Influence of dispersion medium structure on the physicochemical properties of aging colloidal suspensions investigated using the synthetic clay Laponite
Chandeshwar Misra, Venketesh T Ranganathan, Ranjini Bandyopadhyay
TThe role of solvent microstructure on the aging dynamics and rheology ofaqueous suspensions of a soft colloidal clay
Chandeshwar Misra, Venketesh T Ranganathan, and Ranjini Bandyopadhyay ∗ Soft Condensed Matter Group, Raman Research Institute,C. V. Raman Avenue, Sadashivanagar, Bangalore 560 080, INDIA (Dated: September 29, 2020)
Abstract
The influence of solvent microstructure on the microscopic dynamics and rheology of aging colloidalsmectite clay suspensions is investigated by performing dynamic light scattering and rheology experiments.Additives that can either induce or break-up the ordering of water molecules in an aqueous medium areincorporated in aqueous Laponite R (cid:13) clay suspensions. While the addition of sodium chloride, glucose andpotassium chloride accelerates the aging dynamics of the Laponite particles in suspension and leads torapid dynamical arrest, the presence of N,N-Dimethylformamide disrupts the aging and jamming dynamicsof the particles. An increase in temperature leads to the breaking of hydrogen bonds in an aqueous suspen-sion medium. Our experiments reveal an accelerated approach to dynamical arrest when the temperatureof Laponite suspensions is raised. The aging dynamics and rheology of the samples are correlated withtheir microstructural details visualized using cryogenic electron microscopy. Our data demonstrate that themicroscopic dynamics of aging Laponite suspensions show self-similar time-evolution, while their long-time aging behavior and nonlinear rheological responses are sensitive to the temperature of the suspensionmedium and the presence of additive molecules. a r X i v : . [ c ond - m a t . s o f t ] S e p NTRODUCTION
The structure [1–5], dynamics [6–8] and flow properties [9–12] of suspensions of smectite clayminerals have been studied extensively in the literature. Sodium-montmorillonite, also referred toas sodium bentonite, and kaolinite are two examples of clay minerals belonging to the smectitegroup that are available in abundance in nature [13]. The phase and bulk behaviors of smectite clayminerals can be easily tuned by tuning inter-particle interactions. This has led to the widespreadapplications of these clay minerals as rheological modifiers and stabilizers in paints, wellboredrilling fluids, cosmetics, pharmaceuticals, agrochemicals, paper fillers, coating pigments andnanocomposites [14–19]. Aqueous suspensions of clays, eg. sodium-montmorillonite, have beenwidely used in studies to understand large scale geophysical phenomena such as river delta forma-tion, landslides and earthquake-driven liquefaction. [20–22].Hectorite clays, belonging to the smectite group, have layered structures [23]. A Hectorite claymineral that is often used for studying physical aging and glass transition dynamics is the syntheticcolloidal clay Laponite R (cid:13) [24–26]. Laponite particles are monodisperse disks having diameters 25-30 nm and thickness ∼ ff useinto the intra-gallery spaces and hydrate the sodium counterions, thereby causing the tactoids toswell [27]. This results in the dissociation of Na + , which di ff use out of the stacks into the bulkaqueous medium due to osmotic pressure gradients. The dissociation of Na + , which gives rise toexcess negative charges on the surfaces of Laponite platelets [28], increases with increase in clayconcentration [26]. The edge of a Laponite particle consists of anhydrous oxides dominated byMgOH groups. In a medium of pH <
11, the particles acquire positive charges on their edgesdue to the dissociation of OH − [29]. Owing to dissimilar charges on its edges and faces, Laponiteparticles can interact via face-to-face or edge-to-edge repulsions and edge-to-face attractive inter-actions [30, 31]. The screened electrical Debye layer (EDL) formed by the dissociation of Na + from Laponite platelets impose dynamical constraints on the particles in aqueous suspensions. TheEDL evolves with time, and eventually, at a su ffi ciently long waiting time, t w , the suspension getsarrested in a jammed state. In this state, each particle in the aqueous suspension is confined in acage formed by its neighbors.Although the aging dynamics and phase behaviors of Laponite suspensions have been broadlydebated in the literature, almost all studies have focused predominantly on the dynamics of2aponite particles in aqueous suspensions [2, 9, 24, 32–35]. Aqueous suspensions of Laponitewith particle concentrations between 0.5% and 4.0% show a continuous buildup of structure in aphysical aging process that arises due to a spontaneous and gradual evolution of the interparticleelectrostatic interaction [1, 28]. The phase