Confinement of solvents and solutes within nanoporous
materials
frequently leads to the emergence of unique mass transport behaviors
that, once fully understood, may lead to improved chemical separations.
Here, the diffusion of Rhodamine B (RhB) dye within 10 and 20 nm diameter
anodic aluminum oxide (AAO) nanopores filled with binary ethanol/water
mixtures is investigated. Mixture compositions spanning from pure
ethanol to pure water are employed. The results of confocal fluorescence
correlation spectroscopy studies reveal that RhB diffusion occurs
by a two-component mechanism comprising composition-dependent fast
and slow motions, characterized by diffusion coefficients D
f
and D
s
. The results are consistent with those of
previous studies performed under more limited conditions [J. Phys. Chem. C, 2023, 127, 411–420].
The fast component scales with mixture viscosity and is assigned to
hindered bulk-like diffusion in central pore regions. Slow diffusion
likely involves adsorption of RhB to the pore surface and may be described
by a desorption-mediated mechanism. The occurrence of RhB adsorption
to the AAO surface is verified at the single-molecule level by wide-field
fluorescence imaging of membrane cross-sectional surfaces. Unique
composition-dependent trends in the autocorrelation amplitude and
in D
s
that mimic bulk
RhB solubility are revealed. D
s
is found to be smallest in pure ethanol and pure water and
largest in intermediate mixtures. These results suggest that RhB surface
adsorption is strongest in the pure liquids and weakest in mixtures
of intermediate composition, where the dye is least soluble, and most
soluble, respectively. Molecular dynamics simulations reveal that
a water layer appears on the pore surface under most conditions, while
RhB is solvated primarily by ethanol. The composition dependence of
RhB diffusion is concluded to reflect its solvation-dependent interactions
with the pore walls.