When you think of cleaning or removing contamination, the first chemical that usually comes to mind is water. While water is the most commonly used cleaning agent, most people probably know very little about its unique properties. These properties are behind water’s successes and failures as a cleaning agent.How can water be both a boon and a barrier in removal of contamination?
SIMPLICITY, SHAPE, AND FUNCTION=
As we recall, water consists of two atoms of hydrogen (H) and one atom ofoxygen (O) and that its chemical formulais H2O. However, the formula does notconvey the shape of the molecule, and the shape is crucial to the unique propertiesof water. Rather than being a linear molecule (H-O-H), the molecule is bent intoa “V”, with the O at the vertex. It is sometimes depicted as in Figure 1, lookinga bit like Mickey Mouse.
Figure 1: Water molecule
(courtesy of U.S. Geological Survey)
This V-shape gives water its complexity, including polar qualities, and allows hydrogen bonding.
The solvency properties of water are unique. We are often asked why the solvency properties of water are not either totally polar or totally hydrogen bonding. Hanson parameters or a TEAs diagram1,2 show water as a mixture of the three “primary” forces: polar, dispersive, and hydrogen bonding. All molecules have dispersive or non-polar forces. The positive charges associated with the Hs on one side of the water molecule and the negative charges associated with O on the other side confer polarity. Because opposite charges attract, the positive side of a water molecule is attracted to the negative side of another molecule, say another water molecule. It is this polar attraction that causes water to have a much higher boiling temperature than other simple molecules (i.e., molecules with only a few atoms). Without these polar forces, water would be a gas rather than a liquid at typical ambient temperatures and life as we know it could not exist. This attraction would be strongest when oxygen atoms in adjacent water molecules were in a straight line with an H atom between them. This maximum force is defined as “hydrogen bonding” where the H atoms attract both O atoms.3 On average, since there are an infinite number of arrangements where the water molecules do not have a straight line arrangement and only one arrangement that is truly linear, other forces play a role.
Another feature of the water molecule is that it is relatively small compared with other common diatomic or triatomic molecules like O2, N2, and CO2. The water molecules can get closer to each other where the attractive forces are stronger. This makes water particularly dense, another property that contributes to its high boiling point. The higher density of water is advantageous in cleaning in that lighter molecules, such as oils, rise to the top where they can be skimmed off.
A SHARING COMMUNITY
Although the forces of attraction are stronger due to the polar nature of water, at temperatures above the freezing point, these forces are not sufficient to bond adjacent water molecules for long times. Liquid water molecules form transient clusters, sharing Hs among a number of Os (reminiscent of the “free-love” communities of the 1960s). These interactions are quite sensitive to the presence of other substances, giving water much of its solvency characteristics, especially for other polar compounds, such as salts. It may also explain why even relatively modest changes in the make-up of a water-based formulation can have large effects on its ability to dislodge and remove contaminants from surfaces. The transient sharing of Hs also causes water to have a ‘hermaphroditic’ quality in the sense that it behaves like it is simultaneously both an acid and abase.
THE SURFACE
One of the consequences of the strong intra-molecular forces between water molecules is what happens at the surface. Here, the attractive forces that normally completely surround a water molecule emanate only from below the surface, resulting in a relatively high surface tension for water. One of the primary problems associated with a high surface tension is the difficulty of getting into small spaces. Liquids with low surface tension, such as organic solvents, can more easily penetrate small holes or spaces under components. This is one reason why these solvents have such an appeal as cleaning agents.To counteract the high surface tension, additives, such as surfactants, are included in aqueous cleaning agents. While these mixtures can penetrate smaller spaces than pure water, the problem remains of how to adequately rinse. There isa substantial risk of cleaning agent residue.
ARE OILS REALLY HYDROPHOBIC?
Since non-polar oil and water do not mix, it is frequently stated that oil is hydrophobic. This does not tell the full story. The bond between an oil molecule and a water molecule is actually stronger than the bonds between two adjacent oil molecules although it is still weaker than the water-water bond.4Much of this effect is likely due to the smaller size of the water molecule, allowing it to get closer to an oil molecule than another oil moleculecould. When a molecule gets closer, the attractive forces are stronger.
A demonstration of this attraction between oil and water is shown when a drop of oil is added to water. The oil floats to the surface because it is less dense than water. If the oil-oil interactions were stronger than the oil-water interactions, the oil drop would retain much of its near spherical shape. The water-oil attraction overcomes the oil-oil attraction and, thus, the oil spreads out over the water surface, becoming a thin film that may be only one atom deep.
In immersion cleaning, this oil film can completely cover the surface and may make it difficult to completely remove oils. An oil-free component can be re-contaminated when it is lifted out of the cleaning bath through the oil covered surface. Solutions to this problem include using a weir to remove surface oil, having multiple wash cycles prior to rinse, and spraying after immersion.
UTILITY, NOT UNIVERSALITY
The physical and chemical properties of water make it valuable for industrial and critical cleaning. However, some of the same properties that make water useful are also limitations. By understanding its properties, you can bettercontrol the process.
References
- J. Burke, “Solvents and Solubility,” in Handbook for Critical Cleaning, Kanegsberg & Kanegsberg ed., CRC Press 2001.
- J. Burke, “Solubility Parameters: Theory and Application,” http://sul-server-2.stanford.edu/byauth/burke/solpar/.
- M. Chaplin, “Water Structure and Behavior,” http://www.lsbu.ac.uk/water/index.html (an on-line book by Martin Chaplin from London South Bank University with both technical and non-technical portions).
- http://www.ozh2o.com (a non-technical description of Physical and Chemical properties of water).
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical and precision cleaning,surface preparation,and contamination control.They are the editors of Handbook for Critical Cleaning,CRC Press.Contact them at BFK Solutions LLC., 310-459-3614;[email protected];www.bfksolutions.com.