Vapor Pressure Isotope Effects Research Articles

Liquid-phase water isotope separation using graphene-oxide membranes

We report pressure-driven liquid-phase isotope separation (dead-end filtration) to enrich D and 18O in natural water using graphene oxide (G-O) and UV-reduced graphene oxide (UV-rG-O) membranes. The isotope diffusivity (molecular diffusion and adsorption separation) was found to be responsible for isotope separation. Adsorption separation is the dominant mechanism for improvements in D and 18O enrichment via increased G-O loading that leads to the increased number of adsorption sites (epoxy and hydroxyl groups on G-O), and higher degrees of reduction of G-O that result in the narrowing of the nanochannels which decreases the portion of water molecules experiencing molecular diffusion. The best performing membrane was “UV-rG-O” made by exposing a G-O membrane to 24 h UV irradiation from one side, showing enrichment of D of 0.5% for D/H and 18O of 0.08% for 18O/16O in a single-stage experiment, without contribution from the vapor pressure isotope effect. This work improves the understanding of the mechanisms for graphene-based membrane separation of D and 18O enriched water.

Fluctuation aspects of isotope theory - III. Isotopic phase similarity in the set of deuterated methanes CHxD4-x

We have proposed the new principle of isotopic phase similarity (IPS) in the previous works I, II, of this series. The fundamental Born-Oppenheimer approximation maintains separately its applicability within each isotopic phase. The fluctuation-thermodynamic formalism is based on the developed earlier predictive model of CVL (congruent vapor-liquid) – diagram. It changes the mean-field concept of the macroscopic VLE (vapor-liquid equilibrium) – diagram to describe adequately the mesoscopic scale of volumes and time-intervals for a fluid state. The well-established by CVL-diagram fluctuation boundaries of the boil- Pb(T)- and dew-Pd(T)-pressures are important ones. They form the essential mesoscopic expansion of the single Pv(T)- vapor-pressure curve postulated by VLE- diagram. This mesoscopic peculiarity of a non-mean-field phase transition was revealed earlier for any one-component fluids. It provides the new insight into the classic theory of isotope effects. In the first instance the conventional interpretation of the vapor-pressure isotope effect (VPIE) and the molar-volume isotope effect needs an independent consideration by the comparison of CVL-diagrams for isotopes. In two previous works I, II we have introduced the undimensional fluctuation calibration complex Zoc=P0M/(RT0ρc) (P0=101.325kPa, T0=1K, M g/mol – molar mass of isotope, ρc - its critical density of mass) which provides the separation of VPIEs for the variety of isotopic families on two main types: (n) normal (Zocn<1) and (i) inverse (Zoci>1) at low temperatures. In this work III we have applied the same IPS-methodology to the set of partially deuterated methanes CHxD4-x of i-type. The set of exclusively n-type pairs {H2O/D2O/T2O; 36Ar/40Ar; 235U/238U} was considered in works I, II. The reasonable predictive capability of IPS-principle determines systematic shifts in the critical parameters of heavier isotopes if only those for the basic lighter isotope (CH4) are known. The most impressive confirmation of IPS-principle is the reasonable concordance of its dew-pressure isotope effect (DPIE)-predictions with the experimental absolute differences between small vapor pressures of isotopes ΔPv=Pv′−Pv. We have proposed and corroborated the concept of two coupled crossover Bancroft'sTbB and Clapeyron'sTlgC-points. They offer the study of the difference between the vaporization enthalpies of isotopes without virial correction for non-ideality of v-phase.

Computational Investigations of Position-Specific Vapor Pressure Isotope Effects in Ethanol-Toward More Powerful Isotope Models for Food Forensics.

With the advent of new experimental techniques, measurements of individual, per-position, vapor pressure isotope effects (VPIEs) became possible. Frequently, they are in opposite directions (larger and smaller than unity), leading to the cancellation when only bulk values are determined. This progress has not been yet paralleled by the theoretical description of phase change processes that would allow for computational prediction of the values of these isotope effects. Herein, we present the first computational protocol that allowed us to predict carbon VPIEs for ethanol—the molecule of great importance in authentication protocols that rely on the precise information about position-specific isotopic composition. Only the model comprising explicit treatment of the surrounding first-shell molecules provided good agreement with the measured values of isotope effects. Additionally, we find that the internal vibrations of molecules of the model to predict isotope effects work better than the entire set of normal modes of the system.

