What Is Quaternium-91?
Quaternium-91 is a quaternary ammonium compound created by reacting long-chain fatty acids derived from plant oils or tallow with an imidazoline backbone, then neutralizing the result with methyl sulfate to form a stable salt. The finished material, listed on labels as 1-(Docosanamidoethyl)-2-heneicosyl-1-methyl-4,5-dihydroimidazolium methyl sulfate, carries a positive charge that helps it cling to negatively charged hair fibers.
This class of conditioning agents appeared in the late 1980s when formulators looked for alternatives to older cationic surfactants that could cause buildup or irritation. By tailoring the length of the fatty chains, chemists produced Quaternium-91, which offers strong conditioning without leaving heavy residue. Today it is made in specialized reactors where the fatty acid, amidoamine and methyl sulfate are blended under controlled heat and pH, then purified and dried into a paste or liquid concentrate that easily disperses in water.
You are most likely to find Quaternium-91 in rinse-off and leave-in hair conditioners, deep-conditioning masks, curl-defining creams, color-protecting treatments and some 2-in-1 shampoo-conditioner blends. Its mildness and ability to work in both high and low pH systems also make it a helper ingredient in anti-frizz serums and detangling sprays.
Quaternium-91’s Cosmetic Benefits/Uses
This ingredient is prized for one main purpose in hair care formulas
Hair conditioning: The positive charge of Quaternium-91 is attracted to the slightly negative surface of hair strands, forming a thin, even film that smooths the cuticle. This reduces static flyaways, improves wet and dry combability, enhances softness and adds a natural-looking sheen. Its long fatty chains give lasting lubrication so hair feels silky without becoming greasy, making it suitable for daily conditioners as well as intensive repair masks.
Who Can Use Quaternium-91
Quaternium-91 is generally suitable for all hair and scalp types, including sensitive or color-treated scalps, because it stays mainly on the hair shaft and rinses clean without clogging follicles. Those with very oily, fine or low porosity hair might prefer lighter conditioners, as the fatty chains can feel too rich when used in high concentrations.
Most commercial grades are derived from plant oils, yet some manufacturers still use animal-based tallow. If you follow a vegan or vegetarian lifestyle, check the product label or contact the brand to confirm a plant source.
Current safety assessments show no reproductive or developmental hazards, so Quaternium-91 is considered safe for use during pregnancy and while breastfeeding. This is not medical advice; anyone who is pregnant or nursing should review all personal care products with a qualified healthcare provider to be sure they fit individual needs.
The molecule is not known to cause photosensitivity, so it will not make hair or scalp more prone to sun damage. It is also odorless and compatible with most styling products, making it easy to integrate into daily routines.
Potential Side Effects/Adverse Reactions
Responses to topical Quaternium-91 vary from person to person. The following are potential side effects that could occur, although they are uncommon when the ingredient is used correctly in a finished product.
- Scalp irritation mild itching or redness, often linked to overuse or very high concentrations
- Allergic contact dermatitis rare sensitization that can lead to rash or bumps on the scalp or neck
- Eye irritation stinging or watering if the conditioner accidentally drips into the eyes before rinsing
- Product buildup hair may feel coated or weighed down after repeated use without a clarifying wash
If any of these reactions occur discontinue use immediately and consult a healthcare professional or dermatologist.
Comedogenic Rating
Rating: 1 / 5 (very low) Quaternium-91 is a large, positively charged molecule that prefers to stay on the hair shaft rather than sink into skin or clog pores. It is usually found in rinse-off conditioners, so only trace amounts touch the facial area. For these reasons it is considered safe for people who are prone to acne or breakouts.
If a formula pairs Quaternium-91 with heavy oils or butters, the overall product could still feel greasy on acne-prone skin, but the ingredient itself poses little risk of pore blockage.
Summary
Quaternium-91 is a modern hair conditioning agent that smooths the cuticle, tames static, boosts softness and adds a light, natural shine. It does this by using its positive charge and long fatty chains to form a thin, lubricating film on each strand.
While not a household name, it appears in a growing number of mainstream and salon products because it conditions well without leaving heavy residue. Its safety record is strong, with only rare reports of irritation or allergy when used at normal levels.
Most people can use products containing Quaternium-91 with confidence, yet it is always smart to patch test any new hair or skin care formula to make sure it suits your individual needs.