N-alkane Solutions Research Articles

Novel high-precision wax molecular diffusion coefficient correlation based on the diffusion laboratory apparatus coupled with differential scanning calorimeter

Wax molecular diffusion coefficient is a crucial factor determining the wax deposition rate of waxy crude oil, but the calculation accuracy of the traditional Hayduk-Minhas and Wilke-Chang relations is unclear. The Couette wax deposition experiments of Changqing crude oil under different operating conditions (oil temperature, wall temperature, rotation speed and deposition duration) were carry out and find the traditional Hayduk-Minhas relationship will underestimate the wax molecular diffusion coefficient. To increase the calculation accuracy of wax molecular diffusion coefficient, a newly-developed diffusion laboratory apparatus coupled with the Differential Scanning Calorimeter (DSC) were utilized to evaluate the diffusion coefficient of C16, C18, C20, C22, C24 in n-alkane solutions at different temperatures (10, 15, 20, 25, 30,35, and 40 °C). Based on the experimental results, a novel high-precision correlation was put forward and the calculated results are in good agreement with the experimental results, the average relative error is within 3.0 %. The applied solvent of the n-alkane solutions is mineral oil and it has a complex composition. Therefore, the developed correlation can be used to evaluate the wax molecular diffusion coefficient in the real crude oil. The application of the new relationship will lead to more accurate wax deposition calculations.

Low-temperature time-resolved phosphorescence excitation emission matrices for the analysis of phenanthro-thiophenes in chromatographic fractions of complex environmental extracts

The present study investigates the analytical potential of low-temperature photoluminescence spectroscopy for the analysis of seven phenanthrothiophenes with molecular mass 234 g mol−1. The studied PASHs include Phenanthro [1,2-b]thiophene, Phenanthro [2,1-b]thiophene, Phenanthro [2,3-b]thiophene, Phenanthro [3,2-b]thiophene, Phenanthro [3,4-b]thiophene, Phenanthro [4,3-b]thiophene and Phenanthro [9,10-b]thiophene. Excitation and emission spectra recorded from n-alkane solutions at room temperature, 77 K and 4.2 K show phosphorescence emission from all the studied isomers at cryogenic temperatures. The analytical figures of merit obtained under steady state (fluorescence) and time-resolved (phosphorescence) conditions provide limits of detection at the parts-per-billion (ng mL−1) concentration levels. Processing 77 K and 4.2 K phosphorescence data with parallel factor analysis showed to be a robust approach to the determination of phenanthro-thiophenes in complex fluorophore mixtures.

Thermodiffusion, molecular diffusion and Soret coefficients of aromatic+n-alkane binary mixtures.

In the present work, we have measured the thermodiffusion coefficient of 51 binary liquid mixtures at 25 oC. These mixtures correspond to the series of the aromatics toluene and 1-methylnaphthalene with n-alkanes nCi (i = 6, 8, 10, 12, and 14) at different mass fractions in the whole range. For that, we have used the thermogravitational technique. It is shown that the thermodiffusion coefficient is a linear function of the mass fraction in all the mixtures. Extrapolating the lines, we obtain the thermodiffusion coefficient in dilute solutions of n-alkanes for both toluene and 1-methylnaphthalene. These limiting values show a linear dependence with the inverse of the product of the molecular weights. In addition, we have measured the molecular diffusion coefficient of all the mixtures at 0.5 of mass fraction and at 25 oC, by the sliding symmetric tubes technique. It is observed that the product of this coefficient with the viscosity at the same concentrations takes a constant value for each of the series considered. Finally, we have also determined the Soret coefficient of the equimass mixtures by the combination of the measurements of thermodiffusion and molecular diffusion coefficients.

