Journal of Colloid and Interface Science 345 (2010) 168–173

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Journal of Colloid and Interface Science

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Preparation and characterization of coverage-controlled CaCO3 nanoparticles

Yang Li a, Zhong-fu Zhao a,*, Yiu-Ting Richard Lau b, Yong Lin b, Chi-ming Chan b,*

a State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, 158 Zhongshan Road,Dalian 116012, Chinab Department of Chemical Engineering and Advanced Engineering Materials Facility, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong

a r t i c l e i n f o a b s t r a c t

Article history:Received 29 September 2009Accepted 25 January 2010Available online 1 February 2010

Keywords:CaCO3 nanoparticlesSurface modificationTuned coverageExtraction

0021-9797/$ - see front matter � 2010 Elsevier Inc. Adoi:10.1016/j.jcis.2010.01.080

* Corresponding authors.E-mail addresses: zfzhao@dlut.edu.cn (Z.-f. Zha

Chan).

CaCO3 nanoparticles were coated with stearate through a salt-exchange procedure. Their coverage hadbeen successfully controlled by extraction in a Soxhlet apparatus, based on which a series of CaCO3 nano-particles were obtained with different surface coverages. They were characterized with thermogravimet-ric analysis, X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. It was foundthat free stearate, intercalated stearate and chemically-bonded alkyl chains could be extracted sequen-tially with the Soxhlet apparatus. Thus, the coverages of CaCO3 nanoparticles could be adjusted throughcarefully extracting the stearates from the CaCO3 nanoparticles with multi-layer coverage. Spectroscopicresults revealed that the alkyl chains tended to adopt an extended-chain conformation in the monolayercoverage as well as the bi-layer coverage, but less ordered conformation in partially-coated coverage andrandom orientation at the outmost surface of the coated nanoparticles.

� 2010 Elsevier Inc. All rights reserved.

1. Introduction

Compared to micro-size particles, nanoparticles have been paidgreat attention for their excellent contribution to various proper-ties of modified materials. One key advantage of using nanoparti-cles in polymer matrix is that nanoparticles have a very largesurface area, which affects the particle–polymer interactions,leading to significant changes in the mechanical properties of thepolymer nanocomposites [1–3]. As one of most important nano-particles, CaCO3 nanoparticles have been widely studied and uti-lized in academic and industry society because of its commercialavailability and the substantial improvement of mechanical prop-erties of polymers [4–11].

The foremost challenge of using CaCO3 nanoparticles is toachieve their uniform spatial distribution in polymer matrix dueto their incompatible interface. What is more, nano-sized particlestend to aggregate together since they have a large specific surfacearea and high surface tension. It is extremely difficult to homoge-nize polymer matrix with virgin CaCO3 particles just relying on theshearing force in melt-blending. Surface modification of nanoparti-cles is an effective way to reduce their surface tension and increasethe compatibility between them and polymer matrix. Chan et al.successfully increased the Izod impact strength of polypropylenematrix up to 2.5 times with surface modified 44-nm CaCO3 nano-particles [10]. Their transmission electron microscopy images

ll rights reserved.

o), kecmchan@ust.hk (C.-m.

showed that the agglomerates of CaCO3 nanoparticles were brokendown to primary particles in polymer matrix, resulting in good dis-persion of the nanoparticles. Wang et al. also found that CaCO3

nanoparticles uniformly coated with a surfactant benefited the dis-persion of nanoparticles in polymer matrix, leading to high impacttoughness of the nanocomposites [12]. The surfactants bonded onthe surfaces of CaCO3 nanoparticles have weak interactions withthe polymer matrix to facilitate interface de-bonding, which isimportant in releasing plastic constraints and improving thetoughness of the nanocomposites [13–16].

In a polypropylene matrix, the coating nature and coverage ofCaCO3 nanoparticles affect the mechanical properties of their nano-composites through varying the crystallization behavior. The gen-eral agreement about the nucleation effects of micro-sized CaCO3

particles on polypropylene is that the uncoated CaCO3 particleshas a strong nucleation effect on the crystallization rate of polypro-pylene whereas fatty acid coating diminishes the nucleation effect[17,18]. However, only a few studies [19,20] asserted that stearicacid coated CaCO3 nanoparticles do not act as a nucleating agent.The opposite viewpoint is overwhelming, assenting that coatedCaCO3 nanoparticles have nucleating effect [2,9,21,22]. Their dis-crepancy is unclear. Since commercial CaCO3 nanoparticles areusually pre-coated with organic surfactants. The variation of thecoating nature and coverage on various nanoparticles from differentsuppliers makes it difficult to systematically investigate the effect ofthe coating nature and coverage on the crystallization behavior andthe mechanical properties of CaCO3/polymer nanocomposites.

