COLLOIDSANDSURFACES A

Colloids and SurfacesA: Physicochemical and Engineering Aspects 133 (1998) 125-133ELSEVIER

Role of conformation and orientation of surfactants andpolymers in controlling flocculation and dispersion of aqueous

and non-aqueous suspensionsP. Somasundaran, Xiang Yu, S. Krishnakumar *

Department of Chemical Engineering, Materials Science and Mining Engineering, Columbia University, New York,NY 10027, USA

Accepted 1 April 1997

Abstract

Flocculation and dispersion of both aqueous and non-aqueous suspensions using surfactants and polymers havewidespread applications. In this work, the role of adsorbed layer microstructural properties, namely their conformationand orientation at the solid-liquid interface, in controlling dispersion properties is examined. A multi-prongedapproach, involving the use of fluorescence and ESR techniques along with measurements of surface charge andhydrophobicity, was used to explore the structure of the adsorbed layer.

The adsorption isotherm of sodium dodecylsulfate on alumina in aqueous solution interface showed differentregions corresponding to different adsorption mechanisms. The suspension stability also displayed significant changesconcomitant with changes in the structure of the adsorbed layer. When polymers are used their conformation can bemanipulated by changing solution conditions such as pH and/or by the addition of a secondary polymer or surfactant.Such manipulation can be used to obtain desired levels of flocculation or stabilization.

In non-aqueous media, adsorption was found to depend on the solid-solvent-solute interactions as expressed byan effective interaction parameter b~. The suspension stability was dependent on the amount adsorbed and thepacking of molecules in the adsorbed layer. The presence of even trace polar impurities was found to have a criticaleffect on the suspension stability. For example, trace amounts of water were found to cause a significant enhancementof stabilization of alumina-cyclohexane suspension in the presence of adsorbed surfactant layers. However, at higherconcentrations water induced rapid flocculation in these systems. Hydrophobic polymers were found to be effectivestabilizers for alumina in both aqueous and non-aqueous media. In this case the polymer adsorbed at the interface indifferent orientations with the lyophilic side chains dangling into the solution to provide steric repulsion and thusstabilization. e 1998 Elsevier Science B.Y.

Keywords: Aqueous; Dispersion; ESR; Fluorescence; Hydrophobic polymer; Non-aqueous

1. Introduction mers are gaining increasing attention in bothindustrial and academic fields because of its vitalrole in a wide spectrum of areas such as watertreatment, mineral processing, cosmetics and phar-maceutics, and microbial interactions and adhe-sions [1-3). The stability of these suspensions iscontrolled by several forces besides the electrostatic

Stabilization and flocculation of colloidal sus-pensions by the addition of surfactants and poly-

. Corresponding author.

0927-7757/98/$19.00 0 1998 Elsevier Science B. V. All rights reserved.PII80927-7757(97)00128-3

126 P. Somasundaran et aL / Colloids Surfaces A: Physicocherll. EIIg. Aspects /33 (/998) /25-/33

2. Experimental

2.1. Adsorption

The adsorption density in all cases was estimatedby measuring the depletion in the surfactant/polymer concentration after contacting with solidsfor a fixed amount of time under controlledconditions.

2.2. Suspension stabi!it:

The stabilities of suspensions have been eval-uated in terms of several parameters, such as therate of descent of the upper interface (settlingrate). The CAT -scan technique was used to charac-terize the structure of the ftocs. In addition to thesettling rate, the amount of particles settled in agiven period was measured in terms of percentweight or volume of the sediment. In some cases,hydrophobicity of the particles was estimated bymeasuring their ftotability.

2.3. Surface charge

Surface charge was estimated by measuringelectrophoretic mobilities using a Penkem LaserZee meter.

force [4]. Stabilization can also result from stericrepulsion or short-range solvation forces [5]. Forflocculation of suspensions, bridging by adsorbedpolymers is an accepted mechanism that comple-ments charge neutralization.

