Dipolar Substances Research Articles

Modeling Dipolar Molecules with PCP-SAFT: A Vector Group-Contribution Method.

Predicting thermodynamic equilibrium properties is essential to develop chemical and energy conversion processes in the absence of experimental data. For the modeling of thermodynamic properties, statistical associating fluid theory (SAFT)-based equations of state, such as perturbed-chain polar (PCP)-SAFT, have been proven powerful and found broad application. The PCP-SAFT parameters can be predicted by group-contribution (GC) methods. However, their application to the dipole term is substantially limited: current GC methods neglect the dipole term or only allow for a single dipolar group per substance to avoid handling the molecular dipole moment's symmetry effects. Still, substances with multiple dipolar groups are highly relevant, and their description substantially improves by including the dipole term in SAFT models. To overcome these limitations, this work proposes a vector-addition-based (Vector-)GC method for the dipole term of PCP-SAFT that accounts for molecular symmetry. The Vector-GC employs information on the substance's molecular 3D structure to predict the molecular dipole moment through a vector addition of bond contributions. Combining the proposed sum rule for dipole moments with established sum rules for the remaining parameters yields a consistent GC method for PCP-SAFT for dipolar substances. The prediction capabilities of the Vector-GC method are analyzed against experimental data for two substance classes: nonassociating oxygenated and halogenated substances. We demonstrate that the Vector-GC method improves vapor pressure and liquid density predictions compared to neglecting the dipole term. Moreover, we show that the Vector-GC method enables differentiation between cis- and trans-isomers. The Vector-GC method, hence, substantially increases the predictive capabilities and applicability domain of GC methods. All parameters are provided as JSON and CSV files, and the Vector-GC method is available through an open-source python package. Additionally, the developed regression framework for GC methods for PCP-SAFT is openly available. The regression framework can be employed to regress the Vector-GC method to other substance classes and is easily adaptable to other sum rules for PCP-SAFT.

Surface Tension of Binary Mixtures Including Polar Components Modeled by the Density Gradient Theory Combined with the PC-SAFT Equation of State

In this study, the Cahn–Hilliard density gradient theory (GT) is used for predicting the surface tension of various binary mixtures at relatively wide temperature ranges and for testing the application of the GT for predictions of homogeneous nucleation. The GT was combined with two physically based equations of state (EoS), namely the perturbed-chain (PC) statistical associating fluid theory (SAFT) and its modification for polar substances the perturbed-chain polar (PCP) SAFT. The GT applied to the planar phase interface was employed to predict the interfacial tension for various quadrupolar (CO2 and benzene) and dipolar (difluoromethane, i.e., R32; pentafluoroethane, i.e., R125; and 1,1,1,2-tetrafluoroethane, i.e., R134a) substances and for five binary mixtures including polar components (n-decane + CO2, benzene + CO2, R32 + R125, R32 + R134a, R134a + R125). The PCP-SAFT EoS combined with the GT provides more accurate results for both the quadrupolar and dipolar substances than the original PC-SAFT EoS. Besides the planar phase interface, the GT was also applied to the spherical phase interface simulating a critical cluster occurring in homogeneous nucleation of droplets. Carbon dioxide was considered, because it has a relatively high quadrupole moment and because of its relevance to natural gas processing. Application of the PCP-SAFT EoS provides a significant improvement compared to the PC-SAFT EoS, and it is clearly superior to the classical cubic Peng–Robinson EoS, which is still used for modeling droplet nucleation.

Reducing the amount of PCP-SAFT fitting parameters. 1. Non-polar and dipolar components

The pure-component parameters of the Perturbed Chain Polar-Statistical Associating Fluid Theory (PCP-SAFT) equation of state are usually fitted to experimental data over broad temperature ranges. In this work, PCP-SAFT parameter correlations with molar mass were developed for five homologous series of non-polar and three homologous series of dipolar substances. The correlations were identified by using parameters of at least three components within the respective series. Applying these correlations for interpolations as well as for extrapolations within a homologous series, one experimental vapor–pressure point is sufficient for the parameter estimation of the related components. Use of the obtained parameters allows for reliable modeling of pure-component properties as well as of binary phase equilibria.

