The electrical conductivity of Na-montmorillonite suspensions in various salt and clay concentrations was measured. The weight conductance of the clay suspension was found to decrease with increase in clay concentration between 0 and approximately 0·5 g clay/100 ml, then rose to a plateau at 6–10 g clay/100 ml. The weight conductance of the clay suspensions also increased with an increase in the salt solution concentration. If the model of two resistors in parallel is used in interpreting the experimental data, these changes can be attributed to an increase in the mobility of the adsorbed Na ions. It is proposed that the two resistors in series model is more realistic in describing the conductivity of the suspensions. This model predicts the observed weight conductance changes of the suspensions, while the mobility of the adsorbed ions remains constant. A constant mobility of the adsorbed Na ions in clay-water systems of low to medium salt and clay concentrations also is predicted by the diffuse double layer.

Adsorption isotherms for water vapor, c-spacing and heat of immersion in water of Na- and Ca-montmorillonite were measured at 25°C at various r.h. The amount of water adsorbed as a function of the r.h. increased gradually, whereas the c-spacing increased, and the heat of immersion (per mole of adsorbed water) decreased in steps. There was good agreement between the calorimetric data, the heat calculated from the isotherms by use of BET equation, and the calculations from the ion-dipole model. A model is presented to describe the hydration of sodium and calcium montmorillonite.

The integral thermodynamic quantities of adsorbed water on Na- and Ca-montmorillonite have been calculated from water adsorption isotherms on Na- and Ca-montmorillonite at 298° and 313°K and from one adsorption isotherm and calorimetric measurements at 298°K. The integral entropy values decrease and then increase as the amount of adsorbed water approaches zero. In both systems, the curves approach the entropy value of free liquid water at the high content water. The thermodynamics of adsorbate on a non-inert adsorbent is discussed in some detail. The two-isotherm method gives the energy change of the water phase only, whereas the colorimetric method gives the energy change of the whole system (clay, exchangeable cations, and the adsorbed water). Only when the energy changes in the solid phase are negligible (=inert surface) should the two methods give similar results. An hypothesis was developed to explain the entropy-change data of water adsorbed on clay surfaces, in which the clay surface behaves as a non-inert adsorbent.

Equivalent conductivities of adsorbed cations were determined in clays saturated with Na+, Cs+, Ca++ and with mixtures of these cations. Measurements were also made on Ca++ clays which had been forced by previous drying into bundles of platelets or tac-toids. The average mobility of adsorbed Ca++ and Cs+ is much lower than that of adsorbed Na+.

It was concluded that the average mobility of adsorbed Ca++ is low because most of this Ca++ is on the internal surfaces of tactoids. Ca++ adsorbed between these internal surfaces appears to have a mobility much lower than Ca++ on the external surfaces which has a mobility of the same order of magnitude as Na+. Polarization of adsorbed Cs+ accounts at least partially for its low mobility in these clays.

Demixing of adsorbed cations (segregation with Na+ dominant between some platelets and Ca++ between others) is suggested as an initial step leading to breakup of a Na+-Ca++ clay mass into tactoids. The tactoid model, with Ca++ and Na+ preferentially on the internal and external surfaces respectively, furnishes an explanation of the instability of clay and soil aggregates with 15% exchangeable sodium.

Adsorption isotherms for water vapor, c-spacing and heat of immersion in water of mixed Na/Ca-montmorillonite were measured at 25°C at various RH. There was good agreement between the calorimetric data, the heat calculated from the isotherms by use of BET equation, and the calculations from the ion-dipole model. It was concluded that the electrostatic forces between the adsorbed cations and the water molecules are the dominant forces in the hydration of the clay. Thus, at low moisture content, only the adsorbed Ca-ions are hydrated. The heat released when Na-platelets condense to form Ca-packets was measured, and it was suggested that this energy term is the driving force for the demixing phenomena.

An experimentally determined Ca-Mg exchange isotherm of montmorillonite is reported. The selectivity coefficient of this exchange over a wide range of Mg saturation was calculated and found constant.

Standard free energies of exchange, thermodynamic equilibrium constants and activity coefficients of the exchangeable Ca and Mg ions in vermiculite and montmorillonite, were predicted from knowledge of the microstructure of these two clays, assuming that coulombic forces are the main ones playing a role in the interaction between the counterions and the charged clay surface. The standard free energies of exchange (ΔGCa°Mg = 238 cal/mole) predicted a preference for Ca in montmorillonite and a preference for Mg in vermiculite (ΔGCa°Mg = - 1665 cal/mole). The predicted thermodynamic equilibrium constants were compatible with the experimentally determined selectivity coefficients KsCaMg = 0·67 as compared with KsCaMg = 0·68 in montmorillonite, which remains constant over all the range of Mg saturation, and KCaMg = 16·7 as compared with KsCaMg = 13·9 in vermiculite at 95% Mg saturation. The activity coefficients of Ca and Mg counterions in montmorillonite were found to be and , respectively, and to remain constant. The activity coefficients of exchangeable Ca and Mg in vermiculite were found to be and , respectively, at an equivalent fraction of unity. The activity coefficient of exchangeable Mg increased as the saturation with Mg decreased, and was found to be 1·7 × 10-3 in the range of the low Mg saturation.