To make these questions clear, CaCO3 nanoparticles should beprepared with controlled coverage and clear characterization. So

Y. Li et al. / Journal of Colloid and Interface Science 345 (2010) 168–173 169

far, two conventional coating methods are mainly utilized to treatthe raw CaCO3 particles — dry coating and solution coating [23–28]. The dry method involves blending CaCO3 particles with a sur-factant in a high-power mixer at temperatures above 100 �C. In thesolution method, CaCO3 particles are treated with the solution of asurfactant in an apolar organic solvent. However, all these methodscan introduce both chemically- and physically-absorbed surfac-tants in the coverage and do not necessarily guarantee controlledcoverage of the organic molecules on the surfaces of CaCO3

nanoparticles.Characterization of the coating nature and coverage is essential

for the subsequent investigation of CaCO3 nanoparticles and theirpolymeric nanocomposites. The surface coverage has been studiedby using several techniques, such as sedimentation methods[29,30], dissolution method [25], X-ray photoelectron spectroscopy(XPS) [31], and thermogravimetric analysis (TG) [24]. Sedimenta-tion method is phenomenological, unable to determine the abso-lute value of the surface coverage. The dissolution method isunable to accurately determining the monolayer coating becausethe dissolution of physically-absorbed surfactant from the particlesurface into the solvent is actually a desorption–absorption equi-librium process. XPS is a surface-sensitive technique. Theoretically,monolayer coverage is considered to be achieved, if the coatingthickness is equal to the fully extended chain length of stearic acid[28,30]. In fact, the calculated values based on XPS hardly agreeswith the theoretical coating thickness at monolayer coverage[8,13]. Although TG is by far the most convenient and effectiveway to characterize the surface coverage of CaCO3 particles coatedwith fatty acids, it only can roughly differentiated the physically-and chemically-absorbed components of surfactants decomposedat different temperatures due to their different thermal stabilities[24].

In the present study, a systematic methodology will be devel-oped to manipulate the coverage structures of alkyl stearates onCaCO3 nanoparticles. Several techniques will be combined for thecharacterization of the controlled coating coverage of CaCO3

nanoparticles.

2. Materials and methods

2.1. Materials

Partially-coated CaCO3 nanoparticles (CCR) were supplied byGuang Ping Nano Technology Group (Hong Kong). They had anaverage particle size of 44 nm [10]. Stearic acid and calcium stea-rate were purchased from International Laboratory (USA) and Rie-del-de Haën (Germany), respectively. Sodium stearate wasprepared by mixing equivalent mole of stearic acid and sodiumhydroxide in water (80 �C).

2.2. Surface coating

It is accepted that CaCO3 particles react with carboxylic acids ortheir salts to produce a layer of calcium salt on their surfaces [31].In this work, a series of CaCO3 nanoparticles with different cover-age were prepared by a salt-exchange procedure. First, ultrasonictreatment was used to uniformly disperse 10 g of CCR nanoparti-cles with a 40 ml solution mixture of water and alcohol in a volumeratio of 3:1. The suspension was stirred by a magnetic stirrer for2 h to completely wet the particles. Then, the suspension washeated to and maintained at 80 �C. A fixed amount of sodium stea-rate emulsion, prepared by mixing equal amounts of sodium hy-droxide and stearic acid in water at 80 �C, was gradually addedto the suspension drop by drop. After 2 h of reaction time, the mag-netic stir was stopped and the surface-treated CaCO3 nanoparticles

settled at the bottom of a beaker. The surface-coated nanoparticleswere finally filtered out, washed with hot water (80 �C) and finallydried in an oven at 105 �C over night. stearic acid 5, 10, 15 and20 wt.%, respectively, were used to prepare a series of the coatedCaCO3 nanoparticles (including the original coating amount of3.1 wt.% stearic acid on the as-received CCR). The final coatingamounts of the stearic acid on the nanoparticles were 5.0, 9.4,13.7 and 18.2 wt.%, as determined from thermogravimetric analy-sis (TG). Correspondingly, the samples were designated as CCR5,CCR10, CCR14 and CCR19, respectively.