H/D and 12C/13C vapor pressure isotope effects in liquid methylene difluoride studied by density functional theory

H/D and 12C/13C vapor pressure isotope effects (VPIE’s) in liquid CH2F2 were studies at the mPW1PW91/6-31++G(d) level of theory. The optimized monomer and CH2F2 molecules surrounded by other CH2F2 molecules in optimized CH2F2 clusters were used as model molecules of CH2F2 vapor and liquid CH2F2, respectively. The calculations qualitatively reproduced the normal hydrogen VPIE and the inverse carbon VPIE, although they overestimated the former and underestimated the latter at 170 K. The number of CH···F(C) intermolecular interactions between hydrogen and fluorine atoms of neighboring molecules in the liquid phase and their strength were required to be temperature-dependent to explain the experimental temperature dependences of the VPIE’s.

Re-established congruent vapor-liquid diagram and fluctuation aspects of isotope theory-I (H2O-D2O-T2O)

The well-established vapor-pressure isotope effects (VPIEs) are independently corroborated in the set of isotopes H2O-D2O-T2O by the recently proposed congruent vapor-liquid (CVL) diagram only at the vaporization of liquid (l) phase. The striking discrepancies with the expected IE-trends following from the Bigeleisen-Mayer formulation at the condensation of vapor (v) phase have been revealed. The obvious failure of the classical second virial coefficient determined for dilute gas (g) phase to represent the actual predominance of repulsive collective interactions between particles in the so-called overheated v-phase (termed the interphase by CVL-methodology) needs the additional experimental and/or simulation confirmation. This conclusion corresponds qualitatively to that obtained for pair interactions in the about ideal-gas v-phase by Van Hook, Rebelo and Wolfsberg. The crucial distinctions between the fluctuation CVL-diagram and the deterministic, mean-field (mf) by nature, description of the first-order phase transition by its vapor-liquid equilibrium (VLE) diagram are discussed.

Can Path Integral Molecular Dynamics Make a Good Approximation for Vapor Pressure Isotope Effects Prediction for Organic Solvents? A Comparison to ONIOM QM/MM and QM Cluster Calculation.

Isotopic fractionation of volatile organic compounds (VOCs), which are under strict measures of control because of their potential harm to the environment and humans, has an important ecological aspect, as the isotopic composition of compounds may depend on the conditions in which such compounds are distributed in Nature. Therefore, detailed knowledge on isotopic fractionation, not only experimental but also based on theoretical models, is crucial to follow conditions and pathways within which these contaminants are spread throughout the ecosystems. In this work, we present carbon and, for the first time, bromine vapor pressure isotope effect (VPIE) on the evaporation process from pure-phase systems-dibromomethane and bromobenzene, the representatives of aliphatic and aromatic brominated VOCs. We combine isotope effects measurements with their theoretical prediction using three computational techniques, namely path integral molecular dynamics, QM cluster, and hybrid ONIOM models. While evaporation of both compounds resulted in normal bromine VPIEs, the difference in the direction of carbon isotopic fractionation is observed for the aliphatic and aromatic compounds, where VPIEs are inverse and normal, respectively. Even though theoretical models tested here turned out to be insufficient for quantitative agreement with the experimental values, cluster electronic structure calculations, as well as two-layer ONIOM computations, provided better reproduction of experimental trends.

Comment on Vapor Pressure Isotope Effects in Halogenated Organic Compounds and Alcohols Dissolved in Water.

ADVERTISEMENT RETURN TO ISSUEPREVCommentNEXTComment on Vapor Pressure Isotope Effects in Halogenated Organic Compounds and Alcohols Dissolved in WaterFrank-Dieter Kopinke* and Anett GeorgiView Author Information Department of Environmental Engineering, Helmholtz Centre for Environmental Research−UFZ, Leipzig D-04318, Germany*Phone: +49 341 235 1234. Fax: +49 341 235 451234. E-mail: [email protected]Cite this: Anal. Chem. 2017, 89, 19, 10637–10638Publication Date (Web):September 11, 2017Publication History Published online21 September 2017Published inissue 3 October 2017https://doi.org/10.1021/acs.analchem.7b02574Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSArticle Views361Altmetric-Citations2LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (569 KB) Get e-AlertsSUBJECTS:Alcohols,Diffusion,Fractionation,Solution chemistry,Transport properties Get e-Alerts

Hydrogen and Carbon Vapour Pressure Isotope Effects in Liquid Fluoroform Studied by Density Functional Theory

Abstract H/D and 12C/13C vapour pressure isotope effects (VPIEs) in liquid fluoroform (CHF3) were studied at the MPW1PW91/6-31 ++ G(d) level of theory. The CHF3 monomer and CHF3 molecules surrounded by other CHF3 molecules in every direction in CHF3 clusters were used as model molecules of vapour and liquid CHF3. Although experimental results in which the vapour pressure of liquid 12CHF3 is higher than that of liquid 12CDF3 and the vapour pressure of liquid 13CHF3 is higher than that of liquid 12CHF3 between 125 and 212 K were qualitatively reproduced, the present calculations overestimated the H/D VPIE and underestimated the 12C/13C VPIE. Temperature-dependent intermolecular interactions between hydrogen and fluorine atoms of neighbouring molecules were required to explain the temperature dependences of both H/D and 12C/13C VPIEs.

