Journal of Colloid and Interface Science 341 (2010) 298–302

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Journal of Colloid and Interface Science

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Adsorption of sodium polyacrylate in high solids loading calcium carbonate slurries

Joshua J. Taylor a, Wolfgang M. Sigmund a,b,*

a Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, United Statesb Department of energy Engineering, Hanyang University, Seoul, South Korea

a r t i c l e i n f o

Article history:Received 3 June 2009Accepted 24 September 2009Available online 4 October 2009

Keywords:CalciumCarbonateSlurryInfraredPolyacrylateCarboxylateAdsorptionSolidsATR-FTIR

0021-9797/$ - see front matter � 2009 Elsevier Inc. Adoi:10.1016/j.jcis.2009.09.048

* Corresponding author. Address: Department of Maing, University of Florida, Gainesville, FL 32611, United

E-mail address: wsigm@mse.ufl.edu (W.M. Sigmun

a b s t r a c t

The adsorption of sodium polyacrylate (NaPAA) in slurries with up to 75 wt.% calcium carbonate wasinvestigated with the use of attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) and adsorption of probe molecules. Analysis of the IR spectra demonstrated that the carboxylategroups of NaPAA adsorbed onto ground calcium carbonate (GCC) in three different modes. These modeswere shown to be dependent on the solids loading and age of the slurry. Further investigation lead to thedetermination of the chelating ability of NaPAA at high solids loading.

� 2009 Elsevier Inc. All rights reserved.

1. Introduction A five-member chelate ring (consisting of the metal cation and

Polyacrylic acid and its salt sodium polyacrylate (NaPAA) areone of the most frequently applied polyelectrolytes in industryand the household. A few examples include laundering processes,thickening agents, and dispersion of clay and calcium carbonate.NaPAA is used in the calcium carbonate industry as a dispersantfor the mineral because it allows for an increase in the solids load-ing up to 75 wt.% while maintaining the desired viscosity. However,the complete role of stabilization and confirmation of NaPAA in dis-persing calcium carbonate at high solids content is not clear.

The carboxylate group has been shown to interact with metalcations and surfaces in four different modes [1–6], namely, ionic,bridging, bidentate, and unidentate. Several papers have investi-gated the different carboxylate modes with IR spectroscopy be-cause the change in bond symmetry can be detected. The ionic,bridging, and bidentate modes have similar group symmetry. Thetwo oxygen atoms in the bidentate mode are interacting withone metal cation; therefore, there will be a change in the symmet-rical and asymmetrical vibration frequencies. Since the unidentatemode has one oxygen atom coordinated with one metal cation thesymmetrical and asymmetrical vibration frequencies will be simi-lar to a carboxylic acid group.

Geffroy et al. [2] and Dobson and McQuillan [1] have shownthat adsorption is preferred through chelation of dicarboxylates.

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terials Science and Engineer-States. Fax: +1 352 846 3355.d).

dicarboxylates) are the most stable followed by six- and seven-member chelate rings. Lu and Miller [4] further explains that car-boxylate groups interact with calcium ions in three-dimensionalseven- or eightfold coordination and it is common for calcium ionsto coordinate with both carboxylate groups and water. Katz et al.[7] also demonstrate that both unidentate and bidentate coordina-tion of carboxylate groups with calcium cations are possible whenthe calcium ions bind in seven- and eightfold coordination.

The objective of this paper is to investigate the effect of solidsloading and aging of calcite slurries on the adsorption of NaPAA ontoGCC. The attenuated total reflectance-Fourier transform infrared(ATR-FTIR) technique was employed to collect data of ground cal-cium carbonate (GCC) slurries up to 75 wt.%. Previous studies havefocus on the interaction of carboxylate groups with calcium in dilutesystems but did not consider changes in the interactions within highsolids loading slurries or changes due to aging of slurries. All the dif-ferences between dilute systems and high solids loading slurrieshave not been accounted for by the sciences which cause problemsfor industry on a daily basis. This paper is the first to address theadsorption of NaPAA onto GCC in slurries up to 75 wt.%.

2. Materials and methods

2.1. Materials

Ground calcium carbonate (GCC) was provided by Imerys(Sandersville, Georgia) with a d50 = 1 lm measured with a sedi-

1750 1700 1650 1600 1550 15000.00

0.05

0.10

0.15

Log

(1/

R)

(a.u

.)

