Confinement-induced structural changes of water studied by Raman scattering

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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.