Hirofumi Daiguji

Mechanism of absorption enhancement by surfactant

Abstract In absorption refrigeration technology, it is well known that the water vapor absorption into the lithium bromide (LiBr) aqueous solution is enhanced by the addition of surfactant. The Marangoni effect plays a role but its mechanism is not clearly understood yet. In the present study, the existence of instability due to Marangoni effect was investigated using the linear stability analysis, based on the salting out effect. Then the vapor absorption augmentation was estimated by the numerical simulation of cellular convection. Both theoretical results qualitatively agree well with the experimental ones.

Molecular Simulation of Ion Transport in Silica Nanopores

Ion distribution and transport of KCl aqueous solutions at the junction of hydrophobic and hydrophilic regions inside silica nanopores were studied by using two kinds of molecular simulation: grand canonical Monte Carlo (GCMC) simulations and nonequilibrium molecular dynamics (NEMD) simulations. The nanopores were 2 nm diameter silica pores in which surface functional groups, -SiOH, had been modified by hydrophobic surface functional groups, -SiCH(3), within three different lengths along the pore direction (z-direction), L(z0) = 0, 2, and 4 nm. If L(z0) is long enough, water could not enter the hydrophobic region, but for all L(z0) studied here, water entered the hydrophobic region. When an external electric field was applied along the z-direction, ions could not pass through the hydrophobic region when the external electric field was less than a threshold level, E(0), whereas the ionic current increased relatively linearly with increasing electric field strength above E(0). In 2 nm diameter fluidic pores, the electrical potential barrier appeared at the junction between the hydrophilic and hydrophobic regions due to the difference in dipole moment of the surface functional groups, although the overall surface charge of the pore was neutral. This junction formed an electrical potential threshold, and the ionic current could be modulated by adjusting the external electric field.

Enhanced energy harvesting by concentration gradient-driven ion transport in SBA-15 mesoporous silica thin films

Nanofluidic energy harvesting systems have attracted interest in the field of battery application, particularly for miniaturized electrical devices, because they possess excellent energy conversion capability for their size. In this study, a mesoporous silica (MPS)-based nanofluidic energy harvesting system was fabricated and selective ion transport in mesopores as a function of the salt gradient was investigated. Aqueous solutions with three different kinds of monovalent electrolytes-KCl, NaCl, and LiCl-with different diffusion coefficients (D+) were considered. The highest power density was 3.90 W m-2 for KCl, followed by 2.39 W m-2 for NaCl and 1.29 W m-2 for LiCl. Furthermore, the dependency of power density on the type of cation employed indicates that the harvested energy increases as the cation mobility increases, particularly at high concentrations. This cation-specific dependency suggests that the maximum power density increases by increasing the diffusion coefficient ratio of cations to anions, making this ratio a critical parameter in enhancing the performance of nanofluidic energy harvesting systems with extremely small pores ranging from 2 to 3 nm.

Ion Transport in Mesoporous Silica SBA-16 Thin Films with 3D Cubic Structures

Mesoporous silica SBA-16 thin films with highly ordered 3D cubic structures were synthesized on a Si substrate via the dip-coating method. After these films were filled with KCl aqueous solutions, the ionic current passing through the mesopores was measured by applying dc electric fields. At low ion concentrations, the measured I-V curves were nonlinear and the current increased exponentially with respect to voltage. As the ion concentration increased, the I-V curve approached linear behavior. The nonlinear behavior of I-V curves can be reasonably attributed to the electric potential barrier created in nanopores.

Water adsorption–desorption isotherms of two-dimensional hexagonal mesoporous silica around freezing point

Zr-doped mesoporous silica with a diameter of approximately 3.8 nm was synthesized via an evaporation-induced self-assembly process, and the adsorption-desorption isotherms of water vapor were measured in the temperature range of 263-298 K. The measured adsorption-desorption isotherms below 273 K indicated that water confined in the mesopores did not freeze at any relative pressure. All isotherms had a steep curve, resulting from capillary condensation/evaporation, and a pronounced hysteresis. The hysteresis loop, which is associated with a delayed adsorption process, increased with a decrease in temperature. Furthermore, the curvature radius where capillary evaporation/condensation occurs was evaluated by the combined Kelvin and Gibbs-Tolman-Koening-Buff (GTKB) equations for the modification of the interfacial tension due to the interfacial curvature. The thickness of the water adsorption layer for capillary condensation was slightly larger, whereas that for capillary evaporation was slightly smaller than 0.7 nm.

Fabrication of hollow poly-allylamine hydrochloride/poly-sodium styrene sulfonate microcapsules from microbubble templates

Hollow microcapsules made of biodegradable polymers have attracted considerable attention as ultrasound contrast agents and carriers in drug delivery systems (DDSs). A hollow polyelectrolyte microcapsule is one of the promising candidates for such applications. Hollow polyelectrolyte microcapsules are normally fabricated by decomposing a core material after forming a capsule shell. Recently, Shchukin et al. (D. G. Shchukin, K. Kohler, H. Mohwald and G. B. Sukhorukov, Angew. Chem., Int. Ed., 2005, 44, 3310–3314) and Winterhalter et al. (M. Winterhalter and A. F. P. Sonnen, Angew. Chem., Int. Ed., 2006, 45, 2500–2502) proposed a fabrication method of hollow polyelectrolyte microcapsules using microbubbles as templates. In this method, stable microbubbles are formed in a surfactant solution by means of an ultrasound generator, and then, a polyelectrolyte is adsorbed on the microbubble surface, yielding hollow polyelectrolyte microcapsules. In this study, we proposed a new fabrication method for hollow poly-allylamine hydrochloride/poly-sodium styrene sulfonate (PAH/PSS) microcapsules without using surfactants. In a Na2CO3 aqueous solution, PAH becomes colloidal particles of R-NHCOO− and R-NH3+ within a certain pH range. Under such a condition, if microbubbles are generated in the solution, the colloidal particles adsorbed on the microbubble surface stabilize the microbubbles. When PSS is added to the solution sequentially, a bilayer of PAH/PSS can be formed around microbubbles. We successfully fabricated hollow PAH/PSS microcapsules by this method and elucidated the conditions required for the formation of colloidal particles of PAH and a bilayer of PAH/PSS.

