Understanding the Mechanical Properties of Ultradeformable Liposomes Using Molecular Dynamics Simulations.

Improving drug delivery efficiency to solid tumor sites is a central challenge in anticancer therapeutic research. Our previous experimental study (Guo et al., Nat. Commun. 2018, 9, 130) showed that soft, elastic liposomes had increased uptake and accumulation in cancer cells and tumors in vitro and in vivo respectively, relative to rigid particles. As a first step toward understanding how liposomes' molecular structure and composition modulates their elasticity, we performed all-atom and coarse-grained classical molecular dynamics (MD) simulations of lipid bilayers formed by mixing a long-tailed unsaturated phospholipid with a short-tailed saturated lipid with the same headgroup. The former types of phospholipids considered were 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and 1,2-dipalmitoleoyl-sn-glycero-3-phosphocholine (termed here DPMPC). The shorter saturated lipids examined were 1,2-diheptanoyl-sn-glycero-3-phosphocholine (DHPC), 1,2-didecanoyl-sn-glycero-3-phosphocholine (DDPC), 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). Several lipid concentrations and surface tensions were considered. Our results show that DOPC or DPMPC systems having 25-35 mol % of the shortest lipids DHPC or DDPC are the least rigid, having area compressibility moduli KA that are ∼10% smaller than the values observed in pure DOPC or DPMPC bilayers. These results agree with experimental measurements of the stretching modulus and lysis tension in liposomes with the same compositions. These mixed systems also have lower areas per lipid and form more uneven x-y interfaces with water, the tails of both primary and secondary lipids are more disordered, and the terminal methyl groups in the tails of the long lipid DOPC or DPMPC wriggle more in the vertical direction, compared to pure DOPC or DPMPC bilayers or their mixtures with the longer saturated lipid DLPC or DMPC. These observations confirm our hypothesis that adding increasing concentrations of the short unsaturated lipid DHPC or DDPC to DOPC or DPMPC bilayers alters lipid packing and thus makes the resulting liposomes more elastic and less rigid. No formation of lipid nanodomains was noted in our simulations, and no clear trends were observed in the lateral diffusivities of the lipids as the concentration, type of secondary lipid, and surface tension were varied.

Integration of an LPAR1 Antagonist into Liposomes Enhances Their Internalization and Tumor Accumulation in an Animal Model of Human Metastatic Breast Cancer.

Lysophosphatidic acid receptor 1 (LPAR1) is elevated in breast cancer. The deregulation of LPAR1, including the function and level of expression, is linked to cancer initiation, progression, and metastasis. LPAR1 antagonists, AM095 or Ki16425, may be effective therapeutic molecules, yet their limited water solubility hinders in vivo delivery. In this study, we report on the synthesis of two liposomal formulations incorporating AM095 or Ki16425, embedded within the lipid bilayer, as targeted nanocarriers for metastatic breast cancer (MBC). The data show that the Ki16425 liposomal formulation exhibited a 50% increase in internalization by MBC mouse epithelial cells (4T1) and a 100% increase in tumor accumulation in a mouse model of MBC compared with that of a blank liposomal formulation (control). At the same time, normal mouse epithelial cells (EpH-4Ev) internalized the Ki16425 liposomal formulation 25% lesser than the control formulation. Molecular dynamics simulations show that the integration of AM095 or Ki16425 modified the physical and mechanical properties of the lipid bilayer, making it more flexible in these liposomal formulations compared with liposomes without drug. The incorporation of an LPAR1 antagonist within a liposomal drug delivery system represents a viable therapeutic approach for targeting the LPA-LPAR1 axis, which may hinder the progression of MBC.

Molecular Dynamics Simulations of Nanostructures Formed by Hydrophobins and Oil in Seawater.

