Nanoparticles at fluid interfaces

Nanoparticles at fluid interfaces are becoming a central topic in colloid science studies. Unlike in the case of colloids in suspensions, the description of the forces determining the physical behavior of colloids at interfaces still represents an outstanding problem in the modern theory of colloidal interactions. These forces regulate the formation of complex two-dimensional structures, which can be exploited in a number of applications of technological interest; optical devices, catalysis, molecular electronics or emulsions stabilization. From a fundamental viewpoint and typical for colloidal systems, nanoparticles and microparticles at interfaces are ideal experimental and theoretical models for investigating questions of relevance in condensed matter physics, such as the phase behavior of two-dimensional fluids. This review is a topical survey of the stability, self-assembly behavior and mutual interactions of nanoparticles at fluid interfaces. Thermodynamic models offer an intuitive approach to explaining the interfacial stability of nanoparticles in terms of a few material properties, such as the surface and line tensions. A critical discussion of the theoretical basis, accuracy, limitations, and recent predictions of the thermodynamic models is provided. We also review recent work concerned with nanoparticle self-assembly at fluid interfaces. Complex two-dimensional structures varying considerably with the particle nature have been observed in a number of experiments. We discuss the self-assembly behavior in terms of nanoparticle composition, focusing on sterically stabilized, charged and magnetic nanoparticles. The structure of the two-dimensional assemblies is a reflection of complex intercolloidal forces. Unlike the case for bulk colloidal suspensions, which often can be described reasonably well using DLVO (Derjaguin-Landau-Verwey-Overbeek) theory, the description of particles at interfaces requires the consideration of interfacial deformations as well as interfacial thermal fluctuations. We analyze the importance of both deformation and fluctuations, as well as the modification of electrostatic and van der Waals interactions. Finally, we discuss possible future directions in the field of nanoparticles at interfaces.