Keller [2000] Surface Energetics of Calcium Carbonates Using IGC

Colloids and Surfaces

A: Physicochemical and Engineering Aspects 161 (2000) 401–415

Surface energetics of calcium carbonates using inverse gaschromatography�

D. Steven Keller *, Philip LunerEmpire State Paper Research Institute, State Uni6ersity of New York, College of En6ironmental Science and Forestry, Syracuse,

NY 13210, USA

Received 4 November 1998; accepted 19 April 1999

Abstract

Various calcium carbonate samples were characterized by inverse gas chromatography (IGC) at infinite dilution.The interactions between the samples and hydrocarbon probes, C5–C10 were related to the retention volume in gaschromatographic experiments. The surface energetics of chalk, marble and precipitated calcium carbonate (PCC) weredetermined in terms of the apolar component of the surface free energy gS

LW and the differential enthalpy ofadsorption, −DHA

$ . These results were evaluated with respect to bulk chemical composition, BET surface area andpore structure. The results suggest that physi- and chemisorbed water bound at surface sites and within the porestructure significantly influence the surface properties of the calcium carbonates. The values of gS

LW and −�HA$ after

redeposition of the water adlayer suggested reversibility of the water sorption process and the resulting changes in theenergetics of the surfaces. Values of gS

LW for the chalk samples in the initial state were 140–180 mJ m−2, whereas themarble and PCC were :55mJ m−2. Removal of bound and pore water by heating at elevated temperatures causeda progressive increase in the gS

LW to :250 mJ m−2 for the PCC sample. The gSLW of the chalk sample also increased

but to a much lesser extent. �HA$ values for the samples followed the trend chalk\marble\PCC. © 2000 Elsevier

Science B.V. All rights reserved.

Keywords: Adsorption; Alkane; Calcite; Calcium carbonate; Chalk; Enthalpy; Inverse gas chromatography; Surface free energy;Thermodynamics; Water

www.elsevier.nl/locate/colsurfa

1. Introduction

The industrial importance of calcium carbonate(CaCO3) as a filler in composite materials such asplastics and paper is well recognized. As a result,

characterization of the surface energetics is criticalin developing an understanding of the forces ofinteraction between filler and substrate. Severalinvestigators have examined the modification ofthe surface properties of CaCO3 through the useof surface grafted compounds in order to improveparticle cohesion [1–4],. The typical physical char-acteristics of filler particles, i.e. fine particle sizeand irregular shape, precludes the use of moreclassical methods of surface chemical analysisused on large planer surfaces such as contact

� This article was originally submitted to the Per SteniusSpecial issue.

* Corresponding author. Tel.: +1-315-4706907; fax: +1-315-4706945.

E-mail address: [email protected] (D. Steven Keller)

0927-7757/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S0927 -7757 (99 )00212 -5

D. Ste6en Keller, P. Luner / Colloids and Surfaces A: Physicochem. Eng. Aspects 161 (2000) 401–415402

angle measurements and Wilhelmy plate experi-ments to obtain reliable surface energetic informa-tion. The surface energetics of CaCO3 has beenstudied using contact angle on cleaved crystals[5,6] and aggregated powders [7], although therelevance of these results to the fine particle sur-face may be questioned. Other methods have alsobeen used such as immersion calorimetry [8–10]and spectroscopic methods [11]. A general discus-sion of the subject was given by Sanders [12]. Amore direct approach to the characterization ofthe surface chemistry of fine particle calcium car-bonates has been to study the adsorptionisotherms of water vapor [13–17].

Inverse gas chromatography (IGC) is a methodthat was developed to determine the surface en-ergetics of fine powders and liquids by measuringthe dynamic adsorption characteristics of probegases [18]. The convenience of experimentationand control of test conditions has inspired therecent interest in applying IGC for the analysis offiller/pigments such as CaCO3 [19–21]. The effectsof treatment of fibers and calcium carbonates withalkylketene dimer has also been examined [22,23].An estimate of the dispersive (London) compo-nent of the surface free energy, gS

d, may be deter-mined by measuring the interaction of linearalkane probe gases with the surface. Polar gaseousprobes are used to determine the specific interac-tion parameters from which the relative acidityand basicity of the surface can be established[18,22–25].

