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Liquid, glass, gel: The phases of colloidal Laponite

Herman Z. Cummins *

Department of Physics, The City College of CUNY, New York, NY 10031, United States

Available online 30 August 2007

Abstract

Laponite is a synthetic disc-shaped crystalline colloid that is widely used to modify rheological properties of liquids in applicationssuch as cosmetics, paints, and inks so that understanding its flow properties and aging behavior is of considerable practical as well asfundamental importance. However, some recent studies of the liquid–glass and sol–gel transitions in aqueous Laponite suspensions haveproduced results that do not fully agree with each other. Because Laponite is sensitive to sample preparation procedures, it is notstraightforward to compare results reported by different groups. We have begun a study of the dynamics of Laponite suspensions duringaging using photon correlation spectroscopy to explore the consequences of specific sample preparation procedures which may underliethese differences, including: (1) filtration of the sample through filters with different pore sizes before beginning the experiments, (2)adjusting and monitoring the pH of the solution, (3) varying the Laponite concentration, (4) carrying out the sample preparation ineither ambient air or dry nitrogen atmospheres, (5) baking the ‘dry’ powder to remove adsorbed water, and (6) modifying the ion con-centration by the addition of salts. We will compare the effects of different methods of preparation on the intermediate scattering functionF(q, t) and its time evolution. In this report we will describe experiments that explore (1)–(3). The other three will be discussed in a futurepublication.� 2007 Elsevier B.V. All rights reserved.

PACS: 83.80.Hj; 78.35.+c; 67.40.Fd; 82.70.Gg

Keywords: Rayleigh scattering; Transport properties gel; Transport properties – Liquids; Colloids; Nano-clusters

1. Introduction

Most recent experimental studies of the liquid–glasstransition and comparisons of the results with various the-ories have concentrated on fragile molecular glass-formingmaterials. However, several groups have explored theliquid–glass transition in colloidal suspensions and foundthat the data obtained are well suited to testing theories.There are two particular advantages to these systems. First,their relaxation dynamics occur on a considerably longertime scale than for molecular liquids, and can be followedcompletely with the single experimental light scatteringtechnique of photon correlation spectroscopy (PCS). Sec-ond, in carrying out comparisons with predictions of the

mode coupling theory (MCT), the crossover with decreas-ing temperature from cage-effect dominated dynamics tohopping dynamics exhibited by molecular glass-formersdoes not occur, greatly simplifying the analysis. Also, thephase diagrams of colloidal suspension often exhibit richstructure since, if attractive interactions are present, newphases may occur that are absent in molecular glass-formers.

Colloidal particles in dilute suspensions initially undergoindependent diffusional dynamics. With increasing particleconcentration or with aging, particle interactions can leadto more complex dynamical behavior and to transforma-tions to various new phases including fractal or compactclusters, cluster gels, repulsive or attractive glasses, andliquid-crystal phases. The widespread use of colloidal sus-pensions and gels in foods, pharmaceuticals, cosmetics,paints and inks, etc. gives these transformations practicalas well as fundamental interest and has led to an extensive

0022-3093/$ - see front matter � 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2007.02.066

* Tel.: +1 212 650 6921; fax: +1 212 650 6923.E-mail address: [email protected]

www.elsevier.com/locate/jnoncrysol

Journal of Non-Crystalline Solids 353 (2007) 3891–3905


literature of studies using various theoretical and experi-mental techniques (cf., [1–4]).

The simplest colloidal system, hard-spheres that interactonly at contact (HSS) suspended in a neutral solvent,undergoes crystallization to a FCC close-packed structurewhen the particle concentration (volume fraction /) reaches�0.50. However, if there is sufficient polydispersity, crystal-lization may be avoided and, at / � 0.58, a liquid–glasstransition occurs as each colloidal particle becomes trappedin the cage formed by its neighbors. This HSS liquid–glasstransition has been studied extensively by the groups of vanMegen and Pusey [5–11] and Bartsch [12] and has providedcritical tests of theories of the liquid–glass transition.

These studies showed that with increasing volume frac-tion / the relaxation dynamics slows dramatically, andthe quantitative structure of the intermediate scatteringfunction F(q, t) and its evolution with concentration, asdetermined by dynamic light scattering (photon correlationspectroscopy), are closely described by quantitative predic-tions of the mode-coupling theory (MCT) for the hard-sphere system [13–15]. In the experiments, kinetic arrestwas found to occur at a volume fraction of / � 0.58, some-what higher than the MCT ideal glass transition prediction/C = 0.516 for the hard-sphere system (HSS), althoughrecent extensions of MCT to include higher-order termsreportedly lead to an increase in this MCT value [16].

If, in addition to the hard-sphere repulsive potentialthere is a short-range attractive potential, an additionalsoft-solid phase can occur. In 1999, Fabbian et al. carriedout MCT calculations for a system of colloidal spherescharacterized by a hard-sphere potential supplemented bya short-range attractive square-well potential (‘stickyhard-spheres’) [17–20]. They found that this system exhibitstwo glass transitions; first, with increasing strength ofattraction, the volume fraction /C for the usual cage-effectmediated glass transition increases (glass I). Second, withinthe glass I phase, further increase of the attraction causespairs of particles to move together, opening holes in thecages, and causing the glass to melt. Finally, as the attrac-tion increases further, a second transition dominated byattractive forces occurs (glass II). These predictions wereverified in experiments in which the attractive interactionwas produced by adding small polymers to the colloidalsuspension, which causes a short-range depletion attraction[21–23].

