Reprinted from LANGMUIR, 1989, 5, 854.Copyright @ 1989 by the American Chemical Society and reprinted by perm_ion of the copyright OWDer.

Adsorption of Polyacrylamide on Oxide Minerals

L. T. Leet and P. Somasundaran*School of Engineering and Applied Science, Columbia University, New York, New York 10027

Received February 22, 1!NJ8. In Final Form: January 6, 1!NJ9

The adsorption of nonionic polyacrylamide on a series of oxide minerals is studied. The results showthat adsorption of the polymer increases with increase in the point of zero charge of the oxide. A strongpH dependence of the adsorption density is observed for all oxides, which is further correlated with thedistribution of the positive and neutral sites on the surface. This is attributed to the favorable H bondingbetween the electronegative C=() group of the polymer and the proton-donating groups on the oxide surfacethat are constituted by these sites. The solvation of oxide cations is also found to influence polymeradsorption. As the energy of the oxide cation solvation increases, adsorption of the polymer on thecorresponding oxide decreases. Such an inverse relationship is due to competition between oxide--solventand oxide-polymer interactions.

Introduction

Polymer adsorption is encountered in numerous in-dustrial applications, where a good knowledge of the ad-sorption mechanism is important for the economic andtechnical success of the process. In mineral beneficiation,for example, polymers are added for depression, floccula-tion, or dispersion of the minerals. In this case, thepolymer encounters minerals with different surface prop-erties, and the key to successful beneficiation lies in theselectivity of the polymer adsorption. An application ofsuch selectivity is seen in the case of iron ore processingat the Tilden Mines, where starch is added to flocculateselectively the iron oxide from silica in the d~liming stageprior to flotation. 1 Another example is the microflotationof a mixture of hematite and quartz using dodecylaminewhere polyacrylamide is used to depress hematite selec-tively while quartz is completely floated. 2 The reason forthe selectivity of the affinity of polymer for one oxide overanother is not completely understood due to a limitedknowledge of the adsorption mechanism. The adsorptionof polymers can also lead to detrimental effects as in thecase of enhanced oil recovery, where polymer is used asa mobility control agent; in this case, the adsorption ofpolymer onto mineral surfaces leads to a loss in ~ityof the polymer slug resulting in delayed oil recovery. Otherindustrial p~ involving polymers include detergency,food processing, and biotechnology.

In view of the importance of polymer adsorption and itsapplications, the present work entails a study of adsorptionof a nonionic polymer, polyacrylamide, on a series ofwell-characterized oxides to determine the role of surfaceproperties in relation to the nature of polymer-surfaceinteraction.

experimentation in polymer adsorption have not fullyconcurred due to the theoretical requirement of a highdegree of ideality in model systems and of the numerousparameters, which are not readily available. Nevertheless,tlleoretical predictions have provided a useful guide in thestudy of polymer adsorption behavior. However, the basicinformation which reflects the mechanism of the process(such as the type of bonding between the polymer, solvent,and surface, the role of surface properties adsorption sites,and the energy and driving force involved in the process)is still to be obtained experimentally.

Factors Governing Adsorption. The system variablesthat affect polymer adsorption are similar to those thatgovern any adsorption procBS from solution. ~ factorsinclude the physical and chemical nature of the adsorbentand adsorbate, solvent power, and temperature.

1. Nature of the Adsorbent. The type of surface siteis as important as the functional group of the polymer inadsorption. The characteristics of the solid surface, likesurface charge, potential, and the degree of solvation, areall dependent on the solvent properties and temperatureof the system. In the case of oxides, for example, theseproperties depend on the pH of the solution since H+ andOH- are the potential-determining ions.

The solubility behavior of the adsorbent is not to beneglected. For a solid which undergoes significant disso-lution, the kinetics of dissolution and polymer adsorptionhave to be carefully considered, particularly if the adsorbedpolymer can be detached upon dissolution of the adsorbentand the ion-polymer complex does not readsorb. Apartfrom the chemical nature of the adsorbent, the physicalproperties such as porosity and particle size are also im-portant.

