Alkane Systems Research Articles

Relative Resolution: An Analysis with the Kullback-Leibler Entropy.

A novel type of a multiscale approach, called Relative Resolution (RelRes), can correctly retrieve the behavior of various nonpolar liquids while speeding up molecular simulations by almost an order of magnitude. In this approach in a single system, molecules switch their resolution in terms of their relative separation, with near neighbors interacting via fine-grained potentials, yet far neighbors interacting via coarse-grained potentials; notably, these two potentials are analytically parametrized by a multipole approximation. Our current work focuses on analyzing RelRes by relating it with the Kullback-Leibler (KL) entropy, which is a useful metric for multiscale errors. In particular, we thoroughly examine the exact and approximate versions of this informatic measure for several alkane systems. By analyzing its dependency on the system size, we devise a formula for predicting the exact KL entropy of an "infinite" system via the computation of the approximate KL entropy of an "infinitesimal" system. Demonstrating that the KL entropy can holistically capture many multiscale errors, we settle bounds for the KL entropy that ensure a sufficient representation of the structural and thermal behavior by the RelRes algorithm. This, in turn, allows the scientific community to readily determine the ideal switching distance for an arbitrary RelRes system.

Structure of the Hexadecane Rotator Phase: Combination of X-ray Spectra and Molecular Dynamics Simulation.

Rotator phases are rotationally disordered plastic crystals, some of which can form upon freezing of alkane at alkane-water interfaces. Existing X-ray diffraction studies show only partial unit cell information for rotator phases of some alkanes. This includes the rotator phase of n-hexadecane, which is a transient metastable phase in pure alkane systems, but shows remarkable stability at interfaces when mediated by a surfactant. Here, we combine synchrotron X-ray diffraction data and molecular dynamics (MD) simulations, reporting clear evidence of the face-centered orthorhombic RI rotator phase from spectra for two hexadecane emulsions, one stabilized by Brij C10 and another by Tween 40 surfactants. MD simulations of pure hexadecane use the recently developed Williams 7B force field, which is capable of reproducing crystal-to-rotator phase transitions, and it also predicts the crystal structure of the RI phase. Full unit cell information is obtained by combining unit cell dimensions from synchrotron data and molecular orientations from MD simulations. A unit cell model of the RI phase is produced in the crystallographic information file (CIF) format, with each molecule represented by a superposition of four rotational positions, each with 25% occupancy. Powder diffraction spectra computed using this model are in good agreement with the experimental spectra.

Effects of molecular size and orientation on the interfacial properties and wetting behavior of water/n-alkane systems: a molecular-dynamics study.

Molecular dynamics simulations (MD) are performed to study the interfacial structure/tension and wetting behavior of water/n-alkane systems (water/nC5 to water/nC16 where nCx = CxH(2x + 2)). In particular, we study complete-to-partial wetting transitions by changing the n-alkane chain length (NC) at a constant temperature, T = 295 K. Simulations are carried out with a united-atom TraPPE model for n-alkanes and the TIP4P-2005 model of water. Simulation results are in excellent agreement with the initial spreading coefficients and contact angles calculated using experimental values of the surface and interfacial tensions. In addition, it has been determined that water/(nC5-nC7) and water/(nC8-nC16), respectively, exhibit complete and partial initial wetting modes. Simulations show that the interfacial structures of water/(nC5-nC7) are different from water/(nC8-nC16) systems. In the latter, water preferentially orients near the interface to increase the number of hydrogen bonds and the charge and mass densities. Moreover, the orientation of n-alkane molecules at water/(nC8-nC16) interfaces has a long-range persistence, resulting in layered structures that increase with NC. In addition, simulation results of the orientational order parameter Sz show alignment behavior of the n-alkane molecules with respect to the interfaces. Simulations predict that the central segments of n-alkane are strongly packed in the interfaces while the end segments (methyl groups) form smaller peaks in the outer edge of the layer. This observation confirms the "horseshoe" or "C-shaped" structure of n-alkane molecules in the water/n-alkane interfaces. At constant temperature, the interface widths of both water and the n-alkanes decrease with increasing n-alkane molecular length. These results suggest that increasing the n-alkane chain length affects the water/n-alkane interfacial properties in a manner similar to that of cooling.