Classical molecular dynamics simulations using the Martini coarse-grained force field were performed to study oil nanodroplets surrounded by fungal hydrophobin (HP) proteins in seawater. The class I EAS and the class II HFBII HPs were studied along with two model oils, namely, benzene and n-decane. Both HPs exhibit free energy minima at the oil-seawater interface, which is deeper in benzene compared to the n-decane systems. Larger constraint forces are required to keep both HPs within the n-decane phase compared to inside benzene, with HFBII being more affine to benzene compared to EAS. Smaller surface tensions are observed at benzene-seawater interfaces coated with HPs compared to their n-decane counterparts. In the latter the surface tension remains unchanged upon increases in the concentration of HPs, whereas in benzene systems adding more HPs lead to decreases in surface tension. EAS has a larger tendency to cluster together in the interface compared to HFBII, with both HPs having larger coordination numbers when surrounding benzene droplets compared to when they are around n-decane nanoblobs. The HP-oil nanostructures in seawater examined have radii of gyration ranging between 2 and 12 nm, where the n-decane structures are larger and have more irregular shapes compared to the benzene systems. The n-decane molecules within the nanostructures form a compact spherical core, with the HPs partially covering its surface and clustering together, conferring irregular shapes to the nanostructures. The EAS with n-decane structures are larger and have more irregular shapes compared to their HFBII counterparts. In contrast, in the HP-benzene structures both HPs tend to penetrate the oil part of the droplet. The HFBII-benzene structures having the larger oil/HP ratios examined tend to be more compact and spherical compared to their EAS counterparts; however, some of the HFBII-benzene systems that have smaller oil/HP ratios have a more elongated structure compared to their EAS counterparts. This simulation study provides insights into HP-oil nanostructures that are smaller than the oil droplets and gas bubbles recently studied in experiments and, thus, might be challenging to examine with experimental techniques.

Deep Eutectic Solvents Mixed with Fluorinated Refrigerants for Absorption Refrigeration: A Molecular Simulation Study.

Molecular simulations were performed to evaluate mixtures of fluorinated refrigerants with deep eutectic solvents (DESs), for potential use in single-effect absorption refrigeration cycles that use low quality waste heat sources at temperatures of ∼80 °C. The refrigerants considered were the hydrofluorocarbon R245fa and the hydrofluoroolefins R1234zeE and HFO1336mzzE, whereas the DESs evaluated were 1:2 molar mixtures of choline chloride with either ethylene glycol (ethaline) or levulinic acid (levuline) as hydrogen bond donors (HBDs). Assuming the same cycle operating conditions, the waste heat cycle efficiency η was computed for all working fluid mixtures from molecular simulation results of the mixture densities and Henry's law constants of the refrigerants in the DESs, coupled with phase equilibrium calculations and the enthalpies of the pure refrigerants. The largest efficiency was obtained for the mixture R245fa-ethaline (η = 6.82), followed by R245fa-levuline (η = 4.64) and HFO1336mzzE-levuline (η = 2.10). These modest efficiency values could be further increased by tailoring the cycle operating conditions to each particular refrigerant-DES system, as well as optimizing our choice of working fluid mixtures, neither of which we attempted in this study. Strong interactions were observed between the chlorine anions and some of the hydrogen atoms of the refrigerants, but in general the cation-refrigerant and HBD-refrigerant interactions are weaker compared to the refrigerant-refrigerant interactions. Refrigerant molecules have the largest diffusivities and make the cations, anions and HBD to move faster compared to systems of DESs without refrigerant; in general, species in refrigerant-ethaline mixtures have larger diffusivities compared to those for refrigerant-levuline mixtures. We also computed waste heat cycle efficiencies for the same R134a-DES mixtures studied in our previous work, finding significant differences between the efficiencies determined from molecular simulation data and those determined before using the COSMO-RS approach using two standard parametrizations. This observation suggests that further work is needed to improve the accuracy of the COSMO-RS predictions for these systems.

On the Performance of Confined Deep Eutectic Solvents and Ionic Liquids for Separations of Carbon Dioxide from Methane: Molecular Dynamics Simulations.

