  The surface free energy is one of the most fundamental properties of solids,
hence, manipulating the surface energy and thereby the wetting properties of
solids, has tremendous potential for various physical, chemical, biological as
well as industrial processes. Typically, this is achieved by either chemical
modification or by controlling the hierarchical structures of surfaces. Here we
report a phenomenon whereby the wetting properties of vermiculite laminates are
controlled by the hydrated cations on the surface and in the interlamellar
space. We find that by exploiting this mechanism, vermiculite laminates can be
tuned from superhydrophillic to hydrophobic simply by exchanging the cations;
hydrophilicity decreases with increasing cation hydration free energy, except
for lithium. Lithium, which has a higher hydration free energy than potassium,
is found to provide a superhydrophilic surface due to its anomalous hydrated
structure at the vermiculite surface. Building on these findings, we
demonstrate the potential application of superhydrophilic lithium exchanged
vermiculite as a thin coating layer on microfiltration membranes to resist
fouling, and thus, we address a major challenge for oil-water separation
technology.
