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Cite This: Langmuir XXXX, XXX, XXX−XXX
Microstructure of Sodium Montmorillonite Gels with Long Aging Time Scale Yee-Kwong Leong,*,† Mingyong Du,† Pek-Ing Au,†,‡ Peta Clode,§ and Jishan Liu† Department of Chemical Engineering and §Centre of Microscopy, Characterization and Analysis, The University of Western Australia, Crawley 6009, Australia ‡ Department of Chemical Engineering, Curtin University, Miri, Sarawak, Malaysia, 98009 Downloaded via UNIV OF CALIFORNIA SANTA BARBARA on August 13, 2018 at 21:35:50 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Purified sodium montmorillonite (SWy-2) gels of a few percent solids displayed pronounced time-dependent rheological or aging behavior with a long time scale. The aging behavior was characterized by an increasing yield stress with rest time. This increase continued even after a week of rest. An open sponge-like cellular microstructure of the aged gels was captured by cryo-SEM with samples prepared at high pressure. The size of the openings of the cellular structure is small, generally less than 1 μm formed by thin flexible platelet with curling edges. This structure was formed by strong attractive and repulsive forces. The rapid yield stress increase in the early stage of aging is due to rapid bond formation occurring between network platelets and free individual platelet, isolated aggregates, and platelet particles in network with free edges. Over time, all platelets are bonded in the network. During aging, the platelets in the structure would have to adjust continually in response to a net force acting on it by its neighbors. The high concentration of platelets responding to this force imbalance is the cause of the long aging time scale. The operation of the attractive and repulsive forces, and the shape and charge properties of the platelets are responsible for the cellular structure being built. At complete structural recovery, the structure should attain the state of lowest free energy. The repulsive force regulates the development of the microstructure. The aging data of the 3.3 wt % gel were fitted by different aging models.
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van Oss et al.11 nonelectrostatic polar (Lewis acid/base) forces could also be present in smectite particle interactions. Microstructure was suggested to form by electric double layer (EDL) repulsive force6,12,13 immobilizing the montmorillonite particles by caging effect. The particle concentration must be sufficient to occupy the whole gel volume for the EDL caging effect to be effective. Swelling and EDL repulsion should increase the particle number concentration of montmorillonite platelets in water by delamination. Osmotic pressure drives the water into the interlayer of the montmorillonite platelets hydrating the sodium ions and developing an electric double layer on each surface of the interlayer platelet particles. The overlapping of these adjacent double layers gives rise to the EDL force which increases the interlayer separation distance to such an extent that the platelets together with its thick double layer occupying the whole volume of the suspension forming a repulsive gel with a yield stress. Other suggested structures are those formed by positive edge and negative face attraction and van der Waals force. Edge-face attraction formed card-house structure,5,14 zigzag ribbon structure,7 and overlapping band or coin structure13,15
INTRODUCTION The investigation on the time-dependent rheological or aging properties of bentonite (comprised of 70−90 wt % sodium montmorillonite) gels commenced about 100 years ago.1−3 The thixotropic or aging behavior is thus well-known; however, the microstructure responsible for this aging behavior is still a subject of debate. This investigation will use the microstructure captured to explain the aging behavior and, in particular, the time scale of the aging process. Sodium montmorillonite is an important clay with many commercial applications;4 viscosity modifiers, personal care products and others. As bentonite, it is an important ingredient in drilling mud, impermeable slurry wall, facial treatment products, iron making aid and etc. Sodium montmorillonite is a smectite swelling clay displaying pronounced thixotropic or aging behavior at very low solid concentration of a few weight percent.1−3 Several types of microstructures have been proposed to explain the time-dependent rheological properties.5−7 Capturing the microstructural image of this gel clearly has been an issue until recently.8−10 The microstructure is formed by the nature and strength of the surface forces operating between the platelet particles. For ordered structure, these forces also determine the architecture of the microstructure formed. The types of surface forces operating are van der Waals force, electrostatic repulsive (electrostatic double layer or EDL) and electrostatic attractive forces. According to © XXXX American Chemical Society
Received: February 1, 2018 Revised: June 1, 2018 Published: July 27, 2018 A
DOI: 10.1021/acs.langmuir.8b00213 Langmuir XXXX, XXX, XXX−XXX
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mouth (screw top) vessel overcome some of these shortcomings ensuring that many measurements with a small vane can be conducted in the gel with its large exposed surface area. The vane can measure the yield stress in locations not disturbed by previous measurements.20,21 The insertion of the vane was found to have only a negligible effect on the yield stress result.22 In this investigation, the temporal yield stress behavior of aging gels was characterized using this vane technique. As a confirmation of the suitability of this vane technique, the trend of the yield stress increase during aging of transparent nanodiscotic synthetic hectorite gels was identical to that obtained by a nondestructive yield stress technique, the magnetic tweezer-fluorescent particle tracking technique23 developed at MIT.
