Efficiency of methanol induced flocculation of AgI hydrosol stabilized by polyacrylamide

Colloids and Surfaces

A: Physicochemical and Engineering Aspects 194 (2001) 13–23

Efficiency of methanol induced flocculation of AgI hydrosolstabilized by polyacrylamide

Waldemar Nowicki *, Graz� yna NowickaDepartment of Physical Chemistry, Adam Mickiewicz Uni�ersity, Grunwaldzka 6, 60-780 Poznan, Poland

Received 15 June 2000; accepted 7 December 2000

Abstract

AgI hydrosol stabilized by polyacrylamide (PAAm) adsorbed from the dispersion medium was used in the studies.The efficiency of methanol induced flocculation of the system was examined at different polymer and electrolyteconcentrations. Properties of PAAm in water–methanol–electrolyte mixtures were also examined using nephelometricand viscometric methods. Moreover, the solvent power of the medium for PAAm was concluded from experimentson the swelling of PAAm gel in media of different composition. Properties of AgI particles in the absence and in thepresence of PAAm were deduced from their electrophoretic mobilities. The results obtained demonstrated thecomplex behavior of dispersions stabilized by polymer adsorbed from the solution. The important effect of freepolymer concentration in the continuous phase on the dispersion properties was underlined. In particular, its effecton the reduction of efficiency of non-solvent induced flocculation is discussed. © 2001 Elsevier Science B.V. All rightsreserved.

Keywords: Colloids; Stability; Polymers; Mixed solvent; Nonsolvent

www.elsevier.com/locate/colsurfa

1. Introduction

Stability of sterically stabilized dispersions canbe modified by the change of solvency of a stabi-lizing polymer in the dispersion medium. With thedeterioration of the solvent quality, attractiveforces between polymer chains, together withLondon–van der Waals forces between particlecores, lead to the flocculation of suspension.

Polymer solvency may be modified through a

temperature change or addition of the nonsolvent.In practice the solvency of stabilizing polymers isoften modified by the nonsolvent addition. Exam-ples of this procedure can be found in paint, inkand ceramic industries. Solvent induced floccula-tion of sterically stabilized dispersions is a re-versible process, i.e. spontaneous redispersionoccurs if the solvency of stabilizing polymer in thedispersion medium is improved. This reversibilitymight be interesting for easy transportation ofsuspensions.

In practice, the stabilizing polymer often isadded directly to the dispersion medium of asuspension. In this case only a fraction of the

* Corresponding author. Tel.: +48-61-8699181, ext. 457;fax: +48-61-8658008.

E-mail address: [email protected] (W. Nowicki).

0927-7757/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.

PII: S0927-7757(00)00829-3

W. Nowicki, G. Nowicka / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 13–2314

total amount of the polymer introduced is ad-sorbed onto colloidal particles and the non-ad-sorbed residue can interfere the suspensionproperties [1]. Although the effect of the stabiliz-ing polymer solvency on the dispersion stabilityhas been studied extensively [2,3], there are onlyscarce reports dealing with mixed-solvent situa-tion [4–9]. Moreover, the majority of studies wereperformed in the absence of a non-adsorbed poly-mer or its possible effect on the colloid systembehavior was not discussed.

The objective of this study was to determine theeffect of methanol addition on the flocculationefficiency of silver iodide hydrosol stabilized bypolyacrylamide (PAAm). The colloidal AgI is oneof the classical model systems from colloid chem-istry (for general properties see review [10]).PAAm is among water-soluble polymers one ofthe most technologically important and widelyused in processes of stabilization and destabiliza-tion of suspensions. PAAm can precipitate fromaqueous solution by the addition of methanol[11–13]. According to Zhu and Napper [14] it is aresult of the cononsolvency effect that arises fromH-bonding between the alcohol and watermolecules.

The experiments on the methanol induced floc-culation of PAAm-stabilized AgI hydrosol werecarried out at different contents of the stabilizingpolymer in the system. Moreover, since in polarmedia, in addition to steric effects, electrostaticinteractions also affect the suspension stability,the studies were performed at various ionicstrengths of the dispersion medium. The proper-ties of PAAm in water–methanol–electrolytemixtures were also examined and some results arereported. The effect of methanol on the stabilityof AgI hydrosol at different ionic strengths of thedispersion medium is described elsewhere [15–17].

