Where N is the solute concentration in molecules/m 3 , ε 0 is the permittivity of free space, ε(0) is the DC dielectric constant of the solvent, ε(∞) is the high-frequency dielectric constant of the solvent (which can be approximated as the square-root of the refractive index, √ n), ω is the microwave angular frequency, µ is the molecule's dipole moment, k B is the Boltzmann constant, θ is the dipolar relaxation time of the solute [described further in the Molecular Rotation section of the SI], and T is temperature. The function f (ωθ ) describes the variation in experimental sensitivity due to the correlation between the microwave frequency and the frequency of the dipole's relaxation.