Abstract

Hemimorphite, Zn4Si2O7(OH)2·H2O, and its dehydrated analog Zn4Si2O7(OH)2 were studied by low-temperature relaxation microcalorimetry and their heat capacity determined to analyze the behavior of the confined H2O between 5 and 300 K. An analysis of the data, which are corrected for the presence of a phase transition, shows that the CP of H2O in hemimorphite behaves more similar to the CP of ice than to liquid water or steam. The H2O molecule, with its four planar hydrogen bonds in hemimorphite, as well as its tetrahedral coordination in ice, is more rigidly hydrogen bonded in both than in liquid water. This is reflected in their respective CP behavior. The heat capacity and entropy for the dehydration reaction at 298 K are ΔCPrxn = − 2.1 ± 3.6 J/(mol·K) and ΔSrxn = 134.7 ± 4.0 J/(mol·K). CP behavior at 0 < T < 300 K and entropy values at 298 K for confined H2O in hemimorphite and hydrous Mg cordierite are compared to those in several zeolites. The entropy for confined H2O in hemimorphite, analcime, and mordenite is around 54 J/(mol·K) at 298 K. The strength of the interactions (e.g., H bonding) between an H2O molecule and its surroundings increases approximately from steam > cordierite > analcime > hemimorphite ≥ mordenite > heulandite > natrolite ≈ scolecite > liquid H2O > ice and, in the case of microporous silicates, is inversely proportional to the S of the confined H2O.

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