Orientational phase transitions in axially symmetric condensed phases of water-supported amphiphile monolayers are treated in the model within the mean field approximation. Heads of the molecules are assumed to form an orthorhombically distorted hexagonal two-dimensional structure. Interaction of tails is described via the Lennard Jones (LJ) potential. Thermal motion of tails is supposed to be reduced to free rotation of tails around their long axes and to small oscillations near the mean orientation. The free rotation leads to renormalization of the length constant in LJ potential. It is shown that the interaction of tails can be split into two parts : the larger part is referred to as bulk energy and the smaller as interfacial energy. The constant cross-section rule observed experimentally appears to be satisfied if the bulk energy is taken into account only, but this approximation turns out to be insufficient to distinguish phases with tilting direction either to a nearest neighbour (NN) or to a next-nearest neighbour. The addition of interfacial terms allows the transition from NN to vertical state to be found; this transition can be attributed to the L(2)-LS transition observed experimentally. The model transition is found to be weakly first order. The results obtained are compared with other models and with computer simulations.