I'll take a shot at this and see if I can explain:
First off, I've never made mayonnaise, so this is simply from a chemist's perspective. There is no real reaction going on here in the manner of chemicals changing. This is more physical than anything else.
You can think of this as a three part system much akin to cleaning greasy dishes with soap.
( I told you, it was a chemist's perspective!
)
You have a water component which comes from the yolk and lemon juice.
There is a fatty component, the oil.
There is a detergent (called an "ionic surfactant" for those interested) , the lecithin in the yoke, which in simple terms has one end of the molecule that likes water and one that likes fat. The water loving end has a positive charge that will attract the acidic negative charge of the lemon juice.
The fat is broken up into tiny enough globs that the lecithin can surround the blobs in a very fine film with the fat-loving end attracted to the oil, and stabilize them and hold water on the other end of it at the same time, forming little discrete cell-like units (micellae), but not so tiny that a certain coating thickness can't be achieved. This forms an "emulsion", a stable suspension. This is all nin a very fine balance to make it happen and keep it from separating, and why mayo is a skill preparation in cooking. It has to do with a balance between surface tension and viscosity.
The same prinicple applies to how dish soap in water can clean grease off of a fry pan. One end likes water, one end likes fat.
Hope that helps. I apologize if it confuses the issue; knowing something is not the ability to teach it. Sometimes it's difficult to explain things you've known so long that they haven't been verbalized in 35 years.
