First-Order Kinetics: Reaction rate proportional to concentration

ByDr. Celestine (Certified Trichologist)

What is First-Order Kinetics?

First-order kinetics is a chemical principle that describes how quickly a substance breaks down or reacts. The surprising fact is that this same rule that explains how medicine leaves your body also dictates how hair color fades from your strands.

In hair treatments, it means the speed of a chemical reaction depends directly on how much of the active ingredient is present. Think of it like a room full of popping popcorn. At first, many kernels pop rapidly, but as fewer unpopped kernels remain, the popping slows down.

This principle is the hidden clock behind many salon services. It determines how long your color processes or how a relaxer works on your curl pattern.

First-Order Kinetics in Your Hair Color

When you color your hair, the developer breaks down to release oxygen. This reaction follows first-order kinetics. The developer decomposes fastest when it is freshly mixed and full strength.

As the oxygen is released to lift your natural pigment, the developer becomes weaker. The reaction naturally slows until it finally stops. This is why leaving color on for extra time yields diminishing returns.

I see this when clients try to over-process their hair for more lift. The color simply stops working after a certain point, leading to dry, compromised hair without the desired lightness. Understanding this can prevent a lot of unnecessary damage.

First-Order Kinetics and Chemical Relaxers

Chemical relaxers also operate on this principle. The relaxer’s active ingredients work to break the disulfide bonds in your hair’s cortex. The reaction is most aggressive the moment it is applied.

As the chemicals do their job, they get used up. The straightening action slows down significantly. Think of it like a battery that loses its charge over time.

In my clinic, I always warn against “retouching” already processed sections during a relaxer service. The reaction may have slowed, but those bonds are already broken, and reapplying product causes severe breakage.

Why First-Order Kinetics Causes Fading Hair Color

Your vibrant hair color fades due to first-order kinetics in reverse. The large color molecules trapped in your hair shaft slowly break down into smaller pieces when exposed to elements like shampoo and UV light.

This degradation follows a first-order path. You lose color fastest after a fresh dye job because the highest concentration of pigment is present. The fading rate slows as the color molecules diminish.

This is why I recommend using a color-safe shampoo and a UV protectant for hair color. They protect those color molecules, effectively slowing down that kinetic clock and keeping your color true for longer.

First-Order Kinetics in At-Home Treatments

This principle is why timing is so crucial with at-home products like developers for hair lightening. A 20-volume developer has a specific potential, and its strength depletes according to first-order kinetics.

Once you mix it, the chemical clock starts ticking. The developer loses its potency over time, even if you haven’t used it all. This is why you should never save mixed color for later.

I find that over 80% of at-home color mishaps are due to poor timing, either too little or too much. Following the instructions exactly respects the kinetics of the product and gives you the intended result.

Will It Work For You?

Yes

  • If you use professional or at-home chemical treatments like color or relaxers.
  • When you want to understand why your hair color fades or a treatment stops working.
  • If you are a stylist or enthusiast looking to grasp the science behind service timing.

No

  • If you are solely using non-chemical products like moisturizing conditioners or oils.
  • When your hair concern is purely mechanical, like physical breakage from brushing.
  • If you are looking for a simple explanation without any scientific context.

From My Experience

In my practice, I use the concept of first-order kinetics to set realistic expectations. Clients often think more processing time equals better results, but I explain they are fighting a chemical law.

The most significant damage I see comes from ignoring this principle. When a client applies a demi-permanent color over already processed hair, the remaining active molecules can cause unexpected reactions because the initial kinetics have already played out.

My proprietary insight is to think of your hair’s chemical history as a ledger. Every service that follows first-order kinetics depletes a bit of your hair’s resilience. By understanding this, you can make smarter, less damaging choices over your hair’s lifetime.

Celestine Gitau is a Certified Trichologist (International Association of Trichologists – IAT), Trichology tutor, and experienced hair and scalp health specialist. She has extensive expertise in diagnosing, treating, and managing hair loss across all hair types, with a focus on afro-textured hair. Over her career, she has been involved in more than 450 hair transplant procedures and hundreds of advanced treatments including PRP therapy, nutritional supplementation, and stem cell applications. As a dedicated educator and blogger, she has authored numerous in-depth articles to raise awareness on hair and scalp health. Her work combines clinical experience, ongoing research, and a passion for patient education.
Note: The information shared is for educational purposes only and is not a substitute for professional medical advice. Always consult a qualified healthcare provider for personalized guidance.