Modulation of Interfacial Thermal Transport between Fumed Silica Nanoparticles by Surface Chemical Functionalization for Advanced Thermal Insulation.

Since solid-state heat transport in a highly porous nanocomposite strongly depends on the thermal boundary conductance (TBC) between constituent nanomaterials, further suppression of the TBC is important for improving performance of thermal insulators. Here, targeting a nanocomposite fabricated by stamping fumed silica nanoparticles, we perform a wide variety of surface functionalizations on fumed silica nanoparticles by a silane coupling method and investigate the impact on the thermal conductivity (Km). The Km of the silica nanocomposite is approximately 20 and 9 mW/m/K under atmospheric and vacuum conditions at the material density of 0.2 g/cm3 without surface functionalization, respectively, and the experimental results indicate that the Km can be modulated depending on the chemical structure of molecules. The surface modification with a linear alkyl chain of optimal length significantly suppresses Km by approximately 30%, and the suppression can be further enhanced to approximately 50% with an infrared opacifier. The magnitude of suppression was found to sensitively depend on the length of the terminal chain. The magnitude is also related to the number of reactive silanol groups in the chemical structure, where the surface modification with fluorocarbon gives the largest suppression. The surface hydrophobization merits thermal insulation through significant suppression of the TBC, presumably by reducing the water molecules that otherwise would serve as heat conduction channels at the interface. On the other hand, when the chain length is long, the suppression is counteracted by the enhanced phonon transmission through the silane coupling molecules that grow with the chain length. This is supported by the analytical model and present simulation results, leading to prediction of the optimal chemical structure for better thermal insulation.

Augmenting the Carbon Dioxide Uptake and Selectivity of Metal-Organic Frameworks by Metal Substitution: Molecular Simulations of LMOF-202.

Metal organic frameworks (MOFs) are promising porous materials for the adsorption of CO2. Here, we report the study of a luminescent MOF (LMOF), called LMOF-202. We have employed Grand Canonical Monte Carlo (GCMC) simulations to understand and explain the adsorption phenomena inside LMOF-202, and based on the phenomena happening at the molecular level, we have varied the metal ions in LMOF-202 to increase the CO2 affinity and selectivity of the material. We show that the CO2 adsorption capacity and selectivity can be increased by approximately 1.5 times at 1 bar and 298 K by changing the metal ion from Zn to Ba. We also report the feasibility of using this material to capture CO2 from flue gas under realistic conditions (1 bar and 298 K). This work shows that LMOF-202 merits further consideration as a carbon capture adsorbent.

Kinetic analysis of homogeneous droplet nucleation using large-scale molecular dynamics simulations.

Studies on homogeneous nucleation have been conducted for decades, but a large gap between experiment and theory persists when evaluating the nucleation rate because the classical nucleation theory (CNT) with all its modifications still cannot fully incorporate the kinetics of homogeneous nucleation. Recent large-scale molecular dynamics (MD) simulations on homogeneous nucleation estimated a nucleation rate around the same order of magnitude as that obtained in experiments. This immensely improved agreement between experiment and theory is exciting because MD can provide detailed information on molecular trajectories. Therefore, a better understanding of the kinetics of homogeneous nucleation can now be obtained. In this study, large-scale MD simulations on homogeneous nucleation were performed. Through kinetic analysis of the simulation results, the nucleation rate, free energy barrier, and critical cluster size were found. Although the nucleation rates directly obtained from the simulations differed from those calculated from the CNT by 8-13 orders of magnitude, when the parameters calculated from the molecular trajectories were substituted into the classical theory, the discrepancy between the nucleation rates decreased to within an order of magnitude. This proves that the fundamental formulation of the theoretical equation is physically sound. We also calculated the cluster formation free energy and confirmed that the free energy barrier decreases with increasing supersaturation ratio. The estimated barrier height was twice that determined by theory, whereas the critical cluster size showed very good agreement between simulation and theory.

Molecular Dynamics Simulation of Ice Crystal Growth Inhibition by Hexadecyl-trimethyl-ammonium Bromide.

Recent experiments have found hexadecyl-trimethyl-ammonium bromide (CTAB) to have superior ice nucleation inhibition properties [ J. Phys. Chem. B 121, 6580]. The mechanism of how the inhibition takes place remains unclear. Therefore, molecular dynamics was used to simulate ice crystallization of a water/CTAB/ice system. The ice crystallization rate for a pure water system was compared for the basal [0001], first prism [101̅0], and secondary prism plane [112̅0], where the basal plane grew the slowest followed by the first prism plane. When CTAB was added to the ice-liquid water system, crystallization was clearly impeded. Even when ice starts growing away from the CTAB molecule, the hydrophilic head would at some point protrude and get caught in the water/ice interface. Once the head of the CTAB was encapsulated in the advancing interface, the hydrophobic body would wriggle around and disrupt the formation of hydrogen bond networks that are essential for ice growth. When the interface clears the length of the body of the CTAB molecule, ice crystallization resumes at its normal pace. In summary, the inhibition of ice growth is a combination of the hydrophilic head acting as an anchor and the dynamic motion of the hydrophobic tail hindering stable hydrogen bonding for ice growth.

