Confinement-induced structural changes of water studied by Raman scattering

Abstract

The temperature dependence of water confined in the ordered cylindrical nanopores of MCM-41 and SBA-15 materials is studied by means of Raman scattering for different pore sizes covering a diameter range from 2.0 to 8.9 nm. The liquid-solid phase transition temperature of water in confinement can be determined by the analysis of the mode contribution in the OH-stretching region. For pore sizes down to 3 nm, the freezing/melting point depression with decreasing pore size can be consistently described by a modified Gibbs-Thomson equation, with a nonfreezable water layer of 0.6 nm (about two monolayers) close to the pore walls. When the pore size is 2.5 nm or smaller, indication for a first-order phase transition can no longer be found that is in agreement with previously reported differential scanning calorimetry measurements on the same samples. The Raman data further suggest that two spatially separated water phases exist in the smallest pores, i.e., the nonfreezable wall layer and a structurally different water phase in the core of the pores. A distinct tetrahedral hydrogen-bonded network of water molecules is found only in the core part of the pores. In the weakest confinement (8.9-nm pore diameter), the core water is shown to be compatible with crystalline ice with a spectral fingerprint similar to bulk ice. In strong confinement (2.0-nm pore diameter), the core water shows a spectral fingerprint identical to low-density amorphous ice, and there is a gradual transition between these two extremes. These findings suggest that the core part of confined water undergoes considerable structural changes with decreasing pore size, leading us to question recent proposals that aim to extract information about the state of bulk liquid water in the "no man's land" from water in confinement.

Anomalies in a waterlike model confined between plates

“…In some scattering experiments there are indications of the formation of cubic ice instead of the hexagonal ice present in the bulk 12,32 . Several of these experiments show evidences of the presence of layers, one close to the walls and one at the center 11,12,29,30 . For certain type of walls, the central crystallizes before the wall layers 11,12 .…”

“…Several of these experiments show evidences of the presence of layers, one close to the walls and one at the center 11,12,29,30 . For certain type of walls, the central crystallizes before the wall layers 11,12 . Therefore, the experimental results are not conclusive.…”

“…Therefore, the experimental results are not conclusive. They indicate that the crystallization in confined water depends strongly on the size of the porous 11,13,30,33,34 and on the level of hydration water under surfaces 20,32 .…”

“…In particular, x-ray and neutron scattering with water in nanopores show that the liquid state persists down to temperature much lower than in bulk 11,29,30 . In these experiments in the case of hydrophobic walls the liquid-crystal transition occurs at lower temperatures than in the case of hydrophilic walls 29,31 .…”

“…In order to understand the behavior of water under these limitations a number of experiments and simulations of confined water have been performed. Several types of confinement have been explored: experiments in cylindrical porous [11][12][13] and simulations in carbon nanotubes 14,15 , simulations in porous matrices [16][17][18][19] , experiments 20,21 and simulations in rough surfaces [22][23][24][25][26] and simulations in flat plates 23,25,27,28 .…”