Insights into the interactions among Surfactin, betaines, and PAM: surface tension, small-angle neutron scattering, and small-angle X-ray scattering study

TL;DR

This study investigates how Surfactin, betaines, and PAM interact at the molecular level using scattering techniques, revealing how alkyl chain length influences aggregate structures and polymer conformations in solution.

Contribution

It provides new insights into the structural interactions among biosurfactants, betaines, and polymers, highlighting the role of alkyl chain length and the formation of different micellar structures.

Findings

Spherical micelles form at concentrations above CMC and C2.

Transition from spherical to rod-like micelles occurs with Surfactin and SDDAB in presence of PAM.

Polymer conformation changes are observed only in Surfactin/SDDAB/PAM system.

Abstract

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.