Sub-THz complex dielectric constants of montmorillionite clay thin samples with Na⁺/Ca⁺⁺-ions

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We implement a technique to characterize electromagnetic properties at frequencies 100 to 165 GHz (3 cm⁻¹ to 4.95 cm⁻¹) of oriented montmorillionite samples using an open cavity resonator connected to a sub-millimeter wave VNA (Vector Network Analyzer). We measured dielectric constants perpendicular to the bedding plane on oriented Na⁺ and Ca⁺⁺-ion stabilized montmorillionite samples deposited on a glass slide at ambient laboratory conditions (room temperature and room light). The clay layer is much thinner (30 μm) than the glass substrate (2.18 mm). The real part of dielectric constant,εre, is essentially constant over this frequency range but is larger in Na⁺- than in Ca⁺⁺-ioned clay. The total electrical conductivity (associated with the imaginary part of dielectric constant, εim) of both samples increases monotonically at lower frequencies (< 110 GHz), but shows rapid increase for Na⁺ ions in the regime > 110 GHz. The dispersion of the samples display a dependence on the ionic strength in the clay interlayers, i.e., ζ-potential in the Stern layers.