Journal of Colloid and Interface Science 298 (2006) 238–247www.elsevier.com/locate/jcis
Alumina interaction with AMPS–MPEG random copolymersIII. Effect of PEG segment length on adsorption, electrokinetic and
rheological behavior
H. Bouhamed a, A. Magnin b, S. Boufi a,∗
a Laboratoire Sciences des Matériaux et Environnement (LMSE), Faculté des Sciences de Sfax, BP 802-3018 Sfax, Tunisiab Laboratoire de Rhéologie, INPG, UJF Grenoble 1, UMR CNRS 5520, BP 53, 38041 Grenoble Cedex 9, France
Received 12 October 2005; accepted 1 December 2005
Colloidal suspensions, namely those with concentrationswhich exceed 20–30 vol% fractions, are of great practical inter-est and find applications in many fields such as personal care,food products, coatings, ceramics, agrochemical and pharma-ceutical formulations, etc. [1–6]. Throughout their preparation,processing and subsequent use, the colloidal particles have toremain separated from each other and not undergo aggrega-tion in order to preserve their long-term stability against sed-imentation and ensure the lowest viscosity at the highest solidconcentration [5]. Thanks to the great area contact between theparticles and the dispersion medium, long-range van der Waalsforces are relatively strong and must be balanced by other repul-
* Corresponding author.E-mail address: [email protected] (S. Boufi).
sive interactions to prevent the particles from coming too closeto each other to bring about irreversible aggregation [7–10].
In general there are two practical mechanisms for generatingrepulsive forces to stabilize colloidal dispersions: electrosta-tic and steric stabilization. In the former, particles are chargedby adsorption of ionic species, which increases the zeta po-tential and leads to electrical double-layer repulsion [7,10–12].This mode of stabilization is highly sensitive to salt concentra-tion, which controls the double-layer thickness and is effectivemainly in water [12]. Steric stabilization involves adsorbingpolymer layers on the particle surfaces [4,8] that bring aboutphysical barrier as the particles approach each other and thelayers begin to interpenetrate. Steric repulsion is generated asa result of two effects, an osmotic effect due to an increase inthe local concentration of the adsorbed species between the twoparticles, and an entropic effect because the interacting layersbegin to lose certain degrees of freedom due to crowding [4,8].In contrast to electrostatic stabilization, steric stabilization is
H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247 239
less sensitive to salt concentration and can be implemented inboth aqueous and organic media. However, for good steric sta-bilization, it is necessary for the adsorbed polymer layer toprotrude far into the solution, to a sufficient thickness wherevan der Waals attraction is not yet very effective [13].
In order to impart the desired stability to colloidal suspen-sion polymers namely polyelectrolyte are often added. Throughtheir ability to adsorb at a multitude of points on the particle sur-face they enhance stability through a combination of a purelyelectrostatic repulsion and a steric repulsion [14,15]. The rela-tive importance of the respective contributions is closely relatedto the segment density profile at the interface, which is deter-mined by many factors such as the pH, that control the signand density of the surface charge, the degree of dissociation,the conformation of the polymer at the interface and the ionicstrength [7,16,17].
Even though homopolymers are the ones most commonlyused as dispersants for colloidal dispersions, their stabilizationcapability can be enhanced by incorporating the macromolec-ular backbone of the ionic segment, which can adsorb stronglyon the particles, other segments that have a higher affinity forthe dispersion medium and protrude far into the solution. Thiscombined structure, known as a copolymer, is prone to adsorbin a conformation, which increases the thickness of the stericbarrier and will act both as an electrostatic and a steric sta-bilizer [18–23]. Among these copolymers, it is interesting toconsider comb-like copolymers, which have a main chain ofone type of monomers and a side chain grafted on to it consist-ing of another type of monomer units. So far, adsorption andstabilization by comb copolymers has received relatively littleattention in Refs. [18–23] compared to homopolymers or di-block copolymers [20–23].
