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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
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
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.
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Apr 17, 2020
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.

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Apr 17, 2020
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
"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
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
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.

Mar 19, 2021
Yep, I have also tried it but when you add baby shampoo to it it will change the color as well. Have a try. It will be a new thing for you.
Here's a bit of information on how to mix oil and water and the various products produced in the processes.

An emulsion is a temporarily stable mixture of immiscible fluids, such as oil and water, achieved by finely dividing one phase into very small droplets. Common emulsions can be oil suspended in water or aqueous phase (o/w) or water suspended in oil (w/o). There also can be more complex systems, such as oil in water in oil (o/w/o).

Familiar foods illustrate examples: milk is an oil in water emulsion; margarine is a water in oil emulsion; and ice cream is an oil and air in water emulsion with solid ice particles as well. Other food emulsions include mayonnaise, salad dressings, and sauces such as Béarnaise and Hollandaise.

Most emulsions require the use of functional chemicals, called emulsifiers, to stabilize the suspension of small droplets and prevent them from coalescing or coming together to grow larger droplets. The driving force for coalescence is the reduction of interfacial area*, which reduces the thermodynamic energy level of the system. Emulsifiers form physical barriers to prevent droplets from coming together.

The Quadro Ytron® Z emulsifier ensures consistency and control of high-shear process applications.

Food emulsifiers have much in common with detergents in that both classes of chemicals have water-loving and oil-loving (or attracting) regions on the same molecule. The water-attracting portion often is ionic and is described as hydrophilic. The oil-attracting, lipophilic end is often a long-chain hydrocarbon region such as a fatty acid.

There are both natural and synthetic emulsifiers. Lecithin is a phospholipid molecule found in soy and isolated in refining of soy oil. It is an effective and popular food emulsifier. Egg yolk contains two emulsifiers—lecithin, which promotes oil in water emulsions, and cholesterol, which promotes water in oil emulsions. Egg yolk is the traditional emulsifier for mayonnaise and other culinary sauces, but because of its dual functionality, these products can be tricky to make successfully.

Mayonnaise is normally made at room temperature because the oil phase is usually vegetable oil, but other sauces may require mild heating because the oil phase is often butter, which is solid at room temperature. Heating risks irreversibly denaturing the egg yolk (think scrambled eggs) and so must be carefully controlled.

Emulsifiers are characterized by their hydrophilic lipophilic balance (HLB), a number from 1 to 20 that indicates which tendency is more dominant. An HLB less than 6 favors water in oil emulsions; a value greater than 8 favors oil in water emulsions. Values of 7– 9 indicate good wetting agents.

Other common emulsifiers found in foods include proteins, gums, and various esters of fatty acids and poly hydroxyl substrates, such as lactic acid, sucrose, and polysorbates. The mono- and di-glycerides are food emulsifiers made by transesterification followed by molecular distillation. They have different properties depending on which specific fatty acids are included.

The same chemicals that are good emulsifiers are often used in other ways in foods. For example, stearoyl lactylates and mono- and di-glycerides can ****** the process of going stale in baked goods by interfering with starch retrogradation. In chocolate, emulsifiers reduce viscosity, permitting a reduction in the amount of cocoa butter, which reduces both cost and calories. Emulsifiers in cake batter promote better release of cake from pans.

Sucrose esters have been used as fat replacers, including in frying of snacks, to reduce calories. Unfortunately, in that use, the nondigestible fat replacers are said to have some unpleasant effects on one’s digestive system.

In addition to emulsifiers, other factors that affect the stability of emulsions include the viscosity of the continuous phase (higher is better), the droplet size (smaller is better, typically 1–10 μm), and the difference in densities between the two phases (smaller difference is better). Some ingredients can affect viscosity; densities are usually pretty well fixed; so droplet size becomes the one processing variable that can be manipulated.

Emulsification involves making small droplets and having them adequately coated with the appropriate emulsifier. Making small droplets requires adding energy to create a large interfacial area. In the kitchen, this may entail vigorous beating or whisking by hand or by a mixer. In a food plant, the process may be batch or continuous using specialized equipment.

In addition to delivering energy properly, the order of addition of ingredients is critical. The correct procedure is to prepare the continuous phase, including the emulsifier, first. Then the dispersed phase is added slowly with vigorous agitation. Mayonnaise is a good example. Theoretically, uniform spheres can be densely packed to occupy 74% of a given volume. In mayonnaise, oil can be more than 74% of a formula, meaning that while it is the dispersed phase, the continuous phase is a thin film around the many oil droplets. It also means, since egg yolk is the traditional emulsifier, that the mixture runs a risk of inverting, under the influence of the cholesterol unless the mixing is carefully controlled. Specifically, oil must be added slowly so that the lecithin can thoroughly coat the small droplets.

High or low temperatures can destabilize emulsions, so they are not normally frozen. Low temperatures may harden the fat phase, while high temperatures can cause droplets to collide energetically enough to coalesce.

In general, emulsification equipment delivers high shear to the dispersed phase to form small droplets. One approach is the immersion mixer in which a rotor spins at high speed inside a relatively tight cage that has slots or other shaped holes. Fluid is pulled into the cage and expelled through the openings. Additional agitators may be used in a vessel to promote circulation of the mixture so all portions are treated.

Colloid mills are devices in which two plates form a narrow passage through which fluid passes. One or both plates may rotate, and there may be interlacing pins or other shear-inducing features. Usually the clearance can be adjusted and cooling may be applied for temperature control. Depending on the effectiveness of the machine for a given system, one pass may be sufficient, or the mixture may be recirculated for multiple passes.

APV’s Rannie Gaulin homogenizer has a wide range of applications including food and dairy.

A homogenizer is a high pressure pump that forces a fluid through a restrictive valve or two to induce shear. Homogenizers are common in dairy processing and often are incorporated in a pasteurization system where they may also serve as a timing pump. As previously mentioned, fluid milk is an oil in water emulsion after being homogenized. Before that, the fat phase or cream is easily separated. A typical dairy process separates milk into skim milk and cream, then recombines these to make products of the desired fat content, including full-fat milk (3.5% fat), reduced-fat milk (1% or 2%), and skim milk (0%). Yogurt, cheeses, and ice creams of varying fat content are made the same way, by recombining the components. Excess cream is made into butter.


* interfacial area - is the total area of contact between two liquids in a liquid-liquid operation.
Smaller bubbles afford more surface area per unit volume thus increasing the interfacial area for mass transfer.

Liquid-liquid reactors are agitated in order to achieve a good dispersion and a large interfacial area between two immiscible liquids.
The interfacial area is the total area of contact between two liquids in a liquid-liquid operation.


There are proprietary devices from such firms as APV (, Silverson (, and Quadro ( that can emulsify continuous streams. Another approach is the use of ultrasonics, in which a metal tool vibrates at a high frequency to introduce shear to a flowing fluid. A membrane emulsifier forces the dispersed phase through small pores into a flowing continuous emulsified phase.
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