An exact expression for the second virial coefficient for the surface pressure of adsorbed or spread monolayers is first derived using a McMillan-Mayer approach. A formal analysis of several contributions entering into this expression is presented. The role of the solvent is emphasized. It is shown that the calculation of the second virial coefficient requires a knowledge of the potential of mean force at infinite dilution for one and two amphiphile molecules. The assumptions which lead to the standard formula are clarified. In the second part of the paper, the model of the interatomic potential from which the second virial coefficient can be calculated is presented, and some approximations are discussed. In a second paper, these theoretical results will be used to analyze the experimental results concerning two carboxylic acids at the water/air interface.