  Vibrational sum frequency scattering (SFS) has been used to study sub-micron,
catanionic vesicles in solution. The vesicles were synthesized from a binary
mixture of dodecyltrimethylammonium bromide (DTAB) and sodium dodecylsulfate
(SDS) surfactants in deuterated water, which spontaneously assemble into
thermodynamically stable vesicles. The stability of these vesicles is
attributed to a surfactant concentration asymmetry between the inner and outer
bilayer leaflets. This concentration asymmetry should be observable by SFS due
to local inversion symmetry-breaking. Signal corresponding to the symmetric
sulfate stretch mode of the SDS head group is observed at 1044 cm, indicating
that there is indeed asymmetry in the local structure of the leaflets. The
results indicate that it should be possible to measure the interfacial
structure of liposomes in aqueous solution and study in-situ processes like the
binding of sugars and proteins that are important for many processes in
biophysical chemistry.
