Introduction

Pigments are solid colorants, and are sold in the form ofdry powders, slurries, or pressed cakes. For high-techapplications, such as toners for colored copiers [1], pig-ment-based inks for ink-jet printers [2, 3], and otherapplications, pigments composed of uniform particles ofwell-defined sizes [4, 5, 6], crystal structures [7, 8], andchemical composition [9] are highly desirable. Theseparameters affect the dispersion stability and the opticalproperties of the products [10, 11, 12].

The purpose of this study was to produce pigments ofuniform shape and size in the nanometer and microm-eter range. To do so, water soluble dyes, i.e., disodiumsalts of 2-amino, 5-methyl benzosulfonic acid (4B-dye),and 2-amino, 4-chloro, 5-methyl benzosulfonic acid(2B-dye) were precipitated as Ca and Sr salts in aque-ous and water/alcohol solutions in the presence ofstabilizers.

Experimental

Materials. Two dyestuffs, i.e., disodium salt of 2-amino, 5-methylbenzosulfonic acid (4B-dye) and 2-amino, 4-chloro, 5-methyl ben-zosulfonic acid (2B-dye) were supplied by Ciba Specialty Chemi-cals. Surfactants used were Igepal CO-630 (nonylphenolethoxylate, Rh ne-Poulenc), Avanel (sodium alkylethersulfonate,PPG industries), and Daxad 30 (sodium polymethacrylate,Hampshire Chemical Corp.).

The stock solutions of each dye were prepared by dissolvingappropriate amounts (0.5–2 g) of the powder in 500 cm3 of hotwater (70 �C) with constant agitation. The resulting solutions werethen stirred for 1 h at the same temperature, cooled to roomtemperature, filtered through 0.2 lm pore size Nuclepore mem-branes, and stored in glass stoppered Pyrex bottles. They werenever stored for more than two weeks.

Preparation of Ca- and Sr-4B pigment particles. Equal volumes of4B-dye solutions (1·10)3 to 9·10)3 mol dm)3), containing variousamounts (30–70 vol.%) of methanol or ethanol and the desiredsurfactant (0.3–6 wt%), were poured into aqueous solutions ofCa(NO3)2 (2·10)2 to 5·10)2 mol dm)3) or SrCl2 (4·10)2 to 1·10)1

mol dm-3) at room temperature, and the dispersions were then

Ikram ul Haq

Iain Fraser

Egon Matijevic

Preparation and characterization of finelydispersed pigment particles

Received: 17 April 2002Accepted: 22 April 2002Published online: 28 January 2003� Springer-Verlag 2003

Abstract Dispersions of uniformcolloidal pigments were produced byprecipitating solutions of disodiumsalts of 2-amino, 5-methyl benzo-sulfonic acid (4B-dye), and 2-amino,4-chloro, 5-methyl benzosulfonicacid (2B-dye) with Ca(NO3)2, SrCl2,or Ca(NO3)2/SrCl2 solutions in theabsence and in the presence of ad-ditives (surfactants and alcohols).The latter affected the morphologyof the particles, but had a negligibleeffect on their crystal structures.However, drying the Ca-4B pigmentresulted in the change of the crystalphase and morphology. As a rule,

the Sr-4B pigment was spheroidal,while Ca-4B, Ca-2B, and Ca/Sr-4Bconsisted of elongated particles.

Keywords Colored particles ÆNanoparticles Æ Pigments ÆPrecipitation

Colloid Polym Sci (2003) 281: 542–549DOI 10.1007/s00396-002-0815-8 ORIGINAL CONTRIBUTION

I. ul Haq Æ E. Matijevic (&)Center For Advanced Materials Processing,Clarkson University, Potsdam,NY 13699–5814, USAE-mail: matiegon@clarkson.edu

I. FraserCIBA Specialty Chemicals,Hawkhead Road, PA2 7BG, Paisley,Renfrewshire, Scotland

allowed to age for 2 h. The resulting solids were separated from themother liquors by filtration through 0.2-lm Nuclepore membranesand washed extensively with water, ethanol, or methanol.

