is immiscible in
water. The bright rainbow pattern is due to
thin film optical interference.]]
Miscibility () is the property of liquids to mix in all proportions, forming a homogeneous solution. In principle, the term applies also to other phases (solids and gases), but the main focus is usually on the solubility of one liquid in another. Water and ethanol, for example, are miscible since they mix in all proportions.
By contrast, substances are said to be immiscible if in some proportion, they do not form a solution. For example, diethyl ether is fairly soluble in water, but these two solvents are not miscible since they are not soluble in all proportions.
Organic compounds
In
organic compounds, the
weight percent of
hydrocarbon chain often determines the compound's miscibility with water. For example, among the
alcohols,
ethanol has two
carbon atoms and is miscible with water, whereas
octanol with a C
8H
17 substituent is not. Octanol's
immiscibility leads it to be used as a standard for
partition equilibria. This is also the case with
lipids; the very long carbon chains of lipids cause them to almost always be immiscible with water. Analogous situations occur for other
functional groups.
Acetic acid (CH
3COOH) is miscible with water, whereas
valeric acid (C
4H
9COOH) is not. Simple
aldehydes and
ketones tend to be miscible with water, since a
hydrogen bond can form between the hydrogen atom of a
water molecule and the
unbonded (lone) pair of electrons on the
carbonyl oxygen atom.
Metals
Immiscible
metals are unable to form
alloys. Typically, a mixture will be possible in the molten state, but upon freezing the metals separate into layers. This property allows solid
precipitates to be formed by rapidly freezing a molten mixture of immiscible metals. One example of immiscibility in metals is
copper and
cobalt, where rapid freezing to form solid precipitates has been used to create
granular GMR materials.
There are examples of immiscible metals in the liquid state. One with industrial importance is that liquid zinc and liquid silver are immiscible in liquid lead, while silver is miscible in zinc. This leads to the Parkes process, an example of liquid-liquid extraction, whereby lead containing any amount of silver is melted with zinc. The silver migrates to the zinc, which is skimmed off the top of the two phase liquid, and the zinc is boiled away leaving nearly pure silver.
Effect of entropy
Substances with extremely low
configurational entropy, especially
polymers, are unlikely to be miscible in one another even in the liquid state.
Determination
Miscibility of two materials is often determined optically. When the two miscible liquids are combined, the resulting liquid is clear. If the mixture is cloudy the two materials are immiscible. Care must be taken with this determination. If the
indexes of refraction of the two materials are similar, an immiscible mixture may be clear and give an incorrect determination that the two liquids are miscible.
See also
Emulsion
Multiphasic liquid
Heteroazeotrope
References
Category:Chemical properties
