a given system than a low molecuhtr-weight polymer of the same composition. In a related sense, the adsorption of a high molecular-weight polymer will be irreversible. Although each polymer segment may be adsorbed reversibly, since many segments of a given chitin will be adsorbed at any given instant the probability of all adsorbed segments of that chain being desorbcd simuhaneously will be extremely small. ]+his is not the case lbr low molecular-weight fractions, in which there are only a few points tbr attachment. When particles with an adsorbed polymer of a low molecular weight approach, hydrodynamic lbrces may lead to the dcsorption of the protecting molecules, decreasing its protective power, while high molecular-weight polymers will not be displaced so easily [27]. The physic,chemical aspects of polymer adsorption Ibr a number of systems have been pointed out by several workers [28 31]. The increase in adsorption density with
D Sonth(va et aL / Colloids Surfilces A: Physicochem. Eng./Ispects 133 (1998) 157-163
increasing molecular weight has similarly been reported for various systems [2,28,30,32]. 3.2. E l e ctro kin etic s t u d i e s
The electrophoretic mobilities of the alumina particles as a function of pH and polymer concentration for different molecular weights of polyacrylic acid are portrayed in Fig. 4, The isoelectric point (IEP) of the alumina sample is tbund to be located at pH 9.2. On the addition of 0.1 ppm of polyacrylic acid of different molecular weights, the IEP of the alumina-polyacrylic acid system is shifted towards acidic pH values in proportion to the molecular weight of the polymer. In other words, 0.1 ppm of 90000 M,,. polymer results in the maximum shift of the IEP, while the 2000 M w polymer causes the minimum shift for
the same concentration. It is interesting to note that on the addition of I ppm of polymer, the trend with respect to the molecular weight is more or less maintained up to about pH 5. Beyond pH 5, irrespective of the molecular weight of the polymer, at I ppm concentration the electrophoretie mobilities are almost constant up to pH 10. In the pH range of 5 to about 10, at l ppm polymer dosage the alumina particles are quite stable, as attested to by the higher negative mobilities in this pH range. At this polymer dosage, the polymer-coated alumina particles possibly assume the charge characteristic of the polyacrylic acid at the respective pH values. These results are in good agreement with those reported earlier [I,33-35]. Around pH 3 4.5 the polymer is almost uncharged, and addition of 0. I ppm of PAA of different molecular weights leads to good adsorption, resulting in a PAA[M w)
AIz,03( A -11} 10"3M KNO3
4.0
0.~ ppm 1.Oppm 0 2c00 • ,~. 5000 • 0 90,030 • O 90,000 • ~ NO PAA
V.. 3,2
x~...,. ~ ' " ; X ,
\\\\
E
~ 0~.~
:~
0
_
3
\
4
sh,
161
q,',,, _7..
8 \
\
g', ~o
~-1
"4.(3 Fig. 4. Efti:clor pit, mokx:ularweightand com:c,anaionof polyacrylicat:id on the declrophorclicmobilili,.:sof alumina.
16?2
D. Samhoa et al. : Colhnils Sto?lJlce~ .4: Phrshwchcm. Eng. Aspects 133 (19~R) 157 163
decrease in the positive electrophoretic mobilities in proportion to the molecular weight. The less positive value for particles with a higher molecular weight is due to the slipping plane being displaced fimher from the surface. The negative mobilities observed at and above the PZC may be attributed to specific adsorption. In this pH range, it is apparent that non-electrostatic forces are governing the adsorption process.
weight of the polymer, consequent to higher adsorption. The A6"~pvalues calculated are indicative of strong interaction of alumina with polyacrylic acid. Based on the restdts obtained in this study, both hydrogen and electrostatic attraction have been postulated to govern the adsorption process. Recent cxperimems have provided evidence in favour of chemical interaction [I 9.26].
3.3. Adsorption mechanisms 4. Conclusions The specific free energies of adsorption for the alumina-polyacrylic acid systems of different molecular weights are summarized in Table I, following the explicit relationship between the characteristic shift in the isoelectric point (IEP) and the concentration of polymer provided by Pradip [36]: ApHn~n,= 1.0396C, exp(-AG~,~w'RT
( I)
where ApHu~p is the shift in thc isoelectric point at the dispersion conecntration ('o, and - A 6 ~ , reprcscnts the corresponding specific encrgy of interaction between the ceramic powder surface and the dispersant. R and T are the standard gas constant and the temperature in K, respectively. From Table I it is evident that the free energy of adsorption increases with increasing moh.x:ular Table 1 Spccilic fi'cc energies of adsorption lilr die alttmina PAA s}slcnl System
(~, ( p p m l
ApH.~.