have been proposed. Overlapping coin structure is formed by the face of particle overlapping the face near the edge of another particle. The time-scale of the structure forming process should depend on the forces operating and type of microstructure being built. Recently, evidence of other microstructures was reported,9,10 where an open cellular microstructure of SWy-2 sodium montmorillonite gel was imaged by cryo-XRH-SEM, with the gel sample prepared at high pressure.10 Microstructure derived from X-ray tomography was found to lack clarity.9 Cryo-SEM image of gel samples prepared at atmospheric pressure displayed a more recognizable open ordered cellular structure, the honeycomblike structure.8−11 The formation of honeycomb microstructure by clay particles has long been suggested by Terzaghi,16 which was not supported by experimental evidence.17 The time scale for the formation of this open cellular microstructure is unknown and remained not investigated. This study is an attempt to address this issue. In addition, the microstructure of SWy-2 gels prepared at high and atmospheric pressure were compared and discussed. Microstructure of low concentration bentonite suspensions, up to 2.8 wt % solids, has been deduced from the phase state determined as a function of clay concentration and ionic strength by cross-polarizer optical birefringence technique.18,19 Phase states such as isotropic liquid, isotropic gel, nematic gel, and flocculation have been identified. However, for some phase states such as the ordered nematic state it is not possible to deduce detailed features and architecture of the microstructure from this optical technique. The location of the ordered nematic state in terms of clay and salt concentration, corresponded to the attractive gel state. The concentration at sol/gel transition was found to increase with particle size of the Na-montmorillonite.19 The flocculation state occurred at a very high salt concentration, above ∼0.2 M NaCl independent of clay concentration where the EDL thickness is only 0.7 nm. The nematic state occurred at high clay concentration at all salt concentration below 0.2 M NaCl. This state also applied to the highest clay concentration of 2.8 wt %. In this study, the microstructures of nominal 1 and 2 wt % SWy-2 suspensions were presented, in addition to that obtained with the more concentrated gels. The most dilute gel of 3.3 wt % solids used in this study, should have a phase state corresponding to the nematic or attractive gel state. Aging behavior characterizes the kinetics of the structural recovery process of a presheared gel. The recovery of the structure upon resting is reflected by the progressive strengthening of the mechanical properties such as the yield stress and storage modulus or compliance.20,21 In aging study, a well-defined initial state such as an equilibrium breakdown state, is essential for the results to be reproducible and the results of the different studies to be comparable. The preshearing step and its protocol are thus important in obtaining this well-defined initial state.21 With the preshearing step being employed to breakdown the clay microstructure to an equilibrium state, the objective of the aging study was to evaluate the kinetics of structural recovery in a completely undisturbed state. Ideally the sample should not be disturbed in the recovery phase during measurement. A nondestructive test would be ideal. However, this is not always possible. If the recovery phase takes weeks and months, then it will not be possible to follow this process over this time scale in a rheometer without the sample drying out. The use of the vane yield stress technique22 and a sample contained in a wide-open
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EXPERIMENT AND METHODS
The SWy-2 sodium montmorillonite mined from Crook County Wyoming, was sourced from the Clay Mineral Society. As-mined SWy-2 contained 75% smectite, 8% quartz, 16% feldspar, and the remaining 1% comprised of gypsum, mica, and Illite and others.24 A sedimentation method was employed to purify the SWy-2 sodium montmorillonite by exploiting the difference in the sedimentation rate between the platy clay particles and the more spherical shaped impurities. A very dilute concentration of the as-mined SWy-2 suspension was prepared: 40 g clay in 2 L of water. A small sample of the suspension, ∼500 mL, was sonicated at a time with a sonic probe to facilitate the separation of the impurities from clay platelets and the delamination of the platelets. The samples were mixed together, and the suspension was allowed to settle initially for several hours before being decanted into an empty container. Settled impurities located at the bottom of the first container were also collected and kept for accounting purposes. The decanted suspension in the second container was allowed to rest overnight to settle the finer impurities and then decanted again. This was repeated a few times to remove as much of the fine impurities as possible. After that, the 2 L suspension