  The interactions among neutral polymer polyacrylamide (PAM) and the
biosurfactant Surfactin and four betaines,
N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SDDAB),
N-tetradecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (STDAB),
N-hexadecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (SHDAB), and
N-dodecyl-N,N-dimethyl-2-ammonio-acetate (C12BE), in phosphate buffer solution
(PBS) have been studied by surface tension measurements, small-angle neutron
scattering (SANS), small-angle X-ray scattering (SAXS), and rheological
experiments. It has been confirmed that the length of alkyl chain is a key
parameter of interaction between betaines and PAM. Differences in scattering
contrast between X-ray and neutrons for surfactants and PAM molecules provide
the opportunity to separately follow the changes of structure of PAM and
surfactant aggregates. At concentrations of betaines higher than CMC (critical
micelle concentration) and C2 (CMC of surfactant with the presence of polymer),
spherical micelles are formed in betaines and betaines/PAM solutions.
Transition from spherical to rod-like aggregates (micelles) has been observed
in solutions of Surfactin and Surfactin/SDDAB ({\alpha}Surfactin = 0.67 (molar
fraction)) with addition of 0.8 wt % of PAM. The conformation change of PAM
molecules only can be observed for Surfactin/SDDAB/PAM system. Viscosity values
follow the structural changes suggested from scattering measurements i.e.,
gradually increases for mixtures PAM -> Surfactin/PAM -> Surfactin/SDDAB/PAM in
PBS.
