A mean field statistical model is used to calculate the chain organization and the thermodynamics behavior of an amphiphilic monolayer at the surface of water. The configurations of a single chain are generated with the rotational isomeric state model chains. In order to obtain semiquantitative results for comparison with experiments on short alcohol monolayers, the model uses as an external input a relation between the average volume per CH2 and the molecular area experimentally determined in recent reflectivity measurements. Various properties associated with the chain conformations are calculated and compared with molecular dynamics simulations. The model exhibits a phase transition which is identified with the melting transition of short alcohol monolayers and which clearly underlines the major role of the conformational energy.