Minoru Miyahara

Mechanism for stripe pattern formation on hydrophilic surfaces by using convective self-assembly.

We have studied the formation of stripe patterned films of ordered particle arrays on completely solvophilic substrates by using a self-organization technique. In this method, a substrate immersed in a suspension is withdrawn vertically at a controlled temperature. We have also systematically examined the effects of several experimental parameters. Well-defined stripes spontaneously form at the air-solvent-substrate contact line because of a very dilute suspension in a quasi-static process. The stripe width depends on particle concentration, withdrawal rate, and surface tension, while the stripe spacing depends on the thickness of stripes, surface tension, and type of substrate. A stripe width and the adjacent spacing show a clear correlation, strongly indicating the synchronized formation of a stripe and the next spacing. The evaporation rate does not affect stripe width and spacing but determines the growth rate of stripe patterned films. Based on these results, we propose a new mechanism for stripe formation, which is neither a stick-slip motion of the contact line nor dewetting but a negative feedback of particle concentration caused by a concavely curved shape of the meniscus, demonstrating not only its qualitative but also its quantitative validity.

Unveiling thermal transitions of polymers in subnanometre pores.

The thermal transitions of confined polymers are important for the application of polymers in molecular scale devices and advanced nanotechnology. However, thermal transitions of ultrathin polymer assemblies confined in subnanometre spaces are poorly understood. In this study, we show that incorporation of polyethylene glycol (PEG) into nanochannels of porous coordination polymers (PCPs) enabled observation of thermal transitions of the chain assemblies by differential scanning calorimetry. The pore size and surface functionality of PCPs can be tailored to study the transition behaviour of confined polymers. The transition temperature of PEG in PCPs was determined by manipulating the pore size and the pore–polymer interactions. It is also striking that the transition temperature of the confined PEG decreased as the molecular weight of PEG increased.

Determination of adsorption equilibria in pores by molecular dynamics in a unit cell with imaginary gas phase

We developed a new molecular dynamics (MD) scheme, introducing the concept of the potential buffering field through which an adsorbed phase could interact with an imaginary gas phase. This simulation cell allowed us to conduct a MD simulation that allowed a change in the number of molecules to attain equilibrium with given equilibrium pressure, like a grand canonical Monte Carlo simulation. By taking another choice for the setting of the cell, the number of molecules stayed constant but the equilibrium pressure was able to be obtained easily by a new technique of “particle counting method.” The thus obtained equilibrium vapor-phase pressure agreed with that obtained by Widom’s particle insertion method. Some adsorption simulations within slitlike pores of 2 and 3 nm were carried out. Adsorption phenomena could be observed from monolayeradsorption on a pore wall under a low relative pressure to the capillary condensation under a high relative pressure. Thus the adsorption equilibrium relation could be determined. The critical relative pressure for capillary condensation was smaller than that predicted by the modified Kelvin equation. This MD method shall provide much benefit in studying interfaces, which is important for analyzing condensation in pores.

Free energy analysis for adsorption-induced lattice transition of flexible coordination framework

We conduct grand canonical Monte Carlo simulations and free energy analysis for a gate adsorption phenomenon, which is experimentally observed in flexible frameworks of porous coordination polymers. Our calculations demonstrate that the stabilization provided by the guest adsorption drives the structural transition, surmounting the energy cost in creating the adsorption space due to the movement of the host framework. Furthermore, the existence of an energy barrier between two local minima in the free-energy landscape is found to result in hysteretic adsorption.

Coordination and reduction processes in the synthesis of dendrimer-encapsulated Pt nanoparticles.

We synthesized Pt nanoparticles encapsulated in poly(amidoamine) (PAMAM) dendrimers by Pt(2+) coordination and subsequent reduction by NaBH(4). To optimize the experimental conditions for the Pt nanoparticle synthesis, we systematically examined the effects of pH, temperature, coordination time, and surface functional groups of the dendrimers on coordination and NaBH(4) reduction by UV-vis spectroscopy and transmission electron microscopy (TEM) measurements. We used generation-4 dendrimers (hydroxyl-terminated PAMAM dendrimers; G4-OH) and generation-4.5 dendrimers (carboxyl-terminated PAMAM dendrimers; G4.5-COO(-)). According to our results, dendrimer-encapsulated Pt nanoparticles with a narrow size distribution were obtained at high Pt(2+) coordination ratios (alpha), while nonencapsulated Pt nanoparticles were formed at low alpha values. To enhance alpha, it was necessary to use a neutral G4-OH solution or an acidic G4.5-COO(-) solution. Temperature had a marked effect on the coordination rate, with an increase in the temperature from room temperature to 50 degrees C, and the coordination time decreased from 10 days to 1-2 days.