In a previous work we investigated the adsorption propertiesof AMPS/MPEG comb copolymers on colloidal alumina parti-cles and the change in electrokinetic and rheological behaviorof the alumina suspensions in the presence of different copoly-mers [23,24]. Results showed that these copolymers adopt a flatcomb-like configuration at the solid–liquid interface, in whichthe ionic (SO−
3 ) ion groups anchor the polymer to the surfaceand PEG segments protrude from the surface toward the contin-uous medium. However, with PEG segment bearing 10 ethyleneoxide units their length is not enough to overcome van derWaals interaction. Therefore, their steric contribution to stabil-isation is relatively weak and intervene namely at high ionicstrength or at pH close to IEP. Thanks to the primordial role
of PEG segment on the stabilisation mechanism, we investigatein the present work the effect of the PEG length and the ion-ization level of these copolymers on the adsorption properties,electrokinetic and rheological behavior alumina suspensions.
2. Materials and methods
2.1. Starting materials
α-Al2O3 powder (P172-SB, Pechiney, France) was used inall the experiments. This powder is characterized by a meandiameter of 0.4 µm and a specific surface area (BET) of10 m2 g−1. Seven polyelectrolytes are used in this work. Six ofthem are copolymers of 2-acrylamido-2-methylpropanesulfonicacid sodium salt (AMPS) (AMPS-2405 from Lubrizol) andmethoxypolyethyleneglycol methacrylate (MPEG-550, MPEG-2080, and MPEG-5072 MA from Aldrich). The molecularweight of the PEG chains is about 465, 2000, and 5000, re-spectively, on MPEG monomers. The other is a homopolymerof acrylamido-2-methylpropanesulfonic acid sodium salt. Thepolymer structure is depicted in Fig. 1 and corresponding ab-breviations are shown in Table 1. All the polymers were pre-pared in our laboratory. The monomers AMPS, MPEG-550,and MPEG-2080 were used as received without further purifi-cation. However, MPEG-5072 was synthesized in our labora-tory by the reaction of methoxypolyethyleneglycol (5000) withmethacrylic anhydride in presence of anhydrous pyridine as asolvent.
2.2.1. Monomer2-Acrylamido-2-methylpropanesulfonic acid sodium salt
(AMPS) was obtained from 2-acrylamido-2-methylpropanesul-fonic acid by adding equivalent amounts of NaOH solution,
a Adsorbed amount at the saturation plateau of the isotherm.
240 H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247
methoxypolyethyleneglycol methacrylate, Mw = 550 (MPEG-550), and methoxypolyethyleneglycol methacrylate, Mw =2080 (MPEG-2080) purchased from Aldrich. Methoxypolyeth-yleneglycol methacrylate, Mw = 5080 (MPEG-5070) was pre-pared by reacting poly(ethyleneglycol) methylether (PEG)(Mw = 5080) with methacrylic anhydride (MA) in the pres-ence of 4-dimethyl amino pyridine as catalyst. The reactionwas conducted in anhydrous pyridine at 80 ◦C for 8 h in a ni-trogen atmosphere. The stoichiometric ratio MA/PEG was 4,and terbutylcathechol (1 wt%) was added to prevent any riskof monomer polymerization during the reaction. The monomerwas then recovered by precipitation in a mixture of acetone andcylohexane (50/50) and purified by recrystallization. The de-gree of purity was checked by 1H NMR analysis.
2.2.2. Copolymer synthesisAll the copolymerization reactions were carried out in
homogeneous systems using water and potassium persulfate(K2S2O8) as solvent and initiator, respectively, in a nitro-gen gas atmosphere. In order to restrict molecular mass andimpede any possible branching reaction, thioglycolic acid(HSCH2CH2COOH) was added as a chain-transfer agent. Thetwo monomers, AMPS and MPEG, were first weighed and thenadded as a solution to the water with a 30/70 (monomer/water)ratio. The solution was heated to 70 ◦C; then an aqueous so-lution of the initiator was gradually added during 30 min.Copolymerization was carried out at 70 ◦C for 3 h and thenat 80 ◦C for 30 min. The chain transfer agent in the aqueous so-lution was gradually added throughout polymerization. As thereaction progressed, the viscosity of the medium gradually in-creased. With this operating protocol, yields of about 80% wereachieved. The polymer was recovered by precipitation in a mix-ture of acetone and cyclohexane (90/10), dried in nitrogen, andthen dissolved in water.