2. Materials and methods

2.1. Materials

PAAm (BDH) with molecular weight �5×106

was used. No further purification was employed.The degree of PAAm hydrolysis, determined on

the basis of elemental analysis data, was 0.00�0.01.

Negative silver iodide hydrosol was obtainedusing a condensation method as described in [15].Mean AgI particle diameter was 50 nm. Thepotassium nitrate, as an indifferent electrolyte forAgI sol, was used for the adjustment of ionicstrength of the medium. The lowest KNO3 con-centration needed to induce the rapid coagulationof the sol, cKNO3

cc , determined accordingly to themethod described in [18], was equal to 0.118 moledm−3.

In order to obtain PAAm gel, to 100 cm3 ofaqueous solution containing 12.1 g of acrylamideand 2.14 g of N,N �-methylene-diacrylamide, atfirst 5 cm3 of 3% aqueous solution of ammoniumpersulfate and then 195 cm3 of water were intro-duced upon stirring. Next 0.25 cm3 ofN �,N �,N �,N �-tetramethylene-di-amine was addedand the solution was poured into glass pipes of 10mm diameter. After completion of polymerisationthe gel was squeezed out of the pipes and cut intopieces of about 5 g each. Thus prepared gelsamples were washed in flowing water for a fewdays (conductivity of water in a washing compart-ment was controlled), then some of them weredried at 80°C to constant weight and the otherswere left in water. The chemicals used for thepreparation of PAAm gel were purchased fromAldrich.

2.2. Methods

2.2.1. Cloud-point determinationTo series of water–methanol–KNO3 mixtures

of various composition aqueous PAAm solutionwas introduced. PAAm concentration in the sam-ple, cPAAm, was constant and equal to 1.25 gdm−3. After 24 h storage in the dark at 20°C, theturbidity R of samples was measured. Nephelo-metric measurements were carried out at 420 nmusing spectrocolorimeter Spekol (Carl-Zeiss, Ger-many) provided with an FK attachment. PAAmdissolved in 50% aqueous methanol containingthe same KNO3 concentration, cKNO3

, as the sam-ple examined was used as a reference. The volumefraction of methanol corresponding to the inter-section point of tangents to steep and non-steep

W. Nowicki, G. Nowicka / Colloids and Surfaces A: Physicochem. Eng. Aspects 194 (2001) 13–23 15

parts of the curve describing the turbidity of thepolymer solution vs the volume fraction ofmethanol, xMeOH, was assumed as the cloud-pointcomposition, xMeOH

cp .The dependence between the composition of

PAAm–water–methanol–KNO3 mixtures andtheir turbidities was also determined at cPAAm=0.312 g dm−3. In these experiments, after 24 hstorage in the dark, samples were centrifuged for20 min at 2000 rpm and then the supernatantturbidity R was measured. A solid fluorescentstandard was applied as a turbidity reference.

2.2.2. ViscosityThe viscometric measurements were performed

at 25.00�0.01°C using an Ubbelohde viscometerand temperature controller 650 H (Unipan,Poland). Couette–Hagenbach correction was ap-plied. Shear corrections were neglected.

2.2.3. Swelling degreeThe dried PAAm gel samples were weighed,

immersed in 700 cm3 of water–methanol–KNO3

mixtures and allowed to swell to equilibrium.Once the equilibrium was reached the gels wereremoved from vials, blotted with a tissue to re-move surface solvents and then weighed again.The degree of swelling, Q, was defined as:

Q=Wsw−Wp

Wp

, (1)

where Wp and Wsw are the weights of the dry andswollen samples, respectively.

The fact of reaching the equilibrium was judgedfrom parallel experiments in which non-driedPAAm gel samples were equilibrated in water–methanol–electrolyte mixtures. We assumed thatthe equilibrium was reached if Q values obtainedfor dried and non-dried samples equilibrated inmixtures of identical composition were the same.

Swelling experiments were performed at 20°C.