2.3. Characterization

TG (TA TGA 2950) was carried out at a heating rate of 20 �C/minfrom room temperature to 600 �C in an air stream. The weight ofeach specimen was about 4.5 ± 0.1 mg and the weight loss ofparticles was calculated in the temperature range of 200–500 �C.X-ray photoelectron spectroscopy (XPS) measurements were per-formed on a PHI 5600 multi-technique system equipped withmonochromatic Al monochromatic X-ray source. A pass energy of23.5 eV was used to obtain high resolution photoelectron emissionspectra at the take-off angle of 45�. The specimens were also ana-lyzed with a FTS 6000 Fourier Transform Infra Red spectrometerequipped with a diffuse reflectance accessory (DRIFT). One partof the coated particle specimen was diluted by four parts of finelyground KBr powder for the DRIFT measures. These samples werefurther diluted with KBr and characterized with Fourier transforminfrared spectroscopy (FTIR). The resolution was set at 0.5 cm�1.

3. Results and discussion

3.1. Surface coating of CCR particles

CCR is one kind of commercially available CaCO3 nanoparticlespartially pre-covered with �3.1 wt.% of stearates (Fig. 1a). Its driedpowder has a specific surface area of �28 m2/g [10]. A dissolutionmethod developed by Fekete and his coworkers is used to estimatethe amount of stearic acid that is needed to produce a monolayercoverage on the surface of micro-sized CaCO3 particles [25,26].They coated the CaCO3 particle with the solution method. Whena small amount of stearic acid was used, there was no detectableresidual stearic acid in the solvent. With increasing the amountof stearic acid used, a point was reached above which residual stea-ric acid was found in the solvent. This point was denoted as themaximum amount of irreversibly bonded stearate (C100). A plotof the C100 against the specific surface area of CaCO3 fillers showeda good linear relationship [25]. The amount of stearic acid that isneeded to produce a monolayer coverage on the CCR nanoparticlesis estimated to be about 10.2 wt.% based on the extrapolating ofthis curve to the specific surface area of the CCR nanoparticles(28 m2/g). Thus, CCR has a surface coverage far less than amonolayer.

Fig. 1 displays the TG and derivative thermogravimetric analysis(DTG) curves of CCR, CCR5, CCR10, CCR14 and CCR19. Their weightlosses are calculated to be 3.1, 5.0, 9.4, 13.7 and 18.2 wt.%, respec-tively. The relative position of the DTA peaks can infer the charac-teristics of the organic components on the surfaces of CaCO3

particles [24]. CCR and CCR5 only have one maximum peak at330 �C and 335 �C, respectively. As the input content of stearatesis increased, new peaks appear. In the spectrum of CCR10, thereis a second peak at about 110 �C. CCR14 and CCR19 both show a to-tal of three peaks, indicating that there are different species on thesurfaces of the nanoparticles. It should be noted that these peaksare unlikely to be related to the decomposition of stearic acid,calcium stearate and sodium stearate because the DTG curves of

Fig. 1. (a) TG and (b) DTG curves of CCR, CCR5, CCR10, CCR14 and CCR19.

170 Y. Li et al. / Journal of Colloid and Interface Science 345 (2010) 168–173

stearic acid, calcium stearate and sodium stearate show only onemaximum peak at about 280 �C, 490 �C and 510 �C, respectively(see Supporting information).

In Fig. 1b, the peak at 110 �C became more intense when theamount of stearic acid increases from 5 to 18.2 wt.%. This peak isreasonably attributed to the presence of the water molecules ad-sorbed by the polar head of sodium stearate. At the surface cover-age less than a monolayer (see the partially-coated coverage inFig. 2), the stearate is chemically bonded onto the CaCO3 surface,leaving a hydrophobic alkyl chain at the outmost surfaces. There-fore, the absorption of water molecules on the CaCO3 surface isvery limited. As the coverage is more than a monolayer, sodiumstearates intercalate into the inner layer through hydrophobicinteraction among alkyl chains and leave the Na+ head at the out-most surfaces (see bi-layer coverage in Fig. 2), which has a strongaffinity to water, resulting in a higher intensity of the peak at110 �C. Thus, the presence of the peak at 110 �C is indicative ofthe appearance of intercalated sodium stearate molecules (also in-clude the free stearate molecules in the case of multi-layer cover-age), most of which form the outmost layer of bi-layer coverage.With increasing the input content of stearic acid, the shoulder peakadjacent to the main peak, which reasonably corresponds to thedecomposition of the free sodium stearate molecules, also becomesmore intensive (see multi-layer coverage in Fig. 2). The most in-tense peak corresponds to the chemically bonded monolayer ofcalcium stearate and the intercalated sodium stearates. Based onthe facts that the surface area of the CCR nanoparticles is measuredto be 28 m2/g and the area occupied by each Ca+ ion is assumed tobe 0.2 nm2, the amount of stearic acid that is needed for a mono-layer coverage is calculated to be 6.6 wt.% [26]. According to this

Fig. 2. Schematic diagram of CaCO3 nanopar

value, CCR, CCR5, CCR10, CCR14 and CCR19 should have a surfacecoverage of about 0.47, 0.76, 1.42, 2.08 and 2.76 times of mono-layer, respectively.

