  Binding and phase behavior of oppositely charged polyelectrolytes and
surfactants were studied in aqueous bulk solutions and at air-water interfaces.
In particular, we have investigated the polyanion poly(sodium
4-styrenesulfonate) (NaPSS) and the cationic surfactants
dodecyltrimethylammonium bromide (C$_{12}$TAB), tetradecyltrimethylammonium
bromide (C$_{14}$TAB) and cetyltrimethylammonium bromide (C$_{16}$TAB). In
order to reveal surfactant/polyelectrolyte binding, aggregation and phase
separation of the mixtures, we have varied the NaPSS concentration
systematically and have kept the surfactants concentration fixed. Information
on the behavior in the bulk solution was gained by electrophoretic mobility and
turbidity measurements, while the surface properties were interrogated with
surface tension and sum-frequency generation (SFG). This has enabled us to
relate bulk to interfacial properties with respect to the charging state and
the surfactants' binding efficiency. We found that the latter two are strongly
dependent on the alkyl chain length of the surfactant and that binding is much
more efficient as the alkyl chain length of the surfactant increases. This also
results in a different phase behavior as shown by turbidity measurements of the
bulk solutions. Charge neutral aggregates that are forming in the bulk ad-sorb
to the air-water interface, an effect that is likely caused by the increased
hydrophobicity of C$_\text{n}$TAB/PSS complexes. This conclusion is
corroborated by SFG spectroscopy, where we observe a decrease in the intensity
of OH stretching bands, which is indicative for a decrease in surface charging
and the formation of interfaces with negligible net charge. Particularly at
mixing ratios that are in the equilibrium two-phase region, we observe weak OH
intensities and thus surface charging.
