The Combination Of Water And Oil Biology Essay


We have all tried to mix water and oil together, but our results of this experiment are always that oil floats on top of water. This happens because of the physical and chemical properties of water and oil that tend to interfere with their ability to mix completely not allowing for a homogenous mixture to form. Water (H2O) is a very unusual substance with many strange and unique properties that are so important to life on our planet. It is a special substance consisting of two atoms of hydrogen and one atom of oxygen. Two other features of the water molecule are also important for its properties: the small size of the molecule and that the molecule is strongly dipolar. This property makes water an effective solvent, particularly for crystalline salts. The small size of hydrogen atoms makes it possible for molecules of water to effectively bond together or chemically associate, particularly at lower temperatures. Now when it comes to substances that dissolve in water, it's a different story. Although a water molecule has an overall neutral charge (having the same number of electrons and protons), the electrons are asymmetrically distributed, which makes the molecule polar. The oxygen nucleus draws electrons away from the hydrogen nuclei, leaving these nuclei with a small net positive charge. So, when a highly polar substance, such as water, is mixed with a nonpolar or weakly polar substance, such as most oils, the substances will separate into two phases.

Mediators between Water and Oil

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Is there a way to break the rule and actually mix water and oil together? There actually is. Using such a simple substance such as instant mashed potato powder whose emulgator acts as a "mediator" between water and oil. A mediator refers to a substance acting as a medium in transferring something from one place to another. Emulgators are comprised of both a hydrophilic and a hydrophobic group. The hydrophobic portion has an attraction for oils (or fats) and the hydrophilic portion has an attraction for water.

The terms hydrophobic and hydrophilic refers to molecules that react differently when present with water. Hydrophilic ('Water Loving') and hydrophobic ('Water Hating') molecules are greatly different. Substances that dissolve readily in water are termed hydrophilic. They are composed of ions or polar molecules that attract water molecules through electrical charge effects. Water molecules surround each ion or polar molecule on the surface of a solid substance and carry it into solution. Ionic substances such as sodium chloride dissolve because water molecules are attracted to the positive (Na+) or negative (Cl-) charge of each ion. Polar substances such as urea dissolve because their molecules form hydrogen bonds with the surrounding water molecules.

Molecules that contain a preponderance of nonpolar bonds are usually insoluble in water and are termed 'hydrophobic'. This is true, especially, of hydrocarbons, which contain many C-H bonds. Water molecules are not attracted to such molecules as much as they are to other water molecules and so have little tendency to surround them and carry them into solution. But the so-called 'Hydrophobic Effect' does not mean that nonpolar molecules are not attracted to water.

It is commonly believed that individual water and oil molecules repel each other, or at least attract each other very weakly. However, this is clearly wrong and misleading! In fact, an individual oil molecule is attracted to a water molecule by a force that is much greater than the attraction of two oil molecules to each other. This can be demonstrated when a drop of oil is placed onto a clean surface of water. Originally, the oil will be in the shape of a spherical droplet, because the oil molecules are attracted to one another and a spherical shape minimizes the number of oil molecules that are not surrounded by other molecules. When the oil droplet hits the surface of the water, it spreads out to form a thin layer. This happens because the oil and water bonds formed by the oil forming a layer on the surface of the water are stronger than the oil-oil attraction in the oil droplet. If a sufficiently small drop of oil is put on the surface, it will spread to form a single molecular layer of oil.

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Given these strong interactions, why doesn't each oil molecule dive into the water solution and become completely surrounded with water molecules? The reason is that the water-water bonds are much stronger. Displacing the water molecules would cost more energy. Therefore most of the oil molecules stay out of the water, though as many as will fit will hang on to the surface water molecules that do not have a full match of partners. A similar explanation applies for the meniscus that is the curved surface of a liquid in a graduated cylinder or any other small diameter glassware. Water adheres to the sides of any container creating a "cup" of surface tension.

