Book Extract #9: Oil & Water Chemistry #2

Examples of organic molecules are carbohydrates, proteins,  enzymes, oils and fats. These are of course all naturally-occurring molecules (and most are contained in foods you eat) but there are many organic molecules that are either non-edible (crude oil and all its derivatives – petrol and plastics) or fully synthetic (prescription drugs and specialised industrial chemicals).

You can see that carbon-based molecules are all around us – that’s why they have their own category.

We can demonstrate this ability of carbon to link to itself in long chains by looking at a molecule called octane:

 

 

If we count the atoms in this molecule, we arrive at the formula C8H18. Now, since scientists are lazy (but not evil and stupid like marketing people) we like to abbreviate structures like this to make them easier to draw:

 

 

By omitting the atoms all we have to draw are the lines linking them.  The zigzag structure tells us that at each elbow there is a carbon atom, with one on each end.  The hydrogens aren’t considered important enough to put on these abbreviated structures, but if there were any oxygen, sulphur or nitrogens, they would be drawn in.

Octane of course is the chemical that is used when classifying petrol grades.

The other major structure that carbon compounds adopt is the formation of rings, like xylene (paint thinners):

 

 

Once again we can abbreviate this to:

 

 

And of course the sky is the limit in terms of how big these molecules can get

 

 

Inorganic chemicals, on the other hand, tend to have far simpler structures, like common table salt (sodium chloride), which simply has the formula NaCl.

 

You will note the plus sign next to the Na and the minus sign next to the Cl.

This is very important and the whole purpose of this section.  Inorganic compounds have an uneven distribution of charge, whereas organic compounds have an even distribution of charge as they are composed of the same element (carbon).

This is important for one reason alone – the most common liquid on the planet (water) itself has an uneven distribution of charge, and it is impossible to understand the science of stain removal without understanding the chemistry of this remarkable liquid.

You see, strange as it may seem, water is a very unusual molecule.  It is only because it is so abundant that it doesn’t seem unusual, but if it was a lot less abundant than it was, it would be considered quite exotic.  The reason simply is that it is the second most polar liquid in existence (behind ammonia).

By the use of the term polar we simply mean that the charges on the molecule are unevenly distributed.  That is, in just the same way that the earth has the North Pole and the South Pole, and magnets have north and south ends, the water molecule has a positive end and a negative end.

 

 

The oxygen (the blue atom) sucks all the electrons up its end, thereby causing a negative charge.  At the other end of the molecule is the hydrogen (the red atoms).  There is a net positive charge at this end as the oxygen has taken all the electrons away from the hydrogens (we say that the oxygen is more “electronegative” – that is, it likes electrons more).

Another simplified way to look at it might be like this, with a charge separation:

 

 

Now, we all know that opposites attract.  A negative charge is attracted towards a positive charge and a positive charge is attracted towards a negative charge.

So the molecules in water will align themselves to look like this:

 

 

In other words, the negative end of one molecule will be attracted towards the positive end of another molecule, and this provides a very energetically stable structure for water as it aligns itself in this manner.

Now that’s all very well in the bulk of the liquid, but what happens on the surface?  The water molecules want to align themselves with other water molecules to balance their charge, but what happens to the molecules that draw the short straw and find themselves on the surface of the water?

Well, since air contains no charges, the charges on the surface are not balanced.  This causes tension, and is the reason that water has surface tension at all.  They are not at all happy with life, and if such a thing were possible, these molecules would need to see molecular counsellors to deal with the stress that this causes.

Now consider what happens if we mix the octane with the water. If you are a water molecule and you bump into an octane molecule, you’re not going to get along.  The reason is that as a water molecule, you are going to be looking for other water molecules so you can balance your charges.

So if you mix water and octane together all the water molecules will run around until they bump into each other and link up so that the positive and negative charges can be balanced out.  That’s why we get two separate layers – the water molecules want as little interaction with the oil as possible because they want as much interaction with each other.

Or to put it another way, they want to minimise the surface area of the interaction between the water and the oil, as this minimises the number of unbalanced charges.

We could put it this way – the octane is a pretty easy-going molecule – it has no problem with water. it has no particular agendas or issues.  But the water – well – it sure has some agendas and issues.  It needs to balance its charges. So the water has a problem with the octane because it can’t help it balance its charges.

And this affinity or hatred actually expresses itself in chemical terms such as hydrophilic (water loving), hydrophobic (water hating), lipophilic (oil loving), and lipophobic (oil hating).

So in general

hydrophobic = lipohilic

and

hydrophilic = lipophobic.

This is for example why there are two types of paints – solvent based and water-based.  Solvent based paints require turps to clean the brushes whereas water based paints require water.

So the simple rule of thumb is that like dissolves like – polar solids dissolve in polar liquids and nonpolar solids dissolve in nonpolar liquids. An example of a polar substance dissolving in a polar liquid would be table salt (sodium chloride) dissolving in water. If you tried to dissolve table salt in oil, you’d find it wouldn’t dissolve.

We saw above that sodium chloride has and uneven charge distribution.  In fact, the uneven charge distribution is so great that it entirely accounts for the very existence of the molecule.  In other words, they are stuck together by this very charge and nothing else – kind of like two magnets that are stuck together.

Consequently, if they find another source of charge, they will readily separate, and this is exactly what happens when you dissolve salt in water – the positively charged sodium ions surround themselves with the negative ends of many water molecules and the negatively charged chloride ions surround themselves with the positive ends of many water molecules:

 

 

Water is the only polar liquid on the planet in abundance.  Virtually every other liquid that occurs naturally is an oil, and it is the difference between these two, more than anything else, that explains why things stain.

To put it another way, most stains are non-polar, so they don’t wash out in water very well.  That’s why we need detergents, but that’s another story.

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