Classical molecular dynamics simulations were used to investigate the performance of slit graphite and titania (rutile) pores of 5.2 nm in width, partially and completely filled with deep eutectic solvents (DESs) or ionic liquids (ILs), for gas separations of a carbon dioxide-methane mixture of 5:95 molar ratio and temperatures and pressures on the order of 318 K and 100 bar, respectively. The DESs studied were ethaline and levuline (1:2 molar mixtures of choline chloride with ethylene glycol or levulinic acid), and the IL considered was 1- n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [bmim+][NTf2-]. The performance of these systems in terms of solubility selectivity, diffusion selectivity, and permselectivity was compared against the performance of the bulk solvents (which could also be viewed as a model system for the micrometer-sized pores of a supported IL or DES membrane) and against carbon and rutile pores without preadsorbed solvent. The best performance in terms of permselectivity is obtained for bulk levuline and by rutile pores fully filled by ethaline, followed by graphite pores filled by ethaline and the IL. Empty rutile pores have the largest value of solubility selectivity, followed by bulk ethaline and rutile pores completely filled by the IL. The largest values of diffusivity selectivity were observed for bulk levuline, followed by ethaline completely filling a rutile nanopore and a graphite nanopore completely filled with the IL. These observations are rationalized by examining local density profiles and interaction energies among the different entities in our systems. In general, systems of nanopores fully filled by solvents, as well as the bulk solvents, have larger permselectivities than pores partially filled by the IL or the DESs. Drops of 2-3 orders of magnitude are observed in the gas diffusivity in pores filled with solvents with respect to systems of empty pores, which may be problematic if gas permeation is mainly controlled by diffusion. However, if adsorption dominates the gas permeation within the membrane, our results suggest that systems of levuline in the micrometer-sized pores of a supported DES membrane or ethaline confined in the rutile nanopores of a supported DES phase material might represent promising systems for gas separation.

Formation and elasticity of membranes of the class II hydrophobin Cerato-ulmin at oil-water interfaces.

Protein surfactants show great potential to stabilize foams, bubbles, and emulsions. An important family of surface active proteins, the hydrophobins, is secreted by filamentous fungi. Two hydrophobin classes have been recognized, with Class II exhibiting slightly better solubility than Class I, although neither is very soluble in water. Hydrophobins are small proteins (8-14 kDa), but they are larger and more rigid than typical surfactants such as sodium dodecyl sulfate. This rigidity seems to be manifested in the strength of adsorbed hydrophobin layers on oil droplets or air bubbles. A particular Class II hydrophobin, Cerato-ulmin, was characterized at the oil-water interface (the oil was squalane). The results are compared to measurements at the air-water interface, newly extended to lower Cerato-ulmin concentrations. For both oil-water and air-water interfaces, static and dynamic properties were measured during the evolution of the membrane structure. The static measurements reveal that dilute Cerato-ulmin solution efficiently decreases the interfacial tension, whether at oil-water or air-water interfaces. The reduction in surface tension requires several hours. Interfacial mechanics were characterized too, and the dilatational modulus was found to reach large values at both types of interfaces: 339 ±â€¯19 mN/m at the squalane-water interface and at least 764 ±â€¯45 mN/m at the air-water interface. Both values well exceed those typical of small-molecule surfactants, but come closer to those expected of particulate-loaded interfaces. Circular dichroism provides some insight to adsorption-induced molecular rearrangements, which seem to be more prevalent at the oil-water interface than at the air-water interface.

Computational Evaluation of Mixtures of Hydrofluorocarbons and Deep Eutectic Solvents for Absorption Refrigeration Systems.

We used computational tools to evaluate three working fluid mixtures for single-effect absorption refrigeration systems, where the generator (desorber) is powered by waste or solar heat. The mixtures studied here resulted from combining a widely used hydrofluorocarbon (HFC) refrigerant, R134a, with three common deep eutectic solvents (DESs) formed by mixing choline chloride (hydrogen bond acceptor, HBA) with urea, glycerol, or ethylene glycol as the hydrogen bond donor (HBD) species. The COSMOtherm/TmoleX software package was used in combination with refrigerant data from NIST/REFPROP, to perform a thermodynamic evaluation of absorption refrigeration cycles using the proposed working fluid mixtures. Afterward, classical MD simulations of the three mixtures were performed to gain insight on these systems at the molecular level. Larger cycle efficiencies are obtained when R134a is combined with choline chloride and ethylene glycol, followed by the system where glycerol is the HBD, and finally that where the HBD is urea. MD simulations indicate that the local density profiles of all species exhibit very sharp variations in systems containing glycerol or urea; furthermore, the Henry's law constants of R134a in these two systems are larger than those observed for the HFC in choline chloride and ethylene glycol, indicating that R134a is more soluble in the latter DES. Interaction energies indicate that the R134a-R134a interactions are weaker in the system where ethylene glycol is the HBD, as compared to in the other DES. Radial distribution functions confirm that in all systems, the DES species do not form strong directional interactions (e.g., hydrogen bonds) with the R134a molecules. Relatively strong interactions are observed between the Cl anions and the hydrogen atoms in R134a; however, the atom-atom interactions between R134a and the cation and HBD species are weaker and do not play a significant role in the solvation of the refrigerant. In all systems, R134a has the largest diffusion coefficients, followed by the HBD, the anion and the cation; the diffusion coefficients are the largest in the systems containing ethylene glycol, followed by those having glycerol and urea. This work is our first step toward our long-term goal of designing and demonstrating optimal working fluid mixtures for use in absorption refrigeration systems. Our results suggest that COSMO-RS can be used to perform a rapid screening of a large number of working fluid mixtures, and select a few candidates for further exploration using molecular simulations and experiments. These latter approaches can be used to refine the accuracy of the COSMO-RS predictions, and to optimize the selection of optimal working fluid mixtures for demonstration in absorption refrigeration systems powered by solar or waste heat sources.

