Crystalline silicas (crystalline silicic acids) are intracrystalline-reactive layered materials. Two types were studied: H4Si20O42·xH2O and H4Si14O30·xH2O. They show a pronounced preference of methanol and ethanol from mixtures with water, n-butanol, and toluene. The surface excess isotherms are U-shaped with long linear sections extending from x1 = 0.2 or 0.3 to x1 = 1 (x1 molar fraction of methanol or ethanol). Thus, at x1 ≥ 0.2 or 0.3, only methanol or ethanol is adsorbed between the layers. The preference of H4Si20O42 for methanol and ethanol from mixtures with water is as high as in silicalite and NaZSM-5. With longer alcohols (propanol ... n-octanol), the isotherms become S-shaped, and molecules of component 2 (water, n-butanol, toluene) are also adsorbed in the interlayer spaces. Adsorption from mixtures of n-pentanol (or n-hexanol) and n-butanol produces unusual surface excess isotherms. The cause is formation of paraffin-type bilayers of pentanol or hexanol which, at a certain molar fraction of butanol, collapse to monolayers of flat-lying alcohol molecules. When the alcohol molecules lie flat in the interlayer space (basal spacing less than 2.3 nm), the volume of the adsorption phase, Vs, comprises the interlamellar volume Vint (determined by X-ray powder techniques) and the volume Vex of a few layers of alcohol molecules adsorbed at the external surface. The ratio Vs/Vint varies between 1.2 and 1.5. When the layer separation becomes larger (paraffin-type bilayers of alcohols), Vint exceeds Vs. This behaviour is also observed for hydrophobized clay minerals in the presence of polar/unpolar liquid mixtures.
|Number of pages||13|
|Journal||Colloids and Surfaces A: Physicochemical and Engineering Aspects|
|Publication status||Published - Jun 9 1993|
- crystalline silicas
- crystalline silicic acids
- surface excess isotherms.
ASJC Scopus subject areas
- Surfaces and Interfaces
- Physical and Theoretical Chemistry
- Colloid and Surface Chemistry