The relation between the speed of sound (u) in biodiesel and the change in Gibbs energy (G) has not been described in the literature. With the method of Gibbs energy additivity, the relation between u and G can be expressed as ln(u2) = G/RT + A, where R is the universal gas constant, T is the absolute temperature, and A is a constant. Further expansion of G into its enthalpy and entropy, and sub-dividing the molecule of a fatty and methyl ester (FAME) into groups of atoms, the final model is good for estimating the speed of sound in both FAME and biodiesel at various temperatures. Only the numbers of double bonds and carbon atoms of the fatty acid are required for the calculation.

The relationship between the speed of sound (u) in biodiesel and the change in Gibbs energy (ΔG) has not been describe in literature. With the Gibbs energy additivity (GEA ) method, the relation between u and ΔG can be expressed as lnu2 = ΔG/RT + A, where R is the universal gas constant; T is absolute temperature and A is constant. Further expansion of the G to its enthalpy and entropy, and sub-division the molecule of a fatty and methyl ester (FAME) into groups of atoms, the final model was good for estimation the speed of sound in both FAME and biodiesel at various temperatures. Only numbers of double bond and carbon atoms of fatty acid were required for the calculation.