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Liquid-vapor equilibria and interfacial properties of -alkanes and perfluoroalkanes by molecular simulation
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Image of FIG. 1.
FIG. 1.

Density profiles corresponding to C6F14 illustrating the typical temperature dependence of the LV density at orthobaric conditions. The six profiles cover a temperature range between 300 (blue) and 400 K (red). The liquid density is highest at the lowest temperature and decreases with temperature. Conversely, the vapor density is lowest at the lowest-temperature value and increases with temperature. The dotted arrows indicate the direction in which the density changes with increasing temperature. The standard deviation of residuals for the fitted data was less than at all temperatures.

Image of FIG. 2.
FIG. 2.

Time-dependence profiles corresponding to (a) the position of the Gibbs dividing surface, , (b) the interfacial-width parameter, , and (c) the liquid and vapor densities. For purposes of clarity in (a), the curves are displaced by 10 Å for every 20 K increment above 360 K. Solid lines were used to identify the temperatures at which the three properties fully equilibrated. Dashed lines corresponded to either nonequilibrated systems or systems with pronounced fluctuations. Systems corresponding to dashed lines were not included in the calculation of any LV property. The vertical arrows indicate the trend with increasing temperature.

Image of FIG. 3.
FIG. 3.

Pressure profiles corresponding to in a temperature range from 300 to 400 K in increments of 20 K. The vapor pressure increases with increasing temperature, as indicated by the dashed vertical arrow, while the excursion in pressure within the interfacial regions decreases.

Image of FIG. 4.
FIG. 4.

(a) Plots of temperature vs the scaled difference in liquid and vapor densities used to determine the critical temperature, , of the perfluoroalkanes according to Eq. (4) with a -exponent value of 0.325. (b) Plots showing the LV phase coexistence envelopes for the perfluoroalkanes.

Image of FIG. 5.
FIG. 5.

Pressure-tensor difference profiles corresponding to C6F14 at temperatures between 300 and 380 K. The tallest profile corresponds to the lowest temperature and the dotted arrow points in the direction of increasing temperature. The height of the peaks decreases with temperature and is expected to vanish at the critical temperature, .

Image of FIG. 6.
FIG. 6.

Representative temperature dependence of surface tension of perfluoroalkanes (open symbols) and corresponding alkanes (filled symbols); solid lines are the C–H fits to the calculated data. The surface tension decays with temperature and vanishes at the critical point predicted by the UA model. At temperatures below the critical point of the perfluorinated compounds, the surface-tension values of alkanes are always higher than the perfluorinated ones, in agreement with experimental observations (Refs. 18, 23, 27, and 28).


Generic image for table
Table I.

Cell dimensions and number of molecules.

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Table II.

Surface-tension amplitudes obtained from the C–H fits.

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Table III.

Tabulation of surface-tension values for compounds and as given by the UA model in this work and those given by the NERD (Ref. 3), TraPPE (Ref. 29) OPLS (Ref. 31), and Smith (Ref. 32) models from Refs. 30 and 33. The columns titled “% diff.” refer to the differences between model predictions and experimental data (Ref. 18).

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Table IV.

Predicted surface-tension values for as given in this work and by the OPLS-F (Ref. 34) and EXP6 (Refs. 35 and 36) models in Ref. 37 along with the corresponding % differences from the limited experimental data points (Refs. 23–28).


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Liquid-vapor equilibria and interfacial properties of n-alkanes and perfluoroalkanes by molecular simulation