THE WATER ADSORPTION PROPERTIES OF THE POLYMORPHS OF HMX: THE THERMODYNAMICS OF WATER INTERACTION WITH GROUND AND UNGROUND BETA-HMX
PICATINNY ARSENAL (ARMY) DOVER NJ
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Water adsorption isotherms were measured for the polymorphs of octahydro-1,3,5,7-tetranitro-s-tetrazine HMX. Water is found to sorb into the crystal structure in the order, gamma-HMX alpha-HMX, and not at all into beta- HMX. The thermodynamics of water interaction with the surface of beta-HMX was studied as a function of grinding action. Grinding reduces the free surface energy of beta-HMX from 25 to 5 ergssq cm, and increases the hydrophobic character of the surface by a factor of 12. The calculated thermodynamic functions support the concept of adsorption of water in clusters on both ground and unground beta-HMX surfaces. The integral heat of water adsorption increases with increasing hydrophobicity. This is attributed to unhindered dipole interaction of water with the more isolated active sites of like charges. Accordingly, water is shown to become less entropic as the hydrophobicity is increased.
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