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Household ammonia is recognized as being an excellent cleaner for grease and oil. My question is, how does this work? Does ammonia merely put the oils into a suspension, like a detergent does? Or is there a more significant chemical reaction between the two - along the lines of how enzymes affect oils?

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  • $\begingroup$ There is no reaction, just the detergent action. $\endgroup$ – Ivan Neretin Aug 31 '17 at 7:17
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    $\begingroup$ I can't verify your answer, but if you can offer backup, you should make this an answer. Thank you for replying! $\endgroup$ – Corvus B Sep 15 '17 at 1:09
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    $\begingroup$ I found this, a bit of a long winded video but has some relevant information. - nsf.gov/news/special_reports/chemistrynow/chem_cleaners.jsp $\endgroup$ – KalleMP Feb 27 '18 at 20:09
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    $\begingroup$ The comment that there is no reaction, other than a detergent action, is, I am pretty confident, incorrect. There is apparently a chemical reaction between the ammonia and oils, with at least some of the oil being converted to amides. Unfortunately, I don't have enough chemistry knowledge to know what this means in plain English. Starting a bounty. Relevant sources: naldc.nal.usda.gov/download/31213/PDF, aocs.onlinelibrary.wiley.com/doi/10.1007/BF02640174, and a commercial application ossmaine.com/wp-content/uploads/2017/08/… $\endgroup$ – Corvus B Oct 5 '20 at 18:17
  • $\begingroup$ @CorvusB, Are you specifically talking about Windex? The so called ammonia solutions are not pure ammonia solutions. They have alcohols and surfactants. $\endgroup$ – M. Farooq Oct 6 '20 at 2:35
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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:

  1. 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
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    $\begingroup$ Nice try, but I'm pretty sure this is not a good answer. There are, I believe, a number of issues in the answer, beginning with the reference to FFA. The relevance of that link is not established here. 2nd, washing sodas are also traditionally noted cleaning agents, so the alkalinity aspect may be pertinent. However, saponification (soap-making), is not, I think, an emulsification, as the initial ingredients are altered in the process. Also, it is not necessary to apply effort with ammonia to remove grease. However, emulsification IS the question here, as that is all that detergents do to oil. $\endgroup$ – Corvus B Oct 8 '20 at 12:30
  • $\begingroup$ Continuing the above comment. The ammonia in household ammonia is volatile - it readily evaporates - leaving the water behind. This is both the source of the odor, and the reason the odor dissipates. However, the cleaning effect is most certainly not psychological, and in most circumstances is more powerful for dissolving and softening grease and oil than a cleaning soda. Sodas do work because of alkalinity, but primarily as an adjunct to detergents or soaps. One can demonstrate that there is a difference in action by soaking a greasy pot in one or the other overnight. Cont. . . $\endgroup$ – Corvus B Oct 8 '20 at 12:37
  • $\begingroup$ The pot soaked in household ammonia may well be cleaner outright, and any remaining oils will be softened so that the mentioned elbow grease is far more effective than previously. Thanks for trying! $\endgroup$ – Corvus B Oct 8 '20 at 12:41
  • $\begingroup$ I wonder how rapidly the ammonia leaves the aqueous solution compared to how fast the water itself evaporates, since NH3 is so soluble in H2O. The odor threshold of NH3 is <50 ppm, but 4% is 40,000 ppm, so there's lots to go around! $\endgroup$ – James Gaidis Oct 8 '20 at 13:26
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    $\begingroup$ Interesting observation. Odor and pH vs time would be an interesting graph. It could be that household ammonia is made with a concentration only a little above mildly detectable, so it can lose some NH3, yet still be an effective cleaner. The extra NH3 in there is to let the consumer know it's aggressive. HCl at >20% smells strongly acidic; <20%, not so strong. The degree of odor is not linear wrt concentration (at least for HCl in water). $\endgroup$ – James Gaidis Oct 9 '20 at 14:07
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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

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