Gas-Solid Coexistence in Highly Charged ColloidalSuspensions

P. S. Mohanty,† B. V. R. Tata,*,† A. Toyotama,‡ and T. Sawada‡

Materials Science Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102,Tamil Nadu, India, and National Institute for Materials Science, 1-1 Namiki,

Tsukuba, Ibaraki 305-0044, Japan

Received July 12, 2005. In Final Form: September 8, 2005

Aqueous suspensions of highly charged polystyrene particles with different volume fractions have beeninvestigated for structural ordering and phase behavior using static light scattering (SLS) and confocallaser scanning microscope (CLSM). Under deionized conditions, suspensions of high-charge-density colloidalparticles remained disordered whereas suspensions of relatively low charge density showed crystallizationby exhibiting iridescence for the visible light. Though for the unaided eye crystallized suspensions appearedhomogeneous, SLS measurements and CLSM observations have revealed their inhomogeneous nature inthe form of the coexistence of voids with dense ordered regions. CLSM investigations on disorderedsuspensions showed their inhomogeneous nature in the form coexistence of voids with dense disordered(amorphous) regions. Our studies on highly charged colloids confirm the occurrence of gas-solid transitionand are in accordance with predictions of Monte Carlo simulations using a pair-potential having a long-range attractive term [Mohanty, P. S.; Tata, B. V. R. J. Colloid Interface Sci. 2003, 264, 101]. On the basisof our experimental and simulation results, we argue that the reported reentrant disordered state [Yamanakaet al. Phys. Rev. Lett. 1998, 80, 5806 and Toyotama et al. Langmuir 2003, 19, 3236] in charged colloidsobserved at high charge densities is a gas-solid coexistence state.

1. Introduction

Structural ordering in monodisperse charge-stabilizedcolloidal suspensions can be tailored with ease by varyingthe range and the strength of the interparticle interac-tion.1,2 The most dominant interparticle interaction amongthe like-charged colloidal particles in a homogeneouscharge-stabilized suspension is screened Coulomb repul-sion and is given by Derjaguin-Landau-Verwey-Over-beek (DLVO) theory.3 However, there have been severalobservations, viz., reentrant transition observed as afunction of salt concentration,4 vapor-liquid transition,5,6

and observation of stable voids,7-9 which suggested theexistence of a long-range attraction in the effectiveinterparticle interaction, U(r), of like-charged colloidalparticles contradicting the predictions of DLVO theory.The DLVO theory is advanced by Sogami-Ise by con-sidering the counterion mediated attraction and regardingthe macroionic part as a one-component system.10 Theeffective pair-potential Us(r) obtained by Sogami and Ise10

by considering Gibbs free energy is found to have a long-range attractive term in addition to the usual screened

Coulomb repulsive term given by DLVO theory. Tata etal.’s computer simulations using Us(r) could explain theabove-mentioned experimental observations satis-factorily,11-13 and alternative explanation based on volumeterm theory which does not require an attractive term intheeffectivepair-potentialalsoexists.14-15 However, recentcalculations based on Poisson-Boltzmann (PB) cellmodel16,17 have shown that the spinodal instability thatarises in volume term theory is spurious and is due tolinearization of the PB equation. Further, our own pair-potential measurements on very dilute suspensions ofhighly charged colloidal particles have shown the existenceof a long-range attractive term in the U(r) of like-chargedcolloids.18

A suspension of particles having low effective surfacecharge density σe is known to exhibit a homogeneous fluidto homogeneous crystal transition upon lowering the saltconcentration.19 Whereas suspensions of highly chargedcolloidal particles, under deionized conditions, are foundto be in an inhomogeneous state in the form of a rarephase (voids) coexisting with a dense phase havingglasslike disorder.8 The glasslike disorder of the densephase was confirmed using CLSM and ultra-small-angleX-ray scattering (USAXS) studies.8 Though Monte Carlo(MC) simulations have predicted the existence of gas-solid coexistence in the form of voids coexisting withordered regions12 for charge density beyond a critical value,

* To whom correspondence should be addressed.† Indira Gandhi Centre for Atomic Research.‡ National Institute for Materials Science.(1) Tata, B. V. R. Curr. Sci. 2001, 80, 948.(2) Arora, A. K.; Tata, B. V. R. Adv. Colloids Interface Sci. 1998, 78,

49.(3) Verwey, E. J. W.; Overbeek, J. T. G. Theory of Stability of Lyophobic

Colloids; Elsevier: Amsterdam, 1948.(4) Arora, A. K.; Tata, B. V. R.; Sood, A. K.; Kesavmoorthy, R. Phys.

