Introduction of Positive Charges into Zwitterionic Phospholipid Monolayers Disrupts Water Structure Whereas Negative Charges Enhances It

The interfacial water structure and phosphate group hydration of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine monolayers were investigated at air/water interfaces. Both vibrational sum frequency spectroscopy (VSFS) and Langmuir monolayer compression measurements were made. The PC lipids oriented water molecules predominantly through their phosphate-choline (P-N) dipoles and carbonyl moieties. Upon the introduction of low concentrations of 1,2-dioleoyl-3-trimethylammonium propane (DOTAP), a positively charged double chain surfactant, the TAP headgroups were attracted to the phosphate moieties on adjacent PC lipids. This attraction caused the monolayers to contract, expelling water molecules that were hydrogen bonded to the phosphate groups. Moreover, amplitude of the OH stretch signal decreased. At higher DOTAP concentrations, the positive charge on the monolayer caused an increase in the area per headgroup and water molecules in the near-surface bulk region became increasingly aligned. Under these latter conditions, the OH stretch amplitude was linearly proportional to the surface potential. By contrast, introducing 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol, a negatively charged lipid, did not change the area per lipid or the phosphate–water hydrogen bonding network. As the interfacial potential grew more negative, the OH stretch amplitude increased continuously. Significantly, changes in the interfacial water spectrum were independent of the chemistry employed to create the positive or negative interfacial potential. For example, Ca²⁺ and tetracaine (both positively charged) disrupted the water structure similarly to low DOTAP concentrations, whereas SCN^– and ibuprofen (both negatively charged) enhanced the water structure. These results suggest a direct correlation amongst the interfacial water structure, area per lipid, and surface charge density.