Soft-solid phases of colloids held together by attractiveforces are usually considered as gels, but the distinctionbetween gels and attractive glasses is not clear. Analogiesbetween the two have been studied by several groups, e.g.[24,25]. Segre et al. have shown that relaxation dynamicsnear gelation and near the liquid–glass transition areremarkably similar [26]. Bergenholtz and Fuchs [27–29]examined the mode-coupling theory predictions for thebehavior of colloidal suspensions with attractive interac-tions at low volume fractions and concluded that the sol–gel transition could also be described by MCT. They notedthat if the short-range attractive interaction is represented

by a Yukawa potential rather than the square-well poten-tial considered previously, then the liquid–glass II transi-tion line would extend to very low volume fractions,suggesting that the sol–gel transition is a low-/ continua-tion of the glass II transition.

A modification of standard MCT was proposed by Kroyet al. [30] in which two MCT ergodicity-breaking transitionsoccur: a first short length-scale transition involving theformation of clusters, and a second larger length-scale tran-sition in which the clusters aggregate to form a gel (CMCT).A related scenario was identified in simulation studies bySciortino et al. [31,32]. These analyses suggest that the samemechanism underlying the liquid–glass transition alsounderlies the sol–gel transition, so that the characteristicdynamical signatures of the liquid–glass transition shouldalso appear at the sol–gel transition. The quantitativeaspects of these theoretical predictions largely remain tobe explored experimentally, especially those regarding theirdynamics.

If the colloidal particles are electrically charged, addi-tional phases can occur. Kumar and Wu [33] reportedmolecular dynamics simulations of colloids interactingthrough a short-ranged van der Waals attraction and alonger-ranged electrostatic repulsion. They observed a vari-ety of ‘jammed states’ at volume fractions between / = 0.4and / = 0.1, ranging from nearly uniform glass-like struc-tures to network-like gel structures. The relation betweencluster formation and combined short-range attractionand long-range repulsion has been studied by Sciortinoet al. [32]. Lu et al. have reported that suspensions of col-loids with attractive interactions induced by polymers exhi-bit a stable phase of clusters even in the absence of long-range repulsion, and that clusters can percolate across thesample to form a gel [34]. Also, if the colloidal particlesare electrically charged, a third glass phase can occur, sta-bilized by electrostatic repulsion. This phase is sometimescalled the ‘Wigner glass’.

In the colloidal systems described so far, the individualparticles are assumed to be spherical. In the case of asym-metric particles (e.g. round discs as in Laponite), there isalso the possibility of orientational order and liquid-crystalphases. Also, asymmetry of the charge distribution canproduce dense soft-solid phases stabilized by electrostaticinteractions.

1.1. Laponite

Many recent studies of the liquid–glass and liquid–geltransitions have employed the synthetic colloid Laponitewhich has all the characteristics discussed so far: bothattractive and repulsive interactions, anisotropy and netcharge, as well as an anisotropic charge distribution. Itexhibits an array of different phases and behaviors andhas become a widely used model system for testing theoriesof liquid–glass and liquid–gel transitions as well as variousaspects of aging phenomena. However, Laponite is not asimple material to handle, since it is sensitive to the meth-

3892 H.Z. Cummins / Journal of Non-Crystalline Solids 353 (2007) 3891–3905


ods of sample preparation. Comparing the differentreported studies therefore requires evaluating the impor-tance of the different methods of preparation employedby different authors. There are potentially many differentLaponite phases possible, and a major challenge is to sortout which of these phases are observed under particularexperimental conditions, and how they are influenced bythe sample preparation method followed.

Laponite (hydrous sodium lithium magnesium silicate)is a synthetic crystalline layered silicate colloid with crystalstructure and composition closely resembling the naturalsmectite clay hectorite. It is manufactured by RockwoodAdditives Ltd (formerly Laporte Ind. Ltd.), CheshireUK, and Southern Clay Products, Inc., Gonzales, Texas.Chemical analysis of Laponite RD by Levitz et al. [35] gavemean chemical composition: SiO2, 65.82%; MgO, 30.15%;Na2O, 3.20%; LiO2, 0.83%. The melting point is 900 �C.Extensive information on the structure and applicationsof Laponite can be found on the manufacturer’s websiteshttp://www.laponite.com and http://www.scprod.com.

The density of Laponite is 2.53 g m/cm3. Single Lapo-nite crystals are disc shaped and nearly uniform, typically25 nm in diameter by 0.92 nm thick, much smaller thannatural clays. Within a single crystal, each sheet of octahe-drally coordinated aluminum or magnesium oxide is sand-wiched between two layers of tetrahedrally coordinatedsilica.The crystal faces have negative charge; the edges havesmall pH-dependent positive charge, typically �10% of thenegative charge. The overall net negative charge of a singleLaponite disc is approximately 700 electron charges. Thecharge is balanced by interlayer cations which are predom-inantly Na+. In the dry powder, the Laponite crystals forminto stacks with the crystals sharing interlayer Na+ ions.

When dispersed in water, Laponite hydrates and swells toform a clear colloidal dispersion with the Na+ ions formingdouble layers on the faces. The pH for a 2% Laponite sus-pension in pure water is �9.8.

At low ionic strength, electrostatic repulsion keeps theparticles apart. Laponite is decomposed by acids, leadingto an increase in ion concentration with time at low pH.At concentrations of 2% or greater in water a gel will formrapidly. However, gel formation has been observed at con-centration well below 2% in several studies including thepresent one.

Laponite gel is strongly thixotropic, i.e. its viscositydecreases rapidly under shear. After the shear stress isremoved, the gel reforms; the rate of restructuring dependson composition, electrolyte level, age of the dispersion, andtemperature. The addition of salts reduces the thickness ofthe electrical double layer, promoting gel formation.

Laponite contains approximately 8 wt% water which ischemically absorbed into the crystal structure and can onlybe removed by baking at temperatures above 150 �C. Inaddition, Laponite is hygroscopic and will adsorb addi-tional water from the atmosphere, typically up to 15% at50% relative humidity. The structure of individual Lapo-nite particles and a schematic drawing of the proposed‘house of cards’ soft-solid phase are illustrated in Fig. 1.

