Synthesis and X-ray diffraction study of calcium salts of some carboxylic acids

An experimental x-ray diffraction (XRD) study of calcium salts of four carboxylic acids is presented. Calcium salts of propionic, butyric, valeric, and caproic acids were synthesized mixing in a mortar Ca(OH)₂ with the liquid acids. Measuring the thermogravimetric analysis curves it was determined that the salts were actually monohydrates. The densities of the synthesized samples were measured using a density gradient column. The measured values for the densities were as follows: D_{m(propionate)} = 1.38 g/cm³, D_m(butyrate) = 1.30 g/cm³, D_m(valerate) = 1.26 g/cm³, D_m(caproate) = 1.22 g/cm³. The XRD analysis revealed that these compounds have monoclinic cells with symmetry described by the P2₁/a space group. Calcium propionate hydrate has cell parameters: a = 2.437 51 (5) nm, b = 0.681 24(1) nm, c = 0.591 43(1) nm, β = 95.320(2)°. For calcium butyrate hydrate the cell parameters are: a = 2.96684(8) nm, b = 0.68074(2) nm, c = 0.58929(2) nm, β = 95.442(3)°. The cell parameters for calcium valerate hydrate are: a = 3.566 36(4) nm, b = 0.68249(1) nm, c = 0.59277(1) nm, β = 107.280(1)° and for calcium caproate hydrate a = 4.18030(5) nm, b = 0.68261(1) nm, c = 0.592 13(1) nm, β = 110.230(1)°. The calculated density values from the XRD results, taking into account that the number of chemical formulas in the unit cell equals four, agree very well with the measured ones. It was established that the unit cell parameter a grows with the increase of the number of carbon atoms in the aliphatic chain, while parameters b and c remain almost constant. This is an indication of the stacking layer character of the structure as has been suggested for calcium stearate monohydrate. This fact points to the possibility of the refinement of the crystalline structures taking as the starting point the reported structure for calcium stearate monohydrate.