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Our previous quasielastic neutron scattering measurements have revealed a series of freezing transitions of water in proximity to a model zwitterionic (DMPC) and an anionic (DMPG) membrane down to temperatures of 255 K and 200 K, respectively. We have interpreted these freezing/melting transitions as evidence of different water types defined by their local environment [2]. Here we use neutron diffraction to determine the structure of the ice formed in the freezing transitions of both membranes. In all cases, our diffraction patterns reveal the formation of hexagonal ice oriented with its basal plane parallel to the membrane surface. The temperature dependence of D₂O Bragg peak intensities measured at the University of Missouri Research Reactor is in excellent agreement with that of previous incoherent elastic scans conducted on different DMPC and DMPG samples, using the backscattering spectrometer HFBS at the NIST Center for Neutron Research. Moreover, in the case of the DMPG membrane, the diffraction patterns allow us to differentiate between the structure of hexagonal ice formed from bulk-like water and that from confined water by the degree of lattice strain present.

²A. Miskowiec et al., J. Chem. Phys. 146, 125102 (2017)