Using miscibility numbers

Use miscibility numbers to predict the miscibility of a liquid with a standard solvent.

To predict the miscibility of two liquids, subtract the smaller miscibility number (M-number) value from the larger M-number value:

• If the difference between two M-numbers is 15 or less, the two liquids are miscible, in all proportions, at 15 °C.
• A difference of 16 indicates a critical solution temperature from 25 to 75 °C, with 50 °C as the optimal temperature.
• If the difference is 17 or greater, the liquids are immiscible, or their critical solution temperature is above 75 °C.

Some solvents prove immiscible with solvents at both ends of the lipophilicity scale. These solvents receive a dual M-number:

• The first number, always lower than 16, indicates the degree of miscibility with highly lipophilic solvents.
• The second number applies to the opposite end of the scale. A large difference between these two numbers indicates a limited range of miscibility.

For example, some fluorocarbons are immiscible with all the standard solvents and have M-numbers of 0 and 32. Two liquids with dual M-numbers are usually miscible with each other.

A liquid is classified in the M-number system by testing for miscibility with a sequence of standard solvents. A correction term of 15 units is then either added or subtracted from the cutoff point for miscibility.