(443g) Phase Transitions of Water Confined in Nanopores

Phenomena that occur only at very high pressures in the bulk phase are often observed to occur in the confined phase at normal pressures. Abundant experimental evidence suggests that adsorbates confined in nanoporous carbons exhibit high pressures, such as high-pressure crystal structures, high-pressure chemical reactions, and the deformation of pore walls due to the adsorbate. Also molecular simulation studies of the pressure tensor for simple adsorbates in carbon nanopores of slit, cylindrical and spherical geometries show that for modest bulk phase pressures the pressure parallel to the pore walls is of the order of GPa, while the pressure normal to the wall is of the order of hundreds of MPa [1,2].

We report X-ray, neutrons diffraction ( ND) and dielectric studies of ice confined in activated carbon fibres (ACFs). The fibres are built of turbostratic nanoparticles separated by quasi two-dimensional voids, forming narrow slit-shaped pores. The structure of ice in cylindrical nanocarbons pores (CMK-3), which are the reverse carbon replica of silica SBA-15 matrices, was also analyzed. The results show strong evidence for the presence of both hexagonal and cubic forms of ice confined in all of the systems studied. We were not able to obtain a good Rietveld refinement of our diffraction patterns with typical hexagonal and cubic forms of ice; but we observe that the formation of the ice structure in the pores, has been attributed to the restrictions on the size of the crystallite and the quasi-high pressure effect in pores. However, according to recently published work [3], the cubic form of ice is not exactly cubic, but contains stacking faults instead of an ordered arrangement of two dimensional layers. This form of ice is in fact a stacking-disordered material containing cubic sequences interlaced with hexagonal sequences. The structure of the confined ice observed in our experiments in nanocarbons pores corresponds to disordered stacking ice Isd,, which was identified as having the space group P3m1 [3,4]. We observe,that the stacking disorder can vary in complexity depending on the nature of the pore walls and on the prevailing thermal conditions during the ice formation. An analysis of the kind of I_sd formed in ACF and CMK-3 of different pore sizes will be presented. These crystal forms, which occur in bulk water only at temperatures below 180 K in the case of cubic ice, and at pressures of hundreds of MPa are stabilized by the confinement.

Financial support for the NCN grant No UMO-2016/ 22/A/ST4/00092 is gratefully acknowledged

[1] Y. Long, J.Palmer, B.Coasne, M.Sliwinska-Bartkowiak, K.E.Gubbins, Phys. Chem. Chem. Phys , 13 , 17163, (2011)

[2] M.Sliwinska-Bartkowiak, M. Drozdowski, M. Kempiński, Y. Long, J. Palmer, K.E. Gubbins, Phys. Chem.Chem.Phys.,14,7145 (2012);

[3 J.C.Palmer, F.Martelli, Y. Liu, R. Car., A.Z. Panagiotopoulos, P.G. .Debenedetti, , Nature, 510 , 385 (2014); T. Malkin, B. J. Murray, A. Brukhno, J. Anwar and C. G. Salzmann ,Proc. Natl. Acad. Sci. USA, 109 ,1041(2012)

[4] K.Domin, K-Y. Chan, H. Yung, K.E. Gubbins, M. Sliwinska-Bartkowiak,,J.Ch.E.D., 61 (2016), J.Mol. Liq., 43,1580 (2020)