X. Chatellier et al., Adsorption of a cationic polyelectrolyte on Escherichia coli bacteria: 2. Interactions between the bacterial surfaces covered with the polymer, LANGMUIR, 17(9), 2001, pp. 2791-2800
Quaternized polyvinylpyridine (PVPQ) was used as a cationic polymer to dest
abilize an Escherichia coli bacterial suspension. The optical density and t
he fraction of free cells, obtained by light scattering measurements, were
recorded as a function of the introduced polymer amount, as a way to monito
r the stability of the suspension. The flocculation was almost complete for
polymer dosages ranging from about 28 to 47 mg of carbon of PVPQ per dry g
of bacteria. These dosages correspond to still negatively charged cells, a
s shown by xi potential measurements. At low polymer coverages, a less effi
cient flocculation is observed. At higher dosages, the suspension restabili
zes. We interpret these results using our previous study on the adsorption
of the polymer chains. We argue that the flocculation at low dosages is ren
dered possible by the strong inhomogeneities of charge on the bacterial sur
faces because of the self-similar configuration of the adsorbed polymer lay
er and that the restabilization at large dosages is due to the small mesh s
ize of the polymer network on the surface as well as to the Coulombic repul
sion between the cells. The properties of the bacterial aggregates were inv
estigated by light scattering. Destabilized suspensions produce aggregates
with sizes decreasing as the quantity of adsorbed polymer increases. At the
optimum of flocculation, the polydispersity of the aggregates is low, sugg
esting a diffusion-limited aggregation mechanism (DLA). The presence of the
characteristic self-similar structure of DLA aggregates, with a fractal di
mension on the order of 1.9, is suggested by some of the light diffusion ex
periments. On the other hand, at low coverages, that is, when only some reg
ions on the surfaces are covered with polymers, the flocculation seems to o
bey a reaction-limited aggregation, with a large polydispersity in the size
of the aggregates.