The phenomenon of particle framework formation has been predicted and known since the work carried out by McEwen in 1950 [1] and is important phenomenon in membrane technology. Given the size of clay constituents, the electron microscope has been the tool of choice for scientists who study the microstructure of clay suspensions [2,3]. The study of microstructure can be dated to before the advent of the electron microscopy methods of Terzaghi [4] and Casagrande [5], when the classic ‘house of cards’ model was suggested by Goldschmidt in 1926 [6]. The first experimental information about clay microstructure was obtained with the advent of transmission electron microscopy (TEM) and scanning electron microscopy (SEM) [7,8]. The formation of a febrile-like network of particles and existence of a structural framework in sols of Wyoming was proposed in 1950's by McEven and Pratt [9].

Most recently, formation of ribbon like structured networks within clay suspension which hinders involved particles was investigated and found to as the cause of the suspension’s gelling and subsequently resistance to settling [10-13]. Present contribution shows never seen before, completely different particle architecture within “hard gel” where clay particle electrokinetic potential was reduced to zero when no repulsion exists between particles. This gel can be stable for years, display semi-plastic behavior and particles within observed framework are probably in the secondary energy minimum.

The smectite used in this study was SWy-2 sodium montmorillonite, which is a well-known Na-bentonite sample from Wyoming, obtained from the Clay Minerals Society [14]. This bentonite is of the chemical formula Na_0.33[Al_1.67Mg_0.33(O(OH))₂(SiO₂)₄]. Smectite before treatment was soak in 0.1 M NaCl solution overnight and washed by DI water in dialyze tubes. Resuspended smectite sample was repeatable centrifuged on spinning speed 4000 rev/min to separate any sediment. Only colloidal fraction which retained in supernatant was used to further treatment and investigations.

Smectite water suspension was treated by adding aluminium chlorohydrate 50% solution until smectite negative charge was neutralized. This aluminium salts having the general formula Al_nCl(3_nm)( OH)_m and is extensively used in deodorants and antiperspirants and as a coagulant in water purification.

The aluminium chlorohydrate ten times diluted in water solution was titrated into montmorillonite aqueous suspension in sol form, until it rapid transformed into gel. The electrokinetic potential was monitored by Zetasizer (nanoSeries) manufactured by Malvern Ltd. in United Kingdom.

Zeiss Auriga 60 Cryo-SEM/FIB was used in studying vitrified gel sample accordingly to procedure.

The synchrotron-powered transmission X-Ray microscope (TXM) at the National Synchrotron Radiation Research Center (NSRRC) in Taiwan was employed and procedure was described in [15].

General characterization of studied sample was described in TXM micrographs, such as that shown in (Figure 1a), demonstrate that samples display a full flocculated microstructure. Flocculated framework in this scale shows globular aggregates interconnected with each other. Most aggregates looks rather spherical in shape and others more elongated. All aggregates in observed area have diameter between 1 to 2 μm and looks highly porous inside.

Larger scale spherical superstructure was frequently observed like display on a micrograph by dashed circles. Some of this micelle like globular superstructure forms display multilayer, onion style internal texture. In magnified micrographs from computer reconstruction of TXM images in Figure 1B more detailed morphology of highly porous framework can be seen. Total observable porosity is about 70%. Interconnected smectite flakes in many places become denser and forms circular aggregates mostly empty inside.

Figure 1(a): TXM micrographs show gelled smectite suspension spanning three-dimensional framework of interconnected globular in shape microaggregates. Larger scale spherical superstructure was indicated by dashed circles. (b) TXM computer reconstruction display highly porous framework with cellular voids.

Statistical parameters of observed framework were calculated from TXM images using image analyzing technique [16,17]. The aggregate size distribution graph Figure 2 shows strong and broad aggregate distribution maximum between 1 and 3 μm with average aggregate diameter 850 nm. Graph show multimodal particle distribution but microscope resolution limits didn’t allow studying highly porous interaggregate area.

Figure 2: Microaggregate size distribution from TXM micrographs.

Observations of vitrified suspensions using cryogenic scanning electron microscopy (Cryo-SEM) are shown in micrographs in Figure 3. Figure 3a clearly displayed globular flocculated microstructure. Individual smectite aggregates display spherical morphology with diameter 1 to 2 μm. All these globular aggregates lay densely packed in similar way as been observed in TXM images. Some these spheres were partly broken and display empty interior. More complex morphology can be seen in selected aggregates vitrified and partly sublimed in vacuum (Figure 3b). These, partly broken globular aggregates show flexible smectite flakes to be curled into spherical or elaborately spindle like elongated shapes with vitrified water ice visible inside. Many observed larger spherical aggregates display grape like composition of multiple submicrons in diameter smectite spherules.

