Melting and freezing of water in ordered mesoporous silica materials
Abstract
The melting and freezing of water in a series of mesoporous
silica materials with a hexagonal arrangement of unidimensional cylindrical
pores and narrow pore-size distribution (MCM-41 with pore diameters
from 2.9 to 3.7 nm, and SBA-15 with pore diameters from 4.4 to 11.7
nm) was studied by differential scanning calorimetry (DSC). A lowering of
the melting temperature ΔTm
=
Tmb
−
Tm(R) up to 50 K was found for
water in pores of decreasing radius R. The melting point data can
be represented by a modified Gibbs–Thomson equation, ΔTm(R) =
K/(R-t), with K
=
52 K nm and t
= 0.4 nm, in agreement with an earlier study
of water in MCM-41 materials of pore width 2 to 4 nm. The value of K
agrees with an estimate of the Gibbs–Thomson constant based on thermodynamic
data for the normal melting point of ice, and the parameter t can
be taken as the thickness of a surface layer of non-frozen water at
the pore wall. DSC scans of the freezing of H2O and D2O
in partially filled pores of SBA-15 reveal a peak pattern depending
on the degree of pore filling ϕ. The different peaks are attributed
to different states of the liquid in the pore space, iz.
pore water in completely filled regions, and water as an adsorbed film at
the pore wall. These two states coexist in a pore filling range ϕmin<ϕ<1,
but only the film state exists at ϕ<ϕmin.
The freezing peak of pore water appears to be nucleated by external bulk
ice (at ϕ>1) but exhibits substantial supercooling at ϕ<1.
On the other hand, the peak attributed to the freezing of film water exhibits
no significant supercooling and is found at temperatures near 237 K, almost
independent of ϕ and the pore width of the SBA-15 sample.
The nature of a further (small) peak near 233 K is not yet understood. Contrary
to cooling scans, only one DSC peak is observed in heating scans, independent
of the pore filling. This finding indicates that the adsorbed liquid film
is metastable relative to the frozen pore liquid.