Smectite may impact the ability of saline aquifer–caprock systems to store CO2 effectively, because of changes in pressure, temperature, and brine concentration induced by the injection of CO2. These changes influence the molar volume of smectite, affecting the short-term structural and stratigraphic trapping, or the dissolution of smectite via the long-term geochemical trapping. This study investigated the d001 value of an interlayer-cation-exchanged smectite, Na-rich SWy-2 (Na-SWy-2), with Ca or Mg (hereafter CaSWy-2 and MgSWy-2). Molar volume experiments used X-ray diffraction and a high-pressure environmental chamber. The extent of smectite dissolution was simulated at experimental conditions by geochemical modeling using a rate equation derived from the transition state theory. CaSWy-2-CaCl2 and MgSWy-2-MgCl2 brine systems showed that increasing the brine concentration from 0.17 M to saturation results in a <18% decrease in d001 values, and increasing the temperature from approximately 33 to 150°C results in <11% decrease. The effect of the interlayer cation shows the d001 values of MgSWy-2 are <0.4 Å higher compared with CaSWy-2. Geochemical modeling shows the extent of dissolution of Na-SWy-2, CaSWy-2, or MgSWy-2 is only <1.1% in acidic conditions. Furthermore, the calculated swelling pressure needed to decrease the H2O sheets in the interlayer, from 3W to 2W, of MgSWy-2 and CaSWy-2 are higher compared with Na-SWy-2. The swelling pressure was approximated from the sum of the osmotic repulsive pressure, the van der Waals attractive pressure, and the hydration pressure. The data suggest that Na-SWy-2, CaSWy-2, and MgSWy-2 may affect saline aquifer–caprock systems to store CO2. The molar volume is affected by changes in pressure, temperature and brine concentration, or swelling pressure from the injection of CO2. An increase in the d001 value of SWy-2 can enhance the sealing capabilities of a caprock by making saline aquifers less porous and less permeable and thus increasing the capability for CO2 storage. In contrast, a decrease in the d001 value can create cracks in a caprock and thus provide conduits for the CO2 to escape. Furthermore, the CO2 injection will cause a decrease in pH, causing smectite to dissolve until it reaches a steady state. However, despite acidic aquifer conditions, SWy-2 has low solubility.

The three-dimensional order shown by the two-layer hydrates of Na- and Ca-vermiculite, prepared from Mg-vermiculite from Llano, Texas, has enabled clear, two-dimensional Fourier projections of their interlayer structures to be obtained. Structure factor calculations were made in space group C2 and with unit-cell dimensions of a = 5.358 Å, b = 9.232 Å, and ß = 96.82°; for Na-vermiculite C = 14.96 Å and for Ca-vermiculite c = 15.00 Å. In Na-vermiculite the interlayer cations are octahedrally coordinated to water molecules with the sodium-water polyhedra only located between the triads of oxygen atoms forming bases to tetrahedra in adjacent silicate layers. In Ca-vermiculite the interlayer cations are in both octahedral and 8-fold (distorted cubic) coordination with water molecules. The octahedrally coordinated Ca ions are between the bases of tetrahedra in adjacent silicate layers, but the 8-fold coordinated Ca ions are between the ditrigonal cavities. In both Na- and Ca-vermiculite some water molecules are drawn from planar networks appreciably towards the ditrigonal cavities. The three-dimensional order observed for these vermiculites contrasts with the stacking disorder reported for Mg-vermiculite from Llano. The distinct crystallographic behavior of Na+, Ca2+, and Mg2+ in the hydration layers of Llano vermiculite probably depends on cation sizes and field strengths, together with the need to achieve local charge balance near the sites of tetrahedral Al-for-Si substitution.

The 57Fe Mössbauer spectra of a series of untreated and Ca-saturated nontronites showed a predominant Fe3+ resonance which was computer-fitted with two Fe3+ doublets defining iron in non-equivalent cis-FeO4(OH)2 octahedral sites. In most spectra a doublet indicating tetrahedral Fe3+ was fitted and in one untreated sample a doublet indicating interlayer Fe3+ was identified. In a further untreated sample the interlayer iron was present as Fe2+. Upon Ca-saturation the interlayer iron was displaced. It also appears that the interlayer iron was present in at least two different interlayer sites. From the computer-fitted data it was clear that the interlayer cations have a significant effect on the Mössbauer resonances of iron in the two non-equivalent cis-octahedral and the tetrahedral sites of nontronite.

