Isothermal evaporation of ethanol in a dynamic gas atmosphere.

Optimization of evaporation and pyrolysis conditions for ethanol are important in carbon nanotube (CNT) synthesis. The activation enthalpy (ΔH(‡)), the activation entropy (ΔS(‡)), and the free energy barrier (ΔG(‡)) to evaporation have been determined by measuring the molar coefficient of evaporation, k(evap), at nine different temperatures (30-70 °C) and four gas flow rates (25-200 mL/min) using nitrogen and argon as carrier gases. At 70 °C in argon, the effect of the gas flow rate on k(evap) and ΔG(‡) is small. However, this is not true at temperatures as low as 30 °C, where the increase of the gas flow rate from 25 to 200 mL/min results in a nearly 6 times increase of k(evap) and decrease of ΔG(‡) by ~5 kJ/mol. Therefore, at 30 °C, the effect of the gas flow rate on the ethanol evaporation rate is attributed to interactions of ethanol with argon molecules. This is supported by simultaneous infrared spectroscopic analysis of the evolved vapors, which demonstrates the presence of different amounts of linear and cyclic hydrogen bonded ethanol aggregates. While the amount of these aggregates at 30 °C depends upon the gas flow rate, no such dependence was observed during evaporation at 70 °C. When the evaporation was carried out in nitrogen, ΔG(‡) was almost independent of the evaporation temperature (30-70 °C) and the gas flow rate (25-200 mL/min). Thus the evaporation of ethanol in a dynamic gas atmosphere at different temperatures may go via different mechanisms depending on the nature of the carrier gas.