School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, UK.
Division of Pharmacy and Optometry, Faculty of Biology, Medicine and Health, University of Manchester, Stopford Building, Oxford Road, Manchester M13 9PT, UK; Institut Laue-Langevin, 71 Avenue des Martyrs, CS20156, Grenoble 38042, France.
J Colloid Interface Sci. 2022 Jan 15;606(Pt 2):1064-1076. doi: 10.1016/j.jcis.2021.08.014. Epub 2021 Aug 16.
Neutral amphiphilic PEG-g-PVAc co-polymer (a "tardigrade" polymer consisting of a hydrophilic polyethylene glycol, PEG, backbone with hydrophobic polyvinyl acetate, PVAc, grafts) can form complexes at the air-water interface with cationic dodecyltrimethylammonium bromide (DTAB) via self-assembly. Compared to anionic SDS, cationic DTAB headgroups are expected to interact strongly with the negatively charged OH groups from the partial dissociation of the PVAc grafts. We anticipate a transition from synergistic to competitive behaviour, which is expected to be dependent on the surfactant structural characteristics and concentration.
DTAB/PEG-g-PVAc mixtures were investigated using a combination of dynamic and equilibrium surface tension measurements, neutron reflectivity (NR) at the air-water interface, and foaming tests. We varied the concentrations of both the DTAB (0.05 to 5 critical micelle concentration, cmc) and that of PEG-g-PVAc (0.2 and 2 critical aggregation concentration, cac).
Our results show that the interfacial interactions between DTAB and PEG-g-PVAc were both synergistic and antagonistic, depending sensitively on the surfactant concentration. At DTAB concentrations below its cmc, a pronounced cooperative adsorption behaviour was likely driven by the hydrophobic interactions between the DTAB tail and the PVAc grafts and the attraction between the DTAB headgroups and the partially dissociated -O groups in the partially hydrolysed PVAc grafts, forming a mixed layer. This synergistic adsorption behaviour transitioned to a competitive adsorption behaviour at DTAB concentrations above its cmc, leading to polymer-surfactant partition, forming a "hanging" polymer layer underlying a surfactant monolayer at the interface. We postulate that DTAB/PEG-g-PVAc complexation in the bulk contributed to partial depletion of the mixture from the interface. We therefore consider this polymer/surfactant system to be a moderately interacting system at the air-water interface. No discernible differences in the foaming behaviour were observed between the DTAB/PEG-g-PVAc systems and the pure surfactant. Our results suggest that surfactant headgroup characteristics (particularly charges) were crucial in determining the structure and composition of polymer-surfactant complexes at the air-water interface, as well as the foamability and foam stability, whilst the coexistence of the synergistic and competitive adsorption behaviour is attributed to the unique architecture of the tardigrade polymer with amphiphilicity and partial charge, facilitating different surfactant-polymer interactions at different DTAB concentrations.
中性两亲性 PEG-g-PVAc 共聚物（一种“缓步动物”聚合物，由亲水性聚乙二醇（PEG）主链和疏水性聚醋酸乙烯酯（PVAc）接枝组成）可以通过自组装在气-水界面与阳离子十二烷基三甲基溴化铵（DTAB）形成复合物。与阴离子 SDS 相比，预期阳离子 DTAB 头基会与部分解离的 PVAc 接枝的负电荷 OH 基团强烈相互作用。我们预计协同作用会转变为竞争行为，这预计取决于表面活性剂的结构特征和浓度。
使用动态和平衡表面张力测量、空气-水界面的中子反射率（NR）和起泡测试组合研究了 DTAB/PEG-g-PVAc 混合物。我们改变了 DTAB（0.05 至 5 临界胶束浓度（cmc））和 PEG-g-PVAc（0.2 和 2 临界聚集浓度（cac））的浓度。
我们的结果表明，DTAB 和 PEG-g-PVAc 之间的界面相互作用既有协同作用，也有拮抗作用，这取决于表面活性剂的浓度。在 DTAB 浓度低于其 cmc 时，由于 DTAB 尾部和 PVAc 接枝之间的疏水性相互作用以及 DTAB 头基与部分水解的 PVAc 接枝中的部分解离-O 基团之间的吸引力，可能会驱动明显的协同吸附行为，形成混合层。这种协同吸附行为在 DTAB 浓度高于其 cmc 时转变为竞争吸附行为，导致聚合物-表面活性剂的分配，在界面下形成一个“悬挂”聚合物层和一个表面活性剂单层。我们推测，在本体中 DTAB/PEG-g-PVAc 络合有助于从界面部分耗尽混合物。因此，我们认为该聚合物/表面活性剂体系在气-水界面是一种中等相互作用的体系。在 DTAB/PEG-g-PVAc 体系和纯表面活性剂之间没有观察到明显的起泡行为差异。我们的结果表明，表面活性剂头基特性（特别是电荷）对于决定聚合物-表面活性剂复合物在气-水界面的结构和组成以及起泡性和泡沫稳定性至关重要，而协同和竞争吸附行为的共存归因于具有两亲性和部分电荷的缓步动物聚合物的独特结构，这使得在不同的 DTAB 浓度下可以进行不同的表面活性剂-聚合物相互作用。
