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.

Book Extract #8: Oil and Water Chemistry #1

I remember the first time I saw kerosene and water mixed together.  We used to have an old kerosene heater and I was curious to see what would happen if you mixed the kerosene and water together.  Since the kerosene was a nice sky blue colour, I wondered whether it would make the water blue.

Much to my surprise, the kerosene and water separated into two discrete layers, with the kerosene on top.  I was fascinated by this – I’d never seen this before.  Two liquids were mixed together, but not mixing at all.

This phenomenon explains an awful lot of what we see in the world around us, particularly in the kitchen and laundry, so let’s look at – why don’t oil and water mix?

To understand this, we have to go right back to the beginning.  And we’ll work our way slowly forward from there.

The most basic things on earth are elements – everything in existence is composed of one or more of these. These can be represented in a table:

 

Now, here’s the funny bit. In terms of all the things that we see in the world around us, we can essentially divide everything into two categories.

If you look at the table above, in the second row you will see that element number 6 has the chemical symbol “C” – which of course refers to Carbon.

Carbon has a unique property – the ability to link to itself in long chains or rings of infinite length and with an infinite number of structures and shapes. Think of it as chemical Lego – you buy a box of Lego bricks and you can fashion them into any shape you want. So it is with carbon.

For this reason, carbon-based molecules have their own category – organic.

Organic chemistry is also the chemistry of life (hence the term “organic”). We are organic, as we are carbon-based life forms. Inorganic chemistry is everything else.

In other words, the two major branches of chemistry are organic and inorganic. Organic chemistry concerns itself with the chemistry of one element only – carbon, and inorganic chemistry concerns itself with the chemistry of the other 109 elements in the table above.

The Chemistry of Clean #2: How Detergents Work

Here is an exercise for you to try at home: Put some dirty dishes in your sink, and fill it with hot water, without adding any detergent.  Now place a straw on top of the water (so that it floats).  Position yourself in such a way that you can see light reflecting off the surface of the water.  You should see a greasy film on the surface of the water.  Now, while you are watching, get some dishwashing detergent and add a single drop – just one drop – and watch what happens.  When the drop hits the surface of the water, rather than disappearing into the water, you will see it smear instantly across the surface of the water, pushing the oily film out of the way, and it will make the straw move as well.

What just happened?  What are detergents and how do they work?

We have seen that water and oil are fundamentally different in their chemistries.  Whereas water is polar, oil is nonpolar.  Water molecules have an imbalance of charge, and oil molecules do not.

Well, if we know that detergents will allow oil to be dissolved in water, then clearly they are molecules that are able to interact both with water and oil.  And the best way to do this, is where you simply have one part of the molecule as polar and the other part of the molecule nonpolar.

Let’s look at a detergent molecule:

So we can see that one in the molecule is hydrophobic (in other words, hates water) and the other end of the molecule is hydrophilic (in other words loves water).

So what happens when you wash the dishes, is that the oil loving part of the molecule looks for other bits of oil in the water, and wrap themselves around them, like this:

You can see that the non-polar part of the molecule is facing inwards, and is wrapped around the oil, and the polar part of the molecule is facing outwards where it can interact with the water.  The resulting structure is called a micelle.

So the micelle can happily move through the water because the part that is facing the water is polar so it is perfectly happy, and the nonpolar part is locked away inside the micelle.

The more general term for detergents is surfactants.  The word “surfactant” is simply an abbreviation of three words: surface active agent.

They are called surface active agents because they like interfaces.  That is, in the case we just looked at, they look for the interface between the oil drop and the water.  Because the two ends of the molecule are fundamentally different, it is happiest when one end of the molecule is facing a different environment from the other end of the molecule.

So is the surfactant molecule in your sink will go looking for, and attach itself to, interfaces such as the interface between the water and the oil in the sink, water and cutlery or crockery, water and the sides of the sink, or water and air.

So in a demonstration at the beginning of this post, the surfactant molecule will smear across the surface because they are more stable on the surface – the interface between the water and the air.  The reason is that the water has a lot of tension because of the unbalanced charges.

In other words whereas water loves to stick to the water molecules to balance its charge, the water molecules right on the surface have drawn the short straw, because there’s only air above them and has nothing to balance the charge.

Surfactant molecules fulfil this need quite readily.  The polar part of the molecule balances of charge and the nonpolar part sticks up into the air, where no balancing the charge is required.

Surfactants play a huge role in many household cleaning formulations and products, and will look at them in subsequent posts.

Why Don’t Water and Oil Mix? Part 3 – alcohols

We have seen that water and oil don’t mix because of their fundamentally different properties.

Water, because it contains an exposed oxygen atom, is polar, and hydrocarbons (molecules containing only carbon and hydrogen) are nonpolar. They therefore don’t mix, and the liquids when mixed together will separate into two layers.

But here’s a question – are there molecules that are partially non-polar and polar, that may be able to interact with both types of molecule?