The stability of dispersions in organic liquidshas attracted increased attention due to its applica-tion in the processing of high performance ceram-ics, electronic printing and reprographics [6,7].While the past literature reports good correlationbetween zeta potential and suspension stability [8],the theory behind the mechanism of charge devel-opment in organic solvents of low dielectric permit-tivity has been a subject of much controversy. Theelectrostatic effects are further complicated by thepresence of trace amounts of water or other polarimpurities that are almost unavoidable in thesesystems. The choice of stabilizer is quite importantand usually an amphipathic molecule with a stronganchoring group and a tail that has a strongaffinity for the solution phase is used [9].

Hydrophobically modified polymers have beendeveloped recently to increase viscosity and elastic-ity of solutions because they undergo interestingintra- and inter-molecular associations [10]. Owingto their unique structure they can be used tomodify dispersion properties in both polar andnon-polar media.

The stabilization/flocculation provided by theadsorbed layers of surfactants and polymersdepends to a large extent on the orientation ofthese molecules and the microstructure of theadsorbed layer, which contribute to the steric andbridging interactions between the particles [11].The microstructural evolution of the adsorbedlayers has been studied in detail by several workersand correlated with dispersion properties like wet-tability and hydrophobicity of the particles inaqueous suspensions [12]. Several spectroscopictechniques, like fluorescence, electron spin reso-nance (ESR), nuclear magnetic resonance andinfrared spectroscopy have been used widely tostudy in situ the changes in the organization ofthe adsorbed layer [13,14]. In this article, wereview some of our work on the correlationbetween dispersion stability and the orientation/conformation of the adsorbed molecules as evincedby spectroscopic techniques.

2.4. In situ characterization of adsorbed layers

Advanced spectroscopic techniques such asfluorescence and ESR spectroscopy were used tocharacterize the adsorbed layer microstructure.

Fluorescence spectroscopy makes use of thechanges in the photophysical response of a mole-cule such as pyrene which is incorporated into theadsorbed layer [15,16]. Valuable informationregarding the micropolarity and microviscosity ofadsorbed layers can be obtained from these studies.The ratio of the intensity of the peak at 382 nmto that at 374 nm was found to be sensitive to thepolarity of the probe environment and is termedthe polarity parameter (hIIJ. In the case of poly-mers, the excimer fluorescence of pyrene attachedto the polymer chains was used to study theconformation and association of polymers in solu-tion and at the solid-liquid interface [17]. The

121P. Somasundaran et al. / Colloids Surfaces .4..' Physicochem. Eng. Aspects 133 ( 1998) 125-133

ratio of Ie/1m (excimer to monomer intensity) isrelated to the coiling/stretching of the labeledpolymer. In the absence of intermolecular inter-actions a high value of Ie/1m can be considered tobe the result of a coiled conformation, while a lowvalue is associated with a stretched conformation.

ESR spectroscopy monitors the changes in thespin characteristics of a paramagnetic probe thatis incorporated into the system [18]. From thechanges in the spectral line shape it is possible toobtain information on the micropolarity andmicroviscosity of the probe environment throughmeasurements of the hyperfine splitting constantand rotational correlation time respectively [19].In addition, ESR can also provide information onthe orientation of molecules at the solid-liquidinterface from their motional characteristics.

~

1001

I

llXloOIOf IxIO.11

lxlO'

IxIO'

0-6 oS .. -) ~

IxlO IxlO IxlO IxlO IxlO

Residu8 DodecyIaIIIaIe. M

Fig. I. Change in hydrophobicity of alumina particles as deter-

mined by the bubble pick-up technique as a function of dodecyl-

sulfate adsorption.

1&10'" I

ponds to micelle formation in the bulk beyondwhich no further adsorption occurs. Fig. 1 alsoshows the changes in hydrophobicity of the alu-mina particles as measured by the bubble pick-uptechnique; from these results one can concludethat the maximum in hydrophobicity correspondsto region II, where the molecular assemblies renderthe surface hydrophobic [23]. The presence of suchhydrophobic aggregates on the surface have beenconfirmed by in situ studies using techniques suchas fluorescence, Raman and ESR spectroscopy[12,13,24,25]. Fluorescence studies using pyrenehave shown a sharp decrease in micropolarity(I3/IJ corresponding to the onset region II asshown in Fig. 2, which also shows the variation inzeta potential of the alumina particles in the vari-ous adsorption regions. The decrease in hydropho-bicity at higher adsorption densities is speculatedto be due to the formation of surfactant bilayerswhich make the surface hydrophilic.