Wavenumber (cm-1)

NaPAA in D2O

Fig. 1. IR spectrum of the carboxylate stretching region for NaPAA in D2O. The IRspectrum shows that NaPAA is in the ionic form with a peak at 1570 cm�1 for theCOO� and no peak at 1700 cm�1 for the C@O.

1600 1580 1560 1540 1520 1500

-0.0010

-0.0005

0.0000

0.0005

0.0010

ionic

bridging

bidentate

unidentate

NaPAA in D2O

d2 A/d

υ2 (a.

u.)

Wavenumber (cm-1)

75 wt% GCC Slurry

Fig. 2. Second derivative of the IR spectra of the carboxylate region of NaPAA in D2Oand in a 75 wt.% GCC slurry. NaPAA adsorbs onto GCC in unidentate, bridging, andbidentate modes.

J.J. Taylor, W.M. Sigmund / Journal of Colloid and Interface Science 341 (2010) 298–302 299

graph. The sodium polyacrylate (NaPAA) with molecular weightaverage Mw = 5967 g/mol (polydispersity 2.04) was provided byKemira Chemicals, Inc. (Kennesaw, Georgia). Benzoic acid, 99.5%,gallic acid, >99%, and deuterium oxide (D2O), 100.0 at.% D, werepurchased from Fisher Scientific (Acros Organics). D2O is used inthis research because the IR spectrum of D2O allows for analysisof certain NaPAA bands when compared to H2O bands which over-lap critical NaPAA bands.

2.2. Attenuated total reflectance-Fourier transform infrared (ATR-FTIR)

Infrared spectroscopy detects the vibrational characteristics ofchemical groups having a dipole moment. Unlike other infraredtechniques (DRIFTS and transmittance) that require dry or trans-parent samples, the ATR technique allows for measurement of liq-uids, gels, and solids. ATR-FTIR measurements were performedusing the Thermo Electron Magna 760 FTIR Microscope with a ZnSecrystal and a liquid nitrogen cooled MCT/A detector. FTIR spectrawere recorded from 650 cm�1 to 4000 cm�1 with 128 scans anda resolution of 2 cm�1.

2.3. Adsorption measurements

Slurry samples were centrifuged at 3000 rpm for 45 min. Thesupernatant was then decanted into a beaker and diluted with ameasured amount of water. Ultraviolet–Visible Spectroscopy(UV/Vis) was utilized to measure the adsorption intensities atwavelengths of 210 nm and 224 nm for gallic acid and benzoicacid, respectively, with the Beckman DU 640 spectrophotometer.Next, the measured intensities were compared to a calibrationcurve and the amount of probe molecules in the supernatant wasdetermined. The amount of probe molecules adsorbed was calcu-lated from subtracting the measured amount in the supernatantfrom the amount in the slurry.

2.4. Sample preparation

NaPAA was dissolved in D2O. Amount of NaPAA in a sample was1 wt.% of the GCC weight. Next, the GCC was slowly added to thesolution while stirring. The weight percent of GCC is a percent ofthe total GCC and water weight. The aging samples used for testingwere stored in a sealed container during the aging period withoutagitation.

Preparations of slurries containing benzoic acid or gallic acidwere prepared similarly. Amount of benzoic acid or gallic acid ina sample was 1 wt.% of the GCC weight. Either benzoic acid or gallicacid was dissolved into D2O and NaOH was added until a pH of 9.9was reached. Next, the GCC was slowly added to the solution whilestirring.

3. Results and discussion

3.1. Ionic and adsorbed NaPAA

When NaPAA is dissolved into water the carboxylate group willhave a resonance form. This was confirmed in the IR spectra withthe absence of the C@O band which would be located around1700 cm�1 and the presence of a COO� band at 1570 cm�1 as seenin Fig. 1. When calcium carbonate is introduced into the systemwith 75 wt.% solids loading there is a change in the carboxylateband. Fig. 2 shows that the 1570 cm�1 stretching band is split intoa band at 1581 cm�1 and 1567 cm�1 along with a band forming at1524 cm�1 with a shoulder. From the research of Lu and Miller [4]and Young and Miller [6] they have shown that each band repre-

sents a different mode of interaction. The four possible modes ofinteraction are ionic, unidentate, bidentate, and bridging. Mielczar-ski et al. [8,9] assigned the bands at 1575 cm�1 and 1540 cm�1 tounidentate and bidentate adsorption, respectively. Therefore, inFig. 2 the 1581 cm�1 band is representative of unidentate coordi-nation, the 1567 cm�1 band represents bridging coordination,and the bands between 1510 cm�1 and 1545 cm�1 representbidentate coordination with calcium. In high solid loading slurrythe adsorption of NaPAA onto GCC consists of all three coordina-tion modes.

