The natural world is full of mixtures, but sometimes we actually seek to separate them into their components. Toward this goal, scientists have developed advanced separation techniques, enabling us to refine crude oil, desalinate water, and even extract cellular DNA
The phrase ‘like oil and water’ refers to things that do not mix, just like, well – oil and water, whose chemical properties prevent their mixing. But in many cases, what we really need are ways to achieve the opposite – separate mixed substances.
Crude oil is a good example: It is a mixture of numerous carbohydrates (compounds that consist of hydrogen and carbon), some of which are used to power engines as fuel, and others in the manufacturing of plastics or other compounds. A distillation process separates between them. Alongside distillation, there is a range of separation techniques – some physical, others chemical. The former are based on the physical properties of materials, i.e., properties that can be observed or measured without changing the identity of the material (for example, its state of matter), while chemical methods are based on chemical properties – which determine substance identity.
Physical separation methods
Physical separation methods, such as filtration and distillation, are based on the difference in particle size and on the boiling point of the substances in the mixture, respectively. These are properties that can be observed and measured without changing the material itself.
This is the simplest separation method, which can be conducted using basic household equipment. In daily life, it is applied in the separation of solids from the liquids in which they were cooked – pasta, for example.
Filtration is based on the fact that the smaller water particles can easily pass through the filter, which retains the larger solid particles. When straining pasta, our interest lies in the solids, but in other cases, it is the liquid that we seek, so we need to remove the larger solids, keeping those in the filter. The filter hole size determines which materials are retained in the filter and which pass through it.
Sterilization is an example of an industrial application of filtration. To remove a bacterial contamination from a solution, the solution is passed through a filter. The microorganisms are trapped in the filter, while the smaller solution molecules pass through. Also called microfiltration (because of the relatively small size of the molecules), this process is used for sterilization when other techniques, such as heating or ultra-violet radiation, are inapplicable due to their potential harmful effects on the solution’s molecules.
To filter out even smaller molecules, which do not exceed 100 nanometers in size (less than one thousandth of a millimeter), a process known as nanofiltration is used. The main feature of this process is not pore size but the electrical forces at work between the molecules, whose significance increases as the size of the molecule decreases. An example of nanofiltration is reverse osmosis, used for desalination. In reverse osmosis, a selective membrane, which allows water to pass through it but not the dissolved particles, is used. Pressure is applied to reverse the natural flow of the water, forcing them to move from the more concentrated solution to the weaker. Thus, salts and contaminants are concentrated on one side and clean water – on the other.
In distillation, components of a liquid mixture are separated on the basis of the differences in their boiling temperatures. In its simplest form, distillation involves boiling the mixture and collecting the vapor in a condensation vessel, where it turns back into a liquid. The first vapors to appear are those of the substance whose boiling point is the lowest. As the boiling continues, the vapors of the next substance are captured in a different condensation vessel, and so on.
To find out when one component finished boiling and another began, the mixture’s temperature must be constantly monitored. During the boiling of one substance, the mixture’s temperature will be ‘stuck’ at that material’s boiling point, and then will continue to rapidly increase.
Crude oil distillation is based on this principle, which constitutes the first step of separation, whereby oil is separated into fractions – groups of substances with different boiling temperature ranges. Then chemicals are added, with the purpose of improving the quality of the distillates.
The primary fractions produced by the initial distillation of crude oil are naphtha, used to create benzene for car engines and as a raw material for the plastic industry; kerosene, which is used to create fuel for jet engines; diesel fuel used to power diesel engines, and mazut, used as power station fuel.
The refineries in Haifa in the early 1950s. Photography: Wikipedia.
Chemical separation methods
Chemical separation methods, such as extraction and chromatography, are based on chemical interactions between the mixture’s components, or, to be more precise – the differences in the strengths of the interactions of the different materials in the mixture with another material which is not a part of it. Naturally, these interactions are based on the chemical properties of the materials to be separated.
In extraction, a solute is extracted from its solvent. In liquid extraction, two immiscible (non-mixing) liquid phases are used. An organic substance (i.e., a carbon-based compound) dissolved in an aqueous phase moves to an organic phase from which it is extracted, or vice versa – the substance moves from the organic phase to the aqueous phase, from which it is extracted.
An example of an extraction process is the separation of DNA from proteins in a biological sample. In the process of extraction, an aqueous biological sample is mixed with two organic compounds – phenol and chloroform, and then the entire mixture is centrifuged to separate the aqueous phase from the organic phase. The DNA remains in the lighter aqueous phase, while the proteins move to the heavier organic phase.
Chromatography is an analytical chemistry method that enables not only to separate compounds, but also to identify them and determine their quantities. It is therefore an important tool in scientific research, the pharmaceutical industries, and forensics. The word comes from chromo, or “color,” and graphy, or “writing,” since it was originally developed by Mikhail Tsvet, a Russian-Italian botanist who studied chlorophyll, which gives leaves their green color. When separating a mixture of plant pigments he extracted, he noticed individual colored bands characteristic of each of the components. Today, this method is used for the separation of materials unrelated to their color.
All types of chromatography are based on the same principle: Separating substances in a mixture dissolved in a liquid or a gas, or which is itself a liquid or a gas, according to their chemical interactions with the surface or column used for the chromatography. This surface is termed ‘the stationary phase,’ while the liquid or gas in which the mixture is dissolved is termed ‘the mobile phase.’ Because the strength of the interactions between the different components in the stationary is different than that of the mobile phase, the components will travel along it at varying speeds, reaching different points along the stationary phase and can thus be separated.
A graphical representation of chromatographic separation between materials – each peak represents a different component. Source: Wikipedia.
Two types of surface chromatography in frequent use are paper chromatography and TLC – thin layer chromatography. In paper chromatography, the stationary phase is a thin paper strip made of cellulose, on which a tiny drop of the mixture is applied. The paper is dipped in a liquid such as water or ethanol, which ‘climbs’ up the paper because of capillary force. When the liquid reaches the mixture, it ‘drags’ with it the mixture’s components. Mixture components that formed strong interactions with the paper will climb up the paper slowly, while substances that formed weak interactions with the paper will climb rapidly. The end product is a sequence of dots, where each dot represents a different component of the mixture.
Thin layer chromatography is based on a similar principal, but in this case, the stationary phase is made of glass, aluminum paper, or plastic coated by an absorbing material, such as silica or aluminum oxide. This method is faster and more accurate than paper chromatography, and is commonly used in organic chemistry to evaluate the purity of a material and for the initial testing of component quantities in a mixture.