Papirer et al. [2,3] utilized IGC in their investi-gation of precipitated calcium carbonate (PCC)and the effect of surface coverage with stearic acidon the surface properties. They determined thedispersive component of the surface free energy,gS

d, of untreated CaCO3 at 90°C to be 50.5mJm−2 and −�HA

$ for alkanes C5–C8 to rangefrom 9 to 17 kJ mol−1. Ahsan et al. [3] found−�HA

$ to range from 6.4 to 52.6 kJ mol−1 intheir work with PCC using C6–C8. Ahsan sug-gested that the discrepancy could be explained bydifferences in sample preparation or the condi-tioning of the carrier gas. IGC has also been usedto examine the interaction of polar probes withCaCO3 to determine specific interactions, and theacid/base activity of the surface [3,26].

The aim of this investigation was to utilize IGCto determine the gas/solid surface energetics ofseveral calcium carbonates and observe the effectof sample origin. The specific objectives were tocharacterize the interactions of alkanes with PCC,ground marble, and ground chalk using IGC atinfinite dilution. The method facilitates the deter-mination of the apolar component of the surfacefree energy, gS

LW, and the enthalpy of adsorption,�HA

$ , of the alkanes. The sensitivity of the IGCmethod to detect differences in the different sam-ples was also evaluated.

2. Experimental

2.1. Materials

Four samples of commercially available cal-cium carbonate were examined in this investiga-tion. The first was a synthetic PCC, Albacar 5970by Specialty Minerals Inc. A ground marble thatwas mined in Alabama was supplied by ECCAmerica Inc. as Atomite. Two samples, identifiedas chalk I and chalk II, were acquired fromdifferent sources. The chalk I sample was suppliedby Omya, Inc. from Germany. Chalk II was amined sample from Dover, England received innugget form from Ward Natural Science Est. asproduct no. 46E1445, 46E1447, 46E1449, andhand-ground to size immediately prior to analysis.

Particle shapes were examined using scanningelectron microscopy. The PCC particles appearedto be well-formed scalenahedrals as illustrated infig. 1(a). The ground marble particles were ob-served to have rhombohedral form, cf. fig. 1(b),although an increased amount of irregularity isevident. For the two chalk samples, cf. fig. 1(c),(d), discrete particles appear rhombohedral andclusters of coccoliths are clearly visible. The bulkphase crystal structure of the samples was exam-ined using an infrared spectroscopic technique[27] where the dominant structure in all sampleswas found to be calcite.

The particle size distribution of the sampleswere determined using a Sedigraph 5500L(Micromeritics) particle size analyzer. The results,expressed as apparent spherical diameter, are pro-

D. Ste6en Keller, P. Luner / Colloids and Surfaces A: Physicochem. Eng. Aspects 161 (2000) 401–415 403

Fig. 1. Scanning electron micrograph of calcium carbonate samples. (a) Precipitated calcium carbonate; (b) ground marble; (c)mined chalk; and (d) hand ground chalk.


Table 1Physical characteristics

Particle diameter Specific surface area Mercury porosimetry

Pore volume (cc1MaximumMedianSample Initial state Pore diameterOutgassed at 200°Cg−1)(m−2 g−1)(mm) (mm)(mm) (m2 g−1)

4.5 9.0 10.2PCC 0.972.4 1.1012 2.9 3.23.0 0.41Marble 0.6015 2.1 2.1 0.35Chalk I 0.903.2

vided in Table 1. Because of the small particlesizes, all samples required preparation to improvegas permeability in chromatographic columns toprovide acceptable pressure drops. This involvedthe formation of pellets by compression of thepowders to 15 kpsi in a hydraulic press. Pelletswere then ground and sieved to obtain a 250×400mm size fraction which was used to pack the gaschromatographic columns. A scanning electronmicrograph of aggregated PCC is shown in Fig. 2.Aggregation of the particles by this procedure wasfound to have no affect on the measured specificsurface area and permitted the analysis of micronsize particles without having excessively high-pres-sure differential across the chromatographiccolumn.

