Effect of perfluoroalkyl chain length on monolayer behavior of partially fluorinated oleic acid molecules at the air–water interface

• A series of partially fluorinated oleic acid analogs (F-OA) was synthesized.
• Terminal short F-chain in F-OA determines the monolayer stability.
• Lengthening of F-chain in F-OA increases in the monolayer stability.
• Monolayer behavior of F-OA shows even/odd alteration with regard to F-chain length.
• Monolayer behavior of F-OA with even-numbered F-chain is similar to that of OA.

A series of oleic acid (OA) analogs containing terminal perfluoroalkyl groups (CF3, C2F5, n-C3F7, n-C4F9 or n-C8F17) was synthesized to clarify how the fluorinated chain length affects the stability and molecular packing of liquid-expanded OA monolayers at the air–water interface. Although the substitution of terminal CF3 group for CH3 in OA had no effect on monolayer stability, further fluorination led to a gradual increase in monolayer stability at 25 ̊C. Surface pressure–area isotherm revealed that partially fluorinated OA analogs form more expanded monolayers than OA at low surface pressures, and that the monolayer behavior of OA analogs with the even-carbon numbered fluorinated chain is almost the same as that of OA upon monolayer compression, whereas the behavior of OA analogs with the odd-carbon numbered fluorinated chain significantly differs from that of OA. These results indicate: (i) the terminal short part (at least C2 residue) in OA predominantly determines the liquid-expanded monolayer stability; (ii) the molecular packing state of OA may be perturbed by the substitution of a short odd-carbon numbered fluorinated chain; (iii) hence, OA analogs with even-carbon numbered chain are considered to be preferable as hydrophobic building blocks for the synthesis of fluorinated phospholipids.

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