Experimental measurements of the thermal expansion coefficient (a), permeability (k), and diffusivity (D)of water and 1 M solutions of NaCl and CaCl2 are interpreted with the aid of molecular dynamics (MD) simulations of water in a 3 nm gap between glass plates. MD shows that there is a layer ∼6Å thick near the glass surface that has a ∼2.3 times higher and D about an order of magnitude lower than bulk water. The measured D is ∼5 times lower than that for bulk water. However, when the MD results are averaged over the thickness of the 3 nm gap, D is only reduced by ∼30% relative to the bulk, so the measured reduction is attributed primarily to tortuosity of the pore space, not to the reduced mobility near the pore wall. The measured a can be quantitatively explained by a volume-weighted average of the properties of the high-expansion layer and the "normal" water in the middle of the pore. The permeability of the porous glass can be quantitatively predicted by the Carman-Kozeny equation, if 6 Å of water near the pore wall is assumed to be immobile, which is consistent with the MD results. The properties and thickness of the surface-affected layer are not affected significantly by the presence of the dissolved salts.
All Science Journal Classification (ASJC) codes
- Materials Science(all)
- Condensed Matter Physics
- Surfaces and Interfaces