2.2.4. Flocculation measurementsAn aqueous PAAm solution and AgI hydrosol

were mixed in appropriate proportions and thenleft overnight in the dark at room temperature.The polymer concentration in the system neces-sary to produce the stabilizing effect towards the

examined AgI sol was evaluated on the basis ofresults of our earlier studies on PAAm adsorptiononto AgI particles [19]. Subsequently, to a givenvolume of polymer stabilized hydrosol, at firstaqueous KNO3 solution of an appropriate con-centration and then methanol or water were intro-duced. After 24 h storage in the dark sampleswere centrifuged for 20 min at 2000 rpm. Thenthe supernatant was 50 times diluted with waterand nephelometric measurements were performed.The flocculation efficiency, F, was calculated fromEq. (2):

F=1−R

Rref

, (2)

where R and Rref denote turbidities measured forsamples with and without methanol, respectively.Other constituents in both samples were the same.The dilution of samples ensured the sol concentra-tion being in the range of linear dependence be-tween AgI content and R.

The effect of medium composition on the floc-culation efficiency of PAAm stabilized AgI hydro-sol was studied at two different concentrations ofthe polymer. A lower concentration, cPAAm=0.078 g dm−3, corresponded to the polymer con-centration at which a plateau at the isotherm ofadsorption of PAAm on AgI particles in aqueousmedium begins [19], and cPAAm=0.312 g dm−3

was taken as the other polymer concentration.Studies of the reversibility of the methanol in-

duced flocculation were performed in a similarway as described above. The only difference wasthe omission of the sample centrifugation beforedilution with water.

Some flocculation experiments were performedfor PAAm stabilized AgI hydrosol, whose thedispersion medium was depleted of non-adsorbedpolymer. To remove the free polymer, the PAAmstabilized AgI hydrosol, after the overnight incu-bation, was centrifuged at 2000 rpm for 10 minand next the supernatant was replaced with water.Redispersion of the sediment was achieved byultrasonication (5 min, ultrasonic desintegratorUP-11, Techpan, Poland). In order to take intoaccount a possible effect of centrifugation andultrasonication on the system properties, a com-parable flocculation test was performed on the


PAAm–AgI dispersion subjected to the sametreatment except the water substitution for thesupernatant.

All the flocculation studies were performed at20°C.

2.2.5. ElectrophoresisElectrophoretic mobilities were measured with

the aid of Laser Zee dzetameter, model 501 (PenKem, Inc., USA). The samples were prepared inthe same way as for the flocculation reversibilitytest and then appropriately diluted with the wa-ter–methanol–KNO3 mixture of the same com-position as in the dispersion medium.

The effective electrophoretic mobilities, ueff,were calculated from Eq. (3):

ueff=6��e�kT

u, (3)

where � is the viscosity coefficient, e denotes theelementary charge, � dielectric constant, T is abso-lute temperature and u is measured elec-trophoretic mobility.

3. Results and discussion

The determined values of xMeOHcp were practi-

cally independent of the electrolyte content in thesystem and equal to 0.42 Fig. 1(a). The reportedcompositions of water–methanol mixtures corre-sponding to �-solvent of PAAm at room temper-ature range between 0.37 and 0.46 volumefraction of methanol [12,13,20,21]. Moreover, Zhuand Napper [14] found that the coil-to-globuletransition of interfacial PAAm chains in water–methanol mixtures starts to occur aroundxMeOH=0.45. No effect of the salt concentrationon the cloud-point curve parameters was alsoobserved in the PAAm–water–acetone–NaClsystem by Klenina and Lebedeva [21]. Accordingto the authors the lack of dependence of thecloud-point on the electrolyte concentration indi-cates that the electrolyte effect consists in thesuppression of a kinetic barrier.

The effect of water–methanol–KNO3 mixturecomposition on the size of PAAm coils was exam-ined viscometrically. The volume fraction � occu-

pied by coils in the solution was calculated usingthe Guth and Simha [22] equation. Fig. 2 showsthe dependencies between xMeOH and �/�o ratiosat various cKNO3

values; �o denotes the � valueobtained for PAAm dissolved in water. As ex-pected, the size of PAAm coils diminishes with theincreasing methanol and electrolyte content in thesystem. The observed effect of cKNO3

on the �/�o

ratio may result from the competition betweenpolymer and electrolyte for water molecules [14]or from the screening of electrostatic interactionsbetween charged groups on PAAm chains [23,24].Hydrolysis of some amide groups into carboxylicmoieties is possible [25].

