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The diffusion and local structure of eight normal alkanols in 1-octanol at infinite dilution from 298 to 370 K have been investigated via molecular dynamics simulation. For short-chain n-alkanols, the simulated infinite dilute diffusion coefficients D12 are in good agreement with the experimental data, while for long-chain n-alkanols, the simulated D12 overestimate the experimental data. Meanwhile, the local structures are characterized by calculating the radial distribution functions, hydrogen bond, radius of gyration, and end-to-end distance. It is interesting that the average number of hydrogen bonds decreases as the chain length of n-alkanols increases from methanol to 1-heptanol, but unexpectedly, the average number of hydrogen bonds for 1-nonanol begins to increase. The simulation results demonstrate that a kind of intertwist effect through alkyl chain-chain interactions and caused by the molecular flexibility could explain the abnormal change of hydrogen bond number and the serious overestimation of simulated D12 for long-chain n-alkanols.

Based on the linear free energy relationships theory and empirical equations, correlations of mutual diffusion coefficients of solutes in organic solvents at infinite dilution at various temperatures are proposed. Four homologues in 743 binary systems have been regressed and correlation equations for the diffusion coefficients are obtained. The mean relative deviations of the four equations are 8.63%, 8.57%, 2.96%, and 4.85%, and all of the squared correlation coefficients (R2) are larger than 0.96, indicating that the fittings are good. The results reveal that a simple form of the equation exhibits high estimation accuracy, definite theory meaning, wide applicability and wide temperature range. This study successfully combines macroscopic physical properties of substances with their molecular microstructure, providing an alternative approach for calculating of mutual diffusion coefficient.

Over wide temperature and pressure ranges, the molecular dynamics simulation is performed to study the mass transfer of six n-alkanes from n-C5H12 to n-C10H22 in CO2 at infinite dilution by calculating the diffusion coefficients, which have not yet been measured by experiment. Meanwhile, the structural properties of these systems are explored. It is found that under different temperature and pressure conditions, the variation trends of the radial distribution functions of n-alkanes are quite different, while the variation trends of the average coordination number of n-alkanes can be divided into three types. The radius of gyration and the solvent accessible surface area are both affected by temperature and carbon chain length, but their variation trends are different, and it could explain the abnormal variation trends of the radial distribution functions and the average coordination number.

The infinite dilution diffusion coefficients (D12) of methanol, ethanol, 1-propanol, 1-butanol and 1-pentanol in supercritical CO2 (scCO2) at 313.2 K and 10–16 MPa were simulated by molecular dynamics (MD) simulation. The microscopic structure was also analyzed by calculation of the radial distribution function, coordination number (CN) between the center mass of solute and solvent molecules, and the average number of hydrogen bonding of this system. In infinite dilute solution, the probability of forming hydrogen bond between alkanol molecules is greatly reduced relative to pure alkanol fluid, and the weak hydrogen bonds formed between alkanol and CO2 molecules. In general, this work provides a reliable simulation method for transfer properties of solutes in scCO2. The prediction data were provides for the design and development of chemical processing. The results are helpful for one to deeper understand the relationship between microscopic structures of fluid and its transfer properties.

On the basis of the linear free energy relationships theory and thermodynamics formulas, a new method predicting critical temperature (Tc) of pure fluids is proposed for the first time. Sixteen homologues of 616 substances have been regressed and correlation equations between Tc and molecular descriptors are obtained. The mean relative deviations of the 16 equations are from 0.01% to 2.73%, and most of them are under 2%. In addition, the squared correlation coefficients are from 0.90 to 0.98. Moreover, the equations are tested through cross-validation by the leave-one-out procedure and most of the squared correlation coefficients are greater than 0.90. The results reveal that the equations exhibit better effect with simple form of equation, high prediction accuracy, and definitude theory meaning. This study successfully combines macroscopic physical properties of fluids with their molecular microstructure and breaks through the experimental or theoretical application scope, perfecting calculation of criti...

The transfer properties and microscopic structures of methanol, ethanol, 1-propanol, 2-propanol, and 1-pentanol in the temperature range from 290 to 450 K and pressure range from 0.1 to 200 MPa were studied by molecular dynamics (MD) simulation through the calculation of the self-diffusion coefficients, velocity autocorrelation functions (VACF), and radial distribution function (RDF). The calculated self-diffusion coefficients conform to the experimental values on the whole, and the temperature has greater influence, which weaken with the increase of the carbon chain, on self-diffusion coefficient than pressure. The factors affecting self-diffusion coefficients were also analyzed from micro perspective by calculation of VACF and RDF, which is helpful to understand the relationship between microscopic structures of fluid and its transfer properties. This work not only provides a reliable simulation method for transfer properties of alkanols, but also provides the prediction data for design and development of chemical processes.