3.2. Increase solids and aging

As discussed previously, the adsorption of NaPAA occursthrough three coordination states but there are apparent changesin each state with different solids loading. These differences aredemonstrated by preparing slurry samples with 10, 30, 50, and70 wt.% GCC and analyzing the spectra of the IR carboxylate region(Fig. 3). First, the band representative of the unidentate coordina-tion in the 10 wt.% GCC slurry at 1586 cm�1 shifts to lower wave-

1600 1580 1560 1540 1520

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-0.0004

0.0000

0.0004

0.0008

bridging

bidentate

unidentate

d2 A/d

υ2 (a.

u.)

Wavenumber (cm-1)

70 wt% Slurry

10 wt% Slurry 30 wt% Slurry 50 wt% Slurry

Fig. 3. Second derivative of the IR spectra of the carboxylate region with increasingsolids loading. As the solids loading of the slurries increases there is a shift of thebands toward a bridging mode.

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0.0000

0.0005

increasing age

bridgingunidentate

75 wt% Slurry Aged 48 hrs

d2 A/d

υ2 (a.

u.)

Wavenumber (cm-1)

75 wt% Slurry Fresh 75 wt% Slurry Aged 24 hrs

Fig. 4. Second derivative of the IR spectra of the carboxylate region with aging. Theunidentate band shifts from 1585 cm�1 to 1580 cm�1 with increasing age indicatinga shift towards the bridging mode.

Fig. 5. Adsorption of benzoic acid onto GCC at varying solids loading. Benzoic aciddoes not adsorb onto GCC.

300 J.J. Taylor, W.M. Sigmund / Journal of Colloid and Interface Science 341 (2010) 298–302

numbers as the solids loading is increased, eventually to1578 cm�1 in the 70 wt.% GCC slurry. Similarly, the band represen-tative of the bridging coordination in the 10 wt.% GCC slurry at1567 cm�1 shifts to lower wavenumbers until it reaches1560 cm�1 in a 70 wt.% GCC slurry. Also, as the solids loading in-creases the band between 1510 cm�1 and 1545 cm�1 becomes adoublet with band peaks at 1539 cm�1 and 1521 cm�1, represent-ing the bidentate coordination. As the solids loading increasesthere is an increase in the bridging to unidentate ratio. These re-sults can be explained by a decrease in the distance between cal-cium carbonate particles which would decrease the distancebetween calcium allowing for additional bridging. These resultsare important because they demonstrate that the interactionswithin a low solids loading slurry are different than in a high solidsloading slurry.

During the first couple of days after a slurry has been preparedthere are several changes in its physical properties. These changeswere suspected to originate from differences in the adsorption ofthe dispersant. So ATR-FTIR was utilized to determine if anychanges in the interaction of the NaPAA with GCC could be de-tected during the aging process. A 75 wt.% GCC slurry was preparedand analyzed with the ATR-FTIR as it aged. This was performedwhile it was less than an hour old, 24 h old, and 48 h old. As seenin Fig. 4 the interaction of NaPAA with GCC changes with age. Ini-tially, the band representing the unidentate coordination is locatedat 1585 cm�1. As the system ages the band shifts to 1580 cm�1.This indicates that while the slurry ages there is a decrease inthe concentration of unidentate coordination and an increase inthe bridging and/or ionic coordination of the carboxylate groups.These results would support the idea that as the system ages thereis an increase in the amount of dissolved calcium ions which arethen available for bridging coordination, which require two cal-cium ions per carboxylate group, instead of unidentate coordina-tion, which only require one calcium ion per carboxylate group.