Preparation of Ca/Sr-4B pigment particles. In this case, equalvolumes of aqueous solutions in total concentrations of 4.4·10)3 to2.5·10)3 mol dm)3 of Ca(NO3)2 and SrCl2 in different molar ratioswere mixed with 4B-dye solutions (1·10)3 to 9·10)3 mol dm)3),containing the surfactant (0.6–5 wt%), in the absence and in thepresence of methanol (50–70 vol.%) at room temperature, andtreated as described above.

Preparation of Ca-2B pigment particles. Dispersions of the pre-cipitated Ca-2B pigment were prepared by mixing aqueous solu-tions of Ca(NO3)2 (2·10)2 to 5·10)2 mol dm)3) with 2B-dyesolutions (1·10)3 to 3·10)3 mol dm)3) in the absence and in thepresence of the surfactant at room temperature, and processed asabove.

Characterizations. Scanning electron microscopy (SEM), trans-mission electron microscopy (TEM), energy dispersive X-rayanalysis (EDX), X-ray diffractometry (XRD), and thermogravi-metric analysis (TGA) were employed for the characterization ofthe pigments.

Results and discussion

In this study, Ca- and Sr-based pigments were producedby the reaction of calcium and strontium salts with4B- and 2B-dyes in aqueous and aqueous/alcohol solu-tions, containing various surfactants, such as Avanel,Daxad, and Igepal. Preliminary experiments demon-strated that the former two stabilizers were not effective,yielding aggregated particles. In contrast, in the presenceof Igepal well dispersed pigments could be obtained. Theobserved results were attributed to the fact that Avaneland Daxad, being anionic molecules, reacted with thealkaline earth cations [13, 14, 15] . For this reason Igepalwas selected for further studies.

Precipitation of Ca-4B pigment particles

Aqueous solutions

Experiments in which the 4B-dye solutions (2·10-3 to9·10-3 mol dm-3), containing 0.2–5 wt% Igepal, weremixed with the calcium nitrate solutions (1·10-2 to5·10-2 mol dm-3), yielded stable dispersions of thepigment particles. The SEM analysis showed that, inmost cases, needles of nanometer thickness wereobtained.

A transmission electron micrograph (TEM) of atypical pigment sample, prepared as described in thelegend and dried overnight at room temperature, isdisplayed in Fig. 1a. The XRD of these particles(Fig. 2a) showed them to be crystalline. Interestingly,

Fig. 1 a Transmission electron micrograph (TEM) of Ca-4Bpigment particles obtained by mixing 20 cm3 of a 5 · 10)2 moldm)3 Ca(NO3)2 solution with an equal volume of the 4B-dyesolution (9 · 10)3 mol dm)3) containing 3.6 wt% Igepal CO-630 atroom temperature. b Scanning electron micrograph (SEM) ofparticles obtained when the aqueous dispersions of solids shown ina were heated overnight at 90 �C. c SEM of particles shown in aafter drying the separated solids overnight at 90 �C

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when the freshly prepared aqueous dispersion washeated overnight at 90 �C, the initial nanoparticles ag-gregated into large fibers (Fig. 1b), probably due to theloss in stabilizing properties of the dissolved surfactant(Igepal). The crystal structure of the latter solids essen-tially remained the same as that of the original particles,since the major peaks in both patterns coincided.However, the crystallinity (Fig. 2b) of this solid(Fig. 1b) increased as a result of the heat treatment,obviously due to the larger particle size.

When the same pigment powder was dried at 90 �C,the morphology (Fig. 1c) and the crystal structure(Fig. 2c) changed. The dehydration of this solid in thedry state resulted in partial aggregation, and reorgani-zation of the crystallites in the solid matrix.