Rev. Lett. 1988, 60, 2438.(5) Tata, B. V. R.; Rajalakshmi, M.; Arora, A. K. Phys. Rev. Lett.

1992, 69, 3778.(6) Tata, B. V. R.; Arora, A. K. In Ordering and Phase Transitions

in Charged Colloid; Arora, A. K., Tata, Eds.; VCH: New York, 1996;Chapter 6.

(7) Ito, K.; Yoshida, H.; Ise, N. Science 1994, 263, 66.(8) Tata, B. V. R.; Yamahara, E.; Rajamani, P. V.; Ise, N. Phys. Rev.

Lett. 1997, 78, 2660.(9) Tata, B. V. R.; Raj, B. Bull. Mater. Sci. 1998, 21, 263.(10) Sogami, I.; Ise, N. J. Chem. Phys. 1984, 81, 6320.

(11) Tata, B. V. R.; Arora, A. K.; Valsakumar, M. C. Phys. Rev. E1993, 47, 3404.

(12) Tata, B. V. R.; Ise, N. Phys. Rev. B 1996, 54, 6050.(13) Tata, B. V. R.; Ise, N. Phys. Rev. E 1998, 58, 2237.(14) van Roji, R.; Dijkstra, M.; Hansen, J. P. Phys. Rev. E 1999, 59,

2010.(15) Schimitz, K. S.; Bhuiyan, L. B. Phys. Rev. E 2000, 63, 011503.(16) von Grunberg, H. N.; van Roji, R.; Klein, G. Europhys. Lett.

2001, 55, 580.(17) Tamashiro; Schiessel, H. J. Chem. Phys. 2003, 119, 1885.(18) Tata, B. V. R.; Mohanty, P. S.; Yamanaka, J. 22nd International

Conference on Statistical Physics (India). IGC News Letter 2004, 62, 89.(19) Sirota, E. B.; Ou-Yang, H. D.; Sinha, S. K.; Chaikin, P. M.; Axe,

J. D.; Fujji, Y. Phys. Rev. Lett. 1989, 62, 293.

11678 Langmuir 2005, 21, 11678-11683

10.1021/la0518896 CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 10/21/2005

there have been no systematic experimental studiesverifying this prediction. We report here the experimentalverification of this theoretical prediction in deionizedsuspensions of highly charged polystyrene colloids.

Further, Yamanaka et al.20 and Toyotama et al.21 haverecently reported a reentrant solid-liquid transition (alsoknown as reentrant order-disorder transition) by varyingσe on deionized charged colloidal suspensions of silica andpolystyrene particles, respectively. MC simulations22 asa function of σe by Mohanty and Tata have providedunderstanding for these experimental observations. Thesesimulations have clearly shown that the reentered dis-ordered state observed at high charge densities is a gas-solid coexistence state. Further, these simulations haverevealed that the homogeneous ordered phase exists overa narrow range of σe, whereas experiments20,21 haveindicated the existence of an ordered region over a muchwider range of σe. We point out that identification of theordered region by Toyotama et al.21 is completely basedon iridescence from the samples and no detailed inves-tigations have been reported to characterize the structuralordering and for their inhomogeneous/homogeneous na-ture. Motivated by these studies, we have carried outsystematic SLS and CLSM studies on deionized aqueoussuspensions of polystyrene particles for different surfacecharge densities. We report here the observation of gas-solid coexistence in the form of voids coexisting withordered regions in deionized suspensions with particlesof intermediate charge density and voids coexisting withglasslike (amorphous) disordered dense regions at rela-tively high charge densities. Our results thus confirm thepredictions of MC simulations that a homogeneouscrystalline state exists in a narrow range of charge density.This crystalline state becomes inhomogeneous in the formof gas-ordered (crystal) coexistence at intermediate valuesof σe and gas-disordered (amorphous) coexistence atrelatively high values of σe. We have also carried out MCsimulations close to experimental parameters and our MCsimulations results also showed gas-solid coexistence inagreement with experimental results.

