Anomalous Thermal Expansion of Confined Aqueous Solutions
Abstract
The thermal expansion coefficient of water and aqueous solutions is greater in small pores than in bulk liquid. This phenomenon, first reported by Derjaguin et al. [J. Colloid Interface Sci., 109, 586, 1986], has been systematically studied in a series of porous glasses with different pore sizes. We find that the expansion begins to increase when the pore diameter is less than 15 nm. Near room temperature, the thermal expansion coefficient of pure water rises by about a factor of 1.8 in pores with 3.1 nm diameter. The maximum in the density of water is shifted downward from 4 degC in bulk to about -7 degC in 7.4 nm pores. Expansion of a series of salt solutions was measured, and the enhancement was comparable to that of pure water, so there was no indication of an influence of the size of the hydrated ion on the magnitude of the expansion in pores as large as 3.1 nm. Molecular dynamics studies, as well as a variety of spectroscopic measurements, reported in the literature indicate that one or two molecular layers are densely packed against the pore wall. We interpret the high thermal expansion to result from a change in the proportion of molecules included in those layers; as the thermal energy decreases during cooling, more molecules are attracted into the dense layers, and the thermal contraction is therefore enhanced. The layered structure is also supported by measurements of the permeability of the porous glasses. The permeability of a given material decreases as the size of the liquid molecule increases, because the layers bound on the surface reduce the effective pore volume. We find that the permeabilities of our samples can be predicted by assuming that two monolayers are immobilized on the pore wall. Measurements made on saturated cement paste show that the thermal expansion is about 1.6 times greater than that of bulk water, and increases as the permeability decreases, indicating that the expansion is dominated by the smaller pores in the size distribution.
- Publication:
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AGU Spring Meeting Abstracts
- Pub Date:
- May 2004
- Bibcode:
- 2004AGUSMNS44A..02S
- Keywords:
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- 1866 Soil moisture