Probing Biointerfaces: Electrokinetics - Biointerfaces: Where Material Meets Biology - page 63

conductivity, K s , was verified to be insignificant for data analysis. 31 The
density of the carboxyl groups in the collapsed film (low pH) was determined
to be 255 15 mM with a corresponding dissociation pK of 6.6. Further-
more, all data were reproduced using a single value of 0.53 0.03 nm for the
hydrodynamic penetration length, 1/l 0 , of the collapsed film. The extremum
in streaming current versus pH was unambiguously attributed to the for-
mation of an interfacial gradient in polymer segment density upon swelling
with increasing solution pH (leading to an increase of the parameter a, and
to modulations of the segment distribution, as depicted in Figure 3.2). This
expansion of the film increases the friction exerted by the outer tails of the
polymer chains on the fluid flow. As a result, the streaming current de-
creases although the film charge and thus the number of counter-ions in the
soft layer increases with pH. If the charging is considered alone, |I str /DP|
would gradually increase with increasing pH and reach a constant value
reflecting a complete ionization of the carboxyl groups. At high electrolyte
concentrations, the Debye length is suciently small for the electro-
kinetically active zone of the double layer to span the outmost region of the
diffuse part of the film. The streaming current then becomes less sensitive to
the hydrodynamic hindrance of the flow by the heterogeneous film struc-
ture, and the maximum in |I str /DP| becomes thus less pronounced and ul-
timately disappears at high salt concentrations. Conversely, at low electrolyte
concentrations, the double layer encompasses the entire heterogeneous
region at the very interface between the film and bulk electrolyte. Under
these conditions, the impact of a on I str /DP becomes most significant,
leading to the maximum in |I str /DP| with increasing pH (or increasing a).
Finally, it is emphasized that the data analysis indicated a difference of DpK
B

d n 3 r 4 n g | 5

.

0.9 between the pK values of the carboxyl groups in the film derived from
streaming current and surface conductivity. 31 This difference was attributed
to a position-dependent hydrophobicity across the film due to the preferred
orientation of hydrophobic and hydrophilic polymer segments. 31
Within the approximations underlying the applicability of the mean-
field Poisson-Boltzmann equation, 18 the above example demonstrates
how the coupled analysis of streaming current and surface conductivity
allows for a quantitative characterization of structure and electric double
layer properties of soft charged layers. Further examples covering the an-
alysis of streaming current and surface conductivity data obtained for
thermo-responsive films without ionisable groups and for highly charged
polyelectrolyte layers can be found in literature. 30,32 In the latter case, 32
electro-osmosis may significantly contribute to the overall surface conduct-
ivity. K s then reflects not only the key electrostatic properties of the film but
also its hydrodynamic permeability via the quantity 1/l 0 that impacts
V eo (X). 32 Furthermore, for various systems of practical interest, e.g., poly-
electrolyte multilayers, the nature and composition of the films vary across
the substrate-soft film-solution interface. An extension of the theory that
captures the basics of the electrokinetics for such complex systems can be
found in Duval et al. 33