vapor liquid coexistence and critical behavior

7/29/2019 Vapor Liquid Coexistence and Critical Behavior

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VAPOR-LIQUID COEXISTENCE AND

CRITICAL BEHAVIOR OF IONIC LIQUIDS


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ABSTRACT

Ionic Liquids are salts that are liquid near ambienttemperatures.

Experimental determination of critical points and vapor-liquidcoexistence curve is not possible because of thermal stabilityissues as most Ionic Liquids decompose before reaching thecritical point.

The paper reports the first vapor-liquid phase diagram andcritical point for ionic liquids obtained in silico with an atomisticforce field.


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IMPORTANCE OF VLE

VLE gives the nature of intermolecular interactions present inthe liquid and vapor phases.

Useful in developing equations of state and correspondingstate theories

Knowledge of critical points and VLE are key to achievingfundamental understanding of these complex fluids.


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IONIC LIQUID UNDER STUDY

Molecular structure of ionic liquid under study


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EXPERIMENTAL PROBLEMS

Conflicting experimental reports of the way in which propertiessuch as vapor pressure and enthalpy of vaporization vary withfactors such as size of cation, aggregation of ions etc

Most of the experimental uncertainty is due to the pressurebeing too low and temperature being high which makesmeasurements extremely difficult to conduct as even traceamount of impurities can cause a large variations inmeasurements.


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SIMULATION STUDIES

Simulation method used here is Gibbs Ensemble MonteCarlo(GEMC).

Drawback: Reliance on particle exchanges between vapor andliquid phases to equilibrate the chemical potential butconfigurational bias Monte Carlo and other enhanced

sampling techniques are used to circumvent this problem.


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GEMC

Configurational Bias sampling, Parallel computing strategiesand enhanced bias sampling specifically designed for ionicsystems were used to enhance the efficiency.


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COMPUTED VLCC AND CRITICAL POINTS


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ANALYZING VLCC

Compared to non-ionic compounds like alkenes, the VLCCshave a high degree of Asymmetry with a very steep vaporbranch similar to alkyl halides

Effect of a single methylene groupo Critical Temperature decreases with increasing alkyl chain,

Tc decreases by 65K in going from C2 to C6. For alkanesand Alcohols the Tc increases with increasing alkyl chain.

o Critical Density decreases with increasing alkyl chain.


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CRITICAL TEMPERATURE, METHYLENEINCREMENT

It is unclear whether Tc will decrease with increasing chainlength will hold true after C6.

When the chain length becomes very long , it is possible thatthe columbic screening due to alkyl chain becomes saturatedwhile the van der walls interaction due to increasing chain

length continue to increase the cohesive energy, so it ispossible that this trend of decreasing critical temperature withchain length might reverse after certain Cn


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PRESSURE PLOT


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ANALYZING PRESSURE PLOT

Increasing cation alkyl chain length increases the VaporPressure, thus lowering the normal Boiling Point.

One Plausible explanation is increasing the nonpolar contentof the cation weakens the Columbic interaction between ions,resulting in increased volatility

Critical pressure can be computed using critical Temperatureand subcritical saturation pressure.

For ionic Liquids, Pc decreases with increasing alkyl chainlength


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ANALYZING PRESSURE PLOT

Critical compressibility factor (Zc) ranges from 0.038[C1] to0.049[C6]

Zc smaller than simple fluids suggesting highly nonidealbehavior of vapor phase.


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TEMPERATURE DEPENDENCE OF ENTHALPYOF VAPORIZATION


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At high Temperatures, Hvap decreases with increasing sizeof cation, while at lower temperatures Hvap increases withincreasing size of cation.

The Trend reversal maybe due too Significant aggregation in the vapor phaseo Effect of alkyl chain on cohesive energy of condensed

phase upon increasing temperature.


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Hvap ranges from 68-72 KJ/mole which is twice the value ofa H2 bonding liquid like ethanol but lower than energy reqd todissociate.

This suggests that vapor phase exists as ion pairs or largeaggregates and not isolated ions.


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ASSOCIATION IN VAPOR PHASE


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ASSOCIATION OF IONS

Trountons constant for ionic liquids is ~65 J/mol K which islower than nonpolar(~85 J/mol K) and polar (~105 J/mol K).

Associating Liquids like acetic acid also have low Trountonconstants

Significant aggregation of ions in vapor phase lowers the

vapor phase entropy relative to typical fluids where vaporphase consists mainly of isolated molecules.


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CLUSTER ANALYSIS

Cluster analysis performed to address the issue ofaggregation of ions.

2 ions considered part of cluster if their center of mass ofseparation


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DISTRIBUTION OF CLUSTER SIZES AS AFUNCTION OF TEMPERATURE


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TEMPERATURE DEPENDENCE OF IONS INCLUSTER OF SIZE N(AGG)


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AGGREGATION OF IONS

Less than 0.2% of ions are isolated. Single ion pairs account for more than 60% of total clusters. Number of single-ion pairs decreases with increasing

temperature. Number of larger clusters increases with temperature.

Even though individual ion pairs are dominant species in thevapor phase, substantial fraction of ions is present in largeraggregates. This might be the reason for discrepanciesobserved between different experimental techniques.


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CONCLUSION

VLE of Ionic Liquids differs from conventional molecular liquidsbyo Shape of VLCC is highly Asymmetricalo Vapor pressure increases as size of cation increases.o Critical density and temperature decreases as size of

cation increases.o Trountons constant is smaller than that of conventional

fluids, consistent with finding that vapor state consists ofsignificant fraction of aggregated ion pairs.


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REFERENCES

Vapor Liquid Coexistence and critical behavior of Ionic liquids via Molecular Simulations,Neeraj Rai and Edward J. Maginn*


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vapor liquid coexistence and critical behavior

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