Elastic properties of citric acid, citric acid hydrate, trilithium citrate tetrahydrate, trisodium citrate pentahydrate, and tripotassium citrate hydrate

Abstract Single crystals of optical quality of citric acid (monoclinic), citric acid monohydrate (orthorhombic), trilithium citrate tetrahydrate (monoclinic), trisodium citrate pentahydrate (orthorhombic) and tripotassium citrate monohydrate (monoclinic) have been grown from aqueous solutions by standard methods. Elastic constants c ij and thermoelastic constants T ij = d log c ij /d T have been determined from ultrasonic resonance frequencies of plane-parallel plates and their shift with temperature T , respectively. Further, the tensors of thermal expansion have been measured with the aid of an optical Fizeau interferometer. Citric acid and its monohydrate exhibit a distinct depression of their mean elastic stiffness and also stronger thermal volume expansion and mean thermoelastic effect compared to the three citrates. The rules governing the mean elastic stiffness via the S values are also obeyed by the citrates, despite their different structures. The contribution of the Coulomb interaction of a single monovalent cation with the trivalent citric anion to the mean elastic stiffness is somewhat comparable to that with a monovalent anion.