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Why Can’t Oil Be Mixed With Water?



Oil and water just don’t mix - we all know this to be true. The idea has turned into a popular phrase we use to express when two things don’t go together. But why, exactly, are oil and water destined to be enemies? What’s the science? Here’s a look at oil, water, and chemical bonds:

1. Let’s talk chemistry.
Properties of oil and water all come down to chemistry. Water molecules are what is known as polar, which means that the charge on one end is positive, while the other is negative. Oil, on the other hand, is a nonpolar molecule, so it doesn’t have any sort of charge on its ends.



2. We just don’t bond like we used to.
In chemistry, like attracts like. Water molecules will like other polar molecules and be able to bond with them easily. This is why salt dissolves in water - it’s also a polar molecule and can mix together with the water and stick to the molecules. Oil and water don’t mix well because one is polar and the other is nonpolar. Oil isn’t able to break apart the strong bonds that the water molecules have to one another, so it stays separate. Since the water molecules are more tightly bound, they sink to the bottom, and the oil floats on top as a separate layer.



3. They can be forced to get along.
Oil and water may not mix on their own, but there is a workaround. Emulsifiers have one polar end and one nonpolar end. The polar end will attract the water, and the nonpolar one will attract the oil. In this way, you can get them to mix together. In fact, this is exactly how soap works and why it’s able to get oil off your hands when simple water won’t do!
 
Jan 1, 2020
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with oil being much thicker than water- it will carry a bit of weight & so will remain on the top of the water (I think) - theres also molecular reasons as well
 
Jan 6, 2020
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Emulsifiers are kind of like detergents, or soap. The way soap works, is it's a long molecule where one end is polar and the other end is non-polar. Grease and dirt are non-polar, while water is very polar. Polar and non-polar things generally don't mix well, which is why washing grease off your hand with just water doesn't work very well. But if you have a soap or a detergent, the polar end of the soap molecules stick to the polar water, and the nonpolar end of the soap molecules stick to the dirt, therefore bringing the dirt and water together - which is why you can then wash dirt and grease off your hands. The soap acts as a bridge that brings the two things together. Oil and water don't mix for the same reason; oil is very non-polar. But if you add an emulsifier to them, you can get the two to mix in an emulsion. So the emulsifier doesn't chemically change the polar or nonpolar substances, it just acts as a bridge and holds them together.
 
Apr 17, 2020
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The difference between oil and water has generated a large usage of products to bring them together, for example in detergents. These products are called surface active agents, or surfactants for short, because they act at surfaces, for example between oil and water.

Soap is an anionic surfactant, short for surface active agent. The water soluble part is COO- hence the designation anionic.

Fortunately all common surfactants share similar oil soluble parts. They contain between 8 and 18 carbon atoms as CH2 groups. Less than C8 and water solubility is too high and they 'prefer' to stay in the water rather than go to the oil water boundary which, as has been pointed out, is necessary for detergency.
Above C18 the reverse applies.

Anionic surfactants are the biggest tonnage because they are the basis of nearly all cleaning products. They give the emulsion droplets a negative charge. For no particular reason, most common surfaces have a negative charge so this repels the droplets and stops them going back onto the surface, or 'redepositing'.

Cationic surfactants have a positive charge almost always supplied by a nitrogen function such as NH4+. In practice hydrogen is replaced by small hydrocarbon groups like methyl. These give products called 'quaternary ammonium compounds'. These are used where you want the oily material to stick to the surface. When roads were made with bitumen emulsions (an easy way to handle the sticky bitumen) the cationic emulsifier would be pulled out of the emulsion by the negatively charged road surface. This made the road surface 'oily' so the bitumen stuck to it.

Nonionic surfactants are used as emulsifiers in a wide variety of products, including cosmetic creams. They get their water solubility by using a lot of weakly soluble ethylene oxide groups.
They are also used synergistically with anionics in many cleaners.

