Intermolecular forces
Summary
Molecules are held together by chemical bonds, but interact with each other through intermolecular forces. These result either from an asymmetric distribution of charge within a molecule, or from temporary changes in the distribution of charge. In a liquid in a container, cohesive forces attract the molecules of liquid to each other, and adhesive forces attract the molecules of liquid to those of the container. This results in surface tension and the formation of a meniscus. As molecules escape the surface of the liquid, they draw other molecules with the, causing a small movement of the surface of the liquid in the direction of curvature of the meniscus. This is capillary action.
Chemical bonds (see this article) are the forces that hold molecules together. In an ionic bond, one atom gives up an electron to another, resulting in one becoming a positive ion and the other a negative ion. The charge difference between them results in an electromagnetic force between them. In a covalent bond, two atoms share an electron. The interaction of the shared electron with the nuclei of the two atoms similarly gives rise to an electromagnetic force between them.
Molecules also interact with each other, but through weaker “intermolecular forces”. These forces are not strong enough to permanently bind the molecules together into new compounds, but are responsible for many of the physical properties of molecular substances, such as their melting and boiling points, their viscosity, and their surface tension.
Intermolecular forces exist because the electric charge inside a molecule is not evenly distributed. This can be due to the way atoms are arranged in the molecule; with asymmetric arrangements leading to one side being slightly positively charged and the other slightly negative. Such molecules are said to be “polar molecules”. The movement of electrons in their orbitals also causes temporary changes in the distribution of electric charge, resulting in small forces arising between symmetric molecules.
Polar molecules
Water is an example of a polar molecule. In a water molecule, two atoms of hydrogen are bonded (covalently) with a single atom of oxygen. The hydrogen atoms’ electrons are shared with the oxygen atom, leaving the remainder of each hydrogen atom (a proton) on the outside of the molecule. Water is structured asymmetrically, meaning that both such protons are on the same side of the molecule, with the majority of the oxygen atom’s electrons on the opposite side. Even though the overall electric charge on the water molecule is neutral, one side is dominated by the charge on the protons, and the other by the charge on the electrons. This means that the water molecule has two distinct, differently charged, sides or “dipoles”.
Since one dipole is positively charged and the other negatively charged, molecules of water are attracted to each other. The attraction between a hydrogen atom in a molecule such as this and the negative dipole of another molecule is called a “hydrogen bond”.
Water is a molecule formed of one oxygen atom and two hydrogen atoms. The asymmetrical structure of a water molecule results in a positively charged dipole on the side of the hydrogen atoms and a negatively charged dipole on the other side.
Hydrogen bonds are a special case of intramolecular force due to the fact that hydrogen atoms only have one electron and hence that the remainder of the atom consists of a proton only. This makes these forces strong compared to other molecules whose dipole charges are masked to some extent by other electrons. The intramolecular forces between molecules without such an exposed hydrogen atom are called “dipole-dipole” bonds.
Van der Waals forces
Molecules that are symmetric have no permanent dipoles; the charges across the molecule cancel each other out. These molecules cannot then interact through dipole-dipole bonds. However, electrons are not fixed in place within an atom. As they move about, the distribution of charge moves with them. This means that small imbalances of charge occur all the time, creating temporary dipoles.
Both permanent and temporary dipoles induce changes in the distribution of charge in nearby molecules. Small positive charges attract electrons and small negative charges repel them. This causes the electrons to move, shifting the charge distribution in the nearby molecule, again creating a temporary dipole.
These temporary dipoles allow such molecules to interact with others, under the influence of what are known as “van der Waals forces”.
Surface tension
The boundary between a liquid in a tube and a gas curves slightly (in the case of water, it curves inwards away from the gas, but in some other liquids, such as mercury, it can curve outwards), forming a “meniscus”. If the liquid is free falling in the gas, it forms a curved droplet. In effect, the liquid behaves as if it is held together by a thin, elastic skin. This is due to the “surface tension” of the liquid.
Intermolecular forces along the surface of a liquid are stronger than those within the liquid.
Molecules within the liquid are attracted to each other by intermolecular forces. Away from the boundary, these forces balance out, with molecules being pulled equally in all directions. At the surface, however, the forces are unbalanced, since in one direction (that of the gas), there are no molecules of liquid to bond with.
