Electrostatic barrier at biological membranes
The properties of water at the surface, especially at an electrically charged one, differ essentially from those in the bulk phase. The traits of surface water can be inferred from proton pulse experiments with membrane enzymes. In such experiments, protons that are ejected (or captured) by light-triggered enzymes are traced on their way between the membrane surface and the bulk aqueous phase. In several laboratories, as well as in our group, it has been shown that proton exchange between the membrane surface and the bulk aqueous phase takes as much as about 1 ms, being retarded by a kinetic barrier, but could be accelerated by added mobile pH-buffers. We have determined the properties of the barrier, revealed its electrostatic origin, and attributed the barrier to the water layering at the negatively charged surface of phospholipid membranes. Recently this prediction has been confirmed: it has been shown that water is indeed layered at the surface of phospholipid monolayers.
Proton-conducting networks at the surfaces of energy-transducing enzymes of Rhodobacter. The figure is taken taken from 
Protonic coupling over the membrane surface
Understanding the nature of the interfacial electrostatic barrier gives a new perspective for the mechanism of biological energy conversion. Indeed, the height of the barrier for protons could be estimated as ~0.12 eV. While the proton exchange between the surface and the bulk aqueous phase is retarded by the interfacial barrier, the proton diffusion along the membrane, between neighboring enzymes, takes only microseconds. The proton spreading over the membrane is facilitated by the hydrogen-bonded networks at the surface. The membrane-buried layers of these networks can eventually serve as a storage/buffer for protons (proton sponges). As the proton equilibration between the surface and the bulk aqueous phase is slower than the lateral proton diffusion between the "sources" and "sinks", the proton activity at the membrane surface, as sensed by the energy transducing enzymes at steady state, might deviate from that measured in the adjoining water phase. This trait should increase the driving force for ATP synthesis, especially in the case of alkaliphilic bacteria [1,2].
Schematic presentation of a coupling membrane with protons moving from the cytochrome bc1 complex to the ATP synthase along the p-surface. Figure taken from 
4. Mulkidjanian,A.Y., D.A.Cherepanov, J.Heberle, and W.Junge. 2005. Proton transfer dynamics at membrane/water interface and the mechanism of biological energy conversion, Biochemistry-Moscow 70:251-256. (Full text, pdf)
5. Cherepanov, D.A., W. Junge, and A.Y. Mulkidjanian. 2004. Proton transfer dynamics at the membrane/water interface: Dependence on the fixed and mobile pH buffers, the geometry of membrane particles, and the interfacial potential barrier. Biophys. J. 86:665-680.(Full text, pdf)
6. Cherepanov, D.A., B.A. Feniouk, W. Junge, and A.Y. Mulkidjanian. 2003. Low dielectric permittivity of water at the membrane interface: effect on the energy coupling mechanism in biological membranes. Biophys. J. 85:1307-1316.(Full text, pdf)
7. Gopta, O.A., D.A. Cherepanov, W. Junge, and A.Y. Mulkidjanian. 1999. Proton transfer from the bulk to the bound ubiquinone QB of the reaction center in chromatophores of Rhodobacter sphaeroides: Retarded conveyance by neutral water. Proc.Natl.Acad.Sci.U.S.A. 96:13159-13164.(Full text, pdf)