Modelling of mixed thermodynamic model based on new materials

In order to solve the problem of the performance of mixed alkane phase change energy storage materials, this paper proposes the modelling research of mixed thermodynamic model based on new materials. In this paper, the thermodynamic performance of binary mixed alkane system is studied, the influence of phase transition temperature, phase transition enthalpy and its composition on the mixed alkane system is explored, and its solid-liquid phase diagram is drawn. We used different thermodynamic models to predict the phase transition temperature and enthalpy in the melting process of binary mixed alkanes, which were mutually verified with the experimental results. The experimental results show that C9-C10 and C9-C11 systems exhibit eutectic behavior. The eutectic components are 88% (mass fraction, the rest is the same) C9-12% C10 and 90% C9-10% C11. The eutectic temperatures are 218.25 K and 215.15 K, respectively. The C8-C9 system has peritectic phenomenon, C10-C11 system shows complete mutual solubility, and their mini?mum melting points are 200.25 K and 234.35 K, respectively. Conclusion is that the C9-C10 and C9-C11 eutectic systems are suitable for low temperature phase change materials in the temperature range of 210-220 K, providing data reference for their application in low temperature energy storage.

Effect of Branching on Mutual Solubility of Alkane–CO2 Systems by Molecular Simulations

Mutual solubilities of hydrocarbon-CO2 systems are important in a broad range of applications. Experimental data and theoretical understanding of phase behavior of large hydrocarbon molecules and CO2 are limited. This is especially true in relation to the molecular structure of hydrocarbons when the carbon number exceeds 12. In this work, the continuous fractional component Gibbs ensemble Monte Carlo simulations are used to investigate mutual solubility of different alkane and CO2 systems and the molecular structure. We investigate the mutual solubility of n-decane, n-hexadecane, n-eicosane, and the corresponding structural isomers in the CO2-rich and hydrocarbon-rich phase. The focus will be solubility of the heavy normal alkanes and their structural isomers in CO2. The simulation results are verified by comparing the experimental data when measurements are available. The simulation of phase behavior of the n-decane-CO2 system agrees with the experiments. We also present simulation results of n-hexadecane-CO2 and n-eicosane-CO2 systems away from the critical region partly due to the finite size effect. We establish that solubility of the hydrocarbons in CO2 is improved by change of the molecular structure in heavier alkanes. The enhanced solubility is limited in decane isomers, but the isomers of hexadecane and eicosane show 2- to 3-time solubility enhancement. The molecular dynamics simulations suggest that the improvement is from a higher coordination number of CO2 for methyl (CH3) rather than for methylene (CH2) groups. This study sets the stage for molecular engineering and synthesis of hydrocarbons that are soluble in CO2 not only by considering functionality but also by changing the molecular structure. The solubility enhancement is the first step in viscosification of CO2 which broadens the use of CO2.

Dynamic odd-even effect in n-alkane systems: a molecular dynamics study.

Alternation in various properties of n-alkanes (CnH2n+2) as a function of carbon content (n) is termed 'odd-even effect'. Here, we report a comprehensive molecular dynamics simulation study on n-alkane systems carried out with n ranging between 3 (propane) and 8 (octane), examining the odd-even effect in melting point, density, intramolecular conformational ordering, translational and rotational motion. We observe an odd-even alternation in these properties, but with heptane (n = 7) exhibiting anomalous behavior for all except conformational ordering. Our simulations also show the presence of odd-even behavior in rotational and translational dynamics, below and above the melting point, respectively. The results highlight the role of both molecular shape and the variation in density and their interplay in the origins of the odd-even effect.

