Dynamics and hydrogen bonding in liquid ethanol

Molecular dynamics simulations of liquid ethanol at three temperatures have been carried out. The hydrogen bonding states of ethanol molecules have been characterized by the number of hydrogen bonds in which the molecules participate. It is observed that the mean lifetimes of molecules in each hydrogen bonding state are markedly dependent on the temperature. Moreover, molecules with one hydrogen bond are more stable when they are donors than when they are acceptors. The dependence of the reorientational correlation functions on the hydrogen bonding state of molecules has been studied carefully. The decay of these functions is slower for molecules with higher numbers of hydrogen bonds and also becomes slower as temperature decreases. The relaxation for molecules with only one hydrogen bond is faster for those acting as proton donors than for those acting as proton acceptors. Finally, the results obtained by computer simulation are compared with those from recent measurements of the frequency-dependent dielectric permittivity of liquid ethanol.     

限制在疏水板中的新的六边形-四边形三层冰结构 (cited: 1)

对限制在两个光滑的疏水板间的水进行了分子动力学模拟,观察到了两种晶体结构,都满足冰规则. 在1 GPa的压强和1.0 nm的板间距下获得的新的冰相是平坦的六边形-四边形三层冰. 在此结构中,靠近板的两层(外层)中的水分子形成六边形环,中间层的水分子形成四边形环. 对于外层的水分子,其四个氢键中的三个在同一层中,另一个氢键与中间层连接. 对于中间层的水分子,四个氢键中的两个在同一层中,而另外两个氢键与两个不同的外层相连. 虽然三层的形状不同,但其面密度却接近相等. 另一种结构是在0.8 nm的板间距和100     

Structural investigation of n-hexadecanoic acid multilayers on mica surface: Atomic force microscopy study

The structure of n-hexadecanoic acid (HA) multilayers formed by spreading an ethanol solution containing this molecule onto a freshly cleaved mica surface has been studied by atomic force microscopy (AFM). AFM images of multilayers obtained with different coating time showed that HA molecules first formed some sporadic domains on mica surface. With the proceeding of the coating process, these domains gradually enlarged and coalesced, until formed a continuous film finally. It was observed that HA molecules were always adsorbed on mica surface with tilted even-numbered layers structure. The height of the repeated tilted bilayer film was measured to be approximately 3.8 ± 0.2 nm, which implied a ∼60° tilt molecular conformation of the HA bilayers on mica surface. Phase image confirmed that the HA multilayers terminated with the hydrophilic carboxylic acid groups. The formation mechanism of the HA multilayers was discussed in detail. Thus, resulted hydrophilic surfaces are of special interest for further study in biological or man-made member systems.     

Vibrational recognition of hydrogen-bonded water networks on a metal surface

The adsorption of water on Pt(111) surface has been studied with ab initio molecular dynamics simulation. Both the energetics and vibrational dynamics indicate the existence of a well-ordered molecular bilayer on this surface. This conclusion is in contrast to the recent result of water on Ru(0001) surface, but agrees with available experiments. In addition, our calculation identifies two different hydrogen bonds in the bilayer. Both can be directly recognized from the vibrational spectra of the OH stretch modes.     

水在金属表面的氢键网络结构振动谱研究

用从头计算分子动力学模拟方法研究了水在Pt(111)表面上的吸附。总能优化和振动谱分析都表明，在这个表面上水以有序的分子态双层结构存在。这一结论和最近对水在Ru(0001)表面上吸附的计算结果相悖，但和已有的实验相符。此外，文章作者首次确定双层结构中存在两种不同的氢键形式。这两种氢键可以通过0H伸缩振动模的振动谱得到直接证实。     

DNA adsorption and desorption on mica surface studied by atomic force microscopy

The adsorption of DNA molecules on mica surface and the following desorption of DNA molecules at ethanol-mica interface were studied using atomic force microscopy. By changing DNA concentration, different morphologies on mica surface have been observed. A very uniform and orderly monolayer of DNA molecules was constructed on the mica surface with a DNA concentration of 30 ng/μL. When the samples were immersed into ethanol for about 15 min, various desorption degree of DNA from mica (0-99%) was achieved. It was found that with the increase of DNA concentration, the desorption degree of DNA from the mica at ethanol-mica interface decreased. And when the uniform and orderly DNA monolayers were formed on the mica surface, almost no DNA molecule desorbed from the mica surface in this process. The results indicated that the uniform and orderly DNA monolayer is one of the most stable DNA structures formed on the mica surface. In addition, we have studied the structure change of DNA molecules after desorbed from the mica surface with atomic force microscopy, and found that the desorption might be ascribed to the ethanol-induced DNA condensation.     

