Why nonpolar and polar molecules Cannot mix?

Whether two substances mix or not is often determined by how polar or non-polar the two respective substances are. Polar substances have, due to the shape and nature of the molecules they are made of, regions with varying electric charge. Like batteries that have positive and negative poles (see picture), sometimes molecules have parts that are negative or positive in charge. Polar molecules are molecules that have ends that are either positively or negatively charged. Non-polar substances have uniform, neutral electric charge in all parts of the molecule. Non-polar molecules do not have positive or negative regions. 

Polar Molecules - 

The molecular structure of some molecules results in uneven distribution of shared electrons. Recall that electrons have a negative electric charge, while protons have a positive charge. In a part of a molecule that has a higher concentration of electrons, an increase in negative charge  would occur. It is also important to consider what it would mean for other parts of the molecule if one area has a higher concentration of electrons. This would mean that some other part of the molecule would have a lower concentration of electrons, and a higher concentration of positive electric charge from the protons of the atoms. Take water for example. Because of the uneven distribution of the shared electrons, the oxygen side of the molecule has a slight negative charge, while the hydrogen side of the molecule (the side that doesn't get as much time with the electrons) has a slight positive charge. When multiple water molecules interact, the positive part of one water molecule is attracted to the negative side of another water molecule. This +/- attraction occurs throughout a sample of water (whether the sample is in a single drop of water or in the largest ocean) and results in a process called cohesion. Cohesion is when particles of the same substance stick together. 

Mixing polar substances - 

Because polar molecules have both negatively and positively charged regions, they tend to mix well with other polar molecules. Vinegar, also known as acetic acid, is a solution. Most vinegar for sale in grocery stores is about 4% acetic acid. Depending on the kind of vinegar, there will be traces of various other substances that provide subtle (or sometimes bold) flavor variations, but typically the remaining 96% is mostly water. Water is a polar solvent, and the acetic acid is a polar solute. The resulting solution is, by default, a polar solution. The charged parts of a polar solute are attracted to the oppositely-charged polar parts of a polar solvent.

Non-Polar Molecules - 

Some molecules do not have uneven distribution of electrons. Because the electrons and protons are distributed uniformly (evenly), there is no one part of the molecule that is more positively or negatively charged than any other part of the molecule. This lack of variable charge means that such a molecule would be non-polar. Non-polar molecules have uniform electron distribution and have no regions of positive or negative charge.

Mixing Non-Polar Substances - 

Non-polar solvents dissolve non-polar solutes well. Octane is a molecule that is part of a class of compounds known as hydrocarbons. If you have heard of octane before, it's probably because it is a component of a solution that you probably know as gasoline. Gasoline is a solution that contains a variety of petroleum-based compounds with different lengths which are determined by the number of carbon atoms in their carbon backbones. Octane has 8 carbon atoms. According to Elmhurst.edu, gasoline contains about 500 different carbon-based compounds. The non-polarity of these assorted compounds allows them to mix and be a homogeneous solution.

Lipids (more commonly known as fats), such as those found in vegetable oils consist of a glycerol backbone and three attached fatty acids. Because of the relatively even distribution of electrons around the perimeter of these molecules, there are no positively or negatively charged regions. These molecules are also non-polar. 

Mixing Polar and Non-Polar Substances - 

...and so we find ourselves back to vinegar and oil. As discussed earlier, vinegar is a polar solution of acetic acid and water, with a few other compounds present in varying amounts which produce the flavor nuances that make different vinegars unique. 

The olive oil you see in this picture consists of a range of typical lipids, with glycerol backbones and attached fatty acids. The olive oil is non-polar compound. If you have ever tried to mix  oil and vinegar you probably know that they simply don't mix. 

There is cohesion in polar substances (remember that cohesion means something sticking to itself - like water sticking to water), and a similar cohesion in non-polar substances. However, this attraction does not occur between polar and non-polar substances. Polar and non-polar substances repel each other to some degree. Because of the differences discussed above, they simply do no mix. 

But what about soap?

Maybe you've seen dish soap commercials talking about grease cutting power. Dawn, or Palmolive, or whatever other brand of soap tries to convince you that they have the perfect formula to cut through that tough grease... maybe without scrubbing. Grease simply dissolves into the water when the magical soap is applied. How can this true? Water is polar. Grease is just another name for fat, and fat is non-polar. It sounds too good to be true!

Soap molecules are something special. One side of a soap molecule is a fatty acid (think back to the structure of lipids as shown above), and fatty acids are non-polar. The other end of a soap molecule is polar. The polar end of the soap molecule attracts water molecules, while the non-polar portion of the molecule attracts the non-polar fat molecules. This allows grease on pots and pans to break apart and be rinsed down the drain. It's a polar/non-polar interaction miracle!

Soap acts as a bridge between polar and non-polar substances. By having the properties of both polar and non-polar substances, soap allows us to use a polar solvent to remove non-polar materials from our pots, pans, plates, clothes, hands or wherever else they may be. 