Calculating excess volumes of binary solutions with allowance for structural differences between mixed components

Analytical dependences of a volume’s properties on the differences between the geometric structures of initial monosystems are obtained for binary systems simulated by a grain medium. The effect of microstructural parameter k (the ratio of volumes of molecules of mixed components) on the concentration behavior of the relative excess molar volume of different types of real binary solutions is analyzed. It is established that the contribution due to differences between the volumes of molecules and coefficients of the packing density of mixed components is ~80–100% for mutual solutions of n-alkanes and ~55–80% of the experimental value of the relative excess molar volume for water solutions of n-alcohols.

Effect of Temperature on Near-infrared Spectra ofn-Alkanes

Effect of temperature on near-infrared (NIR) spectra has been studied and applied to structural and quantitative analyses. To investigate the effect of temperature on NIR spectra of alkyl organic system, n-alkanes were studied in this work. NIR spectra of pure n-alkanes (hexane to decane), binary (hexane and octane) and ternary (octane, nonane and decane) mix- tures were measured. In the experiments, temperature was controlled to change from 60 to 20 ℃ with a step of ca. 5 ℃. Comparing the spectra at different temperatures, only a little difference in peak intensity of some bands can be found. There- fore, alternating trilinear decomposition (ATLD) algorithm was adopted to analyze the three-order data matrix. The results show that two spectral loadings are obtained because the influence of temperature on the spectra of terminal ethyl (C2H5) groups differs from that of mid-chain methylene (CH2) groups. Furthermore, the temperature scores of CH2 and C2H5 groups decrease linearly with temperature, implying that the temperature effect can be quantitatively described by a quantitative spectra-temperature relationship (QSTR) model. The QSTR model provides an efficient way to predict the temperature of n-alkane solutions. Good linearity also exists between sample scores and carbon number or the relative content of CH2 and C2H5 groups in the molecules of the n-alkanes. Linear models between the two scores and the relative content of CH2 and C2H5 groups are obtained, respectively, using the least square fitting of the score and the relative contents. The model can be used for prediction of the relative content of CH2 and C2H5 groups in mixtures, which can further be used to estimate the composition of the mixtures. Furthermore, the relationship between the scores and the carbon atom numbers is modeled using multivariate linear regression (MLR). The composition of n-alkane mixtures can also be estimated through the predicted car- bon number using the MLR model. These models are validated by binary and ternary mixtures of the n-alkanes. It was indi- cated that the relative contents of CH2 and C2H5 groups or the carbon atom number can be predicted using the models. Therefore, a new way for quantitative estimation of the composition in n-alkane mixtures was developed using the tempera- ture effect of the near-infrared spectra. Keywords near-infrared spectra; temperature effect; n-alkane; alternating trilinear decomposition; quantitative spectra- temperature relationship

Diffusion of Benzene and Alkylbenzenes in n-Alkanes.

The translational diffusion constants, D, of benzene and a series of alkylbenzenes have been determined in four n-alkanes at room temperature using capillary flow techniques. The alkylbenzenes are toluene, ethylbenzene, 1-phenylpropane, 1-phenylpentane, 1-phenyloctane, 1-phenylundecane, 1-phenyltetradecane, and 1-phenylheptadecane. The n-alkanes are n-nonane, n-decane, n-dodecane, and n-pentadecane. Ratios of the solutes' D values are independent of solvent and in general agreement with the predictions of diffusion models for cylinders and lollipops. For the latter, an alkylbenzene's phenyl ring is the lollipop's candy; the alkyl chain is its handle. A model that considers the solutes to be spheres with volumes determined by the van der Waals increments of their constituent atoms is not in agreement with experiment. The diffusion constants of 1-alkene and n-alkane solutes in n-alkane solvents also are compared with the cylinder model; reasonably good agreement is found. The n-alkanes are relatively extended, and this appears to be the case for the alkyl chains of the 1-alkenes and alkylbenzenes as well.