There are several different grades of Laponite availablefor different commercial applications. Laponite RD, themost frequently studied grade, is used in many householdand industrial products including cleansers, surface coat-ings, and ceramic glazes. Laponite XLG is a high-puritygrade of Laponite RD, processed to remove impuritiessuch as heavy metals e.g. lead and arsenic. This grade isused in personal care and cosmetic products including

Fig. 1. Structure of individual Laponite particles and schematic house of cards structure of Laponite gel stabilized by electrostatic interaction between thenegatively charged faces and positively charged edges of the disc-shaped colloidal particles (from southern clay products product information website).

H.Z. Cummins / Journal of Non-Crystalline Solids 353 (2007) 3891–3905 3893


shampoos and sunscreens. Laponite XLG was used in thestudies of Thompson and Butterworth [36] and in the workdiscussed in this report.

1.2. Laponite phases discussed in the literature include the

following

1.2.1. Liquid with clusters

Some studies of Laponite suspensions at concentrationsof 0.9–5 wt% using angle resolved static light, neutron, andX-ray scattering suggested that they contain clusters thathave fractal structure [37,38]. Bonn et al. [39] comparedlight scattering from two samples prepared with 3.5 wt%Laponite and found that the angle-dependent intensitycharacteristic of fractal structures was present in the freshlyprepared solutions, but that filtration through a 0.8 lmmillipore filter resulted in the complete absence of the angledependence. Clusters should be more likely to form in sus-pensions having higher ionic concentration since at low ionconcentrations repulsive electrostatic interactions will keepthe particles apart.

1.2.2. Wigner glassAt low ionic strength, electrostatic repulsion keeps the

colloidal particles apart and can produce a transition toan arrested state stabilized by long-range electrostaticrepulsion [40].

1.2.3. High-density gel

At higher ionic strength, as the screening lengthdecreases, the positive double layers at the edges of plate-lets can approach the negatively charged double layers onthe faces. The high-density gel state of Laponite, calledthe ‘house of cards’ structure, occurs when the screeninglength is sufficiently short so that this attractive interactiondominates. This structure is readily observed if dry Lapo-nite powder is mixed with tap water which typically has ahigh ion concentration.

1.2.4. Low-density gel

Ruzicka et al. [41] studied Laponite suspensions withconcentrations between 0.3 and 3.1 wt%. They found thateven for the lowest concentrations a transition to anarrested phase occurs after a sufficiently long time(�6 months for 0.3 wt%). They also found that the timeevolution of the dynamics differed for concentrationsabove and below �0.17 wt% suggesting that there aretwo different gel structures for this material. One possibilityis that the high-density gel is the ‘House of Cards’ structurewhile the low-density gel consists of a network of chains asone finds in polymer gels, which can form gels at very lowconcentrations. Alternatively, the low-density gel may con-sist of a network of clusters as discussed by Lu et al. [34].

1.2.5. Nematic phasesLemaire et al. [42] studied Laponite gels with SAXS and

found evidence of anisotropy in the scattering patterns,

indicative of nematic orientational order, for Laponite con-centrations above 2 wt%. Gabriel et al. [43] observed sus-pensions of Laponite (Laponite B) between crossedpolarizers and found optical birefringence for concentra-tions above 2.4 wt%, again indicative of nematic order.Agra et al. [44] have shown theoretically how a rich varietyof orientational ordered phases in colloidal crystals can beunderstood.

Previous light scattering studies of Laponite have beenreported in numerous references including [37–41,45–57].

Sample preparation methods differ widely among thepublished studies. Some of the specific aspects of the prep-aration procedures whose importance we are investigating,are:

1. Sample filtration: What type and pore size filter wasused? Was there a delay between mixing and filtration?

2. Is the water pH adjusted before/after adding the Lapo-nite? Is it monitored later?

3. What is the Laponite concentration?4. Is the sample prepared under nitrogen or in a normal

ambient atmosphere?5. Is the sample dried to remove moisture?6. What is the ion concentration (possible modification by

addition of salt)?

In this report we will concentrate on points 1–3. Theothers are currently under study and will be discussed ina future publication.

2. Experimental

2.1. Sample preparation

The Laponite XLG used in the experiments described inthis report was lot 04-239, purchased from Southern ClayProducts in Feb 2005. The certificate of analysis indicates6.8% moisture content, although this should be expectedto increase during handling and transfer to storage jars.The moisture content was measured during preparationof the samples with a Sartorius MA100C moisture analyzerand was found to be �9.8%.

Samples for the PCS experiments were prepared with theLaponite as provided without further drying. Samples wereloaded in screw-top cylindrical glass vials with outsidediameters of either 20 or 28 mm. Three different series ofLaponite samples were prepared. Each series included sev-eral stock solutions with different concentrations preparedfollowing the same procedure. From each stock solution,three (or more) samples were prepared by extracting someof the stock solution with a syringe and forcing it throughvarious Millipore millex sealed syringe filters with 33 mmmixed cellulose ester membranes. For each such prepara-tion, one sample was prepared without a filter. The samplesare listed in Table 1. Concentrations are given in weightpercent of Laponite, uncorrected for water content of thepowder. Using the Laponite density of 2.53 g m/cm3 and



water content of 9.8%, the relation between volume frac-tion / and concentration C is / = 0.9C/(2.5 � 1.35C) (withC = 0.01*C (wt%)). For the samples studied / ranges froma maximum of 5.4E�3 for the 1.5 wt% samples to 1.4E�4for the 0.04 wt% samples. The samples were all preparedunder normal ambient atmosphere. Preparation of samplesin a glove box under dry nitrogen atmosphere is currentlyin progress and will be discussed in a future publication.

