The interactions between ions and lipids control a wide range of membrane properties, including structure, dynamics, stability, and elasticity. Ions can also mediate the interactions between membranes and between membranes and other molecules like proteins. Most earlier studied have focused on the interactions between membranes and cations even though the cations are necessarily accompanied by their anions.

The bicarbonate anion is a ubiquitous and important anion in aqueous media owing to dissolved atmospheric carbon dioxide. We showed a unique effect that bicarbonate has on dipolar lipid membranes (Langmuir 2020).

We discovered that bicarbonate anions can couple with calcium cations and form unique bridges between dipolar (zwitterionic, net neutral) DLPC lipid membranes. These bridges tightly couple the membranes into dehydrated lamellar stacks by a mechanism involving complexes of calcium and bicarbonate ions, which has never been observed before with lipids or other interfaces (neutral or charged). Tight coupling between membranes was only observed between negatively charged membranes in the presence of multivalent cations by a different mechanism that involves strong ion-correlation effects (Langmuir 2019). 

Our conclusions, based on a large data set of solution X-ray scattering experiments and MD simulations, reveal the mechanism by which the lipids and ions interact and complex together, leading to this new type of tight interfacial coupling. We have also shown that other combination of anions and cations do not lead to a similar tight coupling. 

Owing to the ubiquity of bicarbonate in most experimental setups involving lipids, surfactants, and other biological systems, our findings have wide implications in the design and interpretation of experiments involving lipids (as well as other dipolar surfactants) in the presence of salts. In addition to bicarbonate, calcium ions are involved is various cell functions, and are present in unprocessed samples of water.