The structural arrangement of the stearate molecules can be in-ferred from the DRIFT spectra, as shown in Fig. 3. Although thestretching C–H vibrations in the range of 2800–3000 cm�1 aresuperimposed on the first overtone bands of the carbonate anionof CaCO3, the coated CCR gives rise to a higher intensity of this bandsin 2800–3000 cm�1 as well as the absorptions at 1580 cm�1 and1540 cm�1 due to the –COO– group [31]. These three absorptionsconfirm that the alkyl chains of calcium stearates are chemicallybonded to the CaCO3 particle surfaces. A characteristic absorptionat 1110 cm�1, which is not found in the pure calcium carbonate, ex-ists in pure sodium stearates. This absorption, which probably rep-resents the existence of sodium stearates in bi-layer or multi-layercoverage, is detected in the spectra of CCR10, CCR 14 and CCR 19.

3.2. Extraction

CCR, CCR5, CCR10, CCR14 and CCR19 were extracted with etha-nol in a Soxhlet apparatus for 3 days. We performed the sameextraction experiments on pure stearic acid, sodium stearate andcalcium stearate in the Soxhlet apparatus. These chemicals finallydisappeared after 24 h of extraction, showing that the apparatuscould wash away any free stearic acid, sodium stearate and cal-cium stearate from the specimen surfaces. Fig. 4 shows their TGand DTG results after Soxhlet extraction with the weight lossesof 2.3, 3.2, 5.0, 8.8 and 14.2 wt.% for CCR, CCR5, CCR10, CCR14and CCR19, respectively. Clearly, the weight loss decreases after

ticles with various coverage structures.

Fig. 3. DRIFT spectra of CCR, CCR5, CCR10, CCR14, CCR19, pure sodium stearate andCaCO3 as received.

Fig. 5. DRIFT spectra of CCR, CCR5, CCR10, CCR14 and CCR19 after extraction for3 days.

Y. Li et al. / Journal of Colloid and Interface Science 345 (2010) 168–173 171

the Soxhlet extraction, suggesting that the extraction did removesome stearate molecules from the particle surfaces.

After extraction, the main peak shifts to a higher temperature,suggesting that the Soxhlet extraction could wash away the inter-calated and free sodium stearates. Because the free sodium stea-rate molecules were only partially extracted from CCR14 andCCR19, the shoulder peak adjacent to the main peak and thewater-loss peak at 110 �C still can be detected but with a decreasein the intensity. This result suggests that the extraction time is notsufficient for the complete removal of the intercalated sodium ste-arates from CCR14 and CCR19. In addition, there is no obviouswater-loss peak at 110 �C in the DTA curve of CCR10 after extrac-tion, suggesting the absence or less of the intercalated sodiumstearates.

The chemically-bonded alkyl chains could also be extracted asevidenced by the reduced weight loss of CCR and CCR5 afterextraction. The main decomposition peaks become broader andslightly shift to lower temperatures as the exposed surfaces in-crease. This is because HCO�3 can be produced on the exposed sur-faces after cleavage of the alkyl chains by extraction, whereby thecorresponding peak for the thermal decomposition of HCO�3 anionsis about 260 �C.

The samples after the Soxhlet extraction are studied by DRIFTand their spectra are displayed in Fig. 5. As stearate loading in-creases, their absorptions at 2800–3000 cm�1, 1580 cm�1 and1540 cm�1 become stronger. This verifies that the alkyl chains ofstearates remain on the particle surfaces after extraction. As dis-cussed in the previous section, the absorption at 1110 cm�1 indi-cates the presence of intercalated or free sodium stearates. Afterthe extraction, CCR10 has no obvious 1110 cm�1 absorption, indi-cating that the sodium stearates originally present is mostly re-moved. This result is strongly consistent with the DTG result ofCCR10, showing no obvious 110 �C peak in Fig. 4b.

