Impact of Surface Amphiphilicity on the Interfacial Behavior of Janus Particle Layers under Compression

Abstract

Langmuir monolayers of silica/gold Janus particles with two different degrees of amphiphilicity have been examined to study the significance of particle surface amphiphilicity on the structure and mechanical properties of the interfacial layers. The response of the layers to the applied compression provides insight into the nature and strength of the interparticle interactions. Different collapse modes observed for the interfacial layers are linked to the amphiphilicity of Janus particles and their configuration at the interface. Molecular dynamics simulations on nanoparticles with similar contact angles provide insight on the arrangement of particles at the interface and support our conclusion that the interfacial configuration and collapse of anisotropic particles at the air/water interface are controlled by particle amphiphilicity.

Janus Particles at Fluid Interfaces: Stability and Interfacial Rheology

“…The factors that determine the interfacial positioning of the particle are thus the fluid/fluid interfacial tension and the surface tension of the particle with both fluid phases and [ 82 , 107 , 108 ]. Therefore, from a thermodynamic standpoint, particles spontaneously adsorb on a fluid/fluid interface, provided that the surface energy between the two fluids is greater than the difference between the surface energies of the particle with each fluid phase [ 109 ].…”

“…The degree of amphiphilicity, , for a Janus particle is defined as . The surface boundary partitioning the polar and apolar faces is indicated by the angle ; values of or correspond to a homogenous particle, whereas refers to a Janus particle with two equal-sized patches of different wettability, as depicted in Figure 2 a [ 107 ].…”

“…Inspired by the potential of Janus particles at fluid interfaces and the tunability of their behavior, research has boomed in this area with applications spanning from enhanced oil recovery to bi-phasic catalytic reactions [ 6 , 89 , 90 , 118 , 119 ]. However, the rotational behavior of Janus particles cannot be neglected as the particle stability is also influenced by its orientation at the interface [ 78 , 107 , 120 , 121 , 122 , 123 , 124 ]. Monte Carlo simulations performed by Bon and Cheung illustrated that neglecting the Janus nanoparticle rotation at the interface significantly overestimates the detachment energy and thus the orientational freedom of the particle must be considered [ 125 ].…”

“…For instance, hydrophilic colloidal silica particles, which are negatively charged at neutral pH due to dissociation of surface silanol groups, are more likely to fall through the air/water interface upon deposition at the interface given that the interface carries negative charges [ 214 ]; the presence of electrolyte in the sub-phase is shown to enhance the entrapment efficacy of particles in this case and the mechanical properties of the resulting interfacial layer [ 103 , 215 ]. Variation in binding efficacy has also been reported for Janus particles with different degrees of amphiphilicity [ 107 ].…”

“…While these studies suggest that the dilational moduli of interfacial layers are enhanced by employing a Janus character, Razavi et al [ 107 ] further investigated the role of Janus character, specifically the impact of the particle’s degree of amphiphilicity ( ), in dilational properties of particle-laden interfaces. Two sets of Janus particles were studied: μm silica particles capped with a 20 nm-thick layer of gold (low-amphiphilicity, ) and Janus particles with a thiolated gold cap (high-amphiphilicity, ).…”