Structural Organization of Na- and K-Montmorillonite Suspensions in Response to Osmotic and Thermal Stresses

Abstract

Abstract--In order to understand the influence of salt concentration and temperature on the behavior and properties of clays used in drilling muds, we studied montmorillonite supensions (4 g clay/100 g solution) in 0.1, 0.5 and 1 M NaC1 and KC1 solutions. A fraction of each sample was heated to 200 ~ in a closed vessel for 7 d, then cooled at room temperature (RT, 25 ~ Small-angle X-ray scattering (SAXS) spectra were recorded, for all the samples, at RT. The structure of the clay particles was determined by comparing the experimental intensity with the theoretical intensity computed from a model that took into account the number of layers per particle, the hydration state of the layers (0, l, 2, 3 or 4 water layers) and the order in the succession of these states. With this set of parameters, we can compute the mean statistical parameters ~/ (mean number of layers per particle), cl (mean interlayer distance) and g2/~ (parameter describing the disorder of the distribution of interlayer distances). The evolution of these parameters shows that: 1) At low concentration (0.1 M NaC1 or KC1), the samples do not consist of particles but of isolated layers (M = 1). The suspensions form gel-like structures. The difference between Na and K suspensions, or between heated and nonheated samples, is unnoticeable at the studied scale (5-500 ,~).2) An increase in salt concentration (from 0.1 to 0.5 M) brings the sample in a granular state: we notice the appearance of particles at 0.5 M (~7/ ~ 25). Differences appear between NaC1 and KC1 suspensions, and the temperature effect becomes visible. Thus, we noticed that in NaC1 suspensions, particles are composed of hydrated layers (1, 2, 3 or 4 water layers) and internal porositYo(d > 30 A), whereas suspensions in KC1 are characterized by the presence of interlayer distances of 10 A, that is, of collapsed layers. Particles in the KC1 suspensions are much thicker than in the NaCI corresponding ones, and also less hydrated at the interlayer level as well as at the internal porosity level. Further increase in salt concentration (0.5 to 1 M) amplifies this effect. As far as temperature is concerned, its effect is to promote the clay dispersion by breaking up the particles, dehydrating and disordering them. This effect is more important for low salt concentration, that is, when the system is less stressed.

Formation and Restacking of Disordered Smectite Osmotic Hydrates

“…At concentrations of <~0.25 M NaCl for Wyoming bentonite, the separation between layers shows a linear dependence on N ÀÝ , where N is the salt concentration. Remarkably, this trend was observed out to periodicities of~14 nm and was subsequently shown to be a function of salt type and concentration, clay concentration and layer charge, and temperature (Viani et al, 1983;Slade et al, 1991;Zhang et al, 1993;Faisandier et al, 1998;Amorim et al, 2007;Svensson and Hansen, 2013). * E-mail address of corresponding author: jbanfield@berkeley.edu + Equal co-authors DOI:…”

“…Moreover, all models based on DLVO theory fail to depict accurately behavior for highly charged surfaces or non-monovalent ions such as Ca 2+ (Kleijn and Oster, 1982). All XRD and small-angle X-ray scattering (SAXS) studies of smectite osmotic hydrates show evidence of disorder in the interlayer spacings (Quirk and Marčelja, 1997;Faisandier et al, 1998). Whether this is caused by intrinsic heterogeneity of natural smectites, with cation substitutions leading to local variations in layer charge or whether it is simply a consequence of natural fluctuations due to, e.g.…”

NH4-smectite: Characterization, hydration properties and hydro mechanical behaviour

“…The characteristic basal distance d 001 was 1.45 nm for the original smectite and decreased to 1.19 nm after ammonium exchange. The broadened 001 reflections corresponded to heterogeneous systems where hydrated layers coexist with collapsed ones (Faisandier et al, 1998).…”

Small-angle neutron scattering from a nano-layered synthetic silicate

“…In particular the SAXS method has been employed to perform several studies on both clay suspensions and dehydrated clays during recent years. Important questions connected with clay dispersions have been those of size/shape of individual particles and their aggregates [5], influence of salt and temperature [6] or concentration [7] on the ability of clay suspensions to form gel-like structures and granules, and the dynamics of layered silicate crystallization [8]. For the solid-like clay structures, studies have been made on the intercalation of metal ions within the interlayers of clay platelets [9], on the heterogeneity of stacks of platelets in the dehydrated state [10], and on the variation in surface fractal dimension between different natural clays [11].…”