Surface defects on ZnO nanowires: implications for design of sensors

Surface defects are commonly believed to be fundamentally important to gas-sensor performance. We examine the effect of gas coverage and ethanol orientation on its adsorption on the stoichiometric and oxygen deficient (101(-)0) nanowire surface. Our density functional theory calculations show that ethanol adsorbs in multiple stable configurations at coverages between 1/4 and 1 ML, highlighting the ability of ZnO to detect ethanol. Ethanol prefers to bind to a surface Zn via the adsorbate oxygen atom and, if a surface oxygen atom is in close proximity, the molecule is further stabilized by formation of a hydrogen bond between the hydrogen of the hydroxyl group and the surface oxygen. Two primary adsorption configurations were identified and have different binding strengths that could be distinguished experimentally by the magnitude of their OH stretching frequency. Our findings show that ethanol adsorbed on the oxygen deficient ZnO(101(-)0) surface has a reduced binding strength. This is due to either the lack of a hydrogen bond (due to a deficiency in surface oxygen) or to surface reconstruction that occurs on the defect surface that weakens the hydrogen bond interaction. This reduced binding on the oxygen deficient surface is in contrast to the defect enhanced gas-sensor interaction for other gases. Despite this difference, ethanol still acts as a reducing gas, donating electrons to the surface and decreasing the band gap. We show that multiple adsorbed ethanol molecules prefer to be orientated parallel to each other to facilitate the hydrogen bonding to the defect-free surface for enhanced interaction.     

Structure and dynamics of ordered water in a thick nanofilm on ionic surfaces

The structure and dynamics of water in a thick film on an ionic surface are studied by molecular dynamic simulations. We find that there is a dense monolayer of water molecules in the vicinity of the surface. Water molecules within this layer not only show an upright hydrogen-down orientation, but also an upright hydrogen-up orientation. Thus, water molecules in this layer can form hydrogen bonds with water molecules in the next layer. Therefore, the two-dimensional hydrogen bond network of the first layer is disrupted, mainly due to the O atoms in this layer, which are affected by the next layer and are unstable. Moreover, these water molecules exhibit delayed dynamic behavior with relatively long residence time compared with those bulk-like molecules in the other layers. Our study should be helpful to further understand the influence of water film thickness on the interfacial water at the solid-liquid interface.     

How polar hydroxyl groups affect surface hydrophobicity on model talc surfaces

Based on molecular dynamics simulations, we have studied the wetting behaviors of water on the talc-like surface with different surface polarity by modifying the charge distribution of surface hydroxyl (–OH) groups. With the change of the charge of the hydrogen atom (denoted as δ_q) in –OH group, the contact angle decreases from 91° to 50° and then remains constant. On the surfaces with the larger charge of hydrogen atoms (δ_q ≥ 0.2 e), a water droplet is formed above a water monolayer, which is exactly contacted on the surface. Each water molecule in the monolayer forms one hydrogen bond (H bond) with surface –OH groups, without participating in any H bond with the water molecules within the monolayer or with the water molecules above the monolayer. The polarity of the –OH group also has a great influence on the dynamic behaviors of the interface water, such as residence time, hydrogen bond lifetime and self-diffusion coefficient. The diffusion of water molecules in the water monolayer near the highly polar surface is greatly suppressed, and the residence time of water molecules in the water monolayer even exceeds 12 ns.     

Peptide friction in water nanofilm on mica surface

Peptide frictions in water nanofilms of various thicknesses on a mica surface are studied via molecular dynamics simulations. We find that the forced lateral motion of the peptide exhibits stick-slip behaviour at low water coverage; in contrast, the smooth gliding motion is observed at higher water coverage. The adsorbed peptide can form direct peptide-surface hydrogen bonds as well as indirect peptide-water-surface hydrogen bonds with the substrate. We propose that the stick-slip phenomenon is attributed to the overall effects of direct and indirect hydrogen bonds formed between the surface and the peptide.     

Monolayer and aggregate formation of a modified phthalocyanine on mica determined by a delicate balance of surface interactions

An ordered layer of a phthalocyanine modified with ether tails can be formed on muscovite mica if removed from solution and dried. This ordered layer forms on potassium terminated as well as on sodium terminated mica, but not on a hydronium terminated surface. The molecules lie flat on the surface, as shown by AFM and X-ray diffraction, giving a layer thickness of approximately 1 nm. In solution, however, no in-plane ordering exists. The material is attracted to the substrate surface but instead of ordering, it aggregates in a liquid-like mobile fashion. This is likely caused by the fact that the water present in solution has a stronger interaction with the potassium ions on the mica surface than with the ether tails of the phthalocyanine.     