Combined Experimental and Molecular Simulation Investigation of the Individual Effects of Corexit Surfactants on the Aerosolization of Oil Spill Matter.

We report laboratory aerosolization experiments and classical molecular dynamics (MD) simulations, with the objective of investigating the individual effects of the two Corexit surfactants Span 80 (nonionic) and dioctyl sodium sulfosuccinate (DOSS, ionic), on the aerosolization of oil spill matter to the atmosphere. Our simulation results show that Span 80, DOSS, and the oil alkanes n-pentadecane (C15) and n-triacontane (C30) exhibit deep free energy minima at the air/seawater interface. C15 and C30 exhibit deeper free energy minima at the interface when Span 80 is present, as compared to the situation when DOSS or no surfactants are at the interface. These results suggest that Span 80 makes these oil hydrocarbons more likely to be adsorbed at the surface of seawater droplets and carried out to the atmosphere, relative to DOSS or to the situation where no surfactants are present. These simulation trends are in qualitative agreement with our experimental observations in a bubble-column setup, where larger amounts of oil hydrocarbons are ejected when Span 80 is mixed with oil and injected into the column, as compared to when DOSS is used. Our simulations also indicate that Span 80 has a larger thermodynamic incentive than DOSS to move from the seawater phase and into the air/seawater interface. This observation is also in qualitative agreement with our experimental measurements, which indicate that Span 80 is ejected in larger quantities than DOSS. Our simulations also suggest that DOSS predominantly adopts a perpendicular orientation with respect to the air/seawater interface at a dispersant to oil ratio (DOR) of 1:20, but has a slight preference to lie parallel to the interfaces at a DOR = 1:5; in both cases, DOSS molecules have their tails wide open and stretched. In contrast, Span 80 has a slight preference to align parallel to the interfaces with a coiled conformation at both DOR values.

Heterogeneous nucleation from a supercooled ionic liquid on a carbon surface.

Classical molecular dynamics simulations were used to study the nucleation of the crystal phase of the ionic liquid [dmim+][Cl-] from its supercooled liquid phase, both in the bulk and in contact with a graphitic surface of D = 3 nm. By combining the string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)], with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [Santiso and Trout, J. Chem. Phys. 134, 064109 (2011)], we computed minimum free energy paths, the approximate size of the critical nucleus, the free energy barrier, and the rates involved in these nucleation processes. For homogeneous nucleation, the subcooled liquid phase has to overcome a free energy barrier of ∼85 kcal/mol to form a critical nucleus of size ∼3.6 nm, which then grows into the monoclinic crystal phase. This free energy barrier becomes about 42% smaller (∼49 kcal/mol) when the subcooled liquid phase is in contact with a graphitic disk, and the critical nucleus formed is about 17% smaller (∼3.0 nm) than the one observed for homogeneous nucleation. The crystal formed in the heterogeneous nucleation scenario has a structure that is similar to that of the bulk crystal, with the exception of the layers of ions next to the graphene surface, which have larger local density and the cations lie with their imidazolium rings parallel to the graphitic surface. The critical nucleus forms near the graphene surface separated only by these layers of ions. The heterogeneous nucleation rate (∼4.8 × 1011 cm-3 s-1) is about one order of magnitude faster than the homogeneous rate (∼6.6 × 1010 cm-3 s-1). The computed free energy barriers and nucleation rates are in reasonable agreement with experimental and simulation values obtained for the homogeneous and heterogeneous nucleation of other systems (ice, urea, Lennard-Jones spheres, and oxide glasses).

Molecular simulation of homogeneous nucleation of crystals of an ionic liquid from the melt.