behavior, rheological properties and aging dynamics ofaqueous Laponite suspensions, widely investigated in the literature, are very sensitive to the pres-ence of additives such as acid and salt, suspension temperature, applied electric field and prepa-ration protocol [26, 36–39]. It is reported that the sizes of cooperatively rearranging regions ordynamical heterogeneities in aging Laponite suspensions increase monotonically with increasingwaiting time t w and clay concentration [40].In the present study, we focus our attention on the role of solvent microstructure on the agingdynamics and rheology of Laponite suspensions. We achieve control over the solvent microstruc-ture by incorporating additives and by changing sample temperature, both of which alter the pop-ulation and strength of hydrogen bonds in the suspension medium [41–43]. A hydrogen bond inwater is formed due to the dipole-dipole attraction between the positively charged hydrogen atomof a water molecule and the negatively charged oxygen atom of a neighboring water molecule[43]. The hydrogen bonds that form between two water molecules arrange in tetrahedral configu-rations and are recognized as the most energetically favored structure of liquid water [44]. WhenNaCl, a kosmotrope (a molecule inducing structure formation in the aqueous medium [45–47]),is added to liquid water, Na + forms a contact pair with Cl − and the dipolar water molecules formhydration shells around Na + [46]. The strong electrostatic ordering of water molecules leads to areduction in their di ff usion rates. Addition of the kosmotrope glucose to water disrupts the tetra-hedrality of water structure and results in the formation of dense structured layers of water aroundthe glucose molecule [48]. This reduces the number of free OH − groups and increases the popu-lation of hydrogen bonds surrounding each glucose molecule in solution [49]. On the other hand,addition of a chaotrope (a molecule that disrupts the structure of the aqueous medium) such asN,N-Dimethylformamide (DMF) to liquid water disrupts inter-molecular hydrogen bonding, withthe water molecules forming hydrogen bonds with the oxygen sites of DMF [45, 50, 51]. WhenKCl, another chaotrope, is added to liquid water, K + does not form a contact pair with Cl − and aclassic hydration shell is not produced [46, 47]. Even though Na + and K + are both alkali atoms,their distinct interactions with water arise from their di ff erent physical properties such as surfacecharge densities and relative sizes. It is reported that K + accelerates the breakdown of hydrogenbonding in water which results in faster di ff usion of water molecules in the bulk [46]. Since the3isruption of hydrogen bonds by K + dominates over the strong coulombic interactions betweenthe K + ions, KCl, despite being an inorganic salt, acts as a chaotrope. It is to be noted that Cl − isa weak kosmotrope and accumulates water molecules around itself in aqueous solutions of NaCland KCl.We perform dynamic light scattering (DLS) experiments to study the e ff ects of the local struc-ture of the suspension medium on the microscopic dynamics of Laponite particles in aqueoussuspensions. The DLS data is supported by rheological measurements of the viscoelastic moduliof aqueous Laponite suspensions in the presence of di ff erent additives and at several suspensiontemperatures. We observe that the addition of kosmotropes leads to an acceleration in the agingdynamics of aqueous Laponite suspensions. In contrast, the addition of DMF delays the transitionof the samples to a dynamically arrested state. Surprisingly, in spite of its known chaotropic action[46], we observe that KCl accelerates the aging dynamics of Laponite suspensions. Finally, weperform cryogenic scanning electron microscopy (cryo-SEM) experiments to study the morpho-logical changes in aqueous Laponite suspensions with and without additives. Accelerated agingdynamics, resembling the results obtained when kosmotropes are added to Laponite suspensions,are also observed when temperature of the suspension medium is raised. The microscopic agingdynamics are self-similar in the presence of all the additives and at all suspension temperaturesexplored here. Furthemore, the viscoelastic moduli, when scaled appropriately, superpose at lowstrains but are sensitive to both additives and temperature at long times and under large strains. Ourresults, which are interesting from the point of view of achieving control on the spontaneous agingdynamics and rheology of clay suspensions, are applicable to a wide range of minerals belongingto the group of Hectorite clays. EXPERIMENTAL SECTIONSample preparation
