Miscibility (/ˌmɪsɪˈbɪlɪti/) is the property of twosubstances to mix in allproportions (that is, to fullydissolve in each other at anyconcentration), forming ahomogeneousmixture (asolution). Such substances are said to bemiscible (etymologically equivalent to the common term "mixable"). The term is most often applied toliquids, but also applies tosolids andgases. An example in liquids is the miscibility ofwater andethanol as they mix in all proportions.[1]
By contrast, substances are said to beimmiscible if the mixture does not form a solution for certain proportions. For one example,oil is not soluble in water, so these two solvents are immiscible. As another example,butanone (methyl ethyl ketone) is immiscible in water: it is soluble in water up to about 275 grams per liter, but will separate into twophases beyond that.[2]
Inorganic compounds, theweight percent ofhydrocarbon chain often determines the compound's miscibility with water. For example, among thealcohols,ethanol has twocarbonatoms and is miscible with water, whereas1-butanol with four carbons is not.[3]1-Octanol, with eight carbons, is practically insoluble in water, and its immiscibility leads it to be used as a standard forpartition equilibria.[4] The straight-chaincarboxylic acids up tobutanoic acid (with four carbon atoms) are miscible with water,pentanoic acid (with five carbons) is partly soluble, andhexanoic acid (with six) is practically insoluble,[5] as are longerfatty acids and otherlipids; the very long carbon chains of lipids cause them almost always to be immiscible with water. Analogous situations occur for otherfunctional groups such asaldehydes andketones.[citation needed]
Thus a practical rule of thumb for determining the solubility of an organic molecule in water (and/or other similarly polar solvents) is to consider the ratio of carbons in the molecule bound to polar functional groups (such as hydroxyl groups), to the number of simple hydrocarbons. If the molecule has a ratio of roughly 1:4 (Polar-to-non-polar carbons), it is soluble in water. It is however necessary to recognise this as a rule of thumb, and not always indicative.[6]
Immisciblemetals are unable to formalloys with each other. Typically, a mixture will be possible in the molten state, but upon freezing, the metals separate into layers. This property allows solidprecipitates to be formed by rapidly freezing a molten mixture of immiscible metals. One example of immiscibility in metals iscopper andcobalt, where rapid freezing to form solid precipitates has been used to creategranular GMR materials.[7]
Some metals are immiscible in the liquid state. One with industrial importance is that liquidzinc and liquidsilver are immiscible in liquidlead, while silver is miscible in zinc. This leads to theParkes process, an example ofliquid-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 then boiled away, leaving nearly pure silver.[8]
If a mixture ofpolymers has lowerconfigurational entropy than the components, they are likely to be immiscible in one another even in the liquid state.[9][10]
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 theindices 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.[11]
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