Graphene on a Hydrophobic Substrate: Doping Reduction and Hysteresis Suppression under Ambient Conditions

The intrinsic doping level of graphene prepared by mechanical exfoliation and standard lithography procedures on thermally oxidized silicon varies significantly and seems to depend strongly on processing details and the substrate morphology. Moreover, transport properties of such graphene devices suffer from hysteretic behavior under ambient conditions. The hysteresis presumably originates from dipolar adsorbates on the substrate or graphene surface. Here, we demonstrate that it is possible to reliably obtain low intrinsic doping levels and to strongly suppress hysteretic behavior even in ambient air by depositing graphene on top of a thin, hydrophobic self-assembled layer of hexamethyldisilazane (HMDS). The HMDS serves as a reproducible template that prevents the adsorption of dipolar substances. It may also screen the influence of substrate deficiencies.

A density functional theory for vapor-liquid interfaces using the PCP-SAFT equation of state

A Helmholtz energy functional for inhomogeneous fluid phases based on the perturbed-chain polar statistical associating fluid theory (PCP-SAFT) equation of state is proposed. The model is supplemented with a capillary wave contribution to the surface tension to account for long-wavelength fluctuations of a vapor-liquid interface. The functional for the dispersive attraction is based on a nonlocal perturbation theory for chain fluids and the difference of the perturbation theory to the dispersion term of the PCP-SAFT equation of state is treated with a local density approximation. This approach suggested by Gloor et al. [Fluid Phase Equilib. 194, 521 (2002)] leads to full compatibility with the PCP-SAFT equation of state. Several levels of approximation are compared for the nonlocal functional of the dispersive attractions. A first-order non-mean-field description is found to be superior to a mean-field treatment, whereas the inclusion of a second-order perturbation term does not contribute significantly to the results. The proposed functional gives excellent results for the surface tension of nonpolar or only moderately polar fluids, such as alkanes, aromatic substances, ethers, and ethanoates. A local density approximation for the polar interactions is sufficient for carbon dioxide as a strongly quadrupolar compound. The surface tension of acetone, as an archetype dipolar fluid, is overestimated, suggesting that a nonisotropic orientational distribution function across an interface should for strong dipolar substances be accounted for.

Coarse-graining dipolar interactions in simple fluids and polymer solutions: Monte Carlo studies of the phase behavior

In this paper we investigate the phase diagram of pure dipolar substances and their mixtures with short alkanes, using grand canonical Monte Carlo simulations of simplified coarse-grained models. Recently, an efficient coarse-grained model for simple quadrupolar molecules, based on a Lennard-Jones (LJ) interaction plus a spherically averaged quadrupolar potential, has been shown to be successful in predicting single-component and mixture phase diagrams. Motivated by these results, we investigate the phase diagrams of simple dipolar molecules (and their mixtures with alkanes) using a spherically averaged potential. First, we test the model on pure components. A generalized (state-dependent) mapping procedure allows us to recycle Monte Carlo results of the simple Lennard-Jones (LJ) potential. Considering ammonia, nitrous oxide, and hydrogen sulfide, we generally observe improvements in the single-component phase diagram compared to a pure LJ description, but also some discrepancies in the coexistence pressure near the critical point and in the liquid branch of the coexistence densities well below criticality. In addition, we present results for mixtures. We consider mixtures of ammonia (NH3) with methane (CH4), nonane (C9H20) and hexadecane (C16H34)--for which experimental results are available--and compare the predictions from this modeling ansatz with predictions from simple LJ models. We also present results for the hydrogen sulfide-pentane mixture (H2S and C5H12) for which big discrepancies between simulations and experiments are present. Possible explanations for these discrepancies and limitations of the modeling are discussed.