2. Sample Details and Methods2.1.SampleDetails. Monodisperse polystyrene latex particles

synthesized by Dr. Yoshida of Hashimoto Project, ERATO, JST21

are used in the present investigations, and the parameters of thesuspensions are summarized in Table 1. These suspensions are

purified by dialysis and ion-exchange method. After purification,the ion-exchange resin beads (AG501-X8, Bio-Rad Laboratories,Hercules, CA) were introduced into the suspensions, and thesuspensions were kept undisturbed for one week for deionization.The volume fraction of these suspensions was determined by thedrying up method.23 The effective surface charge density σe isdetermined by the conductivity method.1,21

Samples S1-S7 are prepared by diluting the mother suspen-sions with appropriate amounts of H2O and D2O mixtures suchthat the density of polystyrene particles matches with the densityof the medium (50:50 in H2O-D2O mixtures). The conductivityof H2O and D2O used for the dilution is less than 1 µS/cm. Samplesfor SLS studies were prepared in cylindrical quartz cells with 8mm optical path length. For further deionization, a knownamount of mixed-bed ion-exchange resins were added to thesamples. The cells were sealed hermetically and were leftundisturbed for about a week.

2.2. Static Light Scattering. The structural ordering indeionized suspensions is characterized by static light scattering.The scattered intensity Is(Q) [)AP(Q)S(Q)] is recorded as afunction of scattering wave vector Q. Structure factor S(Q) as afunction of Q is calculated after correcting for the particlestructure factor, P(Q),24,25 where Q ) (4πµm/λ)sin(θ/2) is thescattering wave vector, θ is the scattering angle, µm is therefractive index of the dispersion medium, and λ is the wavelengthof laser light. The particle structure factor P(Q) for a sphericalparticle of radius a is given as

2.3. Confocal Laser Microscopy. The internal structure(homogeneous/inhomogeneous) of the deionized samples (S1-S7) was characterized using a confocal laser scanning microscope,M/s Leica TCS-SP2-RS, Germany. The sample cell used for CLSMstudies is shown schematically in Figure 1. The cell is made upof a cylindrical quartz tube of 8 mm in diameter and 25 mm inheight. The cover glass is fixed to the polished end of the quartztube by applying adhesive to the outer surface of the tube.Deionized suspension with known volume fraction is introducedin the cell and a clean nylon bag containing the mixed bed ofion-exchange resins is hung from the top for further deionization.The top of the cell is sealed hermitically using Para-film. The cellis then mounted on CLSM stage for observations.

2.4. Monte Carlo Simulations. To understand the experi-mental observations, MC simulations have been carried out using

(20) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T.Phys. Rev. Lett. 1998, 80, 5806.

(21) Toyotama, A.; Yamanaka, J.; Kitamura, K. Langmuir 2003, 19,3236.

(22) Mohanty, P. S.; Tata, B. V. R. J. Colloid Interface Science, 2003,264, 101.

(23) Mohanty, P. S. Ph.D. Thesis, University of Madras, 2005,Unpublished.

(24) Sood, A. K. Hyperfine Interactions 1987, 37, 265.(25) Tata, B. V. R.; Mohanty, P. S.; Yamanaka, J.; Kawakami, T.

Mol. Simulat. 2004, 30, 153.

Table 1. Sample Details (Effective Surface ChargeDensity σe, Volume Fraction O, Diameter of the Particled, Average Interparticle Separation do), Parameters ofPair-Potential (Position of the Potential Minimum Rmand Its Depth Um), and the Nature of the Coexistence

State as Identified by Static Light Scattering andConfocal Laser Scanning Microscope Studiesa

sampleno.

σe(µC/cm2) φ

d(nm) d0/d Rm/d Um/kBT

coexistencestate

S1 0.24 0.001 136 8.8 6.86 -1.85 G + CS2 0.4 0.001 104 8.8 7.8 -2.04 G + CS3 0.4 0.005 104 5.143 5.146 -2.97 G + CS4 0.4 0.0005 104 11.08 8.83 -1.82 MP (C + G)S5 0.51 0.0005 126 11.08 7.28 -6.3 MP (A+G)S6 0.51 0.001 126 8.8 6.56 -6.94 G + AS7 0.51 0.005 126 5.143 4.23 -10.04 G + A

a Abbreviations G, C, MP, and A represent gas, crystalline,macroscopic phase separation and amorphous, respectively.