2.3. Adsorption isotherm
Polymer adsorption experiments were performed by mixingpolymer solutions at different concentrations at the required pHinto a 10 wt% aqueous Al2O3 suspension. pH was adjustedusing a NaOH or HCl solution. Then the suspensions were son-icated for 15 min at an output power of 200 W and stirred for24 h to reach adsorption equilibrium. The suspensions werecentrifuged at 2500 rpm for 1 h. The supernatant was removedand the amount of free polyelectrolyte in the solution was de-termined by two methods:
(i) The colloid titration technique, using cationic PDMAC(polydiallyldimethylammonium chloride), Mn = 150,000,and orthotoluidine blue as an indicator. This method wasused for AMPS-100, AMPS-85 (550), and AMPS-85(2080). Details of the method are given elsewhere [25,26].
(ii) UV dosage: this procedure is based on the method devel-oped by Nuysink and Koopal for dosing polyethylene oxidesegments [27]. Its principle is related to the ability of poly-ethylene oxide to form a highly insoluble complex withphosphomolybdic acid. The complexing reagent is pre-
pared from 1 g of phosphomolybdic acid (H3Mo10PO32·24H2O), 1 g of BaCl2·2H2O, and 3 ml of concentratedHCl in 500 ml of distilled water. 5 ml of the complexingreagent is added to 5 ml of the supernatant solution con-taining AMPS–MPEG copolymer and gently shaken in acentrifuge tube. Several minutes afterward a precipitate isformed and separated by centrifugation. 1 ml of the super-natant was diluted with water to give 25 ml and absorbanceat 220 nm was determined using a Cecil 7000 UV–vis spec-trophotometer. The concentration of the AMPS–MPEGcopolymer could be determined using a calibration curvedetermined for AMPS–MPEG copolymer with a knownconcentration. The method of analyses is able to detectcopolymer over a concentration range from 10−5 up to10−3 mol/L with reference to PEG segment.
2.4. Measuring zeta potential
A zeta potential analyser (Malvern Zetasizer 5000) wasused to measure the zeta potential (ζ ) of the alumina particlesin the aqueous suspension by electrophoresis. Measurementswere conducted on a 0.05 wt% suspension which was pre-pared as follows: alumina was added to a solution containinga defined amount of polymer. The mixture was ultrasonicatedfor 5 min (output power 110 W); then pH was adjusted byaddition of a NaOH or HCl (0.1 N) solution. Unless statedotherwise, the measurements were conducted at a fixed ionicstrength (5×10−3). Prior to the measurements, the suspensionwas allowed to stand for 8 h, during which time they were me-chanically stirred. Averages of at least four measurements werereported for each sample.
2.5. Measuring size distribution
The dispersive power of a polyelectrolyte is thus determinedby measuring the particle size distribution of a suspension witha dry matter content of the order of 10−2 wt% (2.5×10−3 vol%)using Mastersizer 5000 (Malvern Instruments Ltd). The sus-pensions are prepared by adding alumina powder to a solutioncontaining a defined percentage of dispersant. The suspensionis then subjected to ultrasonic waves for 15 min in order to dis-integrate the powder, and is then magnetically stirred for 24 h toreach adsorption equilibrium. The pH of all the suspensions isbetween 8.5 and 9. The sample is then eluted through the mea-suring cell and kept continuously circulating by a centrifugepump.
2.6. Rheological measurements
Rheological measurements were carried out using a control-led-speed rotating rheometer (ARES-Rheometrics). A cone-plate configuration was adopted. The cone is hollowed in thecenter so as to adapt to the grain size distribution of the alu-mina. To avoid wall slip, the surface of the cone and plate wereroughened [28]. An anti-evaporation cell surrounded the coneto prevent solvent evaporation effects. All the measurementswere carried out at a constant temperature of 25 ± 0.5 ◦C.
H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247 241
To control possible thixotropy effects [23], the mechanicalhistory of the sample was checked by pre-shearing it at 102 s−1
for 1 s. Immediately afterward, the defined shear rate was ap-plied. The transient change in stress was recorded until steadyconditions were achieved. Above steady-state stress values, theflow curve was established for a wide range of shear rates(10−2–102 s−1).
