Supramolecular Association in the System Water−Lysozyme−Lithium Perfluorononanoate

Mixtures containing water, lysozyme (LYS), and a fluorinated surfactant (LiPFN) have been investigated in a wide range of protein-to-surfactant ratios. Depending on composition, sample consistency and coexistence of the different phases, different experimental methods were used. Volumetric, viscometric, surface tension, potentiometric (by a home-built ion-selective electrode), turbidity, optical polarizing microscopy, and 19F NMR experiments were used. The results obtained from the above methods have been interpreted in terms of a combination of electrostatic and hydrophobic contributions to the stability of the different phases formed in the water−LYS−LiPFN system. Solutions, gel phases, and precipitates have been observed in the range investigated in more detail. Multiphase regions have also been observed. Such rich polymorphic behavior implies the existence of interactions between the protein and surfactant. The gel phase is presumably formed by interconnections between micelles and protein-surfactant complexes, held together by protein-bound micelles and forming, presumably, interconnected necklace structures. The overlapping of different protein-surfactant aggregates to form gels requires a significant amount of time. Its formation obeys a volume fraction statistics; the width of the gel phase, in fact, is controlled by the amount of protein−surfactant complex.