Laponite XLG R (cid:13) powder (BYK Additives Inc.) was dried in an oven at 120 ◦ C for more than 18 hto remove moisture. The dried powder was weighed and added to Milli-Q water (Millipore Corp.,resistivity 18.2 M Ω -cm) which was continually agitated using a magnetic stirrer for 45 min. Theadditives NaCl (LABORT Fine Chem Pvt.Ltd.), glucose (Sigma-Aldrich), DMF (SDFCL FineChem Pvt.Ltd) and KCl (Sigma-Aldrich) were measured and added to de-ionized water prior to4he addition of Laponite powder. All the additives were used as received without any furtherpurification. Dynamic Light Scattering
DLS experiments were performed using a Brookhaven Instruments Corporation (BIC) BI-200SM spectrometer attached with a 150 mW solid-state laser (NdYVO4, Coherent Inc., Spectra-Physics) having an emission wavelength of 532 nm. The details of the setup are given elsewhere[25, 26]. A glass cuvette filled with the sample was held in a refractive index matching bath filledwith decaline. For the DLS experiments, a freshly prepared sample was filtered into the glasscuvette through a Millipore filter of diameter 0.45 µ m using a syringe. The increase in the samplewaiting time, t w , was monitored continuously from the time at which filtration was stopped ( t w = t w . The temperature of the sample was controlled with a temperature controller (PolyscienceDigital) equipped with a water circulation unit. A Brookhaven Instruments BI-9000AT digitalautocorrelator was used to measure the intensity autocorrelation function of the scattered light: g (2) ( q , t ) = < I ( q , I ( q , t ) >< I ( q , > = + A | g (1) ( q , t ) | [52]. Here q , I ( q , t ), g (1) ( q , t ) and A are the scatteringwave vector, the intensity at a delay time t , the normalized electric field autocorrelation functionand the coherence factor, respectively. The scattering wave vector q is related to the scatteringangle θ , q = (4 π n /λ ) sin( θ/ n and λ are the refractive index of the medium and the wave-length of the laser, respectively. The decays of the normalized intensity autocorrelation functions, C ( t ) = g (2) ( q , t ) − A , were measured for Laponite suspensions at di ff erent t w . The normalized autocor-relation function for 12.2 mM (2.8% w / v) aqueous Laponite suspensions at several t w are plotted vs . delay time t in Figure 1.The normalized autocorrelation functions, C ( t ) plotted in Figure 1, fit best to functions repre-senting two-step decays for the entire range of waiting times explored. This indicates the presenceof two relaxation time-scales corresponding to two distinct dynamical processes which can befitted to the following equation [25, 26, 40] C ( t ) = [ a exp {− t /τ } + (1 − a ) exp {− ( t /τ ww ) β } ] (1)Fits to the above equation were used to extract τ and τ ww , identified as the time-scales associated5 igure 1: Normalized autocorrelation functions, C ( t ), vs. delay time, t , at 25 ◦ C for 12.2 mM (2.8% w / v)aqueous Laponite suspensions. The solid lines are fits to eq 1. with the β - and α - relaxation processes respectively. The faster β - relaxation involves the rattlingmotion of each particle inside its cage. The slower α - relaxation corresponds to the time-scale atwhich the particle can di ff use out of its cage in a process facilitated by cooperative rearrangementsof the surrounding particles. The parameters a and (1 − a ) represent, respectively, the relativestrengths of β - and α - relaxation processes, while β in eq 1 is a stretching exponent that quantifiesthe distribution of the α - relaxation times-cales. The average α - relaxation time can be defined as < τ ww > = ( τ ww β ) Γ ( β ) [53], where Γ is the Euler Gamma function. The present work studies the ap-proach of Laponite suspensions towards kinetic arrest by systematically monitoring the evolutionof the average α - relaxation time, < τ ww > , with increasing sample waiting time, t w , in the presenceof several additives and at di ff erent suspension temperatures. We have plotted τ and < τ ww > vs . / q in Figure S1 for 12.2 mM aqueous Laponite suspensions without any additive (Figure S1a),in the presence of 260 mM DMF (Figure S1b) and 220 mM glucose (Figure S1c) at di ff erent wait-ing times, t w . The fast and slow di ff usion coe ffi cients, D and D ww , for 12.2 mM aqueous Laponitesuspensions in the presence of 220 mM glucose and 260 mM DMF were calculated according to6he protocol specified in Section 1(a) of the SI and are plotted in Figure S2 at a waiting time t w =
100 min. It can be observed that both di ff usion coe ffi cients of aqueous Laponite suspensionsdecrease in the presence of glucose while they increase in the presence of DMF. This indicates ac-celerated and retarded aging dynamics of aqueous Laponite suspensions in the presence of glucoseand DMF respectively. Rheology
Rheological measurements were performed using a stress controlled Anton Paar MCR 501rheometer. A double gap geometry (DG-26.7) with a gap of 1.886 mm, an e ff ective length of40 mm and requiring sample volume of 3 . r c = . ◦ ,measuring gap d = .
