Introduction:

Switchable solvents, also known as "tunable solvents" or "reversible solvents," are a class of solvents that can undergo reversible changes in their physical properties, such as polarity, acidity/basicity, and solubility, in response to external stimuli such as temperature, pressure, light, or chemical agents. This property makes them particularly attractive for a range of applications, including separation, extraction, catalysis, and chemical synthesis, among others.

One of the earliest reports on the concept of switchable solvents was published in 1997 by Jessop and colleagues, who developed a series of switchable hydrophilicity solvents based on the reversible reaction between amines and carbon dioxide (CO2) to form carbamates. Since then, many other types of switchable solvents have been developed, including switchable ionic liquids, switchable non-ionic solvents, and switchable solvent systems based on organic molecules or polymers.

Switchable solvents have attracted a lot of attention in recent years, as they offer several advantages over conventional solvents, such as reduced toxicity and environmental impact, improved selectivity, and lower energy consumption. They also have the potential to enable new chemical processes that were previously difficult or impossible to perform using traditional solvents.

History:

Switchable solvents have been a topic of research for over a decade, but the concept of using reversible solvents can be traced back to the early 1990s. In 1993, chemists at the University of Nottingham, UK, introduced the idea of "switchable" solvents that can change their properties, such as polarity, in response to external stimuli.

One of the earliest switchable solvents was an aqueous solution of a chemical called 2-(diethylamino)ethanol (DEEA), which was found to switch between two different phases depending on temperature. This discovery led to the development of other switchable solvents that could be triggered by changes in temperature, pressure, or pH.

In 2003, a team of researchers at Queen's University in Belfast, UK, discovered a new class of switchable solvents called "azobenzenes" that could be switched between two different states by exposure to light. This discovery opened up new possibilities for using switchable solvents in a variety of applications, such as chemical synthesis, drug discovery, and environmental remediation.

Since then, researchers around the world have continued to develop new types of switchable solvents with different properties and applications. Today, switchable solvents are a promising area of research that has the potential to revolutionize the field of chemistry and other scientific disciplines.

Types:

1. Room temperature ionic liquids (RTILs): These are solvents composed entirely of ions, and they have a low volatility and high thermal stability. They can be switched by adding or removing an external stimulus such as heat or light.
2. Supercritical fluids: These are gases that have been compressed to a high pressure and temperature, which makes them act as a liquid. They can be switched by changing the pressure and temperature.
3. Switchable polarity solvents: These are solvents that can be switched between polar and nonpolar states by adding or removing an external stimulus such as a salt or acid. They are useful in separating different types of molecules.
4. Reversible ionic liquids: These are solvents that can be switched between two different ionic liquids by adding or removing an external stimulus such as a gas or electric field.
5. Reversible co-solvents: These are solvents that can be switched between two different co-solvents by adding or removing an external stimulus such as a salt or acid. They are useful in controlling the solubility and separation of different types of molecules.

Properties:

Some of the key properties of switchable solvents:

1. High selectivity: Switchable solvents can be designed to selectively dissolve certain compounds, while leaving others untouched. This makes them ideal for separation and purification processes.

2. Low volatility: Switchable solvents have low vapor pressure, which means they are less prone to evaporation and can be used in closed-loop systems without significant losses.

3. High stability: Switchable solvents can be designed to have high stability and low reactivity, which makes them useful for long-term storage and handling.

4. Variable polarity: Switchable solvents can be designed to have varying polarity, depending on the specific application. This allows for fine-tuning of the solvent properties to suit the desired process.

5. Renewable: Many switchable solvents are made from renewable feedstocks, such as plant-based materials. This makes them attractive for green chemistry applications.

6. Non-toxic: Some switchable solvents are non-toxic and non-flammable, which makes them safer to use and handle than traditional solvents.

7. High solubility: Switchable solvents can have high solubility for a wide range of compounds, which makes them useful for dissolving and extracting complex mixtures.

Applications:

1. Separation and purification of chemicals: Switchable solvents can be used to selectively dissolve and separate specific chemicals or compounds from a mixture. This can be useful in chemical and pharmaceutical industries for purification of drugs and other compounds.

2. Industrial cleaning: Switchable solvents can be used as cleaning agents in industrial applications, where they can dissolve and remove various types of contaminants, such as grease, oil, and dirt. They can also be reused multiple times, making them a more sustainable alternative to traditional cleaning solvents.

3. Carbon capture and storage: Switchable solvents can be used to capture carbon dioxide from industrial processes, such as power plants, and then release the gas for storage or utilization. This can help reduce greenhouse gas emissions and mitigate climate change.

4. Biorefinery processes: Switchable solvents can be used in biorefinery processes to separate and extract valuable compounds from biomass, such as lignin, cellulose, and hemicellulose. These compounds can then be converted into biofuels and other renewable chemicals.

5. Electrochemical applications: Switchable solvents can be used as electrolytes in electrochemical applications, such as batteries and supercapacitors. They can improve the performance and stability of these devices, while also being more environmentally friendly than traditional solvents.

Conclusion:

Switchable solvents are a promising new class of chemicals that can be used in various industries, including pharmaceuticals, polymers, and green chemistry. These solvents offer several advantages over traditional solvents, including higher efficiency, lower costs, and reduced environmental impact. The ability to switch between solvents also allows for greater flexibility in chemical processes, making it easier to adapt to changing conditions and optimize production. Switchable solvents offer a promising new approach to green chemistry and sustainable manufacturing. With their unique properties and potential benefits, these solvents are likely to play an increasingly important role in a wide range of industries in the years to come.

References:

1. Jessop, P.G., Mercer, S.M., and Heldebrant, D.J. (2011). Green chemistry: Reversible nonpolar-to-polar solvent. Nature 471, 357-360.

2. Brennecke, J.F. (2010). Switchable solvents: from fundamental studies to applications. Acc. Chem. Res. 43, 1039-1045.

3. Cooper, E.R. and Andrews, A.B. (2016). Switchable solvents: recent advances and applications. Curr. Opin. Green Sustain. Chem. 2, 55-59.

4. Pollet, P., Binnemans, K., and Brooks, N.R. (2018). Switchable solvents: a perspective. Chem. Rev. 118, 732-739.