As with some of the other properties considered it is helpful first to examine the vapour pressures of pure methyl esters. The normal boiling points, at which by definition the vapour pressure is 1 bar, of methyl palmitate, methyl stearate, methyl oleate and methyl linoleate are respectively 338, 352, 349 and 366°C. Pure methyl esters do of course have a single boiling point: a biodiesel, which will comprise several such compounds, has a boiling range. Applications of the Clausius-Clapeyron (C-C) equation to a biodiesel is not fundamentally sound, as the equation is only for a pure chemical compound.
There is however no difficulty with the suggestion that the temperature dependence of the vapour pressure of a biodiesel has the same form as the C-C equation, in which case assignment of a value to the heat of vaporisation becomes arbitrary. The C-C equation can of course be rigorously applied to the pure ester compounds in which case the heat of vaporisation will have a precise value.
Vapour pressures of biodiesels of various known compositions across a temperature range have been studied by Goodrum and, certainly for a limited number of data points in each case, conformity with an equation of the C-C form confirmed. Initial boiling points, that is attainment of a vapour pressure of 100 kPa, were observed at 369°C for a particular rapeseed ester and at 348°C for a particular soybean methyl ester. These fall comfortably within the range of normal boiling points for the pure esters given in.
