Surfactin structures at interfaces and in solution: the effect of pH and cations.

We have investigated the interfacial and bulk phase structures of surfactin at different pH and in the presence of mono/divalent cations using neutron scattering techniques. Neutron reflectivity profiles were recorded at the air/water and sapphire/water interfaces as a function of pH and ionic strength. The air/water results show that surfactin has a hydrophobic ball-like structure and that changes in pH and cations lead to changes in the area per molecule and hydrophilicity of surfactin. However the adsorption of surfactin on the hydrophilic sapphire/water interface is highly pH dependent because of electrostatic interactions with the surface. The bulk phase structures were characterized by small angle neutron scattering (SANS) and are more sensitive to pH and cation than the interfacial structure. At high pH surfactin forms micellar structures with low aggregation numbers, but at low pH values the bulk phase structure becomes rod-like at pH 6.5 and lamellar at pH 5.5. The addition of cations in the subphase tends to neutralize the two acidic groups of the peptide ring and the neutralization seems to be more complete for divalent cations than for monovalent cations.