2. Chemical Nature of Polymer. The type of inter-action between the polymer and the solid surface is de-termined by the chemical structure of the polymer relativeto that of the surface. For example, a nonpolar functionalgroup will react favorably with a hydrophobic site o,n asurface, and a polar group will show affmity for a hydro-philic site. If the polymer is charged, electrostatic inter-action plays a major role in adsorption, and the ionicstrength and pH of the solution become important pa-rameters. For uncharged polymers, electr~tatic forces

Polymer AdsorptionProgress made in the past 30 years on polymer adsorp-

tion has encompassed both experimental and theoreticalaspects. Experimentally, the emphasis has been on thedevelopment of optical and spectr~pic techniques tostudy the adsorbed Jayer. Theoretically, adsorption modelshave evolved from simple gas adsorption models3 to therandom walk approachu to statistical mechanical treat-ment.H The last method has been under extensive de-velopment over the past 2 decades and seems to be areasonable approach to treat polymer adsorption since itemphasizes the configurational entropy and energy. Inspite of these developments, theoretical treatment and

-tP~nt a~ Inatitut Fr~ du P6trole, 1 et., Avenue de

Bois-Pr6au. B.P. 311,92506 Ruen-t.f~lmai80n CedeK, France.

- -(1) Villar, J. W.; Dawe, G. A. Min. ColII'r..T. 1975,40.(2) So_:!D~P8D, p.;~, L T.Int. MiMr. Proce... Co/ll'l'. 14th

1t82, 1V-9.1.(3) HeIJy, T. W.; La Mer, V. K..T. Colloid Interface Sci.lt64,19, 323-(4) Simha, R.; Fri8ch, H. L.; Eirich, F. R..T. ffJya. Chem. Ita, 51, 584.(5) Fri8Ch. H. L; Simba, R..T. Chem. Playa. 1,",24,652-(6) Silberbeq, A..T. ffJya. Chem. IM%,~, 1872-(7) Silberberg, A. .T. ffJy.. Chem. 1HZ, ~, 1884.(8) Scbeutjens, J. M. H. M.; Fleer, G. J..T. PlaY', Chem. Im,83,1619.(9) Scbeutjena, J. M. H. M.; Fleer, G. J..T. Playa. Chem. 1_,84,178.

Adsorption of Polyacrylamide on Oxide Minerals Langmuir, Vol. 5, No.3, 1989 855

Table I. Properties of Oxides

8.42.16.57.86.10.7

F8z03/PAMpH "ti

3.1o-Znol/,.3 NoCI

>50E

-'40c0:

~30..0"ZOc..= 10 ' . ".-0 - -'- -" . . . II

0 10 ZO 30 40 50 800POLYMER CONCE.TRATION. ..'"

Figure 1. r potential of F~Oa 88 a function of PAM ooncen-tration.

of temperature. Note that if polymer adsorption is en-dothermic, the process is entropically driven. Accordingto Gibb's equation

dG = dB - TdS (2)

If dB is positive, then dS also must be positive, and theabsolute magnitude of T dS must be greater than the ab-solute magnitude of dB. Since the polymer loses trans-lational freedom upon adsorption and thereby decreasesthe entropy, a positive dS implies that this decrease isexceeded by the increase in entropy gain from the releaseof bound hydration water around the polymer and/or atthe solid surface.

The present study deals with the nature of the adsorbentsurface and its effects on polymer adsorption. This iscarried out by using different substrates, namely, oxides,and varying their surface properties by changing the so-lution pH.

become unimportant, and H bonding and solvation becomepredominant.

3. Molecular Weight. In general. for nonporous ad-sorbents, polymer adsorption increases with molecularweight, although the degree of dependence is markedlyinfluenced by the solvent power. For a good solvent, allthe statistical theories predict a weak dependence of ad-sorption on molecular weight. &-14 For a poor solvent,lo-14however, the theories of Frisch, Simha, and Eirich4.5 andof Hoeve12-14 predict a square root dependence. Silber-berg's theoryll also predicts a square root dependence forlow molecular weight polymers, but for high molecularweight polymers, a limiting value is predicted. A linearfunction is predicted for low molecular weight from themodels of Roel0 and of Scheutjens and Fleer,s,9 with lev-eling off at high molecular weight for Roe's theory.