The equilibrium swelling degree can be alsoused as a measure of solvent–polymer interac-tions [26]. Fig. 3 presents plots of Q vs xMeOH,determined at various cKNO3

. As expected, withincreasing methanol content in mixture the extentof PAAm gel swelling decreases. However, theelectrolyte concentration practically does not af-fect Q values. The lack of dependence of Q oncKNO3

, which should be attributed to the non-ioniccharacter of the PAAm gel examined [27], seemsto be somewhat contradictory to the observedinfluence of cKNO3

on the size of PAAm coils insolution. Thus, the conclusion can be drawn thatthe dependence of �/�o on cKNO3

may be relatedto the presence of electric charges on PAAmchains.

The effect of methanol concentration on theflocculation efficiency of PAAm stabilized AgI solis presented in Fig. 1(c) and (d). They show thedependencies between F and xMeOH determined attwo different concentrations of PAAm in the dis-persion medium of various ionic strengths. Theeffect of electrolyte concentration on the F=f(xMeOH) dependence for the polymer-free AgI solis also presented Fig. 1(e). According to Lauzonand Matijevic [28] the AgI sol destabilization isessentially governed by the adsorption ofmethanol on the particle surface.

Fig. 4 shows the results of the test of reversibil-ity of the methanol induced flocculation of PAAmstabilized sols. The test has shown that after theappropriate dilution with water the same values ofR were obtained for samples in which flocculationtook place and for the reference samples. As


indicated in Fig. 4, the reversibility was ascer-tained in the whole range of xMeOH andcKNO3

investigated.According to the theories for dispersion stabil-

ity in polar media both steric and electrostaticrepulsive interactions can provide the mechanismfor the stabilization [2]. Their contributions to thetotal interaction energy between particles are af-fected by changes in the composition of the dis-persion medium. The steric interaction isinfluenced mainly by the polymer solvencywhereas the electrostatic interaction depends on

the ionic strength and the dielectric constant ofthe medium. In the system examined an additionof methanol brings about deterioration of PAAmsolvency and a decrease in the dielectric constantof the medium, whereas an increase in the KNO3

concentration reduces, in the first place, the elec-trostatic interaction. Thus, roughly speaking, anincrease in xMeOH involves a decrease in bothsteric and electrostatic interactions, whereas theincrease in cKNO3

mainly screens electrostatic in-teractions. In both cases the flocculation shouldbe promoted. The complex shapes of F vs xMeOH

Fig. 1. Turbidity of PAAm solutions (a, b) and flocculation efficiency (c–e) versus electrolyte concentration and methanol volumefraction: (a) cPAAm=1.25 g dm−3; (b) cPAAm=0.312 g dm−3; (c) cPAAm=0.312 g dm−3; (d) cPAAm=0.078 g dm−3; (e) nopolymer added.


Fig. 2. Relative PAAm volume fraction vs methanol volume fraction.

plots indicate, however, that there are some addi-tional factors influencing the stability of AgI sols.The presence of free polymer in the continuousphase can be considered as a complicating factor.On worsening the polymer solvency the amountof the adsorbed polymer increases [29,30] and inbad solvent conditions even formation of multi-layers can occur [31,32].

The electrophoretic methods are sometimesused in studying the properties of the interfacialregion in the presence of adsorbed neutral poly-mers [33–36]. The methods, however, yield onlyqualitative information [37]. Therefore, we haveperformed a comparative study of the effect ofPAAm on the electrophoretic behavior of AgIparticles as a function of xMeOH and cKNO3

. Theelectrical double layer properties of AgI particlesin the whole range of compositions of water–methanol mixtures were exhaustively discussed byLevine et al. [17]. Moreover, De Roy et al. [14]found that the addition of electrolytes to thesesols results mainly in the compression of theelectric double layer.

The effective electrophoretic mobilities, ueff, ofAgI particles were measured in the absence and inthe presence of PAAm in two concentrations:

0.078 and 0.312 g dm−3. The results obtained arepresented in Fig. 5. The reduction of �ueff� due tothe presence of PAAm is observed at all themedium compositions examined. The decrease inthe electrophoretic mobility of particles caused bythe adsorption of uncharged polymers can beattributed to the shift of the shear plane outwardswith respect to its position in the absence of

Fig. 3. The dependence of the equilibrium swelling degree ofthe PAAm gel on methanol volume fraction at different KNO3

concentrations.