3.3. Probe molecules

Molecules which contain a carboxylate group were chosen toprobe the surface of GCC in high solids loading slurries. The inter-action of the probe molecules with the surface of GCC would pro-vide additional understanding of the interaction between NaPAAand GCC. Benzoic acid and gallic acid were chosen because eachmolecule contains a single carboxylate group along with a benzenering. The benzene ring absorbs ultraviolet light causing the elec-

trons to transition from p (bonding) to p* (anti-bonding) whichwould allow for determination of their concentrations in a solutionwith the use of an UV/Vis spectrometer.

Adsorption experiments for benzoic acid onto GCC were per-formed to demonstrate the carboxylate group adsorption and/orthe hydrophobic adsorption of the benzene ring. Adsorption ofNaPAA is believed to be due to the carboxylate groups interactingwith the calcium ions [2,4,5,10]. Geffroy et al. [2] go into detaildescribing the complexation of carboxylates with the surface ofcalcite required for adsorption. They determine that carboxylatesadsorb through chelation. Since it has been concluded that adsorp-tion modes of carboxylate groups are modified with solids loading,the adsorption of benzoic acid at different solids loading was inves-tigated. Analysis of adsorption measurements in Fig. 5 concludethat benzoic acid does not adsorb onto GCC at any solids loading.Since Fig. 5 does not indicate any adsorption, then the ATR-FTIRwas utilized to confirm that the carboxylate and the benzene ringare not interacting with the surface of the GCC.

Analysis of Fig. 6 demonstrates that benzoic acid does not ad-sorb onto the GCC with increasing solids loading. The carboxylateband at 1548 cm�1 for benzoic acid in D2O does not shift or splitwhen GCC is added to the solution which is an indication thatthe carboxylate is not adsorbing onto the GCC particles. Anotherpossible interaction that could cause adsorption is through hydro-phobic adsorption with the benzene ring. The band for a benzene

1640 1620 1600 1580 1560 1540 1520

-0.008

-0.004

0.000

0.004

C C

COO-

20 wt% GCC Slurry

Wavenumber (cm-1)

57 wt% GCC Slurry

d2 A/d

υ2 (a.

u.)

Benzoic acid in D2O

Fig. 6. Second derivative of the IR spectra of benzoic acid in D2O, 20 wt.% GCCslurry, and 57 wt.% GCC slurry. The bands for the benzene ring and the carboxylatedo not shift, indicating no adsorption.

20 wt% Slurry Gallic acid in D2O

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ring is located at 1596 cm�1 and does not shift with increasingsolids loading so the benzoic acid is not adsorbed through the ben-zene ring.

The results from the adsorption measurements and IR spectra ofbenzoic acid confirm each other and are in good agreement withGeffroy et al. [2] who demonstrated that a molecule with one car-boxylate group will not adsorb onto the surface of GCC. Therefore,the next probe molecule chosen contained OH groups which wereexpected to promote adsorption onto calcite.

Gallic acid was chosen as a probe molecule for the same reasonthat benzoic acid was chosen but it also includes three hydroxylgroups. Hydroxyl groups were chosen in order to determine if che-lation would play an important role in adsorption. The adsorptionof gallic acid onto GCC in several different solids loading slurries isdemonstrated in Fig. 7. The amount of gallic acid adsorbed isdependent on the solids loading of the slurry and decreases withan increase weight percent of GCC. Calculations were performedusing bond angles and bond lengths to determine the adsorbedmonolayer concentration of gallic acid which is between 1.6 mg/m2 and 8.5 mg/m2. The range in adsorbed monolayer concentrationis due to the different modes of adsorption (from flat to vertical)that the gallic acid could accompany while adsorbing. Comparingthe monolayer adsorption calculations to Fig. 7 would suggest that

Fig. 7. Adsorption of gallic acid onto GCC at varying solids loading.

the adsorbed amount of gallic acid at 10 wt.% would contain atleast three adsorbed layers, at 30 wt.% would contain at least twoadsorbed layers, and at 50 wt.% and 70 wt.% would contain onemonolayer with different modes of adsorption.

