Thermal study of accumulation of conformational disorders in the self-assembled monolayers of C8 and C18 alkanethiols on the Au111 surface

The thermal stability of short alkanethiol CH(3)(CH(2))(7)SH (C(8)) and long C(18) self-assembled monolayers (SAMs) is investigated using grazing angle reflection-absorption infrared spectroscopy, cyclic voltammetry, and molecular dynamics simulation. We track the disordering of SAM by untilting and gauche defect accumulation with increasing temperature in the 300-440 K range, a range of interest to tribology. Molecular dynamics simulation with both fully covered and partially covered C(6), C(8), and C(18) monolayers brings out the morphological changes in the SAM, which may be associated with the observed thermal stability characteristics. The molecular dynamics simulations reveal that short-chain C(6) and C(8) alkanethiols are more defective at lower temperature than the long-chain C(18) alkanethiol. With increasing temperature disorder in the SAM, as reflected in both untilting and gauche defect accumulation, tends to saturate at temperatures below 360 K for short-chain SAMs such that any further increase in temperature, until desorption, does not lead to any significant change in conformational order. In contrast the disorder in the long-chain C(18) SAM increases monotonically with temperature beyond 360 K. Thus, in a practical range of temperature, the ability of a SAM to retain order with increasing thermal perturbations is governed by the state of disorder prior to heat treatment. This deduction derived from molecular dynamics simulation helps to rationalize the significant difference we have observed experimentally between the thermal response of short- and long-chain thiol molecules.     

X-ray determination of the mean amplitudes of vibration and debye temperatures of some rare earth monochalcogenides

The x-ray diffraction intensities of Bragg reflections have been measured at room temperature for thulium selenide, samarium sulphide, samarium selenide and samarium telluride. On the basis of a common amplitude approximation, the Debye-Waller factor, the mean amplitude of vibration and the Debye temperature have been evaluated. The values of the Debye temperatures and mean amplitudes of vibration are 176±16°K, 0·185 ± 0·017 Å (TmSe), 155 ± 7°K, 0·244 ± 0·012 Å (SmS), 153 ± 14°K, 0·221 ± 0·020 Å (SmSe) and 151 ± 20°K, 0·204 ± 0·027 Å (SmTe).     

Solid/solid phase transitions in confined thin films: a zero temperature approach

We report a density functional theory study of confinement induced solid/solid phase transitions in a thin film (modeled as methane) at T=0. The solid film is confined by two graphite surfaces represented by a mean-field potential. As the wall separation is varied the structure of the confined film changes, which influences its density and the solvation force. Using the directly accessible grand canonical potential density we determine the stable phases and calculate the exact location of the phase transitions. We observe a series of phases having square and triangular symmetry. At low wall separations we find zig-zag buckling and an asymmetric buckled phase, whose structure is consistent with the strongest buckling instability of a triangular monolayer predicted by Chou and Nelson [Phys. Rev. E 48, 4611 (1993)] but, to our knowledge, has not been observed as a stable phase before. We find that the two-dimensional order parameters Psi(4) (square symmetry) and Psi(6) (triangular symmetry) show unphysical behavior in the transition region between square and triangular symmetry. Thus, in the present model they fail to predict the right location of the phase transitions.     

Freezing of fluids confined between mica surfaces

Using grand ensemble simulations, we show that octamethyl-cyclo-tetra-siloxane (OMCTS) confined between two mica surfaces can form a variety of frozen phases which undergo solid-solid transitions as a function of the separation between the surfaces. For atomically smooth mica surfaces, the following sequence of transitions 1[triangle up] --> 1[triangle up]b --> 2B --> 2 square --> 2[triangle up] are observed in the one- and two-layered regimes, where n[triangle up], n[square], and nB denote triangular, square, and buckled phases, respectively, with the prefix n denoting the number of confined layers. The presence of potassium on mica is seen to have a strong influence on the degree of order induced in the fluid. The sequence of solid-solid transitions that occurs with the smooth mica surface is no longer observed. When equilibrated with a state point near the liquid-solid transition, a counterintuitive freezing scenario is observed in the presence of potassium. Potassium disrupts in-plane ordering in the fluid in contact with the mica surface, and freezing is observed only in the inner confined layers. The largest mica separations at which frozen phases were observed ranged from separations that could accommodate six to seven fluid layers. The extent of freezing and the square-to-triangular lattice transition was found to be sensitive to the presence of potassium as well as the thermodynamic conditions of the bulk fluid. The implications of our results on interpretation of surface force experiments as well as the generic phase behavior of confined soft spheres is discussed.     

Model for dynamics of inhomogeneous and bulk fluids

An accurate model for the density of states (DOS) for strongly inhomogeneous and bulk fluids has been proposed based on gamma distributions. The contribution to the density of states from the collective dynamics is modeled as an incomplete gamma distribution and the high frequency region is obtained from the solution of the memory equation using a sech memory kernel. Using only the frequency moments as input, the model parameters for the collective dynamics are obtained by matching moments of the resulting distribution. The model results in an analytical expression for the self-diffusivity of the fluid. We present results for soft sphere fluids confined in slit-shaped pores as well as bulk soft sphere liquids. Comparisons of the DOS, velocity autocorrelation functions, and memory kernels with molecular dynamics simulations reveal that the model predicts features in the DOS over the entire frequency range and is able to capture changes in the DOS as a function of fluid density and temperature. As a result the predicted VACFs, memory kernels, and self-diffusivities are accurately predicted over a wide range of conditions. Since the frequency moments for bulk liquids can be obtained from pair correlation functions, our method provides a direct route from fluid structure to dynamics. For fluids confined in slit-shaped pores, where the frequency moments are obtained from molecular dynamics simulations, the predicted self-diffusivities capture the resulting oscillations due to variations in the solvation pressure, and in the case of smooth walled pores, the predictions are superior to those obtained using kinetic theory.     