3. Results
3.1. Polymer characterization
With the exception of the AMPS-100, all the polymers pre-pared and used in this work were statistical copolymers ofAMPS and MPEG monomers. Their general chemical struc-ture is given in Fig. 1, where (p/(p + m) × 100) represents theAMPS ionic monomer content, and q the degree of polymer-ization of the PEG segments in the MPEG. q has a value of 10,45, and 113 for the MPEG-550, MPEG-2080, and MPEG-5072,respectively. The characteristics of the various polymers (Mn,ionic content, and radius of gyration) are gathered in Table 1.Hereafter, the polymers are represented by the abbreviationAMPS-X (Y ), in which X is a value representing the AMPSionic monomer in the copolymer and Y corresponds to themolecular mass of the MPEG monomer used.
3.2. Adsorption isotherms
The first part of this study looks at the change in AMPS-Xadsorption as a function of the fraction and the length of PEGsegments within the side chains. An example of isotherms re-lating to the AMPS-X (2000) copolymers with different AMPScontents is shown in Fig. 2. The shape of the isotherms istypical of monolayer adsorption [29]; the adsorption increasesprogressively until it stabilizes above a certain concentration atan adsorption plateau. The lower the MPEG segment content,the higher this plateau is. However, this trend is not common
Fig. 2. Adsorption isotherms of AMPS-X (2080) on alumina P172-SB as afunction of polymer equilibrium concentration for three different ionization lev-els at pH 8.5–9.
to all the copolymers and seems to depend on the length ofthe PEG segments. Indeed, as indicated by the results given inTable 1, in the case of PEG segments bearing 10 ethylene ox-ide (EO) units, incorporating MPEG monomer above 15 mol%fraction enhances adsorption with respect to fully ionic polymerAMPS-100. In contrast, when the degree of polymerization ofPEG increases from 45 to 113, the adsorption plateau falls asthe MPEG content increases. The effect becomes very signifi-cant for MPEG-5072 with a 50% content; here the adsorptionplateau does not exceed 0.1 µmol m−2.
Analysis of Figs. 3a and 3b, representing the change in theadsorption isotherms of AMPS-85 and AMPS-50 copolymerscontaining respectively 15 and 50 mol% of MPEG monomers,with different PEG segment lengths, indicates that at a fixedionization level, i.e., a constant quantity of AMPS, the max-imum amount adsorbed decreases as the length of the PEGsegments increases. Likewise, when the DPn of PEG segmentexceeds 45 a significant decrease in adsorption affinity is notedas revealed by the decrease in the initial isotherms slope. Theseresults are probably associated with the copolymer adsorptionconfiguration at the particle surface. Indeed, it was shown previ-
(a)
(b)
Fig. 3. Adsorption isotherms of (a) AMPS-85 (550), (2080), and (5072), and(b) AMPS-50 (550), (2080), and (5072) on alumina P172-SB as a function ofpolymer equilibrium concentration at pH 8.5–9.
242 H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247
ously [23,24] that at pH 8.5–9 PEG segments do not contributeto adsorption owing to their inability to interact with the Al–OH groups on the particle surfaces. Adsorption is governedmainly by ionic electrostatic interactions of –SO−
3 groups withthe Al–OH+
2 surface sites. As a consequence, the PEG segmentsare strained to adopt a configuration perpendicular to the sur-face, or more precisely a coil-like configuration perpendicularto the surface. The coil configuration is the result of free rota-tion about σ bonds. The radius of the equivalent sphere may beassimilated to the radius of gyration of the PEG segments [30]:
(1)⟨R2
g
⟩1/2 = a√
n√6
√1 + cos(180 − θ)
1 − cos(180 − θ),
where n is the number of bonds in the chain backbone, a isthe bond length, and θ the bond angle. The radii of gyrationof the various segments and the surface area occupied by thechain were determined by referring to relation (1). The resultsare gathered in Table 2.
