Interfacial Self-Organization of Bottlebrush Amphiphiles Enables Stable Monodisperse Submicron Emulsion Droplets:...

Emulsions, in which one liquid is dispersed within another immiscible liquid, are fundamental to a wide range of natural phenomena and technological applications. Because their physical properties critically depend on droplet size, generating droplets with predictable and precisely controlled dimensions has long been a central objective in soft matter science. Classical theory, however, dictates that submicron emulsions are intrinsically metastable, thereby precluding thermodynamic control over their formation. Here, we demonstrate that bottlebrush block copolymer (BBCP) surfactants enable the spontaneous formation of stable, monodisperse water-in-oil (W/O) emulsions with programmable diameters in the submicron range. We find that the rigid BBCP surfactants can generate substantial solvation energy upon interfacial self-assembly, exceeding the increase of interfacial energy. Accordingly, we propose a revised thermodynamic model by including a surfactant-associated free-energy term. Based on the model, a new theory of spontaneous emulsification (SE) is proposed to clearly explain the phenomenon. Moreover, the rigid conformation of the bottlebrush macromolecules balances interfacial tension with intermolecular repulsion, forming an equilibrium interfacial curvature. Consequently, droplet diameter can be quantitatively programmed simply by varying the polymer architecture. This finding establishes emulsions as a previously unrecognized class of equilibrium soft-matter systems, which are highly desirable in multidisciplinary applications.