The eutectic point is a unique temperature at which a combination of different substances melts or solidifies simultaneously. This temperature is the lowest possible melting point that the mixture can attain, lower than any of the substances’ individual melting points.
A eutectic mixture or system, on the other hand, is this particular combination of substances that have the characteristic of melting and solidifying at the eutectic point. In the correct proportions, the mixture components inhibit the crystallization phase of one another. These mixtures are interesting due to their uniform behavior, differing from most mixtures which have variable melting or solidifying ranges.
“Eutectic” comes from the Greek words “eu” meaning “well” or “good” and “teknē” meaning “art”, suggesting the mixtures have ‘good art’ or ‘well crafted’ behavior in their phase transitions. British scientist Frederick Guthrie coined the term in 1884.
Let’s delve into some related terminology to understand eutectics better:
- Eutectic: The term refers to a eutectic system, either as an adjective or noun.
- Eutectics: Eutectics is the study of eutectic systems, points, and the underlying principles.
- Eutectoid: This refers to a three-phase reaction by which, on cooling, a solid solution transforms into two different solid phases at a specific temperature and composition.
- Eutectic Alloy: A eutectic alloy is a mixture of two or more metals that form a eutectic system, where the alloy’s melting point is lower than the melting points of the individual metals. Also, a eutectic alloy melts at a distinct temperature.
- Eutectic Percentage Ratio: This is the specific ratio of the substances in a eutectic mixture that allows the mixture to have the lowest possible melting point.
- Hypoeutectic: This is a mixture or alloy that has a lower percentage of has a smaller percentage of β and a greater percentage of α than a eutectic composition
- Hypereutectic: This is a mixture or alloy that has a higher percentage of α and a lower percentage of β than a eutectic composition.
Examples of Eutectic Mixtures
Several examples of eutectic mixtures exist in both nature and industry:
- Iron-Carbon Alloy forms steel at a ratio of 0.76% carbon. This is an alloy that is critical to human technological advancement.
- Sodium Chloride and Water form a eutectic when mixed in the ratio of 23.3% sodium chloride to 76.7% water. This mixture melts at -21.2°C.
- Tin-Lead Alloy find extensive use in the electronics industry for soldering, it consists of 63% tin and 37% lead, and melts at 183°C.
- Ethanol and Water is a well-known eutectic mixture used in ‘freeze distillation’ of alcoholic beverages. The eutectic point of ethanol and water occurs at a mixture that is approximately 95% ethanol and 5% water by volume. This mixture freezes at -114.1°C, which is lower than the freezing point of either pure ethanol (-114.3°C) or water (0°C).
- Menthol and Camphor: These form a eutectic that is liquid at room temperature but used in ‘solid’ products like Vicks VapoRub.
- Inkjet printer ink is a eutectic mixture that permits printing at a low temperature.
- Galinstan is a liquid metal eutectic consisting of a certain ratio of gallium, indium, and tin. It serves as a less toxic replacement for mercury in some applications.
- Igneous rocks often contain minerals that form eutectic mixtures. Granophyre is an example.
Are All Alloys Eutectic Mixtures?
If you are still confused about what is and is not a eutectic, remember that the defining characteristic of a eutectic mixture is that it has a melting point lower than any of its individual components. Therefore, by definition, the eutectic point cannot be higher than the melting point of one of the components. If a mixture does not exhibit this property, it is not considered a eutectic mixture. So, not all alloys or other mixtures are eutectics.
For example, an amalgam, which is an alloy of mercury with another metal (usually silver, tin, or copper), is not typically a eutectic alloy. Eutectic alloys are specific mixtures of two or more metals that form a eutectic system, where the alloy’s melting point is lower than the melting points of the individual metals. However, in the case of an amalgam, the presence of mercury allows the alloy to be liquid or semi-solid at room temperature. This is not due to a eutectic point, but rather because mercury is liquid at room temperature, reducing the overall melting point of the amalgam.
However, it’s worth noting that some specific proportions of metals in an amalgam do form a eutectic system.
Importance and Use of Eutectics
Understanding eutectic points and mixtures is fundamentally important to several areas of science and technology:
- Metallurgy: Eutectic alloys are essential for creating materials with desirable properties. Steel, a eutectic alloy of iron and carbon, is a key metal for building and manufacturing.
- Electronics: The tin-lead eutectic alloy used for soldering has a melting point low enough not to damage other electronic components during the soldering process.
- Pharmaceuticals: Eutectic mixtures like menthol and camphor find use in topical treatments. Also, eutectics are vital to drug formulation.
- Cryobiology: Knowing the eutectic point of water and solutes helps scientists avoid the damaging effects of ice formation on biological samples.
- Food and Beverage Industry: The ethanol-water eutectic mixture is fundamental to the freeze distillation of beverages like applejack and ice beer.
Predicting Eutectic Mixture Formation
Whether or not a pair of substances forms a eutectic mixture depends primarily on the details of their phase diagram, which plots the state of the substances (solid, liquid, gas) under different temperatures and compositions. This is a complex question that depends on the specifics of the atomic or molecular interactions between the substances. However, there are several general factors that promote the formation of eutectic mixtures:
- Similar Crystal Structures: Substances that have similar crystal structures are more likely to form solid solutions and potentially eutectic mixtures.
- Size of the Atoms or Molecules: If the atoms or molecules of the two substances are of similar size, they are more likely to form a eutectic mixture. This is because similar-sized atoms or molecules more easily form homogeneous mixtures and fit into each other’s crystal lattices.
- Chemical Affinity: Substances with a high degree of chemical affinity for each other are more likely to form eutectic mixtures. This is because a high degree of chemical affinity often leads to the formation of compounds with distinct properties from the constituent elements, which can include a lower melting point.
- Valency and Electronegativity: The valency and electronegativity of the components also impact the formation of eutectic mixtures. For instance, a metal and a non-metal often have strong interactions that can lead to the formation of a eutectic mixture.
- Miscibility: Typically components are miscible as liquids but immiscible in the solid state.
Remember that these factors can increase the likelihood of forming a eutectic mixture but do not guarantee it. Whether a eutectic mixture forms depends on the specifics of the system under consideration. Also, these factors primarily apply to eutectic alloys. Similar considerations apply to other types of eutectic mixtures as well.
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- Smith, William F.; Hashemi, Javad (2006). Foundations of Materials Science and Engineering (4th ed.). McGraw-Hill. ISBN 978-0-07-295358-9.
- Socas-Rodriguez, Bárbara; Torres-Cornejo, Mónica Vanesa; Álvarez-Rivera, Gerardo; Mendiola, Jose A. (May 2021). “Deep Eutectic Solvents for the Extraction of Bioactive Compounds from Natural Sources and Agricultural By-Products”. Applied Sciences. 11 (1): 4897. doi:10.3390/app11114897