Fig. 4. Results of the reversibility test-turbidity of the stabi-lized AgI hydrosol after the dilution with water vs methanolvolume fraction at different KNO3 concentrations.

indicates that, as expected from the adsorptionisotherms [19], the saturation of the particle sur-face with the polymer was achieved already atcPAAm=0.078 g dm−3. However, at higher alco-hol and electrolyte concentrations the larger de-crease in �ueff� was observed for the higher contentof the polymer. This result may indicate thatadditional polymer adsorption took place and itsextent depended on the polymer concentration inthe dispersion medium.

Zhu and Napper [14] observed that PAAmchains, quenched to the latex particles suspendedin aqueous methanol, collapsed gradually whenxMeOH was increased. In their experiment the un-quenched polymer was removed from the system.However, it is well recognised from both theoreti-cal and experimental studies that there is a con-comitant tendency to increase the adsorbedamount of polymer and decrease the adsorbedlayer thickness on worsening of the solvent qual-ity [29]. The higher polymer concentration in thecontinuous phase the larger increase in the ad-sorbed amount with the deterioration of solventquality can be expected. This increase masks, atleast in part, a contraction of the adsorbed layer.Thus, the higher value of �ueff� measured atcPAAm=0.078 g dm−3 in comparison with thatdetermined at cPAAm=0.312 g dm−3 seems toreflect the higher adsorbed amount of PAAm; theshear plane is situated further away from theparticle surface.

On approaching the dispersion medium compo-sition corresponding to the bad solvent of poly-mer the attractive segment-segment interactionstarts to operate causing a considerable increasein the segment density of adsorbed layer. More-over, chains attached to one colloidal particlebegin to interact attractively with those attachedto another particle. If the dispersion was stabi-lized by the steric mechanism alone, the floccula-tion would be observed. However, in the mixturesof water with low alcohols AgI sols are stabilizedby the electrostatic mechanism in the whole rangeof compositions, as it was shown by De Roy et al[16]. As it is seen from Fig. 1(c) and (d) inKNO3-free medium the effective flocculation ofPAAm stabilized AgI sols cannot be obtainedeven at methanol concentrations considerably ex-

polymers [33–36] provided that, however, the vol-ume occupied by the polymer segments in thediffuse part of the double layer is so small that theadsorbed polymer does not alter the charge distri-bution in the diffuse double layer.

Polymer adsorption is generally poor fromgood solvent conditions and is likely to give lowsegment densities in the adsorbed layer; con-versely, high coverage densities are likely underpoor solvent conditions. In systems studied thelow segment density in the adsorbed layer can beassumed for xMeOH�0.42, i.e. where good solventconditions for the polymer are ensured. As seenfrom Fig. 5 in this range of methanol volumefractions a decrease in �ueff� values due to thepresence of PAAm occurred in the whole range ofexamined ionic strengths. The reduction of �ueff�evidences the presence of adsorbed layers ofPAAm on AgI particles. In these good solventconditions the adsorbed polymer produced theeffective steric mechanism for the stabilization ofAgI sols, even at cKNO3

very close to cKNO3

cc , asresults from Fig. 1(c) and (d).

In media of low ionic strength and methanolcontent the ueff values were only slightly depen-dent on cPAAm. As the extend of the reduction of�ueff�, due to the presence of polymer, may in thefirst approximation be considered as a relativemeasure of the thickness of the adsorption layer,no effect of the polymer concentration on ueff


ceeding the limit of polymer solvency, xMeOHcp .

This seems to evidence, although indirectly, thatthere is the electrostatic repulsion sufficient tooffset the attraction energy between particles.Baran et al. [38] reported that even when high-molecular-weight non-ionic polymers are used asstabilizers of dispersions in polar media the elec-trostatic contribution to the interaction energycan be decisive for the dispersion stability pro-vided that the ionic strength is low.

The increase in the ionic strength of themedium reduces the electrostatic repulsion andhence it should be expected to improve the floccu-lation efficiency. Indeed, as it results from Fig.1(c) and (d), the effective flocculation can be

obtained at high electrolyte concentrations. How-ever, the obtained dependencies between F andxMeOH are very complex. At some ionic strengths,for instance, the efficient flocculation aroundxMeOH

cp is followed, upon a further increase inxMeOH, by a minimum in flocculation efficiency.There are also dependencies F vs xMeOH without adistinct minimum, and with the efficient floccula-tion observed only at xMeOH considerably exceed-ing xMeOH

cp . The shape of the plots depends bothon cPAAm and cKNO3

.First, let us analyse the effect of KNO3 and

PAAm content on xMeOH value corresponding tothe onset of flocculation. As results from Fig. 1(c)and (d), if cPAAm is kept constant, the higher

