  In two-dimensional electronic systems, direct first-order phase transitions
are prohibited as a consequence of the long-range Coulomb interaction, which
implies a stiff energetic penalty for macroscopic phase separation. A prominent
proposal is that any direct first-order transition is instead replaced by a
sequence of ``microemulsion" phases, in which the two phases are mixed in
patterns of mesoscopic domains. In this note, we comment on the range $\Delta
n$ of average electron density that such microemulsion phases may occupy. We
point out that, even without knowing the value of a phenomenological parameter
associated with surface tension between the two phases, one can place a fairly
strong upper bound on the value of $\Delta n$. We make numerical estimates for
$\Delta n$ in the case of the Fermi liquid to Wigner crystal transition and
find $\Delta n$ to be on the order of $10^7$\,cm$^{-2}$. This value is much
smaller than the width of the phase transition observed in experiments,
suggesting that disorder is a more likely explanation for the apparent
broadening of the transition.
