Marrink and Berendsen proposed a four-region permeability model for phospholipid bilayers

 Marrink, Siewert-Jan and Herman J. C. Berendsen. "Simulation of Water Transport through a Lipid Membrane." The Journal of Physical Chemistry 98, no. 15 (1994): 4155--4168.

In their paper, Marrink and Berendsen theoretically examined water permeation across phospholipid bilayers through computational means.  They extended the homogenous solubility-diffusion model to what they call the inhomogeneous solubility-diffusion model (ISD), which relates local free energy and diffusion coefficient to permeability.  They show connections between variables in their model to experimentally measurable quantities, like hydrostatic pressure difference.  They performed atomistic molecular dynamics simulations of dipalmitoylphosphatdylcholine (DPPC) membranes and water to compute the permeability coefficient of water through the bilayer by measuring the potential of mean force and local diffusion coefficients of water through the bilayers.  Position-dependent characterization of free energy and diffusion rates, among other properties, indicated an inhomogeneous barrier created by the DPPC bilayer.  Their analysis led them to propose a "Four-region" model of phospholipid bilayers wherein  Marrink and Berendsen designated four regions to describe this inhomogeneity: perturbed water, bound water, high viscosity, and low viscosity.  They claim that combining measurements from their simulations with their ISD model yielded permeability coefficients that were close to permeation rates obtained experimentally.