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

Automated summary

This study introduces new amphiphiles with double and triple hedgehog tails, showing their effectiveness in reducing surface tension at the critical micelle concentration. Interestingly, the surface layer density is found to be a key factor in promoting low surface tension, rather than the total number of carbon and silicon atoms in the hydrophobic tails. Additionally, the exchange of surfactant counterions from Na+ to Mg2+ is shown to significantly reduce the surface tension, making the surfactant comparable to typical fluorinated surfactants.