Experimentally, however, molecular weight dependenceof adsorption can usually be described qualitatively onlydue to the polydispersity of the polymers.

4. Solvent Power. The conformation of a polymer inbulk or at the interface is dependent on the polymer-solvent interaction defined by the Flory-Huggins param-eter which in turn is governed by the solvent power. Ingeneral, the experimentally observed trend is a decreasein adsorption with an increase in solvent power. However,it is possible that a reverse trend can occur if, for example,a poor solvent interacts significantly with the adsorbentsurface. The resultant competition between solvent andpolymer molecules for surface sites can result in a de-creased or even negative adsorption.

5. Temperature. The dependence of polymer ad-sorption on temperature varies with the system studied.For example, the adsorption of polyisobutene from benzeneonto carbon black has been reported to decrease withtemperature,16 while an increase has been found for thesystem poly(vinyl acetate)/metal powders.16 The overallresponse of adsorption depends on the individual effectsof temperature on (i) solubility of the polymer, which ischaracterized by the polymer-solvent interaction and thefree energy of mixing, and (ii) the state of solvation of thesurface and the polymer.

From the temperature dependence, thermodynamicproperties of adsorption can be evaluated. The heat ofadsorption has been calculated this way by using theClausius-Clapeyron equation,16 which under isostericconditions is given by

d(ln C).:illR = d(l/T> (1)

However, this method of enthalpy calculation assumesthat (i) the adsorption process is reversible and (ii) the~enthalpy is constant over the temperature range studied.Neither of these assumptions is unambiguously valid. Infact, a contradiction has been found for the system poly-(ethylene glycol)fsilica!7 where the enthalpy evaluated bythe Clausius-Clapeyron equation is positive while thatobtained calorimetrically is negative. The results obtainedfrom calorimetry should be more reliable since this tech-nique is direct and does not involve any assumptions suchas those inherent in model equations. Using calorimetry,the enthalpy change can also be measured as a function

Materials and MethodsMinerals. The oxide minerals used are Fe203' Cr20a, Al2O3'

TiO2 (anatase), SnO2' and SiO2. All the samples are syntheticoxides acquired commercially from Alfa Chemicals and used asreceived except for SiO2t which is natural Brazilian quartz pre-pared by wet grinding and leaching. The surface areas of thepowdered samples are determined by N2 adsorption using, theBET technique, and the electrokinetic properties measured byusing the Zeta Meter are given in Table I.

Polymer. The polymer is a 14C-labeled nonionic polyacryl-amide (PAM) synthesized by using the radiation-induced pre-cipitation method with a 6OCo source}8 The average molecularweight determined by viscometry is approximately 0.7 milliondaltons.19 The nonionic nature of the polymer has been verifiedby adsorbing the polymer onto F6203 particles and then measuringthe electrokinetic properties of the particles. Figure 1 shows ther potential results of F~O3 at pH 6 as a function of polymerconcentration. In the absence of polymer, the r potential is about+28mV. When the polymer is added up to SOO/mg kg, the rpotential decreases and reaches a constant value of +6m V. Thissuggests that the Fe203 particles are coated with the polymer,and the potential measured is that of the partially polymer~tedsurface. It is deduced that the polymer has not undergone hy-drolysis because if it were hydrolyzed, the negative charge of the

(18) HoUander, A. F. Master's Thesis, Columbia University, NewYork,1979.

(19) Wong, K. Master's Thesis, Columbia University, New York, 1983.

856 Langmuir, Vol. 5, No.3, 1989 Lee and Somasundaran

I.tTable II. r Potential of FezOz in the Presence of PAMF.zOyPAM

pHa 5.1

0 7.0

v e.1

0 11.'

S.~lk_I/"MM;I

.--<>

r potential

+8+7+6

0-9

. 1.2E

...

;-.1.0

>-0-.2.. 0.8Q29:0-.. 0.6~

1..

~~

1.4

~

0.4-."-r..'...zwa

za~..c0..0.

