Synthesis and Surface Properties of CO₂H Type Gemini Surfactant Having Semifluoroalkyl Group as Hydrophobic Group

Novel CO 2 H type gemini surfactants having semifluoroalkyl group (Rf-(CH 2 ) n -: Rf = C 4 F 9 , C 6 F 13 , C 8 F 17 and n = 2, 3) as hydrophobic group were successfully synthesized and their surface properties were studied. (Perfluoroalkyl)ethylmalonic esters were synthesized by the reaction of malonic ester with Rf(CH 2 ) 2 I using NaH as base in THF/DMF. (Perfluoroalkyl)propylmalonic esters were synthesized from allylmalonic ester, namely by the radical addition of Rf-I using AIBN as initiator, and reduction with Zn/AcOH. Those fluoroalkylated malonic esters were connected using Br(CH 2 ) s Br (s = 3, 4) to give dimeric tetraesters. Then, tetraesters were hydrolyzed using KOH/EtOH followed by decarboxylation in AcOH to afford CO 2 H type gemini surfactants. To study their surface properties, both surface tension-concentration and surface pressure-area isotherms were measured. As expected, COOH geminis showed one order of magnitude lower critical micellar concentrations (CMC) than that of conventional 1+1 type surfactant. In the surface pressure-area measurements, as length of fluoroalkyl increased, lift-off area (A L ) decreased, monolayers were clearly formed upon compression and the limited areas (A ∞ ) for C 6 F 13 and C 8 F 17 were smaller than twice of that of C 8 F 17 (CH 2 ) 2 COOH. Especially, COOH geminis having Rf = C 8 F 17 and -(CH 2 ) 3 - spacer showed the formation of solid phase monolayers, and the planar five membered ring-like structure was proposed.