Interaction of Sodium Dodecyl Sulfate with Lipid Monolayer Studied by Sum-Frequency Generation Spectroscopy at Air-Water Interface

We employed vibrational sum-frequency generation (VSFG) spectroscopy to obtain molecular-level understanding of interaction of anionic surfactant sodium dodecyl sulfate (SDS), in low bulk concentration at the micromolar level, with lipid monolayer zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) at the air water interface. These results are different from those reported at higher bulk concentration of SDS at millimolar level. At very low concentration neither DPPC nor SDS produces any VSFG signal in the CH stretch region in the water subphase, but with the same concentration DPPC produces typical VSFG spectra at the SDS subphase due to interaction between these two molecules. The interaction leads to polar ordering of DPPC molecules with enhancement of VSFG intensity in the CH vibrational region of the hydrophobic tails. The interaction between the lipid and SDS molecules is influenced by concentrations of both lipid and SDS. Hydrophobic interactions between long alkyl chains of SDS and DPPC are responsible for an increase in the conformational order of the alkyl chain of DPPC with a decrease in the gauche defect and increase in trans conformer. Similarly, the orientation and concentration of interfacial water molecules of DPPC monolayer at SDS subphase are controlled by concentration of both SDS and DPPC. The VSFG results are complemented by the surface-pressure measurements.