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I’m a little concerned that there is a far stronger mix of antifreeze in my car's coolant system than I am aware. I check the antifreeze level in my car (using a refractometer) when the engine is cold (after being left overnight).
During the night, can the antifreeze settle to the bottom of the cooling system, thus leaving a weak mixture of water and antifreeze within the coolant reservoir up top, where I take a sample with the refractometer?
Ethylene glycol is completely miscible with water in all proportions (as the commenters point out). Therefore, once mixed, the glycol will not separate from the water, ever.
The only exception is if it gets so cold the mixture starts to freeze. Freezing will cause some degree of separation as the water will solidify first. But if it is that cold, separation will be the least of your problems.
In a car radiator below the freezing temperature of water, the water part increasingly turns to slush. The glycol just prevents the mix from freezing hard enough to expand and crack engine blocks, etc.
But, ice is lighter than straight water and I haven't seen that mentioned or addressed here. Ice/slush rises in the system and then might be subsequently melted back into water the following day when the mixture is checked with a refractometer.
So. The glycol doesn't sink as was suspected, but ice/slush rises and then turns into water.
Similarly, separation is the greater of your problems because the slush will not pass through a small tube radiator and it can interfere with the thermostat and water pump bypass. If you fired up an engine for a short period of time, you can cause a log jam of slush in one side of a radiator tank. If the side tank has the radiator cap, I can see getting weak test results the following day when the slush melts.