Fig. 5. Dimensionless electrophoretic mobilities of the AgI hydrosol, PAAm stabilized AgI hydrosol (at the polymer concentrationequal to 0.312 and 0.078 g dm−3) and of the PAAm molecules as a function of methanol volume fraction at different KNO3

concentrations.


cKNO3the lower xMeOH at which the flocculation

starts to occur. However, at high polymer concen-tration, cPAAm=0.312 g dm−3, the xMeOH corre-sponding to the onset of flocculation approachesxMeOH

cp at cKNO3=0.005 mole dm−3 and is nor

much influenced by the further increase in theionic strength. On the other hand, at cPAAm=0.078 g dm−3 the value of xMeOH needed toinduce the flocculation is higher than in the caseof cPAAm=0.312 g dm−3, provided that cKNO3

islow, whereas the stabilizing effect of polymer ispoorer, i.e. if the ionic strength of the medium ishigh the flocculation starts at lower xMeOH incomparison with the value found at cPAAm=0.312g dm−3. And again, different behavior of AgI solsstabilized by PAAm of different concentrationscan be interpreted on the basis of additionalpolymer adsorption. At higher PAAm content inthe system there are conditions for the formationof thick adsorbed layer which can screen theelectrostatic repulsion between particles and hencethe onset of flocculation roughly corresponds tothe xMeOH

cp . In the case of lower PAAm contentthe collapse of adsorbed polymer is less compen-sated by the additional adsorption and hence theelectrostatic repulsion may be still effective for thesol stabilization in poor solvent conditions atsomewhat higher ionic strengths. With a furtherincrease in cKNO3

the electrostatic contribution tothe sol stabilization decreases and mainly thesteric mechanism is operative.

A peculiarity of F vs xMeOH dependencies is thepresence of minima in the flocculation efficiencyin bad solvent conditions. The broad minima werefound in the case of high PAAm content in thesystem, cPAAm=0.312 g dm−3, but a distinct de-crease in F was observed also at cPAAm=0.078 gdm−3. The position of these minima roughly cor-responds to that of the maxima in the turbidity ofPAAm–water–methanol–KNO3 systems. Fig.1(b) shows the influence of xMeOH on the turbidityof PAAm solutions at different cKNO3

. The verti-cal arrows point at the presence of precipitate atthe bottom of a vessel. No distinct change in theturbidity was noticed after a 3 day storage ofsamples.

When the polymer medium is altered from agood to bad solvent, for instance by the addition

of a cononsolvent, the mean force acting betweensegments becomes attractive. In a solution of afinite concentration, processes of coil contractiontogether with aggregation, followed by coales-cence of either the collapsed coils or the aggre-gates occur and as a result the phase separation isobserved. The phase separation can yield col-loidally stable polymer dispersions [39,40]. Stabledispersions of PAAm in water–methanol mixtureswere reported [13]. Also the invariability, withinthe 3 day storage, of turbidity of PAAm solutionsin water–methanol–KNO3 mixtures, correspond-ing to bad solvent conditions, seems to indicatethe colloid stability of the system.

The stability of dispersions of non-ionic homo-polymers may result from different effects like thepresence of residual charges on macromolecules[39] or the difference between solubilities ofcations and anions in the polymer-rich phase thatproduces the electrostatic barrier [41]. A forma-tion of the entropic barrier dependent on theconformational state of polymer chains is alsosuggested [40]. In ref.[39] the authors suggest thatthe colloidal stability of dispersed phase-separatedpolymer may reflect, among others factors, verylow van der Waals attraction between particlesbecause of a high content of dispersion medium inthe polymer-rich phase. In the case of PAAm inwater–methanol–KNO3 mixtures all the effectsare possible to occur.

In the PAAm–AgI system examined the phaseseparation of polymer can be realised through theaggregation of polymer covered particles, the nu-cleation and growth of polymer particles or theprecipitation of PAAm onto AgI particles. Thelast process should be expected to occur morereadily than the nucleation and growth of purelypolymer particles, since the adsorbed polymer lay-ers may act as nuclei for phase separation [42,43].