The 57Fe Mössbauer spectra of untreated, Ca- and K-saturated nontronite from Garfield, Washington, were measured. The spectrum of the untreated sample was computer-fitted to 8 peaks defining two octahedral, a tetrahedral, and an interlayer Fe3+-quadrupole-split doublets. In the Ca- and K-saturated samples interlayer Fe was absent. Spectra of the untreated sample were recorded at increasing increments of background counts from 2.8 × 105 to 9.2 × 106. An evaluation of the initial 4- and 6-peak models and the acceptable 8-peak model, computer-fitted to each spectrum, shows that if the χ2 value is used as a measure of the goodness of the fit, the spectra should be recorded to a background count greater than 3 × 106. The resulting χ2 value then reflects both the validity of the model used and the extent of disorder within the structure. The χ2 value depends linearly on the background counts obtained.

A comparison of the spectra of the Ca- and K-saturated samples with that of the untreated sample shows that the interlayer cations exert a considerable influence on the individual component resonances, particularly the outer octahedral doublet. Hence, it is likely that electrostatic interactions of the nearby tetrahedral Fe3+ and the interlayer cations give rise to two distinct electric field gradients within neighboring cis-[FeO4(OH)2] sites, and hence two octahedral Fe3+ doublets in the Mössbauer spectrum. These results are consistent with earlier electron diffraction data in the literature.

Smectites synthesized from experiments at 5.5 GPa and 1500°C are of high quality, crystals are large at >10 μm, and the 2:1 layers may have a homogeneous charge distribution. Smectite was exchanged with various cations (Na+, Li+, K+, Ca2+, and Mg2+) and the hydration behavior of each sample was observed by an in situ powder X-ray diffraction method under precisely controlled relative humidity (RH). The smectite showed distinct stepwise (discontinuous) hydration versus RH. During the transition between two hydration states, the coexistence of the two states was observed. Randomly interstratified structures with one and two planes of H2O are time-dependent phenomena and relate to hydration and dehydration processes.

Reduction of structural Fe in Na-exchanged dioctahedral smectites decreases swellability in water, but because clay interlayers also collapse in the process the concomitant effect on surface hydration energy is uncertain. This study examined the hydration behavior of oxidized and reduced dioctahedral smectite clays exchanged with polar (Na) and weakly-polar (organic) cations to determine the nature of the surface before and after Fe reduction, and to determine if clay surfaces are hydrophilic or hydrophobic. The H2O content in various dioctahedral smectites decreased if Na was replaced by tetramethylammonium (TMA), trimethylphenylammonium (TMPA), or hexadecyltrimethylammonium (HDTMA). Among the organo-clays, H2O adsorption decreased with increasing complexity of the cation. For oxidized smectites, those exchanged with TMPA retained less H2O than those exchanged with Na at all pressures. The extent of this difference depended on the clay and decreased with increasing applied pressure. Reduction of Fe(III) to Fe(II) in the octahedral sheets decreased the swelling of Na-saturated smectites, apparently causing some previously swelling interlayers to collapse. If the Na interlayer cation was exchanged to alkylammonium after reduction, but prior to swelling-pressure measurements, the swelling increased or remained near constant, suggesting that the organo-cation disrupted the collapse process of the interlayers associated with the reduced smectite layers. Reduced TMPA-saturated smectite surfaces are more strongly hydrated if the octahedral sheet is reduced than if oxidized. Thus, reduction of structural Fe increases the hydration energy of smectite basal surfaces, but swellability could decrease or increase depending on the extent of interlayer collapse occurring with different exchangeable cations.

