We performed 200 ns MD simulations of phosphatidylcholine (PC) bilayers in the liquid crystalline (L-alpha) and gel (L-beta') states to compare the properties of the water-membrane interfaces in these two thermotropic bilayer phases. Our simulations show that the membrane phase determines the behavior of water, ions, and PC head groups. When the membrane was in the gel phase, we found partial dehydration (fewer PC-water interactions), particularly in the carbonyl groups region, as well as an almost complete lack of ionic penetration into this region as compared with the bilayer in the liquid-crystalline phase. In the latter case, there is an exchange of Na+ ions between the water layer and the interfacial region. Ibis is mainly due to the fact that the most stable binding of Na+ in the liquid-crystalline bilayer is to the carbonyl groups. The lack of Na+ binding to the carbonyl groups in the gel phase bilayer can be explained by the more compact structure of the bilayer in this phase.