The influence of the generation number of liquid-crystalline polypropyleneimine dendrimers on the sorption of low-molecular-weight compounds

The thermodynamic characteristics of ultimately dilute solutions of n-alkanes and n-alkanols in columnar and isotropic poly(propyleneimine) dendrimer phases of three generations (G1-G3) were studied by reversed-phase gas chromatography. The influence of the structure of dendrimers and the chemical nature and the size of nonmesogen molecules on their ability to be dissolved in liquid crystalline phases is discussed. It was found that the compatibility of low-molecular-weight compounds with anisotropic phases increased as the number of dendrimer generation grew.

Role of a Carboxylic Acid on the Crystallization, Deposition, and Gelation of Long-Chained n-Alkanes in Solution

n-Alkanes are the primary components of solid wax formed in the wax−oil deposits in subsea oil pipelines. However, there is a wide number of other components present in these deposits that influence crystallization, deposition, and gelation. This work focuses on how a carboxylic acid, stearic acid, can influence the thermodynamics, crystallization, and gelation of two straight-chained n-alkanes, C36H74 and C32H66, in an n-alkane solvent, dodecane. Stearic acid was selected because its solubility in dodecane is very similar to C32H66, and it is known that C32H66 cocrystallizes with C36H74 and influences solution and gel properties. In spite of its similar solubility in dodecane relative to that of C32H66, stearic acid does not cocrystallize with either C32H66 or C36H74, although it influences the nucleation kinetics of the n-alkanes. Deposition experiments showed that adding stearic acid to a C36 solution does not slow deposition and that at high concentrations can actually increase the amount of deposit that forms. Stearic acid influenced gelation only if present at sufficiently high concentrations such that the n-alkanes were more soluble than stearic acid. These results are observed because stearic acid dimerizes in solution by hydrogen-bonding and associates with itself instead of n-alkanes.

Effects of pressure drop, particle size and thermal conditions on retention and efficiency in supercritical fluid chromatography

The effects of particle size and thermal insulation on retention and efficiency in packed-column supercritical fluid chromatography with large pressure drops are described for the separation of a series of model n-alkane solutes. The columns were 2.0 mm i.d. × 150 mm long and were packed with 3, 5, or 10-μm porous octylsilica particles. Separations were performed with pure carbon dioxide at 50 °C at average mobile phase densities of 0.47 g/mL (107 bar) and 0.70 g/mL (151 bar). The three principal causes of band broadening were the normal dispersion processes described by the van Deemter equation, changes in the retention factor due to the axial density gradient, and radial temperature gradients associated with expansion of the mobile phase. At the lower density the use of thermal insulation resulted in significant improvements in efficiency and decreased retention times at large pressure drops. The effects are attributed to the elimination of radial temperature gradients and the concurrent enhancement of the axial temperature gradient. Thermal insulation had no significant effect on chromatographic performance at the higher density. A simple expression to predict the onset of excess efficiency loss due to the radial temperature gradient is proposed.

Thermodynamics of solution in liquid crystalline poly(propyleneimine) dendrimers by inverse gas chromatography

Thermodynamic characteristics of infinitely diluted solutions of n-alkanes and n-alcohols in the columnar and isotropic phases of poly(propyleneimine) dendrimers, generations 1–3 (G1–G3), have been investigated by inverse gas chromatography (IGC). Effects of the dendrimer structure, the chemical nature and molecular size of the non-mesogenes on the ability to get dissolved in the liquid crystalline phases are discussed. The compatibility of the low-molecular compounds with the anisotropic phases is stated to increase with generation G.

