8th Liquid Matter Conference September 6-10, 2011 Wien, Austria ...

Thu 811:10-14:00P7.27Raman scattering study of confined waterMaxim Erko, 1 Nicholas Cade, 2 Alan G. Michette, 2 Gerhard H. Findenegg, 3 andOskar Paris 11 Montanuniversität Leoben, Institute of Physics, 8700, Leoben, Austria2 Physics Department, King’s College London, London, United Kingdom3 Technische Universitaet Berlin, Berlin, GermanyConfinement of water within nanometer pores induces new phenomena such as melting point suppression,and in some cases, water seems even not to freeze at all as shown in recent differentialscanning calorimetry measurements. Thus, when confining water in nanopores its liquid propertiescan be investigated below the homogeneous nucleation temperature of 235 K. Micelle-templatedporous silica materials, such as MCM-41 represent ideal model systems for studying water in nanoconfinement.We performed Raman scattering on water confined in cylindrical pores between 2 - 9nm. The state of confined water in the temperature region between 300 - 120 K was studied by thedetailed analysis of the OH-stretching region. For pore sizes down to 3 nm, the freezing/meltingpoint depression with decreasing pore size can be consistently described by a modified Gibbs-Thomson equation, assuming a non-freezable water layer of about two monolayers thickness closeto the pore walls and a core of bulk-like water. A well-distinct tetrahedral hydrogen-bonded networkof water molecules is found only in the core part of the pores. In the weakest confinementof 9 nm pore diameter, the core water is shown to be compatible with crystalline ice, very similarto bulk ice. In the strongest investigated confinement (2.0 nm pore diameter), however, the corewater shows a spectral signal identical to that of low-density amorphous ice (LDA), and there is agradual confinement-induced transition between these two extremes. These findings suggest thatthe inner part of confined water undergoes considerable structural changes with decreasing poresize, hence questioning recent proposals that aim to extract information about the state of bulkliquid water in the ”no man’s land” from water in confinement.27