Tuning interfacial properties of phospholipid stabilised oil–water interfaces by changing the phospholipid headgroup or fatty acyl chain

Hypothesis: In-plane interactions among adsorbed phospholipids (PL) at an oil–water interfacial film may vary based on the size of the headgroup or the size and saturation of the fatty acyl chain (FA). In general, stronger interactions are expected when the PL can approach each other closer, when 1) the headgroup is smaller and 2) the FA chain is straight, allowing a good alignment. Also, varying pH might alter PL-PL interactions, as electrostatic forces between the adsorbed PL will decrease with decreasing pH (lower number of charged groups). Experiments: The interfacial layers formed by saturated/unsaturated phosphatidylethanolamine and phosphatidylcholine were characterised after a heat-cool cycle as a function of their molecular structure and pH, using dilatational and interfacial shear rheology. Findings: For the same FA chain, a smaller headgroup resulted in a stiffer interface. In the case of the saturated PLs, network formation due to chain crystallisation of the PL's FA chains occurs during cooling, increasing elasticity. The bend in the molecule of unsaturated PLs hinders the PL from packing tightly on the interface, leading to weaker PL-PL interactions and, accordingly, less stiff interfaces. In general, the stiffness of the interface increases with decreasing pH as the degree of ionisation is lowered, electrostatic repulsion forces are reduced and with it, overall in-plane attraction between PLs are enhanced.