01 · Structure

The molecules that don’t break.

Per- and polyfluoroalkyl substances (PFAS) are a family of more than 12 000 synthetic compounds defined by a fully or partially fluorinated carbon backbone. Every C–F bond in the chain is a chemical fortress.

The C–F bond

Fluorine is the most electronegative element (χ = 3.98). The resulting C–F bond is highly polar, short (1.39 Å), and strong — roughly 485 kJ·mol⁻¹, the strongest single bond in organic chemistry. A perfluoroalkyl chain is essentially shielded from nucleophiles, oxidants, and microbial enzymes.

Hydrolysis at neutral pH: k ≈ 0
Atmospheric photolysis half-life of PFOA: > 100 years
No known full enzymatic mineralisation pathway in nature

Bond comparison

Bond	Length (Å)	Energy (kJ/mol)
C–F	1.39	485
C–H	1.09	413
C–Cl	1.78	328
C–O	1.43	358

Three iconic PFAS

Hover any atom. Carbons form the backbone; the fluorine sheath (highlighted in aqua) makes the molecule chemically inert. The polar head group — carboxylate, sulfonate, or ether-acid — controls water solubility and protein binding.

Perfluorooctanoic acid (PFOA) — surfactant tail + carboxylate head.C₈HF₁₅O₂

Perfluorooctanesulfonic acid (PFOS) — the dominant component of legacy AFFF.C₈HF₁₇O₃S

Trifluoroacetic acid (TFA) — terminal degradation product of many short-chain PFAS and HFC refrigerants. Increasingly detected in rainwater worldwide.C₂HF₃O₂

GenX (HFPO-DA)

The replacement chemistry introduced after PFOA was phased out. An ether oxygen breaks the perfluorinated chain, but the molecule is still environmentally persistent and mobile.

CF3-CF2-CF2-O-CF(CF3)-COOH      (HFPO-DA, "GenX")
Molecular formula:  C6HF11O3
Molecular weight:   330.05 g/mol
Detected in Cape Fear River up to ~4500 ng/L

Source: AFFF firefighting foam

AFFF concentrates were developed by the U.S. Naval Research Lab and 3M in the 1960s to extinguish jet-fuel fires. They contain 1–5 % fluorosurfactants whose low surface tension lets the foam spread an aqueous film over hydrocarbon fuel, sealing it from oxygen.

Key reaction — production

PFOS was historically made by electrochemical fluorination (Simons process):

C8H17SO2F  +  17 HF   --(electrolysis, anhydrous HF, ~5 V)-->
C8F17SO2F  +  17 H2   +  by-products (branched isomers)

C8F17SO2F  +  H2O     -->   C8F17SO3H  +  HF       (hydrolysis)

Key reaction — environmental fate

Fluorotelomer alcohol precursors oxidise stepwise to terminal PFCAs:

8:2 FTOH   (C8F17-CH2CH2-OH)
   |  •OH  (atmospheric / microbial)
   v
8:2 FTAL   -->   8:2 FTCA   -->   8:2 FTUCA
   |
   v
PFOA  (C7F15COOH)   +   shorter PFCAs

Conditions: aerobic, ambient T, hydroxyl radical
            or microbial β-oxidation (Houtz & Sedlak, 2012).

Why these chemicals are a concern

Persistence

No environmental sink. Once released, the perfluoroalkyl moiety essentially never degrades — only redistributes.

Mobility

High water solubility + low Koc ⇒ contaminates groundwater kilometres from the source.

Bioaccumulation

Binds serum albumin and liver FABPs in vertebrates; long serum half-lives (years in humans).

Toxicity

Endocrine disruption, immunotoxicity, hepatotoxicity, and carcinogenicity (PFOA — IARC Group 1, 2023).

Detection limits

Toxic at ng/L — at or below routine analytical limits, requiring isotope-dilution LC-MS/MS to quantify.

Treatment difficulty

Resists conventional drinking-water treatment; destruction requires high-energy or specialised methods.