Water and Bonding

“WATER IS A GOOD SOLVENT”
Water is very unique in all facets of its nature. The chemical formula for water is H2O as a water molecule consists of two atoms of the chemical element hydrogen (H) and one atom of the element oxygen (O). Water is the most abundant chemical compound on earth as its mass all exists together, naturally, in its gaseous, liquid and solid state. It is the most precious natural resource to man and is essential to life for all discovered living organisms. One of the unusual things about water is that, unlike any other substance, it is constantly moving in the environment, readily changing from state to state in a cycle which is known as the water cycle or the hydrological cycle. The solid state of water is known as ice and the gaseous state is known as vapour or steam. Water can never be found in the environment as pure H2O. It is always found with some kind of impurity dissolved in it. This is because many compounds can readily dissolve in water from its surrounding environment. Why is that? All of the peculiar characteristics of water are a result of the interactions between the outermost electrons of the constituent hydrogen and oxygen atoms. They are involved in a chemical process known as bonding.
In chemistry there are various types of bonding. They are all a result of electrons interacting with each other. Atoms interact by transferring, sharing or rearranging their electrons in a way that allows them to achieve a full shell of electrons which makes the atom stable. In compounds, there are two main types of bonding; ionic bonding and covalent bonding. Ionic bonding is usually found in compounds of a metal and a non-metal. This type of bonding involves the complete transfer of at least one electron from one atom to another. It is normally the metal atom which donates the electron or electrons to the recipient non-metal atom. The donor metal atom usually has very few or a single electron in its outer most shell which it readily loses leaving a stable, filled inner shell of electrons immediately below it. This produces a positively charged atom called a cation. The atom becomes positively charged because the atom has less negatively charged electrons, than it has positively charged protons, leaving the overall charge of the atom as positive. The donor metal atom gives its electrons to the receiving non-metal atom. The recipient non-metal atom, whose outermost shell is almost complete, accepts the electron (or electrons) to achieve a full and stable outer shell of electrons. The atom gaining the electron (or electrons) becomes a negatively charged anion. The atom gains this charge because it now has more negatively charged electrons, than it has positively charged protons. The electrons move in this direction because the non-metal is, in most cases, more electronegative than the metal component. Electronegativity of an element is a measure of the affinity it has to attract a bonding pair of electrons. A strong ionic bond is now formed as the oppositely charged ions are electro-statically attracted to one another. The molecule formed is known as an ionic compound which will normally be a sub-unit, packed together in a regular structure, forming a giant ionic lattice of that particular compound. Fig.1. Showing ionic bong in Magnesium Chloride. (Taken from Ritchie, 2008)
Bonding in water molecules is, somewhat, different to that of ionic bonding. Atoms in water molecules are held together by a different type of bond known as a covalent bond. This type of bonding is found in compounds where the constituent atoms are non-metals. A covalent bond is formed when “electrons are shared rather than transferred” (Rithchie, 2008). When a covalent bond is formed, each atom involved shares one electron to achieve a shared pair of electrons. This shared pair of electrons is a covalent bond. A compound can also have multiple covalent bonds present between constituent atoms. Depending on the elements involved, different variations of covalent bonds can be observed. A “pure” covalent bond can be found in a molecule where two atoms of the same element are bonded together such as the diatomic elements Hydrogen (H2) and Chlorine (Cl2). In a pure covalent bond the bonding electrons are shared equally amongst the atoms. The atomic s orbitals containing the bonding electrons overlap and concentrate the electrons in the region of space between the two atomic nuclei forming a sigma (σ) bond. (Chester et al. 2003; Sutton et al. 2000). One atom can also share a pair of electrons with another atom. This is known as a dative or coordinate covalent bond. (Wooster, 2008). When the component atoms are not of the same element, the bonding electron pair will be shared unevenly. This unequal distribution of electrons is caused by the difference in electronegativity of the elements. The bonding pair of electrons becomes strongly attracted to the nucleus of the atom with the greater electronegativity. This means that the atom with the greater electronegativity has more electrons present in its vicinity and gains a slightly negative charge (δ-) and the atom that is less electronegative becomes slightly positive (δ+). The covalent bond is known to be polarised or dipolar at this stage. Wooster (2008) stated that “polarisation shows us that ionic bonds and covalent bonds are the two extremes of bonding types and there is a gradual range of bonding in between”.
The oxidation state of a lone oxygen atom is -2 (O-2) which means that oxygen needs to gain two electrons when it forms a compound in order for it to attain its “noble gas” electron structure. The oxidation state of hydrogen is +1 (H+) which means that it needs to share one electron to attain a full shell of electrons. To achieve a stable electron structure, two hydrogen atoms share one electron each with one atom of oxygen. The total oxidation state of a water molecule is zero as the two H+ atoms counterbalance the O-2 atom. A covalent bond is formed and the hydrogen atoms share a pair of electrons each and the oxygen atom shares two pairs.