Taking the above into consideration we havecome to the conclusion that it is the incorporationof AgI particles into PAAm-reach phase which isresponsible for the presence of minima on F vsxMeOH dependencies in bad solvent conditions.Namely, if methanol addition results in a strongsupersaturation of the continuous phase with thepolymer (high values of cPAAm and xMeOH), onemay expect the phase separation of PAAm to


proceed mainly through the polymer depositiononto AgI particles. If particles are very fine, likein these studies, the resulting species may be ofthe average density not much different from thatof pure polymer particles. One may expect theirbehavior to be similar as well, at least in respectof their resistance to sedimentation. Hence, in badsolvent conditions the deposition of non-adsorbedpolymer onto AgI particles may result in thereduction of flocculation efficiency. The extent ofthis reduction depends on the supersaturation ofthe dispersion medium with the polymer. If theresulting supersaturation is small, the aggregationof PAAm-covered AgI particles will mainly occur,provided that there is no interference of the elec-trostatic barrier. As discussed above, at cPAAm=0.312 g dm−3, a low ionic strength is sufficient toscreen the electrostatic repulsion between particlesand therefore in the presence of electrolyte theonset of flocculation is observed around xMeOH

cp .With a further increase in xMeOH, the flocculationefficiency increases at first, as it should be ex-pected for sterically stabilized dispersions. Then,at a certain xMeOH�xMeOH

cp a decrease in F isobserved. It is probably due to the incorporationof AgI particles into the readily formed polymer-rich phase. Finally, the flocculation becomes ef-fective again in methanol-rich media where strongdesolvation of polymer occurs. The electrolytefacilitates the desolvation and hence, improves theflocculation effectiveness. At a low polymer con-tent in the system, cPAAm=0.078 g dm−3, there isless non-adsorbed polymer present in the disper-sion medium, and hence the minima observed arenarrower.

In order to verify the above speculation aboutthe effect of free polymer on the shape of F vsxMeOH plots we performed the flocculation experi-ments on PAAm-protected AgI hydrosol, whosethe dispersion medium was depleted of non-ad-sorbed polymer. As expected, the minima werenot observed. However, the experiments per-formed on the reference system, i.e. PAAm-pro-tected AgI hydrosol subjected to the samecentrifugation and ultrasonic treatment except themedium exchange, did not show the presence ofmaxima as well. In both cases the flocculation wasobserved near xMeOH

cp (at cKNO3=0.020 M). Most

probably the high-speed centrifugation and ultra-sonic treatment caused a partial aggregation ofPAAm-protected AgI particles, by the mechanismparallel to shear induced aggregation [44]. In bothcases the treatment involved the rise in R values incomparison with the parent system that mightreflect the increase in the particle size. As a resultof the deposition of the phase-separated PAAm,the aggregated species could easily undergosedimentation.

The above results have shown that if the stabi-lizing polymer is added to the dispersion medium,the non-adsorbed residue can strongly influencethe dispersion properties. In particular, the pres-ence of free polymer in the continuous phase maystrongly interfere with the flocculation process,especially if stabilized colloidal particles are fine.

References

[1] R. Lambe, T.F. Tadros, B. Vincent, J. Colloid InterfaceSci. 66 (1978) 77.

[2] D.H. Napper, Polymeric Stabilization of Colloidal Dis-persions, Academic Press, London, 1983.

[3] G.J. Fleer, J.M.H.M. Scheutjens, in: B. Dobias (Ed.),Coagulation and Flocculation. Theory and Application,Marcel Dekker, New York, Basel, Hong Kong, 1993.

[4] H.F. Ouada, H. Hommel, A.P. Legrand, J. Chem. Soc.,Faraday Trans. 1 (84) (1988) 3865.

[5] M.C.A. Griffin, J.C. Price, W.G. Griffin, J. Colloid Inter-face Sci. 128 (1989) 223.

[6] A.V. Kerkar, D.L. Feke, Colloids Surfaces 53 (1991) 363.[7] K. Esumi, K. Ishizuki, H. Otsuka, M. Ono, S. Ichikawa,

C. Yanase, J. Colloid Interface Sci. 178 (1996) 549.[8] J.E. Seeberg, J.C. Berg, Colloids Surfaces A: Physic-

ochem. Eng. Aspects 121 (1997) 89.[9] T. Sato, A. Sato, A. Takaaki, Colloids Surfaces A:

Physicochem. Eng. Aspects 142 (1998) 117.[10] B.H. Bijsterbosch, J. Lyklema, Adv. Colloid Interface Sci.