Vapor Pressure Isotope Effects in Halogenated Organic Compounds and Alcohols Dissolved in Water.

Volatilization causes changes in the isotopic composition of organic compounds as a result of different vapor pressures of molecules containing heavy and light isotopes. Both normal and inverse vapor pressure isotope effects (VPIE) have been observed, depending on molecular interactions in the liquid phase and the investigated element. Previous studies have focused mostly on pure compound volatilization or on compounds dissolved in organic liquids. Environmentally relevant scenarios, such as isotope fractionation during volatilization of organics from open water surfaces, have largely been neglected. In the current study, open-system volatilization experiments (focusing thereby on kinetic/-nonequilibrium effects) were carried out at ambient temperatures for trichloromethane, trichloroethene, trichlorofluoromethane, trichlorotrifluoroethane, methanol, and ethanol dissolved in water and, if not previously reported in the literature for these compounds, for volatilization from pure liquids. Stable carbon isotopic signatures were measured using continuous flow isotope ratio mass spectrometry. The results demonstrate that volatilization of the four halogenated compounds from water does not cause a measurable change in the carbon isotopic composition, whereas for pure-phase evaporation, significant inverse isotope effects are consistently observed (+0.3 ‰< ε < + 1.7 ‰). In contrast, methanol and ethanol showed normal isotope effects for evaporation of pure organic liquids (-3.9 ‰ and -1.9 ‰) and for volatilization of compounds dissolved in water (-4.4 ‰ and -2.9 ‰), respectively. This absence of measurable carbon isotope fractionation considerably facilitates the application of isotopic techniques for extraction of field samples and preconcentration of organohalogens-known to be important pollutants in groundwater and in the atmosphere.

Predicting equilibrium vapour pressure isotope effects by using artificial neural networks or multi-linear regression – A quantitative structure property relationship approach

We aim at predicting the effect of structure and isotopic substitutions on the equilibrium vapour pressure isotope effect of various organic compounds (alcohols, acids, alkanes, alkenes and aromatics) at intermediate temperatures. We attempt to explore quantitative structure property relationships by using artificial neural networks (ANN); the multi-layer perceptron (MLP) and compare the performances of it with multi-linear regression (MLR). These approaches are based on the relationship between the molecular structure (organic chain, polar functions, type of functions, type of isotope involved) of the organic compounds, and their equilibrium vapour pressure. A data set of 130 equilibrium vapour pressure isotope effects was used: 112 were used in the training set and the remaining 18 were used for the test/validation dataset. Two sets of descriptors were tested, a set with all the descriptors: number of 12C, 13C, 16O, 18O, 1H, 2H, OH functions, OD functions, CO functions, Connolly Solvent Accessible Surface Area (CSA) and temperature and a reduced set of descriptors. The dependent variable (the output) is the natural logarithm of the ratios of vapour pressures (ln R), expressed as light/heavy as in classical literature. Since the database is rather small, the leave-one-out procedure was used to validate both models. Considering higher determination coefficients and lower error values, it is concluded that the multi-layer perceptron provided better results compared to multi-linear regression. The stepwise regression procedure is a useful tool to reduce the number of descriptors. To our knowledge, a Quantitative Structure Property Relationship (QSPR) approach for isotopic studies is novel.

Measurement of the tritium concentration in the fractionated distillate from environmental water samples

Standard procedures for the measurement of tritium in water samples often require distillation of an appropriate sample aliquot. This distillation process may result in a fractionation of tritiated water and regular light water due to the vapor pressure isotope effect, introducing either a bias or an additional contribution to the total tritium measurement uncertainty. The current study investigates the relative change in vapor pressure isotope effect in the course of the distillation process, distinguishing it from and extending previously published measurements. The separation factor as a quantitative measure of the vapor pressure isotope effect is found to assume values of 1.04 ± 0.036, 1.05 ± 0.026, and 1.07 ± 0.038, depending on the vigor of the boiling process during distillation of the sample. A lower heat setting in the experimental setup, and therefore a less vigorous boiling process, results in a larger value for the separation factor. For a tritium measurement in water samples where the first 5 mL are discarded, the tritium concentration could be underestimated by 4–7%.