Fig. 2. Dot & cross diagrams illustrating covalent bonding in Cl2 and H2O. (Taken from Ritchie, 2008)

In a water molecule, the oxygen nucleus attracts electrons more strongly than the hydrogen nucleus does. This is because oxygen is more electronegative than hydrogen. On the ‘Pauling scale’, the electronegativity of oxygen is 3.5 and 2.1 for hydrogen. (Nelson, 2000; Sutton et al. 2000). This means that the electrons are more frequently in the vicinity of the oxygen atoms more than the hydrogen atoms. The electrons are, therefore, shared unequally between the oxygen and hydrogen atoms. The electron deficient hydrogen atom now has a miniscule shield of electron density around it resulting in two dipoles within the water molecule, one δ+ on each of the hydrogen atoms and a 2δ- on the oxygen atom. Therefore water molecules have a permanent dipole. The positively charged hydrogen nucleus from one water molecule is strongly attracted to the negative charge of the loan pair of electrons on the negatively charged oxygen atom from another molecule. This attractive force is called a hydrogen bond. Fig. 3. Showing dipoles in a water molecule and a hydrogen bond. (Taken from Nelson, 2000)

Another type of intermolecular force known as van der Waal’s forces can occur between molecules as a result of temporary dipole-dipole interactions. Van der Waal’s forces are much weaker than hydrogen bonds and are not exclusive to polar molecules. When two, electrostatically neutral atoms or molecules are very close together. The electron clouds surrounding them can have an influence each other. There are random variations in the positions of electrons around one nucleus which can create a temporary dipole which then induces an oppositely charged temporary dipole in the neighbouring atom or molecule which is then induced onto further molecules. The opposite dipoles attract each other and this is why van der Waal’s forces are relatively weak. Hydrogen bonds are weaker than covalent bonds, but stronger than van der Waal’s forces. Fig. 4. Illustrating van der Waal’s forces amongst atoms. (Taken from bbc.co.uk, 2012)
A single water molecule can bond to two other molecules of water via hydrogen bonding as it has two loan pairs of electrons on the oxygen atom and two partially positive hydrogen atoms. The low relative molecular mass of water (Mr = 18) dictates that it should be a gas at room temperature, but, it is a liquid. The extra intermolecular bonding from hydrogen bonds explains the relatively high melting point, boiling point, high surface tension, viscosity and also why water is less volatile than expected.
In its liquid form, the water molecules are constantly moving and, simultaneously, breaking and reforming their hydrogen bonds. If energy was applied to water, the molecules would begin to move a lot faster. They would have enough energy to break the hydrogen bonds and eventually break away from the surface of the water, changing to its gaseous from known as vapour. This is known as evaporation. If the ambient temperature were to drop to zero degrees Celsius or below, the water molecules would reach maximum density and their hydrogen bonds would cause them be packed in a rigid, hexagonal structure. This structure becomes a solid; however, there is a large surface area between the water molecules due to the hydrogen bonds. This causes ice to float on water as the ice’s large surface area makes it is less dense than water.
The permanent dipoles in water molecules and their ability to form hydrogen bonds determine its ability to suspend substances in solution. The substance which is dissolved in called a solute and the substance in which it is dissolved in is called the solvent. Water is never naturally in its purest form. This is because there are a large variety of substances or solutes that can be dissolved in water, whether they are mixed deliberately or if they have dissolved from the ambient environment. As water is polar solvent, it would only seem logical for polar solutes such as ionic compounds to dissolve in it quite easily. Water dissolves ionic solids by holding the ions in a matrix of polar water molecules. Sutton et al. (2000) states “Cations are bound to water molecules through the negatively polarised oxygen atoms, while anions are bound through the positively polarised hydrogen atoms”. Water molecules bind to the ions, surrounding them, separating them and causing them to diffuse throughout the water. Some non-ionic compounds can dissolve in water. This is very dependent on whether the molecules involved can participate in hydrogen bonding. Some examples of these are sugars, alcohols, carboxylic acids and amines. These compounds contain O—H or N—H bonds. As discussed previously, oxygen and nitrogen are more electronegative than hydrogen and will result in dipoles and eventually hydrogen bonding. Fig. 5. Showing water molecules disrupting an NaCl (Sodium chloride) lattice. Water molecules cluster around the Na+ and Cl- ions. The ionic charges are partially neutralised. (Taken from Nelson, 2000)
