Thermodynamics of structured liquids*

The assembly of nanoparticles at a liquid-liquid interface results in so-called structured liquids:

liquid droplets with solid-like properties. The origins of the solid-like response of these droplets is

rooted in the membrane-like elasticity of the interface that is generated by the nanoparticle assembly.

These materials have often been assumed to be out of equilibrium, with the sluggish dynamics of the

interfacial nanoparticles resulting in a “jammed” interface. Here, we explore the thermodynamics

of these interfacial assemblies by systematically including the effects of interfacial elasticity in the

thermodynamics of liquid and nanoparticle mixtures. Our findings indicate that elasticity acts as a

stabilizing force against emulsification. While the effective surface tension dominates the stability

criteria of large droplets, elasticity plays a significant role with decreasing droplet size and, in the

event of emulsification, can radically increase the size of the resulting microdroplets. In addition to

our theoretical analysis, we conduct molecular dynamics simulations to extract the elastic properties

of the interface. Our findings suggest that structured liquids may in fact be thermodynamically

stable states rather than arrested nonequilibrium materials.