Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 71

Ascher et al. 38 in the COLSYS package is particularly powerful. Indeed, it
eciently approximates the searched electrostatic and hydrodynamic field
distributions through spline-collocation at Gaussian nodes, and selects the
relevant mesh subdivision following an autoadaptative strategy.

d n 3 r 4 n g | 5

A.3 Thin Polymer Films for Streaming Current/Potential
Measurements
The characterization of polymer films by streaming current/potential
measurements requires chemical and mechanical stability (shear forces due
to the hydrodynamic flow) of the films under the experimental conditions. If
possible, covalently attach the polymers to the supporting surface (typically
glass or silicon with a natural or thermal oxide layer) using reactive pre-
coatings. In cases of polymers without reactive groups, the film stability can
be improved by silanization of the carrier surface (for example, with hex-
amethyldisilazane in the case of hydrophobic polymers such as Teflon s AF
or polystyrene). Supported bilayer lipid membranes should be prepared
directly in the measuring cell by injecting the vesicle solution into the
channel formed by the supporting substrates.
The stability of the prepared films/membranes should be checked by re-
peating the measurements in the same pH range at a given concentration of
the background electrolyte. In the ideal case, the streaming current/potential
versus pH plots are identical for all measurements for the same pair of sam-
ples. Shifts in the IEP often indicate chemical variations of the polymer or
delamination. Should stability problems arise, complementary methods like
ellipsometry or photoelectron spectroscopy could help identify the source of
the variation in the 'electrokinetic fingerprint' of the polymer films.

.

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