Figure 3: Cryogenic scanning electron microscope micrographs show (a) Packed globular aggregates, many spherical in morphology. (b) Sophisticated spherical and spindle like aggregates of curled smectite flexible flakes.

In our previous studies flocculated smectite aqueous suspensions was observed in varied ionic strength solutions. All of studied aqueous-clay systems displayed high to low negative electro kinetic potential, usually between -61 to -12 mV. The structure-building phenomenon observed during these TXM and Cryo-SEM investigation was believed to have been triggered by several factors and resulted in suspension coagulation forming “light gel” of semi-liquid properties. The ionic strength of the solution, dry mass density that exceeds the critical coagulation concentration and presence of nanoparticles [18] were mostly cause of this gelation where particles probably found equilibrium in the primary energy minimum. The presence of the extremely small particles (nano-colloids) in a suspension has been reported to enhance sample flocculation [19]. It has been found that a concentration of only 1-1.5 volume % of nano particles is required to produce a space-filling gel network.

In our present investigation, the majority of the sample mass was constituted by montmorillonite nanoparticles and using optimum quantity Aluminium chlorohydrate clay particle electrokinetic potential was reduced to zero, so no electrostatic repulsion exists between particles [20,21].

Nanoparticle presence and lack of repulsive electrostatic forces acting between smectite flakes were most likely responsible formation of spanned network of unique and completely different network fabric and individual aggregates morphology in comparison with gel samples observed before. In previous study, especially dealing with "light gel" formation in the same Wyoming montmorillonite sample SWy-2 sample studied in DI water and moderately salty aqueous solution, flock dimension calculated from TXM stereo images using STIMAN technique, gives median value 312 nm in water and the NaCl solution and 483 nm in the CaCl₂ solution. In our present study newly observed the "hard gel" globular flocs average diameter was twice larger and the flocks dimension range increased from 2.35-3.4 μm observed in the "soft gel" to almost 6 μm as seen within limits of our observation frame. The “hard gel” obtained on way of proof to be strong, dense and stable, displaying semi-plastic behavior. Probably particles within observed framework contacted each other within the secondary energy minimum.

Instead ‘net of flakes’ clay structural model, in which clay particles contact each other in EF and EE pattern and form short and long ribbons, now commonly observed texture in aluminium chlorohydrate modified gel were curled smectite flakes into spherical aggregates. These spherical aggregates were interconnected into multiple spherical superstructures assembled spanned network of globular pattern. Such a pattern can be associated with previously described "pseudoglobular structure model" found commonly in eluvial and hydrothermal clay deposits [22]. Here for the first time this type of microstructure has been observed in gelled montmorillonite suspension. The mechanism behind this curling phenomenon is unknown and need to be studied.

These strongly-built circular aggregates are assembled in a robust multilevel globular framework of gelled suspension where globular and mostly empty spherical aggregates form larger circular micelle-like superstructures. These structures were observed in present TXM and the Cryo-SEM study. The water-encapsulating cellular voids, observed within the gelled suspension, were mostly far below 2 μm in diameter and most of the water retained within the observed network was immobilized in these micro-pores. This phenomenon could explain why gel can retain water without it significant loose, can be employed as moisture rising base and why dewatering of such gelled system can be so complex and difficult task.

These findings on clay particle space arrangement within smectiterich suspension may play a crucial role in the understanding of clay mineral suspension behavior and its aggregating nature. Such an understanding is important in designing the most suitable methods for water, cosmetics and pharmaceutical carriers, for faster particle settling, for water purification and mine tailing dewatering.

When sodium montmorillonite colloidal suspension was titrated by aluminium chlorohydrate aqueous solution to the point of neutral electrokinetic potential, distinctive transition was observed. This transition manifested by rapid change from transparent looking sol to the milky "hard gel" difficult to evacuate from a vessel even by reverting it.

For the first time, new globular flock morphology was observed where flexible smectite flakes were curled and build globular aggregates. These aggregates in many places were observed to assembly multilayer, micelle like globular superstructure.

Average diameter of aggregates observed in studied sample was 850 nm with broad range of aggregates from 0.2-6 μm. Maximum aggregate range was between 1-2 μm.

Globular smectite aggregates looks mostly empty inside and tightly encapsulate water. This may explain it extremely good water retention capability which may be used in membrane technology, pharmaceutical applications, cosmetics industries and others.

Observed smectite gel micro-morphology may be similar to earlier described pseudoglobular microstructural model described in eluvial and hydrothermal clay deposits.