07 mL was used. For the rheo-logical measurements sample was loaded immediately after preparation. Loading memory e ff ectswere erased by shear melting the sample at a high shear rate of 1500 s − for 3 minutes to achievea reproducible starting point for all experiments. The increase in the sample waiting time, t w , wasmonitored continuously from the time shear melting was stopped. The temperature of the samplewas controlled using a water circulation system (Viscotherm VT2). Silicon oil of viscosity 5cStwas used as a solvent trap oil to avoid solvent evaporation. The viscoelastic properties of aqueousLaponite suspensions in the presence of di ff erent additives and at di ff erent suspension tempera-tures were then investigated by performing oscillatory amplitude sweep experiments. By varyingthe strain amplitude ( γ ) from 0.1% to 500%, the storage modulus ( G (cid:48) ) and loss modulus ( G (cid:48)(cid:48) ) weremeasured at a constant angular frequency ( ω ) of 6 rad / s. The time evolutions of storage modulus( G (cid:48) ) and loss modulus ( G (cid:48)(cid:48) ) of aqueous Laponite suspensions, aged to di ff erent t w , were studied inthe presence of the additives and at various suspension temperatures at a constant strain amplitude γ = ω = / s. Cryogenic Scanning Electron Microscopy (cryo-SEM)
Aqueous Laponite suspensions in the presence of kosmotropic and chaotropic molecules werevisualized using a field-e ff ect scanning electron microscope from Carl Zeiss with an electron beamstrength of 5 kV. The samples were loaded in capillary tubes (Capillary Tube Supplies Ltd, UK)7ith a bore size of 1 mm using capillary flow. The ends of the capillaries were sealed. Sam-ples were kept undisturbed before imaging and were then vitrified using liquid nitrogen slush attemperature -190 ◦ C. The vitrified samples were cut and sublimated for 15 min at -90 ◦ C and thencoated with a thin layer of platinum (thickness approximately 1 nm) in vacuum conditions using acryo-transfer system (PP3000T from Quorum Technologies). The cryo-chamber temperature waskept at -190 ◦ C. Backscattered secondary electrons were used to produce surface images of thesamples. ImageJ (Java 1.8.0_172, developed by Wayne Rasband, NIH, US) was used to analyzethe cryo-SEM images.
RESULTS AND DISCUSSIONSE ff ects of additives Figure 2 shows the evolution of the slow α - relaxation time < τ ww > with waiting time, t w , fora 12.2 mM (2.8% w / v) aqueous Laponite suspension ( (cid:68) ) with di ff erent concentrations of kos-motropic and chaotropic additives at a temperature 25 ◦ C. It is observed that when the concentra-tions of NaCl (shown in blue symbols) and glucose (shown in wine symbols) are increased, < τ ww > increases very rapidly as waiting time t w is increased (Figures 2a and 2b). This can be understoodby considering that the osmotic pressure gradients that arise due to formation of tight hydrationshells around Na + and glucose molecules [54] can accelerate aging and lead to the observed rapidtime-evolution of < τ ww > .It is seen from Figure 2c that when DMF (shown in dark yellow symbols) is added to Laponitesuspensions, the divergence of < τ ww > is delayed when compared to the Laponite suspension with-out any additives ( (cid:68) ) at the same temperature. The addition of DMF molecules disrupts hydrogenbonds in water [45, 50, 51] and reduces the di ff usion of the bulk water molecules into the intra-gallery spaces of the Laponite tactoids. This results in a significant reduction in the osmotic pres-sure gradients and a retardation in the aging dynamics. Surprisingly, when KCl (widely regardedas a chaotrope and shown in red symbols) is added to the medium, the aging dynamics of the sus-pensions are seen to accelerate (Figure 2d). We believe that enhanced inter-particle electrostaticscreening, arising from the active participation of K + in the electric double layers (EDLs) formedby the Na + dissociated from Laponite particles, gives rise to the observed acceleration in the sus-pension aging behavior. An increase in the electrostatic screening between Laponite particles can8 igure 2: Mean slow relaxation time < τ ww > vs . waiting time t w for 12.2 mM aqueous Laponite suspensionwithout and with di ff erent concentrations of additives (a) NaCl, (b) glucose (Glu), (c) DMF and (d) KCl.The solid lines are fits to eq 2. (e) Horizontal and vertical shift factors, t ∞ α (Vogel time) and < τ ww > , ofaqueous Laponite suspensions in the presence of di ff erent concentrations of additives. (f) Superposition ofnormalized mean slow relaxation times < τ ww >/ < τ ww > vs . normalized waiting time t w / t ∞ α for aqueousLaponite suspensions without and with di ff erent concentrations of additives. Inset shows conductivity of12.2 mM aqueous Laponite suspensions in the presence of 1 mM NaCl and KCl as a function of waitingtime t w . accelerate the aging dynamics of their suspensions when KCl is added to the suspension medium.Conductivity of Laponite suspensions with equal concentrations of NaCl and KCl, measured usinga Eutech PC 2700 4-probe setup, are seen to be comparable and greater than the conductivity ofLaponite suspensions without any additives (inset of Figure 2f). This indicates that K + behaveslike Na + in aqueous Laponite suspensions. The solid lines in Figures 2a-d are fits to the followingequation 25, 26, 40: < τ ww > = < τ ww > exp[ Dt w t ∞ α − t w ] (2)where D is the fragility parameter and < τ ww > is the mean α - relaxation time when t w →
