Interfacial dilational rheological property and lamella stability of branched alkyl benzene sulfonates solutions

The dynamic dilational properties of branched alkyl benzene sulfonates at the air–water and decane–water interfaces were investigated by drop shape analysis, and their lamella stability was measured. The influences of time, dilational frequency, and bulk concentration on surface dilational elasticity and dilational viscosity were expounded. The results show that the molecular interaction controls the nature of adsorption film during lower concentration range and the film behaves elastic in nature. During higher concentration range, the diffusion-exchange process controls the dynamic dilational properties and the surface film shows remarkable viscoelasticity. An increase in hydrophobic chain length enhances the molecular interaction, which results in the increase of dilational parameters and lamella stability. The data correlation suggests that the ability to form a stable lamella is linked to the intrinsic surface dilational elasticity.     

Numerical Simulation of the Effects of Experimental Error on the Higher Harmonic Components of Fourier Transformed AC Voltammograms

In this simulation study we investigate the effects of realistic levels of random and systematic experimental error in higher harmonics detected by Fourier transformed AC voltammetry for a reversible process. We show that by analyzing in the frequency domain results that are collected in the time domain, we can achieve a very large reduction in the magnitude of random (Gaussian) noise, and complete elimination of the systematic error introduced by mains frequency in both the small and large amplitude cases. As a consequence, we confirm experimental observations that even the eighth harmonic for a reversible process can be readily detected in the presence of very significant noise levels when 100 mV amplitude sine wave perturbations are employed.     

Fully flexible unit cell simulation with recursive thermostat chains

The recursive thermostat chained fully flexible cell molecular dynamic simulation (NsigmaT ensemble) is performed. The ensemble is based on the metric tensor, whose components are used as extended variables. These variables are combined with Nosé-Poincaré recursive thermostat chains. This extended Hamiltonian approach preserves Hamiltonian in structure, and the partition function satisfies the NsigmaT ensemble state in phase space. In the present study, the generalized leap frog method was employed for time integration. The resulting molecular dynamics simulation was performed for bulk and thin film solid materials in the face-centered-cubic crystal structure. Uniaxial tension test and simple shear test are performed to predict the behaviors of a solid material in the bulk state and nanoscale thin film state. The proposed flexible cell method should serve as a powerful tool for the prediction of mechanical and thermal properties of solid materials including nanoscale behavior.     

Recent advances in polymer molecular dynamics simulation and data analysis

Significant advances in molecular simulation methodology over the past decade have greatly reduced the traditional size-timescale bottleneck in molecular dynamics calculations. The development of the geometric statement function method allows for systems up to several hundred thousand atoms to be simulated for up to several nanoseconds in reasonable times on standard workstations. For constant energy simulations, the use of symplectic integrators ensures accurate dynamics, even at long simulation times, without velocity or other artificial rescaling schemes. Finally, new methods of frequency estimation allow for accurate vibrational mode frequency calculations even in the presence of chaotic motion on time scales twenty times shorter than the standard fast Fourier transform, with an additional improvement in the sensitivity of the results when initial dynamics conditions are carefully chosen.     

A complete numerical simulation of the techniques of alternating current linear sweep and cyclic voltammetry: analysis of a reversible process by conventional and fast Fourier transform methods

We describe the method of achieving the first completely general simulation of ac linear sweep and cyclic voltammetry making use of the fully implicit Richtmyer modification (FIRM) method. The simulation technique is applied to a reversible process under conditions where a sinusoidal waveform of any amplitude is superimposed onto the dc potential which is swept at a finite scan rate. Results, where possible, are compared with the existing theory derived at constant dc potential to confirm the fidelity of the simulation. In particular, we demonstrate excellent agreement with the results of Engblom et al. [J. Electroanal. Chem. xxx (1999) xxx] for large amplitude ac voltammetry described in the companion paper immediately preceding this article. The use of conventional and Fourier transform methods of data analysis are compared to highlight the advantages of the use of the fast Fourier transform algorithm in ac voltammetry.     

Anisotropic dynamics of dipolar liquids in narrow slit pores

We report molecular dynamics simulation results for Stockmayer fluids confined to narrow slitlike pores with structureless, nonconducting walls. The translational and rotational dynamics of the dipolar particles have been investigated by calculating autocorrelation functions, diffusion coefficients, and relaxation times for various pore widths (five or less particle diameters) and directions parallel and perpendicular to the walls. The dynamic properties of the confined systems are compared to bulk properties, where corresponding bulk and pore states at the same temperature and chemical potential are determined in parallel grand canonical Monte Carlo simulations. We find that the dynamic behavior inside the pore depends on the distance from the walls and can be strongly anisotropic even in globally isotropic systems. This concerns especially the particles in the surface layers close to the walls, where the single particle and collective dipolar relaxation resemble that of true two-dimensional dipolar fluids with different in-plane and out-of-plane relaxations. On the other hand, bulklike relaxation is observed in the pore center of sufficiently wide pores.     

