Search

Aerogelsof clays can be prepared by freeze-drying of clay water gels. The mechanical strength of the aerogels is improved by the addition of water-soluble polyelectrolytes to the hydrogels. The porosity of the powdered aerogels can be adapted to the requirements for adsorbents in gas chromatography by compounding the hydrogels with suitable amounts of polyelectrolytes. Simultaneously, the surface of the clay may be modified by adsorption of polar or cationic organic compounds in order to achieve specific chromatographic separations. Because of the toughness of the clay-polyelectrolyte aerogels, they can be rolled out to thin sheets which may be applied as chromatographic papers.

Smallangle X-ray scattering curves have been obtained for a series of Na Wyoming Bentonite clay samples containing 10% clay by weight and NaPO3in concentrations ranging from 0 to 100 meq/1. From the scattering data, the relative probability of spacings between parallel clay platelets was computed. For the sample containing no NaPO3, the probability distribution showed a relatively broad maximum at an interparticle spacing of about 180Å. As the concentration of NaPO3increased, the maximum became sharper and occurred for smaller interparticle distances. At NaPO3concentrations between 25 and 100 meq/1, the position dof the maximum was given approximately by the equation d= 21 + 18.4c-1/2, where dis in angstroms, and cis the NaPOs concentration in eq/1. The similarity of this relation to the dependence of don the concentration of NaCl (Norrish and Rausell-Colom, 1963; Norrish, 1954) suggests that the interparticle spacing depends primarily on the sodium ion concentration and not on the concentration of the anion. The value of dappears to be independent of whether the gel was prepared by the method of Norrish and Rausell-Colom, in which a dried flake was allowed to come to equilibrium with an electrolyte solution, or whether, as in this investigation, the gel was obtained by centrifuging a dilute suspension. Since the Na ions act to reduce the double-layer repulsion between platelets, while the anions tend to be adsorbed on the platelet edges and thus reduce the edge-to-face linkages (H. van Olphen, 1962), the value of the most probable interparticle distance appears to be determined primarily by the magnitude of the double-layer repulsion, even though other properties of the clay gels, such as the rheological behavior, are governed mainly by edge-to-face attractions.

In Part I (Deeds and van Olphen, 1961) pycnometric data on expanding clays in water and in other displacement fluids were analyzed in terms of inaccessible void space within aggregates and of possible density anomalies of the adsorbed fraction of the displacement fluids. Further details of this work are reported, and the consequences for the interpretation of core analysis data are discussed. It is concluded that in core analysis the density of the connate water can be assumed to be normal while the fraction of the total pore space which is filled with oil is computed, provided that the total pore space is determined by using water or a polar organic liquid. If a hydrocarbon liquid or an inert gas is used, a small correction for inaccessible pore space can be applied.

An attempt is being made to analyze the forces between unit layers of a bentonite particle and between the flat surfaces of two particles from colloid chemical data and theory. The electric double-layer interaction, the van der Waals attraction, and the hydration energies are considered. From experimental observations the general qualitative shape of the net potential curve of interaction may be derived. In a quantitative discussion, first the long-range interaction is considered by applying the theory of Verwey and Overbeek in which the net interaction curve is found as the sum of the double-layer repulsion and van der Waals attraction. The fact that the layers do not spontaneously dissociate in solution despite the high charge density of the surfaces points to a considerable specific adsorption potential of the counter ions (the exchangeable ions) to the surface (≈0.2 ev). The consequences for ion exchange are briefly discussed. Next, two possibilities for the status and position of the counter ions are considered: the cations are either unhydrated and located in or close to the holes in the tetrahedral sheet in six coordination with the oxygen six rings or they are partly or completely hydrated and with their hydration shell adsorbed on top of the oxygen sheet. Then the short-range interaction of the unit layers is considered. In the case of bentonite there is the fortuitous situation that the net short-range potential curve for the layer interaction can be found experimentally from adsorption isotherms and x-ray diffraction. This net interaction energy is the sum of three contributing energies: the van der Waals attraction, the electrostatic interaction, and the hydration energies. Therefore, on computing the first two for either model mentioned above, the hydration term can be evaluated and its order of magnitude may decide between the two possibilities since ion hydration energy would be expected to be much higher than surface hydration energy. For the model of unhydrated ions the van der Waals attraction is relatively small and partly compensated by the rather small electrostatic repulsion (repulsion of two finite layers of dipoles). Thus the net repulsion energy measured is practically the hydration energy in this case. This hydration energy figure is of the right order of magnitude if interpreted as hydrogen bonding of a Hendricks water layer to the oxygen surface. For the model of the at least partly hydrated cations the van der Waals attraction is small, the same as in the first model, but the electrostatic interaction is now an appreciable attraction since the cations take up a midway position between the negative bentonite layers for at least the mono- and dilayer adsorption complex. This makes the computed hydration energy higher; this is readily explained by the availability of the higher ion hydration energy as compared with the surface hydration energy. Thus the analysis shows both pictures to be acceptable within the limits with which the adsorption isotherms and therefore the net potential curve can be evaluated. More elaborate adsorption data at very low relative humidities would be required to obtain a better estimate of the energies involved in the removal of the last layer of water where the major portion of the ion hydration energy would enter the picture if ion hydration would indeed take place and be the primary cause of interlayer swelling of bentonite. There is a correlation of bentonite hydration energy with the hydration energy of the counter ions present, but this correlation does not constitute positive evidence of ion hydration. A secondary effect of the ions on the formation of ideal Hendricks’ layers (and thus on their bonding energy to the clay surface), which also depends on ion size, is possible. Other experimental and theoretical approaches to the problem are briefly mentioned and finally the differences between swelling and nonswelling clays are dealt with.