0. Theparameter t ∞ α is identified as the Vogel time or waiting time at which the dynamics of the samplefreezes [25]. The time-scales t ∞ α and < τ ww > for aqueous Laponite suspensions with and without9dditives are extracted by fitting the data to eq 2 and are plotted in Figure 2e. In Figure 2f, wesuperpose the < τ ww > data for all the suspensions (Figures 2a-d) on a universal curve by dividingthe horizontal and vertical axes by t ∞ α and < τ ww > respectively. The self-similar curvatures ofthe data indicate a common underlying mechanism in the aging of Laponite suspensions with andwithout additives. The observed increase in < τ ww > (shown in dark navy symbols in Figure 2e)and the simultaneous decrease in t ∞ α (shown in dark purple symbols in Figure 2e) in the presence ofthe kosmotropes, NaCl and glucose, and the chaotrope KCl point to the increased confinement ofLaponite particles in deep potential energy wells and a rapid acceleration in the aging dynamics.However, a decrease in the < τ ww > and an increase in t ∞ α in the presence of DMF indicatesshallower potential wells and retarded aging dynamics. Figure 3: Storage modulus G (cid:48) (solid symbols) and loss modulus G (cid:48)(cid:48) (open symbols) vs . applied oscillatorystrain amplitude γ for 12.2 mM aqueous Laponite suspensions in the presence of di ff erent concentrationsof additives (a) NaCl, (b) glucose (Glu), (c) DMF and (d) KCl at a waiting time t w =
200 min. (e) Plateauvalues of storage modulus and yield stresses, G (cid:48) p and σ y , of Laponite suspensions without and with additivesat t w =
200 min.
We next perform oscillatory strain amplitude sweep experiments to study the viscoelastic prop-erties of aqueous Laponite suspensions with and without additives. The viscoelastic moduli ( G (cid:48) and G (cid:48)(cid:48) ) of 12.2 mM aqueous Laponite suspensions ( t w =
200 min) with di ff erent concentrations10f additives are plotted as a function of applied strain amplitude in Figures 3a-d. At small valuesof the strain (in the linear viscoelastic (LVE) regime) the elastic modulus, G (cid:48) , and the viscousmodulus, G (cid:48)(cid:48) , are independent of the applied strain, with the elastic modulus dominating over theviscous modulus for the samples with NaCl, glucose, KCl, 50 mM and 90 mM DMF. The suspen-sions start yielding with an increase in the applied strain amplitude. The onset of this non-linearregime is characterized by a monotonic decrease in G (cid:48) . Simultaneously, G (cid:48)(cid:48) shows a peak at thecrossover of G (cid:48) and G (cid:48)(cid:48) before decreasing monotonically. These are typical features of soft glassysystems. At very high strain amplitudes, the suspensions show fluid-like behavior indicated by G (cid:48)(cid:48) > G (cid:48) . However, for the Laponite suspensions with the very high DMF concentrations (130 mMand 260 mM DMF in Figure 3e), the viscous modulus is always higher than the elastic modulus,indicating liquid-like behavior in the entire strain window.It is observed from Figure 3 that at identical waiting times ( t w =
200 min), the elasticity ( G (cid:48) ) ofaqueous Laponite suspensions increases with increasing concentration of NaCl and glucose (Fig-ures 3a and b). The addition of DMF, in contrast, reduces the elasticity of the samples (Figure3c). Figure 3d reveals that the elasticity of the sample increases with increase in KCl concentra-tion. This observation is reasonable considering the acceleration of the physical aging process inthe presence of KCl reported earlier using DLS (Figure 2d). In Figure 3e, we plot the plateauvalues of storage modulus, G (cid:48) p (the magnitudes of G (cid:48) extracted at very low applied strain ampli-tude γ are shown in violet symbols), and yield stresses, σ y (shown in wine symbols), of aqueousLaponite suspensions with and without additives at t w =
