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Atomic force microscopy (AFM) can work in a variety of environment with different humidities. When the tip of AFM approaches a sample, the measured adhesion force would be significantly affected by the presence of nanometer-sized liquid bridge. The formation and rupture of liquid bridges can occur either through equilibrium or nonequilibrium process. In this work, the liquid bridges are assumed to be in thermodynamic equilibrium with the surrounding vapor medium. To study theoretically the stability of liquid bridge, a constraint is added into the lattice density functional theory to stabilize a series of bridges with different radii at a given tip-substrate distance. With the help of the constraint, we can identify not only stable and metastable states but also transition states for the formation and rupture of liquid bridges. Using this constrained method we calculate the energy barriers involved in the formation and rupture of the liquid bridges, respectively, and then discuss their stability as well as the origin of the hysteresis behavior observed with atomic force microscope measurements. On the whole, the calculated force-distance curves are found to be qualitatively in agreement with experimental observations. The energy barriers for the formation and rupture of liquid bridges are also analyzed as a function of tip-sample distance, humidity, and tip-fluid interaction. [ABSTRACT FROM AUTHOR]

In this work, extensive lattice Monte Carlo simulations were performed to investigate the influence of confinement on critical micelle concentration (CMC). It is found that the CMC of surfactants in a confined space is shifted from its bulk value, and the shift is affected by the presence of the confining boundaries, which induces both the finite size effect and the wall-surfactant interaction. In general, for strongly confined system (the system with narrow pore size), the finite size effect dominates the CMC shift because the confined space cannot accommodate fully developed micelles, and the rapid increase of the entropic loss due to the decrease of the pore size results in the rapid increase of CMC. In contrast, for a weakly confined space, the CMC shift depends on the interaction between the walls and surfactants. For the systems with two weakly hydrophilic surfaces, the local density depletion of the surfactants near the walls results in lower CMCs than the bulk value, and the CMC shifts to a higher value as the pore size increases. For the systems with moderately hydrophilic surfaces, the shifts of CMCs show a similar behavior as those for weakly hydrophilic surfaces, but the CMCs are near their bulk values in the range of weak confinement. For the systems with strongly attractive wall-surfactant interactions, the strong adsorption also results in lower CMCs than their bulk value, but the CMCs decrease with the increase of pore size. [ABSTRACT FROM AUTHOR]

The dependence of the adsorption isotherm and phase behavior of Lennard-Jones (LJ) fluid in cylindrical pores on the fluid–wall interaction was investigated using a discrete attractive potential model and the density functional theory. The discrete attractive potential contains two parts, a deep well (dw) and a tail. The results show that the chemical potential corresponding to capillary condensation has a weak dependence on the deep well potential near the pore wall, but exhibits a strong dependence on the tail potential near the pore center. On the contrary, the chemical potential of the formation of the first monolayer is strongly dependent on the deep well potential but almost independent of the tail potential. The shape of the adsorption isotherm plotted as density versus chemical potential is determined by the shape of the fluid–wall interaction, namely [variant_greek_epsilon][sub tail]–[variant_greek_epsilon][sub dw], rather than either the deep well potential or the weak tail potential separately. The critical temperature exhibits similar behavior in its dependence on [variant_greek_epsilon][sub tail]–[variant_greek_epsilon][sub dw]. There exists a region where layer transition and capillary condensation intersect. Adsorption isotherms, coexistence curves, and critical temperatures in this region exhibit distinct phase behaviors from other regions. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the development of a quasi-two-dimensional Stockmayer model in which the center of mass of the molecule is confined on a plane while the dipole of the molecule can rotate freely in three dimensional space. Use of the Gibbs ensemble Monte Carlo technique to determine the vapor-liquid equilibria of the model fluids; Systems with different reduced dipole moments studied.

