n-Octyltrimethoxysilane, with the chemical formula C₁₁H₂₆O₃Si and CAS number 3069-40-7, is a colorless and transparent organosilicon liquid. It consists of one n-octyl group (C₈H₁₇—) and three methoxy groups (–OCH₃) linked to a silicon atom, belonging to the typical alkylsilane class of compounds. Due to the simultaneous presence of hydrolyzable methoxy groups and non-reactive long-chain alkyl groups in its molecular structure, it possesses both "inorganic-loving" and "hydrophobic-organic" properties.

At room temperature, n-octyltrimethoxysilane is relatively stable, but upon contact with water or moisture, the methoxy groups gradually hydrolyze to form silanols (Si–OH). These silanols can then undergo condensation reactions with hydroxyl groups on the surfaces of inorganic materials such as glass, metal oxides, and concrete, forming strong Si–O–Si chemical bonds. The octyl group at the other end extends outward, forming a dense hydrophobic layer that effectively blocks water penetration.

This unique "bridging" effect makes n-octyltrimethoxysilane an excellent silane coupling agent and surface modifier. It does not participate in polymerization reactions but significantly enhances the interfacial bonding of organic-inorganic composites. Because it contains no aromatic rings or halogens, it has low toxicity and is environmentally friendly, making it widely used in building protection, coatings, and plastic fillers. With the development of green building materials, these long-chain alkyl silanes are gradually replacing traditional highly polluting waterproofing agents, becoming one of the key additives in modern functional materials.