Molecular simulation of sodium butyl benzene sulfonate at air–water interface and in aqueous solution

Molecular mechanics(MM) calculations for interfacial behaviour of sodium n-butyl benzene sulfonate (NaNBBS), sodium iso-butyl benzene sulfonate (NaIBBS) and sodium tert-butyl benzene sulfonate (NaTBBS) show a significant effect of the butyl group geometry on the surface area occupied by these molecules at the air–water interface. NaNBBS, in comparison with NaIBBS and NaTBBS, shows a closer molecular packing at the interface. The simulation predicts minimum hydrotrope concentration of each hydrotrope to reach surface saturation and molecular surface area at the interface match with good accuracy. The shape, size and charge of the hydrotrope aggregates obtained by molecular dynamics simulation also match well with the results of small angle neutron scattering experiments on the same hydrotrope. The simulation shows non-regular and ellipsoidal hydrotropes aggregates with substantial charge on the surface. The aggregates are also more open structures as compared to surfactant micelles. The water accessible surface area of a NaNBBS aggregate was 25% lower in comparison to that of NaTBBS aggregate, indicating closer packing of NaNBBS molecules. The fractional charge on the NaNBBS aggregate decreases with the increase in the number of NaNBBS molecules in the aggregate indicating more counter-ion association.     

Molecular dynamics simulation of gas diffusion behavior in polyethylene terephthalate/aluminium/polyethylene interface

Molecular dynamics (MD) simulations were performed to estimate the diffusion coefficients of O₂ and H₂O molecules in polyethylene terephthalate/aluminum/polyethylene interface at the temperature of 298 K. It came out that the diffusion coefficient of gasses in the interface is smaller than that of a single polymer, and the diffusion coefficients compare well with experimental data as well as previously published work. Furthermore, the diffusion coefficients of H₂O molecules in the interface are preferable to that of O₂ molecules. Interestingly, the largest diffusion coefficient was detected in the polyethylene terephthalate/aluminum(1 0 0)/polyethylene interface, while the smallest value of the diffusion coefficients was found in the polyethylene terephthalate/aluminum(1 1 1)/polyethylene interface. Calculation and analysis of the interaction between aluminum and polymers indicated that the interaction of polymer/aluminum(1 1 0) has the most interface strength, and crystal density of the metal surface has a definite effect on the planar interface energy. What’s more, the figure of gas molecule concentration is further resulted that the interface make contribution to adsorption of gas molecules. Moreover, the diffusion is belonging to the Einstein diffusion in the multilayer materials, and this work provides some key clues to improve the performance of polymer materials.     

Molecular dynamics study of linear and comb-like polyelectrolytes in aqueous solution: effect of Ca2+ ions

All-atom molecular dynamics simulations were employed to provide microscopic mechanism for the salt tolerance of polyelectrolytes dispersants. The conformational variation of polyelectrolytes and interactions between COO^? groups and counterions/water molecules were also studied via radius of gyration and pair correlations functions. Sodium polyacrylate (NaPA) and sodium salts of poly(acrylic acid)–poly(ethylene oxide) (NaPA–PEO) were selected as the representative linear and comb-like polyelectrolyte, respectively. The results show that Ca²⁺ ions interact with COO^? groups much stronger than Na⁺ ions and can bring ion-bridging interaction between intermolecular COO^? groups in the NaPA systems. While in the NaPA–PEO systems, the introduced PEO side chains can prevent backbone chains from ion-bridging interactions and weaken the conformational changes. The present results can help in selecting and designing new-type efficient polyelectrolyte dispersants with good salt tolerance.     

Molecular dynamics study on water self-diffusion in aqueous mixtures of methanol,ethylene glycol and glycerol: investigations from the point of view of hydrogen bonding

