Similarities in Self-Assembly of Proteins and Surfactants: an Attempt to Bridge the Gap

The area of surfactant self assembly has already received attention for more than half a century. Considerable progress has been made in regards to connecting the molecular properties to the assembly morphology and the phase behaviour, where a multitude of different (rather exotic) types of mesophases with a large variation in topology could be distinguished. In addition, rheological response and even shear induced transitions in morphology and phase behaviour were reported. An important technological area is the assembly of surfactants at an interface. The area of protein assembly and resulting phase behaviour has also received considerable attention. The main focus for many years has been on solubility, crystallisation, interfacial assembly, phase separation and gel formation, in terms of molecular properties of the protein (mixture). Since assemblies of proteins are abundant in food products, the exploration of their morphology, physico-chemical characteristics and mutual interactions is relevant for the engineering of food materials. It is only for the past two decades that the morphology of the assemblies has received more attention. This has been explored in terms of fractal aggregates. The type of protein assembly that is usually found in foods (in 3 dimensions) exhibits a morphology that has many branches per unit length. An extreme case is an infinitely stiff aggregate with zero branches per unit length, i.e. a rod (fractal dimension equals 1). This fibrillar type has been the subject of intensive studies lately, in various science areas, ranging from material science, food science, to medical sciences (beta-amyloid diseases). Apart from the fact that the rod morphology is relevant as it is an extreme case of zero branches, it also has practical relevance, e.g. because the fibrils act as a vehicle to form extremely low weight fraction gels. Another example of a protein aggregate is a hollow capsule (serving as a protection device of many viruses). In this article, the mesoscopic parameters relevant for describing the aggregate morphology, phase behaviour and topology of surfactant systems will first be addressed. Then, a similar scheme will be presented for the use of relevant mesoscopic parameters in describing the morphology and topology of protein aggregates, for fractal dimensions from 1 to 3. The influences of pH, salt concentration, salt type and temperature will be considered. Non-equilibrium effects regarding the particular assembly of proteins into fibrils will also be addressed. Finally, the similarities and differences regarding surfactant versus protein assembly will be discussed.