Mixtures of lysozyme and sodium dodecyl sulfate, SDS, are used to study protein−surfactant interactions. By precipitating the complex salt, CS(12), where DS- ions are the counterions to the positively charged lysozyme, a stoichiometric protein−surfactant component free from simple salts is synthesized. The solubility product of the complex salt is determined, and it provides a measure of the hydrophobic interaction in the precipitate. Aqueous systems of mixtures of the complex salt and the pure surfactant form a true three-component protein−surfactant−water system. The resulting phase behavior is studied and used as a fundament for theoretical discussion as well as modeling. With additional electrolyte excluded, the role of the electrostatic interactions is maximized, and the free energy balance between the different aggregates is shown to be determined by a balance between electrostatic and hydrophobic forces. We find three types of protein−surfactant aggregates:  the insoluble complex salt, an electrostatically swollen gel-like state containing macroscopic aggregates, and a soluble complex containing a single protein molecule.

The lysozyme-sodium dodecyl sulfate-water system features several interesting aggregation phenomena and is also of interest as it constitutes a model system for mixtures of a charged colloid with an oppositely charged surfactant, as both colloid and surfactant are pure and monodisperse compounds. The structure of such mixed protein-surfactant systems has been investigated by means of SANS contrast variation experiments. Two interesting issues of protein-surfactant aggregation are discussed. First, a new set of data on the structure of the protein-surfactant complex in solution is added to the discussion of whether the model of "beads on a necklace", "protein decorated micelles", or "flexible helix" is most appropriate. It is our conclusion that the compact globule of lysozyme does not fit well into any of the mentioned models. Instead, transient clusters of lysozyme-SDS aggregates are proposed for the L 1 phase and more strongly bound locally linear clusters for the gel phase. Second, the structure and formation of a homogeneous, transparent gel at room temperature is analyzed and compared to the well-studied heat-set globular gels. The gel structure in different ionic strengths, lower than the one caused by naturally occurring buffer salts has also been analyzed and it seems that small amounts of salt render the system a more repulsive character than salt-free conditions. In addition to the equilibrated samples studied with the contrast variation technique, the complexation has been studied over time.

By precipitating a complex salt Ly(OS) 8 of the positively charged protein lysozyme and the anionic surfactant octyl sulfate OS -, one can generate a true ternary system: water-Ly(OS) 8 -NaOS. Using NMR diffusometry and UV spectroscopy measurements the thermodynamic parameters of the association processes have been determined. The solubility product of the complex salt, K s , is 10 -28 M 9 . 1 On addition of excess surfactant the complex salt is solubilized into micelles that form at a critical association concentration of 74 mM which is nearly a factor of 2 lower than the CMC of 133 mM. At higher protein and surfactant concentrations these micelles first coexist with gel aggregates. The thermodynamically stable gel phase is observed at protein concentrations higher than 7 wt % and has a stoichiometry of around 28 OS -per protein molecule. Thereafter, in the presence of more than ca. 30 OS -per protein, micelles containing a single lysozyme molecule are formed from the gel aggregates. This rich aggregation pattern can be described as caused by a combination of an attractive hydrophobic interaction between hydrophobic patches on the protein surface and the surfactant hydrocarbon chain, and a composition-dependent electrostatic interaction between charged amino acids and the surfactant headgroup. The net force is attractive up to a ratio of surfactant to protein of 8, after which it becomes increasingly repulsive. The gel phase occurs as a compromise between the attractive hydrophobic interaction and the relatively weak electrostatic repulsion.