~~

Q.tl

~~~~

0 . , . . .0 100 200 300 400 500 600 700 100 100

RESIDUAL POLYMER CONCENTRATION, .,'t,

FigUl'e 3. Effect of pH on PAM adsorption on F820a-

~~

1.~

1.0

0.8

0.8'iI!

-:

~

0 100 ZOO 500 400 500 800 700 ~ tOORESIDUA4. POLYMER CONCENTRATION, ..'"

Figure 2. Adsorption isotherms of PAM on oxides.

polymer adsorbed on the Fe203 would normally be expected tocause a charge reversal and result in a final negative r potential. mSimilar measurements made at other pH values are given in TableII. It can be seen from the above table that at all pH values,the presence of adsorbed polymer reduces the magnitude of ther potential. The nonionicity of the polymer is most evident atpH 7.5 (pzc), where there is no net charge on the F~O3 surfaceand where the adsorbed polymer does not induce any chargeeither.

Experimental Procedure. The depletion method is used todetermine the amount of polymer adsorbed. The mineral isprewetted with the solvent before the addition of polymer. Thepreconditioning (wetting) time is determined by introducing themineral into the solvent and measuring the pH change of thesuspension with time. A constant bulk pH is taken to be anindication that equilibrium is attained. The polymer solutionis then added and the suspension conditioned further at a rmalsolid/liquid = 0.03. The polymer adsorption time is determinedfrom kinetic studies, and adsorption is considered to be completewhen the adsorption density remains constant with time. Bothprewetting and polymer conditioning of the suspension are ac-complished by a wrist-action shaker. For all the studies, thepreconditioning and polymer conditioning times are determined,and the appropriate times chosen are 3 and 4 h, respectively.Longer polymer conditioning times are not used, so that anyPO88ible degradation of polymer during mixing can be minimi7~In tests where the pH is controlled, the equilibrium pH of thesuspension after the preconditioning step is noted, and thepolymer solution is preadjusted to the corresponding pH beforeadding to the suspension. In all cases, the ionic strength ismaintained at 3 x lQ-2 kmolfm3 NaCl, and the temperature rangesfrom 24 to 26 °C.

0 100 200 300 400 500 600 100 800 .00 1000RESIDtJAI. POLYMER CONCENTRATION, -,I..

Figure 4. Effect of pH on PAM adsorption on Cr20a.

followed gradually by a shallower rise with increase inconcentration. This is a high affinity type adsorptionisotherm characteristic of polymer adsorption. The dif-ferent adsorption capacities of the various oxides for thepolyacrylamide are evident from these isotherms. Thepoints of zero charge (pzc) of the oxides measured by usingelectrophoresis are also given in the same figure. A com-parison of the values of pzc with the adsorption densitiesshows that as pzc increases, adsorption of the polymer onthe oxide increases. This correlation is interesting becausethe pH - pzc is a measure of the surface charge of an oxide,and these results show that, in spite of the nonionic natureof the polymer, the surface charge nevertheless plays a rolein the adsorption process.

pH Effects. A more detailed investigation of the sur-face charge effects is carried out by studying adsorptionas a function of pH since the H+ and OH- are the poten-tial-determining ions for oxides. Figures 3-6 show theresults of adsorption densities for Fe20s, Cr203' Al2O3. andTiO2 at different pH values. In all cases, adsorption de-creases with an increase in pH, indicating that adsorptiondecreases as the surface charge of the oxide increases. Thisobservation, which has also been reported by others,21.22is unexpected at first sight since the polymer, being es-

Results and DiscussionAdsorption Isotherms. The adsorption densities of

the nonionic polyacrylamide on six different oxides atnatural pH (from 6 to 8) are plotted in Figure 2 as afunction of residual polymer concentration. All the iso-therms show a steep rise in the low-concentration region

(21) Gerbbardt, J. E.; Fuerstenau, D. W. In Interfacial Phenomenain Mineral Procusinc; Yarar, B., Spottiawood, D. J., Eds.; The Engi-neering Foundation: New York, 1982, p 175.

(22) Griot, 0.; Kitchener, J. A. Trans. Faraday Soc. 1965,61,1026.