Non-canonical mass laws in equilibrium isotopic fractionations: Evidence from the vapor pressure isotope effect of SF6

We report experimental observations of the vapor pressure isotope effect, including 33S/32S and 34S/32S ratios, for SF6 ice between 137 and 173K. The temporal evolution of observed fractionations, mass-balance of reactants and products, and reversal of the fractionation at one temperature (155K) are consistent with a subset of our experiments having reached or closely approached thermodynamic equilibrium. That equilibrium involves a reversed vapor pressure isotope effect; i.e., vapor is between 2‰ and 3‰ higher in 34S/32S than co-existing ice, with the difference increasing with decreasing temperature. At the explored temperatures, the apparent equilibrium fractionation of 33S/32S ratios is 0.551±0.010 times that for 34S/32S ratios—higher than the canonical ratio expected for mass dependent thermodynamic fractionations (∼0.515). Two experiments examining exchange between adsorbed and vapor SF6 suggest the sorbate–vapor fractionation at 180–188K is similar to that for ice–vapor at ∼150K. In contrast, the liquid–vapor fractionation at 228–300K is negligibly small (∼0.1‰ for 34S/32S; the mass law is ill defined due to the low amplitude of fractionation). We hypothesize that the observed vapor pressure isotope for SF6 ice and sorbate is controlled by commonly understood effects of isotopic substitution on vibrational energies of molecules, but leads to both an exotic mass law and reversed fractionation due to the competition between isotope effects on intramolecular vibrations, which promote heavy isotope enrichment in vapor, and isotope effects on intermolecular (lattice) vibrations, which promote heavy isotope enrichment in ice. This explanation implies that a variety of naturally important compounds having diverse modes of vibration (i.e., varying greatly in frequency and particularly, reduced mass) could potentially exhibit similarly non-canonical mass laws for S and O isotope fractionations. We examined this hypothesis using a density function model of SF6 vapor and lattice dynamic model of SF6(ice). These models support the direction of the measured vapor pressure isotope effect, but do not quantitatively agree with the magnitude of the fractionation and poorly match the phonon spectrum of SF6 ice. A strict test of our hypothesis must await a more sophisticated model of the isotopic dependence of the phonon spectrum of SF6 ice.

Revisit to Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

H/D and 16O/18O vapor pressure isotope effects (vpies) of water between liquid water and water vapor were reinvestigated by estimating the reduced partition function ratios, rpfrs, of water clusters, (H2O)n, with n up to 100 at the HF/6-31G(d) level of molecular orbital theory. Large water clusters with n = 80−100 resulted in a reasonable estimate of the 16O/18O vpie, which was in contrast to the one for which only small water clusters had been considered. Contrary to this, no appreciable improvement in the H/D vpie reproduction by using large water clusters was observed.

Revisit to Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

Revisit to Vapor Pressure Isotope Effects of Water Studied by Molecular Orbital Calculations

Quantum path-integral study of the phase diagram and isotope effects of neon

The phase diagram of natural neon has been calculated for temperatures in the range of 17-50 K and pressures between 10(-2) and 2 x 10(3) bar. The phase coexistence between solid, liquid, and gas phases has been determined by the calculation of the separate free energy of each phase as a function of temperature. Thus, for a given pressure, the coexistence temperature was obtained by the condition of equal free energy of coexisting phases. The free energy was calculated by using nonequilibrium techniques such as adiabatic switching and reversible scaling. The phase diagram obtained by classical Monte Carlo simulations has been compared to that obtained by quantum path-integral simulations. Quantum effects related to the finite mass of neon cause that coexistence lines are shifted toward lower temperatures when compared to the classical limit. The shift found in the triple point amounts to 1.5 K, i.e., about 6% of the triple-point temperature. The triple-point isotope effect has been determined for (20)Ne, (21)Ne, (22)Ne, and natural neon. The simulation data show satisfactory agreement to previous experimental results, which report a shift of about 0.15 K between triple-point temperatures of (20)Ne and (22)Ne. The vapor pressure isotope effect has been calculated for both solid and liquid phases at triple-point conditions. The quantum simulations predict that this isotope effect is larger in the solid than in the liquid phase, and the calculated values show nearly quantitative agreement to available experimental data.