was allowed to dry in the oven for several weeks at 60 °C. XRD of the purified and as-received SWy-2 clay samples was characterized. The result is shown in Figure 1a. Quartz and feldspar diffraction peaks have virtually disappeared after purification. A large peak was observed at the high diffraction angle ∼28° for the purified sample and this was due to the diffraction of hkl 005 plane.25 It was not possible to grind the dried purified SWy-2 into fine powder for the XRD characterization. The purified layered agglomerates shown in Figure 1b were found to be highly malleable. Gels of purified SWy-2 at 3.3 (3.26), 4.5, and 6.5 wt % solids were prepared by sonication with a sonic probe. Each gel sample weighed only ∼50 g so that the size of the pH probe or spatula is sufficient to mix the gel very well and uniformly. The total duration of the sonication can be as long as 3 min. The sonication process was stopped occasionally for inspection to ensure that all the particles are wetted and well dispersed and partially wet lumps sticking to the container wall were not present. If present they were dispersed by directing the sonic probe to the area of the container. The freshly prepared gels were rested several days to allow the surface transport processes of hydration, charging and ions transport have time to reach equilibrium.21 To start the aging or structural recovery experiment, the initial state was set up by shearing or agitating the gel to an equilibrium state and the yield stress was measured immediately. This yield stress, the first point of the aging experiment, characterized the gel strength at the equilibrium structural breakdown state. After that the yield stress was measured at regular aging time interval in locations not disturbed by previous measurements.21 In between yield stress measurements, the sample was kept in a 100% humidity environment so that drying did not occur. All gel samples were kept in a 100% humidity environment even for storage after the investigation. B
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Figure 2. EBS determination for 6.5 wt % purified Swy-2 Na montmorillonite gel. The conductivity of the gel is 1.3 mS/cm equivalent to 0.01 M KCl.
charging, hydration, and others have not reached equilibrium. For synthetic hectorite gel this nonequilibrium state was reflected by a temporal varying zeta potential and yield stress.21 A constant yield stress region was reached after 4000 min or 2.8 days. This means that at least 3 days of standing must be allowed for the freshly prepared gel to attain the surface chemical equilibrium state. After that the gel can be sheared or agitated to the EBS for the commencement of the aging or structural recovery experiment. The well-defined initial state, the equilibrium breakdown state, is also shown in Figure 2 in the region after 4000 min, where the yield stress of the agitated gel remained constant. The aging experiment can commence once this EBS is reached. This EBS yield stress takes the value of 77 Pa and is the first point of the aging result at zero aging time. During aging, no agitation of the gel was allowed so that the structure recovery of the presheared gel can be followed and the yield stress measurement conducted at regular time intervals was performed in locations not disturbed by previous measurements. The aging results characterizing the structural recovery behavior are shown in Figure 3a for 6.5 wt % gel and Figure 3b for 4.5 and 3.3 wt % gels. The recovery reflected by the strengthening of the gel structure is characterized by an increasing yield stress with aging time for all three gels. The increase was sharp initially and then became more and more gradual with time. The yield stress is very sensitive to gel solids concentration. For example, the EBS yield stress is 77.6 Pa for the 6.5 wt % gel and only 1.0 Pa for the 3.3 wt % gel. At long aging time of 14 days, the yield stress of the 6.5 wt % gel reached a high value of 578 Pa, representing a 7-fold increase. The yield stress increase continued even after more than 10000 min or 1 week demonstration a long time scale for the structural recovery process. The time scale of the aging process appeared to be shorter for the 3.3 wt % solids gel. The yield stress increase appeared to stop after 8000 min of aging. However, it is not easy to pinpoint the exact time needed to achieve this complete structural recovery state. The yield stress increase continued by a very small amount in the “plateaued” region. For example at times of 8655, 10065, and 15945 min, the yield stress was 22.3, 23, and 26 Pa, respectively. These values should be very close
Figure 1. (a) XRD diffraction results of as-received and purified SWy2 sodium montmorillonite. The quartz Q and feldspar F peaks were virtually eliminated after the purification process. Illite I or mica peak is still present and this material is platelet and difficult to remove. The sharp peak of the purified particles at 2θ ∼ 28° is due to hkl 005 diffraction. (b) SEM image of purified dried SWy-2.
