Which salt is better to use when stabilizing a water-in-oil emulsion — regular table salt or magnesium sulphate? That is the question we will try to answer today. To start, let's talk about what we are actually stabilizing.
Water-in-oil emulsion in simple terms
A water-in-oil (W/O) emulsion consists of tiny water droplets distributed in a continuous oil phase. Our task is to prevent the droplets from destroying the emulsion. To understand why salt is needed, let's first look at exactly how an emulsion can break down.
Droplets merge → "free water" appears → separation. The most obvious way an emulsion breaks down.
Droplets stick together into clusters → texture and viscosity change → risk of coalescence increases.
Small droplets transfer water to larger ones through the oil phase. Small ones disappear, large ones grow → visible puddles of water → separation.
How salt helps the emulsifier
In a water-in-oil emulsion, the main work is done by emulsifiers (the film around the water droplets). Salt is an emulsifier's assistant. It works in two ways.
Salt dissolves → ions bind firmly to water molecules → the thermodynamic drive for water to "escape" is reduced. Salt cannot dissolve in oil — it is trapped inside each droplet. For water to leave, it must "abandon" the ions — which is energetically costly.
Think of salt as an anchor: fresh water can drift freely. Salt water is "tethered" in place.
Salt can "dehydrate" the emulsifier — part of the emulsifier becomes more lipophilic at the interface. The emulsifier packs more tightly, and the film around the droplet becomes stronger.
A stronger film → droplets do not deform or merge, and the emulsion remains stable for longer.
But salt can also do harm:
- Excess → crystals/"sand," especially after temperature cycles or water loss
- The cream formula may become too flocculated (it thickens → loses stability)
- Some components may precipitate
Ostwald ripening: the main enemy
Small droplets have higher internal pressure (highly curved surface). Large ones have lower pressure. Nature dislikes imbalance: water molecules "dissolve," exiting the small droplet, diffusing through the oil, and joining the large one. Over time, the small ones disappear, the large ones grow → separation.
Salt is the best protection against droplet migration during Ostwald ripening. It "anchors" water molecules inside their droplets, making migration through the oil energetically unfavorable.
What if you don't use salt? Does it work?
Many of us, especially beginners, have made water-in-oil emulsions without salt — and it seemed to hold up fine. And it's true: many water-in-oil emulsions remain stable without electrolytes because of:
Sharply slows down the diffusion of water molecules through the oil. If the oil phase is thick enough, water physically cannot migrate quickly between droplets.
Physically immobilize the droplets. Waxes create a three-dimensional network in the oil phase that "freezes" water droplets in place.
If you have a very viscous cream with a large amount of wax — it might work for small volumes (100 g). But for reliable stability, especially when scaling up and under temperature stress, we need salt.
NaCl vs MgSO₄: the main comparison
| Parameter | 🧂 NaCl (table salt) | 🧪 MgSO₄ (magnesium sulphate) |
|---|---|---|
| Ion type | Na⁺ (monovalent) | Mg²⁺ (divalent) |
| Stabilization strength | Mild, predictable | Aggressive, powerful |
| Cream texture | Lighter, less "heavy" | Thick, dense, "body" |
| Thermal stability | Standard | Holds better when heated |
| Compatibility | Wide, few conflicts | May conflict with anionic components |
| Skin analogy | Mimics sweat composition | Less physiological |
| Availability | In your kitchen, cheap | Pharmacy / supplier |
| Ideal for | Light day creams, sunscreens | Rich night creams, cold creams, baby ointments |
| Status | Good choice for beginners | 🏆 "Gold standard" of stability |
- Light day cream or sunscreen
- Need skin compatibility (mimics sweat)
- Simple recipe without complex actives
- You want a lighter texture
- You are a beginner formulator
- Rich night cream, cold cream, baby ointment
- Need "indestructible" stability
- Cream will be in a hot climate / during transport
- Thick, dense texture is a plus
- Complex formula with many actives
Reminder for beginners!
This applies ONLY to water-in-oil emulsions. If you use Polawax, BTMS, Planamuls, or Olivem 1000 — these are oil-in-water emulsions. DO NOT add salt! In oil-in-water emulsions, salt often destroys gelling agents and turns the cream into a watery mess.
Ostwald ripening destabilizes water-in-oil emulsions by allowing water to migrate through the oil phase. Salt counteracts this by "anchoring" water molecules inside their droplets. NaCl is a mild and predictable choice for light formulas. MgSO₄ is the "gold standard" for maximum stability. It is not a panacea, but it is the best protection against droplet migration.
Read also: pH in cosmetics • Xanthan and guar gums
