If you have ever switched shampoos and noticed your hair felt rougher, frizzier, or harder to comb, pH may be the explanation nobody mentioned. It is one of the more consistently misunderstood aspects of hair care, partly because it sounds like a chemistry lesson, and partly because most shampoo labels do not disclose it. But the effect of pH on hair structure is well-documented in dermatology research, and it is directly relevant to anyone making or choosing a shampoo bar.

This article explains what pH is, what it does to the hair shaft, what the research says about shampoo pH specifically, and why syndet bars are formulated to sit in the range that hair actually prefers.

What pH is, briefly

pH is a measure of how acidic or alkaline a solution is, on a scale of 0 to 14. A pH of 7 is neutral. Below 7 is acidic; above 7 is alkaline. The scale is logarithmic, which means each step is ten times more or less acidic than the one adjacent to it. A solution at pH 9 is not slightly more alkaline than one at pH 8, it is ten times more alkaline.

Your scalp maintains a naturally acidic environment, generally sitting around pH 4.5 to 5.5. The hair shaft itself is slightly more acidic than that, with the isoelectric point of hair keratin sitting around pH 3.67. These numbers matter because they define the environment in which your hair functions best (and the environment that most commercial shampoos do not match).

What the hair cuticle is and why it responds to pH

The hair shaft is made up of three layers. The innermost is the cortex, which contains the fibrous keratin proteins that give hair its strength and elasticity. Around that sits the cortex. The outermost layer is the cuticle which is a series of overlapping, scale-like cells that wrap around the shaft in the same way roof tiles overlap each other. The cuticle’s job is protective. When its scales lie flat and close together, they seal the cortex from moisture loss, reduce friction between individual hairs, and give hair its surface smoothness and shine.

The cuticle is pH-sensitive. A 2024 study published in PubMed examining the structural effects of pH on the hair shaft found that exposing the hair shaft to varying pH levels led to specific effects on the cuticle, including cuticle lifting at certain pH levels, with more pronounced effects observed at extreme acidic conditions and alkaline conditions above pH 8. pH levels between 5 and 7 had minimal impact on hair structure and proteins, suggesting that haircare products with pH in this range are ideal for hair-shaft health.

In simple terms: the cuticle scales open when they encounter an alkaline environment and close in an acidic one. This is not a subtle cosmetic effect, it is a measurable structural response.

What happens when the cuticle opens

An open cuticle is a cuticle under stress. When the scales lift away from the shaft, several things happen simultaneously and none of them are good for hair health.

Moisture escapes. The cortex, no longer sealed, loses water more readily. Hair becomes dry and brittle over time not because it lacks conditioning ingredients but because the physical seal that retains moisture has been compromised.

Friction increases. Lifted cuticle scales catch on each other and on adjacent hairs. A 2014 study published in the International Journal of Trichology analysed the pH of 123 shampoos from international brands and concluded that alkaline pH may increase the negative electrical charge of the hair fibre surface and therefore increase friction between fibres, which may lead to cuticle damage and fibre breakage. It is a reality and not a myth that lower pH shampoos may cause less frizzing by generating less negative static electricity on the fibre surface.

That study also found that of the 123 shampoos analysed, 61.78% had a pH above 5.5. The majority of commercial shampoos sit above the range that the research identifies as optimal for hair health.

Protein is lost. An open cuticle allows not just water but keratin proteins to leach from the cortex over time. The same 2024 PubMed study found significant deamidation of keratins types I and II at alkaline conditions (a form of protein degradation that weakens the structural integrity of the hair shaft and accumulates with repeated exposure).

Colour fades faster. For colour-treated hair, an open cuticle is a direct route out for artificial pigment. Dye molecules that have been deposited into the cortex can exit through lifted cuticle scales with every wash. This is why colour longevity is so closely tied to the pH of the products used to maintain it.

