Despite enormous
interest toward graphene oxide (GO) from the research
community, surprisingly, little is known about its solutions. In particular,
the questions related to the structure of the GO/liquid interface
have not been yet properly addressed. In this report, we use a simple
but efficient experimental approach to investigate the distribution
of the four metal cations Na+, Cs+, Ni2+, and Gd3+ at the GO/water interface. We demonstrate that
the concentration of the cations decreases exponentially with the
distance from the GO surface. Such distribution for colloid systems
was theoretically predicted and commonly accepted for a century but,
to the best of our knowledge, has been never proved experimentally.
We further demonstrate that the shape of the counterion distribution
profiles depends on the pH of solution and on the fine chemical structure
of GO. In particular, organic sulfates and vinylogous acids that are
ionizable at different pH levels are responsible for the difference
in the shapes of the concentration profiles. Unlike classical colloid
systems, the diffuse layer in the GO solutions is rather broad (30–55
nm), and the concentration gradient is registered even at distances
of >55 nm from the GO surface, which is typically considered as
the
bulk solution. The latter observation is explained by the immobilized
character of the GO flakes in the nematic phase, impeding the flow
of liquid and the migration of hydrated metal cations. This helps
to establish and maintain the long-range concentration gradient in
the space between the two parallel neighboring GO flakes. Based on
the new findings and on the previously reported data, we formulate
some basic principles of GO solutions.
Gradient materials developed on the basis of poorly compatible epoxy oligomers are proposed. The influence of the composition and curing conditions on the stratification of the compounds was studied. The distribution of the components of the curing systems was determined by Fourier transform infrared and elemental analysis. The microhardness and glass-transition temperature varied across the section as a function of the component content and curing temperature.
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