PEG-40 Hydrogenated Castor Oil (RH 40): The Solubiliser Every DIY Formulator Should Know

PEG-40 Hydrogenated Castor Oil, often labelled RH 40, is a liquid cosmetic ingredient used to blend oil-based materials like fragrance and essential oils into water-based products without them separating. In plain terms, it is a solubiliser and emulsifier that keeps your mists, toners, micellar waters and cleansers clear and stable.

Derived from hydrogenated castor oil, RH 40 is a workhorse in the personal care industry precisely because it is gentle, effective and easy to use. If you have ever wondered why your homemade room spray or face mist turns cloudy and oily on top, chances are you needed exactly this ingredient.

What PEG-40 Hydrogenated Castor Oil is used for

RH 40 has two main jobs, and it does both well:

Solubilising fragrance and essential oils into water so they disperse evenly instead of floating on the surface.
Emulsifying small amounts of oil into water-based formulas to keep them stable over time.

You will find it in products such as:

Face and body mists
Toners and hydrosols with added actives
Micellar cleansing waters
Room and linen sprays
Clear shampoos, body washes and bubble baths
Bath oils and clarifying cleansers

Its key details: chemical formula C38H74O4, CAS number 61788-85-0, colour ranging from white to yellowish, and a liquid form that is easy to weigh and pour.

How to use RH 40 in your formulations

The most reliable method is to pre-mix the RH 40 with your oil phase before adding water. This lets the solubiliser "wrap around" the oil droplets so they disperse cleanly.

Weigh your fragrance or essential oil. A common starting ratio is 3 to 5 parts RH 40 to 1 part oil, adjusted by trial.
Combine the oil and RH 40 in a clean container and stir until fully blended into a uniform liquid.
Slowly add your water phase (distilled water or hydrosol) while stirring continuously.
Check clarity. If the mix is still cloudy, add a little more RH 40 in small increments until it turns clear.

Typical use levels sit around 1–5% of the total formula, but this depends entirely on how much oil you are trying to solubilise. Always run a small test batch first and scale up once you are happy with the result. Do not forget a suitable broad-spectrum preservative in any water-containing product.

Safety and handling notes

RH 40 is meant strictly for external use. Please keep these points in mind:

Do not ingest it, and keep it away from children and pets.
Avoid direct contact with the eyes; rinse thoroughly with water if contact occurs.
As with any new cosmetic ingredient, patch-test finished products on a small area of skin before regular use.
Work in a clean, ventilated space and wipe up spills promptly, as the liquid can be slippery.
Refer to the product MSDS and COA for detailed handling information.

RH 40 is widely regarded as a mild, well-tolerated ingredient at cosmetic use levels, which is one reason it is so popular in leave-on and rinse-off formulas alike.

Buying and storage tips

Azlok stocks RH 40 in a wide range of pack sizes, starting from a handy 100 gm for hobbyists at ₹199, right up to 25 kg for small businesses and regular formulators. Buying a size that matches your batch frequency helps you avoid waste.

Store it in a tightly closed container, away from direct sunlight and heat.
Keep the lid sealed to prevent contamination and moisture pick-up.
The shelf life is around 24 months when stored properly.
Use clean, dry utensils each time you scoop or pour to keep the batch fresh.

For anyone making clear sprays, cleansers or fragranced waters at home or for a small brand, RH 40 is one of the most versatile ingredients you can keep on the shelf.

FAQ

What does PEG-40 Hydrogenated Castor Oil do?

It solubilises oils into water and acts as an emulsifier, allowing fragrance oils, essential oils and small amounts of carrier oil to blend evenly into water-based products without separating.

How much RH 40 should I use?

Use levels usually fall between 1% and 5% of the total formula, and a common starting ratio is 3–5 parts RH 40 to 1 part oil. Adjust in small steps until your mix turns clear.

Is RH 40 safe for skin?

It is a widely used cosmetic ingredient considered mild at normal use levels, and it is intended for external use only. Always patch-test the finished product and keep it away from eyes and children.

Can I use RH 40 to make a room or linen spray?

Yes. Pre-mix the fragrance oil with RH 40, then add water while stirring. This keeps the spray clear and stops the oil from floating on top.

What is the shelf life of RH 40?

When stored in a sealed container away from heat and sunlight, it keeps for about 24 months.