# C18 Silica Gel > modified silica gel used in chromatography **Wikidata**: [Q115220904](https://www.wikidata.org/wiki/Q115220904) **Source**: https://4ort.xyz/entity/c18-silica-gel ## Summary C18 Silica Gel is a modified form of silica gel used in chromatography, featuring an octadecyl (C18) functional group bonded to its surface. It serves as a stationary phase in high-performance liquid chromatography (HPLC), enabling the separation of complex chemical mixtures. This variant is part of a broader class of porous, amorphous silica gels that are also used as desiccants and absorbents in industrial applications. ## Key Facts - **Chemical nature:** C18 Silica Gel is a functionalized silica gel with an octadecyl (C18) bonded phase, derived from synthetic amorphous silicon dioxide (SiO₂). - **Density:** 2.20 g·cm⁻³ (measured at 20°C). - **Melting point:** 3,110°F; **Boiling point:** 4,046°F at 760 mmHg. - **Vapor pressure:** 0 mmHg (practically non-volatile). - **CAS Registry Numbers:** 68855-54-9 (silica gel) and 112926-00-8 (silicon dioxide). - **Hazard limits (NIOSH):** Time-Weighted Average (TWA) exposure limit = 6 mg·m⁻³; Immediately Dangerous to Life or Health (IDLH) = 3,000 mg·m⁻³. - **Primary classifications:** Subclass of silica gel, column-packing material, and absorbent; instance of chemical substance. - **Key identifiers:** KEGG D06521; EC 272-489-0; NIOSH Pocket Guide ID 0552; UMLS C0074530. - **Common aliases:** C18-bonded Silica Gel, C18-modified Silica Gel, Octadecyl-functionalized silica gel, Octadecyl silica, Si-C18. - **Modified forms for chromatography:** Includes C18, C8, Cyano, Amino, and Phenyl silica gels. ## FAQs ### Q: What is C18 Silica Gel used for? A: C18 Silica Gel is primarily used as a stationary phase in high-performance liquid chromatography (HPLC) to separate and analyze complex chemical mixtures. It is also employed as a desiccant and absorbent in industrial processes. ### Q: Is C18 Silica Gel hazardous? A: While C18 Silica Gel itself is chemically inert, exposure to fine particles can be hazardous. NIOSH sets a TWA limit of 6 mg·m⁻³ and an IDLH limit of 3,000 mg·m⁻³ to mitigate inhalation risks. ### Q: What is the chemical composition of C18 Silica Gel? A: It is a porous, amorphous form of silicon dioxide (SiO₂) with an octadecyl (C18) functional group bonded to its surface, produced synthetically as a cross-linked gel. ### Q: Can C18 Silica Gel be regenerated after absorbing moisture? A: Yes, heating can restore its adsorption capacity, though specific regeneration procedures are not detailed in the source material. ### Q: How does C18 Silica Gel differ from regular silica gel? A: Unlike regular silica gel, C18 Silica Gel is functionalized with an octadecyl (C18) group, enhancing its selectivity in chromatographic applications. ## Why It Matters C18 Silica Gel plays a critical role in analytical chemistry by serving as a stationary phase in high-performance liquid chromatography (HPLC). Its unique porous structure and octadecyl functionalization allow for precise separation of complex mixtures, making it indispensable in pharmaceutical, environmental, and biochemical research. Additionally, its hygroscopic properties make it effective as a desiccant for protecting moisture-sensitive materials. Despite its low vapor pressure and high thermal stability, occupational exposure to fine particles is regulated to ensure safety. Overall, C18 Silica Gel balances utility with well-defined health safeguards, making it a foundational material in both industrial preservation and scientific analysis. ## Notable For - **Selectivity in HPLC:** The octadecyl (C18) functional group enhances its ability to separate hydrophobic compounds in chromatographic separations. - **High thermal stability:** Melts at 3,110°F and boils at 4,046°F, allowing safe handling across a wide temperature range. - **Low vapor pressure (0 mmHg):** Remains non-volatile under normal conditions. - **Regulated occupational exposure:** NIOSH sets strict limits (TWA 6 mg·m⁻³, IDLH 3,000 mg·m⁻³) to prevent inhalation hazards. - **Versatility in chromatography:** Part of a family of modified silica gels (C8, Cyano, Amino, Phenyl) tailored for specific separation needs. ## Body ### Composition and Structure - C18 Silica Gel is a synthetic amorphous form of silicon dioxide (SiO₂) with an octadecyl (C18) functional group bonded to its surface. - It consists of a cross-linked network of Si–O bonds, creating a granular, vitreous, and highly porous matrix. ### Physical Properties - **Density:** 2.20 g·cm⁻³ (20°C). - **Melting point:** 3,110°F; **Boiling point:** 4,046°F at standard pressure. - **Vapor pressure:** 0 mmHg, indicating negligible volatility. - **Molecular mass:** Reported as 60.1 Da (source: CDC NIOSH). ### Classification and Related Materials - Subclass of **silica gel**, **column-packing material**, and **absorbent**. - Part of the broader class **synthetic amorphous silica**. - Related modified gels used in chromatography: **C8, Cyano, Amino, and Phenyl** silica gels. ### Applications - **Chromatography:** Serves as the stationary phase in HPLC for separating chemical mixtures; the C18 functionalization enhances hydrophobic compound separation. - **Desiccant:** Adsorbs moisture from sealed environments, protecting electronics, pharmaceuticals, and food. - **Absorbent:** Captures gases and liquids in industrial processes. ### Safety and Exposure Limits - **NIOSH TWA:** 6 mg·m⁻³ (average 8-hour exposure). - **IDLH:** 3,000 mg·m⁻³ (immediate danger to life or health). - Classified as a **hygroscopic agent** and associated with **silica gel exposure** hazards. ### Identifiers and References - **CAS:** 68855-54-9 (silica gel), 112926-00-8 (silicon dioxide). - **KEGG:** D06521. - **EC number:** 272-489-0 (also 601-214-2). - **NIOSH Pocket Guide ID:** 0552. - **UMLS CUI:** C0074530. - **ECHA infocard:** 100.065.880 (68855-54-9) and 100.128.559 (112926-00-8). ### Visual Resources - Images: ![Silica gel indicator](https://commons.wikimedia.org/wiki/Special:FilePath/Indicating-silica-gel.png) and schematic surface view.