The lipid membrane interphase is treated as a binary solution composed of polar head groups, with their hydration shells sunk in water, different from the bulk. This type of water is exchangeable with adjacent media according to the lateral surface pressure and water activity. Thus, the water influxes and effluxes at constant temperature and in the absence of an electric field, driven by the changes in surface pressure (surface tension) and the osmotic gradient (water activity), are coupled through phenomenological coefficients reflecting water-lipid and lipid-water frictions that relate to the diffusion of water in lipids and diffusion of lipids in water as described by the thermodynamics of irreversible processes (TIP) approach. Thus, the experimental surface pressure/area per molecule isotherms for monolayers can be fitted satisfactorily in contrast to the classical gas model. Hydrogen bond arrangements between water and membrane components determine a unique surface tension of water (surface free energy profile). Each surface pressure/water activity pair defines a particular steady state of the interphase, which explains its responsive behavior when chased by physical and chemical perturbations.