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 γCMC ~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 γCMC, the effective hydrophobic-tail layer density (ρlayer) and total number of carbon and silicon atoms in the hydrophobic tails (NC+Si). Interestingly, γCMC is seen to depend on ρlayer rather than NC+Si, and ρlayer ~0.63 g cm−3 appears to be an optimal surface layer density for promoting low γCMC. For a surfactant bearing trimethysilyl (TMS) chain tips, exchanging the surfactant counterions from Na+ to Mg2+ reduced γCMC 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 γCMC and CMC TMS-terminal tips are shown to be key groups for promoting hydrophobicity and/or low surface tensions.