Classification of surfactants

Surfactants are classified according to their chemical structure (head and tail) and their counter ion.

Classification

According to the composition of their tail

The tail of surfactants can be:

• A hydrocarbon chain: aromatic hydrocarbons (arenes), alkanes (alkyl), alkenes, cycloalkanes, alkyne-based
• An alkyl ether chain:
  • Ethoxylated surfactants: polyethylene oxides are inserted to increase the hydrophilic character of a surfactant
  • Propoxylated surfactants: polypropylene oxides are inserted to increase the lipophilic character of a surfactant
• A fluorocarbon chain: fluorosurfactants
• A siloxane chain: siloxane surfactants.

A surfactant can have one or two tails, these are called double-chained.

According to the composition of their head

Surfactant classification according to the composition of their head: nonionic, anionic, cationic, amphoteric.

A surfactant can be classified by the presence of formally charged groups in its head. A non-ionic surfactant has no charge groups in its head. The head of an ionic surfactant carries a net charge. If the charge is negative, the surfactant is more specifically called anionic; if the charge is positive, it is called cationic. If a surfactant contains a head with two oppositely charged groups, it is termed zwitterionic.

Some commonly encountered surfactants of each type include:

• Ionic
  • Anionic: based on permanent anions (sulfate, sulfonate, phosphate) or pH-dependent anions (carboxylate):
    • Sulfates
      • Alkyl sulfates: ammonium lauryl sulfate, sodium lauryl sulfate (SDS, sodium dodecyl sulfate, another name for the compound)
      • Alkyl ether sulfates: sodium laureth sulfate, also known as sodium lauryl ether sulfate (SLES), sodium myreth sulfate
    • Sulfonates:
      • Docusates: dioctyl sodium sulfosuccinate
      • Sulfonate fluorosurfactants: perfluorooctanesulfonate (PFOS), perfluorobutanesulfonate
      • Alkyl benzene sulfonates
    • Phosphates:
      • Alkyl aryl ether phosphate
      • Alkyl ether phosphate
    • Carboxylates:
      • Alkyl carboxylates: Fatty acid salts (soaps): sodium stearate;
      • Sodium lauroyl sarcosinate
      • Carboxylate fluorosurfactants: perfluorononanoate, perfluorooctanoate (PFOA or PFO)

• • Cationic: based on:
    • pH-dependent primary, secondary, or tertiary amines: Primary amines become positively charged at pH < 10, secondary amines become charged at pH < 4:
      • Octenidine dihydrochloride;
    • Permanently charged quaternary ammonium cation:
      • Alkyltrimethylammonium salts: cetyl trimethylammonium bromide (CTAB) a.k.a. hexadecyl trimethyl ammonium bromide, cetyl trimethylammonium chloride (CTAC
      • Cetylpyridinium chloride (CPC)
      • Benzalkonium chloride (BAC)
      • Benzethonium chloride (BZT)
      • 5-Bromo-5-nitro-1,3-dioxane
      • Dimethyldioctadecylammonium chloride
      • Dioctadecyldimethylammonium bromide (DODAB)

• • Zwitterionic (amphoteric): based on primary, secondary, or tertiary amines or quaternary ammonium cation with:
    • Sulfonates:
      • CHAPS (3-[(3-Cholamidopropyl)dimethylammonio]-1-propanesulfonate);
      • Sultaines: cocamidopropyl hydroxysultaine;
    • Carboxylates:
      • Amino acids
      • Imino acids
      • Betaines: cocamidopropyl betaine;
    • Phosphates: lecithin

• Nonionic
  • Fatty alcohols:
    • Cetyl alcohol
    • Stearyl alcohol
    • Cetostearyl alcohol (consisting predominantly of cetyl and stearyl alcohols)
    • Oleyl alcohol
  • Polyoxyethylene glycol alkyl ethers (Brij): CH₃–(CH₂)_10–16–(O-C₂H₄)_1–25–OH:
    • Octaethylene glycol monododecyl ether
    • Pentaethylene glycol monododecyl ether
  • Polyoxypropylene glycol alkyl ethers: CH₃–(CH₂)_10–16–(O-C₃H₆)_1–25–O
  • Glucoside alkyl ethers: CH₃–(CH₂)_10–16–(O-Glucoside)_1–3–OH:
    • Decyl glucoside,
    • Lauryl glucoside
    • Octyl glucoside
  • Polyoxyethylene glycol octylphenol ethers: C₈H₁₇–(C₆H₄)–(O-C₂H₄)_1–25–OH:
    • Triton X-100
  • Polyoxyethylene glycol alkylphenol ethers: C₉H₁₉–(C₆H₄)–(O-C₂H₄)_1–25–OH:
    • Nonoxynol-9
  • Glycerol alkyl esters:
    • Glyceryl laurate
  • Polyoxyethylene glycol sorbitan alkyl esters: Polysorbate
  • Sorbitan alkyl esters: Spans
  • Cocamide MEA, cocamide DEA
  • Dodecyldimethylamine oxide
  • Block copolymers of polyethylene glycol and polypropylene glycol: Poloxamers
  • Polyethoxylated tallow amine (POEA).

According to the composition of their counter-ion

In the case of ionic surfactants, the counter-ion can be:

• Monoatomic / Inorganic:
  • Cations: metals : alkali metal, alkaline earth metal, transition metal
  • Anions: halides: chloride (Cl⁻), bromide (Br⁻), iodide (I⁻)
• Polyatomic / Organic:
  • Cations: ammonium, pyridinium, triethanolamine (TEA)
  • Anions: tosyls, trifluoromethanesulfonates, methylsulfate