Hypothesis: In previous studies we looked at the foam stability of various surfactants with C12 alkyl chains but different head groups and found that stable foams are only generated if the head groups are capable of forming hydrogen bonds with each other. Despite the consistency of the experimental data with the conclusions drawn from it we had no direct proof for our hypothesis that H-bonds are formed between surfactant head groups.
Experiments: To fill this gap, i.e. to demonstrate intersurfactant H-bond formation, we chose the non-ionic sugar surfactant n-dodecyl-β-d-maltoside (β-C12G2) and used molecular dynamics (MD) simulations as well as grazing-incidence X-ray (GIX) scattering and diffraction to study the surfactant-loaded air-water interface.
Findings: (1) In a densely packed monolayer, close to the critical micelle concentration (cmc), each head group of the sugar surfactant is involved in ∼5 intersurfactant H-bonds with other head groups and in ∼5 H-bonds with water molecules. (2) The number of intersurfactant H-bonds decreases, while the number of surfactant-water H-bonds increases with increasing distance between the head groups (below the cmc). (3) Even at very large distances (well below the cmc) there are still intersurfactant H-bonds, which we ascribe to the formation of clusters at the surface. (4) GIX scattering revealed that a homogeneous surfactant monolayer is formed at full coverage (around the cmc), i.e. cluster formation only happens below the cmc.
Keywords: Foam stability; Grazing-incidence X-ray scattering and diffraction; Intersurfactant H-bonds; MD simulations.
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