measured sound velocity of methyl caprate and ethyl caprate at pressures up to 210 MPa in the temperature range 283.15 to 403.15 K.
Based on  data that sound velocity was obtained from experimental and density was obtained from calculated value by (7), [k.sub.m] was calculated in 0.1 to 210 MPa and 303.15 to 383.15 K and the fitting results were presented with isothermal characteristics for methyl caprate and ethyl caprate in Figures 1 and 2.
Accordingly, the calculated results were presented with isobaric characteristics for methyl caprate and ethyl caprate in Figures 3 and 4, and a two-order polynomial fitting in temperature domain was expressed as
At atmospheric pressure (0.1 MPa isobaric) [k.sub.m] of methyl caprate shows a slight downtrend with two-order form described in (10) but not first-order form described in Daridon's formula (5); however for 0.1 MPa isobaric of ethyl caprate is almost horizontal; meanwhile [k.sub.m] show uptrend, which accelerates with increasing pressure, at isobaric with temperature elevated.
Furthermore to calculate molecular compressibility difference [k.sub.m] of ethyl caprate in isothermal line with 20 K step and in isobaric line with 10 MPa step, Figures 5 and 6 can be achieved.
So it can be used to predict [k.sub.m] without experimental data for methyl and ethyl caprate under pressures up to 210 MPa and temperature range from 283.15 to 403.15 K.
[T.sub.ref], which is reference temperature, was considered as the cloud point temperature of an organic liquid under reference pressure [P.sub.ref], for ethyl caprate [T.sub.ref] being 253.15 K and for methyl caprate [T.sub.ref] being 260.15 K.
The results comparison of three formulas was showed in Figures 7 and 8 for methyl caprate and ethyl caprate, respectively.
but the two methods are available only at atmospheric pressure, so we take 0.1013 MPa isobaric value to compare the accuracy of five methods; the comparison results for methyl and ethyl caprate were listed in Figures 9 and 10.
Formula (13) showed bigger difference, while (5) and (22) for ethyl caprate at 0.1 MPa isobaric.
Daridon, "Sound velocity, density, and derivative properties of fatty acid methyl and ethyl esters under high pressure: methyl caprate and ethyl caprate," Journal of Chemical & Engineering Data, vol.