Chemistry - Ammonia for cleaning - what is the interaction between ammonia and oils?
Solution 1:
Common food fats and oils usually contain some free fatty acid1. The alkalinity of household ammonia serves to form a soap which will emulsify the main body of the fat/oil, if enough elbow grease (i.e. vigorous agitation with a scrub brush or wash rag) is applied. Other alkalis do as well - better, if you consider breathable ambient air to be desirable.
Some brands of commercial household ammonia already contain fatty acid soaps and exhibit a cloudy appearance.
It also seems to me that a dilute solution of washing soda ($\ce{Na2CO3}$) would clean quite well, and ammonia should do no better. But when the odor of $\ce{NH3}$ disappears, and the oily/fatty surface is now clean, it is as if there are now two indicators of cleanliness: no odor and no oiliness, therefore the surface must certainly be clean, because all evidence of the cleaning process has been removed. It must be a psychological thing, like foaming with detergents.
Reference:
- Analytical approaches for the assessment of free fatty acids in oils and fats S. A. Mahesar, S. T. H. Sherazi, Abdul Rauf Khaskheli, Aftab A. Kandhro, Siraj uddina, Anal. Methods, 2014 ,6, 4956-4963, DOI: 10.1039/C4AY00344F
Solution 2:
Ammonia is a surfactant, like detergents, but it also reacts chemically with some quantities of oils to convert them into other things. In addition, as a cleaning agent, unlike detergents and soap, ammonia has anti-microbial action.
Ammonia is a historically notable cleaning agent. It was readily available, prior to the advent of the industrial era, as it is a natural by-product of the breakdown of animal (and human) waste (1). Aging urine produces a liquid with a high ammonia content. Ammonia has significant antimicrobial properties (2) as well as anti-grease/oil properties. As an anti-oil agent, it has both surfactant and transmutational properties (3)(4)(5). In plainer English, that means that ammonia emulsifies grease and oils, like soap and detergents, so that the mess can be washed away with water, and it changes the nature of some of the oil.
As a surfactant, ammonia acts like soap and detergents, meaning they pick up and disperse the oil into tiny droplets in the water (emulsification), which then can be rinsed away. Soap and detergents are a relatively unstable emulsifier. Which means that, as the original soap or detergent breaks down, the oils come out of emulsification, and regroup. And, voila, you get your fat back - just in a different location - and probably with lots of other impurities. Ammonia, as a surfactant, does the same thing: i.e. it acts to emulsify the oil. And, when it breaks down, the emulsion ends.
However, ammonia also chemically interacts with the oil, in significant amounts, in a process called ammonolysis, in which it also has catalytic effects. In this process it converts some of the grease and oils into other substances (e.g. amines), which no longer have the same properties as the original grease or oils. (4)(5) Ammonia does this latter through ammonolysis, in which it also may act as a catalyst. This is the part of ammonia's action that is similar to saponification, in which the original substances are altered, and no longer exist in their original form. How much of the oil is converted into amines depends on the temperature of the interaction, the type and characteristics of the oils, etc. However, this change is stable, and permanent, in that the end products do not revert to being oils.
(1) history: Smithsonian Magazine: The science behind historic uses of urine
(2)Ammonia is not currently recognized as a primary antimicrobial product, as, for example, is bleach (chlorine). Chlorine, my example, is far more effective as an antimicrobial. However, in ammonia's favor, is the fact that ammonia is far more effective in this regard than soap or detergents alone. Please note that, on the linked chart, bleach (chlorine) is referred to by its chemical name: sodium hypochlorite.
Antimicrobial Spectrum of Disinfectants
(3) Surfactants: Dawn Chemical: Chemistry of Cleaning
Essential Chemical Industry: Surfactants
(4) Ammonolysis (subset of Solvolysis)
(5) Catalytic Effects in the Ammonolysis of Vegetable Oils