Alunlina + PAA 121100 M~}
alo
(}r711
060 Alumina + P A A ( 5f)ll[I M~)
Alunlina + PAA ( 50 I}(lll M~ }
Alumina + PAA 1911 (lllll M~ )
A(~,'~lRTttllilsl 16.41 +0.30
I00
5911 6"~5
Ib 75±0.51 . 16.31 ±0,23
(}.I0
3,30
- ISS7 ! 11.'~11
0.20 O,N) 11115
5,511 1it11 I.~0
18.67 • 0 Sl} - 1773 t 1123 - 211.~111:t ().4q
0 IO 11,20 (LOg
360 5.40 I,:;I)
21,21) ± 0.13 - 211.911~ 11,50 21.611_.11,42
(I. 11) II.2 0
4.2/) 5.40
--
22,0~) :~ 11.IO 21.60 ± 11.43
Based on this investigation, tile Ibllowing conc[usions can be drawn. ( 1 ) Tile adsorption isotherms of r,olyacrylic acid ( PAA ) onto alumina at acidic pH exhibit highaltinity Langmuirian behaviour for all the molecular weights studied. (2) The adsorption density d~x:rel,;cs with increasing pH due to electrostatic forces of repulsion between the alumina surldce and the polymeric groups in the alkaline pH range. (3) The adsorption density increases with increasing molccuhtr weight of the polymer. (4) Electrokinetic studies show that the electrophoretic mobilities of the alumina particles become more negative following polymer adsorption. At and above the PZC. the negative mobilities are attributed to specific adsorption. 15) The shift in the PZC of the ahtmina PAA system increases proportionally to the increase in the molecular weight of the polymer. 16) The specific free energy of adsorption wits found to increase with increasing molecular weight of Ihe polymer, attesting to higher adsorption.
Acknowledgment The authors gratefully acknowledge the financial support for this research programme granted under the Indo US collaborative scheme coordinated by the Department of Science and Technology. New Delhi. India.
D. S(mtlffva ct t:L ; ('rJlloitL~ Su(];u'es A: PIo'sicochem, Eng. A,~p(,cts 133 (1998) 157-163
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163
[19] E. Ringenbach, G. Chauveteau. E. PeB'erkorn, Colloids Surfaces A: Physicochem. Eng. Aspects 99 ([995) 161. [20] P.C. Hidbcr, TJ. Grau[e, L.I. Gaucklcr, J. Am. Ceram. Soc. 79 119961 1857. [21 ] J. Cesarano, |.A. Aksay, A. B[eier, J. Am. Ceram. Soc. 71 119881 250. [22] WB. Crummett, RA. Hummel, J. Am. Water Works Assoc. 55 ( 1963} 209. [23] N.V. Sastry. J.-M. Scquaris, M.J. Schwuger, J. Colloid Interfat:e Sci. 171 (19951 224. [24] M R . Bohmer, Y. El Attar Soft, A. Eoissy, J. Colloid Interlhce St:i. 164 (1994[ 126. [25] S,N. Vinogradov. R.H. Linnt:lL Hydrogen Bonding. van Nostrand Reinhold, New York. 1971. [26lS. Subramanian, D. ~mthiya. SB. Shanbhag, K.A. Natarajau, S.G. Malghan, in: S.P. Mehrotra, Rally Shekhar ( Eds, I, Mineral Processing: Recent Advances and Future Trends, Allied Publishers. New Delhi, [995, pp. 4 5 52. [27] D. Myers. Surfaces. Inlt:rfaccs and Colloids Principles and Applicalions. VCH, New York, 1991. [2Sl W.F. Linke, R.B. Booth. Trans. AMIE 217 { 19591 _'~4. [29] M E . McCarty. R.S. Olsen. Mining Eng. II { 1959l 61. [30] S.F. Kuzkin. W.P. Nebera. S N . Zolm. in: N. Arbiter (Ed.). Proce~xlings of the Seventh International Minerals Processing Congress. Gordon :~nd Breach, New York, 1965, pp. 347 357. [311 HW. Walker. S.B. Grant. L Colloid InterFace Sci. 179 ( 19961 552. 1321 J. Blaakmeer, M R . Bohmer. M.A. Cohen Stuark G.J. Fleer. Macromoleculcs 23 119911) 23OI. [33] G.C. Sresty. P. Somasundaran. Int. J. Miner. Process. 6 119801 303. [34] ll.M. Rics, Nature 226 ( 19701 72. [35] R S Pradip. A. Prt:machandran. S.G. Malghan. Bull. Mater. St:i. 17 ( 19941 911 136] R.S. Pradip. Trans. Indian Inst. Met. 41 ( 198g[ 15.