It is well known that cleaning your dirty dishes with water alone is just not sufficient and so, we employ soaps like fairy liquid and use dishwasher tablets to help us get rid of the dirt, but why do we need these?
Soap is a generic term for fatty acid salts, with the general formula (RCO2–)nMn+, where R is an alkyl group and CO2– is a carboxylate group that binds to metal Mn+. Soaps are generically formed by reacting a fatty acid with an alkali solution of a metal hydroxide in a reaction termed saponification. This reaction forms soap (the metal salt of the starting fat/oil) and glycerine, a simple polyol. Scents and colourants can be added to the soap whilst it’s still liquid, but the active cleaning agent in the soap is the salt.
The R group in soap is a long alkyl chain, made up of numerous CH2 groups and a terminal CH3 group. These types of groups are very hydrophobic, meaning they do not like to be in water. The metal carboxylate group is the opposite as the charges on the oxygen and the metal make this part of the structure hydrophilic, meaning they interact favourably with the surrounding water.
The opposite ends of the compound make soaps perfect for use as cleaning agents, and this type of structure can often be referred to as an emulsifier. Emulsifiers bring together water and oil and allow them to disperse within each other, something that is not possible in the absence of an emulsifier.
Particles of dirt are relatively greasy compared with water and oily greasy things do not mix well with water, making it difficult to clean dishes with water alone. The hydrophobic nature of dirt particles allows them to favourably interact with the hydrophobic alkyl chains in soap, and the soap compounds surround the dirt particles in a sphere-like way to bury the hydrophobic chains in the centre, favourably exposing the charged metal carboxylate groups to surrounding water. This allows the interacting dirt to be solubilised, forcing it to come off dirty dishes in a soapy water solution.