Membranes composed of cationic lipids can form compact complexes with DNA, making them suitable for non-viral vectors for gene delivery therapy. We employ the mean-field Poisson-Boltzmann theory to investigate the structural behavior of membranes composed of cationic and zwitterionic lipids, taking into account the dipolar nature of the latter. The main finding is that the zwitterionic dipoles orient themselves in such a way that the area per lipid passes through a minimum value as the cationic lipid composition is increased. This result is also supported my a Monte Carlo simulation.