3. Results and discussion

3.1. Alumina/surfactant/water system

The dispersibility of bare alumina in water iscontrolled by the electrostatic forces between theparticles and is hence dependent on the solutionpH. The dispersion properties of alumina can bemodified by adsorption of an ionic surfactant suchas sodium dodecylsulfate (SDS) on the aluminasurface. The adsorption of SDS on alumina hasbeen studied widely, and the isotherm, which con-sists of four distinct regions, is a classical exampleof ionic surfactants adsorbing on charged surfaces.Fig. 1 [20] shows the adsorption of SDS on alu-mina at pH 6.5 under a constant salt concentrationof 10-1 kmol m-3. In region I, which has a slopeof unity under constant ionic strength conditions,adsorption occurs predominantly by electrostaticinteractions. In region II a conspicuous increase inadsorption is observed which is attributed to theonset of surfactant aggregation at the surfacethrough lateral interactions between the hydro-carbon chains. This phenomenon is referred to ashemi-micelle or solloid formation [21,22].Region rn shows a decrease in the slope of theisotherm due to the increasing electrostatic hin-drance to the surfactant adsorption followinginterfacial charge reversal, while region IV corres-

3.2. Alumina/polymer/water system

3.2.1. Manipulation of polymer conformationthrough pH shifting

Flocculation of alumina with polyacrylic acidexhibits marked dependence on the confonnationof the adsorbed polymer rather than the electro-static force. The polyacrylic acid molecules stretchout (Ie/1m decreases) with increase in pH becauseof the ionization of carboxyl groups and the resul-

P. Somasundaran et aI. / Colloids Surfaces A: Physicochem. Eng. Aspects 133 (1998) 12'-113128

~

Fig. 4. Aocculation of alumina with polyacrylic acid underchanging pH conditions; Initial pH=4 ('7 IJIm. 0 transmit-tance. 0 settling rate).Fig. 2. Diagram illustrating the variation in zeta potential and

polarity parameter (4/IJ of the alumina particle surface as afunction of dodecylsulfate.

similar to that observed under fixed pH conditionsin terms of settling rate, and a maximum insupernatant clarity occurred at pH 7 which corres-ponds to the isoelectric point of the polymer-adsorbed alumina. From simultaneous fluores-cence studies, it was clear that the coiled polymer,adsorbed on the alumina surface at low pH,stretched out when the pH was raised and causedsuperior flocculation, while the initially stretchedpolymer adsorbed at high pH remained in thestretched form even when the pH was lowered.

A two-stage flocculation mechanism was pro-posed to explain the superior flocculation obtainedwith pH up-shift. At low pH, the oppositelycharged polymer chains capture all ultrafine par-ticles to form small flocs. When pH was shiftedup, the stretched polymer chains bridge the smallflocs to form large flocs. With all the fines captureddue to charge neutralization, the supernatantobtained is also clearer than that obtainedotherwise.

tant electrostatic repulsion between charged groups(Fig. 3). It can be seen that the stretched polymerresults in better flocculation in tenDS of the settlingrate, while the effect is opposite in tenDS of thesupernatant clarity. Simultaneous CAT scanstudies showed the size of the flocs obtained to besmaller at low pH than at high pH [26].

In order to investigate the effect of on-siteconfonnational changes of adsorbed polymer onflocculation, polyacrylic acid was adsorbed onalumina at pH 4 and pH 10 and the pH was thenshifted up and down respectively. As can be seenfrom Fig. 4 [27], when pH was shifted up,enhanced flocculation was observed in terms ofboth the settling rate and the supernatant clarity.When pH was shifted down, the flocculation was

100 ,

90

10

~ '10

I:

30

20

..,

0.6

t1

~3.2.2. Flocculation with mu/tipolymers

As shown in Fig. 5, a marked difference inflocculation was obtained between the use of indi-vidual and dual polymers. It was found that cat-ionic polyacrylamide used in combination withpolystyrene sulfonate provided excellent floccula-tion, though it did not produce any flocculationby itself [28]. Zeta potential results suggested theflocculation not to be controlled by electrostaticforces. Another feature worthy of note in Fig. 5 is

.."

