The major intermolecular forces between propanoic acid and heptane are dipole-induced dipole forces.
The formula of propanoic acid is
It has a highly polar
The formula of heptane is
It is a nonpolar hydrocarbon, so its strongest intermolecular forces are London dispersion forces.
When the two liquids are mixed, the strong
It induces a dipole in the heptane molecule, so the interaction is called a dipole-induced dipole force.
You would expect propanoic acid to be "insoluble" in heptane, but the dipole-induced dipole forces increase the solubility to about 6 g/100 g heptane.