In this work, a theoretical model describing the interaction between a positivelyor negatively charged nanoparticle and neutral zwitterionic lipid bilayers is presented. It isshown that in the close vicinity of the positively charged nanoparticle, the zwitterionic lipidheadgroups are less extended in the direction perpendicular to the membrane surface, whilein the vicinity of the negatively charged nanoparticle, the headgroups are more extended.This result coincides with the calculated increase in the osmotic pressure between the zwitterionic lipid surface and positively charged nanoparticle and the decrease of osmoticpressure between the zwitterionic lipid surface and the negatively charged nanoparticle.Our theoretical predictions agree well with the experimentally determined fluidity of alipid bilayer membrane in contact with positively or negatively charged nanoparticles. Theprospective significance of the present work is mainly to contribute to better understandingof the interactions of charged nanoparticles with a zwitterionic lipid bilayer, which may beimportant in the efficient design of the lipid/nanoparticle nanostructures (like liposomes withencapsulated nanoparticles), which have diverse biomedical applications, including targetedtherapy (drug delivery) and imaging of cancer cells.

The lipid bilayer is a basic building block of biological membranes and can be pictured as a barrier separating two compartments filled with electrolyte solution. Artificial planar lipid bilayers are therefore commonly used as model systems to study the physical and electrical properties of the cell membranes in contact with electrolyte solution. Among them the glycerol-based polar phospholipids which have dipolar, but electrically neutral head groups, are most frequently used in formation of artificial lipid bilayers. In this work the electrical properties of the lipid layer composed of zwitterionic lipids with non-zero dipole moments are studied theoretically. In the model, the zwitterionic lipid bilayer is assumed to be in contact with aqueous solution of monovalent salt ions. The orientational ordering of water, resulting in spatial variation of permittivity, is explicitly taken into account. It is shown that due to saturation effect in orientational ordering of water dipoles the relative permittivity in the zwitterionic headgroup region is decreased, while the corresponding electric potential becomes strongly negative. Some of the predictions of the presented mean-field theoretical consideration are critically evaluated using the results of molecular dynamics (MD) simulation.