where pUWL is the permeation through an unstirred water layer (UWL), fneutral is the fraction of molecule that is in a non-ionized state in the donor compartment, and pM is the molecule's intrinsic permeability. This equation is plausible for the cases where the permeability of ionized species is negligible. That is often valid, but there are specific examples when this assumption is not fulfilled . UWL is a static layer of water directly adjacent to the surface of a membrane, acting as an additional resistance to permeation. This value can vary with different stirring of donor and acceptor compartments, but for the most common experimental setting of CACO-2/MDCK or PAMPA assay, this value is around -3.9. Intrinsic permeability is often obtained from the calculation based on measuring the permeability scale at different pH levels. This approach can be found in publications by Huque et al. , Avdeef et al. , or Tsinman et al. . Furthermore, other implementations of this approach showed Velický et al. , where the permeabilities are measured at different hydrodynamic regimes and from that, intrinsic permeability can also be calculated.