Monolayers of 14-H-perfluorotetradecane-1-carbonic acid (PFTDA) at the air-water introduce with different metal cations (K+, Cd2+ Cu2+, Al-3+,Al- Fe3+) dissolved in a water subphase were studied. The dependence of the surface pressure P(A) and the jump of the surface potential Delta V on the area per molecule A in the monolayer were studied. Some compression isotherms of the PFTDA monolayers within a wide surface pressure range obey the equation of state P(A-A(0))=kT/n, where A(0) is the effective area of interaction of the molecules in the monolayer, n=1-4 and it depends on the interaction of the polar groups with one another or with the cations of the subphase, On the whole, this is the evidence of a weak cohesive interaction between the hydrophobic radicals in the monolayer.