The homogeneous nucleation of crystals of the ionic liquid [dmim(+)][Cl(-)] from its supercooled liquid phase in the bulk (P = 1 bar, T = 340 K, representing a supercooling of 58 K) was studied using molecular simulations. The string method in collective variables [Maragliano et al., J. Chem. Phys. 125, 024106 (2006)] was used in combination with Markovian milestoning with Voronoi tessellations [Maragliano et al., J. Chem. Theory Comput. 5, 2589-2594 (2009)] and order parameters for molecular crystals [E. E. Santiso and B. L. Trout, J. Chem. Phys. 134, 064109 (2011)] to sketch a minimum free energy path connecting the supercooled liquid and the monoclinic crystal phases, and to determine the free energy and the rates involved in the homogeneous nucleation process. The physical significance of the configurations found along this minimum free energy path is discussed with the help of calculations based on classical nucleation theory and with additional simulation results obtained for a larger system. Our results indicate that, at a supercooling of 58 K, the liquid has to overcome a free energy barrier of the order of 60 kcal/mol and to form a critical nucleus with an average size of about 3.6 nm, before it reaches the thermodynamically stable crystal phase. A simulated homogeneous nucleation rate of 5.0 × 10(10) cm(-3) s(-1) was obtained for our system, which is in reasonable agreement with experimental and simulation rates for homogeneous nucleation of ice at similar degrees of supercooling. This study represents our first step in a series of studies aimed at understanding the nucleation and growth of crystals of organic salts near surfaces and inside nanopores.

Bubble bursting as an aerosol generation mechanism during an oil spill in the deep-sea environment: laboratory experimental demonstration of the transport pathway.

Oil spills in the deep-sea environment such as the 2010 Deep Water Horizon oil spill in the Gulf of Mexico release vast quantities of crude oil into the sea-surface environment. Various investigators have discussed the marine transport and fate of the oil into different environmental compartments (air, water, sediment, and biota). The transport of the oil into the atmosphere in these previous investigations has been limited to only evaporation, a volatility dependent pathway. In this work, we studied the aerosolization of oil spill matter via bursting bubbles as they occur during whitecaps in a laboratory aerosolization reactor. By evaluating the alkane content in oil mousse, crude oil, the gas phase, and particulate matter we clearly demonstrate that aerosolization via bursting bubbles is a solubility and volatility independent transport pathway for alkanes. The signature of alkane fractions in the native oil and aerosolized matter matched well especially for the less volatile alkanes (C20-C29). Scanning electron microscope interfaced with energy dispersive X-ray images identified the carbon fractions associated with salt particles of aerosols. Theoretical molecular dynamics simulations in the accompanying paper lend support to the observed propensity for alkanes at air-salt water interfaces of breaking bubbles and the produced droplets. The presence of a dispersant in the aqueous phase increased the oil ejection rate at the surface especially for the C20-C29 alkanes. The information presented here emphasizes the need to further study sea-spray aerosols as a possible transport vector for spilled oil in the sea surface environment.

Bubble bursting as an aerosol generation mechanism during an oil spill in the deep-sea environment: molecular dynamics simulations of oil alkanes and dispersants in atmospheric air/salt water interfaces.

Potential of mean force (PMF) calculations and molecular dynamics (MD) simulations were performed to investigate the properties of oil n-alkanes [i.e., n-pentadecane (C15), n-icosane (C20) and n-triacontane (C30)], as well as several surfactant species [i.e., the standard anionic surfactant sodium dodecyl sulfate (SDS), and three model dispersants similar to the Tween and Span species present in Corexit 9500A] at air/salt water interfaces. This study was motivated by the 2010 Deepwater Horizon (DWH) oil spill, and our simulation results show that, from the thermodynamic point of view, the n-alkanes and the model dispersants have a strong preference to remain at the air/salt water interface, as indicated by the presence of deep free energy minima at these interfaces. The free energy minimum of these n-alkanes becomes deeper as their chain length increases, and as the concentration of surfactant species at the interface increases. The n-alkanes tend to adopt a flat orientation and form aggregates at the bare air/salt water interface. When this interface is coated with surfactants, the n-alkanes tend to adopt more tilted orientations with respect to the vector normal to the interface. These simulation results are consistent with the experimental findings reported in the accompanying paper [Ehrenhauser et al., Environ. Sci.: Processes Impacts 2013, in press, (DOI: 10.1039/c3em00390f)]. The fact that these long-chain n-alkanes show a strong thermodynamic preference to remain at the air/salt water interfaces, especially if these interfaces are coated with surfactants, makes these species very likely to adsorb at the surface of bubbles or droplets and be ejected to the atmosphere by sea surface processes such as whitecaps (breaking waves) and bubble bursting. Finally, the experimental finding that more oil hydrocarbons are ejected when Corexit 9500A is present in the system is consistent with the deeper free energy minima observed for the n-alkanes at the air/salt water interface at increasing concentrations of surfactant species.