The thermogravimetric analysis (TGA) of the origi-nal pigment, dried at room temperature (Fig. 1a),yielded a weight loss of �82% on heating to �720 �C(Fig. 3a), leaving a white-gray powder. Based on the

chemical structure of pigment Ca-4B (Fig. 4a), describedin the review article by Hao and Iqbal [16], the estab-lished weight loss agrees well with the calculated one(�83%), assuming Ca and S to be the non-volatilecomponents of the ash. The latter was confirmed by theenergy dispersive X-ray analysis (EDX) (Fig. 3b) of the

Fig. 2 X-ray diffraction (XRD) patterns of the Ca-4B pigmentparticles shown in Fig. 1a (pattern a), 1b (pattern b), and 1c(pattern c)

Fig. 3 a Thermogravimetric analysis (TGA) curve obtained withthe Ca-4B pigment particles, shown in Fig. 1a. b Energy-dispersiveX-ray analysis (EDX) pattern of the ash produced in the TGAexperiment, described in (a)

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powder remaining in the TGA experiment, which indi-cated the presence of significant amounts of these twoelements.

Alcohol/water solutions

It is known [17] that the 4B-dye is less soluble in alcohol/water solutions than in pure water. Therefore, onewould expect the properties of particles, precipitated inmixed solvents, to differ from those prepared in aqueousmedia, which justified the efforts to evaluate the effect oftwo alcohols on the formation of this pigment. Fig-ures 5a,b exemplify Ca-4B pigments obtained in meth-anol/water and ethanol/water mixtures, respectively. Itis clear that in both cases ellipsoidal particles were ob-tained, although the aspect ratio of those in the metha-nol/water mixture was higher than in the ethanol/watermixture. The XRD analysis (Fig. 6a,b) demonstratedthat the above samples were of identical crystal struc-ture, and similar to that of the nanometer needles(Fig. 2a).

These results suggested that the nature of the solventhad a negligible effect on the crystal structure, but it didaffect the morphology of the precipitated pigment par-ticles.

Precipitation of Sr-4B pigment particles

Sr-4B particles were precipitated in water, ethanol/water, and methanol/water mixtures. The productsobtained in the first two solvents were eithergelatinous or composed of irregular particles and,therefore, were not further investigated. In contrast,nanosized particles of reasonable uniformity wereobtained in a methanol/water mixture, as displayed in

Fig. 4a,b Molecular structures: a of the Ca-4B complex [16]; b ofthe Sr-4B complex

Fig. 5a,b SEM of Ca-4B pigment particles obtained by mix-ing 20 cm3 of a 0.025 mol dm)3 Ca(NO3)2 solution with an equalvolume of 4B-dye solution (2.25 · 10)3 mol dm)3), contain-ing: a 1.5 wt% Igepal and 33 vol.% methanol; b 1.5 wt% Igepaland 28 vol.% ethanol at room temperature

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Fig. 7a. Their XRD pattern is shown in Fig. 8. Ac-cording to the TGA analysis, this solid lost �73%weight on heating to �800 �C, leaving ash consistingof Sr, S, and C.

It is noteworthy that Ca-4B and Sr-4B pigments,prepared under the same conditions in methanol/watermixtures, are significantly different (Figs. 5b and 7a).The change in the morphology of these pigments may beattributed to the possible difference in their solutecomplexes. For example, it was suggested that the Ca-dye forms initially a dimeric structure [16] (Fig. 4a),while we assume the Sr complex with the same dyemolecule to be a bidentate (Fig. 4b). The latter wouldmore likely form compact (nearly spherical) particles onprecipitation, while the Ca-4B species may preferentiallyaggregate into ellipsoids.

Precipitation of Sr/Ca-4B pigment particles

Attempts to produce Sr/Ca-4B composite pigmentsunder various conditions, described above, yielded in

most cases submicron needles. Figure 7b shows a typicalexample of particles which were produced in the reactantsolutions containing equal amounts of the Ca andSr salts. However, no Ca could be identified in thesesolids by EDX, and the XRD pattern was the same asfor the Sr-4B pigment.

The solubility of the Sr-4B pigment in water is lowerthan that of Ca-4B under the described experimentalconditions; thus, the former preferentially precipitated inthe reactant mixture. However, the presence of Ca2+ didaffect the particle morphology, since under analogousconditions spheroids of Sr-4B are obtained in the ab-sence of Ca2+.

