Calculation of vapor pressure curves for ethyl, propyl, and butyl parabensusing thermogravimetry

The heat treatment of parahydroxy benzoic acid esters follows zero order ra re processes that are in good correlation with their evaporation process. R ising temperature experiments were performed and the optimum conditions wer e found when the heating rate was 10(0)C/min in an atmosphere of dry nitrog en, the flow rate of which was maintained at 100 mL/min. The Antoine constants for methyl paraben were substituted into the Antoine equation to get the corresponding vapor pressures that were then used in th e Langmuir equation to obtain the value for the coefficient of evaporation k. The E-act value, obtained from the Arrhenius plot, was 75.1 kJ mol(-1); the enthalpy of vaporization (DeltaH(vap)) was calculated to be 77.9 kJ mol (-1), in very close agreement with the E-act value. Using the Antoine and Langmuir equations, the value of k was experimentally determined to be 124525 +/- 0.8, units being in the S.I. system. Taking th e value of k to be independent of the substance, the corresponding vapor pr essures were obtained for ethyl, propyl, and butyl parabens to give their v apor pressure plots. The thermogravimetric data was obtained for all the co mpounds under the same experimental conditions. Using the Clausius Clapeyro n equation, the enthalpy of vaporization (DeltaH(vap)) of ethyl, propyl, an d butyl parabens were calculated to be 72.6, 76.5, and 72.2 kJ mol(-1).