Scientific Results

Thermodynamic functions of water and ice confined to 2 nm radius pores

Year: 2005

Authors: Tombari E., Salvetti G., Ferrari C., Johari G.P.

Autors Affiliation: Ist. per I Proc. Chimico-Fisici D., via G. Moruzzi 1, 56124 Pisa, Italy; Dept. of Mat. Sci. and Engineering, McMaster University, Hamilton, Ont. L8S 4L7, Canada

Abstract: The heat capacity C-p of the liquid state of water confined to 2 nm radius pores in Vycor glass was measured by temperature modulation calorimetry in the temperature range of 253-360 K, with an accuracy of 0.5%. On nanoconfinement, C-p of water increases, and the broad minimum in the C-p against T plot shifts to higher temperature. The increase in the C-p of water is attributed to an increase in the phonon and configurational contributions. The apparent heat capacity of the liquid and partially frozen state of confined water was measured by temperature scanning calorimetry in the range of 240-280 K with an accuracy of 2%, both on cooling or heating at 6 K h(-1) rate. The enthalpy, entropy, and free energy of nanoconfined liquid water have been determined. The apparent heat capacity remains higher than that of bulk ice at 240 K and it is concluded that freezing is incomplete at 240 K. This is attributed to the intergranular-water-ice equilibrium in the pores. The nanoconfined sample melts over a 240-268 K range. For 9.6 wt % nanoconfined water concentration (similar to 50% of the maximum filling) at 280 K, the enthalpy of water is 81.6% of the bulk water value and the entropy is 88.5%. For 21.1 wt % (100% filling) the corresponding values are 90.7% and 95.0%. The enthalpy decrease on nanoconfinement is a reflection of the change in the H-bonded structure of water. The use of the Gibbs-Thomson equation for analyzing the data has been discussed and it is found that a distribution of pore size does not entirely explain our results.

Journal/Review: JOURNAL OF CHEMICAL PHYSICS

Volume: 122 (10)      Pages from: 104712-1  to: 104712-9

More Information: College of Natural Resources, University of California Berkeley, CNR. – G.P.J. would like to thank CNR, Rome, Italy, for providing a visiting professorship during the period of this study.
KeyWords: POROUS VYCOR GLASS; PHASE-TRANSITIONS;
DOI: 10.1063/1.1862244

Citations: 77
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