n-Octyltrimethoxysilane (also known as Trimethoxyoctylsilane; CAS No. 3069-40-7) is a typical organosilane surface modifier. With the molecular formula C₁₁H₂₆O₃Si and a molecular weight of approximately 234.41, it is widely recognized in public databases and commercial chemical literature by names such as Octyltrimethoxysilane and N-Octyltrimethoxysilane.

Its true value lies not merely in the presence of silicon, but in a molecular structure that combines two distinct characteristics: a hydrolyzable methoxysilane group at one end and a hydrophobic n-octyl chain at the other.

When n-octyltrimethoxysilane comes into contact with the surface of an inorganic material containing hydroxyl groups, the methoxysilane group undergoes hydrolysis and subsequently condenses with the surface hydroxyls, anchoring the silane structure to the material. Meanwhile, the outward-facing C8 alkyl chain reduces the material's surface hydrophilicity and surface energy, making it difficult for water to spread across or penetrate the surface.

Consequently, it is used for the hydrophobic surface modification of materials such as concrete, glass, inorganic pigments, and mineral fillers. IOTA classifies it as a medium-chain alkyl-functional silane capable of imparting hydrophobicity and creating surfaces with excellent weather and moisture resistance.

In simple terms, it can be viewed as a "molecular-level waterproofing modifier": rather than simply coating the material with a thick waterproof film, it alters the material's surface properties through chemical reaction.