Infinite dilute diffusion coefficients ( D 12 ) of normal alkanes (C1 to C14) in 1-octanol (C8-OH), as well as primary alkanols (C1-OH to C14-OH) in n-octane (C8) have been studied using molecular dynamics simulation in the temperature range from 298 to 374 K and at atmosphere pressure. Simulated D 12 values for short-chain solute molecules show a good agreement with the experimental ones taken from literature, while for the long-chain solute molecules the overestimation is large. Besides, the structures of solute molecules in above fluid mixtures are investigated by calculating radial distribution functions, radius of gyration, end-to-end distance, and root mean square fluctuation of atomic positions. These results confirm long-chain n -alkanes and n -alkanols molecules are curved and curled up, which we call it flexibility. Moreover, flexibilities for solute molecules (5 ≤ n ≤ 9) are symmetric but for much long-chain solute molecules like n-tetradecane (C14) and 1-tetradecanol (C14-OH) are asymmetric. We believe that strong chain-chain interactions are existed between different long-chain molecules, and it will result in the solute and solvent molecules are intertwisted. The serious overestimation of simulated D 12 for long-chain solute molecules can be reasonably explained by the intertwist effect.

Intradiffusion coefficients of N,N-dimethylformamide (DDMF) and water (DW) in DMF–water mixtures were measured as a function of temperature, pressure and composition for the first time using the pulsed field gradient spin echo nuclear magnetic resonance technique. Then, molecular dynamics simulations for the abovementioned system at the same conditions were also conducted. The experimental DDMF and DW exhibited minimums at a DMF mole fraction (xDMF) of 0.250–0.328, which indicates that two kinds of DMF–water complexes formed, namely DMF·3H2O and DMF·2H2O. The simulated DDMF and DW achieved a good agreement with the experimental values of this work, except at xDMF = 0.250 or 0.328 that the overestimation is considerably large. We believe this phenomenon is due to the extremely strong DMF–water hydrogen bonding at such compositions, which is further supported by the followed calculation results of radial distribution functions, average number of hydrogen bonds and total potential energies. Finally, molecular dynamics trajectories of present simulation have successfully captured the hydrogen-bonded DMF–water complexes at the atomic level.

The diffusion coefficients (Ds), viscosities (η), and structural properties of carbon dioxide (CO2) have been studied using molecular dynamics (MD) simulation. Three fully flexible models (MSM-flex, EPM2-flex, and TraPPE-flex) from the literature are used to model CO2. Present simulations have extended the temperature range from 223 K to 450 K and pressures up to 200 MPa for the first time. Generally, the simulation results show a good agreement with the experimental ones. The overall satisfaction of the EPM2-flex model is found to be the best, with an average absolute relative deviation of 6.83 % for Ds and of 2.87 % for η, respectively. However, the TraPPE-flex model performs best at low temperatures below 273 K. Meanwhile, the lifetime of CO2 molecules in the first solvation shell (τs) is calculated, and the qualitative correlation between τs and Ds as well as τs and η is discussed. Finally, the structures of CO2 fluid in different thermodynamic states are investigated by calculating radial distributio...

The diffusion coefficients of 14 n-alkanes (ranging from methane to n-tetradecane) in liquid and supercritical methanol at infinite dilution (at a pressure of 10.5 MPa and at temperatures of 299 K and 515 K) were deduced via molecular dynamics simulations. Values for the radial distribution function, coordination number, and number of hydrogen bonds were then calculated to explore the local structure of each fluid. The flexibility of the n-alkane (as characterized by the computed dihedral distribution, end-to-end distance, and radius of gyration) was found to be a major influence and hydrogen bonding to be a minor influence on the local structure. Hydrogen bonding reduces the flexibility of the n-alkane, whereas increasing the temperature enhances its flexibility, with temperature having a greater effect than hydrogen bonding on flexibility. Graphical abstract The flexibility of the alkane is a major influence and the hydrogen bonding is a minor influence on the first solvation shell; the coordination numbers of long-chain n-alkanes in the first solvation shell are rather low.