The study gives the elimination of two kinds of antibiotics, tetracycline (TC) and cefradine (CF) by adsorption process, on the biochar derived from cornstalk. Dense, multifaceted and thick fragments of raw cornstalk almost all vanished at a pyrolytic temperature above 400°C. The carbon content increased from 60.48% of the raw cornstalk to 75.5% of the cornstalk biochar pyrolyzed at 600°C (BC600), while the oxygen content decreased from 17.31% to 6.94%. The uptake of each TC and CF on the cornstalk biochar was highly pH-dependent. The maximum adsorption capacities of TC and CF at 298 K calculated from the Langmuir mannequin have been 28.0 and 38.0 mg/g, respectively. The Columbic interaction and ?–? electron-donor-acceptor interaction between cornstalk biochar and CF/TC molecules played the main role. The experimental records were well outfitted by way of the ability of the pseudo-second-order kinetics model, indicating a possible chemisorption process to some extent. Isotherm result implied that both adsorption and partitioning contributed to the uptake of TC and CF onto BC600.

In this work, we used the lattice Boltzmann (LB) method to simulate the generation of concentration gradients in microfluidic networks. We first developed a model of microfluidic networks, in which the flow velocity of the laminar flow in the microchannel and the diffusion of solute molecules govern the solute gradients generated, and the obtained results are comparable with experimental data. Our simulation results indicated that the relative positions of the branching plates in different levels, at which a microchannel is split into several daughter microchannels, exert significant effects on the shape of concentration gradients. We also performed extensive simulations to study the dependence of the shape of the concentration gradient on the velocity of the flow u, diffusion coefficients of solutes D, and the length of the microchannel L. A dimensionless parameter, (L/u)/(H2/D), in which His the hydraulic diameter of the main microchannel, was proposed in this work. It is found that for geometrically similar microfluidic networks, the parameter alone determines the shape of the generated concentration gradient. Therefore, the proposed parameter allows one to perform experiments or simulations with reduced models in microchannels and correlate the data to the other flows or geometry sizes. [ABSTRACT FROM AUTHOR]

In this work, a coarse-grained model for a kind of membrane protein, the mechanosensitive channel of small conductance (MscS), is proposed. The basic structure of the MscS is preserved when the protein is coarse-grained. For the coarse-grained model, the channels show two different states, namely the open and closed states, depending on the model parameters. Under the same membrane tension, the state of the ion channel is found to be critically determined by the protein structure, especially the length of the transmembrane α-helix. It is also found that for the protein with certain size, the gating transition occurs when the membrane tension is applied, resembling in a real mechanosensitive channel. [ABSTRACT FROM AUTHOR]

By combining the gauge cell method and lattice model, we study the surface phase transition and adsorption behaviors of surfactants on a solid surface. Two different cases are considered in this work:  macrophase transition and adsorption in a single-phase region. For the case of macrophase transition, where two phases coexist, we investigate the shape and size of the critical nuclei and determine the height of the nucleation barrier. It is found that the nucleation depends on the bulk surfactant concentration. Our simulations show that there exist a critical temperature and critical adsorption energy, below which the transition from low-affinity adsorption to the bilayer structure shows the characteristic of a typical first-order phase transition. Such a surface phase transition in the adsorption isotherm is featured by a hysteresis loop. The hysteresis loop becomes narrower at higher temperature and weaker adsorption energy and finally disappears at the critical value. For the case where no macrophase transition occurs, we study the adsorption isotherm and microphase separation in a single-phase region. The simulation results indicate that the adsorption isotherm in adsorption processes is divided into four regions in a log−log plot, being in agreement with experimental observations. In this work, the four regions are called the low-affinity adsorption region, the hemimicelle region, the morphological transition region, and the plateau region. Simulation results reveal that in the second region the adsorbed monomers aggregate and nucleate hemimicelles, while adsorption in the third region is accompanied by morphological transitions. [ABSTRACT FROM AUTHOR]