Fig. 4. (a) TG and (b) DTG curves of CCR, CCR5, CCR10, CC

3.3. Control of surface coverage

In order to prepare a series of nanoparticles with variable sur-face coverage, CCR19 is selected as the candidate and then thestearate molecules are extracted from this sample for various peri-ods of time. The modified samples are designated as XCCR19,where X represents the extraction time in days. For example,3CCR19 is CCR19 after an extraction time of 3 days. The surfacecoverage is determined by TG (Fig. 6). The weight losses are mea-sured to be 18.2, 14.2, 11.0, 9.0, 6.8 and 4.0 wt.%, respectively, forCCR19, 3CCR19, 5CCR19, 6CCR19, 7CCR19 and 8CCR19. Based onthe calculated 6.6 wt.% of monolayer coverage [26], they shouldhave a surface coverage of about 2.76, 2.15, 1.67, 1.36, 1.03 and0.61 times of monolayer, respectively. It is obvious that the organicmolecules on the particle surfaces can be gradually extracted fromCCR19. The DTG curves, as shown in Fig. 6b, indicate that theshoulder peak and the water-loss peak at 110 �C are graduallyweakened and eventually disappeared with prolonged extractiontimes. The main peak shifts to higher temperature gradually asthe extraction time increases because of the removal of free so-dium stearate, intercalated sodium stearate and finally chemicallybonded calcium stearate. This result demonstrates that CaCO3

nanoparticles with different surface coverage can be preparedthrough careful adjustment of the extraction time.

Fig. 7 shows the DRIFT spectra of CCR19 after various extractiontimes. As mentioned, the existence of the three absorptions at2800–3000 cm�1, 1580 cm�1 and 1540 cm�1 confirms that thereare still alkyl chains on the nanoparticles surface after extractionbut in different amounts. The weakening of the absorption intensi-ties from 7CCR19 to 8CCR19 is due to the partial cleavage of themonolayer of the alkyl chains that were bonded to the particlesurfaces. In contrast, the absorption at 1110 cm�1 that is the mostcharacteristic peak of intercalated or free sodium stearatecompletely disappears for 7CCR19 and 8CCR19, suggesting the

R14 and CCR19 before and after extraction for 3 days.

Fig. 6. (a) TG and (b) DTG curves of XCCR19.

Fig. 7. DRIFT spectra of XCCR19. Fig. 8. FTIR absorbance spectra of XCCR19.

172 Y. Li et al. / Journal of Colloid and Interface Science 345 (2010) 168–173

presence of chemically bonded stearate. Accordingly, 6–7 days isthe optimal extraction time for CCR19 to attain monolayer cover-age. The same conclusion can be obtained by examining the DTGdata, as shown in Fig. 6b, which do not show any water peak(110 �C) in the DTG curves of 7CCR19 and 8CCR19. Therefore, wecan categorize the XCCR19 samples into three groups based onthe DTG (Fig. 6) and DRIFT (Fig. 7) results: (1) samples (7CCR19and 8 CCR19) with a monolayer coverage or partially-coated cover-age, which have neither the shoulder peak, the water-loss peak at110 �C in the DTG curves nor the 1110 cm�1 absorption in theDRIFT spectra; (2) samples (6CCR19 and 5CCR19) with bi-layercoverage, which has no shoulder peak but possesses a water-losspeak at 110 �C in the DTG curves as well as the 1110 cm�1 absorp-tion in the DRIFT spectra; and (3) samples (CCR19 and 3CCR19)with multi-layer coverage, which show both the shoulder peakand the water-loss peak at 110 �C in the DTG curves and the1110 cm�1 absorption in the DRIFT spectra.

3.4. Molecular conformation and coating structures

By monitoring the changes of CH2 symmetric, CH3 symmetric,CH2 asymmetric and CH3 asymmetric stretching C–H vibrations ofalkyl chains in FTIR curves, the conformation and the orientationof alkyl chains on the particle surfaces can be determined [24,32–34]. In general, the frequency and width of CH2 asymmetricstretch, tas(CH2), are sensitive to the gauche/trans conformer ratioand the packing density of methylene. The band shifts from lowerfrequencies, characteristic of highly ordered all-trans conforma-tions, to higher frequencies and increased widths as the numberof gauche conformations along the hydrocarbon chain increases.Fig. 8 displays the FTIR results of XCCR19 series. All the speci-mens show the strong aliphatic absorption bands of the methy-lene modes in the range between 3000 and 2800 cm�l. The

