All molecules also have weak intermolecular forces called
dispersion forces. Dispersion forces arise when the positive nuclei
of the solute molecule atoms attract the negative electrons of the
solvent molecule atoms. This gives the non-polar solvent greater
ability to solvate the solute molecules [18]. The increase in volume
and mass can also be attributed to the extent of higher liquid
absorption as compared to the extraction of soluble components
from the elastomer. Meanwhile, acid number, directly related to
the content of the free fatty acids in fatty acid ester, is considered
as an important parameter to measure the corrosivity and degradation
effect of biodiesel [14]. The acid numbers are 0.32, 0.32
and 0.16 for WCOME, SME and PME respectively, which reflects
the lower solvent power of PME than that of WCOME and SME.