Many packaging materials act in much the same way. Another
application of Eq. (5.27) has been the development of ‘‘edible films,’’ used to
separate compartments or to enrobe foods so that water loss or exchange of
flavor components is considerably diminished. In practice, the situation may
be more complicated. The concentrations on either side of the film do not
stay constant, the geometry may be different, there may be more layers of
different properties, etc.
Eq. (5.27) shows that the diffusional resistance becomes very small if
the layer is very thin. An adsorption layer on the interface between phases,
which mostly is of the order of a nm in thickness, thus causes a negligible
resistance to diffusion. However, highly condensed lipid bilayers, as are
present in cell membranes, may provide a significant diffusive resistance to
large hydrophilic molecules, although the membranes are quite thin; water
itself can pass relatively fast. In a natural tissue there are numerous
membranes in parallel; hence, the total resistance to diffusion may be
substantial. In a living cell, mechanisms for active transport of specific
FIGURE5.17 Example of the concentrationðcÞof a component as a function of
distanceðxÞas it diffuses through a layer of another phaseðbÞ. See text.