Molecular simulations of water adsorption and transport in mesopores with varying hydrophilicity arrangements.

The adsorption and transport of water in an open cylindrical mesopore with two different inner surface arrangements of hydrophilicities were examined by molecular simulations. The first model has a weak hydrophilic surface at both entrances of the pore and a stronger hydrophilic surface in the mid-section. The second pore has stronger hydrophilic surfaces at the entrances and weaker in the middle region. The simulation results show that the water adsorption isotherms obtained from Grand Canonical Monte Carlo simulations and pore filling curves acquired from Grand Canonical Molecular Dynamics simulations change depending on the arrangement of the strong and weak hydrophilic surfaces. In the first model, water condensation focuses on the mid-section forming a liquid bridge or a film, which creates a concave meniscus accelerating subsequent adsorption within the pore. Two bridges form in the entrance regions, where a cavity naturally occurs in between the films, in the second model. The different filling and emptying mechanisms clearly change the adsorption-desorption characteristics for the two pore types, but the second type generally showed faster transitions overall. Flux and meniscus analysis also reveals a circulating flow at the menisci of the interfaces within the pore. The results are expected to be valuable in understanding the effects of interior surface modification of nanopores in future applications.

Self-assembly of peptide amphiphiles by vapor pressure osmometry and dissipative particle dynamics.

Peptide amphiphiles are one of the most promising materials in the biomedical field, so much effort has been devoted to characterizing the mechanism of their self-assembly and thermosensitive gelation. In this work, vapor pressure osmometry measurements were carried out to parameterize the thermosensitivity of interactions between peptide amphiphiles in an aqueous solution. The osmometry measurement verified that the peptides became more hydrophobic as temperature increased, which was quantitatively described with the Flory-Huggins χ parameter. Thereafter, a coarse-grained molecular model was used to simulate peptide amphiphiles dissolved in an aqueous solution. The temperature sensitive coarse-grained parameter a HW, which is the repulsive force between the hydrophilic head of the peptide amphiphile and water was estimated from the aforementioned experimentally obtained χ. Furthermore, the effects of concentration and temperature on the self-assembly behavior of peptide amphiphiles were quantitatively studied by dissipative particle dynamics. The simulation results revealed that a HW plays an important role in self-assembly characteristics and in the resulting microstructure of the peptide amphiphiles, which coincides with previous experimental and computational findings. The methodology in quantitatively linking the coarse-grained parameter from experiment and theory provides a sensible foundation for bridging future simulation studies with experimental work on macromolecules.

Condensation on nanorods by molecular dynamics.

Many recent experimental studies have been conducted on constructing nanorods and nanowires to use in a wide range of applications. In this study, molecular dynamics is used to directly examine the condensation rate of nanorods and the results are compared with other basic configurations such as cubes or spheres. According to previous studies conducted by Suh and Yasuoka [J. Phys. Chem. B 115, 10631 (2011); 116, 14637 (2012)], a simple change in the configuration of the seed produces a shape effect, where the curvature of the solid seed surface directly affects the growth generating an orderly difference depending on the curvature. Nanoscale cuboids or nanorods were studied to find an aspect ratio effect when condensation occurs on the surface. Various aspect ratios were examined for different nanorod sizes over a wide range of supersaturation ratios. The results show that the growth rate of the nanorod is independent of the supersaturation ratio, which was also observed for the sphere and cube. The growth rate for the rod fell between those of the cube and the sphere, and this is due to an increase in the surface area of the nanorod compared to the cube and curvature effect in comparison with the sphere. A clear size dependence of the seed was observed, which is also similar to the cube and sphere. Furthermore, no aspect ratio influence was seen for the growth rate. This does not mean that the actual amount of condensation is the same for longer seeds, but rather from the definition of the growth rate, the amount of accumulation per unit area is the same for all seed lengths.

Nucleation rate analysis of methane hydrate from molecular dynamics simulations.

Clathrate hydrates are solid crystalline structures most commonly formed from solutions that have nucleated to form a mixed solid composed of water and gas. Understanding the mechanism of clathrate hydrate nucleation is essential to grasp the fundamental chemistry of these complex structures and their applications. Molecular dynamics (MD) simulation is an ideal method to study nucleation at the molecular level because the size of the critical nucleus and formation rate occur on the nano scale. Various analysis methods for nucleation have been developed through MD to analyze nucleation. In particular, the mean first-passage time (MFPT) and survival probability (SP) methods have proven to be effective in procuring the nucleation rate and critical nucleus size for monatomic systems. This study assesses the MFPT and SP methods, previously used for monatomic systems, when applied to analyzing clathrate hydrate nucleation. Because clathrate hydrate nucleation is relatively difficult to observe in MD simulations (due to its high free energy barrier), these methods have yet to be applied to clathrate hydrate systems. In this study, we have analyzed the nucleation rate and critical nucleus size of methane hydrate using MFPT and SP methods from data generated by MD simulations at 255 K and 50 MPa. MFPT was modified for clathrate hydrate from the original version by adding the maximum likelihood estimate and growth effect term. The nucleation rates calculated by MFPT and SP methods are within 5%, and the critical nucleus size estimated by the MFPT method was 50% higher, than values obtained through other more rigorous but computationally expensive estimates. These methods can also be extended to the analysis of other clathrate hydrates.