Solid–liquid–liquid phase envelopes from temperature-scanned refractive index data

Abstract A novel method for estimating the upper critical solution temperature (UCST) of N,N-diethyl-m-toluamide (DEET)-polyethylene systems was developed. It was validated using data for the dimethylacetamide (DMA)-alkane systems which showed that refractive index mixing rules, linear in volume fraction, can accurately predict mixture composition for amide-alkane systems. Furthermore, rescaling the composition descriptor with a single adjustable parameter proved adequate to address any asymmetry when modeling the DMA-alkane phase envelopes. This allowed the translation of measured refractive index cooling trajectories of DEET-alkane systems into phase diagrams and facilitated the estimation of the UCST values by fitting the data with an adjusted composition descriptor model. For both the DEET- and DMA-alkane systems, linear behavior of UCST values in either the Flory–Huggins critical interaction parameter, or the alkane critical temperature, with increasing alkane molar mass is evident. The UCST values for polymer diluent systems were estimated by extrapolation using these two complimentary approaches. For the DEET-polyethylene system, values of 183.4 and 180.1 °C respectively were obtained. Both estimates are significantly higher than the melting temperature range of polyethylene. Initial liquid–liquid phase separation is therefore likely to be responsible for the previously reported microporous microstructure of materials formed from this binary system.

Comparison of SAFT-VR-Mie and CP-PC-SAFT in Estimating the Phase Behavior of Acetone + n-Alkane Systems

This study compares the predictions of CP-PC-SAFT and SAFT-VR-Mie for the binary systems of acetone with n-alkanes and some nonassociative gases. CP-PC-SAFT was attached by the 1A association term....

The effect of nanoparticles on the microstructure of alkanes: A molecular dynamics study

Abstract In order to explore the influence of nanoparticles on the microstructure of phase change materials in alkane systems, five different models were studied by molecular dynamics simulation. Eicosane is used to represent paraffin. One of the models was pure eicosane as a control group. The other four models mix four different nanoparticles of Ag, Cu, Al and Fe in eicosane. The mean square displacement, self-diffusion coefficient, end-to-end distance distribution and radial distribution function of each model under different temperature fields were calculated by simulation. Studies have found that the self-diffusion coefficient of eicosane increases with increasing temperature. However, the presence of nanoparticles limits the development of self-diffusion coefficient of eicosane. Through the statistical analysis of the end-to-end distance distribution, it is concluded that the presence of nanoparticles has a certain binding effect on the eicosane molecule. The presence of nanoparticles causes the conformation of more eicosane molecules to change from a linear to a curved state. It is found by the statistics of the radial distribution functions that the presence of the nanoparticles increases the number of particles around the eicosane molecule compared to the number of particles in the unadded nanoparticle model, and the system becomes more compact. The results obtained in this paper hope to provide a certain reference value for exploring the influence mechanism of nanoparticles on organic phase change materials and contribute to the improvement of energy utilization efficiency.

Molecular dynamics investigation on n-alkane-air/water interfaces

Alkanes are the important molecules in crude oil that have applications in different fields. In this project, molecular dynamics simulations have been applied to investigate the n-alkane-air and n-alkane-water interfaces. The surface tension, diffusion coefficients, order parameters, radial distribution function, density profiles and radius of gyration of alkanes in the bulk and on the interfaces with air/water were calculated and compared. It was found that with the n-alkane chain length increasing, the surface tension of the interface system increases. However, with the temperature increasing, the surface tension of the alkanes decreases. The alkanes on the interface can diffuse faster than in the bulk in both kinds of interface systems. While n-alkane diffusion rate in the bulk of interface systems is faster than in the pure alkane systems, the radius of gyration of alkanes in the bulk, on the interface and in the pure alkane systems is consistent. Decomposing the diffusion coefficients into the lateral direction and z-direction, the lateral diffusion rate of alkanes on the interface is about 2–10 times faster than in the z-direction in both kinds of interface systems. Although molecules prefer to pack randomly in the bulk, they are more orderly packed on the interface. The result can help to explain the interface effect in water/air-alkane interface systems. The project also demonstrates that using NAMD program and CHARMM forcfield can predict reasonable structure and properties of n-alkane-air/water systems.

