Miscibility refers to the ability of a gas or liquid to dissolve uniformly in another gas or liquid.Gases mix with each other in all proportions, but this may or may not apply to liquids, where miscibility depends on chemical affinity. Ethanol, an alcohol, and water are miscible because they are chemically similar, but benzene and water are only slightly miscible because of the very large differences in their chemical properties.
Some liquids are essentially insoluble in other liquids, e.g., gasoline in water (and water in gasoline);such liquids are said to be immiscible. Other liquids are only soluble up to a point. In the case of water and ethyl ether (CH3CH2OCH2CH 3), it is possible to dissolve up to about 4 g of ethyl ether in 100 g of water, but the addition of more ethyl ether results in the production of separate layers of the less dense diethyl ether above the denser water layer. And some liquids are completely soluble in other liquids, regardless of the amounts combined; such liquids are said to be miscible in each other in all proportions. (Traditionally, the term solubility has been used interchangeably for miscibility in reference to liquids, even though it should strictly only be applied to solids.)
There are several fundamental rules governing the miscibility of liquids in other liquids. First, the solubility of liquids in liquids increases with increasing temperature. Second, the more similar two compounds are in terms of polarity, the more likely that one is soluble in the other, i.e., polar compounds dissolve in polar compounds, and non-polar compounds dissolve in non-polar compounds. (Polar molecules dissolve in polar molecules because the dipole of one attracts the dipole end of the other.) Thus, benzene and carbon tetrachloride, being both non-polar, dissolve in each other, but neither will appreciably dissolve in water, which is polar.
Both alcohols and ethers with up to three or four carbons are miscible in water because the OH groups in these molecules form hydrogen bonds with the water molecules. Alcohols and ethers with higher molecular weights are not miscible in water, however, because the water molecules can not completely surround those molecules. The molecule 1-heptanol, for example, consists of an alkyl chain of seven carbons and an OH group. The OH group forms hydrogen bonds with water molecules, but the alkyl portion of the molecule exerts no attraction on the water molecules. This part of the molecule is called hydrophobic, meaning water-hating. Because this part of the molecule cannot be surrounded by water, 1-heptanol is immiscible in water.
In aqueous solutions, globular proteins usually turn their polar groups outward toward the aqueous solvent, and their non-polar groups inward, away from the polar molecules. The non-polar groups prefer to interact with each other, and exclude water from these regions, leading to immiscibility. This type of interaction is usually weaker than hydrogen bonding, and usually acts over large surface areas.
Many gases are miscible with liquids. The miscibility of gases in liquids almost always decreases with increasing temperature, and increases with increasing pressure. For example, more oxygen is dissolved in the blood at higher than normal pressures.
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