The microstructure, the isomorphic substitution and the surface charge density provided an understanding of the changes taking place in the activity coefficients of the counterions.

Changes in hydraulic conductivity (HC) and clay dispersion of smectite-sand mixtures as a function of exchangeable Na in Na-Ca and Na-Mg systems were measured. The charge density on the smectites had no effect on Na-Ca and Na-Mg equilibrium, and the affinity of the clays for Na was similar in both systems. A decrease in HC at 0.01 M concentration was found to be due to clay swelling. Mg was found to be less effective than Ca in preventing the breakdown of the packets by low concentrations of exchangeable Na, and Na-Mg-smectite swelled more than Na-Ca-smectites.

Na-Mg-clay particles dispersed more readily than Na-Ca-clay particles when the mixtures were leached with distilled water; however, if the electrolyte concentration in the clay-sand mixture was controlled by the leaching solutions, no difference between the Na-Mg- and Na-Ca-clays was noted. Thus, the effect of Mg on clay mixtures leached with distilled water was apparently due to the effect of Mg on the hydrolysis of the clays. Increase in charge density increased the stability of the R2+ clay packets, and higher concentrations of Na were needed to break the packets. Mg was less effective than Ca in stabilizing the packets, and lower concentrations of Na were needed to break the Mg-packets.

Ca-K.-, Na-K-, and Ca-Na-exchange isotherms on four reference smectites were studied. The electric charge on the four smectites was due predominantly to cationic substitution in the octahedral layer and varied between 90 and 122 meq/g (cmole(+)/kg) clay. The affinity of the clays for K increased with a decrease in the fraction of K in the exchange phase. The selectivity coefficients of the smectites for K increased with an increase in charge density. The effect was similar in the Na-K- and Ca-K-exchange isotherms. Charge densities on the smectites had no effect on the Ca-Na-exchange isotherm. The electrostatic attraction forces between the platelets increased with increase in charge density. The low hydration energy of adsorbed K was apparently not enough to overcome these attraction forces as the charge density on the smectites increased. Equilibrium between the low hydration energy of adsorbed K and the electrostatic attraction energies between the platelets is postulated to explain the results.

The hydrolysis and decomposition of M-hectorite (M being Na, Li, Mg, or Ca) in dilute solutions of M-chlorides were studied by recording the changes in electrical conductivity (EC) of the clay suspensions with time, and by chemical analyses of the interclay solutions and resulting solid phases. The rate of hydrolysis of the hectorites in suspension, as evaluated by the change of EC with time, was found to decrease with increase in salt concentration (to zero for Na-hectorite in NaCl concentration of 47 meq/liter), and to decrease with increase in the valency of the adsorbed cation (the rate of hydrolysis of Ca-hectorite was one sixth of that of Na-hectorite). The rate of Na-hectorite hydrolysis was determined by the concentration of protons at the clay surface as calculated from the diffuse double layer theory.

Conversely, in suspensions of Al-hectorite saturated with Na, the hydrolysis rates were not dependent on the electrolyte concentrations. This result can be explained by assuming that the hydroxy-Al polymers at the clay surface determine the proton concentration at the clay surface. Hectorites saturated with structural cations (such as Mg and Li) are chemically more stable than those saturated with Na and Ca. The presence of Li and Mg at the clay surface slowed down the diffusion release of these octahedral ions to the clay surface, compared with Na and Ca hectorite, respectively.

The specific conductivity, κg, of Na/Ca montmorillonite gels (0.02 and 0.04 g clay/cm3) as a function of the specific conductivity of the interclay solution, κw, was measured. For low values of κw (<3 mmho/cm) the conductivity curves were convex with respect to xw; at higher values they were linear. The κw at which deviation from linearity began increased with increasing sodium content from 0.7 mmho/cm for the Ca-montmorillonite gel to 2.6 mmho/cm for Na-montmorillonite gel. The intercept of the linear portion of the conductivity curve increased and its slope decreased with increasing exchangeable sodium. The experimental results were described by a conductance model comprised of two elements connected in parallel: an element consisting of the solid and solution phases connected in series, and another consisting of the solution phase. At low electrolyte concentrations, the first element accounts for the curvature of the κg-κw curve, whereas at high electrolyte concentrations, the second element dominates, and the curve is linear with a slope determined by a formation factor and an intercept determined by exchangeable cation mobility.

