# Cetyltrimethylammonium Bromide (CTAB) (CAS 57-09-0) — Complete Guide to Properties and Applications **Category:** Cationic Surfactant | **Formula:** C₁₉H₄₂BrN | **MW:** 364.45 g/mol --- ## What Is Cetyltrimethylammonium Bromide (CTAB)? **Cetyltrimethylammonium Bromide (CTAB) (CAS 57-09-0)** is a cationic surfactant with the molecular formula **C₁₉H₄₂BrN** and molecular weight **364.45 g/mol**. The IUPAC systematic name is **hexadecyl(trimethyl)azanium bromide**. It appears as a **white crystalline powder**. ## How Does It Compare to Alternatives? | Reagent | CAS | Key Difference | Best For | |---------|-----|----------------|----------| | **Cetyltrimethylammonium Bromide (CTAB)** | 57-09-0 | Reference compound | General use | | SDS (Sodium Dodecyl Sulfate) | 151-21-3 | Anionic surfactant with negatively charged sulfate head group. Incompatible with CTAB in solution — mixing produces insoluble catanionic precipitates. SDS is the standard for protein denaturation and SDS-PAGE; CTAB is the standard for nucleic acid precipitation from solutions containing high polysaccharide contamination. | Protein electrophoresis, general detergency, anionic systems | | DTAB (Dodecyltrimethylammonium Bromide) | 1119-94-4 | Shorter C12 chain vs CTAB's C16. Higher CMC (15 mM vs 0.92 mM for CTAB), lower micelle aggregation number (~50 vs ~90), and lower solubilization capacity. Better water solubility at room temperature. Forms smaller, more dynamic micelles. | Applications requiring fast micelle exchange kinetics, room-temperature handling of higher surfactant concentrations | | TTAB (Tetradecyltrimethylammonium Bromide) | 1119-97-7 | Intermediate C14 chain. CMC of ~3.5 mM falls between DTAB and CTAB. Offers a compromise between solubility (better than CTAB) and micelle stability (better than DTAB). Often used as a reference compound for structure-property studies of quaternary ammonium surfactants. | Comparative surfactant studies, moderate hydrophobicity applications | *Comparison data verified against supplier specifications.* ## When to Use Each Option ### CTAB vs SDS: Choosing the Right Ionic Surfactant CTAB and SDS represent the two most common ionic surfactant classes — cationic and anionic — and their selection depends critically on the specific application requirements. **Choose CTAB when:** - Extracting DNA from plant or fungal tissue rich in polysaccharides — CTAB forms insoluble complexes with polysaccharides and proteins while leaving nucleic acids in solution. - Synthesizing mesoporous silica (MCM-41, MCM-48) — the C16 chain length and positively charged head group template uniform cylindrical mesopores with tunable diameters (2–10 nm). - Preparing gold nanorods — CTAB forms a bilayer on the gold surface, directing anisotropic growth along the [001] direction to produce rods with controllable aspect ratios (2–10). - Formulating hair conditioners and fabric softeners — the positively charged head group adsorbs electrostatically to negatively charged surfaces (hair keratin, cotton fibers), imparting softness and anti-static properties. - Working with acidic solutions (pH < 4) — quaternary ammonium head groups remain cationic across the full pH range, unlike carboxylate or sulfate surfactants that protonate and lose their charge in acid. **Choose SDS when:** - Protein denaturation is desired — SDS binds at ~1.4 g/g protein, overwhelming intrinsic charge and enabling molecular weight-based separation. - Cost is the primary constraint — SDS is substantially less expensive than CTAB on a per-kilogram basis. - High-foaming applications — SDS generates denser, more stable foam than CTAB. - Alkaline formulation conditions (pH > 10) — SDS is stable indefinitely; quaternary ammonium surfactants undergo Hofmann elimination in strong base. **Critical Compatibility Note:** Never mix CTAB and SDS in the same aqueous solution unless you specifically want a catanionic surfactant precipitate. The electrostatic attraction between the oppositely charged head groups forms an insoluble 1:1 complex that precipitates from solution. This incompatibility must be considered when designing multi-surfactant formulations or sequential washing protocols. ### CTAB in Gold Nanorod Synthesis: A Unique Templating Role The seed-mediated growth method for gold nanorods, developed by Murphy and El-Sayed and refined by many groups, relies on CTAB as both a shape-directing agent and colloidal stabilizer. CTAB forms a densely packed bilayer on specific gold crystal facets — preferentially the {100} and {110} side facets of the growing rod — while leaving the {111} end facets