9 (1978) 147.[11] Polymer Handbook, John Wiley & Sons, New York–

London–Sydney–Toronto, 1975[12] T. Schwartz, J. Sabbadin, J. Francois, Polymer 22 (1981)

609.[13] L.G. Lebedeva, O.V. Klenina, N.G. Hlebtzov, B.I.

Klenin, Vysokomol. Soed. A 33 (1991) 984.[14] W. Zhu, D.H. Napper, J. Colloid Interface Sci. 177 (1996)

343.[15] W. Nowicki, G. Nowicka, J. Kobus, Polish J. Chem. 65

(1991) 1325.[16] N. De Roy, P.L. De Bruyn, J.T.G. Overbeek, J. Colloid

Interface Sci. 75 (1980) 542.


[17] S. Levine, A.L. Smith, E. Matijevic, J. Colloid InterfaceSci. 31 (1969) 409.

[18] W. Nowicki, G. Nowicka, J. Chem. Educ. 68 (1991) 523.[19] W. Nowicki, W. Wojciak, Polish J.Chem. 61 (1987) 821.[20] V.P. Budtov, L.A. Itskovitch, V.Y.a. Kabo, V.A. Maslen-

nikov, L.V. Mineew, Vysokomol. Soed. A 33 (1991) 950.[21] O.V. Klenina, L.G. Lebedeva, Vysokomol. Soed. B 29

(1987) 896.[22] H. Morawetz, Macromolecules in Solution, Interscience,

New York, 1965.[23] K.C. Tam, C. Tiu, Polymer Commun. 30 (1989) 114.[24] T. Schwartz, J. Francois, Macromol. Chem. 182 (1981)

2757.[25] N.V. Sastry, P.N. Dave, M.K. Valand, Eur. Polymer J. 35

(1999) 517.[26] P.J. Flory, Principles of Polymer Chemistry, Cornell Uni-

versity Press, New York, 1953.[27] J.P. Baker, H.W. Blanch, J.M. Prausnitz, Polymer 36

(1995) 1061.[28] R.V. Lauzon, E. Matijeviæ, Z. Kolloid-Z, Polymere 250

(1972) 605.[29] M. Malmsten, F. Tiberg, B. Lindman, K. Holmberg,

Colloids Surfaces A: Physicochem. Eng. Aspects 77 (1993)91.

[30] T. Cosgrove, T.L. Crowley, K. Rayan, J.R.P. Webster,Colloids Surfaces 51 (1990) 255.

[31] H.E. Johnson, S. Granick, Macromolecules 24 (1991)3023.

[32] T. van der Boomgaard, T.A. King, T.F. Tadros, H. TangH, B. Vincent, J. Colloid Interface Sci. 66 (1978) 68.

[33] G.J. Fleer, L.K. Koopal, J. Lyklema, Kolloid-Z. Z. Poly-mere 250 (1972) 689.

[34] L.K. Koopal, V. Hlady, J. Lyklema, J. Colloid InterfaceSci. 121 (1988) 49.

[35] F. Csempesz, S. Rohrsetzer, Colloids Surfaces 11 (1984)173.

[36] F. Csempesz, M. Nagay, S. Rohrsetzer, Colloids SurfacesA: Physicochem. Eng. Aspects 141 (1998) 419.

[37] L. Koopal, Thesis, Agricultural University of Wagenin-gen, 1978

[38] A.A. Baran, Usp. Khim. 54 (1985) 1100.[39] K. Chan, R. Pelton, J. Zhang, Langmuir 15 (1999) 4018.[40] E.G. Timoshenko, Y.A. Kuznetsov, Nuovo. Cimento.

20D (1998) 2359.[41] B.V. Derjaguin, S.S. Dukhin, A.E. Yaroshchuk, J. Col-

loid Interface Sci. 115 (1987) 234.[42] J. Marra, M.L. Hair, Macromolecules 21 (1988) 2356.[43] H.E. Johnson, H.W. Hu, S. Granick, Macromolecules 24

(1991) 1859.[44] S.F. Liu, F. Lafuma, R. Audebert, Colloid Polymer Sci.

272 (1994) 196.

Efficiency of methanol induced flocculation of AgI hydrosol stabilized by polyacrylamide

Documents

Transcript of Efficiency of methanol induced flocculation of AgI hydrosol stabilized by polyacrylamide