(20) Moudgil. B. M. Doctorate Thesia, Columbia University, NewYork, 1981.

Adsorption of Polyacrylamide on Oxide Minerals Langmuir, Vol. 5, No. 3,1989 857

-((H-(H,I-I

-0:(I

"H,

-ICH-CHp-1C=OI

-H2"

MOH/MOH2+.

MO-.

1.4

TIO2/PAMpH

a 1.60 5.9

A 7.1

~

. 1.2.... ,

~ 1.0 I.....Zw 0.80

z9:~o.ec~ I0 I.. 0.4

0,.

0 : ~~~ . . . . . . .0 100 8)0 300 400 500 eoo 700 eoo 800

RESIOUAL POLYMER CONCENTRATION, .,/q

Figure 6. Effect of pH on PAM adsorption on TiO2-

sentially nonionic, is considered to adsorb by H bonding,and electrostatic interaction should not be significant. Athigh pH however, the polymer can undergo hydrolysis, andelectrostatic repulsion between the hydrolyzed polymerand the negative surface charge reduces ads()rption.However, it has been found that polyacrylamide hydrolyzesaround pH 10 (ref 18), and the above pH effects cannotbe totally attributed to such repulsions because in the caseof TiO2 (Figure 6) all the pH values studied are below thepolymer hydrolysis pH.

In considering H bonding between the polyacrylamideand the adsorbent, an examination of the chemicalstructure of the polymer and the oxide surface speciesshows that the most probable situation is where theelectronegative C=O function of the amide acts as a H.bonding base and the oxide surface hydroxyls as H-bonding acid. In this case, not only the neutral undisso-ciated MOH group but also the positive MOH2+ group canact as proton donors (Figure 7a). Note that although Hbonding is not considered as an electrostatic interaction,the fact that it is a bond between an electronegative andan electropositive group could render it charge dependent.Therefore, the positive MOH2 + group should be at leastas favorable or even more favorable than the neutral MOHto form an H bond with the C=O. From Figure 7, it canbe seen that the positive and the neutral sites, acting asH-bonding acids, are more favorable for H bonding withthe electronegative C=O group on the polymer. The

(01 (bl

Figure 7. H bonding between PAM and oxide surface sites.

negative site, MO-, being electronegative, is not favorablefor bonding with the C=O, although at high pH, when the

former is the predominant species, there is a possibilitythat it can bond with the weakly acidic NH2 function,

forming a weaker bond (Figure 7b). The importance ofsurface hydroxyl functions in H bonding has been further

verified by adsorbing the polymer onto pure gold sol, the

surface of which is not oxidized and therefore does not

carry any hydroxyl groups; in this case, no adsorption ofthe polymer is detected.

Thus, the relative acidity/basicity of the chemical groupson the polymer with respect to the oxide surface groups

results in a site preference in adsorption which is mani-fested as a pH dependence. To test this, the site distri-

bution of the oxide with pH is examined in relation to the

pH dependence of the polymer adsorption.Site Distributions on Oxides. Two methods are used

to calculate charge distribution on an oxide surface. Thefirst method, proposed by Parks and de Bruyn, ZJ considers

the hydrolysis of the dissolved metal species and the

subsequent readsorption of the hydroxy-metal complexeson the surface. From the bulk species distribution dia-

gram, and assuming that the ratios of the species at the

interface are similar to those in the bulk, the surface

species distribution can be estimated. Lai and Fuerste-nau,u on the other hand, have argued against this modelon the basis that oxides are quite insoluble. They proposed

that the charging mechanism is due to the adsorption ordesorption of H+ by the surface hydroxyl to form MOH2 +

or MO-, respectively, represented by

MOH + H+ +::t MOH2+ (3)

MOH +::t MO- + H+ (4)

From the law of mass action and the pzc of the oxide, a

charge distribution function can be obtained. The twoassumptions in this model are as follows: (i) the activityis taken to be the fraction of the species on the surface and

(ii) the fraction of neutral sites at the point of zero chargeis an arbitrary value.

The site distribution curves for F~Oa obtained by usingthe models of Parks and de Bruyn and of Lai and Fuer-

