4.2 General Chemistry, Matter & the pH Scale in Hair and Skin Care

4.2 General Chemistry, Matter & the pH Scale in Hair and Skin Care

State Board Essential: Every chemical service—coloring, permanent waving, relaxing, and shampooing—is an applied chemical process. Understanding molecular structure, physical mixtures, surfactant micelles, and the logarithmic pH scale prevents chemical hair breakage and scalp burns.

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1. Structure of Matter: Elements, Atoms & Molecules

Chemistry studies the composition, properties, and changes of matter.

• Matter: Any substance occupying space and possessing mass. It exists in three states: solid (definite shape/volume), liquid (definite volume, takes container shape), and gas (indefinite shape/volume).
• Elements: Simplest chemical forms containing only one type of atom, incapable of being broken down by ordinary chemical methods (118 recognized elements).
• COHNS Elements: Keratin protein in hair, skin, and nails consists of 5 elements: Carbon (51%), Oxygen (21%), Hydrogen (6%), Nitrogen (17%), and Sulfur (5%).
• Atoms & Molecules: Atoms are an element's structural units (protons, neutrons, electrons). Molecules form when atoms combine chemically:
  • Elemental Molecules: Two or more atoms of the same element (e.g., atmospheric oxygen $O_2$, ozone $O_3$).
  • Compound Molecules: Two or more atoms of different elements in fixed proportions (e.g., water $H_2O$, hydrogen peroxide $H_2O_2$, ammonia $NH_3$).

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2. Physical vs. Chemical Properties and Transformations

• Physical Properties: Determined without altering chemical composition (e.g., color, odor, density, melting point, boiling point, hardness).
• Chemical Properties: Determined only by chemical reactions that alter molecular structure (e.g., flammability, oxidation potential, reactivity).

Comparing Transformations in Salon Practice

• Physical Change: Alters form or physical state without creating a new substance.
  • Examples: Ice melting into water; blow-drying or curling hair with a flat iron (temporarily reforming weak physical hydrogen bonds that reset upon exposure to moisture).
• Chemical Change: Alters chemical composition to produce new substances with distinct chemical and physical properties.
  • Examples: Oxidation of melanin by hydrogen peroxide; permanent waving (reducing disulfide bonds with ammonium thioglycolate and reforming them with neutralizer); chemical relaxing (converting cystine to lanthionine); permanent hair coloring.

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3. Pure Substances & Physical Mixtures: Solutions, Suspensions & Emulsions

• Pure Substance: Matter with fixed chemical proportions throughout (e.g., distilled water, elemental sulfur).
• Physical Mixture: Combination of two or more substances united physically in variable proportions without chemical bonds.

Three Types of Physical Mixtures

1. Solutions: Stable, homogeneous mixtures where a solute (dissolved substance) dissolves completely into a solvent (dissolving medium; water is the universal solvent). Solutions are transparent and never separate. Liquids that mix freely are miscible (water and alcohol); those that cannot mix are immiscible (water and oil).
2. Suspensions: Unstable, heterogeneous mixtures containing undissolved solid particles dispersed in liquid. Particles are visible, cloud the liquid, and settle to the bottom upon standing (e.g., calamine lotion, glitter nail polish, certain hair tonics). Suspensions must be shaken vigorously before use.
3. Emulsions: Unstable physical mixtures of two immiscible liquids bound together by an emulsifier (binder or surfactant), remaining stable over extended periods.

Oil-in-Water (O/W) vs. Water-in-Oil (W/O) Emulsions

• Oil-in-Water (O/W): Oil droplets dispersed in continuous water. Rinses cleanly, feels light and non-greasy. Examples: most shampoos, conditioners, moisturizing lotions, facial cleansing milks.
• Water-in-Oil (W/O): Water droplets dispersed in continuous oil. Heavier, water-resistant, and greasy. Examples: cold creams, heavy night barrier creams, massage balms.

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4. Surfactants & the Micelle Cleansing Mechanism

Surfactants (surface active agents) reduce surface tension between immiscible liquids, allowing products to spread and cleanse:

• Hydrophilic Head: Polar, "water-loving" end attracted to water.
• Lipophilic Tail: Nonpolar, "oil-loving" hydrocarbon tail attracted to oil, sebum, and grease.
• Micelle Cleansing Mechanism: During shampooing, lipophilic tails embed into sebum and oils on hair, while hydrophilic heads orient outward into rinse water. This creates spherical micelles trapping oily dirt inside. Rinsing with water pulls the outward heads down the drain, flushing the trapped debris away.

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5. The Logarithmic pH Scale & Acid Mantle Dynamics

pH (potential hydrogen) measures active hydrogen ions ($H^+$) versus hydroxide ions ($OH^-$) in aqueous solutions. Anhydrous products (pure oils, mineral oil, silicones, waxes) contain no water and have no pH.

Logarithmic Scale Mechanics (Powers of 10)

The pH scale spans 0.0 to 14.0:

• pH 0.0 to 6.9: Acidic (excess $H^+$ ions).
• pH 7.0: Neutral (equal $H^+$ and $OH^-$ ions; pure distilled water).
• pH 7.1 to 14.0: Alkaline / Base (excess $OH^-$ ions).

Because the scale is logarithmic, each whole unit change represents a tenfold (10×) difference:

• pH 5 is 10 times more acidic than pH 6.
• pH 5 is 100 times (10 × 10) more acidic than neutral water at pH 7.
• pH 4 is 1,000 times (10 × 10 × 10) more acidic than pH 7.
• Perm solution at pH 9 is 100 times more alkaline than neutral water at pH 7.

Acid Mantle & Cuticle Action

The skin and scalp are shielded by the acid mantle (sebum and sweat blend) with a natural pH of 4.5 to 5.5:

• Acids on Keratin: Acidic solutions below pH 7.0 (especially 4.5–5.5) contract, shrink, tighten, and harden the cuticle, flattening scales, restoring shine, and locking in moisture.
• Alkalis on Keratin: Alkaline solutions above pH 7.0 soften, swell, and open the cuticle, allowing chemicals (colors, perms, relaxers) to penetrate into the cortex. Unchecked alkalinity dissolves keratin and causes chemical burns.
• Chemical Neutralization: Occurs when an acid and alkali react in equal proportions to yield neutral water and a salt (e.g., neutralizing shampoos applied after relaxers).