Hyperbranched hydrocarbon surfactants give fluorocarbon-like low surface energies

Two series of Aerosol-OT-analogue surfactants (sulfosuccinate-type di-BC_nSS and sulfoglutarate-type di-BC_nSG) with hyperbranched alkyl double tails (so-called "hedgehog" groups, carbon number n = 6, 9, 12, and 18) have been synthesized and shown to demonstrate interfacial properties comparable to those seen for related fluorocarbon (FC) systems. Critical micelle concentration (CMC), surface tension at the CMC (γ_CMC), and minimum area per molecule (A_min) were obtained from surface tension measurements of aqueous surfactant solutions. The results were examined for relationships between the structure of the hedgehog group and packing density at the interface. To evaluate A and B values in the Klevens equation for these hedgehog surfactants, log(CMC) was plotted as a function of the total carbon number in the surfactant double tail. A linear relationship was observed, producing B values of 0.20-0.25 for di-BC _nSS and di-BC_nSG, compared to a value of 0.31 for standard double-straight-tail sulfosuccinate surfactants. The lower B values of these hedgehog surfactants highlight their lower hydrophobicity compared to double-straight-tail surfactants. To clarify how hydrocarbon density in the surfactant-tail layer (ρ_layer) affects γ_CMC, the ρ_layer of each double-tail surfactant was calculated and the relationship between γ_CMC and ρ_layer examined. As expected for the design of low surface energy surfactant layers, ρ_layer was identified as an important property for controlling γ_CMC with higher ρ_layer, leading to a lower γ_CMC. Interestingly, surfactants with BC₉ and BC₁₂ tails achieved much lower γ_CMC, even at low ρ_layer values of <0.55 g cm^-3. The lowest surface energy surfactant studied here was di-BC₆SS, which had a γ_CMC of only 23.8 mN m^-1. Such a low γ_CMC is comparable to those obtained with short FC-tail surfactants (e.g., 22.0 mN m^-1 for the sulfosuccinate-type FC-surfactant with R = F(CF₂)₆CH₂CH ₂-).