The Sol Concentration Effect in n-Butylammonium Vermiculite Swelling

Abstract

Abstract--The swelling of n-butylammonium vermiculite in water was investigated as a function of the sol concentration (r), the salt concentration (c) and the temperature (T).The interlayer spacing in the gel phase was investigated as a function of r and c by neutron diffraction and by laboratory experiments which measured how many times its own volume a crystal would absorb. The salt concentration was found to be the stronger variable with the interlayer spacing decreasing proportional to c~ which is consistent with previous results and with the Coulombic attraction theory. The sol concentration was found to affect the swelling for two reasons, the salt fractionation effect and the trapped salt effect. Both of these cause the salt concentration in the supernatant fluid to be greater than that originally added to the crystals and so reduce the swelling.A new method was used for extracting the solution from inside the gels by collapsing the gels by the addition of potassium hydrogen carbonate. The Volhardt titration was carried out on the extracted and supernatant solutions from about 250 gels. The ratio of the external to the internal chloride concentration was found to be approximately constant across the range of salt concentrations. Its average value was equal to 2.6, again in agreement with Coulombic attraction theory and showing the surface potential to be constant at about 70 mV.The (r, c, T) boundary of the two phase colloid region was investigated by three methods. A plot of log c against Tc was linear within experimental accuracy, with a gradient of 0.077 K-~ or 13 K per log unit. This shows that the surface potential varies by only 1 mV per decade in the salt concentration. The system is therefore governed by the Dirichlet boundary condition and not by the Nernst equation.

The Effect of Added Polymers on n-Butylammonium Vermiculite Swelling

“…Before adding polymer, it was necessary to check the properties of the new batch of vermiculite samples and the performance of the instrument by studying the [c, T] behavior of the system with no added polymer. Since the [c, T] behavior of the pure aqueous system has already been extensively studied by neutron scattering (Braganza et al 1990;Williams et al 1994), attention was restricted to c = 0.1 M and 0.01 M and temperatures in the vicinity of the phase transition temperature T c. The c = 0.01 M study is described later.…”

“…This was achieved by rinsing the crystals with 500 cm 3 of distilled water at 60-80 ~ 15 times before drying on filter paper. The distilled water was heated in order to prevent any swelling occurring during this washing process since, although the absorption of distilled water is rapid, it does not occur above 40 ~ as this is above the phase transition temperature for tactoid formation (Williams et al 1994). After drying, the crystals were cut to dimensions of approximately 0.5 • 0.5 • 0.1 cm.…”

“…After weighing, a single vermiculite crystal was placed into a quartz sample cell of dimensions 1 • 1 • 4.5 cm and an appropriate amount of the polymer and n-butylammonium chloride solution (typically 2.5 cm 3) was added to prepare an r = 0.01 sample. The cells were then sealed with parafilm and allowed to stand at 7 ~ for 2 weeks prior to the neutron scattering experiments, to ensure that full equilibrium swelling had been achieved (Williams et al 1994).…”

“…It leads to the formation of coherent gels, which show no tendency to disperse into the surrounding solution. The Figure lb-lc process was discovered more recently (Smalley et al 1989) and has been investigated as a function of the salt concentration c and the temperature T (Braganza et al 1990), hydrostatic pressure P (Smalley et al 1989), uniaxial stress along the swelling axis p (Crawford et al 1991) and, most recently, the volume fraction r of the clay in the condensed matter system (Williams et al 1994).…”

“…Because of this identity we refer to the tactold structure as having "crystalline regions". We recognize that, because of the regular interlayer spacings, the Figure lb structure is itself a type of crystal, actually a 1-dimensional paracrystal (Williams et al 1994), but retain the description "gel" in order to avoid confusion with the Figure 1 a structure, the original crystalline venrticulite. Adding polymers to a well-characterized clay colloid is potentially interesting both from the point of view of clay science and that of polymer science, but care is needed because the problem is a many-variable one.…”