Nanoparticles at fluid interfaces

Abstract

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.

Spatial Distribution of Nanocrystals Imaged at the Liquid-Air Interface

“…3(b), a line tension of À5 Â 10 À12 N is already sufficient to change the adsorption free energy minimum from À3k B T to À9k B T. This value is close to that reported by Wi et al [11] and well within the range of 10 À12 to 10 À6 N reported experimentally [31]. For models that include a line tension contribution, a discontinuous wetting transition is predicted [31], which can explain the sudden transition from adsorption to desorption of the nanoparticles when the solvent is changed from 20% to 25% pentanol. A more systematic study of the dependence of the adsorption energy on particle size would be necessary to determine the magnitude of the line tension.…”

“…The effects of line tension and net charge on the nanocrystals will now be briefly discussed. In contrast to microparticles, nanoparticles are expected to exhibit much stronger line tension effects, which cause an additional contribution to the adsorption energy ÁF ðhÞ ¼ ðR 2 À h 2 Þ 1=2 [31]. As shown in Fig.…”

Contact angle and adsorption energies of nanoparticles at the air–liquid interface determined by neutron reflectivity and molecular dynamics

“…It was found that generally the line tension has a minor impact on the contact angles of particles with radius larger than ∼1 nm. 31 In this report we present NR experiments combined with the contrast variation method to in situ measure the contact angle of NPs deposited on a Langmuir-Blodgett trough at close-packed configuration and at the air-water interface. We present a new approach that enables the determination of contact angles, adsorption energies and interfacial energy differences of core-shell NPs.…”

How stable are amphiphilic dendrimers at the liquid–liquid interface?

“…16 Using molecular simulations it has been shown that macroscopic treatments can accurately predict adsorption strengths for particles as small as ~5nm, however, studies have revealed that macroscopic theories can fail in predicting the strength of interactions for smaller particles and the range of the nanoparticle/interface interactions. 17--20 For example, monitoring the free energy profile of nanoparticles at liquid interfaces, simulations have shown that the interaction between the nanoparticle and the interface (for rigid uniform spherical nanoparticles) was both longer ranged and softer than predicted by theory 17,18 due to broadening of the interface by capillary waves.…”