Separation and cloudiness
Updated
The commonest visible failure, and the one that reads to a customer as the product having gone off.
Three different faults
A layer on top or bottom: something has come out of solution entirely.
Cloudiness without a layer: something is dispersed rather than dissolved.
Sediment: a solid has precipitated.
They look similar and have different causes, and the diagnosis is worth making before changing anything.
Fragrance separating
The usual answer, and it is oil in water. Without a solubilizer it will separate, sooner in the cold and sooner at higher fragrance load.
The fix is a solubilizer or a coupling agent at the level the supplier recommends, added in the right order, with the fragrance pre-mixed into it before it meets the water.
Under-dosing the solubilizer gives a product that is clear on the bench and cloudy in a week, which is the version that reaches customers.
Cold-temperature dropout
A formula near its solubility limit is at its limit at room temperature and past it at four degrees.
A product that spent a night in a delivery van in January arrives cloudy or with sediment. It frequently redissolves on warming and the customer has already decided.
Which is why a cold test is part of development: a week in a fridge and a freeze-thaw cycle, then look.
Fixing it
Identify which of the three it is, then work backwards.
A layer: solubilizer or coupling agent.
Cloudiness: usually the same, or a hardness reaction with an anionic surfactant, which points at a chelator.
Sediment: a precipitate, which usually means an incompatibility or a pH shift rather than a solubility problem.
And test the fix cold, warm and over months, because every one of these faults is time-dependent.