Consideraciones acerca de trayectorias, prácticas y saberes

Swelling of clay minerals is possible because the parallel lamellae in these structures are bonded by van der Waals and electrostatic forces rather than covalently. In smectites such as montmorillonite or hectorite the charge density on the sheets is low and the structure swells rapidly by penetration of polar species between the sheets56. Montmorillonite is able to adsorb water to a greater extent than many other clays because water is not only adsorbed on the external surfaces but also in the interlamellar space.

1 Crystalline swelling.

This is caused by the hydration of the exchangeable cations in the interlayers. In concentrated brine, or in a solution dominated by divalent or multivalent cations, smectites experience only a small volume increase or swelling. In these solutions, water molecules are structured in layers on the clay surface. This type is called crystalline swelling.

2. Osmotic swelling.

This results from the large difference between ion concentration close to the clay surface and in pore water. The essential features of osmotic swelling are not only the large spacings observed between individual clay layers but also the different nature of the forces between the layers. These forces are osmotic and result from a balance of electrostatic forces, van der Waals forces and the osmotic pressure exerted by the interlamellar cations, i.e. in dilute solutions where Na+ is the dominant cation, smectites experience a large volume increase or swelling. In these solutions, an electrical double layer will develop on the clay surface and cause a strong repulsion between the clay platelets. Hence, osmotic swelling is much more damaging in structures such as hydrocarbon reservoirs than crystalline swelling.

When less than three molecular layers of water are present in the interlamellar region, the structure of sorbed water on smectites has been shown to be influenced predominantly by the exchangeable cations59,60. Spectroscopic investigations of smectite- water interactions61,62 suggest that two distinct environments of sorbed water are present:

1. Water molecules co-ordinated directly to exchangeable metal cations.

2. Physisorbed water molecules occupying interstitial pores, interlamellar spaces between exchangeable cations, or polar sites on external surfaces.

In the latter case, water is not strongly bound to the clay surface and these molecules are selectively removed during the desorption process. Upon dehydration, the more labile water is removed from the interlamellar region, leaving water molecules that are co­

ordinated to the exchangeable metal cations and thus reducing intermolecular interactions between water molecules. In addition, the remaining water molecules are polarised by the exchangeable cation

Swelling characteristics of clay minerals can be quantified by x-ray diffraction (XRD) analysis since clay swelling is a result of the increase in interlayer spacing (d0oi spacing) in clay particles63.

The sorption of water by clays is governed by the nature of the saturating cation and by the value and the localisation of layer charge of the adjacent silicate sheets. As discussed above, the net negative charge on smectite clays that arises as a result of isomorphous substitution is neutralised in nature by organic cations such as Na+ or Ca2+ on the clay interlayers and external surfaces. It has also been shown that for a given interlayer cation (Li+, Na+, K+, Mg24 or Ca2+) and for a given octahedral composition (Mg24, Fe2+, Al3+ or Fe3+), the hydration energy generally increases with the layer charge so that most of the minerals of high charge are hydrophilic and should hydrate spontaneously in water64.

Hydration of the interlayer cations in the presence of water initiates a separation of the clay layers causing a separation of the clay (Figure 4). In smectites exchanged with monovalent cations having high hydration energies (e.g. Na+ or Li4) the individual clay platelets may become completely separated in the presence of water65.

T

-1- 1/2 HgO / Ion

J

>1.5Snm

Figure 4. Showing the swelling of montmorillonite in water.

Swelling occurs in a series of discrete steps, corresponding to the intercalation of 0, 1, 2, 3 or 4 layers of water molecules66. Norrish57 showed that the layers of sodium exchanged montmorillonite could be expanded stepwise by the addition of water layers. He observed both the initial stepwise swelling of smectites up to 20 A and an increase in basal spacings > 40 A that varied linearly with the inverse square root of the electrolyte concentration. Above 40 A the layers are taken to be completely disassociated. He found that the d0oi spacing for anhydrous sodium exchanged montmorillonite was 9.6 A

(0.96 nm), which increased to 15.5 A (1.55 nm) for the two-layer hydrate. This series is markedly different to the divalent cation exchanged smectites where the maximum distance between individual clay layers is approximately 19 A (1.9 nm). Divalent cations, such as Ca2+ must associate with charge deficient areas on two clay sheets to achieve charge neutrality, whereas a monovalent cation such as Na+ can associate with a charge deficient area on just one. Hence, when hydrated, single sheets predominate with the monovalent system, compared to a reduced d0oi spacing and therefore reduced surface

— — 1 .2 n m 1 .9nm

r\

n\

Figure 5. a. Showing how divalent cations associate with two different clay platelets, resulting in a small basal spacing upon hydration,

b. Showing the expansion to infinite separation of Na-montmorillonite upon hydration.

Hence the swelling of smectites can be controlled by varying electrolyte concentration. Slade et al.61 investigated the swelling of smectite samples in concentrated NaCl solutions in relation to layer charge. They found that except for the most highly charged smectites, an expansion from «15.5 to «18.5 A occurred as the NaCl concentrations were reduced. This expansion corresponds to the transition from two to three sheets of water between the silicate layers.

It follows then the nature of the exchange cation and the electrolyte concentration will affect the interparticle association of clay suspensions and this will now be discussed.

2.6 INTERPARTICLE ASSOCIATIONS.