Figure 1. Schematic diagram of sample cell used for CLSMinvestigations. (1) Cover glass, (2) adhesive, (3) suspension and(4) nylon bag containing mixed bed of ion-exchange resin.

P(Q) ) {3[sin (Qa) - (Qa) cos(Qa)]

(Qa)3 }2

(1)

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the Metropolis algorithm26 with periodic boundary conditionsfor a canonical ensemble (constant N, V, T where N, V, and T arerespectively the number of particles, the volume, and thetemperature) close to the experimental parameters. Particlesare assumed to interact via a pair potential Us(r) having thefunctional form

where A ) 2[1 + κa coth(κa)]. The inverse Debye screening lengthκ is given as

where Ze is the effective charge on the particle (related to thesurface charge density by σe ) Ze/πd2), Cs is the salt concentration,T is the temperature (298 K), ε is the dielectric constant of water,and kB is the Boltzmann constant. The position of the potentialminimum Rm is given as Rm ) {A + [A(A + 4)]1/2}/2κ and itsdepth Um ) U(Rm). Both Rm and Um depend on σe and Cs.Simulations have been carried out using the procedure reportedin our earlier studies.22

3. Results and Discussion

3.1. Experimental Results. One-week old samples,S1-S3, showed iridescence for the visible light over theentire volume of the suspension, whereas S5-S7 did notexhibit any iridescence even after a month. Iridescence insamples S1 and S3 and the absence of iridescence in sampleS7 are shown in Figure 2, which are arranged in increasingorder of σe. These observations indicate that samples S1-S3 are crystalline and S5-S7 are disordered. However,the information regarding the homogeneous or inhomo-geneous nature of these crystalline samples (S1-S3) anddisordered samples (S5-S7) cannot be known throughvisual observations. Hence, SLS and CLSM studies havebeen carried out to investigate the nature of these samples.

Figure 3 shows the measured S(Q) for the samples S1and S3. The peak positions in S(Q) correspond to a bodycentered cubic (BCC) structure. Sample S2 has also shownBCC structure. Assuming the samples are homogeneous,we have estimated the first peak position Qmax from

the volume fraction of the samples S1-S3 using therelation8,23

The position of Qmax is shown as dotted line in Figure 3.Notice that the first peak in S(Q) occurs at higher valueof Q than at Qmax in both the samples S1 and S3. Similarshift also has been observed for sample S2. The volumefraction (φd) estimated from the peak position in S(Q) isfound to be larger than φ and the corresponding inter-particle separation is found to be smaller than the averageinterparticle separation d0 (d0 ) (x3/2)[πd3/3φ]1/3). Theseobservations imply that the crystallized samples, whichappeared homogeneous for the unaided eye, are inhomo-geneous (i.e., the ordered phase does not occupy the fullvolume of the medium). The fraction of the volume thatis not occupied by the ordered region is expected to appearas particle free regions (voids). The estimated void fractionVf ()[1 - (φ/φd)]) for samples S1-S3 are 0.27, 0.47, and0.36, respectively. Multiple scattering in samples S5-S7is found to be considerable which prevented us to measureS(Q) in these samples.

CLSMstudieshavebeencarriedout in thesamesamplesby transferring the suspensions (S1-S3) from lightscattering cells to CLSM cells. Figure 3 shows the frameaveraged (averaged over 20 frames) CLSM images insamples S1 and S3 at a depths of 55 and 70 µm,respectively, from the cover slip. These images clearlyshow that the crystallites (dense phase) coexist with voids(rare phase). Sample S2 also showed coexistence of

(26) Binder, K. In Monte Carlo Methods in Statistical Physics; Binder,K., Ed.; Springer-Verlag: New York, 1979; p 1.

Figure 2. Photographs of the samples S1, S3, and S7 arrangedin increasing order of σe. Samples S1 and S3 show iridescence,whereas sample S7 shows no iridescence. Mixed bed of ion-exchange resin can be seen at the bottom of the sample cells.

