Miscibility numbers (M-numbers) are used to predict the miscibility of a liquid with one of the standard solvents. To determine the miscibility of the two liquids, subtract the smaller value from the larger.
If the difference between the two M-numbers equals 15 or less, the two liquids are miscible in all proportions at 15oC.
A difference of 16 units shows that the two liquids possess a critical solution temperature between 25 and 75oC, with 50oC as the optimal temperature.
If the difference equals 17 or more, the two liquids are immiscible, or their critical solution temperature is above 75oC.
Interaction between the molecules of the two liquids can sometimes change the expected degree of miscibility. For example, ethers or tertiary amines show unpredicted miscibility with hydroxylic solvents due to hydrogen bonding. Unusually strong hydrogen bonding is also responsible for the miscibility of long-chain alcohols or carboxylic acids with standard solvents of low M-number. Conversely, they show anomalous immiscibility with aprotic solvents of low M-number. Some solvents prove immiscible with solvents at both ends of the lipophilicity scale.