Solvent Considerations
101
C
How to Use Miscibility Numbers (M-Numbers)
Use miscibility numbers (M-numbers) to predict the miscibility of a liquid with a standard
solvent (see
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 between 25 and 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.
5.5
Benzyl alcohol
5.80
205.5
13
––
5.7
Methoxyethanol
1.72
124.6
13
––
6.2
Acetonitrile
0.37
81.6
11, 17
210
6.2
Acetic acid
1.26
117.9
14
––
6.4
Dimethylformamide
0.90
153.0
12
––
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
––
––
Table C-5 Solvent Miscibility (Continued)
Polarity
Index
Solvent
Viscosity
CP, 20 °C
Boiling
Point °C
(@1 atm)
Miscibility
Number
(M)
λ
Cutoff
(nm)