Enthalpies of Vaporization and Vapor Pressures of Some Deuterated Hydrocarbons. Liquid−Vapor Pressure Isotope Effects

Liquid−vapor pressures as a function of temperature and enthalpies of vaporization of a series of both liquid and solid hydrocarbons and their perdeuterated analogues have been determined by correlation−gas chromatography. The applicability of this technique is first demonstrated by reproducing the vapor pressure isotope effect of cyclohexane-d12 and benzene-d6. Other hydrocarbons studied include the perdeuterated forms of hexane, toluene, heptane, the p- and o-xylenes, naphthalene, biphenyl, acenaphthene, phenanthrene, anthracene, hexamethylbenzene, p-terphenyl, perylene, and chrysene. Vapor pressures and vaporization enthalpies of unlabeled hexamethylbenzene and perylene are also reported. Inverse vapor pressure isotope effects of the liquid phase are observed for all the substances examined. Vaporization enthalpy differences between unlabeled and labeled hydrocarbons are small and generally slightly positive. Vapor pressure−temperature relationships are reported in the form of a third-order polynomial that appears to reproduce experimental liquid−vapor pressures from T = 298.15 K to the boiling temperature.

Development of a bipolar electrolysis system for tritium accumulation in HTO

A bipolar electrolysis system for tritium accumulation and HTO waste volume reduction was designed and built. Experiments were performed to achieve the system specific separation factor for iron and stainless steel cathodes in a 1M sodium hydroxide solution. A separation factor of about 6 for stainless steel and 19 to 26 for iron was measured. The vapor pressure isotope effect under experimental conditions was determined as 1.09±0.03 (T = 279 K).

Estimating the Triple-Point Isotope Effect and the Corresponding Uncertainties for Cryogenic Fixed Points

The sensitivities of melting temperatures to isotopic variations in monatomic and diatomic atmospheric gases using both theoretical and semi-empirical methods are estimated. The current state of knowledge of the vapor-pressure isotope effects (VPIE) and triple-point isotope effects (TPIE) is briefly summarized for the noble gases (except He), and for selected diatomic molecules including oxygen. An approximate expression is derived to estimate the relative shift in the melting temperature with isotopic substitution. In general, the magnitude of the effects diminishes with increasing molecular mass and increasing temperature. Knowledge of the VPIE, molar volumes, and heat of fusion are sufficient to estimate the temperature shift or isotopic sensitivity coefficient via the derived expression. The usefulness of this approach is demonstrated in the estimation of isotopic sensitivities and uncertainties for triple points of xenon and molecular oxygen for which few documented estimates were previously available. The calculated sensitivities from this study are considerably higher than previous estimates for Xe, and lower than other estimates in the case of oxygen. In both these cases, the predicted sensitivities are small and the resulting variations in triple point temperatures due to mass fractionation effects are less than 20 μK.

Vapor pressure isotope effects in methyl fluoride studied by density functional theory

H/D and 12C/ 13C vapor pressure isotope effects in methyl fluoride (CH 3F) were studied at the MPW1PW91/6-31++G(d) level of theory. The CH 3F molecule and small CH 3F clusters were used as models of vapor and liquid methyl fluoride, respectively, and their reduced partition function ratios were calculated at their optimized structures. The equilibrium constants of the isotope exchange reactions between the two phases were evaluated as 1.0263 and 0.9991 for the H/D and 12C/ 13C isotope exchanges at 150 K, respectively, which reproduced reasonably well the experimental results. The effect of CH⋯F interaction between neighboring molecules on the vpies was discussed.

Novel Corresponding-States Principle Approach for the Equation of State of Isotopologues: H218O as an Example

The corresponding-states principle (CSP) has been considered for the development of the equations of state (EOS) of minor isotopologues that are usually unknown. We demonstrate that, for isotopologues of a given molecular fluid, a general extended multi-parameter corresponding-states EOS can be reduced to the three-parameter EOS, utilizing the critical parameters (temperature and density) and Pitzer's acentric factor as correlation parameters. Appropriate general CSP mathematical formalism and equations for constructing the EOS of minor isotopologues are described in detail. The formalism and equations were applied to isotopologues of water and demonstrated that the isotopic effect on the critical parameters and the acentric factor of H(2)(18)O can be successfully calculated from the EOS of H2O and experimental data on the isotope effects (liquid-vapor isotope fractionation factor and molar volume isotope effect). We have also shown that the experimental data on the vapor pressure isotope effect (VPIE) for 18O-substituted water are inconsistent within the framework of thermodynamics with the liquid-vapor oxygen isotope fractionation factor. The novel approach of CSP to isotopologues developed in this study creates a new opportunity for constructing the EOS of minor isotopologues for many other molecular fluids.