Assuming that adsorption has a comb-like configuration, thearea occupied by an adsorbed copolymer chain increases withthe MPEG monomer content and with the length of the PEGsegments. Therefore if we assume a monolayer adsorption, themaximum adsorbed amount is reduced. In the case of MPEG-550, it is likely that the length of the PEG segments is relativelysmall to produce steric effects which hamper adsorption.
3.2.1. Effect of pHFor all the copolymers studied a reduction in the pH of the
medium is accompanied by an increase in the amount adsorbedindependently of the MPEG monomer content and length ofthe PEG segments, as illustrated in Fig. 4. This phenomenon isin agreement with polyelectrolyte adsorption on charged sub-
Table 2Gyration radius and area of the PEG segment chains
Fig. 4. Adsorption isotherms of AMPS-85 (2080) on alumina P172-SB as afunction of suspension pH.
strates [31]. It is related to the increase in the density of positivesurface charges on which the –SO−
3 groups, which remain to-tally ionized in a pH range between 2 and 14, are adsorbed. Thecationic sites are generated by protonation of Al–OH groupsinto AlOH+
2 . The increases in positively charged sites enhancepolymer–surface interaction energy to compensate for steric re-pulsion within the PEG units.
3.2.2. Effect of ionic strengthThe effect of ionic strength on AMPS-X copolymer ad-
sorption was also studied. Apart from AMPS-85 (550), rais-ing initial ionic strength by adding KCl (in a range between0 and 10−2 M) does not significantly affect polymer adsorp-tion, as shown in Fig. 5. These results are in contrast to those
Fig. 5. Adsorption isotherms of AMPS-85 (2080) on alumina P172-SB as afunction of initial electrolyte (KCl) concentration at pH 8.5–9.
Fig. 6. Schematic illustration of the configuration of AMPS-X copolymer onalumina surface.
H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247 243
observed in the case of polyelectrolyte, where an increase inionic strength is often accompanied by a rise in the amount ofpolymer adsorbed [32], which is imputed to the reduction inelectrostatic repulsion between the adsorbed segments result-ing from the screening effect of the electrolyte ions.
From this study, it appears that the adsorption of AMPS–MPEG comb-like copolymers on colloidal alumina particles iscontrolled, on the one hand by electrostatic interaction betweenpositively charged surface sites and –SO−
3 groups of the AMPSmonomer, which act as anchoring agents and, on the other handby steric effect through the excluded volume among the PEGunits, which adopt a coil-like conformation protruding from thesurface. Fig. 6 schematically illustrates the copolymer configu-ration at the surface.
3.3. Electrokinetic properties
In the second part of our work electrokinetic behavior is in-vestigated by monitoring the change in zeta potential (ζ ) as afunction of the added polymer content and pH. These measure-
Fig. 7. Electrokinetic behavior of alumina P172-SB in presence of differentamounts of AMPS-50 (2080) polymer.
Fig. 8. Electrokinetic behavior of alumina P172-SB in presence of 1 wt% ofAMPS-100, AMPS-50 (550), (2080), and (5070) polymer.
ments are important as they partly confirm the assumption madeconcerning the mechanisms of copolymer interaction with thealumina surface, and subsequently are helpful in interpretingthe change in rheological behavior in the presence of AMPS-Xcopolymer. Even if ζ measurements are performed in a highlydiluted particle concentration, this does not affect the way theresults are interpreted, as the value of ζ roughly depends ondispersion concentration [2].
The IEP of P172-SB alumina particles are close to 9.2. At thenative pH of the suspension, which is around 8.5, the surfacealumina particles are mainly dominated by Al–OH and Al–OH+
2 species. With the addition of AMPS-X (MPEG) copoly-mer, the ionic –SO−
3 groups of the AMPS units are adsorbed onAl–OH+
2 sites and progressively neutralize the surface positivecharges, which explains the continuous shift in IEP toward acidpH values (Figs. 7 and 8). Above pH 9.5–10, ζ does not changesignificantly in the presence of AMPS-X (2080) and AMPS-X(5072). This is probably related to the poor adsorption capacityof these copolymers beyond a pH of 9.5, as previously indicated(Figs. 2 and 3).
