Lipids bearing net electric charges in their hydrophilic headgroups are ubiquitous in
biological membranes. Recently, the interest in cationic lipids has surged because of their
potential as non-viral transfection vectors. In order to utilize cationic lipids in transfer of
nucleic acids and to elucidate the role of charged lipids in cellular membranes in general,
their complex interactions within the membrane and with the molecules in the
surrounding media need to be thoroughly characterized. Yet, even interactions between
monovalent counter-ions and charged lipids are inadequately understood. We studied
the interactions of the cationic gemini surfactant (2R,3R)-2,3-dimethoxy-1,4-
bis(N-hexadecyl-N,N-dimethylammonium)butane dibromide (RR-1) with chloride, bromide, fluoride, and iodide
as counter-ions by differential scanning calorimetry and Langmuir balance. Chloride
interacts avidly with RR-1, efficiently condensing the monolayer, decreasing the collapse
pressure, and elevating the main transition temperature. With bromide and iodide clearly
different behaviour was observed, indicating specific interactions between RR-1 and these
counter-ions. Moreover, with fluoride as a counter-ion and in pure water identical results
were obtained, demonstrating inefficient electrostatic screening of the headgroups of
RR-1 and suggesting fluoride being depleted on the surface of RR-1 membranes.