A Mathematical Model for Estimating Molecular Compressibility of Fatty-Acid Methyl Ester and Biodiesel

In this study, the molecular compressibility (km) of a fatty-acid methyl ester (FAME) or a biodiesel is correlated with ΔG, ln kmMW ¼ −ΔGkmRT, via the Gibbs energy additivity method, where MW is the molecular weight of the FAME or the average MW of the biodiesel. The Gibbs energy associated with molecular compressibility (ΔGkm ) is further correlated with the structure of FAME. Thus, the relationship between the structure (of a FAME or a biodiesel) and the physical property (km) is established. Thus, km of a FAME at different temperatures can be easily estimated from the carbon numbers of fatty acid (z) and the
number of double bonds (nd) with good accuracy. For biodiesel, km is calculated from the same equation with the average z (z(ave)) and average nd (nd(ave)). km is not temperature independent and a slight change in km depends on the structure of the FAME and biodiesel. For FAME having 14 carbon atoms or less in the fatty acid, km decreases as temperature is increased. On the other hand, for FAME with a longer chain length (16 or higher), km increases as temperature is increased. Similarly, a double bond in the long-chain FAME is more sensitive to temperature than the saturated FAME.