A salad dressed in the morning is limp by lunchtime, while the same ingredients kept separate stay crisp for several days. The difference is a straightforward physical process operating on every cut surface.
Salt draws water out of cells
Plant cells are firm because they are full of water pressing outward against their walls, which is what crispness physically is.
A salted dressing creates a more concentrated solution outside the cell than inside, and water moves across the cell membrane toward that concentration.
As the cells lose water they lose their internal pressure, and the leaf goes from rigid to flexible to limp in a matter of hours.
Acid weakens the structures holding cells together
Vinegar and citrus lower the pH around the tissue, which degrades pectin in the cell walls and the material binding cells to one another.
The effect is slower than the salt effect but permanent, and it is why an acidic dressing eventually turns a leaf translucent rather than merely soft.
Delicate leaves with thin walls show this within an hour, while sturdy cabbage and root vegetables resist it for days.
Oil creates a separate problem
Oil coats a leaf and slows evaporation, which sounds protective, but it also prevents the surface drying and traps moisture against the tissue.
That film blocks the leaf from breathing normally and accelerates the slippery texture that a stored dressed salad develops.
Oil also carries the salt and acid into direct, sustained contact with every surface, which is why an oil-based dressing is more damaging than a squeeze of lemon alone.
Some ingredients improve under exactly these conditions
Firm vegetables with dense cell walls soften into a pleasant texture rather than collapsing, which is the principle behind slaws and marinated bean salads.
Grains, legumes and pasta absorb dressing rather than losing water to it, so they improve over a day as the seasoning penetrates.
Building prepared salads around those components rather than around leaves is what makes a week of them viable.
Order of assembly solves it practically
Keeping dressing separate until service is the obvious answer, and small sealed containers make it possible in a packed lunch.
Layering also works: dressing at the bottom, then the dense ingredients that tolerate it, with leaves on top and out of contact until the container is tipped.
Both approaches accept the chemistry rather than fighting it, which is why they work when refrigeration alone does not.