Hemihydrate dissolution rate

Figure 13. Hemihydrate dissolution rate vs undersaturation. Rate expressed as change in crystal width.

A computer model has been generated which predicts the behaviour of a continuous well mixed gypsum crystallizer fed with a slurry of hemihydrate crystals. In the crystallizer, the hemihydrate dissolves as the gypsum grows. The solution operating calcium concentration must lie in the solubility gap. Growth and dissolution rates are therefore limited. 

Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. 

As the shape of the needle-like hemihydrate crystals changes as they dissolve, it is necessary to convert to the crystal width as a measure of size. In terms of this measure, the dissolution rate is first order with undersaturation and shows only a small temperature effect (activation energy of 10 kJ/mol). 

As far as the gypsum crystals are concerned, the analysis is identical to that for a seeded MSMPR (34). The information required is the growth rate and the mean residence time. For the hemihydrate, the analysis is that for a continuous seeded MSMPR dissolver (35), which parallels that for the crystallizer. The information needed is the dissolution rate and the mean residence time. 

The intrinsic dissolution profiles in water for both hemihydrate forms and the monohydrate form of aspartame were determined using 1 cm diameter compacts [23]. The compacts were mounted so that only one face was exposed to the medium, which was stirred at 50 rpm by a paddle near the solid surface. The three forms gave essentially identical profiles, corresponding to an intrinsic dissolution rate of about 7.3 x 10"4 mg/mL/min/cm2. X-ray powder patterns for the solid recovered from the measurements showed that the two hemihydrate forms had converted in situ to the monohydrate. As all intrinsic dissolution profiles were quite linear, the conversion to the monohydrate appears to be so rapid as to preclude assessment of the dissolution rates for the unaltered hemihydrates. 

Hulsmann et al. in 2000 investigated the use of HME techniques to increase the solubility and dissolution rate of a poorly water-soluble drug, 17-estradiol hemihydrate by preparing solid dispersions of the drug into different compositions of excipients such as PEG 4000, PVP K 30, Kollidon, Sucroester ... 

Chlorobutanol (Chlorobutanol anhydrous and chlorobutanol hemihydrate Ph.Eur. see Fig. 23.21) is now little used because of its poor solubility and strong permeation through rubber and plastic. When heated in water it melts before it dissolves which decreases the dissolution rate further. Furthermore, it decomposes at moderate temperature and during storage it is not sufficiently stable. 

Badens, E., Veesler, S. Sc Boistelle, R. (1999). Ciystallization of gypsum from hemihydrate in presence of additives. Journal of Crystal Growth, 198/199, 704-709 Barton, F. M. Wilde, N. M. (1971). Dissolution rates of polyaystalline samples of gypsum and orthorhombic forms of calcium sulphate by rotating disk method. Transactions of the Faraday Society, 67, 3590-3597. 

Hiilsmann S, Backensfeld T, Keitel S, Bodmeier R. Melt extrusion an alternative method for enhancing the dissolution rate of 17 -estradiol hemihydrate. Eur J Pharm Biopharm 2000 49 237-242. 

The rate of rehydration of the hemihydrate [84] in liquid water passes through a maximum, the position of which depends on the rate of reactant dissolution and of product precipitation. A general kinetic equation was derived which also described previously published data. 

The addition of gypsum is radically modifying the C3A hydration process. A long induction period appears, followed by the ciystallization of ettringite (Fig. 3.48). The preinduction period depends substantially on the rate of sulphate dissolution [ 127]. As it is commonly known, C3 A reacts with water violently and in the absence of gypsum hemihydrate or sodium and potassium sulphates, a certain amount of hexagonal hydrates is formed in the pre-induction period or even the monosul-phoaluminate [128]. 

A decrease in supersaturation due to the separation of a new phase is compensated by the dissolution of new portions of hemihydrate. This allows one to maintain a continuous supersaturated condition and hence ensures further growth of dihydrate crystals. The preservation of supersaturation as well as the time during which the supersaturated state is maintained depends on the ratio between the rates at which the dissolution of hemihydrate and crystallization of dihydrate take place. The existence of sufficiently high supersaturation allows for the formation of seed nuclei for crystallization contacts between crystals of gypsum dihydrate at their points of contact. 

---