Each liquid molecule is able to form the same number of intermolecular bonds, but those at the surface have fewer counterparts with which to bond. As a result, they form stronger bonds with each other along the surface of the liquid (if stronger bonds were formed towards the liquid, those molecules would be pulled into it, and replaced with new surface molecules). The effect of these stronger bonds is to resist deformation of the surface. This is the surface tension.
Stronger forces along the surface resist the weaker forces pulling molecules towards the liquid. This creates tension in the surface.
If the liquid spans a small hole (the size depending on the strength of the surface tension), this resistance to deformation is sufficient to prevent the liquid from falling into the hole. In this way, the surface tension of a liquid determines the extent to which it will flow into narrow channels.
Formation of droplets and a meniscus
Molecules on the surface experience an intermolecular force towards the bulk of the liquid (since there is no corresponding force pulling towards the gas). The strong bonds along the surface itself don’t allow these molecules to be pulled inwards. As a result, the whole of the surface is pulled towards the liquid. Resisting this pressure requires energy, and physical systems arrange themselves into the lowest possible energy state. This is achieved by minimising the surface area, which is why liquids form droplets.
When the liquid is held in a container, another set of intermolecular forces come into effect, those between the liquid and the molecules of the container. The forces between molecules in the liquid are known as “cohesive forces” or “cohesion”, and those between the container and the liquid are known as “adhesive forces” or “adhesion”.
The adhesive forces pull the liquid molecules towards the sides of the container. This is resisted by the cohesive forces within the surface of the liquid (the surface tension). The balance between the two causes the liquid to meet the sides of the container at an angle. Slightly further out from the sides, the adhesive forces are weaker and the angle is consequently less steep. The overall result is to curve the surface of the liquid into a meniscus.
The balance between adhesive forces attracting the liquid towards the container, and
cohesive forces attracting it to itself, result in a curved surface on the liquid, a meniscus.
In a wide container, the meniscus is curved at the edges where the liquid meets the container, then flattens out towards the middle to minimise the surface area of the boundary between the liquid and the gas. In a narrow container, however, the meniscus may not have space to flatten out, and may resemble part of the surface of a sphere.
Whether a meniscus curves inwards or outwards is determined by the relative strength of the cohesive forces within the liquid and the adhesive forces between the liquid and the container. In water, the meniscus curves inwards, but in mercury, for example, the meniscus curves outwards.
Capillary action
Molecules within a liquid are not static, but move around constantly (this is due to the energy stored in the liquid, which gives it its temperature). Sometimes, the energy in a particular molecule may be sufficient for it to overcome the surface tension of the liquid, and escape (this is how liquids evaporate).
When a molecule escapes close to the edge of a container that the liquid is stored in, it may find itself pulled towards the container by adhesive forces, preventing it from travelling far, and meaning that it once again is bound to nearby molecules on the surface through cohesive forces. However, as it did manage to escape, the point of contact with the container is slightly above where the surface was previously.
The surface now extends further up the side of the container, the adhesive and cohesive forces are thrown out of balance by a small amount. In exactly the same way as the meniscus was formed, the surface of the liquid needs to rise by a small amount to bring them back into balance. This is called “capillary action”.
Capillary action causes the liquid to move inside the container in the direction opposite to the curvature of the meniscus (so if the meniscus curves downwards, the liquid will move upwards). Other forces on the liquid (gravity and the internal pressure of the liquid) resist this movement.
If the container is wide, these other forces will win out, pulling the liquid back towards where it was previously. However, if the container is very narrow, the forces resulting in capillary action win out, and the liquid is drawn further into the container.
The strength of capillary forces depends on the surface area of the liquid, whereas those of gravity and pressure depend on the volume of the liquid. As the liquid moves, more and more volume is inside the narrow container, eventually causing the forces to balance and preventing further movement. In wide containers, this happens almost immediately, but in narrow containers, a significant distance can be traversed.
Capillary action is critical to both the storage of water (and hence dissolved nutrients) in soil, and the movement of water into and through plants from the surrounding soil.