Vapor Pressureof Perfluoroalkylalkanes: The Roleof the Dipole.

The vapor pressure of four liquidperfluoroalkylalkanes(CF3(CF2)n(CH2)mCH3; n= 3, m= 4,5,7; n= 5, m= 5) was measured as a function of temperature between278 and 328 K. Molar enthalpies of vaporization were calculated fromthe experimental data, and the results were compared with data fromthe literature for the corresponding alkanes and perfluoroalkanes.The heterosegmented statistical associating fluid theory was usedto interpret the results at the molecular level both with and withoutthe explicit inclusion of the dipolar nature of the molecules. Additionally,ab initio calculations were performed for all perfluoroalkylalkanesstudied to determine the dipole moment to be used in the theoreticalcalculations. We demonstrate that the inclusion of a dipolar termis essential for describing the vapor–liquid equilibria ofperfluoroalkylalkanes. It is also shown that vapor–liquidequilibria in these compounds result from a subtle balance betweendipolar interactions, which decrease the vapor pressure, and the relativelyweak dispersive interactions between the hydrogenated and fluorinatedsegments. [ABSTRACT FROM AUTHOR]