Growth of cholesterol crystals in silica gel

Abstract

depends on its physical and chemical properties. The growth of crystals of cholesterol in aqueous and other solutions have been reported in the literature [1-3]. The growth of cholesterol monohydrate crystals in supersaturated model bile solution has been studied by Toor et al. [4], but no attempt seems to have been made so far to grow cholesterol crystals by a gel technique. Here we report the growth of these crystals in gels. Sodium silicate solution of certain specific gravity, varying from 1.02 to 1.06 g cm 3, was titrated against glacial acetic acid to prepare the gel [5]. Two different approaches were made. Experiments were done at tropical room temperatures. In the first approach the pH of the titrated solution (sodium silicate + glacial acetic acid) was maintained in the range 4 to 7. Depending on the pH value it took 10 to 30 h for the solution to set. Ethanol was allowed to diffuse into the gel medium by pouring ethanol (95%) on the already formed gel medium. Ethanol was replaced periodically every two days. After 14 days the undiffused ethanol was carefully decanted. Meanwhile, cholesterol solutions in ethanol with concentrations varying from 0.25% to 1% were prepared by dissolving cholesterol (reference standard grade) in 95% ethanol. This was poured carefully over the gel without disturbing the latter. Crystals of cholesterol were found to grow in the course of a week. The growth of crystals was complete within a month. Plate-like crystals were obtained within the gel. A typical crystal obtained by this method is shown in Fig. l a. The crystals were approximately (0.1 x 0.03 cm 2) in dimensions. In the second approach, the pH value of the sodium silicate solution was maintained between 4 and 5. A 2:1 ratio was taken for mixing the sodium silicate solution and the 95% ethanol. It took 10 to 48h for the solution to gel. The solution of cholesterol in 95% ethanol was carefully poured on the top surface of the gel. Nucleations were seen to occur inside the gel. Size of the crystals was almost same as that of the crystals obtained by the first method. Plate-like crystals (Fig. lb) were obtained for pH 5 of the gel and 0.5% concentration of the supernatant cholesterol solution. Crystals were analysed using infrared, differential thermal analysis (DTA) and thermo-gravimetric analysis (TGA). DTA and TGA were carried out on a Dupont 990 Modular Thermal Analysis System (heating rate, 10 ° C min-1 ; atmosphere, nitrogen; flow rate, 50 cm 3 min-~; sample weight, 2 to 5 mg). Both of the thermoanalytical methods, namely, DTA and TGA, clearly indicated that the crystals were hydrated. The thermograms are reproduced in Fig. 2a and b. The DTA showed an endothermic peak at about 80°C which is characteristic of the monohydrate cholesterol. The rate of growth of the crystals was slow from the beginning. During the process of diffusion of cholesterol through the gel, ethanol does not react in any way with the gel, but the cholesterol becomes supersaturated. This leads to the nucleation of cholesterol crystals and, subsequently, to the growth and development of the nucleated crystal.