Self-Assembly: From Soap Micelles to Structurally Precise Protein-like Aggregates

Surfactant molecules display an intrinsic duality in their molecular interactions because their structure is composed of a hydrophilic part, referred to as the head, that is covalently linked to a hydrophobic part, referred to as the tail. The forced coexistence of the head and the tail, despite their mutual antipathy, provides the foundation for the self-assembly of surfactants, which gives rise to nanoscale structures of different sizes and shapes. The occurrence of self-assembly in soap solutions was recognized over hundred years ago and the Colloid Symposia that have been held since 1923, consistently featured and highlighted research on surfactants and self-assembly. Over these hundred years, utilizing the concept of the head-tail dichotomy, numerous self-assembling systems have been developed for applications in many critical areas of technology and medicine. Examples of self-assembling systems beyond soaps and surfactants include phospholipids, block copolymers with a solvophilic block functioning as the head and a solvophobic block functioning as the tail, and a variety of non-classical amphiphiles in which polyelectrolytes, dendrimers, DNA, peptides, proteins, and nanoparticles, serve as either or both of the head and tail components. A recent discovery is the self-assembly of calix[n]arene amphiphiles leading to structurally precise aggregates of a unique aggregation number, which can be viewed as the analog of multi-subunit proteins. Since the aggregation number of these micelles can be controlled by molecular design, these structurally precise micelles hold promise as synthetic proteins/enzymes with tunable ligand recognition and catalytic properties. The wide range of experimental advances in the creation of self-assembling systems and their applications are soundly backed by the unified principles of self-assembly built on the concept of opposing forces, coupled to considerations of molecular packing in aggregates of different shapes. In this chapter, we provide a qualitative accounting of the field, mainly tracing the history and the molecular diversity of self-assembling systems. More extensive fundamental discussions on each type of self-assembling system can be found in the cited literature.