Salt concentrations swelling

Addition of a salt can transform the shale by cation exchange to a less sensitive form of clay, or reduce the osmotic swelling effect by reducing the water activity in the mud below that which occurs in the shale. These effects depend on the salt concentration and the nature of the cation. Salts containing sodium, potassium, calcium, magnesium, and ammonium ions ate used to varying degrees. 

When the ionic composition varies within the range of very low salt concentrations, the swelling degree can reach its maximum [101]. This behavior is observed... 

Soils properties are very sensitive to the type of exchangeable ions. Calcium imparts favorable physical properties to the soil, while adsorbed sodium causes clay dispersion and swelling. It is generally recognized that an exchangeable sodium percentage of 10 is sufficient to cause soil dispersion, reduction of soil permeability and impaired growth of some crop plants. On the other hand, excess salt concentration prevents the dispersive effect of adsorbed sodium. 

If salt is present in the solution, counterions as well as co-ions do penetrate into the brush, which leads to additional screening of the Coulomb repulsion inside the brush. The amount of this screening, and the stretching of the polyelectrolyte chains, are now also controlled by the bulk salt concentration. Since the additional salt screening weakens the swelling of the brush caused by the counterion osmotic pressure, salt leads to a brush con-... 

For many systems this is indeed observed. There are, however, important exceptions. One such exception is the swelling of clay [159-161], In the presence of water or even water vapor, clay swells even at high salt concentrations. This cannot be understood based on DLVO theory. To understand phenomena liken the swelling of clay we have to consider the molecular nature of the solvent molecules involved. 

Transport of salt and water into a capsule was considered in [3], Osmotic swelling of the capsule was assumed to be due to Donnan equilibrium between the salt solution outside the capsule and the interior solution which also contained polyelectrolyte molecules. The polyelectrolyte was unable to pass through the membrane which formed the wall of the capsule, but salt could pass freely. A model similar to that used for the clay membrane predicts two relaxation rates, only one of which was observed in experiments in which the salt concentration was varied in the external reservoir [4],... 

The experimental protocol is shown in figure 2 (top). In the first three stages an equilibrium was reached. Thereafter, a faster change in the external salt concentration was applied. In these stages no equilibrium was reached for both the sample height and the electrical potential difference. The values for the ion concentration were chosen such that the shrinking of the sample due to the mechanical load was about the same as the swelling due to the chemical load. 

The ODN adsorption onto cationic microgel poly(N-isopropylacrylamide) particles was reported to be dramatically affected by the salinity of the incubation medium [9] as illustrated in Fig. 6. The observed result was related to (i) the reduction in attractive electrostatic interactions between ODN molecules and the adsorbent and (ii) the drastic effect of ionic strength on the physico-chemical properties of such particles [17, 27]. In fact, the hydrodynamic size, the swelling ability, the electrokinetic properties, and the colloidal stability are dramatically affected by pH, salt concentration, and the medium temperature [27]. 

In polyelectrolyte gels the variation of pH or salt concentration (cs) causes a swelling or shrinkage. Therefore, in this case chemical energy is transformed to mechanical work (artificial muscles). An increase of cs (or a decrease of temperature) makes the gel shrink. Usually, the shrinking process occurs smoothly, but under certain conditions a tiny addition of salt leads to the collapse of the gel [iii, iv]. Hydration of macroions also plays an important role, e.g., in the case of proton-conductive polymers, such as -> Nafion, which are applied in -rfuel cells, -> chlor-alkali electrolysis, effluent treatment, etc. [v]. Polyelectrolytes have to be distinguished from the solid polymer electrolytes [vi] (- polymer electrolytes) inasmuch as the latter usually contain an undissociable polymer and dissolved small electrolytes. 

FIGURE 1.4 Schematic illustration of the swelling of n-butylammonium vermiculite. (a) shows the unexpanded in a 1.0 M butylammonium chloride solution, (b), (c) and (d) show the gels formed in 0.1 M, 0.01 M and 0.001 M solutions, respectively. Note how the extent of swelling is suppressed by an increase in the salt concentration. V represents the volume occupied by the clay, with V the volume of the whole condensed matter system (clay plus excess soaking solution.)... 

As the salt concentration is decreased below 4.5 M, the solid salt phase disappears, but the clay crystals do not swell until c is decreased below 0.2 M (at T = 4°C, P = 1 atm). To a first approximation, this value is also independent of r,... 

FIGURE 5.5 The salt fractionation effect in n-butylammonium vermiculite swelling. The salt fractionation factor s = ceJc x is plotted as a function of cex, the salt concentration in the supernatant fluid. The solid line shows the coulombic attraction theory prediction, s = 2.8. 

In Figure 6.20, the trajectories of the interplate distance dmm (the smaller one) for the potential minimum are drawn for different values of s, showing the increase of the plate separation against the dilution of the salt concentration c. The interplate distance dm]n approaches DU as c —> 0. We note that the swelling is enhanced for... 

---