Shi-aki Hyodo

Dissipative particle dynamics study of spontaneous vesicle formation of amphiphilic molecules

A dissipative particle dynamics (DPD) simulation has been used to study the spontaneous vesicle formation of amphiphilic molecules in aqueous solution. The amphiphilic molecule is represented by a coarse-grained model, which contains a hydrophilic head group and a hydrophobic tail. Water is also modeled by the same size particle as adopted in the amphiphile model, corresponding to a group of several H2O molecules. In the DPD simulation, from both a randomly dispersed system and a bilayer structure of the amphiphile for the initial condition, a spontaneous vesicle formation is observed through the intermediate state of an oblate micelle or a bilayer membrane. The membrane fluctuates and encapsulates water particles and then closes to form a vesicle. During the process of vesicle formation, the hydrophobic interaction energy between the amphiphile and water is diminishing. It is also recognized that the aggregation process is faster in two-tailed amphiphiles than those in the case of single-tailed ones.

Raman intensity study of local structure in non-aqueous electrolyte solutions—I. Cation-solvent interaction in LiClO4/ethylene carbonate

Abstract The local structure of the solvated lithium cation in ethylene carbonate (EC) solutions has been investigated using Raman spectroscopy. The relative intensity change of the split Raman line at ∼ 900 cm−1 in LiClO4/EC was treated quantitatively. This splitting originates from the coexistence of two type EC molecules; one solvates to Li cation and the other does not solvate to Li cation. The resultant intensity data have been applied to the analysis of the Li+-EC solution. The possibilities of the determination of the degree of dissociation of LiClO4 and that of the solvation number of Li+ in a contact ion pair were also discussed. It was found that this Raman line is useful for analysing the solution structure in Li+-EC systems. Basic intensity parameters for such analyses can be determined.

Budding and fission dynamics of two-component vesicles

We studied the shape deformation induced by the phase separation of two-component vesicles using a dissipative particle dynamics simulation. Two types of amphiphiles, which have the same architecture but segregate from each other, are modeled by connecting particles representing the hydrophilic head and hydrophobic tail groups. After vesicle formation using a single component system, some of the amphiphiles are replaced by a second component, and then phase separation on the vesicle is simulated. Under appropriate conditions, typical shape deformations of a vesicle, such as crenated and invaginated shapes, are observed. We demonstrate that the budding and the fission are facilitated by lateral phase separation upon vesicle coupling to an asymmetric transversal distribution of amphiphiles in the bilayer. Moreover, simulations show a possible new pathway for the fission process triggered by cleavage along the domain boundary. Mesoscopic simulation based on a simple coarse-graining model reveals the dynamics of amphiphiles in vesicles that is essentially unpredictable using a conventional continuum model or full atomistic simulation.

Raman intensity study of local structure in non-aqueous electrolyte solutions—II. Cation—solvent interaction in mixed solvent systems and selective solvation

Abstract The selectivity of solvation with lithium cation was discussed in LiClO 4 /ethylene carbonate (EC)—propylene carbonate (PC), LiClO 4 /ECH 2 O, and LiClO 4 /ECCHCl 3 based on the results of Raman intensity measurements. The fundamental parameter for this analysis has been already determined on the ring breathing Raman band of EC. Following from this parameter, the results of the analysis showed that H 2 O molecules solvate to lithium cation selectively in LiClO 4 /ECH 2 O system while EC molecules solvate to lithium cation selectively in LiClO 4 /ECCHCl 3 system, as was expected previously. On the other hand, the results showed an ideal non-selective solvation of lithium cation in LiClO 4 /ECPC system.

Elimination of translational and rotational motions in nuclear orbital plus molecular orbital theory

The nuclear orbital plus molecular orbital (NOMO) theory was developed in order to determine the nonadiabatic nuclear and electronic wave functions. This study presents a formulation to remove the contamination of rotational motion as well as translational motion in the NOMO theory. We have formulated the translation- and rotation-free (TRF)-NOMO theory by introducing the TRF Hamiltonian. The principal moment of inertia, which is the denominator in the rotational Hamiltonian, is expanded in a Taylor series. The zeroth-order of the Taylor expansion corresponds to a rigid-body rotator. The first-order terms contribute the coupling between the vibration and the rotation. Hartree-Fock equations have been derived in the framework of the TRF-NOMO theory. Numerical assessments, which were preformed for H2, D2, T2, mu2 (muon dimmer), and H2O, confirmed the importance of the TRF treatment.

Mesoscopic simulation of the crossing dynamics at an entanglement point of surfactant threadlike micelles.

