Gibbs energy additivity approaches to QSPR in modelling of isentropic compressibility of biodiesel

Isentropic compressibility (Ks, or reciprocal of isentropic bulk modulus) provides an important information about the space among molecules or about how much the substance can be compressed. Direct measurement of Ks of a liquid or a biodiesel cannot be easily performed. The value has generally been obtained from the speed of sound and density through the Newton-Laplace equation. In this study, Ks values of fatty acid methyl ester (FAME) reported in literatures are determined via direct correlation to the Gibbs free energy (associated with isentropic compression, ΔGKs) through the Gibbs energy additivity method. The derived equation lnks=lnA′+ΔGKsRT with the numeric constants is then applied to estimate the Ks of biodiesels with good accuracy. Only the average carbon numbers and average number of double bond(s) of the biodiesel are required for the calculation. The estimated KS values at different temperatures are in good agreement with the conventional method of calculation from the speed of sound and density. The overall AAD for FAMEs was 0.08%, while the overall AAD for 16 different biodiesels (neat and blended) was 0.92%. Thus, the proposed model would facilitate the researchers to estimate the KS a FAME or a biodiesel.