The permeability coefficients of a series of amides, ureas, and diols have been measured on red cells of man and dog using the minimum volume method of Sha'afi et al. When the molecules are grouped according to their ether-water partition coefficients, kether, the behavior of the hydrophilic molecules, with kether less than water, is different from that of the lipophilic molecules, characterized by kether greater than water. The rate of permeation of the hydrophilic molecules through an aqueous pathway is determined by the molar volume, a parameter in which the geometrical measure of molecular volume is modified by hydrogen-bonding ability. This indicates the importance of chemical interactions within the aqueous path. The permeation of the lipophilic molecules is determined in the first instance by kether, taken as a measure of the ease with which the molecule can escape from its aqueous environment. Within the membrane, lipophilic permeability is modified both by steric factors and by the formation of hydrogen bonds with membrane components. These data allow one to infer that lipid-soluble molecules travel through an organized structure within the lipid membrane and come into contact with polar moieties.

Sha’afi, R. I., Gary-Bobo, C. M., & Solomon, A. K. (1971). Permeability of Red Cell Membranes to Small Hydrophilic and Lipophilic Solutes. The Journal of General Physiology, 58(3), 238–258.