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.

将不同长度的烷基链通过多金属氧酸盐（polyoxometalates）的后功能化方法，共价接枝至六钼酸盐（hexamolybdate）表面。所得到的化合物属于典型的所谓巨型表面活性剂结构。令人意外的是，这类以六钼酸盐作为极性头基的表面活性剂能够结晶；通过单晶X射线衍射（single-crystal X-ray diffraction）分析可知，该类表面活性剂的结晶行为高度依赖于烷基链的长度。对于烷基链相对较短的表面活性剂（C6和C10），其烷基链更倾向于与六钼酸盐的反阳离子四丁基铵（tetrabutylammonium）发生相互作用；而对于烷基链更长的表面活性剂（C18），烷基链则倾向于彼此堆叠，形成烷基畴区。其潜在机制可能为：长烷基链无法与四丁基铵的短烷基链（C4）完全相容。