Figure 3. Structure factor S(Q) vs scattering wave vector Qfor samples S1 and S3. The vertical dotted lines shown in thepanels correspond to Qmax calculated from the eq 4 forhomogeneous suspensions. The arrows (from left to right) inpanels S1 and S3 represent the peak positions correspondingto Bragg reflections [(110),(200),(211),(220),(222),(310),(321),-(330),(400),(411),(420),(422)] and [(110),(200),(211)] of the densecrystalline phase, respectively. These reflections characterizethe crystalline state as body centered cubic (BCC). Thecorresponding CLSM images for samples S1 and S3 takenrespectively at distance of 55 µm and 70 µm from the coverglass show coexistence of voids (black regions) with dense phase(white and gray regions) consisting of crystallites. White andgray portions of the image arise from the orientation of thecrystallite surface with respect to the detector. In particular,a crystallite surface which is almost parallel to the detectorsurface will reflect more light into the detector (hence appearsas white) than a crystallite surface which is at some angle withrespect to the detector surface. Such crystallites appear as grayregions in the image. Images are taken using wavelength λ )488 nm of Ar-ion laser and a 40×/0.75 objective. Scale bar )20 µm.

φ ) πd3

61

x2(Qmax

2π )3

(4)

Us(r) ) Z2e2

2ε (sinh(κa)κa )2(A

r- κ) exp(-κr) (2)

κ2 ) 4πe2(npZ + Cs)/(εkBT) (3)

11680 Langmuir, Vol. 21, No. 25, 2005 Mohanty et al.

crystallites with voids. Thus, CLSM observations confirmunambiguously the inhomogeneous nature of crystallinesamples as revealed by static light scattering. Further,CLSM measurements on samples S2 and S3 have revealedthat void fraction increases with decreasingφ. This impliesthat in these samples, the ordered phase is the majorityphase and the voids (rare phase) constitute a minorityphase. The volume fraction of the minority phase (i.e.,void fraction) increases as φ decreases. Below a certain φ,thevoids formamajorityphase,andundersuchconditions,the suspension is expected to exhibit macroscopic phaseseparation (i.e., phase separation is observable by unaidedeye). Indeed we observed macroscopic phase separationin a dilute sample S4 (inset in Figure 4), in the form ofdense phase at the bottom of the cell having iridescenceand a rare phase at the top. S(Q) measured in the topregion shows that the rare phase has gaslike disorder(Figure 4). The appearance of macroscopic phase separa-tion at lower volume fraction can be understood as follows.At higher φ, the ordered phase is the majority phase andis connected. So the suspension appears homogeneous tothe unaided eye at higher φ ()0.001-0.005) and the rarephase (gaslike disorder) is the minority phase whichappears as voids within the dense phase. Below a certainvolume fraction (∼0.001), the voids constitute a majorityphase because void fraction is high. The ordered phase isthe minority phase and appears as clusters9 during thephase separation. Since the density of these dense phaseclusters is more than the density of the solvent, theseclusters settle down due to gravity and coalesce, leadingto a macroscopic phase separation as shown in Figure 4.

CLSM studies also have been carried out on samplesS6 and S7 which did not exhibit iridescence but appearedhomogeneous to the unaided eye. Figure 5 shows theCLSM image of the sample S7 taken at a depth of 60 µmfrom the cover slip. From the image, it can be seen thatthe voids coexist with dense phase which is disordered.The disorder within the dense phase is characterized tobe glasslike (amorphous) by performing averaging overframes. If the disorder is solidlike, the image obtained byframe averaging is expected to be much sharper than asingle image. We found that the CLSM image obtained byaveraging over 20 frames has improved the sharpness of

the image, which suggests that the dense disorderedregions are glasslike (amorphous).8,9 Sample S6 alsoshowed similar behavior. These observations on samplesofhighchargedensityparticles corroboratewith theearlierstudies by Tata et al.8,9 on suspensions of highly chargedpoly(chlorostyrene-styrene sulfonate) particles. The dilutesample S5 of highly charged particles showed macroscopicphase separation in the form of a dense disordered phaseat the bottom of the sample cell and a rare phase at thetop of the cell, similar to that observed in sample S4.

Thus, our studies using SLS and CLSM on deionizedsuspensions of highly charged particles confirm theoccurrence of gas-solid coexistence in the form of voidscoexisting with ordered regions at intermediate chargedensity and voids with disordered regions at higher chargedensity.

