The disjoining pressure P versus membrane separation d(w) in a lamellar structure of DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) bilayers in the gel state in a 30 mM CaCl2 solution was analyzed according to a modified Poisson-Boltzmann equation. The observed P-d(w) curve could not be described by a single association constant, but a linear relationship was obtained from the P-d(w) curve between log10K (where K is the association constant of Ca2+ ion to the DPPC bilayer surface) and the electric field E(b) at the membrane surface. As a result, the binding energy DELTAU lies on a straight line in the DELTAU-E(b) plane, which may be written as DELTAU = alpha0 + alpha1 - E(b). This linearity can be explained by the electrostatic cation-dipole interaction between bound Ca2+ ion and the P--N+ dipole of the polar headgroup of the membrane surface. Two models were formulated for the behavior of the large permanent P--N+ dipole (19 D) : one is a continuous change model (CCM) allowing for continuous conformational change and the other is a discrete change model (DCM). In CCM, the P--N+ dipole is trapped in a harmonically approximated potential with parameter A. The slope alpha1 was calculated for both models to investigate whether these models can express the required change of ion affinity. The derived values of alpha1 (CCM) and alpha1 (DCM) from the models of CCM and DCM, respectively, have the same numerical magnitude in the gel state as alpha1 (P-d(w)) obtained by analysis of the P-d(w) curve. This E(b) dependence of K may vanish in the liquid crystalline state, especially in the continuous change model in relation to the dielectric constant D1 of water in the headgroup region.