帕隆藏布流域堆积体边坡的工程地质特征及稳定性评价

位于藏东南的帕隆藏布流域,处在东喜马拉雅构造结的北缘,是新构造强烈隆升区,断层、褶皱等地质构造发育。在青藏高原隆升及气候变化等内外动力作用下,研究区内堆积体分布广泛,具有独特的工程地质特征并成为该区内地质灾害的主要物源。由于河流的下切及川藏公路的修建,形成了许多高陡的堆积体边坡,以冰碛边坡最为典型。由于其特殊的物源特征及形成过程,冰碛物具有粒度分布范围广、粗粒含量多等与其他沉积物不同的粒度特征。冰碛物粒度分布范围广的特征,导致随着研究尺度的不同,其结构特征也不相同,具有明显的尺度效应。冰碛土的力学强度一般强于其他堆积体,但受不同粒径颗粒的含量及分布、形成时代的影响,其力学特征也有所差异。作者对川藏公路沿线部分典型的剖面进行了量测,所量测的剖面虽然只是整个斜坡的一部分,与经验数据相比,仍表现出高陡的特征,而且很多边坡在既高且陡的形态下仍能保持稳定。本文以极限平衡理论为基础,采用多种方法对这类边坡的稳定性进行计算分析,求出边坡的安全系数。考虑各种地震烈度下边坡的稳定性,得出这些边坡的安全系数与地震加速度具有严格的指数相关关系。     

Molecular dynamic simulations of self-assembled alkylthiolate monolayers on an Au(III) surface

Molecular dynamics simulations incorporating explicit gold atoms in the simulations have been carried out for alkanethiol self-assembled monolayers chemisorbed on the Au(III) surface. The structural properties of the monolayer are evaluated for two force fields: one in which the Au--S--C bond is fixed (FF I), and the other in which it is flexible (FF II). The influence of these force fields on the structural properties of HS(CH2)14CH3 on the structured Au surface is compared at different temperatures. FF I yields greater tilt angles and a smaller film thickness when compared with FF II. Both of the force fields predict that the tilt angles do not follow a monotonic decrease with temperature but show minima around 200 K. Simulations carried out at different chain lengths at 300 K reveal that FF II predicts a greater film thickness than FF I; however, the difference is within 1 A.     

Binary fluids in planar nanopores: Adsorptive selectivity, heat capacity and self-diffusivity

Monte Carlo and molecular dynamics simulations are performed to study fluid adsorption of a two component fluid in slit pores of nanoscopic dimensions. The slit pores are immersed in a binary fluid bath, which is comprised of spherical molecules having a size ratio of 1.43, at constant temperature and composition. Pore width is varied to determine how the heat capacity and self-diffusion coefficient are linked to the composition and structure of the adsorbed fluid. In pores where the fluid structure is most pronounced, we observe: perfect (or near perfect) exclusion of one component by the other component, a heat capacity that rapidly oscillates and is of greater magnitude than in the fluid bath, and self-diffusion coefficients on the order of 10^–8 cm²/s. The behavior of the heat capacity and diffusion coefficients appears to arise from a near solid-like layering of OMCTS that occurs at certain favorable pore widths.     

Non-monotonic, distance-dependent relaxation of water in reverse micelles: propagation of surface induced frustration along hydrogen bond networks

Layer-wise, distance-dependent orientational relaxation of water confined in reverse micelles (RM) is studied using theoretical and computational tools. We use both a newly constructed "spins on a ring" (SOR) Ising-type model (with Shore-Zwanzig rotational dynamics) and atomistic simulations with explicit water. Our study explores the effect of reverse micelle size and role of intermolecular correlations, compromised by the presence of a highly polar surface, on the distance (from the interface) dependence of water relaxation. The "spins on a ring" model can capture some aspects of distance dependence of relaxation, such as acceleration of orientational relaxation at intermediate layers. In atomistic simulations, layer-wise decomposition of hydrogen bond formation pattern clearly reveals that hydrogen bond arrangement of water at a certain distance away from the surface can remain frustrated due to the interaction with the polar surface head groups. This layer-wise analysis also reveals the presence of a non-monotonic slow relaxation component which can be attributed to this frustration effect and which is accentuated in small to intermediate size RMs. For large size RMs, the long time component decreases monotonically from the interface to the interior of the RMs with slowest relaxation observed at the interface.     

Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO(2) and CH(4) to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH(2), H(2)PO(3), NO(2), and CH(3). Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO(2) to exceed that of CH(4) by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH(2), H(2)PO(3), NO(2), and COOH functional groups can significantly enhance gas binding, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO(2)/CH(4) selectivity are discussed.