Three series of samples (A,B,C) were prepared, eachfollowing a different pH adjustment protocol. The pH val-ues measured after completion of the mixing proceduresare shown in the second column of Table 1.

A series Laponite powder was added slowly to distilledor DIUF water while stirring. There was no measure-ment or control of pH during preparation. The pH ofeach stock solution listed in the table was measuredlater.

B series Laponite powder was added slowly while stir-ring; after mixing was complete the pH was adjustedto pH > 10 by addition of 1% NaOH solution, ifrequired. Because the DIUF water pH is �4, some aciddissociation of these B series samples may have occurredbefore the pH was adjusted. Therefore, for the C series,the water pH was adjusted before adding the Laponite.C series Laponite powder was added slowly to DIUFwater with pH adjusted to >10 by addition of 1% NaOHsolution before mixing.

Periodically all samples were removed from the storagerack and tilted slightly to see if gelation had occurred. Thistilting may have caused some slight mixing in those sam-ples that had not gelled. In the right-hand column of Table1 we show the elapsed time (in days) from preparation untila gel was observed. For samples that had not gelled, wegive the elapsed time from sample preparation until the last

Table 1Laponite samples and the results of PCS measurements as discussed in the text

Series Sample Filter Loaded Gelled Bad KWW Last PCS 7/18/06 since load (days)

A (measure pH later) pH = 9.53AA (0.89%) Mix: 6/30/05 AA2 None 7/6/05 1/4/06 9/14/05 GEL (G)182

AA3 0.1 7/6/05 1/4/06 9/14/05 GEL (G)182AA4 0.8 7/14/05 7/10/06 liq(7/17) 368a

AA5 0.45 7/14/05 9/12/05 9/10/05 GEL (G)60

pH = 9.28AB (0.06%) Mix: 7/18/05 AB1 None 7/18/05 2/16/06 9/22/05 liq(7/17) 364

AB2 0.45 7/19/05 2/9/06 2/9/06 liq(7/17) 363AB3 0.8 7/19/05 1/5/06 9/14/05 liq(7/17) 363

pH = 9.90AC (1.50%) Mix: 8/8/05 AC1 None 8/9/05 9/15/05 9/15/05 GEL (G)37

AC2 0.8 8/9/05 6/13/06 3/2/06 3/6/06 GEL (G)308AC3 0.45 8/9/05 6/13/06 3/2/06 GEL (G)308

B (adjust pH after mixing if pH < 10.0) pH = 10.12BA (1.00%) Mix 1/2/06 BA1 None 1/2/06 7/10/06 liq(7/17) 196

BA2 0.8 1/2/06 7/10/06 liq(7/17) 196a

BA3 0.45 1/2/06 7/10/06 liq(7/17) 196

pH = 10.37BB (0.18%) Mix: 2/6/06 BB1 None 2/6/06 7/10/06 liq(7/17) 161

BB2 0.8 2/6/06 7/10/06 liq(7/17) 161BB3 0.45 2/6/06 7/10/06 liq(7/17) 161

C (use water with pH = 10) pH = 10.27CA (0.98%) Mix: 1/3/06 CA1 None 1/3/06 7/10/06 liq(7/17) 195

CA2 0.8 1/3/06 5/3/06 4/11/06 4/11/06 GEL (G)123CA3 0.45 1/3/06 7/10/06 6/5/06 6/26/06 Soft GEL (G)194

pH = 10.29CB (0.04%) Mix: 1/9/06 CB1 None 1/10/06 7/10/06 liq(7/17) 188

CB2 0.8 1/10/06 7/10/06 liq(7/17) 188CB3 0.45 1/10/06 7/10/06 liq(7/17) 188

pH = 10.42CC (0.18%) Mix 1/12/06 CC1 None 1/13/06 7/10/06 liq(7/17) 185

CC2 0.8 1/13/06 7/10/06 liq(7/17) 185CC3 0.45 1/13/06 7/10/06 liq(7/17) 185

The final column shows the elapsed time (in days) before sample gelation (G) or, if gelation was not observed by 7/18/06, the elapsed time since it wasprepared.

a Note: by 10/25/06 samples AA4 and BA2 had also gelled.



observation of the liquid. Note that for samples withC < 0.2 wt% no gelation was observed.

The elapsed time (in days) from sample preparationuntil a gel was observed is shown for all samples by thesolid symbols in Fig. 2. For the samples where gelationwas not seen, the sample is represented by an open symbol.

We note that the filters were used as provided by themanufacturer. Some surprising inconsistencies that weobserved, especially with the 0.8 lm filters, may be relatedto residual traces of detergent or solvent in the filters. Thiscould be checked by rinsing the filters with pure waterbefore passing the Laponite solution through them, butthis has not yet been done.

2.2. PCS measurements

PCS measurements were carried out with a BrookhavenInstruments BI-9000AT digital correlator. Excitation wasprovided by a Coherent Innova I306C Argon laser operat-ing in single-mode at 488 nm with typical output power of150 mW. Power at the sample was approximately 50 mW.All experiments were performed at a 90� scattering anglewith data collection time of 10 min.

