Five basic functional groups were presented with fixed values for all FAEs in the afore-mentioned study . According to the additivity of km, sound velocity value calculated from Eq. (4) present a very high consistency with its experimental value. It clearly indicates that HA14-1 km is related to the specific molecular structure and dependent on temperature. Similarly, the pressure influence to km must be considered for an injection system.
3.1. 2D fitting expressions of molecular compressibility
In order to directly obtain the value of kmkm from experimental data, kmkm was described as a function of density and speed of sound (Eq. (4a)) in terms of:equation(4a)km=Mwcρ32/7Ndiaye et al.  measured the sound velocity of methyl caprate and ethyl caprate under 0.1–210 MPa pressures and 283.15–403.15 K temperature. Density was also measured under the condition of 0.1–100 MPa and 293.15–393.15 K, and then density was determined in the full pressure range (0.1–210 MPa) from speed of sound data by using a tubal pregnancy modified Davis and Gordon’s procedure. Isentropic and isothermal compressibility were also presented in the P−T domain.