Every conversation about damage and repair eventually hits the word "bonds." This article is the working reference. Three bond families hold hair together, each behaves differently under chemical and thermal load, and each is targeted by a different category of salon treatment. Understanding which bond is broken and which reforms in which condition is what separates a defensible protocol from a guess. The glossary covers the chemistry, what breaks each bond, what reforms it, and which product categories legitimately act on which type, with honest disclosure where the evidence is contested.
The glossary feeds the broader recovery framework and is the chemistry layer under every other article in the K02 cluster.
Why bonds matter for damage and repair
Hair is roughly 90 percent keratin protein by dry weight, organized into the cortex (70 to 80 percent of fiber mass) and held together by three bond families plus cell membrane complex lipids. Disulfide bonds carry structural load. Hydrogen bonds carry temporary shape. Salt bonds depend on pH. Different damage breaks different bonds: bleach and permanent color break disulfide bonds, water and heat styling rearrange hydrogen bonds, alkaline residue disrupts salt bonds. Knowing which bond a treatment claims to touch is how you read the marketing claim against the chemistry.
Disulfide bonds: the structural spine
The disulfide bond is a covalent sulfur-to-sulfur link between two cystine residues on adjacent keratin chains. Cystine makes up roughly 17 percent of hair's amino acid composition. The disulfide network gives hair its tensile strength and elasticity, and it is the bond chemistry that bleach and permanent color cleave.
The mechanism is oxidation. Peroxide and atmospheric oxygen convert the cystine sulfur-to-sulfur link into cysteic acid, detectable in damaged hair. Cysteic acid load correlates with bleach exposure history. A single ten-volume permanent color cycle breaks and reforms around 8 to 10 percent of available disulfide bonds; bleach runs significantly higher per service depending on lift. Reformation is slow and incomplete; each cycle leaves a network with fewer intact bonds than it started with.
Which products claim to act on disulfide bonds: bond builders. Maleate ester chemistry (bis-aminopropyl diglycol dimaleate, Olaplex-class), peptide blends (K18-class), and biomimetic acid-and-peptide systems (Envie SOS Express and adjacent formulas) all market disulfide reformation as the mechanism. Patent literature describes the chemistry; independent peer-reviewed evidence for permanent disulfide reformation in clinical conditions is contested. Breakage reduction and improved combability during and after chemical services are well-documented. For the category comparison, see which products claim to act on which bonds.
Hydrogen bonds: weak, numerous, water-sensitive
Hydrogen bonds are non-covalent, weak, and very numerous. They form between hydrogen atoms and electronegative atoms along the keratin backbone. Water breaks them on contact and they reform when the strand dries. This is why wet hair is weaker than dry, why wet hair stretches further before snapping, and why a blowout sets a temporary shape.
Hydrogen bonds matter for damage in two ways. Hygral fatigue: repeated swelling and contraction across wet-dry cycles, on a strand whose cuticle no longer reseals cleanly, progressively damages the CMC and the cortical protein matrix. This is the chemistry behind moisture overload and hygral fatigue. Heat styling: hot tools rearrange hydrogen bonds into a new shape and only damage the strand structurally above the glass transition temperature (around 230 to 250 degrees Fahrenheit dry).
No products act directly on hydrogen bonds; they reform on drying regardless. What products can do is support conditions that let them reform cleanly: acidic closes, conditioners that smooth surface charge, protein steps that reduce cortex gaps. Marketing claims that a product "rebuilds hydrogen bonds" are technically meaningless.
Salt bonds: pH-dependent ionic bonds
Salt bonds (ionic bonds) are medium-strength bonds that depend on the charge state of acidic and basic side chains along the keratin backbone. They engage at slightly acidic pH and disengage when the hair sits alkaline. Hair's isoelectric point sits around pH 3.6. Below it, salt bonds reform reliably; above it, they disperse.
This is the chemistry behind every acidic finishing rinse. After a chemical service, the shaft sits alkaline, typically pH 9 to 10, well above the strand's resting baseline of 4.5 to 5.5. The cuticle does not reseal fully and the salt bond network does not fully re-engage until pH drifts back down, which takes roughly 48 to 72 hours without intervention. An acidic close (pH 3.5 to 4.5) at the bowl accelerates that process to minutes, reseats the cuticle, and re-engages the salt bonds the alkaline service disrupted.
Salt bonds also govern surface charge and ingredient binding. Cationic conditioning polymers (behentrimonium chloride, the polyquaternium family) bind preferentially to anionic damage sites along a chemically stressed cuticle. This is why damaged hair takes conditioning ingredients more aggressively than virgin hair.
Which products claim to act on salt bonds: acidic finishing rinses and pH-balanced treatments. Documented and underused; the acidic close is the most often skipped step in the recovery sequence.
The cuticle layer: structure beyond bonds
The cuticle is its own structural story, though it interacts with bond chemistry. Five to ten overlapping cuticle layers sit outside the cortex. Between cuticle layers, the cell membrane complex (CMC) lipids bind them together. Outside the outermost cuticle, the F-layer (a thin lipid film with 18-methyleicosanoic acid as the principal lipid) gives healthy hair its hydrophobic surface.
