Self-Assembly from Surfactants to Nanoparticles - Head vs. Tail

Abstract

Surfactant molecules are amphiphilic, composed of a polar headgroup that likes water and a nonpolar tail that dislikes water, thus contributing to an intrinsic duality in their molecular characteristics. Despite their mutual antipathy, the headgroup and tail of the surfactant cannot leave one another because they are covalently connected. The dilemma of mutual antipathy and forced coexistence faced by these molecules is resolved in nature by the intriguing phenomenon of molecular self-assembly, wherein the surfactant molecules self-assemble into three-dimensional structures with distinct and separate regions composed of the nonpolar parts and the polar parts, having minimal contact with one another. Block copolymers are an important class of high molecular weight polymer molecules that share great molecular similarity with the surfactants. A diblock copolymer is made up of repeating units A and B, with the repeating units occurring as blocks, covalently connected to one another. If one block (B) is hydrophilic or solvophilic (head) while the other block (A) is hydrophobic or solvophobic (tail), the block copolymer becomes a high molecular weight analog of the low molecular weight surfactant. Surfactants and block copolymers display characteristic molecular self-assembly behavior in solutions, at interfaces as well as in bulk, generating nanoscale structures of different shapes. These nanoscale features determine many macroscopic properties of these amphiphile systems, relevant for their practical applications. The ability to generate desired nanoscale morphologies by synthesizing novel amphiphiles so that the amphiphilic systems can be tailored for specific applications as well as the ability to manipulate the morphologies using chemical and physical stimuli remain active goals of research in this field. Critical to