The crossing dynamics at an entanglement point of surfactant threadlike micelles in an aqueous solution was studied using a mesoscopic simulation method, dissipative particle dynamics, with a coarse-grained surfactant model. The possibility of a phantom crossing, which is the relaxation mechanism for the pronounced viscoelastic behavior of surfactant threadlike micellar solution, was investigated. When two threadlike micelles were encountered at an entanglement point under the condition close to thermal equilibrium, they fused to form a four-armed branch point. Then, a phantom crossing reaction occurred occasionally, or one micelle was cut down at the branch point. Increasing the repulsive forces between hydrophilic parts of the surfactants, fusion occurred less and the threadlike micelle was frequently broken down at an entanglement point. In these three schemes (a phantom crossing cut down at the branch point, and break down at the entanglement point), the breakage occurs at somewhere along the threadlike micelle. The breakage is considered as an essential process in the relaxation mechanism, and a phantom crossing can be seen as a special case of these processes. To explain the experimental evidence that a terminal of threadlike micelles is scarcely observed, a mechanism was also proposed where the generated terminal merges into the connected micelle part between two entanglement points due to the thermal motion.

Nature of water transport and electro-osmosis in nafion: insights from first-principles molecular dynamics simulations under an electric field.

The effects of water content on water transport and electro-osmosis in a representative polymer electrolyte membrane, Nafion, are investigated in detail by means of first-principles molecular dynamics (MD) simulations in the presence of a homogeneous electric field. We have directly evaluated electro-osmotic drag coefficients (the number of water molecules cotransported with proton conduction) from the trajectories of the first-principles MD simulations and also explicitly evaluated factors that contribute to the electro-osmotic drag coefficients. In agreement with previously reported experiments, our calculations show virtually constant values ( approximately 1) of the electro-osmotic drag coefficients for both low and high water content states. Detailed comparisons of each factor contributing to the drag coefficient reveal that an increase in water content increases the occurrence of the Grotthuss-like effective proton transport process, whose contribution results in a decrease in the electro-osmotic drag coefficient. At the same time, an environment that is favorable for the Grotthuss-like effective proton transport process is also favorable for the transport of water arising from water transport occurring beyond the hydration shell around the protons, whose contribution results in an increase in the electro-osmotic drag coefficient. Conversely, an environment that is not favorable for proton conduction is also not favorable for water transport. As a result, the electro-osmotic drag coefficient shows virtually identical values with respect to change in the water content.

An oligosilane bridge model for the origin of the intense visible photoluminescence of porous silicon

Oligosilanes bridging microgaps between silicon microcrystallites were investigated as a possible model for the origin of the intense visible photoluminescence of porous silicon by means of the semiempirical molecular‐orbital method. The incomplete structural relaxation of the oligosilane bridge after the photoexcitation was found to be a key factor to give the visible photoluminescence. The calculated structure‐insensitive photoexcitation energy around 3.3 eV and the structure‐sensitive light emission energy around 1.2–2.1 eV are consistent with the experimental evidence. The sufficient transition probabilities between the concerning electronic states support the high efficiency of photoluminescence. Durabiliy of the structure under the photoexcitation was also suggested. The model is valid even if the silicon microcrystallites are partly or thoroughly replaced by silicon oxides particles as is more realistic for the porous silicon exposed to the air.

Excitation Spectra of the Visible Photoluminescence of Anodized Porous Silicon

Porous silicon samples, photoluminous in orange, pink or greenish yellow, were formed by anodization. The shapes and wavelength positions of their photoluminescence peaks were not influenced by the excitation wavelength. The photoluminescence intensity at any wavelength between 520 nm and 800 nm varied as a function of the excitation wavelength in accordance with the imaginary part of the spectral refractive index of bulk silicon. Thus, the excitation process responsible for the visible photoluminescence of porous silicon seems to be bulk-silicon-like.

Nonresonant third‐order nonlinear optical susceptibility of CdS clusters encapsulated in zeolite A and X

Nonresonant third‐order harmonic generation from CdS clusters encapsulated in zeolite A and X was observed at a fundamental wavelength of 1900 nm. To avoid scattering from the surfaces of the small zeolite crystals, the powder samples were dispersed in a liquid with nearly the same refractive index as that of the samples. The third‐order optical susceptibilities of CdS‐encapsulated zeolite A and X estimated from the intensity of their Maker fringe patterns were 4.1×10−12 and 1.1×10−11 esu, respectively. These values were slightly smaller than those reported for the 1.5 nm surface‐capped CdS cluster. The hyperpolarizabilities of CdS clusters encapsulated in zeolite A and X were estimated by assuming the Lorentz local field to be in the range of 380–480×10−36 and 270–390×10−36 esu, respectively.