0.2

Fig. 3. Flocculation of alumina with 20 ppm polyacrylic acid("7 IJIm, 0 transmittance, 0 settling rate).

P. Somasundaran et al. / Colloids Surfaces A: Physicochem. £IIg. Aspects 133 ( 1991) 12.1-133

~

350

300

250

i 200~Q 150c

t 1000

50

0

1.;~~

.5

!

0 10 20 30 40 50 60 70 80

Solvent dielectric constant

Fig. 6. Settling behavior of alumina as a function of solventdielectric constant at different levels of AOT.

O.6.'6,..I ~

0 20 40 60 80 100

Poiyraero ~ ppm

Fig. 5. Flocculation response of alumina suspension with dualpolymer systems (0 C-PAM alone, <> PSS alone, 6 C-PAMafter PSS, 'V PSS after C-PAM). by the settling rate. The adsorption of the surfac-

tant on alumina from different solvents is shownin Fig. 7. It is clear that the adsorption itself isinfluenced by the solvent polarity with less adsorp-tion from solvents of intermediate polarity. Wehave found in our earlier work that the adsorptionof AOT is less favorable from solvents (20«;<65)in which it does not aggregate significantly [30].This leads to the inference that the changes indispersion behavior are caused only when a sub-stantial amount of surfactant is adsorbed at thesolid-liquid interface. Using interaction parame-ters 15 to characterize the solid-solute-solvent

the marked effect of the mode of polymer addition(sequential or mixture). Adsorption results showedthat the presence of polystyrene sulfonate (eitherin solution or adsorbed at the interface) causedcomplete adsorption of cationic polyacrylamidewhich alone did not adsorb on the positivelycharged alumina particles. Polymer solution vis-cosity studies suggested that the coadsorption wascaused by complexation of the two oppositelycharged polymer at the interface and in the solu-tion. The coadsorption of long chain cationicpolymer provided better interparticle bridging andthus significant enhancement in flocculation withthe use of dual polymer. However, the reason forthe drastic effect of the mode of addition remainsto be explored.

3.3. Alumina/surfactant/organic media

Fig. 6 shows the settling rates of alumina par-ticles in different liquids at three different levels ofsurfactant (Aerosol-OT, AOT) addition [29]. Itcan be seen that the dispersions in the non-polarsolvents are stabilized significantly by the presenceof the surfactant. The dispersions in the highlypolar solvents were also seen to undergo changesin their dispersion behavior. However, dispersionsin the moderately polar liquids do not show anychange in their dispersion behavior as measured

9

8

7

6

5

4

3

2

P. Somasundaran et al. / Colloids Surfaces A: Physicochem. Eng. Aspects m ( 1998) 125-JJ3130

interactions, we have calculated an effective inter.action parameter 150fT as follows [31]:

~.ff = abS{ A 1~lOlid-blOlventl + Blbsolute

bool_l}- blOlvent 1- qblOlid

0 5 10 15

H20/AOT Molar Ratio, Wo

20

where A, Band C are numerical constants. hefTwas found to correlate well with theadsorption/desorption of various surfactants ondifferent solids in non-polar media.

In the low dielectric constant solvents AOTadsorbs on alumina with its polar head groupinteracting with the polar alumina surface and thehydrocarbon tail extending out into the bulk. Thismakes the surface rather hydrophobic with a con-comitant reduction in the Hamaker constant ASLSvalue for the particles, thereby imparting stability.Fig. 7 also shows the changes in mobility of anESR probe (7-doxyl stearic acid) bound to thesurface when the surface is treated with a concen-trated solution of AOT (30 mM) [29]. It can beseen that the probe becomes less mobile due to itsincorporation in the adsorbed surfactant layer inthe non-polar solvents, suggesting that theadsorbed layer in these cases is more-or-less wellpacked.

