A kinetic and microscopic investigation of the thermal dehydration of lithium sulphate monohydrate

Abstract

Lithium sulphate monohydrate loses its water of crystallization in a single rate process Li₂SO₄· H₂O → Li₂SO₄+ H₂O. Scanning electron microscopic studies confirm that this nucleation and growth reaction is completed without melting. During the initiation of reaction there is dense nucleation, and nuclei rapidly coalesce to form a coherent dehydration, approximately planar, surface that progressively advances inwards towards the crystal centre. Pore development in advance of this recrystallization front is evidence of water loss from reactant in the zone beyond the phase transformation interface. At low extents of reaction channels are developed between neighbouring nuclei, thereby accelerating the establishment of a comprehensive superficial layer of product. The structure of the interface is discussed.

These observations are entirely consistent with a parallel isothermal kinetic study. Following an initial brief acceleratory phase, dehydration thereafter is predominantly deceleratory and satisfactorily obeys the contracting volume rate equation. Measurements were based on product yields determined at appropriate time intervals from the pressure of water evolved in a constant-volume vacuum apparatus. Under the present reaction conditions (349–400 K and < 10 Torr water vapour) product accumulation did not appreciably influence kinetic behaviour except, possibly, at the lowest temperatures. The kinetic behaviour of the three reactant preparations studied (single crystals, crushed crystals and rehydrated crystals) are compared.