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RESULTS AND DISCUSSION Aging Behavior. The initial state of the gel chosen in the aging experiment is the equilibrium breakdown state (EBS) after the particles have achieved surface chemical equilibrium. The surface transport processes such as charging, ions transport hydration and others must have reached equilibrium. Leaving the freshly prepared gels to stand for a few days would be sufficient to reach surface chemical equilibrium. The approach employed to determine of the surface chemical equilibrium and the EBS is illustrated with the results in Figure 2, which showed the yield stress versus time since preparation of a 6.5 wt % purified Swy-2 gel. The yield stress at each time interval was measured immediately after the gel was broken down with a spatula or a pH or conductivity probe. The vane measures the static yield stress and not the dynamic yield stress which can be obtained from by fitting Bingham model to the shear stress−shear rate data. The Bingham yield stress is related to the separation energy between two particles interacting attractively.26 This gel displayed a pH of 8.5 and a conductivity of 1.3 mS/cm (equivalent to 0.01 M KCl). The freshly prepared gel displayed a gradual increase in the yield stress with time at the beginning. This showed that the clay platelets have not reached the surface chemical equilibrium state. The surface transport processes such as C
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surface was removed, while remaining the integrity of the microstructure. Samples were imaged at 5 kV with a Zeiss 55 field emission SEM fitted with a Leica EM VCT100 cryo and anticontamination system. a. 0.93 and 1.84 wt % SWy2-gel Gels (Aged for 2 Months). The cryo-SEM image of 0.93 wt % SWy-2 suspension prepared at high pressure in Figure 4 showed that the particles are
Figure 4. Cryo-SEM image of 0.93 wt % SWy-2 suspension with sample prepared at high pressure. The gel has a conductivity of 0.195 mS/cm (equivalent 0.001 M KCl) or a Debye length of 9.6 nm. The red arrow indicate the region where the EDL repulsive force is within the range of 10 nm.
flocculated forming aggregates with an open structure. The suspension with a conductivity of 0.195 mS/cm equivalent to ∼0.001 M KCl should fall in the isotropic liquid phase state.18 Isolated platelet appeared not to be present in this state. The platelet particles are very thin, nanometre in thickness, and highly flexible. They displayed morphology similar to that of a drying leave showing curling edges. This flexible platelet interacting attractively in the edge-face or overlapping edgeface configuration. The right angle edge-face interaction configuration was rarely observed. Angular overlapping edgeface configuration is more likely to occur. The edge of the platelet located far away for this edge-face attractive junction was observed to curl-up. The platelets in the aggregate are separated by face−face repulsion opening up the aggregate. Such repulsion would be strongest at the junctions where the angular face−face separation is shortest. This suspension did not exhibit a yield stress. There are insufficient platelet particles to form a complete and strong 3D network structure occupying the whole volume of the suspension. At the equivalent ionic strength of ∼0.001 M KCl, the range of the electric double layer (EDL) repulsive force is about 9.6 nm. This force is responsible for the separation of faces of the interacting platelet. At this range the EDL force is very effective near the overlapping edge-face (EF) interaction junctions where the separation distance between the faces increased from zero to tens of nanometres further along the platelets. The red arrow in Figure 4 showed where the EDL repulsive force is within range for face−face repulsion. At higher concentration of 1.84 wt % SWy-2 gel the platelet concentration is high enough to form a complete 3D network structure. This gel contained an equivalent salt concentration of 0.0025 M KCl. According to Gabriel et al.,18 this gel should be located in the isotropic gel regime. The network structure
Figure 3. Aging or structural recovery results for (a) 6.5 wt % (inset showing locations of yield stress measurements) and (b) 4.5 and 3.3 wt % purified gels