The diffusion coefficients of n-alkanes (from CH4 to C14H30) in near critical and supercritical carbon dioxide at infinite dilution have been studied by molecular dynamics simulation. The simulation results agree well with experiment, which suggests that the simulation method is a powerful tool to obtain diffusion coefficients of solutes in fluids at high pressures. The local structures of such fluids are further investigated by calculating radial distribution functions and coordination numbers. Meanwhile, the dihedral, end-to-end distance and radius of gyration, which are calculated to characterize the flexibility of n-alkanes, are used to reasonably explain the abnormal trends on radial distribution functions and coordination numbers. Moreover, it is found that the flexibility effects on diffusion in pure n-alkanes and infinitely dilute n-alkane/CO2 system are different. The differences in MD simulation results of molecular diffusion in such systems could be qualitatively explained by the flexibility.

Self-diffusion of methanol, ethanol, 1-propanol and 2-propanol has been studied by molecular dynamics simulation in the temperature range between the melting pressure curve and 478 K at pressures up to 300 MPa. The simulation results on self-diffusion of methanol, ethanol and 2-propanol (for 2-propanol, at high temperatures) agree well with experiment, which suggests that the simulation method is a powerful tool to obtain self-diffusion coefficients over wide range of temperature and pressure, under which it is rather difficult for experiments. The local structures of methanol, ethanol and 2-propanol are investigated by calculating the radial distribution functions, H-bond numbers, coordination numbers and the ratios of H-bond number divided by coordination number. The correlation between self-diffusion and structural properties, and the influence of temperature and pressure on them are discussed. The degree of forming H-bond space network in methanol, ethanol and water is higher than that in 2-propanol, ...

A combined and sequential use of quantum mechanical (QM) calculations and classical molecular dynamics (MD) simulations was made to investigate the σ and π types of hydrogen bond (HB) in benzene-water and pyrrole-water as clusters and as their liquid mixture, respectively. This paper aims at analyzing similarities and differences of these HBs resulted from QM and MD on an equal footing. Based on the optimized geometry at ωb97xD/aug-cc-pVTZ level of theory, the nature and property of σ and π types of HBs are unveiled by means of atoms in molecules (AIM), natural bond orbital (NBO) and energy decomposition analysis (EDA). In light of the above findings, MD simulation with OPLS-AA and SPC model was applied to study the liquid mixture at different temperatures. The MD results further characterize the behavior and structural properties of σ and π types HBs, which are somewhat different but reasonable for the clusters by QM. Finally, we provide a reasonable explanation for the different solubility between benzene/water and pyrrole/water.

Self-diffusion and structural properties of n-alkanes have been studied by molecular dynamics simulation in the temperature range between the melting pressure curve and 600 K at pressures up to 300 MPa. The simulated results of lower n-alkanes are in good agreement with the existing experimental data, and support the reliability of results of the simulations of self-diffusion coefficients obtained at the extreme conditions. We predict the self-diffusion coefficients for methane, ethane, propane and n-butane at the similar reduced temperatures and pressures to draw a comparison between them. Then the correlation between self-diffusion and structural properties are further investigated by calculating the coordination numbers. Moreover, we define four distances and their corresponding relative deviations to characterize the flexibility of long-chain n-alkanes. The simulated results show that the self-diffusion of n-alkane molecules is mainly affected by the close packing, and the flexibility has a strong impact on the self-diffusion of longer n-alkane molecules.

An assessment study is presented about energy decomposition analysis (EDA) in combination with DFT including revised dispersion correction (DFT-D3) with Slater-type orbital (STO) basis set. There has been little knowledge about the performance of the EDA + DFT-D3 concerning STOs. In this assessment such an approach was applied to calculate noncovalent interaction energies and their corresponding components. Complexes in S22 set were used to evaluate the performance of EDA in conjunction with four representative types of GGA-functionals of DFT-D3 (BP86-D3, BLYP-D3, PBE-D3 and SSB-D3) with three STO basis sets ranging in complexity from DZP, TZ2P to QZ4P. The results showed that the approach of EDA + BLYP-D3/TZ2P has a better performance not only in terms of calculating noncovalent interaction energy quantitatively but also in analyzing corresponding energy components qualitatively. This approach (EDA + BLYP-D3/TZ2P) was thus applied further to two representative large-system complexes including porphine dimers and fullerene aggregates to gain a better insight into binding characteristics.

Self-diffusion coefficients D were studied for fluid hydrogen and fluid deuterium at pressures up to 200 MPa and in the temperature range 171–372 K by the spin echo method. Spin–lattice relaxation times of both nuclei were measured in the same p,T range. The diffusion coefficients D are described by the rough hard sphere (RHS) model invoking the rotation translation coupling parameter A = 1. Activation parameters are also derived. The dynamic isotope effect of D given by Dr = D(H2)/D(D2) is a function of p and T and varies from Dr≈1.1 at the lowest pressures and highest temperatures to Dr≈1.3 for the highest pressures and lowest temperatures reached. The spin–lattice relaxation is for both compounds dominated by the spin–rotation interaction. Only for T1(H2) at T = 171 K can a minor contribution from dipole–dipole relaxation be derived from the data.

