2

EFFECT OF SOLIDS CONCENTRATION ONPOLYMER ADSORPTION AND CONFORMATION

Tsung-yuan Chen,. Chidambaram Maltesh,2 and Ponisseril Somasundaranl

IHerny Krumb School of MinesColumbia UniversityNew York, New York 10027

~alco Chemical Co.Naperville, Illinois 60563

1. ABSTRACT

The effect of solids concentration on the adsorption and conformation of polymers atsolid/liquid interface was investigated using depletion adsorption and fluorescence tech-niques. The fluorescence of pyrene labeled polyethylene oxide at silica/water interface asa function of solids concentration revealed a surprisingly measurable change in polymerconformation as the solids content was increased. Adsorption density as well as coiling ofthe polymers was found to decrease with increase in solids concentration. These changesare correlated with the rheological behavior of the suspensions.

2. INTRODUCTION

Concentrated particulate suspensions are widely used for coatings, paints, electronicpastes, clay and ceramic slurries in many industrial processes. To maintain the stability ofthese concentrated suspensions and to facilitate the processing, polymers are frequently usedfor controlling the suspension rheology.' Although polymers are currently used for dispersingconcentrated suspensions, the role of polymer properties, particularly their confonnation andorientation, in stabilization of colloidal particles in concentrated slurries remains unclear.

Recently, we have used fluorescence spectroscopy to estimate the confonnation andinteraction of polymers at surfaces!--4 Infonnation obtained for polymer confonnation atsolid/liquid interface has been correlated with the flocculation properties of particulatesuspensions. In this paper, effects of solids concentration on polymer adsorption and con-fonnation on selected model systems are presented.

Water Soluble Polymers, edited by ArnjadPlenum Press, New YOrlc:, 1998 23

14 Tsung-yuan Chen et at

3. EXPERIMENTAL

3.1. Materials

AKP50 alumina powder (Sumitomo Chemical, New York) of 0.2 mm diameter and10.9 m2/g specific surface area was used for the adsorption study. Particle size distributionmeasured with Photon Correlation Spectroscopy (Brookhaven Instruments, BI-8000AT),shows a log normal distribution with an average particle size of 0.21 mm (Figure I). Thealumina particles are nonporous as indicated by the scanning electron microscopic exami-nation. Purity of the powders is specified by the manufacturer as 99.9%.

For silica, Geltech powder (Geltech Tech., Florida) of 1.0 mm diameter and 4.2 m2/gspecific surface area was used. This silica powder is manufactured using a sol-gel processand SEM examination showed nonporous spherical shape. Particle size distribution meas-ured using the PCS method indicates the silica to be relatively monodisperse with an aver-age diameter of 0.92 mm (Figure I). The density of the silica was measured to be 2.1g/cmJ and their purity is claimed to be 99.9% by the manufacturer.

Polyacrylic acid polymer of molecular weight 90,000 and polyethylene oxide poly-mer of molecular weight 6000 - 7500 were used for the adsorption study. Both polymerswere purchased from Polysciences, Inc. and used as received. Polyethylene oxide polymerwith pyrene probes attached on both ends was synthesized by Kumar and Aguilar of Uni-versity of Florida. The labeling ratio of pyrene molecule on polyethylene oxide polymerwas 2 to I. The attachment of pyrene probes to the polymer was confirmed by NMR meas-urements. Polyvinyl pyrrolidone polymer of molecular weight 40,000 supplied by FlukaChemical Company was also used in the adsorption study.

Sodium nitrate (Fisher Scientific) of ACS grade was used for ionic strength control.Water of resistivity 17 MW after distillation and deionization (Bamstead Nanopure Sys-tem) processes was used for making all solutions.

100

80

0

.~oS~.~~~

20

0500010 100

Diameter, om

1000

Figure 1. Particle size distribution of AKP50 alumina and Geltech silica powders measured by PhotOn ComlationSpectroscopy.

60

40

Effect of Solids Concentration on Polymer Adsorption and Conformation 25

3.2. Methods

3.2.1. Polymer Adsorption. Solutions of polymers at desired concentration were pre-pared in 0.03 M NaNO) solution and the solution pH adjusted. 10 ml of each polymer so-lution was then placed in a 20 ml glass vial and alumina or silica powder was addedgradually into above glass vial while the polymer solution was being stirred. The suspen-sions were then stirred for 30 minutes, pH adjusted, and further stirred for 18 hours forequilibrium adsorption. After adsorption, samples were centrifuged at 2400 rpm for onehour and the supernatants were removed for determination of polymer concentration.Polymer concentration was determined using a DC-90 Total Organic Carbon Analyzer(Dohrmann Instrument). Adsorption density of polymers on the solid surface was calcu-lated from the difference between the initial and residual polymer concentrations.

For the study of polymer conformation at solid/liquid interface, mixtures of pyrenelabeled polymer with unlabeled polymer were used.

