Polymer Brushes for Molecular Transport - Paul Braun Research ...

Our surfaces can be divided into two categories: surfaces on which Prodan diffuses
noticeably during the time scale of the experiment and surfaces on which it does not. The PEG
and CTS surfaces fall under the first category and the clean silica and OTS surfaces fall under
the latter. If one assumes that diffusion takes place on, and not in and through, the layer, which
may not be entirely true on the PEG surface, the surface free energy seems to be the most
important variable to consider. Figure 2.3 illustrates the two diffusion mechanisms of Prodan: on
or in the SAMs. It appears that the PEG and the CTS surfaces which have the intermediate
surface free energies, with water contact angles of 33° and 68° respectively, are more conducive
to Prodan diffusion than OTS surface which have very low surface free energy as shown by its
water contact angle of 109°, or the clean silica surface which has the high surface free energy as
indicated by a water contact angle of 14°.
(a) (b)
Figure 2.3. Schematic representations of the diffusion of an individual probe molecule. (a) on a
surface; (b) in and through the layer.
Prodan is about 5 times more soluble in methanol (~20 mM) than in the less polar solvent
hexane (~4.4 mM). Thus, we expect that it is likely to aggregate on the hydrophobic OTS
derived surface. The fluorescence intensity of samples from the same Prodan concentration on
the OTS surface is significantly lower than on the PEG or CTS surface, which is a strong
indication of aggregation since dye aggregates are commonly known to self-quench.[8, 9] We
24