Osmotic Swelling of Sodium Hectorite in Ternary Solvent Mixtures: Nematic Liquid Crystals in Hydrophobic Media.

The swelling of clay minerals in organic solvents or solvent mixtures is key for the fabrication of polymer nanocomposites with perfectly dispersed filler that contain only individual clay layers. Here, we investigated the swelling behavior of sodium hectorite in different ternary solvent mixtures containing methanol, acetonitrile, ethylene glycol, or glycerol carbonate with minimal amounts of water. We found that in these mixtures, less water is required than in the corresponding binary mixtures to allow for complete delamination by repulsive osmotic swelling. A quantitative study of osmotic swelling in a particular ternary mixture shows that organic solvents resemble swelling behavior in pure water. At hectorite contents larger than 5 vol %, the separation of individual layers scales with ϕ-1. At this concentration, a crossover is observed and swelling continues at a slower pace (ϕ-0.5) below this value.

Layer charge robust delamination of organo-clays.

To date delamination of organo-clays is restricted to highly charged, vermiculite-type layered silicates (e.g. n-butylammonium vermiculites) while - counterintuitively - low charged, smectite-type layered silicates do not delaminate although their Coulomb interactions are much weaker. Guided by previous findings, we now identified organo-cations that allowed for extending the delamination of organo clays to charge densities in the regime of low charged smectites as well. Upon intercalation of protonated amino-sugars like N-methyl-d-glucamine (meglumine) robust delamination of 2 : 1 layered silicates via repulsive osmotic swelling in water is achieved. This process is stable over a wide range of charge densities spanning from smectites (layer charge x ∼ 0.3 charges per formula unit Si4O10F2, p.f.u.) to vermiculites (x ∼ 0.7 p.f.u.). It is evidenced that a combination of first, a sufficiently large charge equivalent area (bulkiness) of meglumine with second, a significant hydrophilicity of meglumine leads to swelling above a threshold d-spacing of ≳17.5 Šin moist air (98% r.h.). Hereby, electrostatic attraction is critically weakened, causing the onset of repulsive osmotic swelling which leads to utter delamination. Moreover, meglumine renders delamination tolerant to charge heterogeneities typically found in natural and synthetic clays.