It is difficult to wash an oil spot out of clothing with plain water, because oil is a hydrocarbon that does not dissolve in water. Oil and water actually repel one another, so that oil adheres even more strongly to clothing in the presence of water. The addition of soap or detergent to water changes the situation; soapy water can dissolve oil from clothing and rinse it away. What is special about the structure of soaps that makes them effective cleaning agents for oils and greases?
Most soaps are soluble sodium or potassium salts of carboxylic acids. The most common commercial soap is sodium stearate, Na[C17H35CO2]. It dissolves in water, forming the sodium and stearate ions. Even though most of the stearate ion is a hydrocarbon chain, it dissolves in water because of the carboxylate group. The carboxylate end is called hydrophilic (water-loving), and the hydrocarbon tail is called hydrophobic (water-fearing).
What is unique about the polarity(charge characteristics) of soap?
It is the long hydrocarbon chains of the stearate anions that dissolve the oils and greases. If water containing dissolved soap is mixed with oil, the hydrocarbon chains strongly attract the oil, while the ionic ends keep the soap dissolved into water. The oil spot is broken up into small droplets and dispersed into the water. The "tails" of many soap anions are needed to remove each oil droplet.
While the sodium salt of stearate ions and the anions of other soaps are soluble in water, the calcium and magnesium salts are not. Hard water contains these metal cations, so the metal salts precipitate, reducing the oil-dissolving efficiency of the soap. "Bathtub ring" originates from the precipitation of soap by hard water. Thus, soaps do not clean well in hard water until most of the metal cations have been precipitated by reacting with the soap. In recent years, this problem has been solved by replacing soaps with detergents, which are generally compounds with long hydrophobic tails and the charged sulfate group such as sodium dodecyl sulfate, Na[CH3(CH2)11OSO3]. The calcium and magnesium salts of detergents generally remain soluble in water.
While the sodium salt of stearate ions and the anions of other soaps are soluble in water, the calcium and magnesium salts are not. Hard water contains these metal cations, so the metal salts precipitate, reducing the oil-dissolving efficiency of the soap. "Bathtub ring" originates from the precipitation of soap by hard water. Thus, soaps do not clean well in hard water until most of the metal cations have been precipitated by reacting with the soap. In recent years, this problem has been solved by replacing soaps with detergents, which are generally compounds with long hydrophobic tails and the charged sulfate group such as sodium dodecyl sulfate, Na[CH3(CH2)11OSO3]. The calcium and magnesium salts of detergents generally remain soluble in water.
How does soap work?
Nearly all compounds fall into one of two categories: hydrophilic ('water-loving') and hydrophobic ('water-hating'). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic compounds just don't mix.
The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).
When grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution, the soap molecules work as a bridge between polar water molecules and non-polar oil molecules. Since soap molecules have both properties of non-polar and polar molecules the soap can act as an emulsifier. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. The soap will form micelles (see below) and trap the fats within the micelle. Since the micelle is soluble in water, it can easily be washed away
Nearly all compounds fall into one of two categories: hydrophilic ('water-loving') and hydrophobic ('water-hating'). Water and anything that will mix with water are hydrophilic. Oil and anything that will mix with oil are hydrophobic. When water and oil are mixed they separate. Hydrophilic and hydrophobic compounds just don't mix.
The cleansing action of soap is determined by its polar and non-polar structures in conjunction with an application of solubility principles. The long hydrocarbon chain is non-polar and hydrophobic (repelled by water). The "salt" end of the soap molecule is ionic and hydrophilic (water soluble).
When grease or oil (non-polar hydrocarbons) are mixed with a soap- water solution, the soap molecules work as a bridge between polar water molecules and non-polar oil molecules. Since soap molecules have both properties of non-polar and polar molecules the soap can act as an emulsifier. An emulsifier is capable of dispersing one liquid into another immiscible liquid. This means that while oil (which attracts dirt) doesn't naturally mix with water, soap can suspend oil/dirt in such a way that it can be removed. The soap will form micelles (see below) and trap the fats within the micelle. Since the micelle is soluble in water, it can easily be washed away
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