Above a concentration corresponding to the appearance ofan adsorption plateau, ζ no longer changes when AMPS-Xcopolymer is added. At this stage, the maximum zeta poten-tial value (in absolute value) is determined both by the MPEGcontent in the chain and by the length of the PEG segments. In-deed, as shown in Figs. 7 and 8, the lower the MPEG monomercontent (Fig. 7), the higher the maximum zeta potential, andfor the same AMPS content, the maximum zeta potential in-creases in absolute value as the length of the PEG segmentsdecreases (Fig. 8). The first observation is fully expected andagrees with studies concerning polyelectrolyte adsorption oncharged surfaces [33]. Indeed, with the increase in AMPS con-tent, in which the ionic –SO−
3 group remains totally dissociatedregardless of the pH, positively charged surface sites (Al–OH+
2 )are progressively neutralized. Above a certain ionic content inthe copolymer, the negative charges from the polymer exceedthe positive surface charges, thus leading to a phenomenon ofovercompensation which causes an inversion in the ζ values.
Fig. 9. Change in ζ at pH 5 as a function of adsorbed –SO−3 groups for AMPS-
85 (550), (2080), (5072), and AMPS-50 (550), (2080), and (5070).
244 H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247
To analyse the effect of PEG segment length on ζ at the sameAMPS content, the change in ζ vs the adsorbed –SO−
3 , deter-mined by multiplying the amount of polymer adsorbed by thefraction of AMPS in the chain, was depicted in Fig. 9. A pHvalue of 5 was selected to enhance copolymer adsorption andbetter amplify the phenomena. A number of conclusions maybe drawn from this figure:
(i) With an adsorbed –SO−3 content below 1 µmol m−2, ζ
varies in the following order: AMPS-85 (550) > AMPS-50(550) > AMPS-85 (2080) > AMPS-85 (5072) > AMPS-50 (2080) > AMPS-50 (5072). Thus, with an equivalentamount of adsorbed negative charge, the longer the PEGsegments and the higher their content, the lower the ζ
values. This phenomenon is probably caused by the PEGsegments which shift the shear plane to greater distances,thus causing a further reduction in ζ .
(ii) In the case of copolymers derived from MPEG 2080 and5072 monomers, the maximum zeta potential correspond-ing to the saturation plateau at pH 5 does not exceed−10 mV, whereas it reaches −50 mV in the presence ofMPEG-550 monomer.
(iii) When the amount of polymer adsorbed reaches the satura-tion plateau, the maximum zeta potential changes as fol-lows: AMPS-50 (5072) > AMPS-50 (2080) > AMPS-85(2080) > AMPS-50 (550) > AMPS-85 (550). The valuesof ζ are −5, −8, −10.7, −25, and −46 mV for AMPS-50 (2000), AMPS-85 (5070), AMPS-85 (2080), AMPS-50(550), and AMPS-85 (550), respectively. This result indi-cates that the effect generated by shear plane shift dependsnot only on the length of the PEG segments in the chainbut also on their density at the surface.
It is clear therefore that the presence of PEG segments inthe copolymer causes a reduction in the absolute zeta potentialvalue after adsorption on the alumina particles. This reductionis a function of the number and length of the PEG segmentsin the copolymer. Thus, in the case of segments comprising113 EO units, the incorporation of 15 mol% of PEG segmentin the copolymer (AMPS-85 (5072)) leads to a reduction in ab-solute zeta potential value in a similar way to a copolymer with50 mol% PEG bearing 45 EO units (AMPS-50 (2080)).
3.4. Copolymer dispersal effect
To analyse the dispersing efficiency of the different copoly-mers prepared, the particle size distribution of a diluted aluminasuspension (2.5×10−3 vol%) in the presence of a polymer wasestablished using the light diffraction technique (Figs. 10a and10b). In the absence of any additive, the suspension displaystwo populations with sizes of 0.3 and 2 µm. The 2 µm particlesare the result of aggregation since at the native pH of aluminasuspension (close to 8.5), the ζ close to 10 mV is not suffi-ciently high to ensure electrostatic stabilization. At pH 4, thesuspension is formed from a single distribution centered around0.3 µm, which corresponds to the individual alumina particles.At this pH the particles have enough Al–OH+ positive charges
2
(a)
(b)
Fig. 10. Grain size distribution of alumina P172-SB in presence of (a) 0.35 wt%and (b) 1 wt% of AMPS-X copolymer.