3.2.2. Polymer Conformation. The conformation of pyrene labeled polyethylene ox-ide polymers in solution and at solid/liquid interface was monitored using the fluorescencespectrum of the pyrene probe using a LS-I fluorescence spectrometer (Photon TechnologyInternational Inc.). Coiling index, defined as the ratio of intensity of the excimer peak(480 om) to that of the monomer peak (373 nm) in the pyrene fluorescence spectrum, wasused as a measure of the polymer conformation (Figure 2). When the polymer is stretched,the possibility of pyrene excimer formation is low and this yields a low excimer peak, andtherefore the coiling index is low. In contrast, when the polymer is coiled, the possibilityof excimer formation is high and the coiling index is higher. Thus, the coiling index of py-rene labeled polymers serves as an in-situ measure of the polymer conformation in the so-lution or at the solid/liquid interface. In all the experiments, suspensions with nearly noresidual polymer are used for confonnation study to minimize the influence of fluores-cence spectra of free polymers in the supernatant.

4. RESULTS AND DISCUSSION

The effect of solids concentration as well as suspension pH on the adsorption ofpolyacrylic acid on alumina was first studied and the results obtained are shown in Fig-

PolYIrB" Stret£hed

LAlwIe/lm

PoIynMJr Coiled

High k/IID

Coiling Index = IeJIm

FIgure 2. Schematic presentation of coiling index and polymer confomlation. Ie and 1m represent the fluores-cence intensities of the excited monomers and excimers, respectively. The coiling index is low when the polymeris stretched and high when polymer is coiled.

Tsung-yuan Chen et aL

Me}~om~8

.~

~{/I

~

Figure 3. Effect of solids concentration on the adsorption of polyacrylic acid on alumina.

ure 3. Interestingly, the solids content was found to markedly decrease the adsorptionwhen it was increased from 0.5 volume percent to 27 volume percent both at pH 4 and pH9. Surprisingly, the decrease in adsorption density occurred even in the low solids concen-tration region when interactions between particles are less than at high concentrations.

The effect of suspension pH on the adsorption of polyacrylic acid on alumina ex-isted in the entire concentration range tested with the adsorption higher at lower pH. Sincethe adsorption of polyacrylic acid on the alumina surface depends on the population andthe ionization status of hydroxyl groups presented on the alumina surface, this suggeststhe electrostatic interaction between the polymer and alumina surface to playa dominantrole in determining the adsorption behavior of polyacrylic acid polymers even when thesolids content of system is high.

The marked effect of solids concentration on polymer adsorption was also observedin the case of polyvinyl pyrrolidone-alumina system. When the solids concentration in thesuspension increased from I volume percent to 20 volume percent, the adsorption densityof polyvinyl pyrrolidone on alumina surface dropped a couple of orders of magnitude(Figure 4). Such huge difference in the polymer adsorption due to solids concentrationchange has not been reported previously to our knowledge.

The effect of solids concentration on the adsorption of polyacrylic acid and polyvi-nyl pyrrolidone on alumina can best be illustrated by plotting log-log the polymer adsorp-tion density against the available surface area per suspension volume (which will betermed as surface area loading later). Figure 5 shows an inverse correlation between poly-mer adsorption density and the surface area loading. The slope of this plot between thepolymer adsorption density and surface solids loading is an indicator of the effect of solidsconcentration on polymer adsorption. For the polyvinyl pyrrolidone system the slope is -0.939, while for the polyacrylic acid system the slopes are -0.231 and -0.257 at pH 4 andpH 9, respectively with the higher slope suggesting the stronger dependence of adsorptionon solids concentration.

Similar effects of solids concentration on adsorption of polyethylene oxide on silicaare illustrated in Figure 6. pH was found to have a measurable effect on polyethylene ox-~

Effect of Solids Concentration on Polymer Adsorption and Conformation 27

Residual conc.. ppm

Figure 4. Adsorption of polyvinyl pyrrolidone on alumina surface at low (1 vol. %) and high (20 vol. %) solids

loadings.

ide adsorption even though polyethylene oxide is a nonionic polymer. This is attributed topH dependence of possible hydrogen bonding as a result of the pH dependent ionization ofhydroxyl groups on silica.

Solids concentration affects the adsorption of polyethylene oxide on silica at pH 4 atsolids loading below 10 volume percent and at pH 9 at solids loading above 25 volumepercent. The change in polymer adsorption behavior at high solids content is particularly

2Swface area per volwne, m /mI

Figure S. Correlation between the polymer adsorption density and surface area per suspension volume forpolyacrylic acid and polyvinyl pyrrolidone/alurnina systems.

