It would be bizarre to say that someone is unfamiliar with soap. This is because soap is an essential part of our daily activities and everyday life.
But have you ever thought, as a chemistry student, whether soap is polar or a non-polar chemical compound? If yes, and you are looking for an answer to this question, then this article is for you.
So let’s find out; is soap polar or non-polar? and what is the chemistry behind it!
A soap contains both polar and non-polar ends. It is thus an amphiphilic chemical compound i.e.; it is both polar and non-polar at the same time.
Chemically, soap is a salt of a fatty acid ester. It thus possesses a hydrophobic (non-polar) alkyl chain and a hydrophilic (polar) anionic end.
It is due to these two contrasting functional group entities that soap develops an affinity with water (a polar solvent) for cleaning non-polar fat and oil stains.
Let’s discuss the chemical composition of soap in detail in order to understand its dual polarity nature.
What is the chemical composition of soap?
As discussed above, soaps are salts (usually sodium or potassium salts) of long-chain fatty acid esters.
Fatty acids are organic acids composed of a carboxyl (COOH) functional group, covalently bonded to a long alkyl chain.
The alkyl chain can be both saturated (containing all C-C single bonds) or unsaturated (containing C=C double and triple bonds in between C-C single bonds). A long-chain fatty acid is usually composed of 12-26 carbon atoms.
For instance, oleic acid (C17H33COOH) is an example of a long-chain fatty acid containing a total of 18 carbon atoms. Other examples of long-chain fatty acids are linoleic acid (C17H31COOH) and palmitic acid (C15H31COOH).
Fatty acid esters are formed by the esterification reaction of the above fatty acid molecules with polyols (alcohols containing multiple O-H functional groups).
Triglyceride is a fatty acid ester formed by the condensation/esterification of three fatty acid molecules with a glycerol molecule.
An ester O-C-O functional group is formed by the loss of 1 H2O molecule. The formation of 3 O-C-O functional groups (as shown below) results in a triglyceride molecule.
The hydrolysis of the given triglyceride in the presence of a base (NaOH or KOH) forms the corresponding salt i.e., soap. This chemical process of soap formation is known as saponification.
Shown below is the structure of sodium palmitate (C15H31COO–Na+), which represents soap.
The structure displays a long alkyl chain i.e., hydrophobic (water-repelling) and a carboxylate functional group which is hydrophilic (water-loving).
The following three factors mainly influence the polarity of a molecule:
Electronegativity
Dipole moment
Molecular shape or geometry
Now, let’s discuss how in light of the above three factors, sodium palmitate (soap) is both polar as well as non-polar.
Factors affecting the polarity of soap
Electronegativity
Electronegativity is defined as the ability of an elemental atom to attract a shared pair of electrons from a covalent chemical bond.
The greater the electronegativity difference between bonded atoms in a molecule, the higher the bond polarity.
As per Pauling’s electronegativity scale, covalent bonds are polar if the bonded atoms possess an electronegativity difference between 0.4 to 1.6 units.
A soap molecule consists of three different types of covalent bonds i.e., C-C bonds, C-H bonds, and C-O or C=O bonds. It also has an O–Na+ bond which is purely ionic in nature.
Ionic bonds are always polar as the bonded atoms possess high electronegativity differences (> 1.6 units). In this case, O– Na+ is extremely polar, with an electronegativity difference of 2.51 units between the sodium (E.N = 0.93) and oxygen (E.N = 3.44) atoms.
The C-O and C=O bonds are also polar, with an electronegativity difference of 0.89 units between the covalently bonded carbon and oxygen atoms.
Polar C-O, C=O, and O–Na+ bonds make the ‘head’’ of a soap molecule extremely polar.
Contrarily, the long alkyl chain (tail) of this molecule is extremely non-polar, taking into account the zero electronegativity difference between identical carbon atoms (in a C-C bond) and the very small (0.35 units) electronegativity difference between a carbon and a hydrogen atom (in a C-H bond).
Therefore, a soap molecule is comprised of both polar and non-polar bonds.
Dipole moment
Dipole moment (µ) is defined as the product of charge (Q) and charge separation (r).
The dipole moment of the polar bond points from the positive center to the center of a negative charge.
In soap (sodium palmitate), the dipole moments of polar C-O and C=O bonds point from Cδ+ to Oδ–.The dipole moment of the polar ionic bond (O–Na+) also points from Na+ to O– (as shown below).
The dipole moment (measured in Debye, D) of an ionic bond is always greater than that of a covalent bond.
Contrarily, the alkyl chain (non-polar end of the soap molecule) possesses negligible dipole moment values.
Molecular shape
In C15H31COO–Na+, a total of four atoms surround each C-atom in the long chain; thus, the shape of the molecule is symmetrical, tetrahedral w.r.t each C-atom.
