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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As novel materials for food contact packaging, inorganic silicon oxide (SiO) films are high barrier property materials that have been developed rapidly and have attracted the attention of many manufacturers. For the safe use of SiO films for food packaging it is vital to study the interaction between SiO layers and food contaminants, as well as the function of a SiO barrier layer in antioxidant migration resistance. In this study, we deposited a SiO layer on polylactic acid (PLA)-based films to prepare SiO/PLA coated films by plasma-enhanced chemical vapour deposition. Additionally, we compared PLA-based films and SiO/PLA coated films in terms of the migration of different antioxidants (e.g. t-butylhydroquinone [TBHQ], butylated hydroxyanisole [BHA], and butylated hydroxytoluene [BHT]) via specific migration experiments and then investigated the effects of a SiO layer on antioxidant migration under different conditions. The results indicate that antioxidant migration from SiO/PLA coated films is similar to that for PLA-based films: with increase of temperature, decrease of food simulant polarity, and increase of single-sided contact time, the antioxidant migration rate and amount in SiO/PLA coated films increase. The SiO barrier layer significantly reduced the amount of migration of antioxidants with small and similar molecular weights and similar physical and chemical properties, while the degree of migration blocking was not significantly different among the studied antioxidants. However, the migration was affected by temperature and food simulant. Depending on the food simulants considered, the migration amount in SiO/PLA coated films was reduced compared with that in PLA-based films by 42-46%, 44-47%, and 44-46% for TBHQ, BHA, and BHT, respectively.

Cationic amphiphilic polymers are often used to coat nanoparticles as they increase chemical stability in solution and exhibit membrane disruption activities. Among these, poly­(oxonorbornenes) (PONs) are tunable membrane disruptors. They can be constructed with either one amine-terminated side chain and one hydrophobic alkyl side chain (PON-50) or two amine-terminated side chains (PON-100) on each repeat unit and can then be conjugated to gold nanoparticles using O-(2-carboxyethyl)-O′-(2-mercaptoethyl) heptaethylene glycol (HEG) spacers. While the amine content and membrane disruption activity of PONs can be controlled, the detailed structural properties of PONs conjugated to gold nanoparticles remain less understood. To address this, we performed molecular dynamics simulations of PON-50 and PON-100 to determine the nonbonded energies of PON structures as a function of amine composition. We found increasing energetic stabilization with decreasing amine composition. These results were consistent with experimental observations obtained with X-ray photoelectron spectroscopy (XPS) in which PON-100 was found to have the lowest conjugation efficiency to gold surfaces out of a range of PON amination ratios. Computationally obtained energetics suggest that replacing the aliphatic amine groups with aromatic amine groups can reverse this behavior and lead to more stable PON structures with increasing amine content. We also found that the curvature of the gold nanoparticle surface affects interactions between the surface and the amine groups of PON-50. Increasing curvature decreased these interactions, resulting in a smaller effective footprint of the HEG-PON-50 structure.

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