Miscibility is the ability of two liquids to mix and form a homogeneous solution. Water and ethanol are miscible, while oil and water are not. Miscibility is expressed as w/w% and some solvents are only partially miscible. Hydrophilic solvents are miscible with water, while lipophilic solvents are miscible with hydrocarbon solvents. Immiscible solvents are used in liquid/liquid extraction to separate compounds with different solubility.
Miscibility is the ability of two liquids to mix with each other to form a homogeneous solution. Water and ethanol, for example, are miscible. They can be mixed in any proportion and the resulting solution will be clear and show only one phase. Oil and water, on the other hand, are immiscible. A mixture of vegetable oil and water will always separate into two layers or phases and will not dissolve into each other.
Miscibility is often expressed as w/w% or weight of a solvent in 100 g of final solution. If two solvents are completely miscible in all proportions, their miscibility is 100%. Other solvents are only partially miscible, which means only some will dissolve in water.
For example, diethyl ether is partially miscible with water. Up to 7 grams of diethyl ether will dissolve in 93 g of water to give a 7% (w/w) solution. If more diethyl ether is added, a separate layer of diethyl ether will appear floating above the water. Most solvents exhibit some miscibility into each other, although it may be very low.
Most of the liquids encountered in everyday life are water-based, called aqueous, or organic, which in the chemical sense means that they contain carbon atoms. These can generally be divided into two broad classes. They are hydrophilic, “water loving” or lipophilic, “fat loving”. Lipophilic solvents are miscible with hydrocarbon solvents, i.e. solvents containing only carbon and hydrogen, such as fats and oils. Hydrophilic solvents are miscible with water.
Hydrophilic solvents generally contain other types of atoms, such as oxygen and nitrogen, which make them capable of hydrogen bonding with water molecules. A hydrophilic solvent may also be referred to as lipophobic, “fearing fat”, while lipophilic solvents are hydrophobic, “fearing water”. Which terms are used is a matter of context.
Immiscible solvents, those that do not mix, are used in the liquid/liquid extraction chemical process, where the compounds of interest can be separated by exploiting their different solubility in water and in a lipophilic solvent. For example, if a mixture of vitamin C, which is highly soluble in water, and vitamin E, which is fat soluble, is stirred with an immiscible mixture of water and hexane, a very lipophilic hydrocarbon solvent, the vitamin C will collect in the aqueous layer. while the vitamin E will collect in the hexane layer. Left to rest, the two layers separate and can be easily isolated and the vitamins recovered by evaporation of the solvents.
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