Why Clays Swell

Abstract

The particularly difficult subject of predicting the swelling behavior of clay minerals is addressed by a combination of molecular dynamics and Monte Carlo sampling techniques. The introduced algorithm essentially mimics the experimental determination of the water adsorption isotherm and quantitatively predicts clay swelling for a montmorillonite-type clay including such details as the occurrence of hydrated states and hysteresis. Furthermore, important insights into the underlying mechanism of clay swelling from the one-layer to the two-layer hydrate are derived. It turns out that, for this case, clay swelling proceeds via the migration of counterions that are initially bound to the mineral surface to the central interlayer plane where they become fully hydrated. The extent of clay swelling strongly depends on the charge locus. This information appears to be transferable to other clay types.

Origins of hydration lubrication

“…Figure 1 shows the normalized force profiles F n (D)/R versus D across water (always taken as a control for system cleanliness before adding salt) and across the 0.1 M salt solutions, revealing the longer-ranged repulsion arising from counterion osmotic pressure 18,20 down to DE2 nm. At Doca 2 nm, we observe the characteristic repulsion arising from the tightly bound hydration shells [14][15][16][17][18] surrounding the Na þ counterions trapped between the negatively charged mica surfaces.…”

“…Over the past decade, the concept of hydration lubrication has been invoked to account for the extremely low sliding friction observed at high pressures between charged surfaces in high-salt solution [3][4][5] , or when coated by surfactants [6][7][8] , liposomes 9 or hydrated polymer brushes [10][11][12] boundary layers resembling those at articular cartilage surfaces 12,13 . According to this, hydration shells formed by water molecules are tenaciously attached to the charges they surround, and so cannot be easily squeezed out on compression [14][15][16][17][18] , yet are labile and so respond to shear in a fluid manner. However, there is little microscopic understanding of this mechanism, especially of the frictional dissipation within the subnanometre hydration shells that form its basic elements.…”

Influence of dust composition on cloud droplet formation

“…Quartz is chosen to represent the insoluble portion of dust particles. We do not consider insoluble smectite clays that swell by acquiring water with increasing relative humidity (Hensen and Smit, 2002;Frinak et al, 2005) 8/21/2006 6 The carbonate content of dust appears to be highest in the coarse size fraction (Pye, 1987;Usher et al, 2003) and varies considerably with source region. For instance, Wang et al (2005) reported that the carbonate content of soils in northern China decreases systematically from west to east from 11.8% to 0.3%.…”

Colloidal crystals and water: Perspectives on liquid–solid nanoscale phenomena in wet particulate media

“…The equilibrium distance also defines the packing and porosity, which are key variables in complex transition phenomena in granular media like jamming and shear banding [42,132] and liquefaction [133], important in phenomena like landslides [134]. It can also lead to better understanding of swelling behavior of clays [27] and cellulose [38].…”

“…For instance, familiar examples are sand, which becomes 'workable' when is wetted by liquid films 5-nm thick or less on the grains [24], and flour in everyone's kitchen, which is sticky on rainy days through adsorption of a few water layers [25]. Set plasters are formed by gypsum crystallites linked by physisorbed water films ~ 1 to 10-nm thick (from dry to saturation conditions) [26]; similarly, clay swelling is typically due to moisture uptake of one to three water layers in the clay interlayer [27]. Friction and wear in soils, along with aging effects, depend on the presence of molecularly thin water films at the nanocontacts between mineral particles [28].…”