Where conventional shampoos sit on the pH scale

Most liquid shampoos are formulated somewhere between pH 5 and pH 7, though as the Trichology study above shows, a significant proportion sit above 5.5. Anti-dandruff shampoos in particular tend to run higher – the same study found that 80.77% of anti-dandruff shampoos presented a pH above 5.5.

The practice of adding citric acid or lactic acid to lower the pH of a shampoo is standard in professional formulation precisely because the base formula without a pH adjuster often sits too high. Without deliberate pH adjustment, most surfactant-based cleansers will land in an alkaline range relative to the hair shaft.

This is not a scandal, just a formulation challenge that many manufacturers address and many don’t. But it is worth understanding, because it means that “shampoo” as a category covers a wide range of pH values with meaningfully different effects on hair structure.

Where soap-based shampoo bars sit

Cold process soap bars (those made by combining oils with sodium hydroxide (lye) through a chemical process called saponification) are inherently alkaline. The saponification reaction produces a finished product with a pH typically between 9 and 10. This cannot be adjusted down without altering the fundamental chemistry of the product. A cold process bar is soap, and soap is alkaline.

For washing skin, this is largely manageable. Skin has a more robust barrier and a slightly less acidic natural pH than the hair shaft. For washing hair, a pH of 9 to 10 sits well outside the optimal range identified by the research. Every wash with a soap-based shampoo bar is an alkaline exposure event for the cuticle meaning the scales lift, friction increases, and the effects described above begin to accumulate.

This is also the source of the ACV rinse recommendation that circulates in cold-process shampoo bar communities. An apple cider vinegar rinse is acidic, and it temporarily restores a lower pH environment to help close the cuticle after a soap wash. It works, but it is a correction for a pH problem built into the product, not an enhancement. A bar that sits in the right pH range to begin with does not need a corrective rinse.

Where syndet bars sit

Syndet bars are built from surfactant bases rather than saponified oils, which means their pH is not fixed by chemistry but rather a deliberate formulation decision. A syndet bar naturally lands in the 5 to 6.5 range. This puts it within or close to the optimal window identified by the research: above the extreme acidity that causes its own structural effects, and below the alkaline threshold where cuticle lifting begins.

The Batch and Bubble oily hair formula or normal hair formula syndet shampoo bar is a great place to start if you want to make your own, ideal pH, shampoo bar.

For a full comparison of syndet bars and their soap-based counterparts, the syndet vs liquid shampoo article covers the broader differences beyond pH. And if you have been using a cold process bar and noticed increased frizz, dryness, or difficulty combing, the frizz and shampoo bars guide covers exactly what is likely happening and how switching bar type addresses it at the source.

Find your personal syndet bar match with the Batch and Bubble hair type quiz:

pH and the scalp microbiome

The scalp’s slightly acidic environment does more than protect the hair shaft, it also maintains conditions that support a healthy microbial balance. The scalp microbiome is a community of bacteria, fungi, and other microorganisms that live on the scalp surface and play a role in scalp health. That community functions best in an acidic environment.

Introducing an alkaline cleanser disrupts the pH of the scalp surface with each wash. As covered in more detail in the biology of the transition period, this disruption can contribute to sebum imbalance and microbiome instability over time. A shampoo that sits within the scalp’s natural pH range causes considerably less disruption to this ecosystem.

A practical note on syndet bars and pH

One of the practical advantages of syndet bars over soap-based bars is that the pH is inherent to the surfactant chemistry rather than something that needs correcting. Surfactants like SCI are mildly acidic by nature, which is why a well-formulated syndet bar sits in the right range without requiring a pH adjuster. This is not a marketing claim, just a consequence of the chemistry the bar is built on.

For home formulators, this means you are not fighting the formula the way cold process soapmakers sometimes do. The ingredients themselves are doing the right thing from the start.

Written by Neets at Batch and Bubble
DIY personal bathroom care recipes for real life.
www.batchandbubble.com