For ergodic samples the normalized intensity correlationfunction g2(t) = C(t)/B (where B is the background) isrelated to the intermediate structure factor F(q, t) by

g2ðtÞ ¼ 1þ ajg1ðtÞj2 ¼ 1þ a½F ðq; tÞ=F ðq; 0Þ�2 ð1Þ

In the simplest case of uncorrelated spherical particles ofradius r undergoing independent translational diffusion,

g2ðtÞ ¼ 1þ a expð�2t=sÞ ¼ 1þ a expð�2Dq2tÞ ð2Þwhere the translational diffusion constant D = kT/6pgr.For a distribution of particle sizes, a simple generalization(which we will use here) is to replace the exponential in Eq.(2) with a KWW stretched exponential function and to usea free baseline b � 1:

g2ðtÞ ¼ bþ a exp �2ðt=sÞbh i

ð3Þ

For 4880 A light and 90� scattering, the mean hydrody-namic radius rh is approximately related to the measuredcorrelation time s by

rh ðnmÞ ¼ sðlsÞ=7:76 ð4ÞWe used Eqs. (3) and (4) to find approximate scatterer sizesfrom the PCS data. For independent single Laponite parti-cles we expect rh � 13 nm. We emphasize that this fittingprocedure was a simple approximation used to provide arough estimate of the time evolution of cluster sizes andpolydispersity under different preparation procedures.Since the experiments were performed at fixed q, the q2

dependence of Eq. (2) was not tested. Furthermore, thecluster size was estimated from Eq. (4) using the value ofs from the fits to Eq. (3). The mean value of s would be in-creased for b < 1, reaching hsi = 2s for b = 0.5.

If the colloidal sample is a gel, then extracting dynamicalinformation from PCS data is much more difficult as dis-cussed in detail by Pusey and van Megen in 1989 [58].We will discuss the PCS data analysis problem for gelsbriefly in Section 3.4 below.

If the correlation function of a monodisperse solution isfit to Eq. (3), the KWW stretching coefficient should beb = 1. If the sample is polydisperse then b < 1. To explorethe dependence of b on polydispersity, we constructed syn-thetic g2(t) data and performed KWW fits for theoreticalpolydisperse solutions with radii ranging from 13 nm to amaximum rmax between 13.1 nm and 300 nm, assumingthat the product of particle concentration and particle scat-tering strength was constant across the range of sizesincluded. For a size distribution whose width is 0.4 timesthe mean size, b is 0.99, still very close to 1. For the mostpolydisperse C(t) considered, with width 1.8 times themean size, b decreased to 0.79. Also, for that fit, there isa small systematic error as C(t) begins to decay from theinitial plateau; the error is very similar to fitting errors seenin the PCS experiments as discussed below.

3. Results

PCS experiments on the Laponite samples listed inTable 1 were performed frequently, beginning soon aftereach sample was prepared. The PCS data was analyzedwith the four-parameter KWW function (Eq. (3)) from

Fig. 2. Elapsed time in days from sample preparation until firstobservation of a gel (solid symbols) vs concentration in wt%. Opensymbols indicate samples that were still liquid at the latest observation.Series A-circles, series B-Squares, series C-triangles. No filter: largesymbols; 0.8 lm filter: medium symbols; 0.45 lm filter: small symbols.



which an estimate of the average size rh of the scattererswas obtained with Eq. (4). The relaxation time s (andestimated radius rh) increased with time for all samplesstudied, although in some cases a small decrease wasobserved during the first few days after sample preparation.

In Fig. 3 we show PCS data for samples CC1, CC2, andCC3 (0.18 wt%), 6 days after preparation (squares) and 137days after preparation (circles). The KWW fits are also

included for the 137-day data. The relaxation slows withincreasing time for all three samples as expected due togrowth of clusters. From the KWW fits with Eq. (3), weobtained average sizes rh at 6 days and 137 days for thethree samples: (CC1) 14.8 nm, 360.4 nm; (CC2) 14.6 nm,154.0 nm; (CC3) 14.3 nm, 53.9 nm. At 6 days, all threesamples had correlation times of about 100 ls and corre-sponding estimated radii of rh � 14 nm, indicating that

Fig. 3. PCS data for 0.18 wt% Laponite samples CC1, CC2, and CC3 six days (squares) and 137 days (circles) after preparation. The solid lines are KWWfits used to extract estimates of the average radius of the scatterers as described in the text.



the scatterers were individual Laponite particles or verysmall clusters. (Rosta and von Gunten concluded that theirLaponite suspensions contained very small clusters ofbetween two and four platelets [57]). Note that at 6 daysrh does not depend on the filter used, but after 137 daysrh of the unfiltered sample has increased the most, whilethe 0.45 lm filtered sample has increased the least. How-ever, as we shall see, the correlation between filter pore sizeand cluster size is not generally consistent.

The systematic departures from the KWW fit in Fig. 3for sample CC1 closely resemble those seen in our fits tosynthetic data for the most polydisperse case, indicating thepresence of considerable polydispersity in this sample.

At longer times, the correlation functions of samplesthat remain liquid often evolve into shapes with long tails,signaling the existence of large slow-moving clusters withlarge polydispersity and limiting the utility of KWW fits.Fig. 4 shows PCS data for samples CC1, CC2, and CC3

Fig. 4. PCS data for 0.18 wt% Laponite samples CC1, CC2, and CC3 166 days after preparation showing the ‘tails’ on C(t) for CC1 and CC2, but not forCC3. The insets for CC1 and CC3 show the counts accumulated during each second of the 10-min runs.



recorded 166 days after preparation. For CC1 and CC3 wealso include the count rate histories as insets which showthe number of photocounts collected each second duringthe 10-min runs. Sample CC1 (unfiltered) has a prominentPCS tail, and the count rate exhibits large fluctuations on atime scale of �1 min, indicating (via Eq. (4)) the presenceof large clusters with rh � 10 lm. Sample CC2 also has aprominent tail, but CC3, which was passed through the0.45 lm filter, has no tail and the count rate history showsno slow fluctuations.

The evolution of rh and b obtained from the KWW fitsfor all samples studied is shown in Figs. 5–7. From these fig-ures, and from Fig. 2, some general observations can bemade. First, for the lowest concentration samples(C < 0.2 wt%), no gelation was observed within the obser-vation time of �1 year. Second, for the highest concentra-tion samples AC, the unfiltered sample gelled first (37days) while the two filtered samples did not gel until 308days. But for the AA 0.89% samples, the unfiltered and0.45 lm filtered samples gelled at 182 days and 60 days,respectively, while the 0.8 lm filtered sample was still liquidafter a full year.

