Organic Enrichment at Aqueous Interfaces: Cooperative Adsorption of Glucuronic Acid to DPPC Monolayers Studied with Vibrational Sum Frequency Generation

Surface tension, surface-specific vibrational spectroscopy and differential scanning calorimetry measurements were all used to test cooperative adsorption of glucuronic acid (GU) to DPPC monolayers adsorbed to the aqueous/vapor interface. Experiments were performed using GU solutions prepared in Millipore water and in carbonate/bicarbonate solutions buffered to a pH of 9.0. The effects of GU on DPPC monolayer structure and organization were carried out with tightly packed monolayers (40 Å2/DPPC) and monolayers in their liquid condensed phase (55 Å2/molecule). Surface tension data show that GU concentrations of 50 mM lead to expanded DPPC monolayers with diminished surface tensions (or higher surface pressures) at a given DPPC coverage relative to monolayers on pure water. With unbuffered solutions, GU induces significant ordering within liquid condensed monolayers although the effects of GU on tightly packed DPPC monolayers are less pronounced. GU also induces a second, higher melting temperature in DPPC vesicles implying that GU (at sufficiently high concentrations) strengthens lipid-lipid cohesion, possibly by replacing water solvating the DPPC headgroups. Together, these observations all support a cooperative adsorption mechanism. In buffer solutions, the effects of dissolved GU on DPPC structure and organization are muted. Only at sufficiently high GU concentrations (when the solution's buffering capacity has been exceeded) do the data again show evidence of cooperative adsorption. These findings place limits on cooperative adsorption's ability to enrich interfacial organic content in alkaline environmental systems such as oceans.