Branched fluorinated nonionic Y-shaped surfactants. Effect of molecular geometry on liquid crystalline phase behavior

The synthesis of branched partially fluorinated nonionic Y-shaped surfactants, with the general formula (CnC2N)-C-F(E(m)M)(2), where C-n(F) denotes fluorinated alkyl chain F(CF2)(n) with n = 4-10, C-2 is C2H2, and E(m)M is oligooxyethylene monomethyl ether with m = 2, 3, and their hydrocarbon analogs Cn+2N(E(m)M)(2) (C-n equals hydrocarbon chain) is described. These surfactants are synthesized to analyze the influence of the fluorinated alkyl chain on the packing effects to amphiphilic layers in a binary surfactant/water system. The lyotropic phase behavior of these surfactants is investigated by polarizing microscopy. The liquid crystalline (LC) phase polymorphism of the fluorinated (CnC2N)-C-F(E(3)M)(2) surfactants strongly changes with n from hexagonal H-1 phase (n = 6) to lamellar L(alpha) phase (n = 10). In contrast to other Y-shaped hydrocarbon oligooxyethylene surfactants C(m)G(E(m)M)(2) (where G denotes a glycerol unit) which form only cubic I-1 and H-1 phases for n = 12-16 and m = 3-5, the phase behavior of the (CnC2N)-C-F(E(m)M)(2) surfactant/water systems shows a remarkable change in the LC phase polymorphism (from H-1 to L(alpha) phase) with n and m. Compared to hydrocarbon surfactants these results distinctly confirm the influence of more rigid fluorinated alkyl chain on the phase behavior.