Third, for the samples that gelled, there was a rapidincrease in size and corresponding decrease in b, indicating

increasing polydispersity, that precedes gelation (see, e.g.,BA2, BB2, and CA2).

A series: Of the samples with 0.89 wt% concentration, allbut one (AA4) had gelled within 75 days of prep-aration while the third sample (AA4) was stillliquid after 250 days. The 1.50 wt% AC samplesall gelled, with the unfiltered sample AC1 after60 days and the other two after �340 days.The AB 0.06 wt% samples showed constantlyincreasing cluster sizes but did not gel withinone year.

B series: Samples BB1, BB2, and BB3 (0.18 wt%) PCSdata obtained for up to 154 days after prepara-tion. Note that the unfiltered sample (BB1) andthe 0.45 lm filtered sample (BB3) haverh � 12.3 nm indicating that no significant aggre-gation has occurred while sample BB2, filteredwith a 0.8 lm filter, has rh � 56 nm indicatingconsiderable aggregation.

C series: Aggregation of the C samples proceeded as leastas fast as the B samples. This indicates that thereis no advantage to adjusting the water pH beforeaddition of the Laponite.

70

60

50

40

30

20

10

radi

us (n

m)=

tau

(mic

rose

c)/7

.76

350300250200150100500elapsed time since loading (days)

A_ rh-vs-et .pxpHydrodynamic radius vs elapsed time - samples AA,AB,AC (12 JULY06)

AA : 0.89 wt% - mixed inpureDIUF water - no pH controlred AA2 (no filter)green AA4 (0.8micron filter)blue AA5 (0.45 micron filter)black AA3 (0.1micron filter)

AA2 gelledafter 14 Sept

AA3 gelledafter 14 Sept

AA5 gelled12 Sept

2

46

100

2

46

1000

2

4

20015010050

red AB1 (no filter)green AB3 (0.8micron gilter)blue AB2 (0.45 micron filter)

NOTE: Beyond ~ 60 days, PCS datanotdescribed by KWW - have big tailsBUT AB SAMPLES DONOT GEL

AB: 0.06 wt% - mixed in DIUFwater (no pH adjustment)

10

2

468100

2

4681000

250200150100500

AC 1.50 wt % - mixed in oure DIUF water - no pH adjustmentred AC1 ( no filter)green AC2 (0.8micron filter)blue AC3 (0.45 micron filter)

gel (60 days)

9 Feb

5 Jan

6 March

AA4

10July06(361 days)

AC2 & AC3Poor KWWfi ts ; gelled after~340 days

0.89 wt%

0. 06 wt %

1. 50 wt %

1. 0

0. 9

0. 8

0. 7

0. 6

0. 5

KWW

stre

tchi

ng p

aram

eter

β

350300250200150100500elapsed time since sample loading (days)

A_ bet a-vs-et .pxpKWW parameter β vs elapsed time - samples AA, AB, AC ( 12 July 06)

red: AA2 (no filter)green: AA4 (0.8micron filter)blue: AA5 (0.45 micron filter)black: AA3 (0.1micron filter)

0.8

0.6

0.4

0.2

0.0

16014012010080604020

red: AB1 (no filter)green: AB3 (0.8 micron filter)blue: AB2 (0.45 micron filter)

0.7

0.6

0.5

0.4

0.3

0.2

300250200150100500

red: AC1 (no filter)green: AC2 (0.8 micron filter)blue: AC3 (0.45 micron filter)

10July06t=361 days

9 Feb06

6 March06

Fig. 5. Approximate hydrodynamics radius (left) and KWW stretching parameter b (right) vs elapsed time since sample preparation in days from KWWfits for all samples in series A.



3.1. Cluster formation vs gel formation

Most samples showed increasing s (and rh) and decreas-ing b with increasing time, in some cases following an ini-tial short-time decrease in rh, demonstrating that bothmean cluster size and polydispersity generally increase asaging proceeds. For some samples, the intercept/back-ground ratio a/b suddenly decreased from �1 to 0.5 or less,and tipping these samples then showed that a gel hadformed. The dates and corresponding elapsed times sincepreparation when a gel was first observed for each sampleare also shown in Table 1. For other samples, especiallythose prepared at low concentrations, s (and rh) continuedto increase while a/b remained at �1. For these samples,the correlation function C(t) usually developed a long hightail indicating that cluster size and polydispersity continueto increase, but the samples remained liquid. Also, thecount rate record for these samples show very slow fluctu-ations, indicating the presence of very large clusters. Thesetwo distinct patterns of time evolution of the PCS data areillustrated in Fig. 8.

For polymer suspensions, as the particles aggregate, theform of the aggregates (or clusters) can take on differentstructures depending primarily on the coagulation rate.When coagulation is rapid, the cluster structure is open

and can be characterized as a fractal structure with fractaldimension D in the range 1.7 < D < 2.2. When coagulationis slow, the aggregates tend to be much more dense [2]. Thisdistinction was discussed by Lin et al. [2] for colloid aggre-gation and may underlie the two routes to gelationreported by Ruzicka et al. [41].

