Effect of the fluorination degree of hydrophobic chains on the monolayer behavior of unsaturated diacylphosphatidylcholines bearing partially fluorinated 9-octadecynoyl (stearoloyl) groups at the air–water interface

The effect of the fluorination degree of hydrophobic chains on the monolayer behavior of unsaturated diacylphosphatidylcholines (PCs) was examined by employing a series of PCs bearing partially fluorinated 9-octadecynoyl (stearoloyl) groups (DFnStPCs, n: the number of fluorinated carbon atoms in a stearoloyl group; n = 1, 2, 4, 8), including their hydrophobic parts – partially fluorinated stearolic acids (FnStAs) – at the air–water interface. π–A isotherm measurements and Brewster angle microscope observations revealed: (i) all DFnStPCs including FnStAs form monolayers of liquid character at 25 °C; (ii) they form more expanded monolayers than their non-fluorinated counterparts, distearoloyl-PC (DStPC) and stearolic acid, while the monolayer stability increases with n; (iii) compared with DStPC and DF8StPC, DFnStPCs (n = 1, 2, 4) in the low-π region tend to show a weakening in their self-aggregation property and an increase in the work required for monolayer compression; (iv) although DF8StPC forms the most expanded monolayer, the behavior of DF8StPC resembles that of DStPC rather than that of DFnStPCs (n = 1, 2, 4). The monolayer behavior of DFnStPCs (n = 1, 2, 4) is explained by postulating a flatly-lying conformation of hydrophobic chains, in which three polar parts (ester group, triple bond, CF2CH2 linkage) in chains are immersed in the subphase at large areas. DStPC and DF8StPC lacking a CF2CH2 linkage, however, do not likely adopt such a conformation.