  We found theoretically that competition between ~Kq^4 and ~Qq^2 terms in the
Fourier transformed conformational energy of a single lipid chain, in
combination with inter-chain entropic repulsion in the hydrophobic part of the
lipid (bi)layer, may cause a crossover on the bilayer pressure-area isotherm
P(A)~(A-A_0)^{-n}. The crossover manifests itself in the transition from n=5/3
to n=3. Our microscopic model represents a single lipid molecule as a worm-like
chain with finite irreducible cross-section area A_0, flexural rigidity K and
stretching modulus Q in a parabolic potential with self-consistent curvature
B(A) formed by entropic interactions between hydrocarbon chains in the lipid
layer. The crossover area per lipid A* obeys relation Q^2/(KB(A*))~1 . We
predict a peculiar possibility to deduce effective elastic moduli K and Q of
the individual hydrocarbon chain from the analysis of the isotherm possessing
such crossover. Also calculated is crossover-related behavior of the area
compressibility modulus K_a, equilibrium area per lipid A_t, and chain order
parameter S.