stenau are plotted in Figure 8. The sum of the positiveand the neutral sites is represented in boldface. The pH

dependence of the adsorption density at a constant re-sidual polymer concentration is also plotted in the same

figure. It is clear from this fIgure that the sum of the

positive and neutral sites gives a better correlation with

adsorption density that either of the two alone: thissupports the postulate that both the positive and neutralgroups on the Fe20a are the adsorption sites. The same

correlations are also obtained for Cr203' Al2Os, and TiO2

(Figures 9-11), showing that this hypothesis holds true alsofor other oxides.

From the above correlations, it appears that the positiveand the neutral surface groups are the preferred adsorptionsites and that H bonding can be charge or pH dependent

due to the relative acidity and basicity of the surface

(23) Parb, G. A.; de Bruyn, P. L. J. Phys. Chern. 1M2, 66, 967.(24) Lai, R. W. M.; Fuerstenau, D. W. TrIms. Soc. Min. Eng. AIME

1916,26Q, 104.

858 Langmuir, Vol. 5, No.3, 1989 Lee and Somasundaran

W0-M

#

pH

Figure 8. (a, Top) Comparison of PAM adsorption on FetOs withsurface site distributions calculated by using the model of Parksand de Bruyn. (b, Bottom) Comparison of PAM adsorption onFezO3 with surface site distributions calculated by using the modelof Lai and Fuerstenau.

groups with respect to the polymer functional groups.This being the case, can the differences in adsorption

densities between oxides (see Figure 2) be attributed solelyto the variations in surface charge densities? One way toisolate the extent of the charge effect is to plot the ad-sorption densities as a function of the difference, pH - pzc.Figure 12 shows such a plot for F~Oa, Cr20a, A12Oa, andTiO2- In this figure, for a fixed value of pH - pzc, all theoxides represented should ideally have the same surfacepotential since according to the Nernstian equation25

~o = -o.O59(pH - pzc) (5)

Hence, if charge were the only governing factor in ad-sorption, all the curves for the oxides should be superim-posable. But this is not the case; clearly, charge effectsalone cannot account for the characteristic adsorptiondensities for the oxides.. It is possible that the oxides are

0 10

(25) De Bruyn, P. L; Agar, G. E. In Froth Flotation; 50th AnniversaryVol.; Fuentenau, D. W.. Ed.; AIME: New York, 1962; p 91.

1E...I-

MzWQ

~~..E0..Q.

Adsorption of Polyacrylamide on Oxide Minerals Langmuir, Vol. 5, No.3, 1989 859

1.8

1.8

1.9

3.2

3.2

3.5

From Table ill, it can be seen that as the solvation energyincreases from Fe3+ to Si.+ the polymer adsorption densityon the corresponding oxide decreases (see Figure 13).Such an inverse relation is comprehensible since a highdegree of solvation implies a strong interaction of the solidand the solvent, and the affmity of the polymer for thesurface is consequently decreased or hindered by thesolvation layer on the solid surface.

The above calculations consider the solvation of freeunhydroxylated cations only. Since the mineral is inequilibrium with several dissolved species such as Me3+,Me(OH)2+, Me(OH)2+, and Me(OH).-, a more completecalculation can be carried out by taking all these speciesinto account. For each species, the solvation energy isestimated by using the Born equation and multiplied bythe fraction of the species present in the solution at a givenpH (evaluated from the species distribution diagram).Hence, at each pH, the total solvation is the sum of sol-vation of all the major dissolved species present. Note thatalthough solvation of the dissolved species in solution isnot equivalent to the solvation of the oxide surface, thedissolved species, nevertheless, playa role in determiningthe surface properties, as proposed by Parks and deBruyn.23 According to their hypothesis, the charge dis-tribution on an oxide surface is due to the hydrolysis ofdissolved metal species and the subsequent readsorptionof the hydroxy-metal complex on the surface. Therefore,following the above hypothesis, the solvation behavior ofdissolved species can reflect that of the oxide surface.

