Synthesis and Crystallization Behavior of Surfactants with Hexamolybdate as the Polar Headgroup

Alkyl chains with different lengths
were covalently grafted onto the surface of hexamolybdate through
the postfunctionalization protocol of polyoxometalates. The obtained
compounds represent typical structures of the so-called giant surfactants.
Unexpectedly, those surfactants with hexamolybdates as polar headgroups
are able to crystallize, while single-crystal X-ray diffraction reveals
that the crystallization behavior of the surfactants is highly dependent
on the length of the alkyl chains. For surfactants with comparatively
short alkyl chains (C6 and C10), the alkyl chains prefer to interact
with tetrabutylammonium, the countercation of hexamolybdate. However,
the alkyl chains tend to pack with each other to form a domain of
alkyl chains in the surfactant with a longer alkyl chain (C18). The
possible mechanism is that a long alkyl chain cannot be fully compatible
with the short chain (C4) of tetrabutylammonium.