to representing the yield stress at the state of complete structural recovery. The yield stress of the 4.5 wt % gel is 20 Pa at EBS and rose to 60 Pa after 9000 min of aging. The trend showed a continued increase in the yield stress after 9000 min. The long time scale of structural recovery lasting several days, must reflect the type of microstructure formed and the nature and strength of the interparticle forces controlling this recovery process. Gel Microstructures. Ice formation during cryo-freezing of gel materials was reported to affect the microstructure formed.27,28 In order to prevent ice crystal formation in the samples during cryo-freezing, the sample was subjected to a very high pressure. A Leica EM PACT2 equipment designed for this purpose was used. A sample of SWy-2 slurries aged for two months was placed on two flat carriers each with a diameter of 1.5 mm and a depth of 0.2 mm and then pressed together before being placed in a loader for pressurization and rapid freezing in a liquid nitrogen bath. The sample was pressurized up to 2000 bar. A freezing rate as high as 25000 °C/sec was achieved to prevent ice formation. The frozen sample was transferred to a preparation system (Leica EM MED020 preparation system fitted with a Leica EM VCT100 control system) and sublimated at −100 °C for 10 min before coating with 8 nm Pt. The sublimation time was optimized empirically to make sure the frozen water from the exposed D
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overlapping edge-face junctions. This cellular structure is formed by strong attractive and repulsive force. Attractive force occurred at the junctions where the platelet particles are interacting in the overlapping EF configuration. Strong EDL repulsive force occurred near the junctions where the face− face separation is in range of the EDL force. The angle between the faces at overlapping edge-face junction opened up in response. As many of the cells are very small with diameters of less than a hundred nanometre or a few hundred nanometres, very close contact repulsion between neighboring platelets that are not junction partners, are possible. The curving or bending of the sheets are in response to this EDL repulsive force. b. 3.6 wt % SWy-2 Gel (Aged for 2 Months). At 3.6 wt %, the gel displayed the sponge-like cellular microstructure with well-defined openings as shown in Figure 6a. This gel with an equivalent salt concentration of 0.005 M KCl should fall in the nematic gel regime18 by extrapolation as phase state data were not available at clay concentration beyond 3 wt %. This means that the well-developed sponge-like microstructure is nematic in nature, that is, it has certain degree of directional order. However, the openings of cells came in all shapes and sizes. The size of the openings were measured using images with a higher magnification shown in Figure 6b and 6c. The boundary wall of the openings formed by the thin flexible platelets is more complete. The size of the openings is also more uniform in size. From 10 measurements it ranged from 250 to 707 nm with an average of 543 nm. All the platelets forming the network are bend with curling edges. With a conductivity of 0.7 mS/cm or 0.005 M KCl, the EDL repulsive force range is slightly shorter, ∼4.3 nm. With a smaller opening, the platelets are closer together providing a greater opportunity for both EDL repulsion between faces and attraction in the overlapping EF configuration to occur. c. 6.5 wt % SWy-2 Gel (Aged for 2 Years). The cryo-SEM images of the 6.5 wt % gel prepared at high pressure are shown in Figure 7a and at higher magnification in Figure 7b. This gel has been aged for 2 years and was used in the aging experiment. Again, several flexible curve platelets with curling edges are clearly seen in this image. The cellular microstructure is formed by a higher density of platelet interactions. All the flexible platelet particles are interconnected, that is, they come together to form a strong 3D network, as reflected by its high yield stress. The size of the openings are thus smaller. Over 12 measurements, the average maximum length of the opening is 409 nm calculated for sizes ranging from 240 to 709 nm. At a conductivity of 1.36 mS/cm or 0.01 M KCl, with a Debye length of 3 nm, the range of the EDL repulsive force is even shorter. This, however, compensated by the higher platelet concentration bring the platelet closer together. The curving of the sheets and the curling-up of the edges could be in response to the interplay of the attractive and repulsive forces. The curling up of the edges and the orientation of the platelets enhanced overlapping attractive EF interactions with other platelets in all types of position and orientation. Face− face EDL repulsion is diminished to a certain extent as the platelet curving and curling hindered face−face alignment. d. Microstructures of Samples Prepared at High and Atmospheric Pressure. For the 6.5 wt % gel, the microstructure of the samples prepared at high and atmospheric pressure appeared to be similar, see the comparison in Figure 8a and b. This suggests that the microstructure of concentrated gel is less affected by pressure. Nakazawa et al.29 found that the
shown in the cryo-SEM image in Figure 5a and 5b showed a cellular microstructure of flexible platelet interacting attrac-
Figure 5. Cryo-SEM image of 1.84 wt % gel prepared at high pressure (a) and (b) at lower magnification.
