Study of moisture sorption behavior of urea inclusion compounds with aliphatic guests and with linear polymers

Urea is known to form stable inclusion compounds (IC) with a wide variety of guests, which include aliphatic organic substances as well as linear polymers. Urea in its native form is also known to undergo deliquescence when exposed to an atmosphere of higher water activity. The purpose of the present study was to determine and compare the moisture sorption profiles of the uncomplexed urea as well as ICs of urea with different guests. Urea ICs with two types of guest moieties were prepared, that is, either monosubstituted unbranched alkane derivatives possessing an anchor length of 16 carbons or linear polymer poly (ethylene glycol) of different molecular weights. The formation of ICs was confirmed through thermal analysis and powder X-ray diffraction/single crystal diffraction. The ICs were exposed to environment of varying water activity in dynamic water sorption analyzer, and the rate and extent of moisture uptake were determined. Critical relative humidity of urea and representative ICs was determined by two techniques. Urea ICs were found to demonstrate greater resistant to deliquescence as exhibited by increase in critical relative humidity in comparison with uncomplexed urea. Thermodynamic considerations involved in the process of deliquescence are discussed.