Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia.
Department of Physics, Technische Universität Darmstadt, Hochschulstrasse 8, 64289 Darmstadt, Germany.
J Colloid Interface Sci. 2021 Mar 15;586:588-595. doi: 10.1016/j.jcis.2020.10.125. Epub 2020 Nov 3.
In previous studies we looked at the foam stability of various surfactants with C 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.
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-CG) 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.
(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.
在之前的研究中，我们研究了具有 C 烷基链但不同头基的各种表面活性剂的泡沫稳定性，发现只有当头基能够相互形成氢键时，才会产生稳定的泡沫。尽管实验数据与从中得出的结论一致，但我们没有直接证据证明我们的假设，即表面活性剂头基之间形成氢键。
为了填补这一空白，即证明表面活性剂之间形成氢键，我们选择了非离子糖表面活性剂 n-十二基-β-d-麦芽糖苷（β-CG），并使用分子动力学（MD）模拟以及掠入射 X 射线（GIX）散射和衍射来研究表面活性剂负载的气-液界面。
（1）在紧密堆积的单层中，接近临界胶束浓度（cmc）时，糖表面活性剂的每个头基都与其他头基形成约 5 个氢键，并与水分子形成约 5 个氢键。（2）随着头基之间的距离（低于 cmc）增加，氢键的数量减少，而表面活性剂-水氢键的数量增加。（3）即使在非常大的距离（远低于 cmc）处，仍然存在氢键，我们将其归因于表面形成的簇。（4）GIX 散射表明，在完全覆盖时（约在 cmc 处）形成均匀的表面活性剂单层，即簇形成仅发生在 cmc 以下。
