Interactions between nanoparticles and cell membranes may play a crucial role in determining the cytotoxicity of nanoparticles as well as their potential application as drug delivery vehicles or therapeutic agents. It has been shown that such interactions are often determined not by biochemical but by physicochemical factors (e.g., nanoparticle size, hydrophobicity, and surface charge density). Here, we propose a mesoscale thermodynamic model describing the transitions in membrane morphology observed after exposure to various types of nanoparticles. Our simulations demonstrate under which conditions (determined by particle size and hydrophilic/hydrophobic interactions) the particles can adsorb into the membrane or compromise the membrane integrity to result in the formation of nanosized holes. The model could be refined to include a more accurate description of various phospholipid membranes, and its results could be applied in the design of specific nanoparticles for various biomedical applications.

Semicrystalline polymers exhibit metastable interphases, which must simultaneously accommodate molecular connectivity and disorder across the interlamellar phase. Off-lattice molecular simulations, previously used to study the {001} interphase in freely rotating chains, are used here to study the interlamellar phases between the {101}, {201}, and {502} crystal facets (polymer chains tilted to the lamellar normal by 19°, 34.4°, and 41°, respectively). The order-to-disorder transition from the crystal to the amorphous region occurs with an interface approximately 10−12 Å thick, for all cases studied. The interfacial potential energies for the {101}, {201}, and {502} facets are computed to be 100, 70, and 90 mJ/m2, respectively, compared to 140 mJ/m2 for the {001} facet. The topology in the interlamellar phase shifts away from tight folding as the tilt angle of the chains exiting the crystal increases. Whereas [110] loops dominate the {001} interface, loop reentry along [200] and [310] directions is more common in the interfaces with tilted chains. The chain length distributions associated with tilted chains more closely approximate the ideal distribution suggested by a model of Gaussian chains, which indicates that entropy favors tilting of polymer chains away from the lamellar normal. These results are consistent with the frequent observation of {201} oriented interfaces in polyethylene and offer a thermodynamic explanation for the selection of interface orientation in semicrystalline polyethylene.

Monte Carlo simulations of the γ-β, α-γ, and α-β phase transitions of nitrogen

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The study of structure and properties of semi‐crystalline polymer inter‐phases is important to explain and extend polymer applications. In this region, polymer chains exist in three distinct populations: tie chains that bridge the two crystals, chain folds and chain ends. The distribution of these populations influences the properties of the interphase. We have developed off‐lattice Monte Carlo simulations of constrained interphases of semi‐crystalline polymers which utilize robust off‐lattice moves. A united atom model with polyethylene‐like interactions and with freely rotating bonds is used to mimic the prototypical flexible chain structure. These simulations capture the limiting distributions of tight and loose chain folds and of tie chains within the metastable phase. The dissipation in order and density between the crystal and amorphous regions has been studied, and results for freely rotating chains indicate that the characteristic decay of anisotropy occurs in a length scale of ca. 10 Å. Simulation results for the effect of system size and molecular weight for freely rotating chains have also been investigated.