关键词: 顶空气相色谱, 化学反应, 硅羟基, 气相二氧化硅, 格氏试剂, 甲烷

Abstract:

Fumed silica is prepared by flame pyrolysis, where silicon halide is combusted in an oxygen-hydrogen flame, resulting in finely dispersed and thermally stable silicon dioxide. Because of its unique physical and chemical properties, including high porosity, large pore volumes, large specific area, and high chemical activity, fumed silica is widely used in rubbers, plastics, adhesives, paints, and printing inks for reinforcement, as well as in thixotropy, anti-setting, and anti-sagging applications. These functional properties of fumed silica are related to the silanol group on its surface. However, there is no accurate and convenient test method to determine the silanol group content on the surface of fumed silica. This work explores a novel method to determine the silanol group content on the surface of fumed silica by chemical reaction-headspace gas chromatography (HS-GC). Theoretically, by this method, the silanol group can rapidly react with the Grignard reagent and generate methane, the amount of which can be determined accurately by GC analysis. GC analysis was conducted using a headspace flask as a closed reactor to transform the silanol group into a volatile component through a chemical reaction, so as to realize the accurate determination of silica hydroxyl. The amount of methane produced in the reaction was directly proportional to the content of silanol groups on the surface of fumed silica. Therefore, the silanol group content was calculated using the chemical reaction equation. Before the experiment, fumed silica was dried for 2 h in an oven at 105 ℃ to remove adsorbed moisture. The dried fumed silica sample was then reacted with the Grignard reagent dispersed in toluene in an airtight reaction bottle. Toluene was used as a dispersion agent to promote contact and reaction between the fumed silica sample and Grignard reagent. The methane produced by the reaction was injected into a gas chromatograph for separation and further detected using a flame ionization detector (FID). Methane was quantified from the peak areas of the GC signals using the external standard method, and the silanol content in the sample was obtained. Simultaneously, factors influencing the outcome of the method, such as the dosage of the Grignard reagent and reaction time with it, were optimized by a comparison test. Accordingly, 2.0 mL of 0.3 mol/L Grignard reagent and a reaction time of 15 min were found to be optimal for testing. The test results showed that there was good linear correlation between the content of the silanol group and the GC signals, with a correlation coefficient of 0.9990. The limit of detection was 0.30 mg/g, and the limit of quantification was 1.00 mg/g. The relative standard deviation of reproducibility was less than 3%. Based on an interlaboratory test conducted by four laboratories on five samples with different silanol group contents, the repeatability limit (r) was less than 2.5%, and the reproducibility limit (R) was less than 6.5%. Compared with the traditional chemical method, the method involving HS-GC presents distinct advantages in terms of lower reagent consumption, high sensitivity, good stability, and reliability. It is suitable for the rapid detection of the silanol group content on the surface of fumed silica, and can aid in the quality control of fumed silica during its production and application. This method has important theoretical and practical significance for developing accurate methods to determine silica hydroxyl in the silicon industry for standards and the optimization of industrial technology. This study serves as a foundation to standardize and promote the rapid development of silicon material-related industries.

Key words: headspace gas chromatography (HS-GC), chemical reaction, silanol group, fumed silica, Grignard reagent, methane