The water molecules they are polar (charged) and form bonds or hydrogen bonds with each other. A hydrogen bond forms between an acceptor oxygen (from one water molecule) of negative charge density and a donor hydrogen of positive charge density (from another water molecule). This polarity allows it to dissociate ions (salts) and join other polar substances such as alcohols and acids, dissolving them.
The hydrogen bond It accounts for many of its unique properties, such as having a solid form that is less dense than its liquid form, a relatively high boiling point of 100°C for its mass, and a high heat capacity.
Again, these unique properties of water are due to hydrogen bonds. Water molecules are constantly moving past each other, and hydrogen bonds are continually being broken and formed on very fast time scales of 2 × 10−13 seconds. However, these bonds are strong enough to create many of water’s unique properties, some of which make it an integral part of life.
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A thorough understanding of the nature and strength of the hydrogen bond in a system can provide insights into chemical and biochemical processes fundamental, for example, nucleic acids, protein folding and enzymatic interactions. Also on how it directly affects the final properties of many materials and physicochemical processes. Furthermore, such an understanding is essential in the design of sophisticated materials that use hydrogen bonding motifs.
Normally, water molecules in the liquid state can form an average of four hydrogen bonds, but it has been observed -in temperature-dependent experiments by infrared spectroscopy- that there is a certain number of water molecules in the liquid that form fewer than four bonds.
This very peculiar behavior allows us to approach studies of changes induced by substances that dissolve in the structure of water and their influence on the interactions mediated by hydrogen bonding, as well as their role in the dynamic ensemble formation on a large scale of compounds of oily, polar and ionic molecules.
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The direct application of these investigations can be of great help in industry and the environment. Normally, organic solvents are a common part of industrial processes and everyday products such as plastics, synthetic adhesives, paints and enamels, or pesticides. They are used for extraction and as media for chemical reactions. Unfortunately, they have disadvantages such as their toxicity and polluting capacity, with the additional fact that many are volatile and, therefore, can affect not only the environment but also human beings, if they are absorbed by inhalation, skin or mucosa.
From the point of view of green chemistry, however, it has been found a promising alternative in the so-called natural deep eutectic solvents (NADES), a mixture of two or more solid or liquid components that, in certain proportions, exhibit a large decrease in melting point. Therefore, it can be a liquid mixture at room temperature. And they have good properties as an organic solvent and for extraction of oily or intermediate polarity compounds. NADES are nothing more than solvents based on primary and natural metabolites of plant origin such as sugars, amino acids or organic acids.
Their advantage, over other alternatives, is that they are non-toxic, have a relatively low cost, are highly biodegradable and, therefore, compatible with the environmentin addition to the fact that, for the most part, they are not volatile.
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But what do NADES have to do with water? That the chemical-physical behavior that allows the formation and stabilization of these mixtures is the formation of bonds or hydrogen bonds between the intervening components. In fact, in many examples of NADES, the main component is water.
Natural deep eutectic solvents, then, fit very well with the principles of sustainable chemistry and have great potential for various applications. They are expected to be the next generation of solvents and, although more study and research time is needed for their optimization, they will great contributions to a more sustainable industrial development.
* Nicholas Spegazzini. Director of the Pharmacy career at the University of Belgrano.
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