The size-dependent behavior of small unilamellar vesicles is explored by dissipative particle dynamics, including the membrane characteristics and mechanical properties. The spontaneously formed vesicles are in the metastable state and the vesicle size is controlled by the concentration of model lipids. As the vesicle size decreases, the bilayer gets thinner and the area density of heads declines. Nonetheless, the area density in the inner leaflet is higher than that in the outer. The packing parameters are calculated for both leaflets. The result indicates that the shape of lipid in the outer leaflet is like a truncated cone but that in the inner leaflet resembles an inverted truncated cone. Based on a local order parameter, our simulations indication that the orientation order of lipid molecules decreases as the size of the vesicle reduces and this fact reveals that the bilayer becoming thinner for smaller vesicle is mainly attributed to the orientation disorder of the lipids. The membrane tension can be obtained through the Young-Laplace equation. The tension is found to grow with reducing vesicle size. Therefore, small vesicles are less stable against fusion. Using the inflation method, the area stretching and bending moduli can be determined and those moduli are found to grow with reducing size. Nonetheless, a general equation with a single numerical constant can relate bending modulus, area stretching modulus, and bilayer thickness irrespective of the vesicle size. Finally, a simple metastable model is proposed to explain the size-dependent behavior of bilayer thickness, orientation, and tension.
A vesicle can be swollen in response to the imposed osmotic pressure gradient and vesicular membrane properties are changed. The swelling of small unilamellar vesicles is explored by dissipative particle dynamics, including the growth, rupture, and fusion processes. The spontaneously formed vesicles are inflated by water addition into the vesicle lumen. As the vesicle size grows due to the increment of lumenal contents, the bilayer gets thinner and the area density of the inner lipid head declines much more significantly than that of the outer does. Both area densities converge to the same value before membrane rupture. The packing parameters for both leaflets also decrease and the shape of lipids in the inner leaflet changes from an inverted truncated cone to a truncated cone. The distribution of lipid tails indicates that an interdigitated structure takes place within the bilayer of an inflated vesicle. The outcome of local order parameter demonstrates that the orientation order decays with inflation, revealing that vesicular membrane stretching is accompanied with entropy increment, which is opposite to stretched elastic membrane with entropy reduction. The Young-Laplace equation is adopted to estimate tension, which rises with inflation. Although vesicles of different sizes rupture at distinct degrees of inflation, membrane properties, such as membrane thickness, packing parameter, order parameter, and tension increment must reach the same critical values for all vesicle sizes before rupture. It is also found that the vesicle fusion process is greatly facilitated by the increment of tension owing to the substantial reduction in the time periods for adhesion and hemifusion processes.
InGaN-based light emitting diodes (LEDs) with a top pattern-nanoporous p-type GaN:Mg surface were fabricated by using a photoelectrochemical (PEC) process. The peak wavelengths of electroluminescence (EL) and operating voltages were measured as 461.2 nm (3.1 V), 459.6 nm (9.2 V), and 460.1 nm (3.3 V) for conventional, nanoporous, and pattern-nanoporous LEDs using 20 mA operation current. The EL spectrum of the nanoporous LED had a larger blueshift phenomenon as a result of a partial compression strain release in the InGaN active layer through the formation of a top nanoporous surface. The light output power had 12.1% and 26.4% enhancements for the nanoporous and the pattern-nanoporous LEDs compared with conventional LEDs. The larger operating voltage of the nanoporous LED was due to the non-ohmic contact on the PEC treated p-type GaN:Mg surface. By using a pattern-nanoporous p-type GaN:Mg structure, the operating voltage of the pattern-nanoporous LED was reduced to 3.3 V. A lower compression strain in the InGaN active layer and a higher light extraction efficiency at the top nanoporous surface were observed in pattern-nanoporous LEDs for higher efficiency nitride-based LED applications
Novel 1-substituted imidazole derivatives (4-10) were synthesized by imidazole and the corresponding substituted reagents (chloromethylpivalate, diphenylphosphinicchloride, di-tert-butyldicarbonate, 1,1 0 -oxalylchloride, pyrazine, phneylisocyanat, and p-toluensulfonylchloride). Polymerization of diglycidyl ether of bisphenol A (DGEBA) with 1-substituted imidazole derivatives, two commercial available catalysts (imidazole and 1-cyanoethyl-2-ethyl-4methylimidazole) and N-benzylpyrazinium hexafluoroantimonate were investigated as model reactions of epoxy resin systems with respect to the thermal latency and storage stability of the catalysts. The catalytic activity of 1-substituted imidazole derivatives 4-10 depended on the steric and withdrawing electronic effect of the substitution groups. To characterize the cure activation energy and the viscosity-storage time, the order of thermally latent activity is 1-tosylimidazole (6) > 1,1 0
The interactions between surfactants and vesicles formed by double-tail amphiphiles are investigated by the dissipative particle dynamics. As the surfactant concentration is increased, vesicle solubilization can be generally described by the three-stage hypothesis including vesicular region, vesicle-micelle coexistence, and mixed micellar region. We study the partition of surfactants between the bilayer phase and the aqueous phase where a higher value of K indicates that more surfactant molecules are incorporated in the bilayer. It is found that ln(K(-1)) is proportional to the hydrophile-lipophile balance (HLB), which depicts the degree of hydrophilicity associated with a surfactant. As the overall hydrophilicity of surfactants increases, i.e., higher HLB, K declines and vice versa. When the amounts of surfactants reach a critical point, the solubilization begins and the coexistence of vesicles and mixed micelles is observed. Further increase in the surfactant concentration results in total collapse of the vesicle. Consistent with experimental observations, the three stages are identified through the vesicle size-surfactant concentration relation. Our simulations clearly demonstrate the process of the vesicle solubilization and confirm the validity of the three-stage hypothesis.
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