Molecular dynamics simulations of the ionic liquid [EMIM+][TFMSI-] confined inside rutile (110) slit nanopores.

The structure and dynamics of the ionic liquid (IL) [EMIM(+)][TFMSI(-)] inside a rutile (110) slit nanopore of width H = 5.2 nm at T = 333 K are studied using classical molecular dynamics (MD) simulations. These results are compared against those obtained in our previous study (N. N. Rajput et al., J. Phys. Chem. C, 2012, 116, 5169-5181) for the same IL inside a slit graphitic nanopore of the same width. Electrostatic and dispersion interactions are present between the IL and the rutile walls, whereas only weaker van der Waals interactions are present between the IL and the graphitic walls. Our results suggest that the strength of the interactions between the pore walls and the IL can significantly affect the structure and dynamics of the confined IL. Layering effects were more pronounced for the IL inside a rutile pore as compared to inside a graphitic pore. The ions near the rutile pore walls had a liquid structure that was significantly different from that of the bulk IL; in contrast, the same ions near graphitic pore walls had a liquid structure that was similar to that of the bulk IL. Cations and anions adopted multiple orientations near the rutile walls, which contrast with the parallel orientations that were uniformly observed for the same ions near graphitic walls. The dynamics of [EMIM(+)][TFMSI(-)] inside a slit rutile pore are significantly slower than those observed inside a slit graphitic pore. Near the rutile walls, the dynamics of the ions were about an order of magnitude slower than those of ions near graphitic walls. The ions in the center of a rutile pore exhibit enhanced mobilities, but still about 2-4 times slower than those observed for ions in the center of a graphitic pore. The effects of variations in the amount of IL on the dynamics were very marked inside a rutile pore, with reductions of up to 4 times in the mobilities of the ions in the different regions of the pore; in contrast, pore loading seems to cause smaller variations in the dynamics of ILs inside a graphitic slit nanopore.

Molecular modeling of the green leaf volatile methyl salicylate on atmospheric air/water interfaces.

Methyl salicylate (MeSA) is a green leaf volatile (GLV) compound that is emitted in significant amounts by plants, especially when they are under stress conditions. GLVs can then undergo chemical reactions with atmospheric oxidants, yielding compounds that contribute to the formation of secondary organic aerosols (SOAs). We investigated the adsorption of MeSA on atmospheric air/water interfaces at 298 K using thermodynamic integration (TI), potential of mean force (PMF) calculations, and classical molecular dynamics (MD) simulations. Our molecular models can reproduce experimental results of the 1-octanol/water partition coefficient of MeSA. A deep free energy minimum was found for MeSA at the air/water interface, which is mainly driven by energetic interactions between MeSA and water. At the interface, the oxygenated groups in MeSA tend to point toward the water side of the interface, with the aromatic group of MeSA lying farther away from water. Increases in the concentrations of MeSA lead to reductions in the height of the peaks in the MeSA-MeSA g(r) functions, a slowing down of the dynamics of both MeSA and water at the interface, and a reduction in the interfacial surface tension. Our results indicate that MeSA has a strong thermodynamic preference to remain at the air/water interface, and thus chemical reactions with atmospheric oxidants are more likely to take place at this interface, rather than in the water phase of atmospheric water droplets or in the gas phase.

Molecular simulations of green leaf volatiles and atmospheric oxidants on air/water interfaces.

