  The electric dipole moments of $(H_{2}O)_{n}DCl$ ($n=3-9$) clusters have been
measured by the beam deflection method. Reflecting the (dynamical) charge
distribution within the system, the dipole moment contributes information about
the microscopic structure of nanoscale solvation. The addition of a DCl
molecule to a water cluster results in a strongly enhanced susceptibility.
There is evidence for a noticeable rise in the dipole moment occurring at
$n\approx5-6$. This size is consistent with predictions for the onset of ionic
dissociation. Additionally, a molecular dynamics model suggests that even with
a nominally bound impurity an enhanced dipole moment can arise due to the
thermal and zero point motion of the proton and the water molecules. The
experimental measurements and the calculations draw attention to the importance
of fluctuations in defining the polarity of water-based nanoclusters, and
generally to the essential role played by motional effects in determining the
response of fluxional nanoscale systems under realistic conditions.