Measurement of critical temperature and critical pressure of tert-butanol and alkane mixtures

Tert-butanol is a good additive for improving gasoline performance, but as two of the most important thermodynamic properties, the critical temperature (Tc) and critical pressure (pc) of tert-butanol and alkane systems have not been studied. In this work, the experimental data of the Tc and pc of five binary systems (tert-butanol + cyclohexane, tert-butanol + n-hexane, tert-butanol + n-heptane, tert-butanol + n-octane, and tert-butanol + n-nonane) and a ternary system (tert-butanol + n-heptane + n-octane) were obtained using a flow method. The Tc and pc of the binary systems were fitted using the Redlich–Kister equations with maximum average absolute relative deviations (AARDs) of 0.19% and 0.36% for Tc and pc, respectively. The Tc and pc for tert-butanol + n-heptane + n-octane were correlated by Singh and Sharma's equations and Cibulka's equations with AARDs less than 0.09% and 0.32%, respectively. The experimental data of Tc and pc were also used to verify and improve the accuracy of our previous prediction model.

Molecular Dynamics Study on the Suitable Compatibility Conditions of a CO2-Cosolvent-Light Hydrocarbon System by Calculating the Solubility Parameters

Molecular dynamics simulations were performed to calculate the solubility parameters of oil molecules (such as straight-chain alkanes, cycloalkanes, and aromatics) and CO₂ + cosolvent systems. It was found that the solubility parameter of supercritical carbon dioxide (SC-CO₂) increases with the increase of pressure at a fixed temperature, decreases with the increase of temperature at a fixed pressure, and increases linearly with the increase of the SC-CO₂ system density. The simulated results showed that the nonbonding van der Waals interaction plays a dominant role in the solubility parameter of the SC-CO₂ system. Furthermore, adding a certain amount of cosolvent into the SC-CO₂ system can evidently improve the solubility parameters of the system. The simulated solubility parameters showed that the CO₂ + 20% ethanol molecules are more effective in extracting light hydrocarbons. When the solubility parameters of CO₂ + cosolvent systems are identical to those of alkane, cycloalkane, or aromatic systems, the linear relationship between the pressure and temperature can give the guidance for selecting the cosolvent in the SC-CO₂ system in the future. Through the simulation, we concluded that the investigations about the light hydrocarbons and CO₂ + cosolvent systems can provide a theoretical model for selecting suitable external conditions for CO₂ flooding technology.

Predicting wax appearance temperature and precipitation profile of normal alkane systems: An explicit co-crystal model

Wax precipitation is a major flow assurance issue for petroleum production, oil blending, downstream processing, and oil products usage. While numerous thermodynamic models have been proposed to calculate wax appearance temperature (WAT) and precipitation profile at temperatures below WAT, the existing models are correlative in nature and fail to comply with phase behavior experimentally observed for wax precipitation. This work presents a predictive thermodynamic model for wax precipitation based on an explicit co-crystal formation assumption for precipitated wax, a treatment consistent with experimental findings. With a simple Flory-Huggins expression accounting for the liquid phase nonideality and the treatment of lamella structure for wax precipitation below WAT, the model satisfactorily predicts WAT at low pressures for multicomponent paraffin mixtures and wax precipitation amounts and contents as the temperature drops below WAT.

Liquid–Liquid Equilibrium Data for Ternary Systems Containing Alkanes (n-Pentane, n-Hexane, n-Heptane, and n-Octane) + Alcohol (Methanol and Ethanol) + Protic Ionic Liquid (2-HEAF)

Azeotropic mixtures can occur in alcohol and alkane systems because of their close boiling points and highly nonideal molecular force interactions, leading to vapor–liquid coexistence envelopes wit...

Estimation of diffusion coefficients for multiple penetrant/polyolefin systems based on sorption data

The binary and ternary diffusion coefficients for gas/polymer systems can be estimated from solubility data and an equation of state to the free volume of penetrant/polymer systems. The system free volume can be calculated by the Sanchez Lacombe equation-of-state (SL-EoS), eliminating the need for viscoelastic data in determination of free volume parameters. Furthermore, this approach accounts for polymer swelling. The model gives reliable predictions for both binary and ternary systems of alkanes and alkenes in polyolefins.

Low-Pressure VLE Measurements and Thermodynamic Modeling, with PSRK and NRTL, of Binary 1-Alcohol + n-Alkane Systems

New subatmospheric vapor–liquid equilibrium (VLE) data for five binary 1-alcohol + n-alkane systems are presented. An all-glass dynamic recirculating still was used to measure the phase behavior of...

