Stacking of Monolayer Graphene Particles at a Water-Vapor Interface

DM Goggin and RH Bei and R Anderson and DA Gomez-Gualdron and JR Samaniuk, JOURNAL OF PHYSICAL CHEMISTRY C, 125, 7880-7888 (2021).

DOI: 10.1021/acs.jpcc.0c11447

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.

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