Arrested state of clay-water suspensions: gel or glass?

0.25 MB·English

by  B. Ruzicka

Arrested state of clay-water suspensions: gel or glass?
1 1 1 2 2 1
B. Ruzicka , L. Zulian , R. Angelini , M. Sztucki , A. Moussa¨ıd , G. Ruocco
1 SOFT INFM-CNR and Dipartimento di Fisica, Sapienza Universita` di Roma, I-00185, Italy.
2 European Synchrotron Radiation Facility. B.P. 220 F-38043 Grenoble, Cedex France.
(Dated: February 3, 2008)
The aging of a charged colloidal system has been studied by Small Angle X-rays Scattering, in
theexchanged momentum rangeQ=0.03 ÷ 5nm−1,and byDynamicLight Scattering, at different
clay concentrations (Cw =0.6 % ÷ 2.8 %). The static structure factor, S(Q), has been determined
as a function of both aging time and concentration. This is the first direct experimental evidence
of the existence and evolution with aging time of two different arrested states in a single system
8
simply obtained only by changing its volume fraction: an inhomogeneous state is reached at low
0
concentrations, while a homogenous oneis found at high concentrations.
0
2
PACSnumbers: 61.10.Eq,82.70.Dd,82.70.Gg,64.70.Pf

n
a
J Inrecentyears,dynamicalarrestincolloidal,andmore turesofgelsandglassesincolloidsisnecessaryandlively

4 generallyinsoftmattersystems,hasgainedincreasingat- debated in the up-to-date literature.
2 tention [1]. Specifically, much effort has been devoted to In Laponite suspensions -the system under investiga-
clarify the dynamical behavior at large packing fraction, tion in the present work- nanometric size disks form a
]
t where the dynamical arrest is commonly identified as a charged colloidal dispersion with a rich phase diagram.
f
o kind of ”glass transition”. In colloids where both short The competition between attractive and repulsive inter-
s range repulsion and attraction are present, a rich phe- actions and/or the anisotropy of the potential that orig-
t. nomenology is found: a re-entrant liquid-glass line, two inatesthe complexityofthe phasediagramandthe exis-
a kindsofglasses(named”attractive”and”repulsive”)and tence of several aggregation processes. Therefore, in re-
m
a glass-glass transition line have been predicted and ex- cent years, Laponite suspensions have been widely stud-
d- perimentally observed [2]. Very recently an increasing ied not only for the important industrial applications [6]
n attention has also been devoted to the arrest at much butespeciallyfortheirpeculiarexperimental/theoretical
o smaller densities, in the usually called ”gel region” of properties [7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19].
c the phase diagram [1]. Experimental and simulation [3] The first Dynamic Light Scattering (DLS) study [8] on
[ studies have proven that, for hard-core plus spherically this system has shown a slowing down of its dynamics
1 symmetric pair-wise attractive potentials, arrest at low (aging)asit evolvestowardsequilibrium. Thereforeevo-
v density occurs only through an interrupted phase sep- lution with respect to waiting time from a liquid to a
8 aration. More complex attraction is necessary to pro- gel/glass state has been usually investigated for samples
6 duce gelation in equilibrium which takes place when the at clay concentration of Cw =3% and at salt concentra-
7 gel state is reached continuously from an ergodic phase. tionofCs =1×10−4M. ArecentcompleteDLSstudyin
3
This occurswhen along-rangerepulsion,inducedfor ex- a wide range of clay and salt concentrations has permit-
.
1 ample by residual charges on the colloidal particles, is ted to individuate a surprising final arrested state not
0 added to the short range depletion attraction [4]. Very only for high but also for very low clay concentrations
8
recently a suppression of the phase separation has also [13], at variance to previous [7] and recent [11] proposed
0
: been achievedby using an anisotropicinteractionpoten- phase diagrams. The aging time evolution from the ini-
v tial[5]. Inthis scenarioloweringthe ”coordinationnum- tial liquid to the final arrested state occurs in a time
Xi ber”,i.e. thenumberofnearestneighborsallowedbythe that strongly depends on salt and clay concentrations
interaction potential, pushes the spinodal line to lower andthat increasesas clay and/orsaltconcentrationsare
r
a and lower packing fraction opening up the possibility to decreased [13], reaching the order of some months for
reach very low temperature (and hence states with ex- Cs = 1 ×10−4M and Cw = 0.3%. The differences in
tremely longbondlifetimes) withoutencounteringphase the agingtime evolutionofboththe rawspectraandthe
separation. This permits the formationofanequilibrium parameters obtained by their analysis have also permit-
bonding gel,i.e. aspanningnetworkoflong-living”phys- ted to distinguish between two different routes to reach
ical” bonds at very low colloids concentrations [1]. two final non ergodic states at low and high clay con-
The study of different arrested states in colloidal sys- centrations[13](seeFig. 6of[20]). Themechanismsthat
tems is therefore of crucial importance. In fact gels and originatetheexistenceoftwodifferentnonergodicstates
glasseshave often been viewed in an unifying framework are a very interesting point. Moreover, whether or not
due to unambiguous similarities in the phenomenology these two states correspondto really different structures
accompanying the transition to the kinetically arrested and what is the nature of these arrestedstates are up to
state. However, the limits of this unifying scenario are now open and intriguing questions.
emerging as the matter is further investigated: a deeper In this letter we give the first direct experimental evi-
comprehension of the differences and the common fea- dence of the existence of two different arrested states in
2