The self-diffusion coefficients D of the isomers of liquid N-isopropyl-N-methylacetamide (IPMAA) and acetylacetone (AC AC) are studied as functions of temperature. For AC AC also pressure dependent studies were undertaken. In IPMAA the eis and trans conformers have almost identical D. In liquid ACAC, DKclo is always smaller than Denol , this difference being 15% around 300 K.

Self-diffusion coefficients for monomethylamine and trimethylamine are given, taken in the temperature range between the melting pressure curve and 423 K at pressures up to 200 MPa. In the same T,p-range intradiffusion coefficients for an equimolar mixture of trimethylamine and ammonia were measured. For the evaluation of the diffusion data the densities of trimethylamine were determined between 273 and 373 K up to 200 MPa. These data permit the description of diffusion in this fluid by the rough hard sphere (RHS) model. The result of the data analysis of all systems studied under inclusion of the previously investigated ammonia is that hydrogen bonding has no influence upon the dynamics in any of these systems.

Self-diffusion and structural properties of ammonia, methylamine and trimethylamine have been studied by molecular dynamics simulation in the temperature range between the melting pressure curve and 700 K at pressures up to 400 MPa. The calculation results agree well with the experiment, which suggests that one can use the simulation method as a powerful tool to obtain self-diffusion coefficients over wide range of temperatures and pressures, under which it is rather difficult for experiments. The local structures of such fluids are investigated by calculating radial distribution functions (RDFs), the numbers of hydrogen bonds and coordination numbers. The correlation between self-diffusion and structural properties, and the influence of temperature and pressure on them are discussed. The simulation results demonstrate that the temperature effects are more pronounced than the pressure effects on self-diffusion and structural properties, and the effect of hydrogen bonding on the translational dynamics in any of these systems is a minor factor, while it is mainly affected by the close packing of amine molecules.

This chapter examines the p ,T- dependence of molecular mobility in supercritical fluids studied by high pressure NMR. Three binary mixtures of CO 2 with nonpolar molecules have been studied. The mixtures with benzene as the second component show the typical behavior for this group of compounds that is also observed in ammonia mixtures. In the next two binary fluids presented, molecules that may form hydrogen bonds were added to carbon dioxide. The temperature and pressure coefficients of the translational molecular mobility in the binary systems that are made up of nonpolar as well as highly polar substances are a collective property of the fluid. Within the limits of the precision of the values derived, they are identical for the two components and do not reflect the individual properties of the individual compounds such as mass, shape, and dipole moment. The system CO2/CH3OH in which hydrogen bonded associates can be formed is an exception to this observation. The diffusional mobility of the alcohol molecules decreases much faster with decreasing temperature than the Dii(CO2), reflecting the formation of hydrogen bonded associates between the alcohol molecules only. The isobaric temperature dependence of the Dii(CH3OH) in this system is also described in this chapter.

The binary infinite dilute diffusion coefficients, D₁₂(∞), of some alkylbenzenes (Ph-C(n), from Ph-H to Ph-C12) from 313 K to 333 K at 15 MPa in supercritical carbon dioxide (scCO2) have been studied by molecular dynamics (MD) simulation. The MD values agree well with the experimental ones, which indicate MD simulation technique is a powerful way to predict and obtain diffusion coefficients of solutes in supercritical fluids. Besides, the local structures of Ph-C(n)/CO2 fluids are further investigated by calculating radial distribution functions and coordination numbers. It qualitatively convinces that the first solvation shell of Ph-C(n) in scCO2 is significantly influenced by the structure of Ph-C(n) solute. Meanwhile, the mean end-to-end distance, the mean radius of gyration and dihedral angle distribution are calculated to gain an insight into the structural properties of Ph-C(n) in scCO2. The abnormal trends of radial distribution functions and coordination numbers can be reasonably explained in term of molecular flexibility. Moreover, the computed results of dihedral angle clarify that flexibility of long-chain Ph-C(n) is the result of internal rotation of C-C single bond (σ(c-c)) in alkyl chain. It is interesting that compared with n-alkane, because of the existence of benzene ring, the flexibility of alkyl chain in Ph-C(n) with same carbon atom number is significantly reduced, as a result, the carbon chain dependence of diffusion behaviors for long-chain n-alkane (n ≥ 5) and long-chain Ph-C(n) (n ≥ 4) in scCO2 are different.