tively. Curling of the edges and curving/bending of the platelet can also be seen. Some of the platelets appeared to be lying flat with the edges curling upward which is highlighted in red dash circle and labeled as C&C. The cellular microstructure did not show any order as the platelets particles could be lying in any position; flat and at various angles, are interacting attractively via overlapping edge-face configuration with other platelet particles lying in all sort of positions. The high flexibility and edge curling of the platelets facilitate edge-face interactions in all positions. The cellular structure showed some cell openings did not have clear complete boundary. The size of the maximum dimension of the openings with clear boundary ranged from 250 to 1065 nm with an average of 604 nm based on 7 measurements. Some of the larger openings have within it smaller cells of size less than 100 nm, examples as highlighted in green dash circle. There is no regular pattern in this cellular structure. The morphology of this network structure appeared to be similar to that formed by curve dried leafs with curl edges coming together forming an interlocking structure. The interlocking force is friction. However, with SWy-2 gel, the overlapping edge-face attractive is holding the network structure together. The range of EDL repulsive force in this gel with a conductivity of 0.33 mS/cm or 0.0025 M KCl is ∼6.0 nm. Again EDL repulsive can only operate at the E
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Figure 7. Cryo-SEM image of 6.5 wt % SWy-2 gel prepared at high pressure: (a) low and (b) higher magnification.
also more rigid and orientated in the vertical direction forming the honeycomb-like microstructure. At lower solids of 3.3 wt %, the contrast of the gel microstructure is even greater, see Figure 8c. The size of the openings are several microns in diameter compared to 500 nm for the high pressure sample. The larger and thicker platelet particle is highlighted in the figure. Ice crystals formed in the freezing process caused the platelets to come together to form large multilayer platelets. The thicker and larger platelets driven to form by the ice crystals must have the platelets adhered in the overlapping EF configuration. Only in this configuration will the platelet particle be able to grow in size, as shown in Figure 9a. The increase in the thickness can also occur in these interaction configuration. The rigid multilayer platelets formed by ice crystals can be as large as several hundred microns, as shown in Figure 9b. e. Microstructure Development and Aging. The face or basal plane of smectite platelet is negatively charged while the edge possesses pH-dependent charges. The negative face charge is permanent due to isomorphic substitution with a lower valency metal ions such as Al with Mg in the octahedral alumina sheet and Si with Al in the tetrahedral silica sheet. For this SWy-2 Na montmorillonite, the substitution occurred mainly in the octahedral alumina sheet with a net negative charge of −0.53. The total tetrahedral charge of the silica sheet is only −0.02. The interlayer charge is therefore equal to −0.55. This face charges are responsible for the EDL repulsion between faces in the gel opening up the microstructure. The
Figure 6. Cryo-SEM of 3.6 wt % gel with the sample prepared at high pressure: (a) low, (b) higher, and (c) high magnifications.
effect of the freezing rate, which affects ice crystal formation on the cellular sponge-like microstructure was diminished if concentrated bentonite gel was used. The particle interaction configurations and the shape of some of the openings were quite similar for the samples prepared at high and ambient pressure. A significant difference is the scale of the structure. The one frozen at ambient pressure is much larger, 3× larger. The diameter of a similar opening highlighted in red dash circle in both figures showed that the sample prepared at ambient pressure is roughly 3× larger. There are fewer individual platelets in the atmospheric pressure prepared sample. These platelets are consumed in the formation of multilayered and larger platelets. The resultant platelets are F
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Figure 9. (a) Schematic showing the particle interaction configuration in the formation of large and thicker multilayer platelet by ice crystals. Top view. (b) Some of the platelets formed are several hundred microns in length for 3.3 wt % gel.