What makes some liquids mix together, like water and methanol, but others like water and oil, do not?To investigate, we experimented in class with 7 solvents and tested to see whether certain combinations would mix together. We used;

• Water
• Methanol
• Pentanol
• Glycerol
• Hexane
• Kerosene
• Iodine

I've colour coded the observations to make it easier to read - green text for those that mixed together, red for those that did not, orange for those that showed some signs of mixing but only slightly, and pale brown for the test that I would like to repeat again in the future to confirm.

So before we get to why some things dissolve in others, let's examine the structures of each of these solvents in a bit more detail, and see if they have any clues within them as to why they mix or not.

Let's start with water. Water is a bent molecule comprising Oxygen and Hydrogen. What gives water most of its interesting properties is because it is so very polar. Polar means an opposite character or tendency - in this case we are talking about charge - a positively charged pole, and a negatively charged pole. The oxygen atom is much more electronegative than the hydrogen - this means that oxygen has a greater attraction for electrons than hydrogen has. This will cause an unfair sharing of electrons between them across the covalent bond - most of the time the electrons will be slightly closer to the oxygen than the Hydrogen. As a result, the Oxygen will be slightly negative, and the Hydrogen will be slightly positive, and since water has one of these polar bonds on each side (and they don't cancel each other out) - the overall molecule has a permanent dipole.

This dipole enables the water molecules to stick together with other water molecules by the electrostatic attraction between the slightly positively charged pole of the Hydrogens, and the slightly negatively charged poles of the oxygen. These are Hydrogen bonds - which are special cases of dipole-dipole intermolecular forces, that only occur when Hydrogen is covalently bonded to strongly electronegative elements like Fluorine, Oxygen, and Nitrogen. In those cases, the Hydrogen's nucleus becomes unshielded, and can allow other polar molecules to come closer - resulting in a stronger bond. A Hydrogen Bond.

Methanol (top) and Pentan-1-ol (bottom)

Let's look at Methanol and Pentanol. From each of these structures we can see that they too, have a polar Oxygen-Hydrogen bond, which looks similar to what water has, but the other side have Carbon-Hydrogen bonds, which are very weakly polar since the electronegativity differences between Carbon and Hydrogen is quite small. We usually consider hydrocarbon chains to be non-polar when discussing solubility, and so overall Methanol and Pentanol are hybrids - they have a polar group on one end, and a non-polar group on the other.

Glycerol has three polar Oxygen-Hydrogen bonds but these are attached to hydrocarbon chains which are relatively non-polar.

Hexane C6H14

Hexane is a 6 membered hydrocarbon chain, and since Carbon-Hydrogen bonds are weakly polar, we consider the Hexane molecule to be relatively non-polar.

Kerosene C12H26 up to C15H32

Kerosene is similar to Hexane in polarity. It's non-polar, but has more of a tree-like structure.

Iodine is a non-polar molecule, with only one bond - with itself - and so there cannot be any unfair sharing of electrons to any degree, so it is completely non-polar.

When it comes to explaining or determining solubility of solvents, we use a helpful phrase that goes like this: Like dissolves Like. This means that polar molecules will dissolve polar molecules, and non-polar molecules will dissolve with non-polar molecules. The reason why this occurs is because polar molecules can undergo dipole-dipole force attractions - these are simply the electrostatic attractions between the slightly positive and slightly negatively charged poles of each molecule. Non-polar molecules can't do this because they don't have poles, instead they attract each other via dispersion forces. These are the weakest forces between molecules, where all the protons of one molecule can weakly attract the electrons of a neighbouring molecule, and vice versa. The problem is that most molecules are moving too fast for these molecules to have a chance to draw each other in, but if the molecules are sufficiently long enough in length, then the contact area is increased which allows for stronger attraction to occur.

Let's look at some examples from our observations table: Methanol and Water underwent complete dissolution - this agrees with the theory that polar molecules dissolve in other polar molecules. Similarly with Pentanol and Methanol. In fact, I have colour-coded the upper left half of the table in blue to denote the polar molecules, and orange in the lower right to denote the non-polar molecules, and they seem to agree with the the rule except for some exceptions.

Water and PentanolWater and pentanol have a lower solubility than expected. Why? Although both solvents have polar groups attached, the proximity and strength of the hydrogen bonds between water molecules will be much greater than when pentanol is in the mix with its long non-polar hydrocarbon tail interrupting the hydrogen bonding of the water molecules surrounding it. The water molecules will expel the hydrocarbon tail out of the water as a result, but keeping the polar Oxygen-Hydrogen bond close by to partake in hydrogen bonding. This will result in fewer molecules of Pentanol being able to dissolve.

Glycerol and Pentanol

Iodine and Glycerol

Iodine and Pentanol

Glycerol and Hexane