Stacking of Monolayer Graphene Particles at a Water–Vapor Interface

Abstract

Two-dimensional (2D) materials such as graphene prefer to interact in a face-to-face manner when colloidally suspended but are forced to interact in an edge-to-edge manner when trapped at a fluid−fluid interface. However, molecular dynamics (MD) simulations suggest these platelet-like particles can spontaneously stack and adopt the preferred face-to-face orientation after lateral edge-to-edge assembly, while experiments tend to contradict these findings. Thus, conditions under which these stacking events occur are unknown. Herein, MD simulations are employed to elucidate the physical origin of the free-energy barrier inhibiting instantaneous particle stacking: the surface energy penalty associated with deforming a fluid−fluid interface. Simulations suggest stacking kinetics are governed by a Boltzmann-like relation between the time to stack and the particle−particle contact edge length, and thus, the interfacial area deformed. A thermodynamic model is also shown to predict the change in excess interfacial free-energy as particles transition from the laterally aggregated to vertically stacked state at a fluid interface. Finally, experimental evidence is presented that corroborates these results. These results suggest that the existence of nanometer-scale edge defects is expected to influence the stacking behavior of 2D particles at fluid interfaces, which has broad, practical implications spanning from emulsion stability to the integrity of Langmuir film morphology.

Adsorption of Single and Multiple Graphene-Oxide Nanoparticles at a Water–Vapor Interface

“…Understanding the dynamics of particle agglomeration and stacking is crucial as it can explain certain differences between numerical and experimental observations. For instance, in the case of pristine graphene, the typical stacking time for particles trapped at a water-vapor interface was found to increase exponentially with the contact edge length of the particles [41] . Therefore, stacking is more likely to be observed with nanometric particles during molecular dynamics simulations, than it is during experiments that typically involve micrometric particles [41,49] .…”

“…23 For GO particles with a low or a zero degree of basal oxidation (respectively, red disks and black triangles in Figure 8), nonlinearities of ΔE ads as a function of N are found, with the adsorption of the test particle being the less favorable for N = 2 and the most favorable for largest values of N. We link these nonlinearities with the tendency of pristine graphene to form clusters in bulk water as well as at the interface between water and vapor. 41 By forming clusters, graphene particles reduce the contact area with water while increasing the more energetically favorable graphene−graphene contact. 42,43 However, such a simple picture does not account for minimums of |ΔE ads | for N = 2.…”

“…For instance, in the case of pristine graphene, the typical stacking time for particles trapped at a water−vapor interface was found to increase exponentially with the contact edge length of the particles. 41 Therefore, stacking is more likely to be observed with nanometric particles during molecular dynamics simulations than it is during experiments that typically involve micrometric particles. 41,49 In the case of GO, oxygen groups at the edges and basal plane of the particles are known to affect the particle−particle energy of interactions, 50 and therefore, their presence is expected to lead to a different stacking time compared to the one measured with pristine graphene.…”

Adsorption of Single and Multiple Graphene-Oxide Nanoparticles at a WaterVapor Interface

“…8), non-linearities of ∆E ads as a function of N are found, with the adsorption of the test particle being the less favorable for N = 2, and the most favorable for largest values of N . We link these non-linearities with the tendency of pristine graphene to form clusters in bulk water as well as at the interface between water and vapor [41] . By forming clusters, graphene particles reduce the contact area with water, while increasing the more energetically favorable graphene-graphene contact.…”

“…For instance, in the case of pristine graphene, the typical stacking time for particles trapped at a water-vapor interface was found to increase exponentially with the contact edge length of the particles [41] . Therefore, stacking is more likely to be observed with nanometric particles during molecular dynamics simulations, than it is during experiments that typically involve micrometric particles [41,49] . In the case of GO, oxygen groups at the edges and basal plane of the particles are known to affect the particle-particle energy of…”