Molecular dynamics simulations have been performed to investigate the aqueous binary mixtures of alcohols, including methanol, ethylene glycol (EG) and glycerol of molalities ranging from 1 to 5 m at the temperatures of 273, 288 and 298 K, respectively. The primary purpose of this paper is to investigate the mechanism of water self-diffusion in water-alcohol mixtures from the point of view of hydrogen bonding. The effects of temperature and concentration on water self-diffusion coefficient are evaluated quantitatively in this work. Temperature and concentration to some extent affect the hydrogen bonding statistics and dynamics of the binary mixtures. It is shown that the self-diffusion coefficient of water molecules decreases as the concentration increases or the temperature decreases. Moreover, calculations of mean square displacements of water molecules initially with different number n of H-bonds indicate that the water self-diffusion coefficient decreases as n increases. We also studied the aggregation of alcohol molecules by the hydrophobic alkyl groups. The largest cluster size of the alkyl groups clearly increases as the concentration increases, implying the emergence of a closely connected network of water and alcohols. The clusters of water and alcohol that interacted could block the movement of water molecules in binary mixtures. These findings provide insight into the mechanisms of water self-diffusion in aqueous binary mixtures of methanol, EG and glycerol.     

The structure,dynamics and solvation mechanisms of ions in water from long time molecular dynamics simulations: a case study of CaCl2 (aq) aqueous solutions

Systematic long time (5–20 ns) molecular dynamics (MD) simulations have been carried out to study the structural and dynamical properties of CaCl₂ aqueous solutions over a wide range of concentrations (≤9.26 m) in this study. Our simulations reveal totally different structural characteristics of those yielded from short time (≤1 ns) MD simulations [A.A. Chialvo and J.M. Simonson, J. Chem. Phys. 119, 8052 (2003); T. Megyes, I. Bako, S. Balint, T. Grosz, and T. Radnai, J. Mol. Liq. 129, 63 (2006)]. An apparent discontinuity was found at 4–5 m of CaCl₂ in various properties including ion–water coordination number and self-diffusion coefficient of ions, which were first noticed by Phutela and Pitzer in their thermodynamic modelling [R.C. Phutela and K.S. Pitzer, J. Sol. Chem. 12, 201 (1983)]. In this study, residence time was first taken into consideration in the study of Ca²⁺–Cl^? ion pairing, and it was found that contact ion pair and solvent-sharing ion pair start to form at the CaCl₂(aq) concentrations of about 4.5 and 4 m, respectively, which may be responsible for the apparent discontinuity. In addition, the residence time of water molecules around Ca²⁺ or Cl^? showed that the hydration structures of Ca²⁺ and Cl^? are flexible with short residence time (<1 ns). It needs to be pointed out that it takes much longer simulation time for the CaCl₂–H₂O system to reach equilibrium than what was assumed in previous studies.     

Assessment of Compatibility in Polypropylene/Poly(lactic acid)/Ethylene Vinyl Alcohol Ternary Blends: Relating Experiments and Molecular Dynamics Simulation Results

Two systems of polypropylene (PP), poly(lactic acid) (PLA) and ethylene vinyl alcohol copolymer (EVOH) ternary blends having different compositions were extruded in a co-rotating twin screw extruder. The first system was PP/PLA (75/25) with various EVOH contents, the second one was PP/EVOH (75/25) having various PLA contents. The effects of composition on the morphology and the tensile and impact properties of the blends were investigated. There were increases in the tensile modulus and tensile strength with an increase in the EVOH and PLA contents in the first and second systems, respectively. A molecular dynamics (MD) simulation was used to investigate the compatibility between the components. Prediction of the miscibility of the blends was carried out by determining the interaction parameters (χ), mixing energies (ΔH_mix), phase diagrams and Gibbs free energies. The MD simulation showed a UCST behavior for the components. Moreover, the simulation results showed a compatibilizer effect for the EVOH component. The experimental values of the dynamic mechanical thermal analysis (DMTA) and mechanical properties were correlated to the MD results. There was a good correlation between the MD and DMTA results. The modulus values using the parallel and Davis models were near to the experimental ones. A good fitting to the mixture law with addition of EVOH confirmed a good compatibilzing effect of it between the PP and PLA components.     

激光场中一维和三维氢分子离子模型经典动力学行为的比较

应用经典轨迹方法,采用辛算法数值求解激光场中的一维和三维氢分子离子(H2 )的Hamilton正则方程,得到氢分子离子在激光场作用下的经典轨迹,并比较分析氢分子离子一维模型与三维模型的存活、解离、电离和库仑爆炸等动力学行为,以及电子的运动情况的相似之处.数值结果表明,采用一维模型能近似定性反映氢分子离子的动力学行为,并且简便可行.