Approximation of molecular surfaces is of central importance in numerous scientific fields. In this study we theoretically derive a physical model to relate phase-change thermodynamics to molecular surfaces. The model allows accurately predicting vaporization enthalpy of compounds for a wide temperature range without requiring any empirical parameter. Through the new model, we conceptualize thermodynamically effective molecular surfaces and show that they, although only marginally different than van der Waals surfaces, substantially improve predictability of multiple thermodynamic quantities.