Crystal morphology and surface reactivity studies of calcium hydroxide

Abstract

Calcium hydroxide samples, obtained as a precipitate from the mixing of solutions of sodium hydroxide with a variety of calcium salts in the optional presence of ethanol and a silylating agent, have been characterised by the techniques of nitrogen adsorption, water vapour adsorption , thermogravimetric analysis, Fourier-transform infra-red spectroscopy, ultra-violet spectroscopy and electron microscopy. The morphology of the samples varied from octahedral crystals to hexagonal prisms to sheets. The individual morphology depends on the conditions of precipitation, the nature and concentration of the calcium salt(s) and the nature of the precipitation medium. It has been shown that a large excess of sodium hydroxide results in octahedral forms, calcium salt(s) in approximate equimolar amount or in large excess to the sodium hydroxide results in hexagonal forms, and sheets are formed in the presence of ethanolic precipitation medium. A poisoning mechanism has been suggested in terms of the morphological properties of the samples. It has been found that the sheet forms of calcium hydroxide can be stabilised by reaction with a silylating agent, resulting in greater thermal stability and chemical stability of the surface. Silylation was found not to occur by direct contact between the precipitated calcium hydroxide samples and silylating agents. Silylation only occurred when precipitation was carried out in the presence of a solution of the silylating agent in absolute ethanol. The silylation of the surface of calcium hydroxide has been found to stabilise the material. Reaction of this with 3-aminopropyltriethoxysilane formed an intermediate which subsequently reacted with p-nitrobenzoyl chloride to form an inorganic - organic composite. The technique of thermogravimetric analysis has been employed to measure the thermal stability of the samples. It has been found that the samples exhibit major weight losses at around 688K for the non-silylated samples and around 918K for the silylated samples. The BETnitrogen and the BET-H20 surface areas of the samples range typically from --1 to 43 m2g-1 , and from 9 to 798 m2g-1 respectively. On the basis of this evidence, taken together with the isotherm shapes it has been shown that: the samples are non-porous; non-silylated calcium hydroxide samples are more hydrophobic than the silylated samples, but upon heat-treatment in air below the decomposition temperature the silylated samples became more hydrophobic, whereas the non-silylated samples became more hydrophilic; samples heated in air at above the decomposition temperature exhibit a dramatic increase in hydrophilicity, the H 20-BET surface areas becoming —800 m2g-1 for non silylated samples, compared to 368 - 600 m 2g-1 for silylated samples, indicative of chemisorption following decomposition of the calcium hydroxide to form calcium oxide.