The specific surface areas of the samples weredetermined using a multipoint BET method withnitrogen as the adsorbate in a Quantasorb Sorp-tion System (Quantachrome). Pore volume analy-sis was performed on the PCC and chalk I samplesusing an ASAP 2000 (Micromeritics). The porevolume distributions were calculated using theBJH method [28,29]. Both samples were analyzedin two states. The first analysis was conductedwithout outgassing of the adsorbed water to ob-tain information on the pore size distribution ofthe samples in the ‘initial state’. The samples werethen examined following outgassing at 200°C for24 h, which was similar to the exposure conditionsused in IGC analyses. The results of all surfaceanalyses are shown in Table 1. Bulk phase elemen-tal analysis was conducted on the samples usinginductive coupled argon plasma/atomic emissionspectroscopy, ICP/AES using an FMA-03 (Spec-tro Analytical Instruments). The results are pro-vided in Table 2 as oxide equivalents.

IGC experiments were conducted using a HP5890 Series II gas chromatograph (Hewlett-Pack-ard) equipped with a flame ionization detector.The instrument was directly interfaced to a PCcomputer for system control and acquisition ofchromatograms. Stainless steel columns with adiameter of 5.34 mm and a length of 60.0 cm werepacked with approximately 10 g of aggregatedCaCO3 particles. Preconditioning of the columnswas carried with the columns in the gas chro-matograph. High purity nitrogen (99.999%) thathad been passed through two moisture/oxygentraps was used as the carrier gas. The flow rate ofthe N2 was controlled at 20.590.1 ml min−1.Reagent grade alkanes, C5–C10 were used as test

Fig. 2. Scanning electron micrograph of precipitated calciumcarbonate (PCC) aggregates prepared by compression for usein inverse gas chromatography (IGC) column packing.


Table 2Trace chemical composition in wt% oxide equivalent

SiO2 Na2OSample Fe2O3MgO MnO2

0.06 0.21PCC 0.050.26 –0.64 0.190.55 0.06Marble –3.16 0.31Chalk I 0.140.26 .040.44 0.260.20 0.07Chalk II .04

quired to completely elute the adsorbate, less thedead volume of the column, per unit mass orsurface area of adsorbent. Many of the earlierstudies in IGC, especially with polymers, involvedthe analysis across the gas/liquid interface whereit was useful to express the specific retentionvolume in terms of adsorbent mass, Vg°. However,for the analysis of solid surfaces, as in the presentstudy, expression in terms of specific surfacearea,VS°, is more appropriate.

The mass and area specific retention volumesare related to the retention time, t, of injectedprobes and the specific surface area, Asp, by therelationship [31]

VSo=

Vgo

Asp

=jFcol(tp− tm)

Aspw(1)

The subscripts p and m identify the retentiontimes of the probe and a non-interacting marker,respectively. The parameter w is the weight ofadsorbent in the column, j represents the James–Martin correction for compressibility, and Fcol isthe flow rate of carrier gas within the column thathas been corrected for column temperature. Amore detailed discussion of experimental parame-ters is given elsewhere [20,23].

The partition coefficient or Henry’s law con-stant, KS, may be expressed as [32]:

KS= limc�0

�dGdc

�A, T

(2)

where c is the molar concentration of the adsor-bate in the gas phase and G is the molar concen-tration per unit surface area of the adsorbent,assuming constant surface area, A, and tempera-ture, T. For analysis at infinite dilution, (or zerocoverage), Eq. (2) becomes the simple ratio of G/cand may take the form:

VSo=

Vgo

Asp

=KS

RTC

(3)

where R is the gas constant and TC is the columntemperature.

