Azelaic acid is one of those skincare ingredients that sounds simple until you actually try to formulate with it.

You may look at a formula and think:

Why not just dissolve 10% azelaic acid in a solvent such as 1,3-propanediol and leave the water out completely?

It is a very reasonable question.

Water can seem like an unnecessary filler. But in an azelaic acid formula, water can actually change how the azelaic acid behaves.

You do not need to be a chemist to understand why.

First: azelaic acid has to be available before your skin can use it

Imagine putting a spoonful of sugar into a glass of water.

Once the sugar dissolves, the individual sugar molecules are free to move around.

Azelaic acid works in a similar way.

If azelaic acid is sitting in a skincare product as large undissolved crystals, your skin cannot simply absorb those crystals.

The azelaic acid first needs to become available at the skin surface.

So when we formulate with azelaic acid, one important question is not simply:

“How much azelaic acid did I put into the formula?”

It is:

“How much of that azelaic acid is actually available to leave the formula and enter the skin?”

That is where solvents, water and pH start to matter.

What happens when azelaic acid meets water?

Azelaic acid is an acid.

When it is placed in water, something interesting happens.

Some azelaic acid molecules stay in their original form.

Others lose a tiny positively charged hydrogen particle.

They become charged molecules.

We call this process ionisation.

You can think of it very simply:

Azelaic acid can exist in two useful forms:

Neutral azelaic acid

and

Charged azelaic acid

The two forms can change back and forth.

Why should we care?

Because they behave differently.

The charged form likes water more.

The neutral form generally has an easier time moving into the oily, lipid-rich outer layers of our skin.

That immediately creates an interesting formulation problem.

We want enough azelaic acid dissolved and available.

But we also want it to leave the product and move into the skin.

So is charged azelaic acid better or worse?

Neither.

This is where skincare chemistry becomes more interesting than simply saying:

“This ingredient penetrates well.”

Charged azelaic acid is generally happier in water.

That can help us keep more azelaic acid dissolved and available.

Neutral azelaic acid is generally better suited to moving from the product into the skin.

So the two forms can work together.

Imagine you have a little reservoir:

Charged azelaic acid ⇄ Neutral azelaic acid → Skin

As some neutral azelaic acid leaves the formula and enters the skin, more molecules can change form and replace it.

The formula keeps supplying more azelaic acid to the skin surface.

And this is where pH comes in

You have probably seen skincare formulators talk constantly about pH.

For azelaic acid, pH is not just about whether a product feels acidic.

pH influences how much azelaic acid is neutral and how much is charged.

At different pH levels, that balance changes.

This can change:

• how much azelaic acid stays dissolved,
• how much is available at the skin surface,
• and how easily it can leave the product and enter the skin.

This is why simply making a product “more acidic” does not automatically mean better penetration.

Sometimes a slightly higher pH can allow more azelaic acid to stay available in solution.

And having more dissolved azelaic acid available can sometimes matter more than having the theoretical “best penetrating” form but very little of it dissolved.

Why can't we simply do this in pure 1,3-propanediol?

You can make an anhydrous azelaic acid formula.

And 1,3-propanediol can be a very useful solvent.

But there is an important difference between:

Azelaic Acid + 1,3-Propanediol

and

Azelaic Acid + 1,3-Propanediol + Water

Once meaningful amounts of water are present, we can measure and adjust the formula's pH.

That lets us deliberately influence the balance between neutral and charged azelaic acid.

In a completely waterless product, normal cosmetic pH does not work in the same way.

You cannot simply say:

“My anhydrous serum is pH 4.5.”

pH is primarily a measurement describing what is happening in an aqueous environment.

That does not mean azelaic acid suddenly stops working without water.

It simply means that with water present, the formulator gains another very useful tool for controlling how the ingredient behaves.

Water does something else too: it hydrates the skin

The outermost layer of your skin is called the stratum corneum.

Think of it as a protective wall.

When that wall becomes hydrated, it can become slightly more flexible and more permeable to certain ingredients.

So water in an azelaic acid serum can have two jobs.

First, it helps create an environment where azelaic acid can ionise and where pH can be controlled.

Second, it hydrates the surface of the skin.

That can help create a more favourable environment for ingredient delivery.

So why add 1,3-propanediol as well?

Because azelaic acid does not dissolve particularly well in water by itself.

This is where a solvent such as 1,3-propanediol becomes very useful.

Think of water and propanediol as doing slightly different jobs.

1,3-Propanediol

helps dissolve azelaic acid and keeps it available.

Water

allows us to control pH, ionisation and skin hydration.

Together, they can create a much more interesting delivery system than either ingredient alone.

You could imagine it like this:

Azelaic Acid

↓

Propanediol helps dissolve it

↓

Water allows pH and ionisation to be controlled

↓

The formula sits against hydrated skin

↓

Dissolved azelaic acid leaves the formula

↓

Azelaic acid enters the skin

But doesn't dissolving more always mean better penetration?

Not necessarily.

This is one of the most important ideas in topical formulation.

A solvent's job is to help get an ingredient into the formula.

But eventually we need that ingredient to get out of the formula again.

If an ingredient loves its solvent too much, it may be very comfortable staying there.

So the perfect formula is not necessarily the one that dissolves the absolute maximum amount of azelaic acid.

We want enough solubility to make azelaic acid available, while still giving it a reason to leave the product and move into the skin.

Formulators call this partitioning.

In simple language:

Does azelaic acid prefer staying in the serum, or does it also want to move into your skin?

That balance matters enormously.

What happens after you apply the serum?

The formula does not stay exactly the same after it reaches your face.

Imagine a serum containing:

Azelaic Acid + Water + 1,3-Propanediol

You spread a thin layer onto your skin.

Some of the water gradually evaporates.

The remaining ingredients become more concentrated.

The balance of the formula changes.

Meanwhile, dissolved azelaic acid starts moving toward the skin.

As some leaves the serum, more can become available behind it.

So the product in the bottle and the tiny film sitting on your skin several minutes later are actually slightly different delivery environments.

That is one of the fascinating parts of cosmetic formulation.

Does this mean we need liposomes or complicated delivery systems?

Not necessarily.

Liposomes, phospholipids, microemulsions and other specialised delivery systems can be very interesting.

But a complicated formula is not automatically a better formula.

A well-designed simple system containing:

Azelaic Acid + 1,3-Propanediol + Water

can already provide:

• solubilisation,
• controlled pH,
• controlled ionisation,
• skin hydration,
• and a pathway for azelaic acid to leave the formula and enter the skin.

A sophisticated delivery system should therefore prove that it performs better than a good simple formulation.

More complicated does not always mean more effective.

The simplest way to remember it

When thinking about azelaic acid, there are three separate questions:

1. Can we get azelaic acid into the formula?

This is solubility.

2. What form is the azelaic acid in?

Water and pH influence whether it is mainly neutral or ionised.

3. Will azelaic acid leave the formula and enter the skin?

This is release and partitioning.

And that is why water is not necessarily just filler in an azelaic acid serum.

It can be part of the delivery system itself.

The goal of formulation isn't simply to put 10% azelaic acid into a bottle.

The more interesting question is:

How do we create an environment that keeps enough azelaic acid available — and then encourages it to leave that environment when it reaches the skin?

That is where formulation science begins.