The dynamical crossover phenomenon in strongly confined and mildly supercooled water in graphene oxide (GO) nanopores is investigated using molecular dynamics simulation. Since the extent of hydrophilicity can be varied on graphene oxide surfaces, they offer energetically heterogeneous environments that can potentially modulate the rotational and translational relaxation dynamics of confined water. The role of the extent of hydrophobicity on the dynamical transitions is investigated by varying the extent of hydrophobicity on the confining surfaces that are placed at the separation distance of 10 Å. Analysis of density distributions reveals that water confined between the GO surfaces exhibits two distinct layers where the bulk-like nature is absent. We observed a significant slowing down in all dynamical quantities as the temperature is lowered from 298 to 200 K; The nature of the dynamical crossover is, however, a strong function of the surface type. Water confined between surfaces consisting of alternating hydrophilic and hydrophobic regions show a strong-to-strong dynamical transition in the diffusion coefficients and rotational relaxation times at a crossover temperature of 237 K and show a fragile-to-strong transition in the α-relaxation time at 238 K. We notice that the observed dynamical transition temperature is much higher than the freezing point of the SPC/E water model used in this study, indicating that these dynamical transitions can occur with mild supercooling under strong confinement in the absence of bulk-like water. Interestingly, water confined in a hydrophilic nanopore exhibits a single Arrhenius energy barrier over the entire temperature range studied. Our results strongly suggest that in addition to confinement, the nature of the surface can play a significant role in the dynamical transitions of water upon supercooling.

Effect of surface hydrophilicity on dynamical transitions of confined water in graphene oxide pores: A molecular dynamics study

ACS Spring Meeting 2021

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