Prediction of the phase diagram of liquid–liquid phase separation during crystallization via a molecular mechanism study

•The molecular mechanism of oiling-out phenomenon was proposed.•Solute dimers are emphasized to be the key factor in oiling-out.•A new prediction method was established for phase diagram of oiling-out.•The oiling-out curves and solvent systems of the five substances were predicted.•The purity of pyraclostrobin was significantly increased from 42.76% to 99.32%.