Role of three-body interactions in formation of bulk viscosity in liquid argon

With the aim of locating the origin of discrepancy between experimental and computer simulation results on bulk viscosity of liquid argon, a molecular dynamic simulation of argon interacting via ab initio pair potential and triple-dipole three-body potential has been undertaken. Bulk viscosity, obtained using Green-Kubo formula, is different from the values obtained from modeling argon using Lennard-Jones potential, the former being closer to the experimental data. The conclusion is made that many-body inter-atomic interaction plays a significant role in formation of bulk viscosity.     

Quantum effects on adsorption and diffusion of hydrogen and deuterium in microporous materials

Monte Carlo and molecular dynamics simulations and neutron scattering experiments are used to study the adsorption and diffusion of hydrogen and deuterium in zeolite Rho in the temperature range of 30-150 K. In the molecular simulations, quantum effects are incorporated via the Feynman-Hibbs variational approach. We suggest a new set of potential parameters for hydrogen, which can be used when Feynman-Hibbs variational approach is used for quantum corrections. The dynamic properties obtained from molecular dynamics simulations are in excellent agreement with the experimental results and show significant quantum effects on the transport at very low temperature. The molecular dynamics simulation results show that the quantum effect is very sensitive to pore dimensions and under suitable conditions can lead to a reverse kinetic molecular sieving with deuterium diffusing faster than hydrogen.     

Air-liquid interfaces of aqueous solutions containing ammonium and sulfate: spectroscopic and molecular dynamics studies

Investigations of the air-liquid interface of aqueous salt solutions containing ammonium (NH(4)(+)) and sulfate (SO(4)(2-)) ions were carried out using molecular dynamics simulations and vibrational sum frequency generation spectroscopy. The molecular dynamics simulations show that the predominant effect of SO(4)(2-) ions, which are strongly repelled from the surface, is to increase the thickness of the interfacial region. The vibrational spectra reported are in the O-H stretching region of liquid water. Isotropic Raman and ATR-FTIR (attenuated total reflection Fourier transform infrared) spectroscopies were used to study the effect of ammonium and sulfate ions on the bulk structure of water, whereas surface sum frequency generation spectroscopy was used to study the effect of these ions on the interfacial structure of water. Analysis of the interfacial and bulk vibrational spectra reveal that aqueous solutions containing SO(4)(2-) perturb the interfacial water structure differently than the bulk and, consistent with the molecular dynamics simulations, reveal an increase in the thickness of the interfacial region.     

Surface,thermal and mechanical characteristics of polymeric solids

Surface molecular motions of amorphous polymeric solids have been directly measured on the basis of lateral force microscopic (LFM) and scanning viscoelasticity microscopic (SVM) measurements. SVM measurement revealed that the molecular motion at the surface of the monodisperse polystyrene (PS) film with Mn less than ca.30k was fairly activated compared with that in a bulk region, mainly due to the surface segregation of chain end groups. Temperature dependent LFM and SVM measurement revealed that the surface glass transition temperature, Tg of the monodisperse PS film was lower than the bulk one, even though Mn was fairly large as 140k and also, that the time-temperature superposition was applicable to the surface relaxation process. The chain end group segregation at the air/PS interface was verified from the dynamic secondary ion mass spectroscopic (DSIMS) depth profiling of the proton and deuterium ion for the end-labeled deutrated-PS (dPS) film. These results suggest that the surface Tg is depressed due to an increase in free volume near surface region, being induced by the preferential surface localization of chain end groups.     

原油活性组分油水界面膜扩张粘弹性研究

研究了用超临界萃取分馏法（SFEF）从伊朗重质原油中分离的两个具有不同平均分子量的原油界面活性组分在正癸烷/水界面的扩张粘弹性行为以及温度对体系扩张粘弹性的影响.研究发现平均分子量大的样品能在油水界面形成更为牢固的界面膜.从扩张模量幅度对扩张频率的双对数曲线和扩张模量相角的频率依赖关系可以推断所有实验体系界面膜的主要的弛豫过程不是扩散弛豫,而可能主要是通过吸脱附势垒的弛豫过程.温度对两个样品的扩张粘弹性参数都有强烈的影响.升高温度可以降低膜的强度和粘度,并且改变相角的频率响应.     