In a previous paper (van Olphen, 1956) data were presented for the rheological and sedimentation behavior of sodium bentonite suspensions in water. From these data particle interaction energies were derived. Analogous data have been collected for calcium bentonite suspensions. It was observed that more clay is required to obtain a gel or a certain sediment volume with calcium bentonite than with sodium bentonite. This result can be quantitatively interpreted on the basis of a larger average particle thickness in the calcium bentonite suspensions. It is furthermore concluded that the interparticle forces are of the same order in both calcium and sodium bentonite suspensions.

The physical process of compaction of clay sediments and the inverse process of swelling can be discussed in terms of particle interaction forces. The swelling pressure in a given situation equals the nct repulsive force per unit area between the particles. The net repulsive force is the resultant of various repulsive and attractive forces. Two stages of swelling or compaction can be distinguished, one in which the particles are relatively far apart, and a second in which they arc separated by only a few monomolecular layers of water. In the first stage the swelling pressure is primarily due to double-layer repulsion (“osmotic swelling”). The pressures range from a fraction of one atmosphere to several times ten atmospheres. These pressures are of particular interest for foundation engineering problems. In the second stage the swelling pressures are dominated by the work of adsorption of the water layers on the clay surfaces. Tile pressures may reach several thousand atmospheres. Quantitative data on the swelling pressures in this range are derived from water vapor sorption isotherms combined with X-ray data on the c-spacing of expanding clays during dehydration. It is concluded that overburden pressures in nature will seldom be high enough to remove all adsorbed water from between parallel grain surfaces.

The concept of surface conductance and its role in the interpretation of the formation resistivity factor of reservoir rocks is reviewed. The specific surface conductance of a clay is determined for four narrow particle-size fractions of sodium Wyoming bentonite in conductivity water. The values are derived from the specific conductance of gels of these clays on the basis of the picture of an idealized cubic network structure of the clay plates. The average surface conductance for internal and external surfaces of the bentonites is found to be 3.2 × 10−19ohm−1, which is 0.55 × the value expected when all exchangeable ions are assigned their infinite dilution mobility. The data are independent of particle size. In addition, conductivity data are presented for clay suspensions in a wide range of concentrations. In extremely dilute suspensions, the contribution of the clay to the conductivity when expressed per unit weight of clay (the so-called “weight conductance”) is smaller than expected from an independent movement of particle and counter ions. This is interpreted to be partly a result of both electrophoretic and relaxation retardation, indicating that the electrical double layer on sodium bentonite has a condensed character. Probably part of the counter ions are entirely immobilized by adsorption on the surface. With increasing clay concentration, further compression of the double layer increases the retardation effects and a further decrease of the weight conductance is observed. A minimum is reached at a concentration of a few tenths of one percent clay at which, according to viscosity and light absorption data, particle interaction becomes apparent. At this point, immobilization of the particles and bridging between the electrodes gradually eliminate the retardation effects, and surface conductance takes over. The weight conductance increases gradually until a constant value is reached in a concentration range of 5 to 12 g of clay per 100 ml gel, from which the above value of the specific surface conductance was computed.

Purified sodium bentonite suspensions in water display rigidity at concentrations as low as about 2 percent. On addition of NaCl the yield stress decreases, but beyond about 5 meq/ liter of NaCl it increases. The same is true for the relative viscosities and the sediment volumes of more dilute suspensions. This is interpreted as a flocculation-deflocculation-floccu-lation sequence. The paradoxical phenomenon of flocculation of the clean suspensions is explained by the concept of positive edge charge of the clay plates, allowing positive edge to negative flat-surface flocculation. Both the purified and the salt-flocculated gels show Bingham flow behavior. When Good-eve’s analysis of Bingham flow is applied, the linking force between the clay plates in the pure gels is computed at about 10−4dyne. The salt-flocculated gels show thixotropic hysteresis, whereas the pure gels set immediately after stirring. This means that there is no energy barrier for gelation in the pure gels, which is in line with the proposed mechanism of association due to Coulomb attraction. In the salt-flocculated gels there must be an energy barrier causing thixotropic hysteresis. This might be expected from the summation of residual double-layer repulsion and van der Waals attraction. Considering thixotropic stiffening as slow coagulation, the height of the energy barrier could be calculated from the thixotropic increase of the modulus of elasticity with time after previous shear breakdown of the structure. The reasonable value of 7 kTwas found for the height of the energy barrier of a gel containing 30 meq/liter of NaCl.