The differential enthalpy of adsorption, �HAo ,

may be determined by measuring values for VSo at

different column temperatures and applying therelationship [31]:

probes. Methane was used as the non-interactingreference marker to determine the dead volume ofthe column. Chromatograms were recorded foreach probe. All calculations were conducted usingthe first moment of the peak as the retention timeand symmetry of the peak was verified by com-parison of these results with the time at peakmaximum.

Two approaches were taken in the examinationof the CaCO3 samples using IGC analysis. Thefirst, referred to as sequential isothermal analysis,involved monitoring changes in V s° at differentintervals up to 150 h as the sample was held at aconstant temperature under a :20 ml min−1 N2

purge. New columns were prepared for each tem-perature (60, 100, or 140°C). The second experi-mental approach, referred to as preconditionedanalysis, involved the outgassing of the column atspecified elevated temperatures, Tp, before IGCanalysis [3,22,30]. The effect of preconditioningtemperature was examined because general agree-ment on an ideal pretreatment temperature forCaCO3 did not exist. Therefore, samples wereconditioned at Tp=100, 200, or 300°C for 24 hprior to IGC analysis. The retention times of thealkane probes were then determined at Tp and atseveral lower temperatures to determine gS

LW and�HA

$ .

2.2. IGC theory

Calculation of the thermodynamics of adsorp-tion from IGC analysis is based on the determina-tion of the specific retention volume, V°, of aknown adsorbate probe as it passes through achromatographic column containing the adsor-bent under examination. The specific retentionvolume represents the volume of carrier gas re-


�HAo = −R

d(lnVSo)

d(TC−1)

(4)

is obtained graphically from the slope of theplot of −R lnV s° versus 1/TC.

The standard molar free energy of adsorption,�GA

o , of the probe on the adsorbent is relatedto the Henry’s law constant by the relationship

�GAo = −RT ln

�KSpsg

ps

�= −RT lnVS

o+C (5)

Here a standard reference gas pressure, psg,and the spreading pressure, ps, are defined bythe chosen reference state [33,34]. The integra-tion constant, C, is a function of these parame-ters and may be assumed constant for a givenadsorbent and a homologous series of probes.

When non-polar molecules, such as n-alkanesare used as probe gases in IGC, the energeticsof adsorption are considered non-coulombic andresult entirely from Lifshitz–van der Waals(LW) interactions. The LW contributions to thesurface free energy, gS

LW, encompasses the elec-tromagnetic interactions including the dispersive(London), gS

d, induction (Debye), gSi , and orien-

tation (Keesom), gSm, components so that:

gSLW=gS

d+gSi +gS

m (6)

In the case of IGC experiments conducted atinfinite dilution of the gaseous probe, the pair-wise-addition approximation [35] provides ameans to interpret the significance each separatecomponent to the overall LW interaction. Theassumption in this case is that intermoleculardistances between probe molecules are large andthe Debye and Keesom components are notsuppressed by neighboring molecules as in con-densed phase-condensed phase interactions, e.g.liquid/solid or solid/solid. The impact of thisperspective on existing theory for the determina-tion of gS

d by IGC, and experimental evidencefor the significant effect that surface polarity hason the apolar interactions that occur in IGC aregiven in this section.