Green leaf volatiles (GLVs) are oxygenated hydrocarbons that are emitted by plants, especially under stress conditions such as mechanical damage and local weather changes. GLVs can react with photochemically-generated oxidants (e.g., OH radicals) in atmospheric water drops, and contribute to the formation of secondary organic aerosols (SOAs). Here we investigated the adsorption of a gas phase GLV, 2-methyl-3-buten-2-ol (MBO) and OH radicals on atmospheric air/water interfaces using classical molecular dynamics (MD) simulations and potential of mean force (PMF) calculations. Our models can reproduce experimental values of the free energy of hydration of MBO and ˙OH, as well as 1-octanol/water partition coefficients of MBO determined experimentally in this study. Both MBO and ˙OH have a strong thermodynamic incentive to remain at the air/water interface, with their density profiles overlapping significantly at the interface. These results suggest that chemical reactions between MBO and ˙OH are more likely to take place at the interface, rather than inside the bulk of water droplets or in the vapor phase. We found a significant number of contacts between MBO and ˙OH in our simulations, which could lead to reactions between these two species.

Ice growth from supercooled aqueous solutions of benzene, naphthalene, and phenanthrene.

Classical molecular dynamics (MD) were performed to investigate the growth of ice from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene. The main objective of this study is to explore the fate of those aromatic molecules after freezing of the supercooled aqueous solutions, i.e., if these molecules become trapped inside the ice lattice or if they are displaced to the QLL or to the interface with air. Ice growth from supercooled aqueous solutions of benzene, naphthalene, or phenanthrene result in the formation of quasi-liquid layers (QLLs) at the air/ice interface that are thicker than those observed when pure supercooled water freezes. Naphthalene and phenanthrene molecules in the supercooled aqueous solutions are displaced to the air/ice interface during the freezing process at both 270 and 260 K; no incorporation of these aromatics into the ice lattice is observed throughout the freezing process. Similar trends were observed during freezing of supercooled aqueous solutions of benzene at 270 K. In contrast, a fraction of the benzene molecules become trapped inside the ice lattice during the freezing process at 260 K, with the rest of the benzene molecules being displaced to the air/ice interface. These results suggest that the size of the aromatic molecule in the supercooled aqueous solution is an important parameter in determining whether these molecules become trapped inside the ice crystals. Finally, we also report potential of mean force (PMF) calculations aimed at studying the adsorption of gas-phase benzene and phenanthrene on atmospheric air/ice interfaces. Our PMF calculations indicate the presence of deep free energy minima for both benzene and phenanthrene at the air/ice interface, with these molecules adopting a flat orientation at the air/ice interface.

Adsorption of naphthalene and ozone on atmospheric air/ice interfaces coated with surfactants: a molecular simulation study.

The adsorption of gas-phase naphthalene and ozone molecules onto air/ice interfaces coated with different surfactant species (1-octanol, 1-hexadecanol, or 1-octanal) was investigated using classical molecular dynamics (MD) simulations. Naphthalene and ozone exhibit a strong preference to be adsorbed at the surfactant-coated air/ice interfaces, as opposed to either being dissolved into the bulk of the quasi-liquid layer (QLL) or being incorporated into the ice crystals. The QLL becomes thinner when the air/ice interface is coated with surfactant molecules. The adsorption of both naphthalene and ozone onto surfactant-coated air/ice interfaces is enhanced when compared to bare air/ice interface. Both naphthalene and ozone tend to stay dissolved in the surfactant layer and close to the QLL, rather than adsorbing on top of the surfactant molecules and close to the air region of our systems. Surfactants prefer to orient at a tilted angle with respect to the air/ice interface; the angular distribution and the most preferred angle vary depending on the hydrophilic end group, the length of the hydrophobic tail, and the surfactant concentration at the air/ice interface. Naphthalene prefers to have a flat orientation on the surfactant coated air/ice interface, except at high concentrations of 1-hexadecanol at the air/ice interface; the angular distribution of naphthalene depends on the specific surfactant and its concentration at the air/ice interface. The dynamics of naphthalene molecules at the surfactant-coated air/ice interface slow down as compared to those observed at bare air/ice interfaces. The presence of surfactants does not seem to affect the self-association of naphthalene molecules at the air/ice interface, at least for the specific surfactants and the range of concentrations considered in this study.

Molecular simulation study of the adsorption of naphthalene and ozone on atmospheric air/ice interfaces.