Solid-liquid phase transition of Lennard-Jones fluid in slit pores under tensile condition

The effect of equilibrium vapor-phase pressure onto freezing of a simple fluid in a nanopore is examined. We employ a molecular dynamics (MD) technique in a unit cell with imaginary gas phase, which has the benefit of easy determination of equilibrium vapor pressure. The method is shown to give consistent results with those by the grand canonical Monte Carlo (GCMC) method, and to have better feature of smaller degree of hysteresis between freezing and melting. The MD simulations showed liquid–solid phase transitions, at a constant temperature, with the variation in the equilibrium vapor-phase pressure below the saturated one. Thus-determined solid–liquid coexistence lines exhibited significant dependence of the freezing point against small changes in the bulk–phase vapor pressure, which implies the importance of tensile effect on freezing in nanopores. The capillary effect on the shift in freezing point was successfully described by a simple model based on continuum and isotropic assumption, even in a por...

Triple point of Lennard-Jones fluid in slit nanopore: Solidification of critical condensate

We report the results of a molecular dynamics simulation that looked for the triple point of Lennard-Jones fluid in slit-shaped nanopores. The simulation method employed for this purpose is able to maintain vapor-liquid coexistence in a nanopore at a specific equilibrium bulk-phase pressure. The triple point is the freezing point of the critical condensate. The triple-point temperature could be higher or lower than the bulk triple point, depending on the pore size. This is thought to be due to two opposing factors: the elevating effect of the pore-wall potential energy, and the depressing effect of the capillary condensates tensile condition. Because of the cancellation, the deviation of the triple-point temperature from the bulk triple-point temperature was not considered significant. The pressure of the triple point, however, was significantly different from that of the bulk triple point. A simple model to describe the triple point is developed and shown to agree well with the results of the simulation. The importance of the two factors in nanoscale pores, which cannot be described by the classic Gibbs-Thomson equation, is emphasized.

Wetting-induced interaction between rigid nanoparticle and plate: A Monte Carlo study

The interaction forces between a nanosphere and a flat plate in undersaturated vapors are examined. We perform grand canonical Monte Carlo simulations, where the surfaces of the sphere and the plate are treated as rigid smooth and the vapors are modeled as a Lennard-Jones fluid of nonpolar spherical molecules. The following results are obtained: (i) The force between the sphere and plate becomes attractive at the surface distances where capillary condensation takes place in the gap between the surfaces; (ii) the onset of the attractive force becomes farther as the relative vapor pressure increases; (iii) the curve of the pull-off force (or the adhesion force) as a function of the relative vapor pressure has a peak, where the peak position shifts to a higher relative pressure and the peak height becomes smaller with decreasing the attractive interaction of the surfaces with a fluid molecule; (iv) at the relative vapor pressure where the pull-off force becomes maximum, the coverage of the surface by fluid m...

Simulation study for adsorption-induced structural transition in stacked-layer porous coordination polymers: Equilibrium and hysteretic adsorption behaviors

We conduct grand canonical Monte Carlo simulations and a free-energy analysis for a simplified model of a stacked-layer porous coordination polymer to understand the gate phenomenon, which is a structural transition of a host framework induced by the adsorption of guest particles. Our calculations demonstrate that stabilization of the system due to the guest adsorption causes host deformation under thermodynamic equilibrium. We also investigate spontaneous transition behaviors (gate opening and closing under metastable conditions). The structural transition should occur when the required activation energy, which is determined using the free-energy analysis, becomes equal to the system energy fluctuation. To estimate the system energy fluctuation, we construct a kinetic transition model based on the transition state theory. In this model, the system energy fluctuation can be calculated by setting the adsorption time and transition domain size of the host framework. The model demonstrates that a smaller domain size results in a gate-opening transition at lower pressure. Furthermore, we reveal that the slope of the logarithm of the equilibrium structural transition pressure versus reciprocal temperature shows transition enthalpy, and that slopes of the gate-opening and -closing transition pressures versus reciprocal temperature show activation enthalpies.

Sublimation phenomena of Lennard-Jones fluids in slit nanopores

Using molecular dynamics simulations, The authors studied the solid-vapor coexistence states of Lennard-Jones methane confined in slit-shaped graphite nanopores. Both the intrapore solid and extrapore vapor were simulated using a unit cell which they previously developed. Frozen critical condensates in the pores were cooled stepwise, and the equilibrium vapor pressure was determined at each temperature. The obtained solid-vapor coexistence curves were remarkably lower than that of the bulk phase. Their thermodynamic model successfully predicts the simulation results without the need to introduce any adjustable parameter, and thus proves its reliability.