How to use miscibility numbers (M-numbers) D-5
How to use miscibility numbers (M-numbers)
Miscibility numbers (M-numbers) are used to predict the miscibility of a liquid
with one of the standard solvents.
To predict 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 15 °C.
• A difference of 16 units shows that the two liquids possess a critical
solution temperature between 25 and 75 °C, with 50 °C as the optimal
temperature.
• If the difference equals 17 or more, the two liquids are immiscible or
their critical solution temperature is above 75 °C.
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. These solvents receive a dual M-number. The first number, always
lower than 16, indicates the degree of miscibility with highly lipophilic
6.5 Dimethylsul-
foxide
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 C.00 100.0 — —
a. Adapted from Godfrey, Norman B., Solvent Selection via Miscibility Number, CHEMTECH, 359-363
(1972).
Physical properties of solvents
a
(Continued)
Polarity
index
Solvent
Viscosity
CP, 20 °C
Boiling
Point °C
(1 atm)
Miscibility
Number
(M)
Wavelength
Cutoffs (nm)
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