Fig. 8. Diagram illustrating the settling behavior (---) of alu-mina suspensious in cyclohexane as a function of water concen-tration in the presence of adsorbed AOT. Also shown are thewater adsorption density (-) and the order parameter (.)as detennined by ESR.

decrease in the surface charge as measured by theESA. However, this decrease in surface chargecannot explain fully the massive floccu1ationobserved in this case. From the water adsorptionisotherm also shown in Fig. 8 it can be seen thatthe water adsorption increases sharply close to theonset of flocculation.

The water-induced flocculation was furtherinvestigated by monitoring the spectral responseof 7-doxyl stearic acid which is co-adsorbed alongwith the surfactant. 7-Doxyl stearic acid adsorbedon the alumina surface gave a characteristic aniso-tropic spectrum which was quantified using theorder parameter which can be calculated from thespectral characteristics. The order parameter variesbetween S = I (highly ordered, restricted) and S =0 (highly random and mobile) and reflects thestate of the environment in which the probe resides.The order parameter calculated in this casedecreases with increase in water concentration andlevels off at approximately the water concentrationcorresponding to that of the onset of floccu1ation(Fig. 8). These results indicate that there is asignificant change in the structure of the adsorbedlayer, with the surfactant layer becoming morefluid and mobile as water adsorbs at the interface.This opens up the possibility for the water layerson different particles to penetrate across the surfac-tant layers and bridge the particles together during

3.3.1. Effect of water on suspension stabilityFig. 8 shows the effect of water on the stability

of alumina suspensions which have been dispersedin cyclohexane by the adsorption of a monolayerofAOT [14]. In an extremely dry state the disper-sions flocculated rapidly and the addition of traceamounts of water stabilized the suspensions quitesignificantly. Thereafter the suspensions remainedstable over a range of water concentration andthen flocculated very rapidly at higher water con-centrations. The concentration of water at whichthe suspension reflocculates correlated well withthe solubility of water in the bulk solution. Surfacecharge measurements made using the electrokineticsonic amplitude (ESA) system have shown clearlythat addition of trace amounts of water can inducea significant amount of charge at the surface. Athigher water concentrations there was a slight

inP. SomaSWlliaran et aI. / Colloids Surfaces.4.: Physicochem. Eng. Aspects 133 ( 1998/12'-133

*l~zocollision, thereby causing the massive flocculationobserved.

100

_10" ri.ift

3.4. Alumina/hydrophobic polymer/aqueous or non-aqueous system { 32 J 4x10'

#

)60

I~hlO'

~.

-»)

..10

0,,\0.-100 200 400 600 1000

1- DAPRAL.-, -

Fig. 10. Adsorption of DAPRAL on alumina and its effect onthe stability and zeta potential (pH=8, SjL=2.5%, 'V zetapotential, 0 adsorption, 0 settling rate).

-

carbon solvents at a DAPRAL concentration of500 ppm and then remained unchanged. Relativehydrophobicity obtained with two-phase partitionshowed agreement with the fluorescence results atlow DAPRAL concentration, but a decrease inhydrophobicity was found at high concentrations.Continued adsorption even after charge reversalwithout any change in zeta potential was attributedto such aggregation of DAPRAL on the alumina

particles.Results obtained for the settling rate of alumina

in toluene are shown in Fig. 11 as a function ofDAPRAL concentration. In the absence ofDAPRAL settling occurs rapidly, since aluminaparticles tend to form aggregates in apolarsolvents and the suspension stability increasesmarkedly with its addition. Adsorption testsshowed the polymer to adsorb completely on alu-mina in the concentration range studied (up to500 ppm). The wavelength of the emission maxi-mum of 7-dirnethyl amino 4-methylcoumarin(coumarin 311) was correlated with the hydropho-bicity of its environment, changing from 391 to471 om when the solvent is changed from hexaneto water. It is seen that, with the increase inpolymer concentration, the ma:ximum emissionwavelength shifts towards the shorter wavelengthrange, reaching the value for hydrocarbon solvents(390 om) at 500 ppm of DAPRAL. This observa-tion supports the mechanism proposed earlier forpolymer adsorption in toluene. The shift of theemission maximum correlated well with the sus-pension stability, both undergoing significant