200 min. The yield stress is calculatedfrom amplitude sweep data following the method proposed by Laurati et al. [55]. The details ofthe analysis and a representative plot are presented in section 1(b) and Figure S3 of the SI. Both G (cid:48) p and σ y , which estimate the strength of the underlying sample microstructures [56], are observedto increase with increasing concentrations of NaCl, glucose and KCl, while they decrease with in-creasing DMF concentration at t w =
200 min. The increase in G (cid:48) p and σ y for Laponite suspensionsin the presence of NaCl, glucose and KCl and their decrease in the presence of DMF arise due tothe altered aging dynamics of the samples in the presence of these additives and agree with theDLS results reported earlier.Figures 4a-d show the evolution of the storage and loss moduli ( G (cid:48) and G (cid:48)(cid:48) ) with waiting time t w for aqueous Laponite suspensions without and with additives. Soon after preparation, G (cid:48)(cid:48) dom-inates over G (cid:48) for all the suspensions, thereby indicating viscoelastic liquid-like behavior. As timeprogresses, both moduli increase, with G (cid:48) exceeding G (cid:48)(cid:48) at a transition time, t r . The transition11 igure 4: Storage modulus G (cid:48) (solid symbols) and loss modulus G (cid:48)(cid:48) (open symbols) vs. t w for 12.2 mMaqueous Laponite suspensions in the presence of di ff erent concentrations of additives (a) NaCl, (b) glucose(Glu), (c) DMF and (d) KCl. (e) The transition time t r at which a viscoelastic liquid transforms to aviscoelastic solid (time at which G (cid:48) = G (cid:48)(cid:48) ) for Laponite suspensions with and without additives. times, t r , at which the viscoelastic liquid transforms to a viscoelastic solid are plotted in Figure4e. It is observed from this figure that the transition of aqueous Laponite suspensions from vis-coelastic liquid to viscoelastic solid occurs earlier when the kosmotropes, NaCl and glucose, andthe chaotrope KCl are added to the suspension. In contrast, the transition is retarded when thechaotrope DMF is added. We attempt to superpose the storage and loss moduli measured in oscil-latory strain sweep and time test rheological experiments in Figure S4. It is observed that whilethe microscopic dynamics and rheological responses in the limit of low strains collapse well, thelong-term aging behavior and the mechanical response of Laponite suspensions at high strains aresensitive to the presence of additives.Finally, we perform cryo-SEM experiments to investigate the morphologies of the samplesstudied here. Figures 5a-e display the gel-like microstructures of aqueous Laponite suspensionswith and without additives at a waiting time of 24 h. Honeycomb like network structures are seenin all samples, with pore sizes that are sensitive to the additive present in the suspension. We notethe existence of holes on the flat surfaces of the network branches, indicating the possibility of12 igure 5: Representative cryo-SEM micrograph for (a) 12.2 mM aqueous Laponite suspension and Laponitesuspensions in the presence of (b) 1 mM NaCl, (c) 90 mM glucose, (d) 90 mM DMF and (e) 1 mM KCl at t w =
24 h. (f) Average pore area (black) and plateau values of storage modulus (red) for aqueous Laponitesuspensions without and with additives at t w =
24 h. overlapping coin configurations (OC) of the Laponite platelets [56, 57]. Monte Carlo simulationsof suspensions of charged disklike particles at various volume fractions and with di ff erent saltconcentrations [30, 58] have reported that an OC configuration arises when the positive edge of aplatelet parallelly attaches to the edge of the negatively charged face of another platelet, leading to13he formation of long sheets. The magnified cryo-SEM micrographs of all these gels are providedin Figure S5.We adopt a protocol used earlier [56] to quantify the gel structures by estimating pore areas(data at t w =
24 h is shown in black symbols in Figure 5f) and branch thicknesses (Figure S6). Inthe analysis of cryo-SEM images, the presence of vitrified water on the structures can lead to theoverestimation of the network branch thickness and the simultaneous underestimation of averagepore area. However, since the sublimation time (15 min) after cutting the vitrified samples isidentical in all the experiments, an equal sublimation-depth is expected for all the samples studiedusing cryo-SEM. We next correlate the network morphologies with their mechanical responsesobtained in rheological experiments. The plateau values of the storage moduli, G (cid:48) p (magnitude of G (cid:48) of these gels at t w =