CMC lipids strip during chemical services, particularly bleach. The F-layer strips with the first alkaline pass. Once lost, neither rebuilds; the strand sits more hydrophilic, more porous, and more prone to the swelling cycle that drives hygral fatigue. For the cuticle-versus-cortex framing, the surface-versus-recovery article is the reference.
Which products claim to act on which bonds
The honest mapping:
| Bond family | Product category | Claim | Evidence position |
|---|---|---|---|
| Disulfide | Bond builders | Permanent disulfide reformation | Contested in peer review; breakage reduction well-documented |
| Hydrogen | None directly | "Rebuild" claims are meaningless; bonds rebuild on every dry cycle | n/a |
| Salt | Acidic finishing rinses (pH 3.5-4.5) | Re-engage salt bonds, reseat cuticle | Well-documented |
| Cuticle/F-layer | Conditioners, masks, leave-ins | Smooth cuticle, restore surface feel | Well-documented; surface and temporary |
| Cortex protein | Protein treatments (hydrolyzed keratin, silk amino acids) | Fill cortex gaps, restore strength | Well-documented; effect lasts 4-8 wash cycles |
Protein treatments and acidic closes do exactly what they claim. Bond builders do something useful; the permanence claim is contested. Hydrogen bond claims in product marketing are usually meaningless.
How bond damage progresses across services
Virgin hair has the full disulfide network, intact F-layer, full salt bond and CMC integrity. A single-process permanent color breaks and reforms 8 to 10 percent of disulfide bonds, strips most of the F-layer, and disrupts salt bonds for 48 to 72 hours. Double-process breaks more disulfide bonds and strips CMC lipids more aggressively. Repeated lift compounds the damage; each pass adds cysteic acid load and progresses the cortex toward Tier 2 and Tier 3 elasticity loss.
The progression is not linear. The first two services produce visible recovery between cycles; by the fourth or fifth stacked service, recovery plateaus and the strand sits in a raised damage state. This is the inflection point that gates whether further chemical service is responsible.
Real signals that bond integrity has dropped
Loss of elasticity. The wet stretch test snaps below 20 percent stretch or stretches past 40 percent without rebound. The method sits in the elasticity test article.
Mid-shaft breakage. Short broken hairs on the pillow, brush, or shoulders, particularly in a halo at the crown or a band at the regrowth line. The breakage pattern map covers this in detail.
Banding. A visible band of fragile hair at a chemical regrowth line indicates disulfide depletion concentrated at the chemistry boundary.
Slow-dry mushy strands. Stretch-and-stay behavior combined with extended drying time indicates hygral fatigue, a downstream effect of long-term cuticle and CMC damage.
Embedded FAQ
How many types of bonds hold hair together?
Three categories: disulfide bonds (covalent, sulfur-to-sulfur, strongest), hydrogen bonds (weak, very numerous, water-sensitive), and salt or ionic bonds (medium strength, pH-dependent). Cuticle CMC lipids and the F-layer are sometimes counted as a fourth structural element, though strictly speaking they are not bonds.
Which bonds does bleach break?
Primarily disulfide bonds, through oxidation of cystine into cysteic acid. Bleach also disrupts melanin (the visible lightening effect), strips the F-layer, and degrades the cell membrane complex lipids. The structural damage that matters for hair integrity is at the disulfide level; lipid loss governs porosity drift and surface feel.
Which bonds reform when hair dries?
Hydrogen bonds. This is why wet hair is weaker and stretches more than dry, and why heat styling sets a temporary shape: the heat-and-tension state rearranges hydrogen bonds until the strand gets wet again. Salt bonds also re-engage as the strand pH drops back to baseline, on a longer timeline.
Can salon products rebuild disulfide bonds?
The claim is contested. Patent literature for maleate ester systems (Olaplex-class), peptide blends (K18-class), and biomimetic acid-and-peptide systems (Envie SOS Express and adjacent) describes mechanisms that bridge or reform disulfide-adjacent structures. Independent peer-reviewed evidence for permanent reformation in field conditions is not settled. Breakage reduction and improved combability during and after chemical services are well-documented.
Why does pH matter for hair bonds?
Salt bonds depend on the charge state of acidic and basic side chains. At pH below the isoelectric point (around 3.6), salt bonds reform reliably. Above it, they disperse. This is the chemistry behind acidic finishing rinses at pH 3.5 to 4.5: the rinse reseats the cuticle and re-engages the salt bond network the alkaline service disrupted.
Do conditioners and masks act on bonds?
Indirectly. Conditioners and masks act on the cuticle surface and F-layer through cationic polymer binding and emollient deposition. They smooth the strand and support hydrogen bond reformation as the hair dries. They do not act on disulfide bonds inside the cortex. Marketing that claims a conditioner rebuilds bonds is using the word loosely.
Where this fits
Use this glossary as the chemistry reference for the rest of the recovery cluster. Disulfide bonds for structural integrity, hydrogen bonds for shape and water behavior, salt bonds for pH and cuticle re-engagement, CMC and F-layer for cuticle structure. Each bond family is targeted by a different product category, with honest disclosure where marketing outruns evidence.
CTA
The Envie SOS Express 3-Step Recovery is built around the three bond families: a bond-building step for disulfide support, a light protein step for cortex gap filling, and an acidic close for salt bond and cuticle re-engagement. Match the right step to the right bond.