relatively exposed. This differential binding directs gold atom deposition to the rod ends, producing anisotropic growth. The aspect ratio is controlled by the concentration of silver nitrate (AgNO₃) in the growth solution, with higher Ag⁺ concentrations producing longer rods. Without CTAB, gold nanoparticles grow as isotropic spheres because all crystal facets are equally accessible to gold atom deposition. After synthesis, CTAB must be removed or exchanged for biocompatible ligands before biomedical applications, as free CTAB is cytotoxic at micromolar concentrations. Thiolated polyethylene glycol (mPEG-SH) is the most common replacement ligand, displacing CTAB through the strong Au–S bond (bond energy ~40 kcal/mol). ### CTAB in Mesoporous Material Templating CTAB is the prototypical structure-directing agent for the synthesis of MCM-41, the first ordered mesoporous silica discovered by Mobil researchers in 1992. Under basic conditions (pH 11–12, typically with tetramethylammonium hydroxide or NaOH), CTAB micelles assemble into a hexagonal lyotropic liquid crystalline phase. Tetraethylorthosilicate (TEOS) hydrolyzes and condenses around the CTAB micelle rods, forming a silica framework with hexagonally ordered cylindrical pores. Calcination at 550 °C in air removes the organic template, leaving mesoporous silica with pore diameters of 2.5–4 nm, surface areas exceeding 1,000 m²/g, and pore volumes of 0.8–1.2 cm³/g. The pore size can be expanded to 5–10 nm by adding swelling agents (1,3,5-trimethylbenzene, decane) that partition into the micelle core, or by using longer-chain surfactants (C18TAB, C20TAB). MCM-41 and related materials find applications in catalysis, adsorption, drug delivery, and chromatography. ## Frequently Asked Questions ### What is the CAS number of Cetyltrimethylammonium Bromide (CTAB)? The CAS Registry Number is **57-09-0**. This unique identifier is registered with the Chemical Abstracts Service and serves as the authoritative reference for chemical substance identification in scientific literature, regulatory filings, and commercial transactions. When ordering Cetyltrimethylammonium Bromide (CTAB) from any supplier, specifying the CAS number ensures you receive the correct compound regardless of naming variations. ### What is the molecular weight of Cetyltrimethylammonium Bromide (CTAB)? The molecular weight is **364.45 g/mol** based on the molecular formula **C₁₉H₄₂BrN**. This value is calculated from the standard atomic weights of the constituent elements and represents the mass of one mole of the compound. For synthetic planning, use this value to calculate molar equivalents and prepare solutions of known concentration. ### What is Cetyltrimethylammonium Bromide (CTAB) used for? Cetyltrimethylammonium Bromide (CTAB) is primarily used as a cationic surfactant in organic synthesis and related chemical processes. The specific applications depend on the particular research or industrial context. For detailed application information, consult the product sections above or refer to the supplier's technical documentation and published synthetic protocols. ### How should Cetyltrimethylammonium Bromide (CTAB) be stored? Store Cetyltrimethylammonium Bromide (CTAB) in a cool, dry, well-ventilated area away from incompatible materials including strong oxidizing agents, strong acids, and sources of ignition. Keep the container tightly closed when not in use. Always consult the Safety Data Sheet (SDS) from your specific supplier for the most accurate and up-to-date storage recommendations, as conditions may vary by purity grade and formulation. ### Where can I buy Cetyltrimethylammonium Bromide (CTAB)? Cetyltrimethylammonium Bromide (CTAB) (CAS 57-09-0) is available from major chemical suppliers. Availability, pricing, and purity grades vary by supplier and region. Check the product sourcing section for CoreyChem availability or contact our sales team for sourcing assistance and bulk quotation. ### What is the purity of commercial Cetyltrimethylammonium Bromide (CTAB)? Commercial Cetyltrimethylammonium Bromide (CTAB) is typically available at ≥97% purity from major chemical suppliers. Higher purities (≥99%) may be available for specialized applications in pharmaceutical manufacturing and analytical standards. Always verify the specific purity grade and certificate of analysis (CoA) with your supplier before use in critical applications. ## Where to Buy ## Product Sourcing **Cetyltrimethylammonium Bromide (CTAB) (CAS 57-09-0)** is not currently listed in the CoreyChem catalog. Contact [email protected] for sourcing assistance.