Fig. 6 XRD patterns of the Ca-4B pigment particles shown inFig. 5a (a), and 5b (b)

Fig. 7a,b SEM of the Sr-4B pigment particles obtained by mixing20 cm3 of a 4B-dye solution (2.02 · 10)3 mol dm)3), containing0.7 wt% Igepal and 54 vol.% methanol with equal volume ofaqueous solution of either: a 0.02 mol dm)3 SrCl2 or; b 0.01 moldm)3, each with respect to SrCl2 and Ca(NO3)2 at roomtemperature

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Precipitation of Ca-2B pigment particles

Mixing aqueous solutions of Ca(NO3)2 with 2B-dyesolutions produced a needle-like pigment, exemplified inFig. 9, which was crystalline (Fig. 10). These particleswere somewhat larger, and had a rather different crystalstructure than those of the Ca-4B pigment particles(Fig. 1a). The observed differences in the properties ofthese two pigments may be attributed to the difference inthe chemical compositions of their precursor dyes, givenabove.

Optical evaluation

The optical properties of the 4B-dye and its precipitatedpigment Ca-4B are displayed in Fig. 11. The corre-sponding chromaticity values were calculated accordingto CIE standard colorimetric system for luminant D65 at10� observer. Table 1 lists the results, including thedominant wavelength, hue, luminosity (Y%), and thecolor purity for the 4B-dye and for the Ca-4B pigmentsample shown in Fig. 1a. For the purpose of the com-parison with the original dye solution, the chromaticityvalues of the pigment (Fig. 12) were based on theextinction/absorption spectra.

As shown in Table 1, there is a significant differ-ence in optical properties of the dye solutions and ofthe Ca-4B pigment of the same dye. Furthermore,the color of the same pigment sample changes with

Fig. 8 XRD pattern of the Sr-4B pigment particles shown inFig. 7a

Fig. 9 TEM of the Ca-2B pigment particles obtained by mixing a20 cm3 of 0.02 mol dm-3 Ca(NO3)2 with an equal volume of 2B-dyesolution (1.5 · 10-3 mol dm-3 ), containing 0.3 wt% Igepal, at roomtemperature

Fig. 10 XRD pattern of the Ca-2B pigment particles, shown inFig. 9

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increasing solid content of the sample. As expected,the latter also affects the lightness (Y%) and the pu-rity of the colorants. One possible explanation is that,by increasing the concentration of the solid, the ag-gregation of nanosized needles changes the packing,which should affect the shape of the pigment particlesand their light scattering, causing the change in theoptical properties.

Fig. 11 a Absorption spectra of aqueous solutions of the 4B-dye atvarious concentrations (0.25–0.05 mg cm-3). b Extinction spectra atvarious concentrations (0.25–0.05 mg cm-2) of the deposited Ca-4Bpigment, shown in Fig. 1a. The concentrations in a are given in mgcm3 for comparison purposes

Table 1 Chromaticity data for the 4B-dye and its precipitated pigment [Ca-4B, displayed in Fig. 1a], shown in Fig. 12

Code Sample ID Chromaticitycoordinates

Dominantwave-length (nm)

Hue Y (%) Purity (%) Figure

x y

1 4B-dye (ba) 0.591 0.406 593 Orange red 40 100 11a2 4B-dye (ca) 0.580 0.409 592 Orange red 43 99 11a3 4B-dye (da) 0.552 0.406 590 Orange red 47 92 11a4 Ca-4B (ba) 0.509 0.247 496Cb Reddish magenta 19 64 11b5 Ca-4B (ca) 0.442 0.220 500C Magenta 23 58 11b6 Ca-4B (da) 0.387 0.207 520C Magenta 25 55 11b

aRefers to the absorption/extinction curve in respective figuresbComplementary color

Fig. 12 Chromaticity data for the Ca-4B dye and its precipitatedpigment (Ca-4B). The numbers next to each symbol refer to thecode in Table 1

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Acknowledgement This work was supported by CIBA SpeciallyChemicals, Basel, Switzerland. Ikram ul Haq was on study leave

from the National Center for Excellence in Physical Chemistry,University of Peshawar, NWFP, Pakistan.

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