Molecular structure of monolayers formed at the interface between a Au(111) surface and solutions containing n-alkanes has been studied by in situ scanning tunneling microscopy at room temperature. For increasing CnH2n+2 length from n = 10 up to 50 with even n values, we found that rectangular and tilted arrangements of alkanes within the self-organized layers alternate. This alternation is related to the periodic decrease of the sliding force for molecules displaying a length close to mT (m-integer), where T is the period of commensurability between the CH2-CH2-CH2 period along alkyl chains and the interatomic distance along Au <110>. The observation of rectangular lamellae for all the alkane monolayers grown at higher temperature supports this model.