2.6 Salon Chemistry: Matter, Solutions, Emulsions, Ingredients & the pH Scale
Key Takeaways
- The pH scale runs 0 to 14 and is logarithmic: each whole number is a tenfold change, so pH 9 is 100 times more alkaline than pH 7.
- Hair and skin sit at pH 4.5 to 5.5, the acid mantle; products above that swell and open the cuticle, products below it contract and close it.
- A solution is a stable uniform mixture, a suspension separates on standing and must be shaken, and an emulsion is two immiscible liquids held together by a surfactant.
- Oil-in-water emulsions feel light and rinse readily; water-in-oil emulsions feel rich and occlusive, which is why cold cream and pressing oil behave as they do.
- Alkanolamines replace ammonia in low-odour haircolor but still raise pH to swell the cuticle, so "ammonia-free" does not mean "damage-free."
2.6 Salon Chemistry: Matter, Solutions, Emulsions, Ingredients & the pH Scale
The NIC theory outline devotes an entire sub-domain to basic chemistry used in cosmetology: recognising the purpose and effects of ingredients, interactions between chemicals, chemical reactions such as overexposure and chemical burn, and the values of the pH scale. Virginia backs this with 55 curriculum hours of general sciences covering chemical usage and safety. Everything in chapters 5 and 6 of this guide is downstream of what follows.
Matter, and the difference between a physical and a chemical change
Matter is anything with mass that occupies space. It exists as solid (definite shape and volume), liquid (definite volume, takes the shape of its container), and gas (neither definite shape nor volume). Salon work moves between all three constantly — solid polymer powder, liquid monomer, and the vapour that rises from both.
| Physical change | Chemical change | |
|---|---|---|
| What changes | Form or state only | Molecular identity |
| Reversible? | Usually | Usually not |
| Salon example | Wet-setting hair: water breaks hydrogen bonds, drying reforms them | Permanent waving: thio breaks disulfide bonds and neutraliser rebuilds them elsewhere |
| Another example | Melting hard wax; blow-drying a set | Oxidative haircolor developing; relaxer lanthionisation; acrylic polymerisation |
The wet-set-versus-perm contrast is the classic exam pairing. A wet set is a temporary physical change because water reversibly disrupts hydrogen bonds; a perm is a permanent chemical change because disulfide bonds are cleaved and reformed in a new configuration.
Solutions, suspensions and emulsions
| Mixture | Description | Behaviour | Salon example |
|---|---|---|---|
| Solution | Solute fully and uniformly dissolved in a solvent | Stable, clear, does not separate | Salt water; alcohol-based toner; many liquid astringents |
| Suspension | Larger particles dispersed but not dissolved | Separates on standing — shake before use | Calamine lotion; some liquid makeup and powder lighteners in oil |
| Emulsion | Two immiscible liquids held in a stable mix by an emulsifier (surfactant) | Stable for a defined shelf life | Conditioners, cream haircolor, cleansing milks, hand lotion |
Miscible liquids mix (water and alcohol); immiscible liquids do not (water and oil). An emulsifier bridges them because its molecule has a hydrophilic (water-loving) head and a lipophilic (oil-loving) tail.
Two emulsion directions, two very different feels:
- Oil-in-water (O/W) — oil droplets suspended in a water base. Lighter, less greasy, rinses easily. Most conditioners, cream rinses and lotions.
- Water-in-oil (W/O) — water droplets suspended in an oil base. Richer, heavier, occlusive. Cold creams, pressing oils, heavy barrier and base creams.
That distinction explains a practical rule from chapter 5: a base cream applied before a sodium hydroxide relaxer is water-in-oil precisely because you want an occlusive film the alkaline solution cannot penetrate.