order of minutes. Platelets bonding would occur immediately upon contact leading to production of a relatively compact microstructure. Thin flexible curve platelets with curl edges are clearly seen in the microstructure of the clay gels at 1.84−6.5 wt %. The size of these platelets is usually less than 1 μm in length. Its thickness is of the order of tenth of nanometers. These platelets formed EF bonds in the microstructure. The pHdependent edge charge contribution to the total charge of sodium montmorillonite platelets was reported to be quite small, 5−10%, estimated based on the contribution of the edge area to the total surface area.30 It is quite remarkable that such charge distribution could produce a strong EF attraction. The curving of the platelets and the curling of its edges facilitated EF interaction by enabling platelets in more positions to participate in such inteaction. The curve sheets make face−face alignment more difficult. The platelets in the network structure are restricted in movement due to overlapping edge-face attraction at several positions on the edges. There are two possible causes for the curving or bending of the sheets. Multiple overlapping EF attractive contacts with other sheets in a restricted network environment is one such cause. Strong repulsive interaction with neighboring platelets is another. The platelets can be very close as reflected by the small size of the cells, much smaller than 1 μm for the maximum dimension of the cell. A mixture or combination of both mechanisms is most likely to occur in the network. The sharp increase in the yield stress of a presheared gel in the initial period of aging is an indication of the rapid reformation of platelet−platelet bonds in the network. The rapid bond formation could be via (i) individual platelets navigating through the complex force field to attach on to platelets in the network, (ii) isolated aggregates with an open structure, as seen Figure 4, formed bonds with the network, and (iii) platelets in the network with free ends formed bonds with other platelets in the network. All platelets do not have straight edges. This allowed overlapping EF interactions to occur at several locations with other platelets, thereby increasing the probability of such interactions. At longer aging time, all the platelets should be bonded in the network. Each of these platelets experiences both attractive and
Figure 8. Microstructure of (a) 6.5 wt % gel prepared at high pressure, (b) 6.5 wt % prepared at atmospheric pressure, and (c) 3.3 wt % of gel prepared at atmospheric pressure.
range of the EDL repulsive force in the 1.8−6.5 wt % gels is 3− 6 nm based on the electrical conductivity data measured. The platelets came together to form the open sponge-like cellular structure presented earlier. Both the repulsive and attractive forces formed this microstructure. The repulsive force however regulates the formation of this structure and the two platelets should approach to form the overlapping EF attraction. The formation of overlapping EF bond is more favorable if the faces of two platelets approach at an angle. Without the repulsive force, the microstructure formed will not be that open and may even not be cellular and sponge-like. In contrast, the microstructure formed by attractive forces alone even with platelets should be very short, with time scale, on the G
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structural recovery or equilibrium state yield stress (Pa), 1/ Kr is the time constant, τy∞, τy0, and the model time constant need to be determined from experimental yield stress−time data.32−34 The two-parameter model of Chow31 is given by t τy(t ) = β ln tm (2)
repulsive forces in the network. The net force experienced by any platelet in the network is not zero initially, that is, there is a net force acting on it by neighboring platelets. If this net repulsive force becomes too large, the platelet will break free at the weak EF bonds from some of its neighbors at the joints or bend or move in response. Any movement, however slight, of a platelet in the network will cause force imbalance to be experienced by all neighboring platelets. All these platelets will have to move and adjust or bend in response. This action percolates down the network with large number of platelets responding. This force imbalance is experienced continually by the platelets in the network during the period of structural recovery or rejuvenation. The continual adjustment or movement of the platelets could be large initially but will become smaller as the structure reformation approaches the fully recovered equilibrium state. This particle adjustment process involving high concentration of platelets takes time and is responsible for the long time scale of the aging process lasting many days. At the end of the aging process, each platelet in the network should be in stress free equilibrium state. The maximum yield stress at the state of complete structural recovery is a reflection of the network attaining maximum strength. This maximum strength state usually denotes the attainment of the state of minimum free energy. A similar microstructure obtained by extra high resolution cryo-SEM of sodium montmorillonite SWy-2 gel prepared at high pressure was reported by Mouzon et al.10 The sequence of cryo-XHR-SEM images captured at various time of sublimation of the high pressure frozen samples also showed a similar open cellular structure. According to the phase state diagram,18 the phase of the 1.84 