  The adsorption of three different polycations at the negatively charged
surface of tubular J-aggregates of the amphiphilic cyanine dye
3,3'-bis(2-sulfopropyl)-5,5',6,6'-tetrachloro-1,1'-dioctylbenzimidacarbocyanine
(C8S3) is investigated by means of cryogenic electron microscopy and optical
absorption spectroscopy. All three polycations could be adsorbed at the
aggregates without flocculation or precipitation when added in molar amounts of
monomers sufficiently smaller than of the dye molecules. Preferably the
aggregates are either coated by the polycations in total or left completely
uncoated. For the coated aggregates, the adsorption leads to charge reversal of
the aggregate surface as supported by zeta-potential measurements. The
morphology of the coating differs significantly for the three polycations: The
branched polycation polyethlyenimine (PEI) attaches to the tubular aggregate by
hit-and-stick adsorption of the coiled state in solution forming irregular
clot-like coatings; the flexible and weakly cationic poly(allylamine
hydrochloride) (PAH) forms a more homogeneous coating but destroys the
integrity of the dye aggregate; the more hydrophobic and strong polycation
poly(diallyldimethylammonium chloride) (PDADMAC) forms a thin and homogeneous
layer, supposedly by wrapping around the tubular aggregate. For the latter,
growth of a second dye double layer of the aggregates seems to grow. The
different morphologies of the coatings are rather due to the details of the
chemical structure of the polycations than due to physical parameters. The
possible adsorption of polyelectrolytes at these amphiphilic tubular
structures, stabilized by non-covalent bonds only, is far from obvious but
demonstrates an applicable route to the build-up of more complex nanostructures
by means of a self-assembly process.