A canonical Monte Carlo simulation is performed to investigate the microstructure and the electrical double layer (EDL) of polyelectrolytes around macroions in the bulk systems based on the primitive model. We explore the influences of particles size, chain length, and charge density of polyelectrolytes on the microscopic behavior of the macroions-polyelectrolytes systems. The simulation results show that the surface charge density and the chain length of the polyelectrolytes are two key factors that affect the microstructure of polyelectrolytes around the macroions and potential of mean force between the macroions as well as the potential of the spherical EDL constructed by polyelectrolytes. The high surface charge density of a polyelectrolyte leads to the polyelectrolyte acting as a bridge for the aggregation of macroions, causing the presence of the attraction between macroions. The polyelectrolytes with a long chain length present a cooperativity effect for the adsorption of the polyelectrolytes on the surface of the macroions. Furthermore, the two key factors both induce the overcharge of the macroions. The longer the chain length and the higher surface charge density of the polyelectrolytes, the stronger is the overcharge. [ABSTRACT FROM AUTHOR]

We use a hybrid density functional approach to investigate the microstructure and self-assembly of inhomogeneous rigid rodlike chains between two neutral surfaces, i.e., two hard walls. In the calculation, the rodlike molecule is modeled as a rigid rod linearly connected by the tangent sphere beads. The hybrid method combines a single-chain Monte Carlo (MC) simulation for the ideal-gas part of Helmholtz energy and a DFT approach for the excess Helmholtz energy. The DFT approach includes a modified fundamental measure theory for the excluded-volume effect, the first order thermodynamics perturbation theory for chain connectivity, and the mean field approximation for the van der Waals attraction. We investigate the effect of the chain length (i.e., aspect ratio) of the rodlike molecule and the separation between two surfaces on the microstructure and self-assembly of inhomogeneous rigid rodlike chains. For the athermal systems, the rodlike chain fluids present a smaller partitioning coefficient compared to the flexible chain fluids. For the thermal systems, lamellar thin films formed by the rigid rodlike molecules perpendicular to the neutral surface are observed. The effects of the head−head interaction and the separation on the self-assembly of the rodlike chain fluids in the slit are investigated. [ABSTRACT FROM AUTHOR]

The South-to-North Water Diversion Project in China is an important measure to promote the ecological civilization construction in the receiving areas. Since the operation of the east and middle routes, the comprehensive benefits are more and more remarkable. It is very significant to quantitatively evaluate the comprehensive benefits of the South-to-North Water Diversion Project. The idea of quantitative evaluation by the improved degree of the carrying capacity of regional water environment after water diversion is put forward in this paper. On the basis, combining with variable fuzzy set and cloud theory, a new comprehensive assessment model is established. According to the evaluation index system of water environmental carrying capacity and the index values of research area, the linear difference function value is generated randomly by triangular forward cloud generator, and then the variable fuzzy comprehensive evaluation is carried out. Taking Haihe River Basin and Beijing City as typical research areas respectively, the comprehensive benefits of water supply from the diversion project in 2030 are analysed. The comprehensive evaluation method with variable cloud model considers both “good” and “bad” aspects and considers both randomness and fuzziness. Results show that the model is feasible and efficient. It is worth popularizing and applying.

We report a theoretical study of nanobubble stabilization on a substrate by contact line pinning. In particular, we predict the magnitude of the pinning force required to stabilize a nanobubble and the threshold values of the pinning force that the substrate can provide. We show that the substrate chemistry and the local structures of substrate heterogeneity together determine whether or not surface nanobubbles are stable. We find that for stable nanobubbles, the contact angles are independent of substrate chemistry as its effects are cancelled out by the pinning effect. This prediction is in agreement with available experimental data. [ABSTRACT FROM AUTHOR]