Aggregation in Solvent Extraction Systems Containing a Malonamide, a Dialkylphosphoric Acid and their Mixtures

Aggregation phenomena in n-alkane solutions of di-n-hexylphosphoric acid (HDHP), N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), and their mixtures, were investigated by electrospray ionization–mass spectrometry (ESI-MS), vapor pressure osmometry (VPO), and small-angle X-ray and neutron scattering (SAXS and SANS). The objective of the study was to probe the formation of mixed HDHP-DMDOHEMA species before and after extraction of trivalent lanthanide and actinide (M3+) nitrates. The most important species formed by HDHP upon metal extraction has the composition M(DHP)3(HDHP)3(H2O). These species exist as spherical aggregates of the reverse micelle type with a polar core diameter of ∼ 7 Å and total diameter of ∼ 11 to ∼ 15 Å. The aggregation of DMDOHEMA is a progressive phenomenon, with an average aggregation number of ∼ 2 in the 0.2 to 0.6 M range and larger aggregates forming at higher concentrations. The metal loaded DMDOHEMA aggregates can be considered as interacting spheres with a polar core diameter between ∼ 11 and ∼ 16 Å, depending on composition, a total diameter of up to ∼ 25 Å, and a weight-average aggregation number of ∼ 9. The results obtained in this work provide strong evidence for the formation of mixed aggregates when mixtures of HDHP and DMDOHEMA are used for the extraction of trivalent Ln and An cations. These mixed reverse micelles have a diameter between 19 and 24 Å with a polar core diameter of 10 and to 14 Å. The most recurrent micellar composition is 2 HDHP and either 4 or 5 DMDOHEMA molecules.

Photochemical Modification of a Boron-doped Diamond Electrode Surface with Vinylferrocene

Boron-doped diamond (BDD) surfaces were photochemically modified with vinylferrocene (VFC), and the modified surfaces were analyzed both qualitatively and quantitatively by electrochemical techniques. VFC-modified BDD (VFC-BDD) was prepared by ultraviolet (254 nm) irradiation of hydrogen-terminated BDD in mesitylene, acetonitrile, or n-alkane solutions of VFC. Cyclic voltammetry (CV) results indicated facile electron transfer between ferrocenyl groups and the BDD substrate. The surface coverage of VFC-modified polycrystalline and single-crystal (100) BDD was also estimated by CV and was found to reach saturation at a value possibly corresponding to monolayer coverage by ferrocenyl groups. The electrochemical stability of VFC-BDD to successive potential cycling in acetonitrile was found to be much higher than that of a self-assembled ferrocenylalkanethiol monolayer on an Au electrode surface. This study is the first to show that appropriate solutions of a terminal alkene can be used in the photochemical modification of diamond, yielding monolayer coverage of the surface.

Diffusion of 1-alkenes and cyclohexene in alkane solvents

The translational diffusion constant, D, has been measured for each of the 1-alkenes 1-C 6H 12, 1-C 8H 16, 1-C 12H 24, and 1-C 14H 28 in each of the even n-alkanes n-C 6H 14– n-C 14H 30; the D values have also been measured for 1-C 10H 20 in each of the even n-alkanes n-C 8H 18– n-C 14H 30. Cyclohexene has been studied in each of the even n-alkanes n-C 8H 18– n-C 14H 30 and cyclohexane. Deviations from the Stokes–Einstein (SE) relation ( D = k B T/6 πηr) were found. For a given solute, the hydrodynamic radius r decreased as the viscosity η increased. Analyses of literature data for n-alkane solutes in n-alkane solvents, including self-diffusion, also gave values of r that decreased as η increased. These solvent-dependent r values are discussed in terms of the relative sizes of the solutes and solvents. The data also were analyzed using D/ T = A/ η p ( p = 1 for the SE relation). The p values for the 1-alkenes and the n-alkane solutes with six or more carbon atoms were all <1 and were not a strong function of size; those for the 1-alkenes ranged from 0.637 ± 0.027 for 1-hexene to 0.725 ± 0.017 for 1-tetradecene. The p values for the analogous n-alkane solutes were roughly the same and indicated that the similar shapes and polarities of the two types of hydrocarbon play key roles in determining their diffusion. In the n-alkane solvents, the p value of the more globular cyclohexene is somewhat larger than those of both 1-hexene and n-hexane.