3.2. MC Simulation Results. Occurrence of gas-solidtransition in highly charged colloids suggest the existenceof long-range attraction in the effective pair-potential.Hence, Us(r) is chosen as the effective pair-potential andMC simulations have been carried out close to theexperimental parameters to understand the experimentalobservations. Here we present the simulation results forsuspension parameters close to samples S3, S7, and S4.Figure 6A shows the pair-correlation function g(r) forsuspension S3. The peaks in g(r) correspond to bcccrystalline order. The dotted vertical line shows theposition of the average interparticle separation, d0,calculated for a homogeneous suspension at φ ) 0.005.Notice that the first peak of g(r) occurs at distance lessthan r/d0, implying that the particles in the suspensiondo not occupy the full volume of the MC cell. Thecorresponding projected particles in the MC cell (Figure6B) show particle-free regions (voids) coexisting withordered regions. Thus, MC simulation results for sampleS3 agree with experimental results of SLS and CLSMstudies. Simulations on samples S1 and S2 also showedgas-solid(crystal) coexistence which is in agreement withexperiments.

The pair-correlation function g(r) corresponding tosample S7 (Figure 6C) shows a split second peak anddecays as a function of r. This suggests glasslike disorderin the suspension. Further the first peak position occursat a distance smaller than r/d0 indicating that thedisordered region does not occupy the full volume of theMC cell. The corresponding projected particles in the MC

Figure 4. Photograph of the sample S4 (shown as an inset)exhibiting macroscopic phase separation in the form of densephase at the bottom of the cell coexists with a rare phase at thetop. Mixed bed ion-exchange resins can be seen at the bottomof the cell. The iridescence for the visible light from the densephase implies that it has crystalline order. S(Q) vs Q measuredin the rare phase shows that the rare phase has gaslike disorder.

Figure 5. CLSM image after averaging over 20 frames showscoexistence of voids (dark region) with dense disordered regions(gray region) for the sample S7 taken at a distance of 60 µmfrom the cover glass. Images are taken using λ ) 488 nm ofAr-ion laser and a 40×/0.75 objective. Scale bar ) 20 µm.

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simulation cell (Figure 6D) showed particle free regions(voids) coexisting with dense disordered regions. Thedisorder in the dense phase is identified to be glasslikefrom the spilt second peak in g(r) and from the saturationbehavior of mean square displacement (MSD) (inset ofFigure 6C). Simulations for sample S6 also showedcoexistence of voids with dense disordered (glasslike)regions. These MC simulation results are in agreementwith the CLSM observations on sample S6 and S7.

Figure 6E shows g(r) as a function of r/d0 for sample S4.The peaks in g(r) and the gradual decay of g(r) to 1 asfunction of r/d0 suggest the existence of dense phaseclusters with a liquidlike structural ordering within theclusters.11 Further, the first peak position in g(r) is closeto the Rm value of the Us(r). The corresponding projectionsof particles in the MC cell (Figure 6F) confirm the existenceof dense disordered clusters. In an experimental situation,these clusters settle down due to gravity, leading to amacroscopic phase separation in the form of a dense phase(at the bottom) coexisting with a rare phase (at the top).

The above simulation results can be understood fromthe behavior of Us(r) on σe and φ. Figure 7A shows thepotential Us(r) for values of σe ) 0.4, 0.51 µC/cm2 at φ )0.005. The vertical line shows d0 corresponding to φ )0.005. If Rm is less than d0 and well depth Um . kBT (Table1), particles which are initially at a distance of d0experience strong attractive forces and get trapped intosuch deep wells with a new interparticle spacing close toRm, leading to an inhomogeneous suspension. As the totalvolume of the suspension remains unchanged, this alsoleads to the appearance of voids. If the difference betweenRm and d0 is small (≈0.04 d0, see Figure 7A, curve a, Table1), the dense phase appears as crystalline. Note that ifthe difference between Rm and d0 is more (≈0.2 d0, Figure7A, curve b, Table 1) and well depths are large, then the

particles suffer large random displacements due to thestrong attractive forces and get arranged randomly duringthe dense phase formation, resulting in an amorphousstructure in these suspensions. When this differencebetween Rm and d0 is sufficiently large (>0.3 d0, see Figure7B), the dense phase is expected to be initially in the formof clusters as seen in Figure 6F. In an experimentalsituation, such clusters sediment is due to gravity leadingto a macroscopic phase separation.

