Very low surface tensions with "Hedgehog" surfactants.

Previous studies [Langmuir, 2014, 30, 21, 6057–6063, Phys. Chem. Chem. Phys., 2017,19, 23869–23877] have shown that surfactants bearing highly-methylated alkyl tails (so-called "hedgehog" groups) are able to reduce the limiting surface tension at the aqueous critical micelle concentration (CMC) to γ CMC ~24 mN m−1, which is considerably lower than for common n -alkyl tail surfactants (30–40 mN m−1). In the quest to develop even more effective and efficient non-fluorinated surfactants, this study introduces new amphiphiles having double and triple hedgehog tails and examines relationships between surfactant structure and aqueous solution properties. Of particular interest are links between γ CMC , the effective hydrophobic-tail layer density (ρ layer) and total number of carbon and silicon atoms in the hydrophobic tails (N C+Si). Interestingly, γ CMC is seen to depend on ρ layer rather than N C+Si , and ρ layer ~0.63 g cm−3 appears to be an optimal surface layer density for promoting low γ CMC. For a surfactant bearing trimethysilyl (TMS) chain tips, exchanging the surfactant counterions from Na+ to Mg2+ reduced γ CMC from 23.8 mN m−1 to 21.5 mN m−1, which is very low for a hydrocarbon surfactant, and comparable to typical fluorinated surfactants. A new measure of the ability of different surfactants to lower surface tension is proposed, which is helpful for targeting low surface energy (tension) non-fluorinated surfactants. In terms of both γ CMC and CMC TMS-terminal tips are shown to be key groups for promoting hydrophobicity and/or low surface tensions. [Display omitted] • Highly-methyl-branched-tail (hedgehog) surfactants were synthesized and evaluated. • Surfactants with Mg2+ and TMS tips attained aqueous surface tensions of ~21.5 mN m−1. • Low surface tensions were achieved with apparent hydrophobic layer densities of ~0.63 g cm−3. • TMS tips resulted in larger hydrophobicity and/or surface tension reducing ability per CH 2 unit. [ABSTRACT FROM AUTHOR]