A method for estimating the London-disper-sive component of surface free energy, gS

d, fromIGC data was proposed Dorris and Gray [31].Their approach involved the use of Fowkes [36]

equation for the work of adhesion of a non-po-lar liquid and a surface in terms of the incre-mental change in free energy per methylenegroup, �GA

CH2. Values for �GACH2 are accessed

from the difference in �GAo for adjacent mem-

bers of a homologous series of linear alkanesfrom the relationship:

�GACH2= −RT ln

�KSn+1

KSn

�= −RT ln

�VSn+1

VSn

�(7)

�GACH2values are determined graphically as the

slope of a plot of VSo versus the number of car-

bons in the alkane, n. The equation that Dorrisand Gray [31] derived may be rearranged tosolve for gS

d so that

gSd=

1gCH2

��GACH2

2Na�2

(8)

where N is Avogadro’s number, a is the crosssectional area of the methylene group (6A, 2) andgCH2

is the surface energy of a solid consisting ofonly -CH2- groups, i.e. polyethylene.

Since the interactions of the alkanes with thesurface will involve all of LW interactions, thesolution to Eq. (8) is more appropriately givenas gS

LW [37]. Furthermore, because the probesare in the dilute gaseous state, the Debye induc-tion interactions, gS

i , that result from dipoles in-duced in the alkane molecules by the electricfields at the adsorbent surface may be consider-able [35], especially for high energy materialssuch as mineral fillers [38]. Dorris and Grayconsidered the significance of the inductionforces even for the low energy cellulosic surfacesthat they studied [31]. This aspect, however, hasbeen overlooked in many subsequent investiga-tions where gS

d derived from IGC experimentshave been reported for both low- and high-en-ergy materials. It may be valid to assume thatthe interaction of gaseous alkanes with low en-ergy surfaces, absent of appreciable electricfields, are dominated by dispersive (London) in-teractions. It should be noted that for thealkane/solid system, the Keesom orientationterm, gS

m, is zero because n-alkane probe gasesdo not possess a permanent dipole.


3. Results

3.1. Sequential isothermal IGC analysis

The relationship between VSo and the exposure

time, t, for the PCC sample with Tc=100°C isshown in Fig. 3. In this logarithmic plot, VS

o

continually increases with the progression of theexperiment for each of the alkanes tested. Thechange in VS

o is a direct indication of Ks becausethe change in specific surface area is minimal, cf.Table 1. This indicates that the probe has anincreased affinity for the PCC surface with theduration of the experiments. In all of the curves,the rate of change appears to occur in two phases.In the first phase a linear change occurs until:30–40 h when the rate of change diminishes.This may indicate either a change in the mecha-nism of adsorption of the gases or a change in thesurface characteristics of the adsorbent.

The effect of column temperature, Tc, was ex-amined for PCC with hexane and octane at 60,

100, and 140°C as shown in Fig. 4. Although VSo

is observed to increase with exposure time at thethree Tc values, there does not appear to be asignificant change in the trend with temperature.At the lower temperatures of 60 and 100°C thegS

LW values for the different CaCO3 did not varysignificantly with time.

The PCC was tested at 140°C where its initialvalue of gS

LW was 50 mJ m−2 and increased to 65mJ m−2 after 100 h. This evidently resulted fromthe effects of conditioning temperature on gS

LW

which will be examined in detail in the nextsection.

The increase of VSo with exposure time was also

determined with the marble and chalk I samplesat 100°C. Selected results are illustrated in Fig. 5where VS

o values of heptane are plotted as afunction of t for each of the CaCO3 samples. Thevariation in VS

o is substantially different for thethree samples. The greatest relative increase in VS

o

was found for marble followed by PCC, and thenchalk I, which changed very little. The range of C7

Fig. 3. Sequential isothermal analysis of precipitated calcium carbonate (PCC). Temperature of column during test (TC) was 100°C.Specific retention volume of various alkanes is plotted as a function of exposure time.


Fig. 4. Sequential isothermal analysis of precipitated calcium carbonate (PCC). Temperature of column during test (TC) was heldat either 60, 100, or 140°C. Specific retention volume of hexane and octane is plotted as a function of exposure time.

retention times, tp, cf. Eq. (1), that correspond tothe curves illustrated in Fig. 5 are: PCC 0.97–2.1min, marble 0.66–1.44 min, and chalk I 3.68–6.36min. These values are subject to an experimentalerror of B1%. The behavior observed in theseexperiments indicates that the CaCO3 surfacescontinually undergo physical and/or chemicalchanges even beyond 100 h. In view of theseresults, the CaCO3 samples were then examinedfollowing heating pretreatment.

