Interactions of surfactants with a water treatment protein from Moringa oleifera seeds in solution studied by zeta-potential and light scattering measurements.

Protein extracted from Moringa oleifera (MO) seeds has been advocated as a cheap and environmental friendly alternative to ionic flocculants for water purification. However, the nature and mechanism of its interaction with particles in water, as well as with dissolved surface-active molecules, are not well understood. In this article, we report studies of the protein and its interaction with four surfactants using dynamic light scattering (DLS), zeta-potential and turbidity measurements. Zeta-potential measurements identified points of charge reversal and the turbidity and DLS measurements were used to characterize the microstructure and size of protein-surfactant complexes. From the points of charge reversal, it was estimated that 7 anions are required to neutralize the positive charges of each protein molecule at pH 7. For protein mixtures with sodium dodecyl sulfate and dodecyl di-acid sodium salt, the peak in turbidity corresponds to concentrations with a large change in zeta-potential. No turbidity was observed for protein mixtures with either the nonionic surfactant Triton X-100 or the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate. Changes of pH in the range 4-10 have little effect on the zeta-potential, turbidity, and the hydrodynamic radius reflecting the high isoelectric point of the protein. Addition of small amounts of salt has little effect on the size of protein in solution. These results are discussed in the context of the use of the MO protein in water treatment.