Heterylation of 3-R1-5-R2-1,2,4-Triazoles with Derivatives of 3,5-Dinitro-1,2,4-Triazole

Heterylation of 3-R1-5-R2-1'2'4-triazoles (pKa 3-12) with N-alkyl-, N-alkenyl-, N-alkoxy-carbonyl-, N-oxoalkyl-, N-nitroxyalkyl, N-nitroaminoalkyl-3'5-dinitro-1'2'4-triazoles results insubstitution of a nitro group in 5 position of the dinitro compound yielding 1-R-methyl-3-nitro-5-(3-R1-5-R2-1,2,4-triazolyl)-1,2,4-triazoles. The side processes: Hydroxide-ion attack on C5 and (or) N1 of the ring both in the substrate and in the target compound afford 1-R-methyl3-nitro-1,2,4-triazol-5-ones, 3,5-dinitro-1,2,4-triazole and NH-acids of N-C-bitriazole series. Optimal reaction media are aprotic dipolar substances, and for compounds prone to heterolysis ethyl acetate-water systems. The azole pKa is the decisive factor controlling the composition and the ratio of reaction products. The process is promising for azoles with pKa > 5, and the optimal range of pKa is 8-10.

Multicomponent solvents for cellulose triacetate

The effect of dipolar aprotic substances (urea or acetone) and also of protogenic molecules (acetic acid) or of the protophylic acetate ion on the optical and rheological properties of cellulose triacetate spinning solutions has been examined. It has been established that protolytes and dipolar aprotic substances in definite quantities aid in deaggregation of the microgel particles and reduce the viscosity of the spinning solution. Thereupon, the protolytes exert a greater effect on the viscosity of the spinning solution than the dipolar aprotic solvents.

Interaction of electric dipoles with phospholipid head groups. A deuterium and phosphorus-31 NMR study of phloretin and phloretin analogs in phosphatidylcholine membranes

Phloretin, 4-hydroxyvalerophenone, and 2-hydroxy-omega-phenylpropiophenone are lipophilic dipolar substances that modify ionic conductances of bilayer membranes. The structural changes at the level of the head groups and the hydrocarbon chains as induced by the incorporation of phloretin and its analogues were investigated with deuterium and phosphorus nuclear magnetic resonance. Membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were selectively deuterated at the choline head group and at the hydrocarbon chains, and 2H and 31P NMR spectra were recorded with varying concentrations of dipolar agents. Incorporation of phloretin leaves the bilayer structure intact, induces only a small disordering of the hydrocarbon chains, and has no significant effect on the head-group dynamics. On the other hand, quite distinct structural changes are observed for the phosphocholine head group. While the -P-N+ dipole is oriented approximately parallel to the membrane surface for pure POPC bilayers, addition of phloretin, and to a lesser extent 4-hydroxyvalerophenone and 2-hydroxy-omega-phenylpropiophenone, rotates the N+ end of the -P-N+ dipole closer to the hydrocarbon layer. The resulting normal component of the -P-N+ dipole partly compensates the electric field of the dipolar agents. In addition to this structural change, phloretin also modifies the hydration layer at the lipid-water interface. Much less 2H2O is adsorbed to the membrane surface when the bilayer contains phloretin, 4-hydroxyvalerophenone, or 2-hydroxy-omega-phenylpropiophenone. Moreover, a rather large change in the residual phosphorus chemical shielding anisotropy argues in favor of hydrogen-bond formation between the phosphate segment and the phloretin hydroxyl groups.

13C Nuclear Magnetic Resonance Studies on Molecular Association. I. Self-association of Dipolar Molecules

Self-association of dipolar substances is studied by 13C magnetic resonance. The dilution shifts in carbon tetrachloride are analyzed in terms of the monomer–dimer equilibrium. Downfield 13C shifts of the nitrile and carbonyl groups are observed for the formation of the dimer of acetone, acetonitrile, and ethyl acetate. Upfield 13C shifts, on the other hand, are observed in the methyl and methylene groups of acetone, methylethyl ketone, ethyl acetate, acetonitrile, propionitrile, nitromethane, nitroethane, N,N-di-methylformamide, and N,N-dimethylacetamide. The equilibrium constants are 0.1–0.5 in units of mole fraction. The downfield 13C shifts of the nitrile and carbonyl groups are interpreted in terms of electric (reaction) field effects imposed by another molecule through dimer formation. The alternation of polarity by polarization of the X–Y group explains the upfield 13C shifts of methyl and methylene groups as shown in the following scheme: Hs+—C−—X+—Y−, where X—Y stands for C=O, C≡N, and NO2 groups which have been classified as –I− groups by Pople and Gordon.