Molecular dynamics simulation of surface segregation in a (110) B2-NiAl thin film

Surface segregation in (110) B2-NiAl film approximately 3 nm thick is investigated by using molecular dynamics simulation with a reliable embedded-atom potential. The simulation is performed for the stoichiometric composition at a temperature of 1500 K, just below the melting temperature of the film model. It is found that the (110) surface is structurally stable but develops adatoms, vacancies and antisites. The coverage of an adatom layer is estimated to be ～0.07 ML (monatomic layers) and it contains on average ～95% of Al atoms. The top (surface) and second (subsurface) layers of the (110) surface is the most enriched in Ni relative to the bulk composition. These layers contain on average ～51% of Ni atoms. The Ni fraction in the third and forth layers of the film is estimated as ～50.5%. The deeper layers have essentially the bulk composition. Vacancies in the film model are found only on the Ni sublattice. The vacancy concentration on the Ni sublattice in the top layer is ～7.5%. The second layer almost does not contain vacancies. The next layers have essentially the constant bulk vacancy composition which can be estimated as ～1.3-1.4%.     

Transitory response of confined polymer films subjected to oscillatory shear

Molecular-dynamics simulations were used to study the response of a nanometer thin polymer film to oscillatory shear. Several types of response occur, depending on the amplitude of the shear. At low amplitude, the film deforms elastically. At intermediate ones it deforms plastically. Short-range stress-induced structured crystalline domains occur. This flexible elastic state is very dynamic. The crystalline domains oscillate with the applied stress. In the course of repeated cycling, they slowly increase in size. These mesoscopic domains may account for experimentally observed memory behavior. Ultra-thin polymer films typically possess relaxation times that are orders of magnitudes larger than those of the individual polymers. When oscillated at even higher amplitude, stick-slip is observed. In our constant pressure simulations, the film yields when wall spacing is increased to a value at which the polymer segments can smoothly rearrange and hence relax the internal stress.     

Dynamic ft-ir spectroscopy of polymer films and liquid crystals

The use of continuous-scan and step-scan Fourier transform infrared (FT-IR) spectroscopic techniques to study the dynamics of the response of polymer films and liquid crystals to external perturbations is described here. The first application of dynamic stepscan FT-IR deals with the response of various polymer films to sinusoidally modulated tensile strain. In these experiments, a small amplitude sinusoidal stress is applied to a polymeric film and the transition dipole responses are monitored as a phase lag with respect to the external perturbation. The degree of deformation is small enough to cause only linear reversible responses to the sample. The main advantage of the technique is that it can provide valuable information at the molecular level that can be used to interpret the macroscopic properties of the polymeric material under investigation. Results for heterogeneous polymers include semicrystalline high density/low density polyethylene blends and the micro-phase separated copolymer Kraton^r̀ are presented. In addition, continuous-scan stroboscopic FT-IR was used to explore the submolecular (functional group) contributions to the reorientation dynamics of the liquid crystal director in response to pulsed (DC) electric fields. For the nematic liquid crystal molecules, 4-pentyl-4'-cyanobiphenyl (5CB) and 4-pentyl-4'-cyanophenylcyclohexane (5PCH), the individual response of the rigid and floppy parts was examined.     

Effect of alkyl chain length on the surface dilational rheological and foam properties of N-acyltaurate amphiphiles

The dynamic dilational properties of sodium 2-(2-(alkylaryloxy)-alkylamido)ethanesulfonates (12+nB-Ts) at the air–water interfaces were investigated by drop shape analysis, and their foam properties were also measured. The influences of time and bulk concentration on surface dilational properties were expounded. The results show that the molecular interaction controls the nature of adsorption film during lower concentration range, and the film behaves elastic in nature. During higher concentration range, the diffusion exchange process controls the dynamic dilational properties and the surface film shows remarkable viscoelasticity. An increase in hydrophobic chain length enhances the molecular interaction at low concentration and speeds up the diffusion exchange process at high concentration, which results in the different variations of modulus at different concentration regions. For 12+nB-Ts, too short a chain probably produces bad film elasticity, whereas too great a length produces too fast liquid drainage. Therefore the optimal length in the branched chain leads to the best foam stability.     

Dynamic bulk and shear relaxation in glassy polymers. I. Experimental techniques and results on PMMA

In this paper the authors describe in detail the experimental techniques for the simultaneous measurement of the dynamic Young's modulus and the dynamic Poisson's ratio, from which the dynamic bulk and shear moduli can be calculated. Experimental results are presented on the effects of temperature, frequency, and tensile strain on these properties of poly(methyl methacrylate) (PMMA). The temperature and frequency effects indicate that the β relaxation in PMMA is not a purely internal motion but is coupled to the bulk.     