Surfactants, the workhorse molecule
A surfactant (surface active agent) reduces the surface tension between two substances. In shampoo, the lipophilic tail buries itself in sebum and grime while the hydrophilic head stays in the water, so the soil is lifted into a spherical micelle and rinsed away.
| Surfactant class | Charge | Where it shows up |
|---|---|---|
| Anionic | Negative | Primary detergents — sulfates in most shampoos; strong cleansing, high foam |
| Cationic | Positive | Conditioners and quats; the positive charge is attracted to negatively charged damaged hair |
| Non-ionic | None | Mild co-washes and emulsifiers; low irritation |
| Amphoteric | Both, depending on pH | Baby and low-irritation shampoos; cocamidopropyl betaine |
Because damaged and porous hair carries a more negative surface charge, cationic conditioning agents deposit preferentially on the areas that need them most. That is a real molecular explanation, not marketing.
The pH scale
pH measures the concentration of hydrogen ions in a water-based solution on a scale of 0 to 14. 7 is neutral. Below 7 is acidic; above 7 is alkaline.
The scale is logarithmic. Each whole number is a tenfold change:
- pH 8 is 10 times more alkaline than pH 7.
- pH 9 is 100 times more alkaline than pH 7.
- pH 10 is 1,000 times more alkaline than pH 7.
That is why a perm at pH 9.2 and a relaxer at pH 13 are not "a bit different" — they are four orders of magnitude apart in hydrogen-ion concentration.
| Approximate pH | Product or tissue | Effect on the cuticle |
|---|---|---|
| 0–3.5 | Strong acids; some professional neutralising and finishing rinses at 2.5–4.5 | Contracts and hardens; closes the cuticle |
| 4.5–5.5 | Natural hair and skin — the acid mantle | Baseline: cuticle lies flat |
| 5–7 | Acid-balanced shampoos, conditioners, demi-permanent colour | Minimal swelling |
| 7–8.5 | Some clarifying and alkaline shampoos | Mild swelling; cuticle begins to lift |
| 8.5–9.6 | Alkaline perms; permanent haircolor with developer | Significant swelling; cuticle opens for penetration |
| 12.5–13.5 | Hydroxide relaxers | Extreme swelling; permanent bond cleavage |
The single most useful generalisation in cosmetology chemistry: alkalinity swells and opens; acidity contracts and closes. Every chemical service is an application of it — raise pH to get product in, then lower pH to close the cuticle and stop the reaction.
Ingredients you are expected to recognise
- Water (aqua) — the universal solvent and, by volume, the first ingredient in most salon products.
- Alcohols — solvents and antiseptics; isopropyl alcohol at 70% or above is the standard for wiping the gel inhibition layer. Higher concentrations evaporate too fast to be effective antiseptics.
- Ammonia (NH₃) — alkalising agent in permanent haircolor; swells the cuticle and helps activate peroxide.
- Alkanolamines (for example monoethanolamine) — ammonia substitutes in "low-odour" or "ammonia-free" colour. They raise pH with far less smell, but they still raise pH. Do not let a client hear "ammonia-free" as "gentle."
- Hydrogen peroxide (H₂O₂) — the oxidising developer; volume expresses available oxygen.
- Thioglycolic acid / ammonium thioglycolate (ATG) — the reducing agent in alkaline perms and thio relaxers.
- Sodium, calcium, lithium, guanidine hydroxides — the extremely alkaline relaxer bases.
- Silicones (dimethicone, cyclomethicone) — smoothing film-formers; heavy non-soluble silicones build up and can block chemical penetration, which is why clarifying before a chemical service matters.
- Humectants (glycerin, panthenol, hyaluronic acid) — attract and hold water.
- Proteins and hydrolysed keratin — temporarily fill and reinforce porous cuticle and cortex.
- Preservatives — necessary in any water-containing product; the leading cause of "clean" formulations spoiling is their absence.
Overexposure and incompatibility, in one paragraph
Two chemical hazards recur in the NIC blueprint. Overexposure is repeated or prolonged contact building toward sensitisation — an acquired allergic response that, once developed, is usually permanent. Incompatibility is two products reacting destructively: hydroxide over thio, metallic salts under peroxide, or chlorine bleach mixed with an acid or ammonia product. Both are treated in detail in section 2.7.
How much more alkaline is a product at pH 9 than one at pH 7?
Which statement correctly distinguishes an oil-in-water emulsion from a water-in-oil emulsion?
A client insists on an "ammonia-free" permanent haircolor because she believes it will not open the cuticle. What is the accurate response?