bands at 2920 and 2850 cm�1 are attributed to the CH2 asymmet-ric stretch, tas(CH2), and CH2 symmetric stretch, ts(CH2), respec-tively. Among these specimens, 7CCR19 and 8CCR19, whichhave a monolayer coverage or partially-coated coverage, possessthe highest frequency of tas(CH2). From 6CCR19 to 5CCR19,tas(CH2) shifts to a lower frequency when the extraction time isreduced. Thus, the concentration of trans conformation increaseswith the formation of bi-layer coverage. These results suggestthat the alkyl chains on the particle surfaces are inclined to adopta chain-extended arrangement (all-trans conformations), as in-duced by the intercalation of second layer of sodium stearate intothe inner chemically bonded layer. When the extraction time isreduced from 5 to 3 days, the tas(CH2) peak not only slightlyshifts back to a higher frequency but also increases its intensity.Clearly, the presence of the free sodium stearates at the outer-most particle surface decreases the degree of order of the alkylchains. It is highly possible that the free sodium stearates lie onthe particle surface with random orientation because of highermobility at the surface. As a result, the tas(CH2) vibration ofCCR19, including the signals from the free sodium stearates andthe ordered alkyl chains in the inner layers, is stronger and itspositions lie at higher frequency.

The building up of stearates on the surface of the particles wasinvestigated by XPS [26]. In Fig. 9, the intensity ratio between thehydrocarbon component of the C1s peak (285.0 eV) to the carbon-ate component of the C1s peak (290.0 eV) of the XCCR19 samples isplotted against the extraction time to investigate the relationshipbetween the coating structures and the coating amount of theCaCO3 nanoparticles.

The peak-intensity ratio changes slightly for the extraction timeup to 5 days. Then it decreases dramatically afterward. It is well-known that the XPS signal strongly depends on the escaping depthof the concerned electrons. The peak-intensity-ratio between the

Fig. 9. XPS intensity ratio of XCCR19 between the C1s peak characteristic of thehydrocarbons (I(CH2–CH2), 285.0 eV) of the alkyl chains and the C1s peakcharacteristic of the carbonate anions (I(CO2�

3 ), 290.0 eV) at different extractiontime X.

Y. Li et al. / Journal of Colloid and Interface Science 345 (2010) 168–173 173

hydrocarbon and carbonate components varies only slightlyamong CCR19, 3CCR19 and 5CCR19, which contain free stearatemolecules as discussed above, indicating that the removal of thefree sodium stearate molecules as a result of the extraction doesnot significantly change the average thickness of the coating dueto their random orientations. After the free sodium stearate mole-cules are completely removed in the extraction process, the so-dium stearate molecules in the intercalated layer are extracted.Since the intercalated stearate molecules adopts an all-trans con-formation and are fully extended, the disappearance of the interca-lated stearate molecules significantly reduces the average coatingthickness. Therefore, the carbonate component of the C1s becomesmore intense and is very sensitive to extraction time due to theconsiderable change in the average coating thickness. The XPS re-sults support the deduction from the DIRFT spectra of the extractedspecimens that the intercalated stearate molecules adopt an all-trans conformation and free stearate molecules lie on the outmostsurface with random orientations.

4. Summary

The surface coverage of the CaCO3 nanoparticles could be con-trolled with the developed extraction system. The excessiveamount of stearate was successfully extracted with a Soxhlet appa-ratus. Free sodium stearate molecules, intercalated sodium stea-rate molecules and chemically bonded calcium stearates weresequentially removed from the particle surfaces as a function ofextraction time. The water-loss peak at 110 �C in the DTG curvesand absorption peak at 1110 cm�1 in the DIRFT spectra can be usedto differentiate the monolayer and bi-layer coverage. The shoulderpeak adjacent to the main peak in DTG curves represents the pres-ence of the free stearate. DIRFT and XPS results suggest that thefree stearate molecules assume random orientations at the out-most surface of the nanoparticles whereas the first and second lay-ers of the stearate molecules adopt an all-trans conformation andare fully extended.

Disclosure statement

There is no actual or potential conflict of interest including anyfinancial, personal or other relationships with other people or orga-nizations within 3 years of beginning the work submitted thatcould inappropriately influence (bias) their work.

Acknowledgments

We thank the financial supports from the National Natural Sci-ence Foundation of China (No. 50843058) and the Hong Kong Gov-ernment Research Grants Council under Grants HKUST (No.610906).

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jcis.2010.01.080.

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