The results of total solvation of the dissolved oxidespecies calculated as a function of pH are given in Figure14. For each oxide, the energy of solvation goes thro~ha minimum as a function of pH. This means that ron-sidering the contribution of solvation energy alone, ad-sorption should go through a maximum. But from theearlier discussion, it has been shown that surface chargeplays an important role and that polymer adsorptioncorrelates with surface site distribution as a function ofpH. Therefore, for an individual oxide, the predominantforce responsible for polymer adsorption is H bonding, andthe resultant pH dependence of adsorption is due to thesite dependence of H bonding. Between various oxides,

non-Nernstian in behavior, possibly due to solvation ef-fects.

Therefore, another factor considered to account for thedifferences in adsorption capacities of the oxides is sol-vation energy since, due to the polar nature of the system,the solvation factor can play an important role in theoverall adsorption process.

Effects of Cation Solvation. The extent of solvationof the surface species on a particle is an important con-sideration because it reflects the affmity of the solvent forthe solid, which in turn affects the surface-adsorbate in-teraction. Minerals undergo various degrees of interactionwith the solvent depending on the nature of the surfacespecies. The general equation for the calculation of sol-vation energy for a charge species according to the Bornmodel is28

-(~) Bockris, J. O.'M.; Reddy, A. K. N. Modem Electrochemistry;

Plenum P~ New York, 1970; Vol. 1. (27) Pauling, L. The NGtUI'e of the Chemical Bond; CorneU UniversityPress: New York. 1960.

860 Langmuir, Vol. 5, No.3, 1989 Lee and Somasundaran

. determines the differences in polymer adsorption betweenvarious oxides.

6 - Fe,.--- Co,.- AltO.

.~0-G 0.!..00- -2 \ ~

\ /...

\ \ \-6 "- ,'"-8 /""

,.--10 ' . . . , . .

0 2 4 6 8 10 12 14

pH

Figure 14. Total solvation of predominant oxide SpeclM insolution 88 a function of pH.

however, the degree of mineral-solvent interaction canexplain the differences in polymer adsorption on theseoxides. This is shown in Figure 14, where the energy ofsolvation increases from F~O3 to Cr203 to Al2Oa, a trendwhich corresponds to the inverse of that for polymer ad-sorption; this is attributed to competition between solid-solvent and solid-polymer interactions.

Thus, the present study has shown that, for the ad-sorption of nomonic polyacrylamide on oxides, pH is animportant parameter due to the dependence of H bondingon the nature of the surface sites and that the extent ofmineral solvation constitutes an additional factor which

ConclusionsThe adsorption of non ionic polyacrylamide on oxide

minerals is strongly dependent on the solution pH, anda correlation is found between the adsorption density andthe pzc of the oxide. This indicates that even though thepolymer is essentially nonionic the surface charge of theoxide plays an important role in the adsorption process.

The major driving force controlling polymer adsorption,H bonding, is proposed to take place between the elec-tronegative C=O group on the polyacrylamide and theproton-donating oxide surface hydroxyls, the positiveMOH2+ and the neutral MOH. The negative site, MO-,being electronegative, is not as favorable to interact withthe polymer. A correlation is obtained for the adsorptiondensity and the distribution of the positive and neutralsites as a function of pH. The site preference of thepolymer is thus explained in terms of the relative acidi-ty fbasicity of the polymer functional groups with respectto the oxide surface groups.

Another driving force that influences adsorption issolvation. The solvation energies of the ionic species fromthe oxides estimated by using the Born equation are foundto follow an inverse relationship with the adsorptiondensities of the polymer on the oxides. This is attributedto competition between oxide-solvent and oxide-polymerinteractions.

Acknowledgment. We thank the National ScienceFoundation (NSF-DAR-79-09295 and NSF-CP~18163)for financial support of this work.

Registry No. Fe20a, 1309-37-1; Cr20a1 1308-38-9; TiOz,13463-67-7; 8UO20 18282-10-5; 8i02, 7631-86-9; ~~. 1344-28-1;polyacrylamide, 9003-05-8.