Nanoparticle growth analysis by molecular dynamics: cubic seed.

Growth of a cubic nanoparticle was studied by classical molecular dynamics simulation for three seed sizes in nine different supersaturation ratios. Similar to the spherical seed in our previous study, for high supersaturation ratios, a two-stage phenomenon that consists of an initial heterogeneous growth around the seed and homogeneous nucleation at various sites within the system was observed. A decomposition of the distinct phenomenon was carried out and the results were compared to that from the spherical seed. The homogeneous nucleation characteristics for high supersaturation ratios show no significant difference, but as the supersaturation ratio decreases, the ratio of the nucleation rate for the systems with different seed shapes shows a deviation from unity. Other tendencies are nearly identical to that seen from the spherical seed study and the physical rationales are alike. The heterogeneous growth rate was greater by a factor of 3 to 10, even though the number of molecules in each seed class was nearly identical, which is evidence of a shape effect. Furthermore, cluster formation free energy analysis was conducted and the results were compared with the classical nucleation theory and condensation theory. The disk-shape modification of the classical nucleation theory was used for growth on the cubic seed and produced a similar deviation to that of the spherical seed, which used a cap-shape modification. Additionally, the condensation theory showed a better agreement compared with the sphere. Finally, the nanoparticle growth mechanism on the seeds along with the packing and surface diffusion characteristics was analyzed and showed why and how the nanoparticle grows.

An Improved Isotropic Periodic Sum Method That Uses Linear Combinations of Basis Potentials.

Isotropic periodic sum (IPS) is a technique that calculates long-range interactions differently than conventional lattice sum methods. The difference between IPS and lattice sum methods lies in the shape and distribution of remote images for long-range interaction calculations. The images used in lattice sum calculations are identical to those generated from periodic boundary conditions and are discretely positioned at lattice points in space. The images for IPS calculations are "imaginary", which means they do not explicitly exist in a simulation system and are distributed isotropically and periodically around each particle. Two different versions of the original IPS method exist. The IPSn method is applied to calculations for point charges, whereas the IPSp method calculates polar molecules. However, both IPSn and IPSp have their advantages and disadvantages in simulating bulk water or water-vapor interfacial systems. In bulk water systems, the cutoff radius effect of IPSn strongly affects the configuration, whereas IPSp does not provide adequate estimations of water-vapor interfacial systems unless very long cutoff radii are used. To extend the applicability of the IPS technique, an improved IPS method, which has better accuracy in both homogeneous and heterogeneous systems has been developed and named the linear-combination-based isotropic periodic sum (LIPS) method. This improved IPS method uses linear combinations of basis potentials. We performed molecular dynamics (MD) simulations of bulk water and water-vapor interfacial systems to evaluate the accuracy of the LIPS method. For bulk water systems, the LIPS method has better accuracy than IPSn in estimating thermodynamic and configurational properties without the countercharge assumption, which is used for IPSp. For water-vapor interfacial systems, LIPS has better accuracy than IPSp and properly estimates thermodynamic and configurational properties. In conclusion, the LIPS method can successfully estimate homogeneous and heterogeneous systems of polar molecular systems with good accuracy.

Nanoparticle growth analysis by molecular dynamics: spherical seed.

Three-dimensional condensation on a spherical nanoscale seed was simulated by classical molecular dynamics. In order to observe the effects of the dimension of seeds and thermodynamic conditions on the condensation characteristics, initial seed size and system supersaturation ratio were the factors that were examined. At supersaturation ratios above the critical value, two stages of nucleation were found to exist within the system, where the first stage is from the seed growth and the second from homogeneous nucleation. Therefore, the growth and homogeneous nucleation characteristics were each decomposed and analyzed separately. The Yasuoka-Matsumoto method was used to calculate the nucleation and growth rate. The homogeneous nucleation characteristics coincided with the classical nucleation theory. The condensation characteristics, however, showed a discrepancy with the modified classical nucleation theory for completely wetted heterogeneous nucleation, where no supersaturation ratio influence could be observed. The seed size was found to have a reciprocal effect on the growth rate, but showed to be insignificant on the homogeneous nucleation characteristics for this system. The critical nucleus size from kinetic analysis showed a greater difference compared to the first nucleation theorem, classical nucleation theory, or free energy analysis. All in all, the classical nucleation theory showed relatively good agreement compared to previous homogeneous nucleation studies by molecular dynamics, but a modification was found to be necessary when applying to heterogeneous growth of nanoparticles.