When we think of solvents, a very specific image comes to mind: that transparent liquid we add to a can of paint, to make it spread better over the surface we are covering. We can also think of products such as acetone in nail polish, or even alcohols in disinfectants and hand gels. However, an older solvent was already around long before humans could even comprehend the concept of dissolving. This is water, which approximately 3.8 billion years ago acted as the precursor that enabled our planet’s oceans to form by dissolving sodium chloride and other salts.
Mankind has spent centuries exploring the possibilities of various solvents, only to ultimately realise that the wonders of water remain unmatched.
Let’s not fool ourselves: we owe everything to water. According to the primordial soup theory, water is the very reason we exist today. The formation of complex organic molecules such as DNA, RNA and proteins —the building blocks of the earliest life forms— required a universal solvent, one capable of dissolving an enormous variety of substances and facilitating essential chemical reactions. We only need to think about the first human civilisations, which settled along riverbeds, to appreciate water’s fundamental role in our lives.
Although water is considered the universal solvent, and its role in this story is indisputable, there are thousands of other substances that are used for the same purpose. But what exactly does this process entail, and what is it for?
A simple glass of water
Solvents are substances capable of dissolving other materials, resulting in a homogeneous mixture in which the individual components are indistinguishable. Let’s look at a simple example: a glass of water with salt added. At first, the salt settles to the bottom of the glass. However, when stirred with a spoon, the salt dissolves in the water, forming a homogeneous solution in which the sodium chloride is no longer visible to the naked eye. This mixture is called a solution and consists of two main components: the solvent, the most abundant substance in the mixture, and the solute, the material that is dissolved.
At a molecular level, solvation is a different story. One of the characteristics of solutions is that substances do not usually react chemically with each other. In a solution, the solvent molecules surround and attract the solute, which helps it to distribute and stabilise evenly. Also, when you get a solution, the physical properties of the parts change. In aqueous solutions, for examples, solutes make the boiling point higher and the freezing point lower: the reason behind adding salt to icy roads.
But solutions are not limited to the liquid state. Think of the air, when it has a high concentration of humidity or when it carries dust. This means we can also see solutions where the two components are solids, gaseous or liquids, or where the substances are in different states.
One substance to dilute them all
It all started with water. For many centuries, water has remained the most widely used solvent and, although it has many advantages, it is not without its limitations. As our knowledge of organic chemistry grew, we started finding other substances that could be more attractive for certain functions.
Perhaps we are getting a little ahead of ourselves: even before the 18th century, alchemists were pursuing the idea of a much more powerful substance capable of dissolving absolutely any solute, known as Alkahest. At that time, the line between science and magic –and alchemy and chemistry – was still slightly blurred.
In the 15th and 16th centuries alchemy, which was considered a proto-scientific discipline, reached its golden age: its forerunners combined the principles of physics and chemistry with elements of medicine, astrology and spiritualism. As we said, the line was very thin.
The history of science has relegated the alchemists to an obscure place, treating them as madmenmad people, but in reality, many of them were educated people, driven by a genuine thirst for knowledge. Isaac Newton himself devoted much more time to alchemy than to physics.
Alkahest never appeared. However, the history of solvents is much older: apart from water, the first known substance of this kind was ethyl alcohol (commonly called ethanol), which according to the records we have today was first prepared 13,000 years ago in a brewery in the middle East. In addition to ethyl alcohol, ubiquitous across cultures thanks to beer, wine, and liquors, humans discovered other alcohols, such as isopropyl, amyl and methyl alcohol. The first uses of these solvents were closely related to sedation for ailments or surgical interventions. As early as the late 18th century, chemists began to explore a new use for solvents, mostly focused on extracting chemical compounds and active ingredients from plants.
After the industrial revolution, the use of solvents became widespread and implemented in fields as diverse as pharmaceuticals, cosmetics and paints. And, with the arrival of the 20th century, petroleum appeared, and with it solvents such as hexane and toluene. And so on, up to the present day, where we know of hundreds of such compounds.
Dissolving, extracting, purifying… Action!
As previously stated, solvents have the capacity to dilute a range of substances, creating a uniform blend. However, their true value lies in their versatility, which makes them an essential tool in the chemical industry, hence their use is implemented for a wide variety of purposes.
In the production of oil-based paints, used on artistic canvases, solvents include, for example, turpentine and acetates. The former dissolves the pigments, while the latter helps an even and efficient paint application, making it spread properly. In industrial paints, we can also find other substances derived from petroleum.
Solvents are also used extensively in the textile industry at various stages of the production process, from the preparation and processing of synthetic fibres to the synthesis of inks and prints for garments. Substances such as acetone are included to enhance the quality of the final product and remove any residual materials that may have been produced during the manufacturing process.
They are also widely used in cosmetic products in various forms: as disinfectants, to retain moisture in certain creams or even as natural moisturisers, like glycerine.
In addition, solvents are involved in the manufacture of countless cleaning products: here, alcohols are the real stars. They are used to dissolve oils and fats as well as to disinfect bacteria and viruses on surfaces. Of course, these substances are also present in the manufacture of plastics and polymers, in the petrochemical industry and in the creation of pesticides and herbicides. Yes, solvents are everywhere.
But one of the most important uses of solvents is the synthesis and purification of pharmaceuticals. Solvents appear at practically every link in the production chain of medicinal products: they are essential in the synthesis of active ingredients, facilitating chemical reactions between the substances. They also key in purification processes, techniques such as crystallisation and column chromatography, both of which help to separate the different substances present in a mixture and remove residues, resulting in very pure active ingredients.
Solvents are also used in medicines that are administered in liquid form, such as syrups, or by injection. Ethanol, for example, is used to form a protective film that ensures that the drug is released in an appropriate way, ensuring that it is not diluted as soon as it enters the body.
Although their use is highly standardised, solvents have significant drawbacks and pose health problems. In this sense, mechanochemistry could establish itself as a safe and environmentally friendly option by achieving the same end products, but eliminating solvents from the equation.
Find out more about the different types of these substances currently used in the industry and the most promising alternatives the following week in our upcoming article.