(zeta potential around 60 mV) to ensure efficient electrosta-tic stabilization. In the presence of 0.35 wt% of AMPS-100,the suspension is thoroughly stable and well dispersed as ev-idenced by the low particle size. This stability results fromthe adsorption of the polymer, which imparts enough negativecharges (zeta potential around −50 mV) to ensure good elec-trostatic stabilization. In the presence of AMPS-X copolymers,the size distribution at 0.35 wt% displays, in addition to themain peak centered at 0.3 µm, a second peak of much lowerintensity around 30 µm, which disappears at 1% polymer con-tent (Fig. 10b). At this level the quantity of polymer adsorbedis enough to ensure good particle dispersion. The stabilizationmode differs depending on the structure of the copolymer. Byreferring to the electrokinetic study, it may be stated that inthe presence of AMPS-100 and AMPS-85 (550), stabilizationis dominated by electrostatic effects, as ζ reaches −60 mVin these conditions. In the presence of copolymers AMPS-50(550) and AMPS-85 (2080), stabilization involves a combina-tion of electrostatic and steric effects as ζ is situated between−30 and −20 mV. On the other hand, in the presence of AMPS-50 (2080), AMPS-85 (5072), and AMPS-50 (5072), stabiliza-tion is probably dominated by the steric contribution generatedby the PEG segments, which protrude from the surface to form a
H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247 245
Fig. 11. Viscosity vs shear rate for 30 vol% alumina P172-SB suspension at pH8.5–9 in the presence of different amounts of AMPS-85 (MPEG-5072).
sufficiently thick physical barrier to withstand the van der Waalsinter-particle attraction forces. Indeed, in the presence of thesecopolymers the zeta potential does not exceed −15 mV, whichis too low to bring about an electrostatic contribution.
3.5. Rheological behavior
Viscosity η and shear stress σ were measured at differ-ent shear rates γ̇ to analyse the effect of adding AMPS-Xcopolymers on the rheological behavior of alumina suspen-sions. Fig. 11 shows an example of flow curves η = f (γ̇ ) for asuspension of P172-S alumina at 30 vol% with various AMPS-85 (5070) copolymer contents. At very low shear rates, viscos-ity tends toward infinity and has a −1 slope in log–log scales.The corresponding shear stress then tends toward a constantvalue characteristic of a yield stress. The continuous decreasein viscosity with shear rate indicates a shear-thinning viscousbehavior.
In order to demonstrate more clearly the effect of addingAMPS-X copolymers on the viscosity of the suspension, weplotted the change in η at a constant shear rate of 1 s−1 vsthe amount of polymer added (Fig. 12). With all the polymersstudied, η at 1 s−1 decreases until an optimum is reached at aconcentration varying from 0.3 to 0.5 wt%. Above this, the vis-cosity remains practically unchanged in the presence of AMPS–MPEG copolymer with an MPEG content over 85%, while it in-creases again in the presence of AMPS-85 and AMPS-100. Theincrease is, however, distinctly greater in the case of AMPS-100. It is also observed that the higher the ionic content in thepolymer, the lower the optimum viscosity value. The optimumvalues of η at 1 s−1 and the corresponding polymer content forthe various polymers are collected in Table 3.
In the absence of polymer, the low surface charge of thealumina particles leads to particle aggregation, which justifiesthe relatively high viscosity values and plastic character of thesuspension. After adding AMPS-X polymer, the continuous de-crease in η is the consequence of polymer adsorption, whichreinforces the stabilization of the suspension by increasing therepulsion forces and therefore reduces particle aggregation. In
Fig. 12. Change in viscosity at shear rate 1 s−1 vs AMPS-X (2080) concentra-tion at pH 8.5–9 and 30 vol% alumina concentration.
