Cation effects on phosphatidic acid monolayers at various pH conditions.

The impact of pH and cations on phase behavior, stability, and surface morphology for dipalmitoylphosphatidic acid (DPPA) monolayers was investigated. At pH<10, DPPA monolayers on water are predominantly populated by neutral species and display the highest packing density. Cations are found to expand and stabilize the monolayer in the following order of increasing magnitude at pH 5.6: Na+>K+∼Mg2+>Ca2+. Additionally, cation complexation is tied to the pH and protonation state of DPPA, which are the primary factors controlling the monolayer surface behavior. The binding affinity of cations to the headgroup and thus deprotonation capability of the cation, ranked in the order of Ca2+>Mg2+>Na+>K+, is found to be well explained by the law of matching water affinities. Nucleation of surface 3D lipid structures is observed from Ca2+, Mg2+, and Na+, but not from K+, consistent with the lowest binding affinity of K+. Unraveling cation and pH effects on DPPA monolayers is useful in further understanding the surface properties of complex systems such as organic-coated marine aerosols where organic films are directly influenced by the pH and ionic composition of the underlying aqueous phase.