Water surface tension

So, last week a new post-Doc started at Cranfield, and I took a little time out to show her how to use some of the equipment I work with. While I was explaining all the various kit and the chemistry behind it, it occurred to me that I really should do a blog post on some this chemistry – I’ll try and keep it interesting, promise.

Covalent bonding….sort of. I may have glossed over a few details to make the diagram work

The work that I do relies heavily on one quite well known property of liquids called ‘Surface Tension‘ so that is the best place to start. One of the most interesting properties of water is the forces that act within it – called ‘hydrogen bonding‘. A water molecule is made up of two hydrogens which are covalently bonded to an oxygen molecule. A covalent bond can be thought of as the sharing of electrons between molecules, however the electrons are not shared equally and in the case of an O-H bond, the electrons are preferentially drawn towards the oxygen. This creates what is called a dipole within the molecules, where the H is left slightly positive and the O slightly negative.

When two or more water molecules interact they form what is known as a hydrogen bond, between the negative hydrogen and the positive oxygen of an adjacent molecule. This bond is short lived and has much less energy that a covalent bond but still provide water with a great deal of inter-molecular (between molecules) attraction. Without this attraction, water would be a gas a room temperature – in fact, the boiling point of water without hydrogen bonds would be around -150℃. This inter-molecular attraction also gives water its strong surface tension force.

When floating free in solution, the water molecules will form 3.86 hydrogen bonds (it’s an average – they don’t form 0.86 of a bond! and while we’re at it can people stop make jokes about the 2.4 children statistic, we get it, you don’t understand how averaging works!) to the surrounding water molecules. These surrounding bonds ‘pull’ the water molecule in all directions, resulting in a net force of 0. However, on the surface of the water, these water molecules aren’t surrounded by other molecules they are open to the air, limiting the number of hydrogen bonds they can form in one direction (up). So the surface molecules of water have a net force inwards towards the bulk of the water – and to minimise this force, the water will always try to have the smallest possible surface area. In the video above, this is demonstrated quite well with a brush – the wet hairs of the brush are ‘pulled’ in by the water as it tries to maintain the lowest possible surface area.

This inwards force of the water means that the surface of water behaves not like a liquid but more like an elastic film. As the surface is trying to constantly maintain the lowest possible surface area, it will preferentially curve around non-wetted objects placed on the surface. If the weight of the object is smaller than the intermolecular hydrogen bonding forces, then the surface of the water will support it. One way you can show this is to float a paper clip or safety pin on the surface of a small cup of water.

In the video above you can see the safety pin floating happily on the surface of the water until it suddenly sinks. This was caused by me adding a little bit of alcohol to the water, which disrupts the hydrogen bonding force and dramatically reduces the force of the inter-molecular forces – and therefore the surface tension. Water forms ~3.86 hydrogen bonds, where as ethanol can only form ~2 – so by diluting the water with ethanol, you greatly reduce the capacity of the liquid for inter-molecular hydrogen bonding. You can see this effect even more strikingly if you place a drop of water and a drop of dilute alcohol on a glass slide.

Left – Pure water, Right – 25% Alcohol (equivalent to a good whiskey, or for that matter a bad whiskey)

You can see the drop of water (left) forms a nice bead as it tries to maintain the lowest possible surface area between it, the surrounding air and the glass plate. By contrast, the water with 25% alcohol spreads out widely across the glass as it has weaker inter-molecular forces. However, rather than interfering with the bulk water you can also alter the surface tension of the water by changing the interface/air above it. As I mentioned, the surface tension forces are the result of the interface between the air and the water however, if bock this interaction then you can alter the surface tension.

‘Stuff’ on the surface changes reduces the net inwards force

By adding other materials (e.g. oils) to the surface of the water, these oils form hydrogen bonds with the surface water molecules and neutralise the force in-blanace lowering the surface tension of the liquid. It is this property that is most interesting to anyone wanting to study membranes and molecular systems, as the way the surface tension changes in relation to the addition of these materials can tell us a lot about their properties. Something I’ll explain more in a later blog post 🙂