Enthalpies of solution of n-alkanes in mixtures of MeCN with alkan-1-ols at 298.15 K. Relations between the thermodynamic properties of binary and ternary systems

The solution enthalpies of n-hexane, n-nonane, and n-undecane in mixtures of MeCN with EtOH, PrOH, and BuOH were determined by calorimetric method at 298.15 K. Relations between the thermodynamic properties of ternary systems and the properties of the binary solvents and the solute are discussed. To predict the enthalpies of solution of hydrocarbons in mixed solvents whose components have strongly different molar volumes (V2/V1 > 1.5), it is proposed to use the values that are additive in the volume fraction scale. The solution enthalpies of hydrocarbons in mixtures characterized by close molar volumes of the components strongly depend on the excess properties of the binary solvent.

Effect of molecular packing on radiolytic processes of n-alkanes

Both ionic and radical processes in the radiolysis of n-alkanes may be affected by the packing arrangements of the respective reagents. On the ionic side, proton transfer from alkane radical cations to alkane molecules is greatly affected by the accessibility of secondary C–H bonds in alkane molecules to planar C–H bonds in alkane radical cations from which proton donation takes place. As a result, the proton transfer from ground-state radical cations is not an efficient process in neat n-alkane crystals but occurs readily in the presence of longer-chain n-alkane solutes. With respect to radical reactions, the effect of molecular packing on alkyl radical production (resulting from the preferential migration of hydrogen atoms in the intermolecular boundary region) and on radical transformation and recombination (resulting from intermolecular radical-site transfer) is being discussed. The effect of crystalline structure on radical transformation by intermolecular radical-site transfer is utilized to determine the physical state of n-alkane aggregates in CCl 3F.

Recovery of semivolatile organic compounds during sample preparation: implications for characterization of airborne particulate matter.

Semivolatile compounds present special analytical challenges not met by conventional methods for analysis of ambient particulate matter (PM). Accurate quantification of PM-associated organic compounds requires validation of the laboratory procedures for recovery over a wide volatility and polarity range. To meet these challenges, solutions of n-alkanes (nC12-nC40) and polycyclic aromatic hydrocarbons PAHs (naphthalene to benzo[ghi]perylene) were reduced in volume from a solvent mixture (equal volumes of hexane, dichloromethane and methanol), to examine recovery after reduction in volume. When the extract solution volume reached 0.5 mL the solvent was entirely methanol, and the recovery averaged 60% for n-alkanes nC12-nC25 and PAHs from naphthalene to chrysene. Recovery of higher MW compounds decreased with MW, because of their insolubility in methanol. When the walls of the flasks were washed with 1 mL of equal parts hexane and dichloromethane (to reconstruct the original solvent composition), the recovery of nC18 and higher MW compounds increased dramatically, up to 100% for nC22-nC32 and then slowly decreasing with MW due to insolubility. To examine recovery during extraction of the components of the High Capacity Integrated Gas and Particle Sampler, the same standards were used to spike its denuders and filters. For XAD-4 coated denuders and filters, normalized recovery was >95% after two extractions. Recovery from spiked quartz filters matched the recovery from the coated surfaces for alkanes nC18 and larger and for fluoranthene and larger PAHs. Lower MW compounds evaporated from the quartz filter with the spiking solvent. This careful approach allowed quantification of organics by correcting for volatility- and solubility-related sample preparation losses. This method is illustrated for an ambient sample collected with this sampler during the Texas Air Quality Study 2000.

An upper bound on the theoretical activity coefficient of non-electrolytes.