The adsorption of gas-phase naphthalene and ozone on atmospheric air/ice interfaces was investigated using classical molecular dynamics (MD) simulations and potential of mean force (PMF) calculations. Naphthalene and ozone exhibit a strong preference to be adsorbed at the air/ice interface, rather than being dissolved into the bulk of the quasi-liquid layer (QLL) or incorporated into the ice crystals. When the air/ice interface is coated with increasing concentrations of naphthalene molecules, the QLL becomes thinner and surface adsorption of ozone is enhanced. Furthermore, ozone tends to adsorb on top of the naphthalene film, although significant penetration of ozone into this film is also observed. Naphthalene molecules tend to adopt a flat orientation on the air/ice interface. Less variation in the orientation was observed for lower concentrations of naphthalene, whereas variations in the ozone concentration do not affect the orientation of naphthalene molecules. However, as the concentration of ozone increases, most of the naphthalene molecules still prefer to stay close to the mobile water molecules in the QLL, but a significant fraction of the naphthalene molecules spends a considerable amount of time inside the thicker layer of ozone. We also monitored the number of contacts between naphthalene and ozone at the air/ice interface upon variations in the concentrations of these two species. These contacts were assumed to be proportional to the reaction rate between these two species. When the number of ozone molecules was held constant, the number of contacts showed a linear relationship to the number of naphthalene molecules. However, when the naphthalene concentration was held constant, for all systems we observed a linear relationship at low ozone concentrations and a plateau at high ozone concentrations.

Nontemplated approach to tuning the spectral properties of cyanine-based fluorescent nanoGUMBOS.

Template-free controlled aggregation and spectral properties in fluorescent organic nanoparticles (FONs) is highly desirable for various applications. Herein, we report a nontemplated method for controlling the aggregation in near-infrared (NIR) cyanine-based nanoparticles derived from a group of uniform materials based on organic salts (GUMBOS). Cationic heptamethine cyanine dye 1,1',3,3,3',3'-hexamethylindotricarbocyanine (HMT) was coupled with five different anions, viz., [NTf(2)(-)], [BETI(-)], [TFPB(-)], [AOT(-)], and [TFP4B(-)], by an ion-exchange method to obtain the respective GUMBOS. The nanoGUMBOS obtained via a reprecipitation method were primarily amorphous and spherical (30-100 nm) as suggested by selected area electron diffraction (SAED) and transmission electron microscopy (TEM). The formation of tunable self-assemblies within the nanoGUMBOS was characterized using absorption and fluorescence spectroscopy in conjunction with molecular dynamics simulations. Counterion-controlled spectral properties observed in the nanoGUMBOS were attributed to variations in J/H ratios with different anions. Association with the [AOT(-)] anion afforded predominant J aggregation enabling the highest fluorescence intensity, whereas [TFP4B(-)] disabled the fluorescence due to predominant H aggregation in the nanoparticles. Analyses of the stacking angle of the cations based on molecular dynamic simulation results in [HMT][NTf(2)], [HMT][BETI], and [HMT][AOT] dispersed in water and a visual analysis of the representative simulation snapshots also imply that the type of aggregation was controlled through the counterion associated with the dye cation.

Quadrupolar particles in a nematic liquid crystal: effects of particle size and shape.

We investigate the effects of particle size and shape on the quadrupolar (Saturn-ring-like) defect structures formed by a nematic liquid crystal around nm-sized and mum -sized particles with spherical and spherocylindrical shapes. We also report results for the potentials of mean force in our systems, calculated using a mesoscale theory for the tensor order parameter Q of the nematic. Our results indicate that for pairs of nm-sized particles in close proximity, the nematic forms "entangled hyperbolic" defect structures regardless of the shape of the nanoparticles. In our calculations with nanoparticles we did not observe any other entangled or unentangled defect structures, in contrast to what was reported for pairs of mum -sized spherical particles. Such a finding suggests that the "entangled hyperbolic" defect structures are the most stable for pairs of nanoparticles in close proximity. For pairs of mum -sized particles, our results indicate that the nematic forms entangled "figure-of-eight" defect structures around pairs of spheres and spherocylinders. Our results suggest that the transition between "entangled hyperbolic" and figure-of-eight defect structures takes place when the diameter of the particle is between D=100 nm and 1 microm . We have also calculated the torques that develop when pairs of spherocylindrical nanoparticles in a nematic approach each other. Our calculations suggest that the nematic-mediated interactions between the nm-sized particles are fairly strong, up to 5700 k{B}T for the case of pairs of spherocylindrical nanoparticles arranged with their long axis parallel to each other. Furthermore, these interactions can make the particles to bind together at specific locations, and thus could be used to assemble the particles into ordered structures with different morphologies.