Yes, there are – but only a very small number.  To fall into this category, the structure of the molecule must be such that neither the polar nor the nonpolar part of the molecule dominate the behaviour.  For this to be the case, the molecule must have an exposed electronegative atom (typically oxygen) and a short hydrocarbon chain.  If the hydrocarbon chain is too long, it will dominate the behaviour of the molecule, and the molecule will behave as a nonpolar molecule.

There are essentially only two molecules that we can put in this category (apart from surfactants, which are a special case that we will talk about later).

Let’s look at acetone and isopropanol:

Acetone looks like this:

this is an abbreviated form of a molecule, that kind of looks like a man with no arms.  The full structure looks like this:

 

Now let’s look at isopropanol in its abbreviated form:

They are remarkably similar, aren’t they?

Each of them has a hydrocarbon chain of three carbons, and each of them has an oxygen coming off the central carbon. The difference is that the acetone has an exposed (ketone) oxygen, whereas the isopropanol has an oxygen attached to a hydrogen (which makes it an alcohol).

The combination of the single polar group, with a short hydrocarbon chain, means both of these molecules will interact with both hydrocarbons and water.  For this reason, they are both very common solvents in laboratories across the world – mainly used for rinsing glassware, as they will dissolve what ever is on the glassware, whether lipophilic or hydrophilic.  That is, they will dissolve both oils and water from the glassware.  Acetone is particularly useful, as it is also very volatile, and evaporates very quickly, so the glassware can be cleaned and dried very easily.

Both of these solvents find their way into the marketplace.  Acetone is nail polish remover, and isopropanol is “rubbing alcohol”.

In terms of the chemical properties, acetone is the more aggressive of the two.  It will chemically attack many plastics and paints, which is why it is used as nail polish remover – nail polish is essentially an acrylic paint.

Isopropanol, on the other hand, is far more useful.  It is simply the best solvent in existence for general-purpose cleaning.  As the name “rubbing alcohol” suggests, it can be used for rubbing excess oil off your skin, as a skin cleanser.  Because it is miscible with water, it is also an excellent general-purpose solvent for all other forms of cleaning.  Used on your kitchen benchtop for example, it will successfully wipe up whatever might happen to be on there, with a water-based or oil based.

For some reason, the isopropanol has not found its way into any kitchen products unlike “vanilla fresh” which is an alcohol based cleaner (ethanol) which will simply not work as well on oil based things as the isopropanol will.

So go and get yourself a bottle of “rubbing alcohol”.  Whether it’s cleaning a kitchen benchtop, leather seats, or even texta or ink, it will do a good job of removing it– is a remarkable chemical, and the second-best general-purpose cleaning compound in existence.

 

Why Don’t Water and Oil Mix? Part 2

We know then water and oil don’t mix. We are considering the reasons for this in terms of their various chemistries.

Water, as the have seen, is polar – that is, there is an uneven distribution of charge, and so water molecules like attaching themselves to other water molecules so that they can balance their charges.

Let’s now look at oil.  Oil is organic in nature.  By organic, we mean carbon-based.  Broadly speaking, all chemistry is split up into organic chemistry and inorganic chemistry.  Carbon atoms have the ability to form themselves into long chains, and these long chains are what make up all living organisms – this is called carbon chemistry, organic chemistry, or simply life chemistry.

It is quite funny really – one entire branch of chemistry is devoted to one element in the periodic table – carbon – whereas the other major branch of chemistry, inorganic chemistry, is devoted to the other hundred and seven elements.

So carbon has this incredible ability to link itself together in long chains, rather like Lego.  In just the same way that we can take a box of Lego bricks, and put them together into all sorts of shapes and configurations, carbon atoms can be linked together to form an infinite number of molecules, these molecules are the amino acids, the carbohydrates, the proteins that make up our bodies and make up life.  They also the chemistry of oils and fats.  So it’s look at them now.

Let’s consider the simplest organic molecule that there is: methane.

Methane consists of a single carbon atom, with four hydrogens attached to it in the shape of a tetrahedron:

because the molecule contains no electron withdrawing atoms, like oxygen, there is no uneven distribution of charge.

But of course methane is a gas.  Let’s consider a simple organic molecule such as octane:

We can see immediately that there is no imbalance of charge on this molecule.  There is therefore no requirement for it to mix with anything to balance a charge.

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 with each other so that the positive and negative charges can be balanced out.  That’s why we get to 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.

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 – ensure handsome agendas and issues.  It needs to balance its charges. So the water has a problem with the octane because you 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 we can see now why oil and water don’t mix.  Tomorrow, we’ll look at some molecules that kind of fall between these two categories – that is, they are partially hydrophilic and partially hydrophobic, and by their nature these molecules make excellent cleaning compounds.

And then will go on look at the wonderful world of surfactants (detergents) – how they work and how they clean your dishes and your clothes.  Stay tuned