Using first-principles calculations, we have systematically studied the quantum size effects of ultrathin Pb(111) films on the adsorption energies and penetration energy barriers of oxygen atoms. For the on-surface adsorption of oxygen atoms at different coverages, all the adsorption energies are found to show bilayer oscillation behaviors. It is also found that the work function of Pb(111) films still keeps the bilayer-oscillation behavior after the adsorption of oxygen atoms, with the values being enlarged by 2.10–2.62 eV. For the penetration of the adsorbed oxygen atoms, it is found that the energy barriers are all oscillating with a bilayer period on different Pb(111) films because of the modulation of quantum size effects. Our studies indicate that the quantum size effect in ultrathin metal films can modulate a lot of processes during surface oxidation. [ABSTRACT FROM AUTHOR]

In this work, lattice Monte Carlo was used to study the effects of crowding on the self-assembly of surfactants. Simulation results show that crowding strongly shifts the critical micelle concentration (CMC) of surfactants from the bulk value. Two effects originated from crowding are found to govern the CMC shift: one is the depletion effect by crowding agents and the other is the available volume for micelle formation. The depletion effects inevitably result in the enrichment of surfactants in crowding-free regions and cause the decrease in CMC. On the other hand, the appearance of crowding agents decreases the available volume for micelle formation, which reduces the conformational entropy and impedes the micelle formation. Three factors, including the radius of crowding agents, the arrangement of crowding agents, and the volume fraction of crowding agents, are considered in this work to study the crowding effects. The trends of CMC shifts are interpreted from the competition between the depletion effects and the available volume for micelle formation. [ABSTRACT FROM AUTHOR]

It is known that the thermoacidophilic archaebacterium Sulfolobus acidocaldariuscan grow in hot springs at 65−80 °C and live in acidic environments (pH 2−3); however, the origin of its unusual thermal stability remains unclear. In this work, using a vesicle as a model, we study the thermal stability and rupture of archaebacterial cell membrane. We perform a simulation investigation of the structure−property relationship of monolayer membrane formed by bolaform lipids and compare it with that of bilayer membrane formed by monopolar lipids. The origin of the unusually thermal stability of archaebacterial cell and the mechanism for its rupture are presented in molecular details. [ABSTRACT FROM AUTHOR]

Abstract: The crystal growth, space group, Raman and IR spectra of the thiocyanato manganese mercury-N,N-dimethylacetamide (MnHg(SCN)4(H2O)2 (C4H9NO)2, MMTWD) single crystal are introduced. The experiments show that the characteristic vibrational modes of MMTWD crystal consists of eight frequency regions: 50–300cm−1, lattice modes; 300–550cm−1, NCS bending; 550–700cm−1, H2O wresting; 700–1200cm−1, SCN stretching; 1200–1400cm−1, CH3 bending; 1400–1500cm−1, OH bending; 1500–1700cm−1, CH2 bending or CO stretching; 2000–3400cm−1, CN, CH, and OH stretching. Spectra analysis reveals that MMTWD crystal belongs to the two-dimensional layer net structure. The complex network layers are piled up in the c axis direction, and the water and dimethylacetamide (C4H9NO, DMA) molecules occupy the interlayer space. It is the additive and coordinates of water and DMA molecules in the layers that provides the crystal with large interaction force between molecules within a layer, which in turn, induces large nonlinear optical properties and higher environmental stability. [Copyright &y& Elsevier]

A concentration gradient along a fluid-fluid interface can cause flow. On a microscopic level, this so-called Marangoni effect can be viewed as being caused by a gradient in the pressures acting on the fluid elements or as the chemical-potential gradients acting on the excess densities of different species at the interface. If the interface thickness can be ignored, all approaches should result in the same flow profile away from the interface. However, on a more microscopic scale, the different expressions result in different flow profiles, only one of which can be correct. Here we compare the results of direct nonequilibrium molecular dynamics simulations with the flows that are generated by pressure and chemical-potential gradients. We find that the approach based on the chemical-potential gradients agrees with the direct simulations, whereas the calculations based on the pressure gradients do not. [ABSTRACT FROM AUTHOR]