不同结构芳香侧链酰基牛磺酸钠的表面扩张性质

利用悬挂滴方法研究了N-(α-苯氧基)十四酸牛磺酸钠(12+B-T)和N-(α-对乙基苯氧基)十四酸牛磺酸钠(12+2B-T)在空气/水表面上的动态扩张粘弹性质, 考察了时间、扩张频率及摩尔浓度对扩张模量和相角的影响, 测定了不同摩尔浓度条件下的泡沫性能. 研究发现: 低浓度条件下, 表面分子间相互作用决定表面活性剂吸附膜的性质, 膜以弹性为主; 高浓度条件下, 扩散交换过程起主导作用, 吸附膜表现出粘弹特性. 表面活性剂芳环支链上增加一个乙基, 分子间相互作用增强, 扩张模量增大, 泡沫更加稳定.     

Controlling the Bonding Site of Photosynthetic Reaction Centers on Au Electrode by Different Bifunctional Agents

The orientation of the immobilized protein is the key factor to effect the surface electrochemistry of the redox couples therein. The photosynthetic reaction centers (RCs) composite film was fabricated by self-assembled monolayers (SAMs) on Au electrode. The bifunctional reagents e.g. 4-aminothiophenol (ATP), 2-mercaptoehtylamine (MEA), 2-mercaptoethanol (ME), 2-mercaptoacetic acid (MAA), and Poly(dimethyldiallylammonium) chloride(PDDA) were found to bond RCs at different sites. The square wave voltammetry (SWV), bulk electrolysis were employed for characterizing the composite film. The electrochemically-driven electron transfer (ET) behavior in films was found driven by SWV or bulk electrolysis. In case the electrode was modified directly by amino compound reagent, the greater force was needed. If the film was activated by SWV with frequency of 15 Hz, the SWV amplitude of about lOOmV for MAA-PDDA-RC and 250 mV or more for MA-RC film was needed respectively. When the ME-PDDA-RC composite film was stimulated by SWV, the peak for the bacteriochlorophyll dimmer (P) and secondary quinone (Q_B) appeared at first, then the peak of primary quinone (Q_A) came into being. Further more,if the simulated signal of SWV was continually applied, all of the peak current increased. On the other hand, the opposite process was observed while the potential of -0.4 V was applied by bulk electrolysis. For all the composite films,the redox potential for P⁺/P declined with frequency increasing from 30 to 150 Hz.As reported before, the redox potential of species in RC were:0.45-0.60 V (vs.NHE)for P⁺/p, +0.05~-0.15 V for Q_A and -0.20~-0.35 V for Q_B.     

A combined molecular dynamics+quantum mechanics method for investigation of dynamic effects on local surface structures

A combined molecular dynamics (MD)+quantum mechanics (QM) method for studying processes on ionic surfaces is presented. Through the combination of classical MD and ab initio embedded-cluster calculations, this method allows the modeling of surface processes involving both the structural and dynamic features of the substrate, even for large-scale systems. The embedding approach used to link the information from the MD simulation to the cluster calculation is presented, and rigorous tests have been carried out to ensure the feasibility of the method. The electrostatic potential and electron density resulting from our embedded-cluster model have been compared with periodic slab results, and confirm the satisfying quality of our embedding scheme as well as the importance of applying embedding in our combined MD+QM approach. We show that a highly accurate representation of the Madelung potential becomes a prerequisite when the embedded-cluster approach is applied to temperature-distorted surface snapshots from the MD simulation.     

端羟基化聚苯乙烯的表面性质

利用Wilhelmy片技术和躺滴法研究了端羟基化聚苯乙烯的表面性质.结果表明,端羟基化对聚苯乙烯在空气面的接触角基本没有影响(89°),而在玻璃面的接触角则大大降低(66°),其降低幅度与分子量及分子量分布有关.这与动态接触角的测定结果基本一致,而且宽分子量分布的端羟基化聚苯乙烯的前进接触角(θa)随着温度的升高而降低,于40℃时达到最低值.而窄分子量分布样品的动态接触角基本不变.样品与不同温度水接触后表面接触角的变化也基本相似.DMA研究结果表明,样品损耗模量、储能模量和tanδ从40℃开始发生突变,刚好与接触角最低值的温度相对应.这是由于宽分子量分布样品中的较低分子量组分在表面聚集,导致表面分子具有较高的活动能力.接触角随温度的变化趋势可能是聚合物表面分子运动能力增加和结晶程度变化等因素综合作用的结果.