The rehydration and rehydroxylation properties of homoionic rectorites (saturated with Ca2+, Mg2+, Na+, or K+) were further investigated. The rehydration properties of the rectorite were characterized as follows: (1) basal spacings of rehydrated materials after heating above 500°C changed to 22.5 Å for H2O-complexes, 26.85 Å for ethylene glycol-complexes, and 27.65 Å for glycerol-complexes; (2) rehydrated Ca- and Mg-materials exhibited single layer hydrates at <50% RH, and rehydrated K-material showed double layer hydrates at 80% RH; (3) IR absorption spectra due to rehydrated H2O and OH exhibited the same or very close absorption intensities and frequencies to each other; (4) DTA-TGA curves of rehydrated materials indicated that the amount of rehydrated H2O approached about 4.2 wt. %, and about one-half of OH was rehydroxylated after heating at 800°C; and (5) interlayer cations of expandable layer components became non-exchangeable after heating above 500°C. These results suggest the following rehydration mechanism of rectorite: the interlayer cations migrate into the hexagonal holes of the SiO4 network by thermal dehydration. The cations migrated below 400°C easily return to the interlayer space and their original hydrated configurations have been recovered completely on rehydration. However, those migrated above 500°C are fixed to the hexagonal holes but water molecules are regained in the interlayer space. Consequently, electrostatic effects of interlayer cations on formation of water molecule layers are considerably reduced.

Theoretical models of the mechanical properties of hydrated smectites, saturated with a variety of cations, are of much value in determining the potential for their use in various applications, including clay-polymer nanocomposites, but the development of such models is still in its infancy. The purpose of this study was to calculate the effects of divalent cations on the structural and mechanical elasticity of montmorillonite under different degrees of hydration. A theoretical study of the swelling and hydration behavior of montmorillonite was, therefore, undertaken using density functional theory (DFT) to investigate the basal spacing behavior of the homoionic montmorillonite with varying amounts of water in the interlayer space. The effect of the degree of the hydration of divalent interlayer cations (Mg2+/Ca2+/ Sr2+/Ba2+) on the structure expansion of the interlayer space was analyzed. In addition, the results obtained were compared to calculations performed on the montmorillonite model with a monovalent cation (Na+). The basal spacing (d001) is governed by the size and the degree of hydration of the countercations. The structures containing divalent cations are more compact than structures with monovalent cations. Ba-exchanged montmorillonite was found to have the largest d001 value for any degree of hydration (‘dry,’ one water layer, or two layers). The basal spacings of ‘dry’ montmorillonite exchanged with small cations, Mg2+ and Ca2+, are very similar. In hydrated models, the d001 expansion correlates with the ionic radius of the interlayer cation. The dependence of the total electronic energy on the volume expansion was calculated. From the energetic curves, bulk modulus (B0) was obtained by fitting in order to show how the compliance of the material depends on the type of interlayer cation and on the degree of hydration. With increasing water content in the interlayer space, the bulk modulus decreased, suggesting that the c-axial compression becomes easier with increasing hydration of the clay mineral. The values of the bulk modulus in hydrated systems are less sensitive to the type of the interlayer cation.

The effects of temperature on the swelling properties of smectites are important for a variety of different geological conditions, but studies on this topic have been rather limited. The purpose of this study was to investigate the swelling behavior of Na- and Ca-montmorillonite at various temperatures greater than room temperature, up to 150°C, using in situ X-ray diffraction (XRD) analysis. A sample chamber was designed, the temperature and humidity of which were controlled precisely, for environmental in situ measurements. The XRD measurements were performed at small relative humidity (RH) intervals for precise observation of the swelling behavior.

The swelling behavior of Na-montmorillonite showed distinct zero-, one-, and two-layer hydration states. The basal spacings of Na-montmorillonite changed continuously with RH for various temperatures in the transition region between the zero- and one-layer hydration states, and the swelling curves of the transition region moved to greater degrees of RH with increasing temperature. The basal spacings jumped from the one- to two-layer hydration states for all temperatures at almost the same RH.

The basal spacings of Ca-montmorillonite changed continuously from the zero- to the two-layer hydration states at all temperatures. This behavior is remarkably different from that of Na-montmorillonite. At low-RH conditions, the d001 value of Ca-montmorillonite decreased with increasing temperature. The swelling curves of Ca-montmorillonite did not show a plateau at any temperature for the one-layer hydration state. The swelling curves of Ca-montmorillonite moved to greater RH with temperature, similar to the transformation region between the zero- and one-layer hydration states in Na-montmorillonite. These differences between Na- and Ca-montmorillonite are related to the hydration powers of exchangeable cations.