No or little enhancement due to electroosmosis was
observed at AC frequency Q0.5 Hz at 0.5 and 2.5 V. This is
consistent with previous findings that AC iontophoresis at
moderate frequencies does not enhance the transport of
neutral permeants (3,4). Previous studies have shown that
flux enhancement due to electroosmosis is generally less than
20% (average of around 10%) of that due to electrophoresis
for small permeants (23). The ARA data in the present
study fall in this 5 to 20% range. According to the TEA and
ARA results in Fig. 4, flux enhancement due to electroosmosis
at 0.5 V is approximately 20% of the total electrotransport
enhancement at the low frequency plateau, and the
contribution of electroosmosis to total electrotransport at
2.5 V is approximately 5% in the same frequency region. In
addition, flux enhancement at 0.5 V and 0.001 Hz is
essentially the same as that at 2.5 V and 0.001 Hz for ARA.
The decrease in electroosmosis contribution from 0.5 to 2.5 V
suggests different pore charge densities between the electric
field-induced pores and the pre-existing pores in SC. These
results are consistent with a previous finding (18).