Laponite suspensions and of their nature. The study of 1.0 DLS
the aging time evolution of both the dynamic and the
0.8
static structure factors has been performed through the
c(SoAmXbiSn)attieocnhnoifqDueLsS. TanhdesSemmaellasAunregmleeXnt-srapyesrmSciattttoeroinbg- g(t)-1200..46 tw
servedirectlytheevolutionwithagingtimeofthesystem
and to distinguish between final inhomogeneous and ho- 0.2
Cw=1.9% (A)
mogeneousstates resolvingthe longstanding controversy
0.0
about the gel or glass nature of Laponite arrested state 1.0
[10, 11]. Moregenerally,while the signaturefor the time
0.8
evolution of a gel was quite well known [21] this is the
firstexperimentalevidenceoftheS(Q)timeevolutionfor 0.6
two different arrested states obtained only by changing g(t)-120.4 tw
sample’s volume fraction.
0.2
Laponitesuspensionswerepreparedinagloveboxun- C=2.8%
w (B)
der N2 flux and were always kept in safe atmosphere to 0.0
avoid samples degradation [22]. The powder, manufac- 10-2 10-1 100 101 102 103
t (m s)
tured by Laporte Ltd, was firstly dried in an oven at
SAXS
1
T=400 C for 4 hours and it was then dispersed in pure
deionizedwater(Cs ≃10−4 M),stirredvigorouslyfor30 0.8 tw
Q)
minutesandfilteredsoonafterthrough0.45µmporesize MS(0.6
Millipore filters. The same identical protocol has been
strictly followed for the preparationof each sample, fun-
0.4
damental condition to obtain reliable and reproducible C=1.9%
w
resultsasalsorecentlyreportedby[23]. Thestartingag- tw (C)

ing time (tw=0) is defined as the time when the suspen-
sion is filtered. Samples were placed and sealed in thin 1 tw
tw
glasscapillarieswithadiameterof2mmtobeusedboth 0.8
forDLSandSAXSmeasurements. Asalreadydiscussed, MS(Q) 0.6
thewaitingtimeevolutionoftheagingdynamicsrequires
fewhoursforhighclayconcentrationsandmonthsforlow
0.4
ones. Thereforeonlytheagingofsamplesathighconcen- Cw=2.8%
trations can be directly followed during allocated SAXS (D)
beamtime. Toinvestigatesamplesatlowconcentrations, 0.05 0.1 0.5
nominallyidenticalconcentrationsampleshavebeenpre- Q(nm-1)
paredat differentdates starting three months before the
FIG. 1: (COLOR ONLINE) Evolution of the time autocor-
plannedexperiment,sotohavedifferentwaitingtimesat
relationfunctions(toppanels)andofthemeasuredstructure
the moment of the measurements. factors (bottom panels) as a function of the waiting time tw
DLS experiments were carried out using an ALV go- (same waiting times - same lines) for two different Laponite
niometer fitted with a toluene bath. The incident laser concentrations: a low one, Cw = 1.9% ((A) and (C) panels)
wavelengthwas a diode pumped, frequency doubled Nd- and a high one, Cw =2.8% ((B) and (D) panels).
YAG 30 mW laser (λ = 532 nm). The scattered inten-
sity was recorded with an avalanche photodiode. The
intensity correlation function was directly obtained as auto-correlationfunctions(toppanels)andthemeasured
2
g2(Q,t) =< I(Q,t)I(Q,0) > / < I(Q,0) > , where Q structure factors (low panels) for samples in the low and
is the modulus of the scattering wave vector defined as high concentration regions, at Cw = 1.9% ((A) and (C)
o
Q= (4πn/λ)sin(θ/2). In the present experiment θ=90 panels)andCw =2.8%((B)and(D)panels)respectively.
and the acquisition time of each g2(Q,t) was 120 s. Thesamesamplewasmeasuredwiththetwotechniques,
SAXSmeasurementswereperformedatthe HighBril- DLSandSAXS(samelinesinthetopandbottompanels
liancebeamline (ID2)atthe EuropeanSynchrotronRa- correspond to roughly same waiting time) and therefore
diationFacility(ESRF)inGrenoble,Franceusinga10m a clear and direct comparison between the dynamic and
pinholeSAXSinstrument. Theincidentx-rayenergywas staticstructurefactorbehaviorscanbeobtained. While,
fixedat12.6keV.TheformfactorF(Q)wasmeasuredus- infact, staticlightscatteringmeasurementsonLaponite
ing a flow-through capillary cell. In SAXS data analysis samples have been previously carried out [8, 10, 15], the
the correctionsforempty cellandwaterhavebeentaken full evolutions of the structure factor with waiting times
intoaccount. Themeasuredstructurefactorhasbeenob- for both low and high concentration samples have never
tainedasSM(Q)=I(Q)/F(Q). Figure1showsasanex- been reported.
amplethewaitingtimeevolutionofboththeclaydensity From Fig. 1 the difference in the behavior of low and
3