wt % Swy-2 gels should be isotropic gel and that at 3.6 wt % should be isotropic gel at the salt concentration of ∼0.005 M KCl. However, their microstructures captured in this study was found to be similar. The 6.5 wt % gel with salt concentration of 0.01 M KCl is very similar to the 3.6 wt % gel. This microstructure more resembled that in the attractive gel state. Bentonite in suspension is well-known to intercalate a range of compounds including water-soluble “hydrophobic” molecules such as surfactants. These molecules can reside within the interior of the micron size cells without the need to interact with the particle surfaces. Upon drying, the cellular sponge-like structure collapsed forming layered platelets, as shown in Figure 1b, trapping the molecules in the interlayer. Aging Modeling. The aging data of the 3.3 wt % gel showed that its structure recovery process is close to completion. In some aging models, the yield stress at this complete structural recovery state is required. Two aging models were used to fit the aging data for this purified 3.3 wt % gel; the three-parameter Leong model and the two-parameter model developed by Chow31 for the physical aging of glassy polymer and adopted by Rich et al.23 for describing the yield stress of aging synthetic hectorite gel. The Leong model is given by 3/2 ij ij τy0 yz yzz jj 1 − j z jj j τy∞ z zzzz jj k { zz τy(t ) = τy ∞jj1 − jj 1 + K rt zzzz jj zz jj z k {
where β is a constant and tm is the microstructural development time constant. If these models described the aging behavior accurately then having a single time constant is more advantageous than those models needing a relaxation time spectrum to describe the behavior. There is no need to invoke a multiple relaxation mode. The results of the model fit to the aging data are shown in Figure 10, a log−log plot of yield stress versus aging time. The
Figure 10. Model fit to the aging data of the 3.3 wt % gel.
Leong model provided a better fit of the data. The parameters used in the Leong model fit have the following values; τy0 = 1 Pa, τy∞ = 29 Pa, and Kr = 0.00022 min−1 or 1/Kr = 3.15 days. For the two-parameter model, the values of the model parameter used are β = 3.5 Pa and tm = 44 min. This model predicts a zero yield stress at t = tm and negative yield stress at t < tm. At t just greater than tm, the aging yield stress is much smaller than the experimental value.
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CONCLUSIONS The time scale of the aging process takes a long time, lasting several days and increases with clay concentration. An open cellular sponge-like structure of the aged gels prepared at high pressure was captured by cryo-SEM. The length scale of the opening of the cells is generally less than 1 μm. The platelet particles within the cells are very close together. The thin platelets forming the structure are flexible and bent with curling edges enhancing its probability of overlapping edgeface interaction and limiting its ability for face−face repulsive interaction. This structure is formed by attractive and repulsive forces operating simultaneously. The repulsive interaction opened up the structure, while the overlapping edge-face attraction formed the network. The bending of the platelets could be in response to the forces acting on it in the network structure. Attractive and repulsive forces occurred at the network junctions and close-by
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(1)
where τy(t) is time-dependent yield stress, τy0 is the agitated state equilibrium yield stress (Pa), τy∞ is the complete H
DOI: 10.1021/acs.langmuir.8b00213 Langmuir XXXX, XXX, XXX−XXX
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Langmuir
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neighboring platelets can also exert a repulsive force. In the early stage of structural recovery, rapid bond formation occurs with (i) individual platelet having navigated through the repulsive force field of the recovering network, (ii) isolated aggregates, and (iii) platelet particles in network with free ends or edges. The platelets forming the bond will all have to approach at a favorable angle for EF bond formation in the network. Initially, the platelets in the network are not stressfree as they will experience both repulsive and attractive forces. The platelets forming the network junctions in particular will encounter more strongly these opposing forces. These platelets will response to this force imbalance by movement or bending or bond breaking. This, in turn, causes force imbalance to be experienced by the neighboring particles. The high concentration of particles in the structure continually responding to this force imbalance is a reason for the long time scale of the aging process. The repulsive force regulates the development of the open cellular sponge-like microstructure.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yee-Kwong Leong: 0000-0001-7864-5931 Mingyong Du: 0000-0001-7006-2125 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors thank Lyn Kirilak and Jeremy Shaw for technical support and acknowledge the use of the facilities of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy, Characterization and Analysis, and the University of Western Australia, a facility funded by the University, State and Commonwealth Governments. We wish to thank the reviewer for making this a better paper.
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DOI: 10.1021/acs.langmuir.8b00213 Langmuir XXXX, XXX, XXX−XXX
