= Adsorption confinement free energy
{title2=$\Delta F_{\rm conf}=Nk_{\rm B}T\ln[V/(A\Lambda)]$}
The translational <Helmholtz free energy> cost of restricting a dilute <ideal gas> from a volume $V$ to an <area> $A$ at the same <temperature>. For identical particle number and <mass>,
$$
F_{2D}-F_{3D}=-Nk_{\rm B}T\ln\frac{A/\Lambda^2}{V/\Lambda^3}
=Nk_{\rm B}T\ln\frac{V}{A\Lambda}.
$$
Here $\Lambda$ is the <thermal de Broglie wavelength>. This contribution is positive for $V/A\gg\Lambda$; a favourable binding <energy> must be added separately to predict net <adsorption>.
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