Table 3Optimum viscosity at 1 s−1 and the corresponding amount of AMPS-X
view of the particular structure of the copolymers, the repul-sion forces include an electrostatic contribution from the AMPSionic units and a steric contribution resulting from the PEG seg-ments. The electrostatic contribution is all the more importantas the AMPS content is high. Likewise, the steric contributionmust depend not only on the length of the PEG segments butalso on their fraction in the copolymer. This aspect will be de-veloped in more detail in another publication, which will alsoexamine the effect of adding salt on the change in rheologicalbehavior in the presence of these different copolymers.
To confirm clearly the contribution of PEG segments to thesteric stabilization process, rheological measurements were per-formed in the presence of 0.8 wt% AMPS-50 (550), (2080),and (5072) at a pH value corresponding to the isoelectric point,so as to annul the electrostatic contribution to the stabilizationprocess. By referring to the electrokinetic study, the pH cor-responding to the PIE is between 4.5 and 5 depending on thepolymer. The measurements were performed as follows: the40.4 vol% alumina suspension was first brought to a pH 4.5–5by adding a solution of HCl. The necessary quantity of AMPS-50 was then added to the suspension. After stirring for 12 h, therheological measurements were performed. In addition, checkswere made beforehand to ensure that the zeta potential of theparticles was close to zero. The measurements were performedat a volume fraction of 40.4 vol% to obtain sufficiently high
246 H. Bouhamed et al. / Journal of Colloid and Interface Science 298 (2006) 238–247
Fig. 13. Viscosity vs shear rate for 40.5 vol% alumina suspension at a pH cor-responding to the IEP and in presence of 0.8 wt% AMPS-50 (550), (2080), and(5072).
viscosities to carry out the measurements using the same cone-plate measuring device.
The flow curves η = f (γ̇ ) for the various systems at the IEPare presented in Fig. 13. A plastic behavior, characterized bya yield point at low shear rates and a shear-thinning behaviorat high shear rates, was observed for all these systems. How-ever, in the presence of one of the AMPS-50 polymers, theviscosity at a given shear rate is much lower that in the caseof alumina alone. Thus, η at 1 s−1 is 1000 Pa s for aluminaat its IEP and 12, 0.65, and 2.6 Pa s in the presence of 0.8%,AMPS-50 (550), AMPS-50 (2080), and AMPS-50 (5070) re-spectively. The much lower values of η at the IEP in the pres-ence of AMPS-50 can only be imputed to the steric contributionof the PEG segments, as in these conditions there is no electro-static contribution. In addition, the decrease in η at 1 s−1 whenchanging from AMPS-50 (550) to AMPS-50 (5070) suggeststhat the longer the PEG segments, the greater the steric con-tribution. This result is in agreement with the general rules ofsteric stabilization [4].
4. Conclusions
The purpose of this work was to analyse the interactionof comb-like copolymers with colloidal alumina particles. Thecopolymers were prepared by radical polymerization of AMPSand MPEG containing PEG segments of different lengths.
Study of the adsorption isotherms reveals that the adsorp-tion is controlled on the one hand by electrostatic interactionbetween the positively charged surface sites and the negative–SO−
3 groups of AMPS monomer which act as anchoringagents and on the other hand by steric repulsion between thePEG units, which adopt a coil-like conformation protrudingfrom the surface.
The interaction of AMPS-X copolymers with P172-SBalumina particles leads to considerable changes in the elec-trokinetic properties of the particles. Independently of theAMPS/MPEG ratio, polymer adsorption is accompanied by ashift in the IEP toward acid pH values and may lead to a charge
inversion above a certain level. Likewise, it was shown that theparticular conformation of PEG segments on the particle sur-faces leads to a shift in the shear plane, which results in a majordecrease in the absolute zeta potential.
A particle size analysis and rheological analysis confirmedthe stabilizing effect of AMPS-X copolymers. The additionof copolymers to the alumina suspension is accompanied bya reduction in viscosity, which greatly depends on the AMPScontent in the polymer and the PEG segment length. The fall inviscosity is attributed to polymer adsorption at the particle sur-face, which generates repulsive forces that oppose the van derWaals attraction. These repulsive forces involve both an elec-trostatic contribution from the AMPS ionic units and a stericcontribution derived from the PEG segments. The longer thePEG segments, the greater the steric contribution.
Acknowledgment
We thank Pr. M. Milas for his help with GPC measurementand interpretation.
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