  While ion transport processes in concentrated electrolytes, e.g. based on
ionic liquids (IL), are a subject of intense research, the role of conservation
laws and reference frames is still a matter of debate. Employ-ing
electrophoretic NMR, we show that momentum conservation, a typical prerequisite
in molecular dynamics (MD) simulations, is not governing ion transport.
Involving density measurements to deter-mine molar volumes of distinct ion
species, we propose that conservation of local molar species volumes is the
governing constraint for ion transport. The experimentally quantified net
volume flux is found as zero, implying a non-zero local momentum flux, as
tested in pure ILs and IL-based electrolytes for a broad variety of
concentrations and chemical compositions. This constraint is consistent with
incom-pressibility, but not with a local application of momentum conservation.
The constraint affects the calcu-lation of transference numbers as well as
comparisons of MD results to experimental findings.
