SiOH

SiOH is the notation for a silicon–hydroxyl moiety, commonly represented as Si–OH. It most often refers to silanol groups on silicon-containing oxides, particularly on silica (SiO2) surfaces and related silicate minerals, where silicon atoms are terminated by hydroxyl groups after hydrolysis or surface hydration.

Structure and bonding

A silanol group consists of a silicon atom bonded to an oxygen that carries a hydrogen (Si–O–H). On a silica surface, silicon is typically four-coordinate, linked to three oxide neighbors and one hydroxyl. The Si–O bond is a single bond and the O–H bond is a typical hydroxyl; silanol OH stretching vibrations appear in infrared spectra near 3700 cm−1, with shifts influenced by hydrogen bonding. On surfaces, silanol groups exist in various environments, including isolated Si–OH and silanols involved in hydrogen-bond networks.

Occurrence, reactivity, and applications

Silanol groups are central to surface chemistry of silicon oxides. They act as Brønsted acid sites, can participate in condensation reactions to form siloxane (Si–O–Si) bridges with loss of water, and can be deprotonated to form silanolate species. They influence surface hydrophilicity, adsorption properties, and catalytic behavior. Silanols are commonly used as anchoring points for the grafting of organosilanes and for functionalizing silica-based materials in catalysis, sensing, and chromatography.

Characterization

Silanol presence and environment are probed by infrared spectroscopy (OH stretch and H-bond signatures) and solid-state NMR (29Si, 1H). Temperature-programmed desorption and adsorption studies help quantify silanol density and reactivity, which vary with sample history, pretreatment, and environment.

Gas-phase context

SiOH can also denote a transient gas-phase silicon hydroxide species in high-energy or combustion processes. In such contexts, SiOH is a reactive intermediate rather than a stable bulk compound.