从常温常压到超临界乙醇的分子动力学模拟

采用分子动力学方法系统地研究了从常温常压到超临界状态乙醇的热力学性质、结构性质和动力学性质.模拟发现随着温度的升高,体系焓值增大,乙醇分子间的氢键作用减弱,自扩散系数增大;随着压强的增大,乙醇分子间的氢键作用增强,自扩散系数减小;乙醇自扩散系数在液相区随温度变化不明显,在气相区随压强增大很快减小,超临界区乙醇的自扩散系数比液相区大十几倍.温度和压强对乙醇自扩散系数的影响可通过密度来体现.与常温常压相比,超临界条件下的乙醇体系因密度涨落存在分子聚集现象,且在低密度区域更显著;乙醇分子间的氢键作用明显减弱,结     

用分子动力学模拟甲烷水合物热激法分解

用分子动力学模拟方法研究甲烷水合物热激法分解，系统地研究注入340 K液态水的结构Ⅰ型甲烷水合物的分解机理.模拟显示水合物表层水分子与高温液态水分子接触获得热能，分子运动激烈，摆脱水分子间的氢键束缚，笼状结构被破坏.甲烷分子获得热能从笼中挣脱，向外体系扩散.热能通过分子碰撞从外层传递给内层水分子，水合物逐层分解.对比注入277K液态水体系模拟结果，得出热激法促进水合物分解. 关键词： 甲烷水合物 分子动力学模拟 热激法     

AES studies of palladium films formed on copper and mica

Thin epitaxial films of palladium were grown on epitaxial copper films and cleaved mica in ultra high vacuum. The growth modes of these films were investigated by Auger electron spectroscopy (AES), low energy electron diffraction (LEED), transmission electron microscopy (TEM), and TEM replica techniques. Layer by layer growth of Pd on Cu and mica was observed and inelastic mean free paths of Auger electrons for energies of 60 eV (Cu MMM) and 329 eV (Pd MNN) were calculated. These values were 5.7 and 6.9 Å respectively. The thermal stability of monocrystalline and polycrystalline Pd/Cu bilayer films at 483 K was also investigated by AES and TEM. It was found that Pd agglomerates on the Cu at this temperature to form a Stranski-Krastanov growth morphology. The agglomeration is much more rapid on polycrystalline films, suggesting that high surface diffusivity paths (grain boundaries and possibly other defects) enhance the surface diffusion of Pd on Cu.     

Influence of (phospho)lipases on properties of mica supported phospholipid layers

The effect of enzymes: lipase from Candida cylindracea (L_Cc), phospholipase A₂ from hog pancreas (PLA₂) and phospholipase C from Bacillus cereus (PLC) to modulate wetting properties of solid supported phospholipid bilayers was studied via advancing and receding contact angle measurements of water, formamide and diiodomethane, and calculation of the surface free energy and its components from van Oss et al. (LWAB) and contact angle hysteresis (CAH) approaches. Simultaneously, topography of the studied layers was determined by Atomic Force Microscopy (AFM). The investigated lipid bilayers were transferred on mica plates from subphase of pure water by means of Langmuir-Blodgett and Langmuir-Schaefer techniques. The investigated phospolipid layers were: saturated DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine), unsaturated DOPC (1,2-dioleoyl-sn-glycero-3-phosphocholine), and their mixture DPPC/DOPC. The obtained results revealed that the lipid membrane degradation by the enzymes caused increase in its surface free energy due to the amphiphilic hydrolysis products, which may accumulate in the lipid bilayer. In result activity of the enzymes may increase and then break down the bilayer structure takes place. It is likely that after dissolution of the hydrolysis reaction products in the bulk phase, patches of bare mica surface are accessible, which contribute to the apparent surface free energy changes. Comparison of AFM images and the free energy changes of the layers gives better insight into changes of their properties. The observed gradual increase in the layer surface free energy allows controlling of the hydrolysis process to obtain the surfaces of defined properties.     