Fabrication of hollow poly(lactic acid) microcapsules from microbubble templates.

Hollow microcapsules are expected to be integral components of drug delivery systems in medical and pharmaceutical applications. Among the methods studied, the bubble template method (Makuta et al. Mater. Lett. 2009, 63, 703-705) should easily fabricate uniform hollow microcapsules covered with biodegradable polymers. In this study, we clarified the two conditions required to fabricate uniform hollow microcapsules using the bubble template method: the stability of uniformly sized microbubbles in a liquid droplet and the release of hollow microcapsules from the droplet. Furthermore, our experiments evaluated the radius distributions of template microbubbles and fabricated hollow poly(lactic acid) microcapsules.

Molecular dynamics study of n-alcohols adsorbed on an aqueous electrolyte solution

The distribution of normal alcohol (n-alcohol) on water and the effect of salt on the structural and dynamical properties of n-alcohol on aqueous electrolyte solutions were investigated using molecular dynamics simulation. The stability of the alcohol distribution was studied for three types of n-alcohol (n-propanol, C3H7OH; n-heptanol, C7H15OH; and n-undecanol, C11H23OH), four or five concentrations of alcohol, and three concentrations of salt. The simulation results reveal the following. The distribution of n-propanol on water is homogeneous at all n-alcohol concentrations studied here and the distribution of n-heptanol and n-undecanol on water is heterogeneous. The n-alcohol concentration at which fluctuations in the alcohol distribution begin to increase depends on the length of the hydrocarbon chain of the n-alcohol. Salt concentration affects the surface excess concentration of n-alcohol and the stability of the adsorbed layer of n-alcohol. The degree of each effect depends on the length of the hydrocarbon chain of the n-alcohol. For n-undecanol, the surface structure of n-alcohol is independent of salt concentration because interaction between the hydrocarbon chains is sufficiently strong. In absorption refrigeration technology, to enhance the absorption rate of water vapor into a highly concentrated aqueous electrolyte solution, a small amount of alcohols is added to the aqueous electrolyte solution, which induces cellular convection referred to as Marangoni instability. Among the three types of n-alcohol studied here, only n-heptanol induces strong cellular convection. The simulations reveal two required conditions for Marangoni instability: generation of fluctuations in the alcohol distribution on water, and strong correlation between the structural and dynamical properties and salt concentration. Among the three types of n-alcohol studied here, based on the simulations, only n-heptanol satisfies both conditions.The distribution of normal alcohol (n-alcohol) on water and the effect of salt on the structural and dynamical properties of n-alcohol on aqueous electrolyte solutions were investigated using molecular dynamics simulation. The stability of the alcohol distribution was studied for three types of n-alcohol (n-propanol, C3H7OH; n-heptanol, C7H15OH; and n-undecanol, C11H23OH), four or five concentrations of alcohol, and three concentrations of salt. The simulation results reveal the following. The distribution of n-propanol on water is homogeneous at all n-alcohol concentrations studied here and the distribution of n-heptanol and n-undecanol on water is heterogeneous. The n-alcohol concentration at which fluctuations in the alcohol distribution begin to increase depends on the length of the hydrocarbon chain of the n-alcohol. Salt concentration affects the surface excess concentration of n-alcohol and the stability of the adsorbed layer of n-alcohol. The degree of each effect depends on the length of the hydr...

Effect of withdrawal speed on film thickness and hexagonal pore-array dimensions of SBA-15 mesoporous silica thin film.

Two-dimensional hexagonal mesoporous silica thin films of SBA-15 were synthesized on Si substrates via dip-coating using an evaporation-induced self-assembly process. The effect of the withdrawal speed on the thicknesses, one-dimensional pore alignments, and two-dimensional hexagonal pore arrays of the films was elucidated. Detailed analyses of FE-SEM and TEM images and XRD and XRR patterns of the synthesized thin films clarified that the pore sizes, interplanar spacings, and film thicknesses depend on the withdrawal speed. Furthermore, the same films were synthesized on Si substrates with microtrenches. The local flow of coating solutions around microtrenches affects the pore direction as well as the film thickness. In order to form well-ordered mesoporous silica thin films with large surface areas, it is important to control the synthetic conditions such as the local flow of the coating solutions as well as the physicochemical properties of the silica precursor solutions or template molecules.

Effect of Organic Solvents on the Pore Structure of Catalyst Layers in Polymer Electrolyte Membrane Fuel Cells

The effect of organic solvents in catalyst inks on the pore structure of catalyst layers (CLs) in polymer electrolyte membrane fuel cells was investigated. The pore size distribution of CLs fabricated from catalyst inks containing either ethylene glycol (EG) (CL EG ), propylene glycol (PG) (CL PG ), or 1,3-propanediol (PDO) (CL PDO ) was measured. The dielectric constants of these three organic solvents were > 10, and perfluorosulfonate ionomer was dissolved in the catalyst inks. The pore volume (v) of CL PDO , CL EG , and CL PG increased in this order: v PDO < v EG < v PG , suggesting that the pore structure of the CLs depended on the solvent evaporation process.