As a frequent poster on several of the major hair boards, this is a question that often comes up. What’s the difference between regular conditioner and deep conditioner? A lot of people say that it’s just the length of time that you leave the conditioner on your hair that determines if it is “deep” or not. Others say that you can add stuff “oils/butters/humectants” to a regular conditioner to get the effects of a deep conditioner. In my opinion, to actually be able to answer that question, you have to delve a bit deeper into the science of hair, specifically the structure of the actual hair.

Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair and makes up the majority of the hair shaft; and the cuticle, the hard outer layer that protects the shaft. The cuticle is formed by tightly packed scales in an overlapping structure similar to roof shingles. On hair that is in optimal condition, the scales on the cuticle lie flat and are tightly packed.

Here is a diagram that I found that best show what I'm describing

Most conditioners that we encounter/use are not deep conditioners (yes, even if they say they are) and are really rinse out conditioners. Regular/rinse out conditioners can only coat the hair regardless of how long you let it sit on your hair. In order for a conditioner to be considered a deep conditioner, it must be either used with heat (which opens up the cuticles of the hair and allows for penetration of the ingredients) or it must contain various oils/ingredients that naturally penetrate the hair shaft. Most deep conditioners use both (penetrative ingredients AND advise the application of a heat source).

• Coconut oil, being a triglyceride of lauric acid (principal fatty acid), has a high affinity for hair proteins and, because of its low molecular weight and straight linear chain, is able to penetrate inside the hair shaft.
• Coconut Oil, Avocado Oil, Olive oil penetrate the hair shaft to make it stronger (J. Cosmet.Sci 52, 169-184, 2001)

• Henna and henna extracts, specifically the lawsone molecule that is responsible for the red dying effect of henna, has an affinity for keratin and protein. This molecule is able to penetrate the hair shaft and impart color, while also providing conditioning effects.
• Panthenol is the only vitamin (Vitamin B5) known to penetrate the hair shaft. It has historically been used to build healthy follicles and reduce graying.
• Hydrolyzed Keratin Proteins are the basic building blocks of the hair shaft.

• Aloe Leaf Extract provides building blocks for proteins and is a soothing anti-inflammatory that protects against environmental damages to the hair and scalp.
• Dimethyl Sulfoxide is used to facilitate absorption of key ingredients into the scalp and follicles.
• Linoleic Acid is a fatty acid that is necessary for the growth of healthy hair follicles.
• Tocopherol Acetate (Vitamin E) is a known anti-oxidant for skin and scalp and helps promote red blood cell count.
• Cetyl Trimethyl Ammonium Chloride is used to emulsify all of the ingredients and can soften, soothe and condition the scalp.
• Glycerin is a naturally derived fatty component that absorbs water to naturally moisturize the hair shaft and root.
• Silica is an important mineral that helps the body form collagen, which is instrumental to promoting strong, healthy hair.