Li掺杂对MgH2(001)表面H2分子扩散释放影响的第一性原理研究

本文利用基于密度泛函理论的第一性原理分别研究了MgH₂(001)表面H原子扩散形成H₂分子释放出去的可能路径及金属Li原子掺杂对其影响. 研究结果表明: 干净MgH₂(001)表面第一层释放H原子形成H₂分子有两种可能路径, 其释放能垒分别为2.29和2.50 eV; 当将Li原子替代Mg原子时, 两种H原子扩散释放路径的能垒分别降到了0.31和0.22 eV, 由此表明Li原子掺杂使MgH₂(001)表面H原子扩散形成H₂释放更加容易.     

The effects of chain number and state of lipid derivatives of nucleosides on hydrogen bonding and self-assembly through the investigation of Langmuir-Blodgett films

The long-chain lipid derivatives of acyclovir—a nucleoside analogue were used to prepare Langmuir-Blodgett (LB) films, including the single-chained derivative (SGSA) and the double-chained derivative (DASA). The bilayer LB film of DASA or the SGSA/cholesterol (SGSA/Chol) mixture (1:1, mol/mol) on quartz plates was investigated with ultraviolet absorption spectroscopy, and the blue-shifted absorption with 4 nm (DASA) or 18 nm (SGSA/Chol) wavelength changes was observed in comparison with their solutions in chloroform. The rigid double chains of DASA prevented adjacent molecules from approach, while the flexible single chains of SGSA did not. Then the strength of intermolecular hydrogen bonding between the nucleoside moieties of DASA was much more weaker than one of SGSA, and their blue-shifted wavelength in LB films was different. DASA and SGSA/Chol also showed the different bilayer LB films on mica according to the atomic force microscopic observation. The former was prone to tilting on solid supports while the latter would like to stand vertically with the help of cholesterol that could insert into the flexible single chains of SGSA. The chain number (one or two) and state (flexible or rigid) of lipid derivatives of nucleosides strongly impact intermolecular hydrogen bonding and self-assembly behavior.     

The chemisorption of hydrogen on silicon and silicon carbide (1 0 0) surfaces

The chemisorption of hydrogen onto semiconductor surfaces is examined. The hydrogen bonds to the dangling bond of a surface atom. These dangling bonds also dictate the reconstruction of the crystal surface. The chemisorbed hydrogen therefore modifies the reconstructed surface topology. In this work theoretical calculations of the surface structures of both covalent elemental silicon and polar silicon carbide are presented. The periodic MINDO method is employed to determine the topologies for the 2 × 1 reconstructed (1 0 0) surfaces. These topologies are obtained from the minimisation of the total energy, for silicon and silicon carbide films of 14 layer thickness, with respect to the atomic co-ordinates of the hydrogen adsorbate and the first four layers of the substrate. The results show that the formation of the hydrogen bond to the substrate leads to a general lengthening of the surface dimer bond. In addition, the buckling of the silicon dimer determined for silicon terminated SiC is removed by hydrogen chemisorption.     

Diffusion in supported lipid bilayers: Influence of substrate and preparation technique on the internal dynamics

The diffusion law of DMPC and DPPC in Supported Lipid Bilayers (SLB), on different substrates, has been investigated in details by Fluorescence Recovery After Patterned Photobleaching (FRAPP). Over micrometer length scales, we demonstrate the validity of a purely Brownian diffusive law both in the gel and the fluid phases of the lipids. Measuring the diffusion coefficient as a function of temperature, we characterize the gel-to-liquid phase transition of DMPC and DPPC. It is shown that, depending on the type of substrate and the method used for bilayer preparation, completely different behaviours can be observed. On glass substrates, using the Langmuir-Blodgett deposition technique, both leaflets of the bilayer have the same dynamics. On mica, the dynamics of the proximal leaflet is slower than the dynamics of the distal leaflet, although the transition temperature is the same for both layers. Preparing bilayers from vesicle fusion in same conditions leads to more random behaviours and shifted transition temperatures.     

Influence of the reversible adsorption of methyl alcohol on magnetization reversal in ferromagnets

A change in the dynamics of 180° domain walls on the surface of a soft amorphous ferromagnet in methyl alcohol atmosphere is established by means of a magnetooptic method. A reversible decrease in the relaxation frequency of the domain walls near the surface in the presence of methyl alcohol admolecules is observed. This effect is related to the magnetic defects resulting from methyl alcohol adsorption on the ferromagnet surface, which proceeds through hydrogen bonding. Based on earlier data for the influence of the reversible adsorption of water molecules on the domain wall dynamics in ferromagnets, it is concluded that reversible adsorption through the mechanism of hydrogen bond formation considerably affects the domain wall dynamics in soft ferromagnets.