An upper bound on the activity coefficient, calculated from its theoretical definition, is formulated and discussed in detail for binary solutions of non-electrolytes. It is compared with a previous order-of-magnitude analysis that predicted it to be e. The existence of an upper bound points to a major discrepancy between theoretical and actual (experimentally measured) activity coefficients in highly nonideal solutions: The actual activity coefficient may be much higher than the theoretical upper bound. It is shown that the theoretical activity coefficient of the more volatile component must indeed be much smaller than e for slightly soluble components. For the less volatile component, the upper bound on the theoretical activity coefficient is shown to be approximately equal to the relative volatility. However, it is demonstrated that this upper bound may be grossly exaggerated when the relative volatility is high. In addition, as shown for aqueous solutions of n-alkanes that are less volatile than water, even when the upper bound is much higher than e, it may still be much smaller than the actual activity coefficient.

Radical cations of n-alkanes in irradiated solutions as studied by time-resolved magnetic field effects

The time-resolved magnetic field effect in the recombination fluorescence of spin-correlated radical ion pairs has been measured to study n-alkane radical cations in irradiated solutions at room temperature. The magnetic field effect was recorded as a ratio of fluorescence decay curves in the 0.1 T and zero magnetic fields for solutions of C8, C9, C10, C12, and C16 n-alkanes in n-hexane with addition of 3 × 10−5 M p-terphenyl-d 14. A distinct maximum at 10–30 ns followed by a slowly decaying plateau was observed for all the solutions. Simulation shows that the maximum corresponds to an unresolved ESR spectrum with the peak-to-peak line-width ranging from about 1.6mT to 0.5mT for C8 to C16 radical cations. The unresolved structure is believed to result from the hyperfine couplings with many protons of the radical cation, the increase in the number of interacting protons compared with low temperature matrices being caused by the methyl group rotation and conformational motion of the carbon chain. With increase in concentration of dissolved n-alkane, the maximum in the curves first moves to longer times and finally disappears; this was attributed to the narrowing of ESR spectrum contour due to degenerate electron exchange.

Infinite dilution activity coefficients of n-alkane solutes, butane to decane, in n-alkane solvents, heptane to hexatriacontane

The literature data for the infinite dilution activity coefficients, γ i ∞, of alkane solutes having short carbon chains in alkane solvents having long carbon chains have been investigated. The total number of the literature values is 215, covering the alkane solutes from butane to decane, alkane solvents from heptane to hexatriacontane, and temperatures from 280.15 to 373.15 K. The values of ln γ i ∞ are independent of the measuring techniques that included chromatography, gas stripping (GS) and a static equilibrium cell (SEC). The values of ln γ i ∞ obtained at different temperatures for given solute–solvent systems are the same within experimental error. The values of ln γ i ∞ are directly proportional to ( q i − q j )/ q i , where q i and q j , respectively, denote the measures of the molecular surface areas of solute i and solvent j. The range in the variation of ln γ i ∞ is extensively negative as −0.50 to 0.0, indicating molecular associations between both solute–solvent and solvent–solvent alkanes, probably forming parallel frameworks. The relationship between ln γ i ∞ and ( q i − q j )/ q i cannot be estimated by the combinatorial entropy terms from the conventional solution theories such as the ideal solution, the regular solution, and the Flory–Huggins and the UNIQUAC models.

Simulation Studies of Retention in Isotropic or Oriented Liquidn-Octadecane

In gas−liquid chromatography with liquid-crystalline stationary phases and in liquid−liquid chromatography with polymeric bonded phases, the retention selectivity has been linked to the orientation of the retentive phase. In an effort to explore the nature of this effect, molecular simulations utilizing the configurational-bias Monte Carlo technique in the Gibbs ensemble were performed. Through this simulation technique, the partition coefficients of benzene, naphthalene, n-heptane, and n-dodecane were calculated when partitioning takes place between an oriented or an isotropic liquid n-octadecane phase of equal density and a helium gas phase. The calculated partition coefficients demonstrate that the n-alkane solutes prefer the oriented n-octadecane phase over the isotropic one, while the opposite behavior (but to a smaller extent) is observed for benzene and naphthalene solutes. The n-alkane solute transfer into the oriented n-octadecane phase is favored by a smaller entropic penalty and a minor enthalp...