3.2. Preconditioned IGC analysis

Fig. 6 shows the results from the analysis of thevarious CaCO3 samples and are expressed as gS

LW

(determined at 100°C) versus preconditioningtemperature, Tp. The marble and the PCC exhibitsimilar behavior gS

LW increases dramatically withTp\150°C. The results calculated from the dataof Ahsan et al. [19,30], Schmitt et al. [3] andLundqvist et al. [21] are also included. There isclose agreement between the results of these stud-ies and this investigation for calcium carbonate

samples. There is also agreement between theresults obtained for the marble sample, 57 mJm−2, and those reported by Janczuk et al. [6]using contact angle on marble plates 64mJ m−2.The influence of Tp on the observed gS

LW are alsoapparent on the three other samples and theselection of a single preconditioning temperatureto characterize the CaCO3 appears inappropriate.Chalk samples I and II both have high gLW

S valuesand only small variation with Tp as compared toPCC and marble. It appears that the surfaces ofthe chalk samples require minimal conditioning toattain higher energies as compared to the PCCand marble (that require higher Tp to attain highenergy surfaces).

Fig. 7 shows the −�HAo values for chalk I and

II, PCC, and marble samples that were condi-tioned at 200°C. As with gS

LW, the chalk sampleshad the highest energy of interaction and PCChad the lowest. The slope of −�HA

o versuscarbon number was also greater for the chalk Isample than the PCC and marble samples. This


originates in higher values of �GACH2 and thus

higher gSLW values.

Fig. 8 shows the effect of the conditioningtemperature on enthalpy of adsorption, −�HA

o ,for the PCC. A 25% increase in −�HA

o wasobserved between heating at 200 and 300°C. Di-rect determination of −�HA

o for temperaturesB200°C were not made because of the variation ofV°S with time. In order to approximate −�HA

o ofthe samples before surface changes occur, a newparameter was introduced which is termed theintrinsic enthalpy of adsorption, [�HA

o ]o. Deter-mination of [�HA

o ]o involved the extrapolation ofVS

o values, cf. Fig. 4, to t=1 min for each of thetemperatures tested. Three temperatures, 60, 100,and 140°C, were included in the extrapolation forthe PCC sample shown in Fig. 8. The [�HA

o ]ofalls well below the preconditioned samples andalso the standard heat of condensation, −�HC

o .This could result from the mode of condensationof the alkane on surface sites as discussed theoret-ically by Kiselev [39].

4. Discussion

4.1. Pore structure

The variation in adsorption characteristics thatoccurs in the sequential isothermal IGC analysismay at first appear to result from capillary con-densation of the alkane probes in newly formedpores [40]. Carrott and Sing [41] in their workwith porous carbons demonstrated that the ad-sorption energy, i.e. −�HA

o and gSLW, signifi-

cantly increased for samples with greater porevolume. The chalk I and chalk II sample gaverelatively high, −�HA

o and gSLW as compared to

the PCC and marble samples which was at firstspeculated to result from molecular sieving [25] inthe internal porous structure. However, the BETsurface area analysis, cf. Table 1, indicated thatthe chalk I sample had the lowest Asp which didnot support the conjecture. In order to clearlyresolve the influence of the pore structure, a moredetailed analysis was conducted to quantify the

Fig. 5. Sequential isothermal analysis of calcium carbonates. Specific retention volume of chalk I (mined), precipitated calciumcarbonate (PCC), and ground marble are is plotted as a function of exposure time.