The already negligibly small dipole moments of C-H bonds further cancel out each other, equally in opposite directions, due to this symmetrical molecular arrangement or shape, resulting in a non-polar tail.
The shape of the molecule w.r.t terminal C-atom is trigonal planar. The C-O, C=O, and O–Na+ dipole moments do not get canceled here, thus endorsing the overall polarity effect at the head region.
In this way, soap is amphiphilic i.e., it contains a polar head (net µ> 0) and a non-polar tail (net µ = 0).
Soap possesses dual polarity. It acts as a polar as well as a non-polar chemical compound simultaneously. Chemically, soaps are salts of long-chain fatty acid esters.
The long hydrocarbon (alkyl) chain of a soap molecule acts as its non-polar end. Contrarily, the carboxylate (COO–) functional group bonded to a metal cation (Na+) acts as a polar end in soap.
What do we mean when we say soap is amphiphilic?
Amphiphilic is a chemical compound that displays both hydrophobic (water-repelling) as well as hydrophilic (water-loving) behavior at the same time.
Soap is amphiphilic as its chemical composition comprises a hydrophilic polar head and a hydrophobic non-polar tail.
Conversely, it can develop intermolecular forces of attraction with both polar and non-polar chemical compounds or solvents and get solubilized in both.
The polar head of soap attaches to ———- while the non-polar tail attaches to ————-.
Which of the following options fills the blanks given in the above statement in the correct order?
A) oil, water B) water, oil C) oil, lather D) lather, oil
Answer: Option (B) is the correct answer.
Explanation:
Like dissolves like. As water is polar in nature, the polar head of soap attaches to water. Contrarily, oil is just the liquid form of fat which implies it is non-polar in nature; composed of long hydrocarbon chains. Thus, the non-polar tail of soap develops an affinity with non-polar oil molecules.
Writing a combined statement: The polar head of soap attaches to water while the non-polar tail attaches to oil.
Does soap dissolve in water?
Yes, soap dissolves in water and forms a lather that is used for cleaning stains.
Soap uses its hydrophilic polar head (COO–Na+) to develop ion-dipole and dipole-dipole forces of attraction with polar H2O molecules and get solubilized.
How dish soap helps polar water molecules to clean non-polar grease stains?
Soap has both polar and non-polar ends in its chemical structure. On the one side, soap develops an affinity with polar water molecules using its polar head.
On the other end, it develops hydrophobic forces of attraction with non-polar grease stains, breaking them into small particles, and increasing their surface area so that they can be removed and washed away with water.
In this way, soap acts as a mediator between grease stains and water, supporting its cleaning action.
Why soaps do not work well in hard water?
Soaps are soluble salts of fatty acid esters, predominantly comprising Na+ or K+ ions. Hard water contains Ca2+ and Mg2+ ions.
When soap is added to hard water, the Na+ and/or K+ ions of soap get displaced by small radius, hard cations such as Ca2+ and Mg2+.
This results in the formation of insoluble fatty acid salts (called scum). The soap thus stays insoluble in hard water and does not form any lather to facilitate stain cleaning.
Does adding soap to a mixture of oil and water make it mix?
Yes. Oil and water don’t mix otherwise. They are immiscible i.e., they form two separate layers when tried to mix, owing to their opposite polarities. However, soap can act as a mediator between the two.
The soap contains both polar as well as a non-polar ends, so it can develop attractive forces with both polar H2O and non-polar oil molecules, not allowing the two to separate out into different layers.
Summary
Soap has both polar and non-polar ends. It is an amphiphilic chemical compound.
Soaps are soluble salts of long-chain fatty acid esters.
A fatty acid ester is formed by the condensation reaction between fatty acids and polyols.
The hydrolysis of a fatty acid ester in the presence of a base such as sodium hydroxide (caustic soda) or potassium hydroxide (caustic potash) produces soap. This process is called saponification.
The long hydrocarbon chain containing C-C and C-H bonds is non-polar (net µ = 0), while the carboxylate group bonded to metal cation (COO–Na+) is polar (net µ > 0).
Like dissolves like, so the polar head of soap is water-soluble (hydrophilic), while its non-polar tail develops an affinity with fat or oil particles (hydrophobic) to facilitate the cleaning action of dish soap.
References
Science Ready. “Soaps and Detergents”, https://scienceready.com.au/pages/soaps-and-detergents.
Numerade. “Why does soap need to be both polar and nonpolar? What is the purpose of curing your soap?”. Numerade, https://www.numerade.com/ask/question/1-why-does-soap-need-to-be-both-polar-and-nonpolar-2-what-is-the-purpose-of-curing-your-soap-21657/.
LibreTexts. “Liquids, Solids, and Intermolecular Forces.” LibreTexts, 2021, https://chem.libretexts.org/Courses/Riverland_Community_College/CHEM_1000_Introduction_to_Chemistry_(Riverland)/14%3A_Liquids_Solids_And_Intermolecular_Forces.
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