Cationic Hofmeister Series of Wettability Alteration in Mica-Water-Alkane Systems.

The specific interaction of ions with macromolecules and solid–liquid interfaces is of crucial importance to many processes in biochemistry, colloid science, and engineering, as first pointed out by Hofmeister in the context of (de)stabilization of protein solutions. Here, we use contact angle goniometry to demonstrate that the macroscopic contact angle of aqueous chloride salt solutions on mica immersed in ambient alkane increases from near-zero to values exceeding 10°, depending on the type and concentration of cations and pH. Our observations result in a series of increasing ability of cations to induce partial wetting in the order Na+, K+ < Li+ < Rb+ < Cs+ < Ca2+ < Mg2+ < Ba2+. Complementary atomic force microscopy measurements show that the transition to partial wetting is accompanied by cation adsorption to the mica–electrolyte interface, which leads to charge reversal in the case of divalent cations. In addition to electrostatics, hydration forces seem to play an important role, in particular for the monovalent cations.

Phase behaviour and characterization of microemulsion stabilized by a novel synthesized surfactant: Implications for enhanced oil recovery

The phase behavior of microemulsion systems comprised of methyl ester sulfonate (MES) as surfactant, propan-1-ol as cosurfactant, brine and alkane oils with varying chain lengths were studied for application in enhanced oil recovery. The aggregation and adsorption behavior of Jatropha oil-derived anionic surfactant were studied to investigate its favourability for use in microemulsion systems. Oil and water solubilisation parameters were determined in order to identify the optimal salinity values. The relative phase volumes were also studied as a salinity scan of microemulsion systems. The interfacial tension values between microemulsion and alkane systems were found to be much lower than that of surfactant and alkane systems. The prepared microemulsions were characterized by dynamic light scattering analysis and the particle sizes have been obtained in the range of 5–80 nm. Rheological studies revealed that the microemulsions exhibited non-Newtonian behaviour with favourably high viscosity values. Sandpack flooding using microemulsion systems was conducted at laboratory scale to predict the performance of microemulsion in oil recovery. It was found that the injected microemulsion formulations can achieve about 30% oil recovery close over conventional secondary water-flooding.

Adsorption Isotherms of Liquid Isomeric Mixtures

To separate linear and branched molecules in a liquid state, adsorption on porous materials is a promising separation method. To calculate the adsorption isotherms, a combination of lattice cluster theory and density functional theory was introduced recently, allowing the prediction of branched molecules’ adsorption isotherms based on the knowledge of the adsorption isotherms of the pure linear substances. However, these models are not practicable for process simulation and optimization because of their high numerical effort. Therefore, a simpler adsorption model based on the lattice cluster theory was developed to provide the results of the density functional theory approach for process development. In addition to the adsorption isotherm calculations, the model also considers the overall mass balance of the adsorption process. The model was validated for the adsorption of two binary, liquid alkane systems on three different adsorbents. Therefore, adsorption isotherms of these mixtures on activated coal, ...

Liquid–Liquid Equilibria for 2-Phenylethan-1-ol + Alkane Systems

The liquid–liquid equilibrium (LLE) curves for 2-phenylethan-1-ol (2-phenylethanol, 2PhEtOH) + octane, + decane, + dodecane, + tetradecane or + 2,2,4-trimethylpentane have been determined by a method of turbidimetry using a laser scattering technique. Experimental results reveal that the systems are characterized by an upper critical solution temperature (UCST), which increases linearly with the number of C atoms of the n-alkane. In addition, the LLE curves have a rather horizontal top and become skewed to higher mole fractions of the n-alkane, when its size increases. For a given n-alkane, UCST decreases as follows: phenol > phenylmethanol > 2-PhEtOH, indicating that dipolar interactions decrease in the same sequence. This has been ascribed to a weakening in the same order of the proximity effects between the phenyl and OH groups of the aromatic alkanols. DISQUAC interaction parameters for OH/aliphatic and OH/aromatic contacts in the investigated systems are reported. Phenol, or phenylmethanol or 2-PhEtO...