Further investigation of the chelating ability of gallic acid wasperformed with FTIR spectroscopy. Analysis of the IR spectra con-firmed that gallic acid adsorbs onto the surface of GCC. As seen inFig. 8, the carboxylate band at 1550 cm�1 shifts to 1547 cm�1 whena 20 wt.% GCC slurry is prepared. The carboxylate shift is a confir-mation that the gallic acid is chelating with the surface of the GCC.Another important band includes the benzene ring band located at1600 cm�1 for the gallic acid and D2O system at a pH of 9.9. The IRband for the benzene ring shifts to 1594 cm�1 for a 20 wt.% GCCslurry containing gallic acid. This is an additional indication of che-lation because the benzene ring state is changed due to its OHgroup’s participation in the chelate ring.

The inclusion of the hydroxyl groups allows for gallic acid tochelate with the GCC surface forming a seven-bond ring throughone hydroxyl group, calcium ion, and carboxylate group (similarresults from Geffroy et al. [2] with different molecules). A seven-member chelate ring is not as stable as a five-member chelate ringbut the chelating ability of gallic acid promotes adsorption unlikebenzoic acid which does not have the ability to form a chelate withcalcium. Since it has been demonstrated that the adsorption of acarboxylate group must be performed through chelation then theadsorption mechanism for NaPAA onto calcium must be throughan eight-member chelate ring utilizing two adjacent carboxylategroups. This is important because the eight-member chelate ringis not very stable and could be an explanation for the physicalchanges of the slurry with aging.

Previously, it has been concluded that the adsorption modes ofcarboxylate groups are modified with solids loading; therefore, theadsorption of gallic acid at different solids loading was also inves-tigated. Similar to the results for NaPAA, there are changes in theadsorption modes with increase solids loading. Fig. 9 shows a shiftof the carboxylate band at 1547 cm�1 for a 20 wt.% GCC slurry to1560 cm�1 for a 67 wt.% GCC slurry. Along with the carboxylateshift there is a shift of the benzene band from 1594 cm�1 to1584 cm�1 for a 20–67 wt.% slurry. Comparing Fig. 9 and Fig. 3,both of them demonstrate an increase in the ratio of bridging tobidentate coordination with increasing solids loading. Again, theseresults can be explained by a decrease in the distance between cal-cium carbonate particles which would decrease the distance be-tween calcium allowing for additional bridging.

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-0.0016

-0.0008

0.0000

0.0008

bidentatebridging

C Cd2 A/d

υ2 (a.

u.)

Wavenumber (cm-1)

Fig. 8. Second derivative of the IR spectra of gallic acid in D2O and a 20 wt.% GCCslurry. The bands for the benzene ring and the carboxylate shift, indicatingadsorption.

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-0.0004

0.0000

0.0004

bidentate

bridging

67 wt% Slurry

C C

d2 A/d

υ2 (a.

u.)

Wavenumber (cm-1)

20 wt% Slurry

Fig. 9. Second derivative of the IR spectra of gallic acid in a 20 wt.% and 67 wt.% GCCslurry. The bands for the benzene ring and the carboxylate shift, indicating changein the coordination of the gallic acid with increasing solids loading.

302 J.J. Taylor, W.M. Sigmund / Journal of Colloid and Interface Science 341 (2010) 298–302

4. Conclusions

Several published works have focused on the interaction of car-boxylate groups with calcium and have provided valuable insight.But these works have been performed in dilute systems and do notaccount for the behavioral differences experienced in high solidsloading slurries. This paper is the first to address the adsorptionof NaPAA onto GCC in slurries up to 75 wt.%. First, the differencebetween a dilute system and a high solids loading system is dem-onstrated to be due to the change in adsorption coordination of thecarboxylate. As the solids loading increases there is an increase in

the bridging/unidentate and bridging/bidentate ratios which aredemonstrated in both the NaPAA and gallic acid systems. Second,as a high solids loading slurry ages there is a decrease in the con-centration of unidentate coordination of carboxylate groups and anincrease in the bridging and/or ionic coordination of the carboxyl-ate groups. This can be explained by an increase in the amount ofdissolved calcium ions with age which become available for theunidentate coordinated carboxylate groups to form bridging coor-dination. Third, NaPAA was demonstrated to adsorb onto calciumcarbonate through an eight-member chelate ring consisting oftwo adjacent carboxylate groups and does not adsorb through sin-gle carboxylate groups alone.

Acknowledgments

This research was partially supported by Kemira Chemicals, Inc.and Imerys Clays, Inc.

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