24 h extracted at very low applied strain amplitude γ (Figure S7)), arealso plotted in Figure 5f (shown in red symbols). While the branch thicknesses of the gels remainunchanged in all the samples studied here, it is noted that at a fixed waiting time, the average porearea decreases for Laponite suspensions in the presence of NaCl ( (cid:78) ), glucose ( (cid:95) ) and KCl ( ⬢ ).Figure 5f displays a simultaneous increase in the elasticity of these samples. On the other hand, anincrease in the average pore area and a simultaneous decrease in elasticity is noted when DMF ( (cid:32) )is added to Laponite suspensions (Figure 5f). Many small dangling branches are observed in thewalls of the gel network in Figure 5d indicating incomplete structure formation and retarded agingdynamics due to the addition of 90 mM DMF to aqueous Laponite suspensions. In Figure S8, weshow the microstructures of aqueous Laponite suspensions in the presence of all the additives at t w =
200 min. When NaCl, glucose and KCl are added to Laponite suspensions, gel-like structureswith pore areas larger than those observed in the same samples at a longer waiting time ( t w = E ff ect of temperature Figure 6a plots the dependence of the mean slow α - relaxation time, < τ ww > , estimated fromDLS experiments, on t w for aqueous Laponite suspensions at di ff erent temperatures. Since an14 igure 6: (a) Mean slow relaxation time < τ ww > vs. waiting time t w for 12.2 mM aqueous Laponite suspen-sions at di ff erent temperatures. The solid lines are fits to eq 2. (b) Superposition of normalized mean slowrelaxation times < τ ww >/ < τ ww > vs . normalized waiting times t w / t ∞ α . Inset shows the conductivity of theaqueous Laponite suspensions at di ff erent temperatures as a function of t w . (c) Horizontal and vertical shiftfactors, t ∞ α (Vogel time) and < τ ww > , of aqueous Laponite suspensions at di ff erent temperatures. increase in temperature ruptures the hydrogen bonds in liquid water, increasing the suspensiontemperature is expected to result in delayed aging dynamics. Surprisingly, the reverse trend isobserved for the suspensions studied here, with higher suspension temperatures resulting in ac-celerated aging dynamics. This unexpected behavior has been reported in an earlier work [26]and can be explained by considering the competition between the chaotropic action of temper-ature and the kosmotropic action of Na + ions that di ff use out of the spaces between Laponiteparticles. The di ff usion of Na + ions from the basal planes of Laponite particles is enhanced whentemperature is increased (conductivity plotted in the inset of Figure 6b). This e ff ect dominatesover the chaotropic action of temperature and results in enhanced inter-particle screening due tothe increased participation of Na + in EDL formation. The aging dynamics of aqueous Laponitesuspensions are therefore accelerated when the temperature is raised. The superposition plot of the < τ ww > data (Figure 6b), obtained by shifting the horizontal and vertical axes, shows self-similarbehavior, thereby revealing an indistinguishable aging mechanism of aqueous Laponite suspen-sions at all temperatures [26]. The horizontal and vertical shift factors, t ∞ α and < τ ww > (extractedby fitting the data in Figure 6a to eq 2) respectively, are plotted in Figure 6c. The observed de-creases in < τ ww > (displayed in Figure 6c using red symbols) reveals the initial chaotropic e ff ectof the temperature. On the other hand, a decrease in t ∞ α (displayed in Figure 6c using blue sym-bols) with increasing temperature indicates an increase in the rate of structure buildup and theacceleration of aging dynamics in the samples.Next, oscillatory amplitude sweep experiments are performed to study the viscoelastic behav-15 igure 7: (a) Storage modulus G (cid:48) (solid symbol) and loss modulus G (cid:48)(cid:48) (open symbol) vs. applied oscillatorystrain amplitude γ for 12.2 mM aqueous Laponite suspension at di ff erent temperatures at t w =
200 min.(b) Plateau values of storage modulus and yield stresses, G (cid:48) p and σ y , of Laponite at t w =
200 min fortemperatures in the range 15-55 ◦ C. ior of aqueous Laponite suspensions at di ff erent suspension temperatures. Figure 7a shows thetemperature dependence of the viscoelastic moduli ( G (cid:48) and G (cid:48)(cid:48) ) of 12.2 mM aqueous Laponitesuspensions at t w =
200 min as a function of applied strain amplitude, γ . It is noted that the plateauvalues of the elastic modulus, G (cid:48) p (olive symbols), and yield stresses, σ y (orange symbols), ofaqueous Laponite suspensions increase with temperature (Figure 7(b)). This observed increasesin G (cid:48) p and σ y reveal enhancement in the rate of structure buildup [56] and hence accelerated agingdynamics of the samples with increasing suspension temperature and are in agreement with theresults of the DLS experiments reported in Figure 6.The evolution of the viscoelastic moduli ( G (cid:48) and G (cid:48)(cid:48) ) with waiting time, t w , at di ff erent tem-peratures is shown in Figure 8a. Soon after sample preparation, G (cid:48)(cid:48) dominates over G (cid:48) , showingviscoelastic liquid-like