Water is the universal solvent of choice in most industries, because it is not just a good solvent, but it is also a good coolant. All living organisms have evolved with water being an essential part of their life processes. However, not everything can dissolve in water so is it the most ideal solvent? There are many other compounds that can be widely used as a solvent. Many solvents are organic such as Dimethyl sulfoxide, also referred to as DMSO. It is an organic sulphur compound and its chemical formula is C2H6OS. DMSO has been used as a commercial solvent since 1953. It is a covalent compound made up of two methyl groups (CH3) bonded to the sulphur atom of a sulfinyl group. DMSO is also a polar compound so it can easily dissolve ionic compounds just like water. As well as dissolving ionic compounds, DMSO can also dissolve other types of compounds such as aromatic hydrocarbons, alkanes, esters, aldehydes, ketones, fats and oils, whereas, water can only dissolve polar solutes or compounds capable of hydrogen bonding. (Muir, 2001). Fig. 6. Illustrating the displayed formula and 3D chemical structure of dimethyl sulfoxide. (Taken from wikedpedia.org, 2012)
Water cannot dissolve hydrophobic solutes such as hexane or benzene because it is energetically unfavourable for the substances to mix. When any solute is mixed with water, it interferes, weakly, with the hydrogen bonding amongst the water molecules often breaking a few hydrogen bonds. This causes a change in the energetic equilibrium as water-water hydrogen bonds are broken. Because hydrogen bonds are broken between water molecules, energy is taken up from the system. “Ionic solutes compensate for lost water-water hydrogen bonds by forming new solute-water interactions”. (Nelson, 2000). The water molecules form a cage like structure around the hydrophobic molecules and eventually push them out, separating them from the water molecules. Fig. 7. Illustrating molecules forming a cage structure around a hydrophobic molecule. (Taken from Nelson, 2000)
Therefore, non-polar compounds are immiscible with water, but are miscible with DMSO. Like water, DMSO is capable of hydrogen bonding and is miscible with other compounds that are capable of forming hydrogen bonds. DMSO is also used industrially as paint stripper and coating remover and can also solvate long chain polymers. DMSO is a clear viscous liquid, more viscous than water at room temperature. It is colourless with a slight garlic odour and has few known toxic symptoms to humans. However, it is still more toxic to humans and the environment than water. (Brobyn, 2001). DMSO has been recognised to have many applications in the medical industry. It has been found to have multiple medicinal qualities. DMSO has been established as a topical analgesic for severe pain and as an anti-inflammatory. DMSO can easily penetrate cell membranes and has been documented to increase their permeability resulting in its importance to scientist researching drug design. Researchers have found that DMSO has the ability to transport or carry other molecules across cell membranes. It can also be used as cell extractant in biology and biochemistry. This is an important application in medicine but, can also be hazardous to humans and the environment at the same time. (Muir, 2001).
Overall, DMSO has many characteristics which make it more of an ideal solvent than water. It can dissolve polar and ionic compounds, non-polar compounds and most other compounds that water cannot. However the properties of water make it the more physically practical solvent to use. Water is easier to manually manipulate. It can easily be converted to its solid, or gaseous from by changing the ambient temperature to zero or a hundred degrees Celsius respectively. This would also use less energy than DMSO because the melting point for DMSO is eighteen degrees Celsius and its boiling point is 189°celsius. Water doesn’t pose any risk to human health industrially in the lab or to the environment. DMSO also poses a few risks but, they are still risks none the less. DMSO is a hygroscopic compound which means that it readily absorbs moisture from its ambient surroundings, so if there were an instance where pure DMSO was required, it would be quite hard to achieve unless it is kept under controlled lab conditions. Water is never found in its pure form but, can easily be distilled to achieve its purest form. Water is a protic compound which means that it can donate H+ ions because the molecule contains a hydrogen atom attached by a single bond to a more electronegative oxygen atom (hydroxyl group). Water molecules can dissociate into H+ ions and OH- ions which contributes to its amphoteric nature. This means that water can act as proton donor or an acid and it can also act as a proton acceptor or base. DMSO has no dissociable ions and is therefore not amphoteric like water. Water’s amphoteric nature may make it more useful in areas where DMSO may not be.
In conclusion water and dimethyl sulfoxide both have properties which make them good solvents. The main characteristic that DMSO has over water is the versatility in the types of compounds it can solvate. The term “good solvent” would suggest that the solvent in question would be capable of dissolving most solutes or a wide variety of them, at least. Water is not as versatile as DMSO in the types of compounds it can dissolve. However, water makes up for this flaw by being readily available. Water is also the more practical choice as it a lot cheaper than DMSO because it doesn’t need to be synthesised and it requires less energy for heating and cooling. It is a naturally occurring and renewable resource as it can be recycled. It can be manipulated easily into its three states, depending on the conditions required for a chemical reaction. These factors ultimately make water more economically viable as they make water the cheaper option with less potential side effects to humans, animals and the environment. So it would be fair to conclude that water is not as good a solvent as DMSO, but, water is still a good solvent because there are many other favourable qualities that water has which makes it the ideal choice of solvent for many applications.