  Can we control the crystallization of solid CaCO3 from supersaturated aqueous
solutions and thus mimic a natural process predicted to occur in living
organisms that produce biominerals? Here we show how we achieved this by
confining the reaction between Ca2+ and CO32- ions to the environment of
nanosized water cores of water-in-oil microemulsions. Using a combination of in
situ small-angle X-ray scattering, high-energy X-ray diffraction, and low-dose
liquid-cell scanning transmission electron microscopy, we elucidate how the
presence of micellar interfaces leads to the formation of a solute CaCO3 phase
that can be stabilized for extended periods of time inside micellar water
nano-droplets. We can also control and >>on-demand<< trigger the actual
precipitation and crystallization of solid CaCO3 phases through the targeted
removal of the organic-inorganic interfaces.
