This is an overview of what Acids and Bases are, what lye really is, how it is used to make soap, and how different fatty acids can affect the outcome of a soap. Explained as simply as possible for everyone to understand.
When it comes to solutions, whether it be at home, in a lab, or making a soap, everything falls into three different categories. These three categories are acidic, basic and neutral solutions. Some acids you may be familiar with include vinegar, car battery fluid, and lesser known, aspirin. Some examples of bases are soap, baking soda, toothpaste and ammonia. Neutral solutions include water, oil, sugar, and salt. I will be explaining exactly what makes things acidic, basic or neutral, some properties of them, and how this applies to soap making.
Acids : Unveiling the Properties and Definitions Behind Their Chemistry
Acids and Bases were first discovered by a Swedish Chemist named Svante Arrhenius who gave them several properties distinguishing the two, let’s first start with Acids. According to Arrhenius, an acid was a substance that tasted sour, this is observable in lemon juice, vinegar, and almost every acid that won’t kill you upon putting it in your body. Arrhenius also discovered that when dissociated in water, an acid produces a positively charged hydrogen, called a hydrogen ion.
This hydrogen ion is commonly referred to as a proton, and this leads us to a second definition of an acid. This hydrogen ion can be donated to other molecules in a chemical reaction. This is known as the Bronsted-Lowry definition of an acid (Bronsted-Lowry Acid) and is observable when the acid is dissociated in something other than water. There is a third definition, however to understand it, a deeper knowledge of chemistry is required.
The common way to tell if an acid is an acid, is by its pH value. A pH value is given by a logarithmic equation involving the hydrogen ion population in a solution, and the value lies on a scale. The scale goes from 0 to 14, with 0 being the most acidic, the middle number, 7, being neutral, and 14 being the most basic. Acids are denoted as having a value less then seven. The strongest acid, Hydrochloric Acid in its highest concentration having a pH of .1, vinegar is around 2.4, lemon juice from 2 to 3.
Bases : Exploring Their Slippery Nature and Alkaline Properties
Bases (also known as alkalis) were also first discovered by Arrhenius and he first described them as slippery. This is because the base dissolves the oils and acids from your hands, making soap (more on this later). He also described them as bitter, which is a reason many medicines do not taste well if you do not swallow them. He also discovered that when bases are dissociated in water, they produce a negatively charged hydroxide ion (this is a negative charge due to excess electrons on the molecule made from one oxygen and one hydrogen).
The hydroxide ion is an interesting ion, it is what makes an alcohol (ethanol) from regular ethane, and it also acts like a proton accepter. The negative charge on the hydroxide, makes it want to bond with protons, or other molecules, hence leads to it being called a proton accepter. This was observed by Bronsted-Lowry, and this is the definition of a Bronsted-Lowry Base.
Bases can be found by their pH as well, as bases have a pH value greater than 7. Baking soda sits at around 8.3, Ammonia is so great at cleaning because of its pH of 11, and Sodium Hydroxide in its most evil form sits right at 14.
The Chemistry Behind Soap: How Acids, Bases, and Saponification Cleanse Our Skin
Acids and Bases when put together undergo a reaction known as neutralization, creating a new molecule, water, and neutralizing the pH. If we look at the strongest acid and base put in a reaction together (Hydrochloric Acid (HCl) and Sodium Hydroxide (NaOH)) the hydrogen from the acid donates itself to the hydroxide in the base, making H2O and NaCl, which is table salt. This is the story of how neutrality is achieved.
Soap is something that cleans us, taking the oil off the body. The way this happened and why was discovered by Arrhenius, however is has been used for long before Arrhenius came around. Since about 2800 B.C. soap has been made and used originating from the Babylonians, who used it for cleaning, and for medicine.
Soap is made using a substance called Lye, which is a base. Lye is the most dangerous of bases, as it is Sodium Hydroxide, however Potassium Hydroxide is sometimes used, which in a high concentration has a pH of 14, and can cause chemical burns on the skin, and if ingested, death. So why do we use such a dangerous chemical in soap, and why doesn’t it hurt us? The answer lies within a chemical reaction called saponification.
Understanding Saponification: Chemistry of Soap
Saponification is a very complex reaction, often not taught until late in the second semester of an organic chemistry class, as it requires knowledge of bases, mechanisms for the reaction, and fats. Saponification uses a substance known as a triglyceride in academia. A triglyceride is just a fancy word for a fatty acid ester, which is just fancy for animal fat (human fat in Fight Club, but that is to not be spoken of) or vegetable oils (commonly used are Coconut Oil and Olive Oil). When Lye and a triglyceride react, the result is glycerol (a sugar/amino acid alcohol) and a soap base of whatever triglyceride you used.
Read Also: Choosing Oils For DIY Shampoo
The type of triglyceride you use can affect the outcome of the soap greatly. A saturated fat (often solid at room temperature, examples are animal fats, the white substance in your nice steak or coconut oil) is often used in the production of bulk cheap soap, and results in a solid bar. The type of saturated fat judges how hard the bar is (the bigger the molecule, the harder to break, and such the harder the soap). An unsaturated fat (often a liquid) is used to balance the soap and give it more of the properties that are wanted for moisturizing.
Thank you for reading, hopefully we helped cleared some things up for you. If you’d like to learn more we have more stuff for you. We have a great article on Lye. We wrote an article on How To Choose Oils For Soap if you’d like to know which oils do what after saponification. We have an Easy Soap Recipe as well.
Written by Samuel Brown (Mechanical and Aerospace Engineering Student at NMSU) and Steven Johnson.