An adjustable parameter in the conductivity term for the first element, which accounts for the thickness of the clay particles, increased with exchangeable Na, and its numerical value indicated that the solid phase also includes the diffuse double layer. The mobility of the exchangeable cation relative to that in water increased from 4% for Ca to 41% for Na with the largest increase occurring between 20 and 40% exchangeable Na. Similar effects of exchangeable Na were evident on the axial ratio of the clay particles calculated from the slopes of the linear lines. These observations are consistent with an exchangeable-ion demixing model where the initial exchangeable Na is adsorbed on the external surfaces of the Ca-montmorillonite tactoids without affecting exchangeable ion mobility or tactoid size. Further additions of exchangeable Na result in Na adsorption on the internal surfaces of the tactoid and tactoid breakdown with a subsequent increase in both exchangeable ion mobility and the axial ratio of the clay particles.

Changes in hydraulic conductivity of smectite-sand mixtures (using four reference smectites) as a function of the concentration (0.01, 0.003, 0.002, 0.001 M Cl- and distilled water) and potassium adsorption ratio (of 2, 4, and 6) of the percolating solution were measured. Swelling and dispersion of the clays were evaluated from the changes in hydraulic conductivity of the mixture and from the clay concentration in the effluent.

The effect of exchangeable potassium percentage (EPP) on the hydraulic conductivity of the smectites depended on the charge density of the clays. The effect of potassium at EPPs <20 on the hydraulic conductivity of smectites having high charge density was negligible. Conversely, the hydraulic conductivity of smectites having low charge density (smectites from Wyoming and Belle Fourche, South Dakota), changed markedly when leached with dilute solutions as the EPP of the clay increased. The dispersive effect of exchangeable potassium on low-charge smectites was similar to that of exchangeable sodium. The low hydration energy of the K+ cations, coupled with the strong electrostatic attraction forces between platelets of smectites with high charge density account for the “inefficiency” of K+ in dispersing these smectites.

Mixtures of kaolinite clay and quartz sand were packed in columns and leached with NaCl-CaCl2 solutions of different concentrations (ranging from 0.5 M to distilled water) having sodium adsorption ratios (SAR) of 5, 10 and 20. Sodium hexametaphosphate (NaHMP) (10 g/m3) was added to each solution. Changes in the hydraulic conductivity (HC) and clay concentration in the effluent were measured, and the dispersion of the clay was evaluated. In the absence of NaHMP, using distilled water instead of 0.01 M solutions caused some reduction in the HC; the reduction became more severe as the SAR increased. At SAR 20 the HC dropped to 73% of that obtained when the 0.5 M salt solution was used. This reduction in HC was related to increases in the pH of the effluent and partial dispersion of the clay. On adding the NaHMP to the leaching solutions, a marked decrease in the HC and severe clay dispersion were observed at all SAR levels, if the kaolinite-sand mixtures were leached with dilute solutions and distilled water. The HC dropped to 40% of that obtained when leaching with 0.5 M salt solution, and dispersed clay in the effluent peaked at about 1% in the SAR 20 treatment following leaching with distilled water. The observed reduction in the HC of the kaolinite-sand mixture when NaHMP was added to the leaching solutions was due to the dispersive effect of NaHMP and sodium on the kaolinite. The polyanion NaHMP reduced the edge-to-face particle interactions, thereby enabling the dimise double-layer repulsion forces to predominate. Following addition of the polyphosphate, the response of kaolinite to sodium and electrolyte concentration was similar to that reported in the literature for smectite.

Changes in hydraulic conductivity and clay dispersivity of clay-sand mixtures (four reference smectites and Fithian illite) as a function of concentration (0.01 M Cl− and distilled water) and sodium adsorption ratio (SAR ≤ 30) of the percolating solution were measured. In addition, the effect of sand percentage, sand particle size, and addition of AlCl3 and FeCl3 on the hydraulic conductivity of the mixtures were measured.

Clay dispersion and migration out of the 3% clay columns was substantial. The clay dispersed only in the distilled water system; dispersion increased with an increase in the percentage of exchangeable Na and was about the same for the Wyoming montmorillonite and Fithian illite. Conversely, the clay swelled in the 0.01 M Cl− solution. The swelling of the montmorillonites increased in the order: Upton, Wyoming = Belle Fourche, South Dakota > Polkville, Mississippi > Otay, California, and was higher than that of the Fithian illite. The swelling and dispersion of the clay accounted for the changes in hydraulic conductivity.

Mixtures treated with FeCl3 and AlCl3 were leached with NaCl-CaCl2 solutions until the pH of the effluent exceeded 6.5. The composition of the exchangeable phase was then determined by the SAR of the leach solutions. At pH > 6.5, the polycations hydrolyzed and were present as the hydroxy-polymer species. The hydraulic conductivity of the mixtures decreased as exchangeable Na increased, but the decrease was less than in untreated mixtures, AlCl3 was more effective in maintaining hydraulic conductivity than FeCl3. In montmorillonite clay with an ESP of 20, less than 5% of a complete Al-interlayer was enough to prevent a reduction in hydraulic conductivity. Packets in the day systems tested explain the high efficiency of the Fe and Al polycations.