behavior. Both moduli increase with t w and a transition from a viscoelasticliquid to a viscoelastic solid ( G (cid:48) = G (cid:48)(cid:48) ) is noted at a time t r . The observed decrease in t r withincreasing suspension temperature (Figure 8b) indicates a suppression of the chaotropic actionof temperature by the kosmotropic action of the Na + ions that form the EDL. This results in theobserved acceleration of the aging dynamics and the rapid approach to kinetic arrest when sus-pension temperature is raised. In Figure S9, we attempt to superpose the storage and loss modulimeasured in oscillatory amplitude sweep and time test rheological experiments. Similar to theobservations in Laponite suspensions with additives, we note that while the microscopic dynam-16 igure 8: (a) Storage modulus G (cid:48) (solid symbol) and loss modulus G (cid:48)(cid:48) (open symbol) vs. waiting time, t w , for 12.2 mM aqueous Laponite suspension at di ff erent suspension temperatures. (b) Transition times t r (for transformation of the viscoelastic liquid to a viscoelastic solid, designated by G (cid:48) = G (cid:48)(cid:48) ) at di ff erenttemperatures. ics and linear rheological responses collapse well at all temperatures, the bulk dynamics do notsuperpose over the entire experimental time and strain windows. Clearly, the long-term aging andnonlinear rheology of the samples are sensitive to changes in suspension temperature. SUMMARY AND CONCLUSIONS
In this study, we have shown that the structure and temperature of the medium significantlya ff ect the aging dynamics of aqueous suspensions of the colloidal clay Laponite. An acceler-ation in the aging dynamics of the Laponite suspensions was observed when the population ofhydrogen bonds in water was increased by adding the kosmotropic molecules NaCl and glucose[45–48]. The enhancement in the local population of hydrogen bonds, induced by the additionof these molecules, results in large osmotic pressure gradients that are directly responsible forthe accelerated aging dynamics of the Laponite suspensions studied here. On the contrary, theaging mechanism, facilitated by the sodium counterions present in the intra-tactoidal spaces, wassuppressed when the hydrogen bonds in the suspension medium were disrupted by adding DMF[45, 50, 51]. Surprisingly, an acceleration in the aging dynamics was observed when KCl, knownto exhibit chaotropic activity in water [46], was added. This result was explained by consider-ing the participation of K + in the electric double layer formed by Na + that are dissociated from17he faces of the Laponite platelets in aqueous suspension. Rheological experiments, performedto study the e ff ects of the structure of the suspension medium on the aging dynamics of aqueousLaponite suspensions, verify the results of our DLS experiments. Direct imaging using cryogenicscanning electron microscopy (cryo-SEM) reveal that aging Laponite suspensions, in the presenceof di ff erent additives, form honeycomb like network structures through attractive overlapping coin(OC) configurations. Changes in pore area, estimated from cryo-SEM micrographs, can be corre-lated to changes in the gel elasticity and the speed of the aging dynamics of the suspensions.The e ff ect of temperature on the aging dynamics of aqueous Laponite suspensions was alsostudied. Even though an increase in the temperature of the suspension medium is expected to havea chaotropic e ff ect, the increased dissociation of sodium counterions, confirmed by conductivitymeasurements, leads to accelerated aging when medium temperature is raised. The strength andpopulation of hydrogen bonds can be altered both by incorporating additives or by changing thesuspension temperature. Interestingly, the changes in the microscopic dynamics and rheologicalproperties of aqueous Laponite suspensions were found to be comparable in both situations.Our results are su ffi ciently general and can be extended to suspensions of other charged clayssuch as sodium bentonite and kaolinite. The results reported here will be useful in many applica-tions involving the non-equilibrium dynamics of materials, such as in cosmetics, pharmaceuticalproducts, food industries etc. and in understanding large scale geophysical phenomena such aslandslides, earthquakes and the formation of wet quicksands and river deltas. ACKNOWLEDGEMENTS
We thank Ananya Saha for her help during the initial stages of the experiments and K. M.Yatheendran for his help with cryo-SEM imaging. We thank Raman Research Instistute for fund-ing our research and DST SERB (grant number EMR / / REFERENCES ∗ Electronic address: [email protected][1] B. Ruzicka, E. Zaccarelli. A fresh look at the Laponite phase diagram. Soft Matter , 7, 1268-1286.
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