There is another group of surfactants called amphoterics. They have both positive and negative groups and are now used in large amounts to reduce the irritation of anionics in shampoos and other skin contact products like bubble bath.
They have some remarkable properties and can give excellent cleaning when mixed with anionics in hand cleaners, despite the mildness. They are, however, expensive to produce because of their complexity.

You can see from the above that the intrinsic difference between oil and water has given rise to large industries by needing surfactants to mediate between them. There are all sorts of advantages. Water is cheap and can thin oils when emulsified, giving easier hndling. Cleaning, of course, is a major industry. More recently consumers have demanded less irritant products and surfactants have benn modified accordingly.

Cat
 
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Apr 17, 2020
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Oil is heavier than water
"Oil" is a very broad term but, generally, oil is less dense (lighter for the same volume) than water.

Just go in your kitchen now and pour some cooking oil and some water into a closed bottle. Which is on top?
Give it a good shake (after closing cap) and look again. Which is on top? Consider changing or deleting that post?

Cat :)
 
Apr 17, 2020
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"with oil being much thicker than water- it will carry a bit of weight & so will remain on the top of the water (I think) - there're also molecular reasons as well"

Yes and No. Oil covers a multitude of different types from almos gas to heavy tar or bitumen used on roads. Some are lighter, some are heavier than water. Most are lighter and sit on top.

sarajo, you are correct. There are molecular reasons for this. Excuse me if I make a simple analogy.
Oils are mostly simple hydrocarbons going from gas like butane to liquid like octane. They just consist of carbon and hydrogen. So, if you have started any organic chemistry you know they don't react much. They will burn with oxygen but otherwise are not reactive. Let's say they are like people clasping their own hands and not reacting with other people.
Now consider water molecules. They do interact weakly by what is called hydrogen bonding. Let's say these are like people who can shake hands with other people.
If you now mix a lot of water and oil , people who can shake hands and people who cannot, what will happen? The people who can shake hands will connect together and those who cannot will be excluded, so you will get two groups. You might find, in a room, the 'connecting; people will gather together whilst the others will be separate.
Coming back to oil and water, the water molecules are loosely attached. When they meet oil molecules, they cannot connect. The oil is usually lighter and will go to the top. In this case like attracts like and this bringsabout phase separation. Oil phase and water phase. They separate because they have different ways of connecting.
This leads on to surface tension, but that is another story.
I hope this helps you to understand from whatever level you are at in your study of chemistry.

:)
 
Jan 1, 2020
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Many thanks for that -yes it does help -I only did it at junior & senior high school so my knowledge of chemistry is nowhere near the extrememly highest of levels you are at -Judging by everyones conversations Im the only one who is at such a low level -In general reading everyones comments - & think WOW

Yeah it helps & thanks again
 
Apr 17, 2020
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sarajo
If you or anyone has questions about surfactants, I shall be only too pleased to answer. For years, after I retired from my main job (amphoteric surfactants) my wife and I ran an international training company on nothing but surfactants.
Guess what? Our biggest clients were THE largest detergent producers. Of course, they knew far more than I did about surfactants generally but they said their best people who could have done the training were better employed in their 'proper' jobs. So they employed us!
Anyway, ask away if I can be of any help.
Let me guess. Surface tension is one of the most difficult things to understand when you first meet it.

Cat :)

I should explain my interest now is everything from the Big Bang (and 'before'), from cosmology to astronomy to planetary sciences to the results we walk on (geology). My adopted name Catastrophe comes from asteroid collisions. If there is one piece of advice I can offer to younger generations who are now studying it is read around your subject. I was always number 3 or 4 in a very intelligent class. Looking back, I can see that the top 1 or 2 took the time and effort to go beyond the class notes. I read the 'same' subject from many different sources - Wiki - Encyclopaedia Britannica - you name it". Connect links from different sources.

Sorry to digress somewhat, but I am very interested on the learning process and I found my classmates like oil and water when it came to learning ability. Interest is a great driver.

Cat
 

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