An Investigation of the Bound Water in Tendon by Dielectric Measurement

The dielectric constant of natural tendon in the frequency range of 200 Hz to 100 kHz has been determined as a function of temperature (300-450 degrees K) for water concentrations ranging from about 6 to 16% by weight. The results compare well with the approach taken by Haggis and his coworkers. Based on the assumption that at low concentration of water the probability of water-water bonding is small and hence may be disregarded, a structure for water in the collagen matrix is proposed in which the water is either bonded in one of four possible states to the polar groups of the polypeptide chains, or is unbound. In determining the distribution of the water among these states an approach similar to that of Haggis and his co-workers, in conjunction with the order-disorder theory of Bragg and Williams, is used. The number of water molecules per unit volume is then determined experimentally by relating it to weight loss as a function of temperature, as determined by thermogravimetric analysis. The dispersion which is normally found in dipolar substances has not been found for tendon. A maximum in the value of the dielectric constant is observed to occur between 25 and 80 degrees C, the temperature depending upon the heating rate.

The dielectric constant of dipolar substances in the environment of ions produced by irradiation

The time-behavior of the dielectric constant of dipolar substances polarized exclusively by the ions within the system is described by a simple exponential function. The dielectric constant varies only with the time elapsed after the formation of the ions.

Behavior of Ion Spurs and Tracks in Irradiated Dipolar Systems

The problem of electron moderation and subsequent recapture or escape in irradiated dipolar substances is investigated theoretically. By considering the dielectric relaxation, the electric energy of a spur or track consisting of positive ions and diffusing electrons is evaluated. The moderation being fast as compared with the dielectric relaxation time, the energy content of the spur or track decreases due to an increase in the dielectric constant after the moderation has been completed. At the expense of this energy decrease, some thermalized electrons may diffuse away from the field of the positive ions if they are slow enough not to increase the spur energy faster than it is decreased by dielectric relaxation. The theory is tentatively applied to γ and α-irradiated water and ice and, while a substantial fraction of electrons is found capable of escaping in the liquid state, for the crystalline state complete recapture is predicted.

Torsional vibrations of long-chain hydrocarbons

The torsional vibrations of long-chain hydrocarbons represent rotational oscillations of small amplitude around the carbon-carbon bonds. On the assumption that the forces hindering rotation around the various C— C bonds are equal and independent of each other, the frequencies of isolated long-chain hydrocarbon molecules in the plane configuration are calculated exactly. In the case of L carbon atoms, there are L — 3 m odes of torsional vibration. The frequencies extend from a maximum value ω m in the far infra-red region, down to low values The introduction of a polar group into the molecule makes the torsional vibrations active in the absorption spectrum. In a subsequent paper, the present results will be used to derive the torsional absorption spectrum of dipolar long-chain substances in the solid state .

The Frequency Dependence of the Dielectric Properties of Dipolar Substances

A critical review of theories of the frequency dependence of the dielectric constant of dipolar substances is given and inconsistencies are pointed out in previous attempts to account for dipolar interaction. A new theory is presented based on Debye's theory of molecular relaxation and using Onsager's method for treating electrostatic inter- action. Fluctuations are considered explicitly both in the orientation of an individual dipole and in the local field acting on it ; a distinction is also made between the experimentally measurable time constants and the relaxation times governing molecular processes. The results are presented m the form of a series development in pourers of the dipolar concentration. In first approximation the theory leads to a Debye frequency dependence of the dielectric constant ; in second approximation a Debye dependence can be made consistent with the molecular mechanism only for a more restricted (two-position) model.

A DILUTION EFFECT IN REACTIONS BETWEEN DIPOLAR MOLECULES IN SOLUTION

A study has been made of the variation of the reaction rate constant with increasing dilution in the case of three reactions between pairs of dipolar substances in ethanol solution. The reactions examined were those between aniline and 2,4-dinitrochlorobenzene, p-toluidine and 2,4-dinitrochlorobenzene, and p-toluidine and 2,4-dinitrobromobenzene. As the concentrations of the reactants in the first two cases are increased 50-fold, the bimolecular velocity coefficient decreases by approximately 12% in the case of the first reaction, and by 7% in the case of the second one. The rates decrease sharply as the solubility limits of the products in ethanol are approached, while they become relatively constant as high dilutions are attained.