Fig. 6. The apolar component of surface free energy for various calcium carbonates plotted as a function of the preconditioningtemperature. The temperature of column was held at 100°C during the test. The results from other investigators are also shown onthe plot.

size distributions of the pores using nitrogen ad-sorption and mercury intrusion porosimetry. Fig.9 illustrates the results of nitrogen adsorptionporosimetry for the chalk I and PCC samplesfollowing a standard pretreatment at 200°C for 24h. Clearly the chalk I sample has less fine porevolume than the PCC sample. It may be inferredthat the relatively high surface energetics observedwith the chalk I sample is not the result of in-creased condensation of the alkanes in the porestructure.

Earlier investigations [15,16] suggested that anincrease in pore volume is associated with theremoval of adsorbed water CaCO3. This aspectwas explored in detail by analyzing the pore vol-ume distributions of the PCC and chalk I samplesbefore and after sample conditioning at 200°C.The results are shown in Fig. 9 as a plot ofDV/DD at various diameter intervals. For bothCaCO3 samples, the pore volume increases withheating at 200°C for diameters less than 100 A, .Table 3 provides the cumulative increase in pore

volume that occurs for PCC sample and chalk Ifor DB100 A, following preconditioning at200°C. The chalk I sample, while having lessoverall pore volume, had a greater relative in-crease in the pore volume following heating.Whereas, it has already been demonstrated thatthe chalk I sample changed the least during se-quential isothermal IGC analysis, cf. Fig. 5. Thisfortifies the conclusion that changes in the porestructure that occur during dewatering are notsignificantly responsible for the differences in sur-face energetics that are observed in the IGCanalyses.

4.2. The role of water

The evolution of water from the samples bydesorption of the physisorbed water or by dehy-droxylation of Ca(OH)2 was expected to occur inall of the CaCO3 samples under the conditions ofthe sequential isothermal IGC analysis and inpreconditioning [17]. To quantify the effects of


adsorbed water on V°S, columns that had beenpreviously heated to 100 °C for more than 100 hwere purged with saturated water vapor for 24 h,and again subjected to sequential isothermal IGCanalysis. The results for PCC with hexane, octaneand decane are shown in Fig. 10. The very closeagreement between the two curves suggests thatthe changes in adsorption are directly related tothe presence or absence of water at the surface.

A similar experiment was conducted using CO2

instead of water vapor to purge the column. TheVS

o values were unaffected by CO2 treatment indi-cating insignificant change in the surface energet-ics. Several investigators [8,9,17] havedemonstrated that the CaCO3 decomposes veryslowly at temperatures below 300°C. The resultingCaO may provide sites that give different VS

o

values and are readily hydroxylated to chemisorbwater.

The effects of chemisorbed water on the surfaceenergetics of mineral oxides were investigated bySuda et al. [38,42]. They found that the electro-static field strength of the materials was inverselyproportional to the surface hydroxyl concentra-tion. Fig. 11 shows a plot of gS

LW versus the

Fig. 8. Differential enthalpy of adsorption of precipitatedcalcium carbonate after preconditioning at 100, 200, or 300°C.The plot shows the heats of condensation of the alkanes (DHC)Also shown are the DHa indicating the initial state valuesdetermined by extrapolation of sequential isothermal plots.

estimated monolayer coverage of water for thethree CaCO3 samples. Monolayer water coveragewas calculated using the total weight loss duringconditioning and a molecular area of water of10.4 A, 2 [29]. The estimation assumes that theweight loss is entirely as a result of water from thesurface and that the water is completely desorbed

Fig. 7. Differential enthalpy of adsorption of alkanes on theCaCO3 surfaces. All samples were preconditioned at 200°C for24 h.

Fig. 9. Pore size distribution of precipitated calcium carbonate(PCC) and chalk with and without preconditioning at 200°C.


