The structure of mixed monolayers of the zwitterionic phospholipid dipalmitoylglycerophosphocholine (DPPC) containing nonionic surfactants of the polyethylene glycol monododecyl ether variety (C12En with n = 2, 4, 6, 8) is studied at the air/water interface using a combination of neutron reflectivity with film balance techniques. The two methods applied simultaneously gave good agreement of the molecular area data using the most simple single-layer model for fitting the reflectivity curves. Isotopic substitution of both lipids and surfactants has been employed in order to determine structural parameters of both components separately. All measurements were done at lipid to surfactant molar ratios rL/s > 2 and at surfactant concentrations well below their critical micelle concentration. Measurements were performed in the liquid-condensed (LC) and in the liquid-expanded (LE) phase states of the monolayer. The results show that all surfactants studied readily associate with the DPPC in the LE phase to a mixed monolayer but get completely transferred from the monolayer into the subphase at the transition to the LC phase. A determination of the surfactant partition between the mixed monolayer and subphase at the LE phase state provides evidence that virtually all surfactant is associated with the monolayer. The area per surfactant molecule in the mixed monolayer is found to increase rather linearly from 45 ± 5 Å2 (n = 2) to 165 ± 5 Å2 (n = 8) by an increment of 22 ± 2.5 Å2 per ethylene glycol group (film pressure 5 mN/m). The thickness of the surfactant alkyl chain was found to be 14 ± 2 Å for C12E2 and 8 ± 2 Å for C12E4 which indicates a very high degree of chain disorder. The thickness of the fatty acyl region of the DPPC is not affected by the surfactant for n = 2 and 4. The headgroup region thickness of the mixed monolayer is 14 ± 2 Å for C12E2 and 11 ± 2 Å for C12E4. While the diethylene glycol group exhibits a rather extended conformation, longer surfactant headgroups are prone to a significant incidence of gauche conformations. Moreover, the data indicate that the interaction of C12En with the monolayer is markedly different for n = 2 compared to all other ethylene glycol groups (n = 4, 6, 8) studied. © 1994, American Chemical Society. All rights reserved.