28 Tsung-yuan Chen et aL

Figure 6. Effects of solids concentration and suspension pH on polyethylene adsorption at silica/water interface.

interesting since most industrial processes use such highly concentrated suspensions. Tounderstand the reasons for the behavior, fluorescence experiments were done with mix-tures of pyrene labeled polymers in unlabeled polymers along with their adsorption iso-therms. In Figure 7, the coiling index of the adsorbed polymer is plotted against solidsloading along with polymer adsorption density. There is a clear correlation between thepolymer conformation change and the decrease in polymer adsorption density with the in-crease in solids concentration. The coiling index of adsorbed polymers drops from 0.75 at5 volume percent of solids to 0.3 at 35 volume percent solids of concentration. This sug-

0 10 20 30 40

Solids loading, vol. %

Figure 7. C~lation between polymer confonnation and the polymer adsorption density.

Effect of Solids Concentration on Polymer Adsorption and Conformation 29

gests that polymers become stretched when the solids content is increased above 25 vol-ume percent. Confonnational changes correlate well with this change in polymer adsorp-tion density since a stretched polymer will occupy more area on the surface than a coiledpolymer. This suggests that the change in polymer adsorption density results primarilyfrom the stretching of polymers at silica surfaces rather than the change in the nature ofpolymer-surface interaction.

The effect of polymer coverage on confonnation of adsorbed polymers at two differ-ent solids concentrations is shown in Figure 8. Interestingly, in the complete range of ad-sorption density, the coiling index of the adsorbed polymer at high solids loading isalways lower than that at lower solids loading. The adsorbed polymer is thus morestretched at high solids loading even when the particle surface is not completely covered.The rheology data for silica suspension sheds some lights on the possible reasons for thiseffect. Zamrnan et. al.5 have reported the relative viscosity of silica suspensions of thesame composition but without polymers to show an increase from the value predicted byBatchelor model6 when the solids concentration was above 25 volume percent. This sug-gests a stronger particle-particle interaction than that dictated by the Batchelor model atsolids concentration above 25 volume percent. Such interaction can indeed cause the poly-mer to stretch on the surface. The observed increase in polymer stretching can be ac-counted for by the enhanced interaction between particles that are closer to each other athigh solids loading.

5. SUMMARY

Solids concentration exhibits a marked effect on the adsorption of polyacrylic acid,polyvinyl pyrrolidone, and polyethylene oxide polymers on solids. Adsorption ofpolyacrylic acid and polyvinyl pyrrolidone on alumina was found to decrease as the solidsconcentration was increased from 0.5 to 27 volume percent with a logarithmic correlation

0.1

.§--u- 0.6

jco 0.4

~'0U 0.2

00 0.1 0.2 0.3

Adsorption density, mg/m2

0.4

Figure 8. Effects of solids concentration and polymer adsorption density on conformation of adsorbed polyethyl-ene oxide polymers.

30 Tsung-yuan Chen et oL

of the adsorption with solids surface area loading. The effect of solid5 c,oncentration onadsorption was measurably higher for polyvinyl pyrrolidone than for polyacrylic acid.

In the case of adsorption of polyethylene oxide on silica at high pH the adsorptiondensity decreased at solids concentration above 25 volume percent. Investigation of thepolymer conformation at silica/water interface using fluorescence spectroscopy indicatespolymer to become more stretched as the solids loading was increased. The decrease inpolymer adsorption density with solids can be accounted for by considering the stretchingat high solids concentration. With more stretched polymers on the surface, the area occu-pied per polymer is larger; and hence the total number of polymer molecules on the sur-face and hence adsorption density is lower. It was also found that the conformation of theadsorbed polymer at low solids loading is different from that at high solids concentrationeven at low adsorption densities. This suggests strong influence of particle interaction onadsorbed polymer conformation even when the surface coverage is low. Correlation be-tween the suspension rheology and the polymer adsorption density and conformationchange suggests that increased interaction between particles at high solids to be a possiblereason for the observed effect of solids loading on polymer conformation and adsorptiondensity.

ACKNOWLEDGMENTS

The authors acknowledge the financial support of the Engineering Research Center(ERC) for Particle Science and Technology at the University of Florida, the National Sci-ence Foundation (NSF) grant #EEC-94-02989, and the Industrial Partners of the ERC.

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3. Somasundaran P and Krishnakumar S, "In-situ spectroscopic investigations of adsorbed surfactant andpolymer layers in aqueous and nonaqueous systems", Colloids and Surfaces, 1994;93:79-95.

4. Xiang Yu and Somasundaran P, "Role of Polymer Conformation in Interparticle-Bridging Dominated Floc-culation", Journal of Colloid and Interface Science, 1996; 177:283-287.

5. Zaman AA, Moudgil BM, Fricke Al, and EI-Shali H, "Rheological Behavior of Highly ConcentratedAqueous Silica Suspensions in the Presence of Sodium Nitrate and Polyethylene Oxide", J. Rheology,1996;40(6):1191-1210.

6. Batchelor OK, "The Effect of Brownian Motion on the Bulk Stress in a Suspension of Spherical Particles",J. Fluid Mech.. 1977;83:97-117.