highconcentrationsamplesis striking. For the highcon- 2
centration sample ((B) and (D) panels of Fig. 1) there C=0.6 % f=0
arenosignificativechangesinthestructurefactorprofiles f(Q,¥ )DLS„ 0 Cww=0.6 % f„0
C=1.2 % f=0
while the sample crosses the ergodic/non ergodic tran- w
C=1.2 % f„0
w
sition (from dot to dashdot and dash curves). As the C=1.9 % f=0
w
system is performing aging there is in fact only a pro- tw Cw=1.9 % f„0
gressiveand slow decrease in the intensity at very low Q Q) 1
M(
and a very small shift of the main peak to higher Q val- S
ues. On the contrary, for the low concentration sample
((A) and(C) panels ofFig.1)there is anevident change
of the static structure factor as the sample ages and ap- f(Q,¥ ) =0
proachesthe non ergodicstate (from dotto dashdotand DLS
0
dashcurves)specificallyaprogressiveincreasedexcessof 0.1
scattering is observed at low Q values. Moreover, also a Q(nm-1)
change in the shape of the curve and a shift of the main
peak to higher Q values can be recognized. These dif-
FIG. 2: (COLOR ONLINE)Waiting time evolution of the
ferences are the clear evidence that the two samples are static structure factors from full decay (f(Q,∞) = 0) to in-
reaching the final non ergodic states following different completedecay(f(Q,∞)6=0)inthecorrespondingDLSspec-
routes [13] and, furthermore, that the two final arrested traforthreedifferentlowLaponiteconcentrationsattw ≈1.2
states are actually different. t∞w.
The same behavior shownin panels A and C has been
found for all the investigatedlow concentrationsamples.
state.
In Fig. 2 the static structure factors measured for three
differentlow(Cw<2.0%)Laponiteconcentrationsincor- The present experimental findings, together with the
respondence of the full decay (small waiting time, lines) phenomenology associated to gel and glass formation in
andincompletedecay(longwaitingtime,symbols)ofthe colloids allow to speculate on the nature of the observed
DLS spectra are shown. Due to different aging ”veloc- arrested states. Despite the apparent simple definition
ity”, we show here the SM(Q) at the same tw/t∞w, being that ”colloidal gels are arrested state of matter at low
t∞w theCw-dependentarresttime,asdefinedinRef. [13]. density where the particles are tightly bonded to each
It is clear that in the low Q region the same increase other so that the thermo-reversible bonds is compara-
of intensity is found as the sample arrests. Moreover ble or longer than the experimental observation time”,
this excess of scattering at low Q increases as Laponite the essence of the (colloidal) gel state is still under de-
concentration is decreased. This indicates the existence bate [24]. In this context, it is important to stress that
of strong inhomogeneity in the suspension, that we at- sometimes the discussion on what is a gel and a glass
tribute to the formation of a network which -as the ag- becomes a nominalistic dispute. In fact, the formation
ing time goes by- grows and eventually forms a gelled of a bonding gel, which in a part of the colloidal com-
network. Also observable is the disappearance of the munity provides indication of gel formation, for another
main peak and/or its shift to higher Q values. On the part of the scientific community, would be rather classi-
contrary, none of these features is observed for the high fied as glass formation [24]. In this sense we can follow
concentration (Cw> 2.0 %) samples where all the mea- the proposition that in a ”gel” the attraction between
surements have shown the same trend as the one drawn colloidal particles is the leading mechanism for gelation,
in panel (D) of Fig. 1. This indicates the homogeneity while a ”glassy state” can be driven either by repulsion
of the high Cw arrested state. The observed concentra- (hard-sphere or Wigner glass) or by attraction (attrac-
tion dependence of the low Q scattering intensity seems tiveglass). Moreover,whilethe gelstateischaracterized
to suggest that the transition between the two different by structural inhomogeneities (signalled by a non-trivial
finalstatesisnotdiscontinuousbutthearrestedstatebe- low-Qsignalinthescatteringintensity)theglassisstruc-
comes more and more homogeneous increasing clay con- turally homogeneous [1].
centration. Summarizing, the data reported here give a On the ground of the previous discussion the low clay