The structure produced through the interaction with water molecules is very important as it is related to the structure and function of membranes which are very characteristic of life as we know it. Membranes in bacteria are composed of phospholipids and proteins. Phospholipids contain a charged or polar group (often phosphate) attached to a 3 carbon glycerol back bone. There are also two fatty acid chains dangling from the other carbons of glycerol. The phosphate end of the molecule is hydrophilic and is attracted to water. The fatty acids are hydrophobic and are driven away from water.

Because phospholipids have hydrophobic and hydrophilic portions, they do remarkable things. When placed in an aqueous environment, the hydrophobic portions stick together, as do the hydrophilic bits. A very stable form of this arrangement is the phospholipid bilayer. This way the hydrophobic parts of the molecule form one layer, as do the hydrophilic. Lipid bilayers form spontaneously if phospholipids are placed in an aqueous environment. The cytoplasmic membrane is stabilized by hydrophobic interactions between neighboring lipids and by hydrogen bonds between neighboring lipids. Hydrogen bonds can also form between membrane proteins and lipids. These are known as membrane vesicles and are used to study membrane properties experimentally. There is some evidence that these structures may form abiotically (not containing or supporting life) and may occur on particles that rain down on earth from space.

One of the great truths of life, that oil and water do not mix, has been turned on its head. The secret to making them mix without chemicals, according to Ric Pashley, a chemist at Canberra's Australian National University, is extracting all the dissolved air from the water."It makes an emulsion, not quite as cloudy as milk," the chemist said. The discovery, which could lead to everything from new medicines to paints and perfumes, has delighted scientists around the world. In 1982, Professor Pashley discovered something called long-range hydrophobic force, now accepted as the reason oil and water do not normally mix. He explained that oil droplets can attract each other over a distance as large as their own radius. As a result, oil droplets merge rather than disperse in water.

In his experiments, a typical liter of water contains about two milliliters of dissolved air. Suspecting that was the problem, he extracted 99.999 per cent of the dissolved air from some water. To his joy, it mixed with oil, forming an emulsion that did not separate.

Experiment Plan:

For our experiment we will be using instant mashed potato powder, paprika, oil and water to try to mix the substances together. In order to mix the substances, 2 containers of equal size should be used. The initial plan is to conduct the experiment several times, observing the results and recording them in a way that will make it easy to view and understand. We will also use a different a substance as a mediator, to try to compare their different results.

What will happen once we add the substances is that they will mix completely, but how does that happen?

The insolubility of oil in water is due to the fact that oil is non polar and hydrophobic; oil is also less dense, so it will be in the upper layer. Paprika is soluble in oil and here it is used to indicate color. The emulgator also called "emulsifier" of the instant mashed potato powder acts as a "mediator" between water and oil. Due to the amphiphilic nature, these molecules are adsorbed at the interface or the boundary between oil and water. Thus the value of the interfacial tension is decreased enough that the two fluids can mix.

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The emulgator in instant mashed potato powder consists of mono- and diglycerides of fatty acids. Mono- and diglycerides are made from oil, usually soybean, cottonseed, sunflower, or palm oil, act as emulsifiers In the case of a diglyceride the two fatty acid "tails" on the molecule are hydrophobic (lipophilic). The glycerin backbone is hydrophilic.

Emulsifiers are used in situations where fats or oils must form for a certain time a stable mixture in aqueous media. Therefore they are a component of foods, detergents, and cosmetics. Lecithin is often used as an additive in such processed foods as ice cream, margarine, and salad dressings, because it helps blend or emulsify fats with water. Lecithin is a natural emulgator. Pure lecithin is white and waxy and darkens when exposed to air. Commercial lecithin is brown to light yellow, and its consistency varies from plastic to liquid.

Among the products in which it is used are animal feeds, baking products and mixes, chocolate, cosmetics and soaps, dyes, insecticides, paints, and plastics. The word "lecithin" is derived from the Greek term "lekithos", which means "yolk of an egg", where it was first discovered. But soybeans are the most important source of commercial lecithin.

With lecithin's hydrophobic and hydrophilic portions, it can simultaneously interact with both oil and water, making it an effective emulsifier. The hydrophobic "tail" contains two long hydrocarbon chains of two fatty acids. The hydrophilic head consists of carbonyl groups, a glycerol bridge and a phosphatidylcholine region.