In addition to the wide use as rheological modi-fiers, hydrophobically modified polymers findincreased application in controlling the stability ofsuspensions. Fig. 9 shows the effect of DAPRALaddition on the stability of alumina water suspen-sions along with the zeta potential and the adsorp-tion isotherm (the structure ofDAPRAL is shownin the inset). A comparison of the zeta potentialdata with stability data shows the suspension sta-bility to remain unchanged up to a polymer con-centration of 150 ppm, even though the zetapotential of alumina particles did undergo amarked change (from + II to -50 mY) in thisconcentration range. Furthermore, the stability ofthe suspension increased markedly above theDAPRAL concentration of 150 ppm even thoughthere was only a minor change in the zeta potential.It is clear that stabilization cannot be attributedin this case to the electrostatic forces.

Using pyrene fluorescence spectroscopy, hydro-phobicity of alumina particles was studied; theresults are shown in Fig. 10 as a function ofDAPRAL concentration. At a DAPRAL concen-tration of 5 ppm the 13/1. value begins to increase,from 0.6 (typical value of water solution),approaching a typical value> 1 of the hydro-

~~'5~

11-.~~

Fig. 9. Changes in surface hydrophobicity and polarity parame-ter of alumina particles as a function of DAPRAL adsorption.

P. Somasundaran et aL / Colloids Surfaces A: Physicochem. Eng. Aspect.f 133 (1998) 125-133132

0 200 400 600 800 1000

[DAPRAL], ppm

Fig. II. Effect of DAPRAL on the stability and hydrophobicityof alumina in toluene.

changes when the polymer concentration is below200 ppm and leveling off at higher concentrations.

The dispersion behavior of alumina suspensionsin the presence of DAPRAL can be explained asfollows. In aqueous media, at low polymer concen-trations such interactions can permit formation ofaggregates of polymer-"coated" alumina particlesin spite of the electrostatic repulsion. At higherpolymer concentrations, the possibility for inter-actions between adsorbed polymer molecules withthose in the bulk solution increases. Such inter-actions can cause formation ofbilayers with hydro-philic side chains dangling into the solution, assuggested by the observed decrease in the hydro-phobicity. In non-aqueous media, DAPRALadsorbs on the alumina surface with the hydro-carbon side chains stretched out into solution toprovide steric repulsion.

ticles. However, in organic liquids, where suchsurface aggregation is not observed, higher surfacecoverages are required to obtain sufficient hydro-phobicity. In non-polar media, stability can alsobe affected by the presence of trace polar impuri-ties, such as water, that can induce ionization inthe adsorbed layer.

The flocculation efficiency of polyelectrolytes isdependent critically on their conformational stateat the interface, as this governs their ability tocapture other particles in the "bridging" process.The conformation itself can be controlled byadjusting solution conditions during the adsorp-tion and flocculation processes. Sequential addi-tion of polymers improved the flocculationefficiency considerably, possibly due to formationof complexes at the interface that provide betterbridging conditions.

Hydrophobically modified polymers such asDAPRAL, due to their unique structure, canadsorb at interfaces with different orientations,depending upon the properties of both the sub-strates and the media. In water the hydrocarbonside chains of DAPRAL molecules on an aluminasurface cause either hydrophobic or steric inter-actions, depending on the polymer concentration.In the alumina-toluene system the stabilization ofthe suspensions is achieved by the steric hindranceprovided by lyophilic side chains.

Ackno~'ledgment

The authors wish to acknowledge financial sup-port of this work by the National ScienceFoundation, Department of Energy, UnileverResearch Inc. and Nalco Chemical Company.

4. Concludin~ remarks

The role of adsorbed layer microstructure onthe dispersion properties of suspensions in aqueousand non-aqueous media is seen to be prominent.Using advanced spectroscopic techniques we havecorrelated the nature of the adsorbed layer to thestabilization/flocculation behavior. In aqueous sys-tems the surfactant adsorption generates hydro-phobic patches at relatively low surface coveragesand imparts sufficient hydophobicity to the par-

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