Solvent miscibility C-5
How to use miscibility numbers
Use miscibility numbers (M-numbers) to predict the miscibility of a liquid
with a standard solvent (see page C-3.)
To predict the miscibility of two liquids, subtract the smaller M-number value
from the larger M-number value.
• If the difference between the 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 °Cto
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, 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.
6.5 Dimethylsulfoxide 2.24 189.0 9 ––
6.6 Methanol 0.60 64.7 12 210
7.3 Formamide 3.76 210.5 3 ––
9.0 Water 1.00 100.0 –– ––
Solvent miscibility (Continued)
Polarity
Index
Solvent
Viscosity
CP, 20 °C
Boiling
Point °C
(1 atm)
Miscibility
Number
(M)
λ Cutoff
(nm)
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