Insights into the Interactions among Surfactin, Betaines, and PAM: Surface Tension, Small-Angle Neutron Scattering, and Small-Angle X-ray Scattering Study

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 surfacetension measurements, small-angle neutron scattering (SANS), small-angle X-rayscattering (SAXS), and rheological experiments. It has been confirmed that thelength of alkyl chain is a key parameter of interaction between betaines and PAM.Differences in scattering contrast between X-ray and neutrons for surfactants andPAM molecules provide the opportunity to separately follow the changes ofstructure of PAM and surfactant aggregates. At concentrations of betaines higherthan CMC (critical micelle concentration) and C2 (CMC of surfactant with thepresence of polymer), spherical micelles are formed in betaines and betaines/PAM solutions. Transition from spherical to rodlikeaggregates (micelles) has been observed in solutions of Surfactin and Surfactin/SDDAB (ƒ¿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 PB