  Confined in two dimensional planes, polymer chains comprising dense monolayer
solution are segregated from each other due to topological interaction.
Although the segregation is inherent in two dimensions (2D), the solution may
display different properties depending on the solvent quality. Among others, it
is well known in both theory and experiment that the osmotic pressure ($\Pi$)
in the semi-dilute regime displays solvent quality-dependent increases with the
area fraction ($\phi$) (or monomer concentration, $\rho$), that is, $\Pi\sim
\phi^3$ for good solvent and $\Pi\sim \phi^8$ for $\Theta$ solvent. The osmotic
pressure can be associated with the Flory exponent (or the correlation length
exponent) for the chain size and the pair distribution function of monomers;
however, they do not necessarily offer a detailed microscopic picture leading
to the difference. To gain microscopic understanding into the different surface
pressure isotherms of polymer solution under the two distinct solvent
conditions, we study the chain configurations of polymer solution based on our
numerical simulations that semi-quantitatively reproduce the expected scaling
behaviors. Notably, at the same value of $\phi$, polymer chains in $\Theta$
solvent occupy the surface in a more \emph{inhomogeneous} manner than the
chains in good solvent, yielding on average a greater and more heterogeneous
interstitial void size, which is related to the fact that the polymer in
$\Theta$ solvent has a greater correlation length. The polymer configurations
and interstitial voids visualized and quantitatively analyzed in this study
offer microscopic understanding to the origin of the solvent quality dependent
osmotic pressure of 2D polymer solutions.
