frequency plot suggests that the homogeneous barrier model is
inadequate. The shape of the TEA AC flux enhancement vs AC
frequency profile at 2.5 V is similar to those of previous model
simulation with synthetic Nuclepore membranes (2). In this
previous simulation study, which involved a model of multiple
barriers with water reservoirs in series, the relationship of TEA
flux vs AC frequency under a constant electric field could be
divided into five main regions as pointed out in the Introduction.
At the high AC frequency extreme, the flux approached
the passive permeability limit. At the low AC frequency
extreme, the flux approached the upper limit of permeability
corresponding to one-half of the expected permeability for DC
iontophoresis; when the half cycle time period is significantly
larger than the transport lag time, the square-wave AC field is
essentially acting as 50% of a DC field to enhancing transport.
In the middle region between these two extremes, a constant
flux relatively independent of frequency was observed. This
middle region is the main difference between a homogenous
barrier model and the multiple-barrier model. The present AC
enhancement data for TEA obtained at 2.5 V appear to
demonstrate this multi-region behavior