SOLID-STATE THERMAL DESOLVATION AND STRUCTURAL STUDIES OF OXALIC ACID DIHYDRATE, ZINC ACETATE DIHYDRATE, COPPER (II) SULFATE PENTAHYDRATE AND NICKEL (II) SULFATE HEXAHYDRATE

Abstract

For many years it has been widely accepted that crystal dehydration can be related to internal crystal structure. However, there is very little empirical data or experiments on this work. This work makes an attempt to use thermal studies to understand the internal structure of crystals. Three inorganic solvates (nickel (II) sulfate hexahydrate, copper (II) sulfate pentahydrate and zinc acetate dihydrate) and an organic solvate (oxalic acid dihydrate) were chosen and studied as model compounds. This was done through study of their crystallization habits, thermo chemical properties, dehydration kinetics and other related studies. The physiochemical properties of the known compounds were determined using DSC and TGA. Data mining was carried out in which crystallographic data of study materials were obtained from crystallographic data base. Using mined crystal data, modeling was carried out on the internal structures of the study materials. The observed internal structure obtained using modeling was then explained using thermal study observations. It was observed that the rate of dehydration is directly related to crystal packing. Compact molecular packing meant low rate of dehydration and vice versa. The numbers of hydrogen bonds were observed to also have impact on molecular packing and therefore thermal dehydration. The presence of tunnels within the crystal structure provided channels through which desolvated water left the crystal and hence increased rate of thermal dehydration. The nature of the DSC/TGA curves were used to compare rates of desolvation of the solvates in that the solvates whose curve has a sharper gradient has a higher rate of desolvation implying that its structure is less compact or more open facilitating easy escape of solvent. Copper (II) sulfate pentahydrate and zinc acetate dihydrate had lower energies of desolvation at 159.6 J/mole and 235.0 J/mole respectively. Oxalic acid dihydrate and nickel (II) sulfate hexahydrate had higher energies of desolvation at 503.6 J/mole and 241.8 J/mole respectively. Copper (II) sulfate pentahydrate and zinc acetate dihydrate salts had higher rates of desolvation and from modeling results their structures are more open compared to the structures of oxalic acid and nickel sulfate. These salts of oxalic acid dihydrate and nickel (II) sulfate hexahydrate had more packed structures due to the presence of many hydrogen bonds compared to salts of copper (II) sulfate pentahydrate and zinc acetate dihydrate. The presence of tunnels and the open nature of the crystals packing in various solvates facilitated escape of solvent.