Amino-Acids Surfactants and n-Octanol Mixtures RSustainable, Efficient, and Dynamically Triggered Foaming Systems

Surfactants with amino-acid heads (AASs), namely N-lauroyl-l-alanine (C-12-ALA), N-lauroyl-l-leucine (C-12-LEU), and N-lauroyl-l phenylalanine (C-12-PHE)were mixedwith n-octanol (C8OH) and thoroughly investigatedvia surface tension and foamability experiments. Significant differencesbetween the selected AAS were observed. Moreover, the results obtainedunder equilibrium and dynamic conditions were remarkably differentin terms of surface activity. The surface tension measurements (equilibriumconditions) indicated that the C-12-ALA/C(8)OHmixture exhibits the highest synergistic effect. For the C-12-LEU/C8OH system, a moderate synergism was observed, whilefor C-12-PHE/C8OH, the effect was antagonistic.In contrast, in foamability experiments (dynamic conditions), allof the studied AAS/C8OH mixtures exhibited a spectacularsynergistic effect at a wide concentration range. The observed effectwas referred to as convection-activated surface activity of AAS/C8OH mixtures. The obtained experimental results were interpretedon the molecular level via all-atom detail molecular dynamics simulations(MD). The observed phenomenon was connected with the hydrogen bond-mediatedaggregate formation in the bulk solution. Such aggregates act as reservoirsof surfactant molecules for supplementation of the adsorption coverageat the freshly formed liquid/gas interface. Additionally, the differencesbetween the specific AAS were explained via the interplay of AAS-AASand AAS-C8OH hydrogen bond affinities. The presented resultsshowed an interesting example of a foaming system in which the surfaceactivity can be controlled in situ via convection. This finding alsosignificantly expands the range of potentially interesting moleculesthat can be used as efficient foaming additives and impacts the currentunderstanding of the role of hydrogen bonding in designing of tuneablesurfactant mixtures.