3.2. When does aging begin?

It has sometimes been asserted that when stock Lapo-nite solutions are passed through a filter into a sample cell,all clusters are broken up and the sample aging processeffectively starts over, so the aging time clock is reset tozero. However, we observed three effects that appear tocontradict this claim:

(1) Stock solution AA was mixed on 6/30/05 with concen-tration C = 0.89 wt%. Samples AA2 through AA5were loaded within the next two weeks. Another sam-ple, AA6, was loaded 88 days after mixing. SamplesAA5 and AA6 were both prepared using 0.45 lm fil-ters. The first PCS run for sample AA6, carried outon the same day that the sample was prepared, gavean initial s value of 1020 ls, �8 times larger than the125 ls found for sample AA5. Presumably, some clus-

10

2

3

45678

100

2

3

45678

radi

us(n

m) =

tau(

mic

rose

c)/7

.76


BA: 1.0 wt% mixed in pure DIUF water adjust pH = 10 afterwards if needed with 1% NaOH

red: BA1 (no filter)green: BA2 (0.8 micron filter)

blue: BA3 (0.45 micron filter)

Hydrodynamic radius vs elapsed time - samples BA, BB (10July06) B_rh-vs-et .pxp

10

2

3

4

5

6

7

89

100

160140120100806040200

BB:0.18 wt% mixed in pure DIUF water, adjust pH = 10 afterwards if needed

red: BB1 (no filter) green: BB2 (0.8 micron filter) blue: BB3 (0.45 micron filter)

10 July 06189 days

BA:1.0 wt %

BB: 0. 18 wt %

10 July 06154 days

0.8

0.7

0.6

0.5

0.4

0.3

150100500

BA: beta vs elapsed timered: BA1 (1.0 wt%)green: BA2blue: BA3

Stretching coefficient beta vs elapsed time - samples BA, BB [10 July 06]

0.90

0.88

0.86

0.84

0.82

0.80

0.78

0.76

160140120100806040200

BB: beta vs elapsed timered: BB1 (0.18 wt%)green: BB2blue: BB3

B_beta -vs-et .pxp

10 July 06

10 July06

Fig. 6. Approximate hydrodynamics radius (left) and KWW stretching parameter b (right) vs elapsed time since sample preparation in days from KWWfits for all samples in series B.



ters that had formed in the AA stock solution duringthe 88 days after it was mixed were not fully brokenup by filtration in the preparation of sample AA6.

(2) For many of the samples (e.g. BA) the value of sdecreased for several days after the sample wasloaded and then began to increase again (seeFig. 6). This observation suggests that some smallaggregates present in the dry powder survive severalhours of mixing and filtration but do dissolve slowlyin the sample cells after several days.

(3) The records of radius vs elapsed time shown in Figs5–7 allow a comparison of results for different filtersizes. From the figures, there is no clear correlationof radius with filter size. In fact, for some samplesprepared with no filter (e.g. CA1 and CB1) the meancluster size increases less with time than the samplesprepared with 0.45 or 0.8 lm filters. The origin of thisinconsistency is currently unknown.

3.3. Anisotropy vs polydispersity

The fits of Laponite PCS data to Eq. (3) were primarilyused to estimate rh, but some of the fits were poor, espe-cially in the region of the initial decay away from the pla-teau. Departures becomes more visible as the mean size

increases (for PCS spectra of a standard 22 nm polystyrenesuspension, the same procedure gives excellent KWW fits).To see if this effect is due to anisotropy or polydispersity,we carried out several runs with polarization selection,using samples contained in square optical cuvettes to avoidpolarization distortion. The experiments were performedwith the incident light polarized vertically, perpendicularto the scattering plane (V) and the scattered light polariza-tion was selected as either vertical (VV), horizontal (VH) orall scattered light was collected (VT).

For a 1% Laponite dispersion (CA) the Laponite VT andVV fits were nearly identical, giving rh = 12.6 and 12.7 nm,respectively. The VH spectrum was very weak, with intensityabout 3% of the VV intensity. This indicates that the anisot-ropy of the Laponite particles is not a significant factor in thePCS data, and that the typical departure from the KWW fit,visible in the short-time behavior of the VT and VV spectra,is due to polydispersity and not to anisotropy.

3.4. Gels

As the colloidal solution transforms from a sol to a gelthere are dramatic changes in the structure and dynamicsthat continue to evolve as the sample ages. We intend toexplore this aspect of Laponite in detail. So far, however,

10

2

4

68

100

2

4

6

radi

us (n

m) =

tau

(mic

rons

)/7.7

6


C_rh-vs-et .pxpHydrodynamic radius vs elapsed ti me - samples CA, CB, CC (13July06)

CA : 0.89 wt% mixed in pH=10 DIUF water (adjusted with 1% NaOH) red: CA1 (no filter)green: CA2 (0.8 micron filter)

(98days BIG TAIL - stop KWW) blue: CA3 (0.45 micron filter)

10

2

46

100

2

46

1000

2

200150100500

CB: 0.04 wt% mixed in pH=10 DIUF waterred: CB1 (no filter)

green: CBA2 (0.8 micron filter)blue: CB3 (0.45 micron filter)

10

2

4

68

100

2

4

68

1000

150100500

CC: 0.18 wt% mixed in pH=10 DIUF water red: CC1 (no filter) green: CC2 (0.8 micron filter) blue: CC3 (0.45 micron filter)

10July06

10July06

13July06

CA: 0.89 wt%

CB: 0.04 wt%

CC: 0.18 wt%

CA2Gel-3May120 days)

CA3 Gel - 13July 174 days

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

150100500

CA: beta vs elapsed timered - CA1 (0.89%)green - CA2 (stop at 98)blue - CA3

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

150100500

CB : beta vs elapsed timered - CB1 (0.04%)green - CB2blue - CB3

C_beta -vs-et .pxpDependence of KWW beta on elapsed time - Series C (13July2006)

1.0

0.9

0.8

0.7

0.6

0.5

0.4

150100500

CC: beta vs elapsed timered - CC1 (0. 18% )green - CC2blue - CC3

10July06

10July06

10July06

CA2 Gel-3May

(120 days)

0.89 wt %

0.04 wt %

0. 18 wt %

CA3GEL-13July(188 days)

Fig. 7. Approximate hydrodynamics radius (left) and KWW stretching parameter b (right) vs elapsed time since sample preparation in days from KWWfits for all samples in series C.



we have only carried out a preliminary study of one aspectof the gel transition: how the PCS data are affected by theonset of nonergodicity as described briefly below.