Table 3Pore volume characteristics of PCC and chalk I

Pore volume BSample D Pore volume100 A,

(10−3cc−1 g−1) (mg−1 m−2)(10−3cc−1 g−1)

PCC3.47Not

conditioned4.17 0.70Conditioned 0.88

at 200°C

Chalk I1.23Not

conditioned1.71Conditioned 0.48 1.38

at 200°C

curves as gSLW decreases with the adsorbed water

concentration. Dorris and Gray observed a similartrend with water on silica [44].

The effect of water on decreasing the interactionsof gaseous alkanes with CaCO3 surfaces has beenreported by several investigators [7,14]. The wateracts to present a low energy surface to the non-po-lar molecules vapors [45,46] and interacts mostlybecause of dispersion (London) forces. In IGC, thisis manifested as shorter observed retention times.When dry heating desorbs the water layer, highlypolar sites are exposed which increases the magni-tude of the dispersion (London) and induction(Debye) forces substantially [38]. Fig. 12 illustratesa model of the surface that accounts for the changein gS

LW that accompanies the desorption of waterfrom the CaCO3 surface. Trace amounts of con-taminants at the surface, e.g. silica or iron, maycontribute to the increased surface energies of thechalk samples. Although important differences inthe surface energetics of the three CaCO3 samplesexist, identification of the origin of such differencesis still under investigation.

at 300°C. The difference between the differentforms of CaCO3 is apparent. The results fromChassin et al. [43] in their investigation of Ca-mont-morillonite using contact angle measurements arealso included in Fig. 11. Note the similarity in the

Fig. 10. A comparison of the sequential isothermal analysis of precipitated calcium carbonate before (solid symbols) and after (opensymbols) regeneration with water vapor. The temperature of column was held at 100°C during the test.


Fig. 11. The apolar component of the surface free energy plotted as a function of the apparent monolayer water coverage of CaCO3

samples.

5. Conclusions

The usefulness of IGC for identifying differ-ences in the surface chemistry of calcium carbon-ates was demonstrated in this investigation. Themethod appears quite sensitive to the changes insurface energetics that result from desorption ofsurface water at submonolayer concentrations.Non-polar probe gases were used to monitor thechange in the apolar component of surface freeenergy as samples were subjected to precondition-ing at temperatures between 100 and 300°C, re-ferred to as preconditioned analysis. PCCexhibited a substantial increase from :50 mJm−2m to :250 mJ m−2. Ground marble showedsimilar behavior to the PCC. Two chalk sampleshad much greater apolar components (:140 and180 mJ m−2) and showed less change with pre-conditioning temperature.

Sequential isothermal analysis was introducedas a method for monitoring the change in the test

sample imparted by the carrier gas sweep of theIGC test. Dehydration that occurs during the testchanges the energetics of interaction of the probeadsorbates well beyond the conventional condi-tioning time of 24 h. The dynamic state of the testcolumn can interfere with the measurment of en-thalpy of adsorption, and apolar component ofsurface energy if such changes are not considered.

Continuation of this investigation has includedIGC experiments under equilibrium conditionswhere the water in the carrier gas is held constantat very low concentrations.

Acknowledgements

The authors would like to thank N. Sanders ofSpecialty Minerals Inc. for providing PCC sam-ples, and P.R. Suitch and R.J. Pruett of ECCInternational for conducting sample analyses. Theefforts of D. Driscoll, R. Hanna, and A. Day of


Fig. 12. Proposed mechanism for the increase in the apolarcomponent of surface free energy with thermal pretreatmentand dry sweep dehydration. With mild preconditioning, boundwater is retained at the calcium carbonate surface and tends toshield the electric field. The induced (Debye) component isnegligible. As the heat and dry gas sweep of rigorous precondi-tioning remove water, the electric field of the crystal surface isexposed inducing a dipole in the dilute alkane molecules. Theinduced (Debye) component is by the increased interactionbetween the permanent dipole of the CaCO3 crystal and theinduced dipoles in the alkane molecules.

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Keller [2000] Surface Energetics of Calcium Carbonates Using IGC

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