3.3. Discussion. As mentioned earlier, Yamanaka etal.20 and Toyotama et al.21 have reported that initially ahomogeneous disordered (liquidlike ordered) deionizedsuspension undergoes crystallization upon increasing σeon the particles. This crystalline order (solid phase) turnsdisorder once again on further increase of σe. Thisreentrant order-disorder transition (also referred to asreentrant solid-liquid transition) is understood using MCsimulations24 with Us(r) as the pair-potential. Thesesimulations showed existence of homogeneous orderedphase in a narrow range of σe, whereas the solid regionreported by Yamanaka et al20 and Toyotama et al.21 existedrelatively over a wide range of σe. It may be mentionedhere that the solid region reported by Toyotama et al.21

is based on observing iridescence, whereas that reportedby Yamanaka et al.20 is by USAXS study. The presentinvestigations using SLS and CLSM have reveled that,for σe values in the range of 0.24-0.4 µC/cm2, the suspen-sions do not exhibit homogeneous crystalline state butare in an inhomogeneous state and it corresponds to gas-solid (crystal) coexistence. This gas-solid (crystal) co-existing state becomes inhomogeneous and disorderedupon increasing σe beyond 0.4 µC/cm2. Yamanaka et al.20

and Toyotama et al.21 have identified this inhomogeneousdisordered region as reentrant liquid region. However,present studies have clearly revealed that the reenteredliquid region is also a gas-solid coexistence state, wherethe structural ordering of particles in the solid correspondsto a glasslike disorder. Thus, we conclude that phasediagrams reported by Yamanaka et al.20 and Toyotama etal.21 consists of a homogeneous liquidlike region for lowvalues of σe (<0.2 µC/cm2) and a homogeneous crystallineregion for σe < 0.24 µC/cm2. An inhomogeneous crystallineregion exists in the form of voids coexisting with denseordered regions up to σe e 0.4 µC/cm2. For charge densitiesbeyond this σe, deionized suspensions will be in aninhomogeneous disordered state which will be in the formof coexistence of voids with a dense phase having glasslikedisorder. Hence, we conclude that a dilute charged colloidalsuspension under deionized conditions exhibit a homo-geneous liquid to homogeneous crystal transition uponincreasing σe. This homogeneous crystalline state once

Figure 6. Pair-correlation function g(r) vs r/d0 and thecorresponding projection of the particle coordinates for samplesS3, S7, and S4 are shown in A and B; C and D; and E and F,respectively. The white regions in panels B, D, and E correspondto voids and dots represent particles in the dense phase (solid).It can be seen the dense phase is ordered in panel B anddisordered (amorphous) in panels C and D. The vertical dottedlines in A, C, and E correspond to d0 for homogeneoussuspensions with φ ) 0.005, 0.005, and 0.0005, respectively.

Figure 7. Pair-potential Us(r)/kBT vs r/d0 for (A) sample S3(curve a) and sample S7 (curve b) and (B) for sample S4. Thevertical dotted lines shown in (A) and (B) correspond to d0 forhomogeneous suspensions with φ ) 0.005 and φ ) 0.0005,respectively.

11682 Langmuir, Vol. 21, No. 25, 2005 Mohanty et al.

again becomes inhomogeneous in the form of a gas-solidcoexistence state upon further increase of σe. Presentresults also suggest that the reentrant liquid or reentrantdisordered state reported by Yamanaka et al.20 andToyotama et al.21 is a gas-solid coexistence state andsuspensions of highly charged colloidal particles exhibitgas-solid transition upon deionization (i.e. lowering thesalt concentration).

4. ConclusionsFor the first time, an inhomogeneous crystalline phase

in the form of voids coexisting with ordered regions indeionized dilute charged colloidal suspensions have beeninvestigated using static light scattering and confocal laserscanning microscope. This inhomogeneous crystallineregion occurs in a relatively narrow range of σe. For charge

densities beyond this range, suspensions remain in aninhomogeneous disordered state in the form of voidscoexisting with glasslike disordered regions. Thus highlycharged colloids exhibit gas-solid coexistence underdeionized conditions. Occurrence of gas-solid transitionin charged colloids provide a strong evidence for counterionmediated long-range attraction in the effective pair-potential. Present results are expected to motivate theo-reticians to propose microscopic mechanism for counterionmediation leading to long-range attraction between like-charged colloids.

Acknowledgment. We thank Dr. Junpei Yamanakafor fruitful collaboration and for useful discussions.

LA0518896

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