When a colloidal dispersion gels, the range of motion ofeach particles becomes limited and the dynamics becomesnonergodic. Time averages and ensemble averages are nolonger equivalent and Eq. (1) and (2) are then not valid. Toovercome the problem of nonergodicity, several methodshave been described. First, the sample can be slowly rotated[23] or translated during the PCS measurement so that manyindependent scattering volumes are sampled sequentially,making the time-averaged PCS data effectively an ensembleaverage. Second, scattered light can be collected simulta-neously over a range of scattering vectors and the multispec-kle correlation functions averaged over the different spots,again resulting in an ensemble average [5,6,58–61].

In their 1989 paper, Pusey and van Megen [58] suggestedanother way to overcome the nonergodicity problem bylooking for a place in the scattering volume where the staticcomponent of the scattering is very weak. Fig. 9 shows PCSspectra of Laponite sample AA5 (0.89 wt%). In the upperpanel, the sample is a liquid with a/b ratio �0.95, at timesof 0, 20, and 25 days after loading the sample. The initialhydrodynamic radius is �15 nm, increasing to �70 nm by25 days. By 62 days after loading, the sample has gelledand the a/b ratio has dropped from 0.95 to �0.35. ThePCS spectra shown in the lower panel are all at 62 daysor later. The a/b ratio varies between a maximum of�0.8 to a minimum of �0.05, depending on location inthe sample. The higher a/b ratios corresponded to loweraverage count rates. This extreme variation occurs becausethe detected signal consists of a dynamical component

Fig. 8. Correlation data with KWW fits for sample AA5 (top) and AB1 (bottom). AA5 was a gel by 62 days after preparation and shows a drop in its a/bratio. AB1 remained liquid, but developed a long tail indicating the presence of large clusters.



superimposed on a static component which, if the particleswere immobile, would produce the familiar speckle patterncharacteristic of scattering from a system of random fixedscatterers. As Pusey and van Megen noted, this randomspatial property of the static component is the reasonwhy the a/b ratio is so variable, and can be exploited bymoving the sample around until a value near zero is foundfor the static component, resulting in an a/b ratio near to1.0. As shown in Fig. 9, one spectrum has an a/b ratio of�0.8 and is therefore close to the case they described. Also,it appears that the apparent decay time becomes longer asthe a/b ratio decreases, but we have not attempted to verifythis correlation quantitatively.

We also recorded PCS spectra of some gelled sampleswhile slowly translating the sample tube vertically. Thea/b ratio was then nearly 1.0 as expected if ergodicity isrestored, and C(t) exhibits a high plateau that decays atlong times. We also recorded count rate histories for thesespectra. For the stationary sample cases, the count rate is

largest for the small a/b ratio runs (large static intensitycauses a small a/b ratio) and is relatively constant. Forthe translated samples, the count rate is very large and fluc-tuates wildly as the sample moves. The decay of C(t) attimes of �0.1 s observed for these translated samples isdue to the motion of the sample and does not relate tothe intrinsic dynamics of the colloidal particles.

4. Discussion

We have carried out PCS measurements on aqueoussolutions of Laponite XLG for three different preparationmethods, for a range of concentrations, and with differentfiltration procedures. As in previous studies we found thatat concentrations below �1 wt% the aging process is veryslow and the PCS data are still evolving at times approach-ing one year. Samples prepared without pH control aggre-gated fastest in general, although one sample in this series(AA4) had not gelled a full year after preparation. For

Fig. 9. PCS data for sample AA5. Upper panel: C(t) and KWW fits after zero days (circles). 20 days (triangles), and 25 days (squares) when the sample is aliquid. Lower panel: C(t) after 62 days the sample has gelled. Different data correspond to different heights in the cell and show the large variation in a/bratio caused by the random nature of the static scattering intensity.



the samples prepared with pH control, there was little differ-ence between those for which the pH was adjusted to a value>�10 after mixing was complete and those mixed withwater whose pH had already been adjusted to a value >�10.

Comparing samples prepared without filtration, filtra-tion with a 0.8 lm pore size filter or with a 0.45 lm filtergave ambiguous results. In some cases the unfiltered sam-ple gelled first, the 0.8 lm sample second, and the0.45 lm sample last, with the rate of increase in cluster sizefollowing the same sequence. But in some samples thisorder was permuted or reversed. The filters were used asobtained from the manufacturer (Millipore) and may con-tain small residues of detergent or solvent that influencesthe cluster growth and gelation processes. In future exper-iments the effect of flushing the filters with pure waterbefore use will be explored as will the effects of preparingsamples under a dry nitrogen atmosphere.

5. Conclusions

We conclude that the aging behavior of Laponite sus-pensions is strongly affected by the sample preparation pro-cedure, making it essentially impossible to compare theresults of experiments that follow different methods ofpreparation. First, the speed with which Laponite particlesaggregate to form growing clusters is significantly higherfor samples with no pH adjustment than for those withthe pH >10. Second, filtration affects the rate of aggrega-tion, but the relation between filter pore size and aggrega-tion rate is not consistent. It is possible that residualimpurities in the filters used play a role, a possibility thatrequires further study. Finally, in contrast to previousclaims, we conclude that filtration does not completelybreak up the existing clusters and that aging that takesplace between mixing and filtration is not completelyreversed by filtration.

Acknowledgement

This research was supported by the NSF under GrantNo. DMR-0243471.

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