clear proof of the existence of two structurally different concentration inhomogeneous arrested state can be con-
arrested states in Laponite suspensions, completing the sideredasagelwhilethehighconcentratedhomogeneous
information-alreadyreported in literature [13]- that low asaglass. Moreover,inLaponite,increasingsaltconcen-
and high clay concentrations reaches the arrested state trations the arrested states are still observed and occur
following two different routes. Static structure factor even more rapidly [10, 13]. This observation is in clear
measurements,indeed,showanexcessofscatteringinthe contradiction with the formation of a colloidal glass in-
low Q region for the low concentration samples, thus in- ducedbystrongrepulsiveinteractionsthataredecreased
dicating the formation of an inhomogeneous state, while (due to effect of screening) as the ionic strength is in-
M
forthehighconcentrationsamplestheS (Q)dataarein creased. Therefore, the attractive interaction must be
agreementwiththeformationofahomogeneousarrested responsible of the final arrested state. For these reasons
4
wecancallthehomogeneousstateathighclayconcentra- itative agreement with Fig.5(b) of Ref.[5]: moving away
tions an attractive glass, i.e. a homogeneous state gov- from the spinodal line (increasing Cw) the Q → 0 peak
erned by attractive interactions (that can also be called decreasesandthenearestneighborpeakgrows,signaling
homogeneousgel). Onthecontrary,inthelowconcentra- the increasing importance of the packing. It is impor-
tion region, the inhomogeneous state is compatible with tantto underline that, besides the qualitative agreement
the formation of a gel state where the aging dynamics is betweenthe behaviorofthe modelandLaponitesuspen-
supposed to be characterized by the growth of region of sions no quantitative comparison can be done between
strongly correlated platelets which increases in size and, the systems due to Laponite peculiarity. Therefore, at
eventually,spanalloverthesystemsatthegeltransition. present, we are not also able to locate the spinodal line
It is worth to note that the features of this concentra- inLaponitesuspensions. Itcouldbeinfactpossiblethat
tionregionfit with thoseof the newly discoveredequilib- some of the samples at very low concentrations are ”in-
rium bonding gel region [1], a state found in low density side”thespinodallineandthatforthosesystemsasepa-
colloids with anisotropic interactions. In fact, the inter- rationphase atverylong waitingtimes couldtake place.
action potential between Laponite platelets is certainly
In conclusion, the results reported here, resolve the
nonspherical,withprivilegedrelativeorientation,anas-
longstanding controversy about the final arrested state
pectthatprobablyhasnotbeenproperlyaccountedforin
inLaponitesuspensions[10,11]: bothaninhomogeneous
previousstudies. Moreover,thewaitingtimeevolutionof
and a homogeneous state exist. They are reached -as
S(Q)inFig.1C stronglyresemblesthe temperatureevo-
the system ages- following two different dynamic routes
lution of S(Q) of Fig.5 (a) of Ref.[5] obtained in simula-
[13]. Theexistenceoftwodifferentarrestedstatesinthis
tion of a valence-limited colloidal system. The increased
charged colloidal system with a non spherical potential
intensity at small wave vectors of S(Q) as waiting time
(furthercomplicatedbythepresenceofcharges),depend-
(temperature)is increased(decreased)indicatesthatthe
ing only on its concentration, gives a proof of the strong
systembecomes moreand more compressible,with large
connectionexistingbetweenthe”gel”and”glass”states.
inhomogeneitiesthatcanbeseenasanechoofthenearby
A common experimental and theoretical effort for the
phase separation or, equivalently, as a consequence of
understanding of the still puzzling liquid-gel/glass tran-
building up a fully connected network. In fact one can
sition and a theoretical investigation about the features
suppose that as the time goes by the system feels more
ofthedifferentarrestedstatesinpresenceofchargednon
andmoretheattractionapproaching,throughsuccessive
spherical potentials, are needed.
equilibrium states, the final arrested state as the ideal
systems [5] do decreasing temperature. Also the evolu- We acknowledge P. Bartlett, T. Nicolai, F. Sciortino
tion of the S(Q) with concentrationseems to be in qual- and E. Zaccarelli for helpful discussions.
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